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17.4: Drugs Targeting Other Microorganisms - Biology

17.4: Drugs Targeting Other Microorganisms - Biology


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Skills to Develop

  • Explain the differences between modes of action of drugs that target fungi, protozoa, helminths, and viruses

Because fungi, protozoa, and helminths are eukaryotic, their cells are very similar to human cells, making it more difficult to develop drugs with selective toxicity. Additionally, viruses replicate within human host cells, making it difficult to develop drugs that are selectively toxic to viruses or virus-infected cells. Despite these challenges, there are antimicrobial drugs that target fungi, protozoa, helminths, and viruses, and some even target more than one type of microbe. Table (PageIndex{1}), Table (PageIndex{2}), Table (PageIndex{3}), and Table (PageIndex{4}) provide examples for antimicrobial drugs in these various classes.

Antifungal Drugs

The most common mode of action for antifungal drugs is the disruption of the cell membrane. Antifungals take advantage of small differences between fungi and humans in the biochemical pathways that synthesize sterols. The sterols are important in maintaining proper membrane fluidity and, hence, proper function of the cell membrane. For most fungi, the predominant membrane sterol is ergosterol. Because human cell membranes use cholesterol, instead of ergosterol, antifungal drugs that target ergosterol synthesis are selectively toxic (Figure (PageIndex{1})).

Figure (PageIndex{1}): The predominant sterol found in human cells is cholesterol, whereas the predominant sterol found in fungi is ergosterol, making ergosterol a good target for antifungal drug development.

The imidazoles are synthetic fungicides that disrupt ergosterol biosynthesis; they are commonly used in medical applications and also in agriculture to keep seeds and harvested crops from molding. Examples include miconazole, ketoconazole, and clotrimazole, which are used to treat fungal skin infections such as ringworm, specifically tinea pedis (athlete’s foot), tinea cruris (jock itch), and tinea corporis. These infections are commonly caused by dermatophytes of the genera Trichophyton, Epidermophyton, and Microsporum. Miconazole is also used predominantly for the treatment of vaginal yeast infections caused by the fungus Candida, and ketoconazole is used for the treatment of tinea versicolor and dandruff, which both can be caused by the fungus Malassezia.

The triazole drugs, including fluconazole, also inhibit ergosterol biosynthesis. However, they can be administered orally or intravenously for the treatment of several types of systemic yeast infections, including oral thrush and cryptococcal meningitis, both of which are prevalent in patients with AIDS. The triazoles also exhibit more selective toxicity, compared with the imidazoles, and are associated with fewer side effects.

The allylamines, a structurally different class of synthetic antifungal drugs, inhibit an earlier step in ergosterol biosynthesis. The most commonly used allylamine is terbinafine (marketed under the brand name Lamisil), which is used topically for the treatment of dermatophytic skin infections like athlete’s foot, ringworm, and jock itch. Oral treatment with terbinafine is also used for the treatment of fingernail and toenail fungus, but it can be associated with the rare side effect of hepatotoxicity.

The polyenes are a class of antifungal agents naturally produced by certain actinomycete soil bacteria and are structurally related to macrolides. These large, lipophilic molecules bind to ergosterol in fungal cytoplasmic membranes, thus creating pores. Common examples include nystatin and amphotericin B. Nystatin is typically used as a topical treatment for yeast infections of the skin, mouth, and vagina, but may also be used for intestinal fungal infections. The drug amphotericin B is used for systemic fungal infections like aspergillosis, cryptococcal meningitis, histoplasmosis, blastomycosis, and candidiasis. Amphotericin B was the only antifungal drug available for several decades, but its use is associated with some serious side effects, including nephrotoxicity (kidney toxicity).

Amphotericin B is often used in combination with flucytosine, a fluorinated pyrimidine analog that is converted by a fungal-specific enzyme into a toxic product that interferes with both DNA replication and protein synthesis in fungi. Flucytosine is also associated with hepatotoxicity (liver toxicity) and bone marrow depression.

Beyond targeting ergosterol in fungal cell membranes, there are a few antifungal drugs that target other fungal structures (Figure (PageIndex{2})). The echinocandins, including caspofungin, are a group of naturally produced antifungal compounds that block the synthesis of β(1→3) glucan found in fungal cell walls but not found in human cells. This drug class has the nickname “penicillin for fungi.” Caspofungin is used for the treatment of aspergillosis as well as systemic yeast infections.

Although chitin is only a minor constituent of fungal cell walls, it is also absent in human cells, making it a selective target. The polyoxins and nikkomycins are naturally produced antifungals that target chitin synthesis. Polyoxins are used to control fungi for agricultural purposes, and nikkomycin Z is currently under development for use in humans to treat yeast infections and Valley fever (coccidioidomycosis), a fungal disease prevalent in the southwestern US.1

The naturally produced antifungal griseofulvin is thought to specifically disrupt fungal cell division by interfering with microtubules involved in spindle formation during mitosis. It was one of the first antifungals, but its use is associated with hepatotoxicity. It is typically administered orally to treat various types of dermatophytic skin infections when other topical antifungal treatments are ineffective.

There are a few drugs that act as antimetabolites against fungal processes. For example, atovaquone, a representative of the naphthoquinone drug class, is a semisynthetic antimetabolite for fungal and protozoal versions of a mitochondrial cytochrome important in electron transport. Structurally, it is an analog of coenzyme Q, with which it competes for electron binding. It is particularly useful for the treatment of Pneumocystis pneumonia caused by Pneumocystis jirovecii. The antibacterial sulfamethoxazole-trimethoprim combination also acts as an antimetabolite against P. jirovecii.

Table (PageIndex{1}) shows the various therapeutic classes of antifungal drugs, categorized by mode of action, with examples of each.

Figure (PageIndex{2}): Antifungal drugs target several different cell structures. (credit right: modification of work by “Maya and Rike”/Wikimedia Commons)

Table (PageIndex{1}): Common Antifungal Drugs
Mechanism of ActionDrug ClassSpecific DrugsClinical Uses
Inhibit ergosterol synthesisImidazolesMiconazole, ketoconazole, clotrimazoleFungal skin infections and vaginal yeast infections
TriazolesFluconazoleSystemic yeast infections, oral thrush, and cryptococcal meningitis
AllylaminesTerbinafineDermatophytic skin infections (athlete’s foot, ring worm, jock itch), and infections of fingernails and toenails
Bind ergosterol in the cell membrane and create pores that disrupt the membranePolyenesNystatinUsed topically for yeast infections of skin, mouth, and vagina; also used for fungal infections of the intestine
Amphotericin BVariety systemic fungal infections
Inhibit cell wall synthesisEchinocandinsCaspofunginAspergillosis and systemic yeast infections
Not applicableNikkomycin ZCoccidioidomycosis (Valley fever) and yeast infections
Inhibit microtubules and cell divisionNot applicableGriseofulvinDermatophytic skin infections

Exercise (PageIndex{1})

How is disruption of ergosterol biosynthesis an effective mode of action for antifungals?

TREATING A FUNGAL INFECTION OF THE LUNGS

Jack, a 48-year-old engineer, is HIV positive but generally healthy thanks to antiretroviral therapy (ART). However, after a particularly intense week at work, he developed a fever and a dry cough. He assumed that he just had a cold or mild flu due to overexertion and didn’t think much of it. However, after about a week, he began to experience fatigue, weight loss, and shortness of breath. He decided to visit his physician, who found that Jack had a low level of blood oxygenation. The physician ordered blood testing, a chest X-ray, and the collection of an induced sputum sample for analysis. His X-ray showed a fine cloudiness and several pneumatoceles (thin-walled pockets of air), which indicated Pneumocystis pneumonia (PCP), a type of pneumonia caused by the fungus Pneumocystis jirovecii. Jack’s physician admitted him to the hospital and prescribed Bactrim, a combination of sulfamethoxazole and trimethoprim, to be administered intravenously.

P. jirovecii is a yeast-like fungus with a life cycle similar to that of protozoans. As such, it was classified as a protozoan until the 1980s. It lives only in the lung tissue of infected persons and is transmitted from person to person, with many people exposed as children. Typically, P. jirovecii only causes pneumonia in immunocompromised individuals. Healthy people may carry the fungus in their lungs with no symptoms of disease. PCP is particularly problematic among HIV patients with compromised immune systems.

PCP is usually treated with oral or intravenous Bactrim, but atovaquone or pentamidine(another antiparasitic drug) are alternatives. If not treated, PCP can progress, leading to a collapsed lung and nearly 100% mortality. Even with antimicrobial drug therapy, PCP still is responsible for 10% of HIV-related deaths.

The cytological examination, using direct immunofluorescence assay (DFA), of a smear from Jack’s sputum sample confirmed the presence of P. jirovecii (Figure (PageIndex{3})). Additionally, the results of Jack’s blood tests revealed that his white blood cell count had dipped, making him more susceptible to the fungus. His physician reviewed his ART regimen and made adjustments. After a few days of hospitalization, Jack was released to continue his antimicrobial therapy at home. With the adjustments to his ART therapy, Jack’s CD4 counts began to increase and he was able to go back to work.

Figure (PageIndex{3}): Microscopic examination of an induced sputum sample or bronchoaveolar lavage sample typically reveals the organism, as shown here. (credit: modification of work by the Centers for Disease Control and Prevention)

Antiprotozoan Drugs

There are a few mechanisms by which antiprotozoan drugs target infectious protozoans (Table (PageIndex{3})). Some are antimetabolites, such as atovaquone, proguanil, and artemisinins. Atovaquone, in addition to being antifungal, blocks electron transport in protozoans and is used for the treatment of protozoan infections including malaria, babesiosis, and toxoplasmosis. Proguanil is another synthetic antimetabolite that is processed in parasitic cells into its active form, which inhibits protozoan folic acid synthesis. It is often used in combination with atovaquone, and the combination is marketed as Malarone for both malaria treatment and prevention.

Artemisinin, a plant-derived antifungal first discovered by Chinese scientists in the 1970s, is quite effective against malaria. Semisynthetic derivatives of artemisinin are more water soluble than the natural version, which makes them more bioavailable. Although the exact mechanism of action is unclear, artemisinins appear to act as prodrugs that are metabolized by target cells to produce reactive oxygen species (ROS) that damage target cells. Due to the rise in resistance to antimalarial drugs, artemisinins are also commonly used in combination with other antimalarial compounds in artemisinin-based combination therapy (ACT).

Several antimetabolites are used for the treatment of toxoplasmosis caused by the parasite Toxoplasma gondii. The synthetic sulfa drug sulfadiazine competitively inhibits an enzyme in folic acid production in parasites and can be used to treat malaria and toxoplasmosis. Pyrimethamine is a synthetic drug that inhibits a different enzyme in the folic acid production pathway and is often used in combination with sulfadoxine (another sulfa drug) for the treatment of malariaor in combination with sulfadiazine for the treatment of toxoplasmosis. Side effects of pyrimethamine include decreased bone marrow activity that may cause increased bruising and low red blood cell counts. When toxicity is a concern, spiramycin, a macrolide protein synthesis inhibitor, is typically administered for the treatment of toxoplasmosis.

Two classes of antiprotozoan drugs interfere with nucleic acid synthesis: nitroimidazoles and quinolines. Nitroimidazoles, including semisynthetic metronidazole, which was discussed previously as an antibacterial drug, and synthetic tinidazole, are useful in combating a wide variety of protozoan pathogens, such as Giardia lamblia, Entamoeba histolytica, and Trichomonas vaginalis. Upon introduction into these cells in low-oxygen environments, nitroimidazoles become activated and introduce DNA strand breakage, interfering with DNA replication in target cells. Unfortunately, metronidazole is associated with carcinogenesis (the development of cancer) in humans.

Another type of synthetic antiprotozoan drug that has long been thought to specifically interfere with DNA replication in certain pathogens is pentamidine. It has historically been used for the treatment of African sleeping sickness (caused by the protozoan Trypanosoma brucei) and leishmaniasis (caused by protozoa of the genus Leishmania), but it is also an alternative treatment for the fungus Pneumocystis. Some studies indicate that it specifically binds to the DNA found within kinetoplasts (kDNA; long mitochondrion-like structures unique to trypanosomes), leading to the cleavage of kDNA. However, nuclear DNA of both the parasite and host remain unaffected. It also appears to bind to tRNA, inhibiting the addition of amino acids to tRNA, thus preventing protein synthesis. Possible side effects of pentamidine use include pancreatic dysfunction and liver damage.

The quinolines are a class of synthetic compounds related to quinine, which has a long history of use against malaria. Quinolines are thought to interfere with heme detoxification, which is necessary for the parasite’s effective breakdown of hemoglobin into amino acids inside red blood cells. The synthetic derivatives chloroquine, quinacrine (also called mepacrine), and mefloquine are commonly used as antimalarials, and chloroquine is also used to treat amebiasis typically caused by Entamoeba histolytica. Long-term prophylactic use of chloroquine or mefloquine may result in serious side effects, including hallucinations or cardiac issues. Patients with glucose-6-phosphate dehydrogenase deficiency experience severe anemia when treated with chloroquine.

Table (PageIndex{2}):Common Antiprotozoan Drugs
Mechanism of ActionDrug ClassSpecific DrugsClinical Uses
Inhibit electron transport in mitochondriaNaphthoquinoneAtovaquoneMalaria, babesiosis, and toxoplasmosis
Inhibit folic acid synthesisNot applicableProquanilCombination therapy with atovaquone for malaria treatment and prevention
SulfonamideSulfadiazineMalaria and toxoplasmosis
Not applicablePyrimethamineCombination therapy with sulfadoxine (sulfa drug) for malaria
Produces damaging reactive oxygen speciesNot applicableArtemisininCombination therapy to treat malaria
Inhibit DNA synthesisNitroimidazolesMetronidazole, tinidazoleInfections caused by Giardia lamblia, Entamoeba histolytica, and Trichomonas vaginalis
Not applicablePentamidineAfrican sleeping sickness and leishmaniasis
Inhibit heme detoxificationQuinolinesChloroquineMalaria and infections with E. histolytica
Mepacrine, mefloquineMalaria

Exercise (PageIndex{2})

List two modes of action for antiprotozoan drugs.

Because helminths are multicellular eukaryotes like humans, developing drugs with selective toxicity against them is extremely challenging. Despite this, several effective classes have been developed (Table (PageIndex{3})). Synthetic benzimidazoles, like mebendazole and albendazole, bind to helminthic β-tubulin, preventing microtubule formation. Microtubules in the intestinal cells of the worms seem to be particularly affected, leading to a reduction in glucose uptake. Besides their activity against a broad range of helminths, benzimidazoles are also active against many protozoans, fungi, and viruses, and their use for inhibiting mitosis and cell cycle progression in cancer cells is under study.2 Possible side effects of their use include liver damage and bone marrow suppression.

The avermectins are members of the macrolide family that were first discovered from a Japanese soil isolate, Streptomyces avermectinius. A more potent semisynthetic derivative of avermectin is ivermectin, which binds to glutamate-gated chloride channels specific to invertebrates including helminths, blocking neuronal transmission and causing starvation, paralysis, and death of the worms. Ivermectin is used to treat roundworm diseases, including onchocerciasis (also called river blindness, caused by the worm Onchocerca volvulus) and strongyloidiasis (caused by the worm Strongyloides stercoralis or S. fuelleborni). Ivermectin also can also treat parasitic insects like mites, lice, and bed bugs, and is nontoxic to humans.

Niclosamide is a synthetic drug that has been used for over 50 years to treat tapeworm infections. Although its mode of action is not entirely clear, niclosamide appears to inhibit ATP formation under anaerobic conditions and inhibit oxidative phosphorylation in the mitochondria of its target pathogens. Niclosamide is not absorbed from the gastrointestinal tract, thus it can achieve high localized intestinal concentrations in patients. Recently, it has been shown to also have antibacterial, antiviral, and antitumor activities.345

Another synthetic antihelminthic drug is praziquantel, which used for the treatment of parasitic tapeworms and liver flukes, and is particularly useful for the treatment of schistosomiasis (caused by blood flukes from three genera of Schistosoma). Its mode of action remains unclear, but it appears to cause the influx of calcium into the worm, resulting in intense spasm and paralysis of the worm. It is often used as a preferred alternative to niclosamide in the treatment of tapeworms when gastrointestinal discomfort limits niclosamide use.

The thioxanthenones, another class of synthetic drugs structurally related to quinine, exhibit antischistosomal activity by inhibiting RNA synthesis. The thioxanthenone lucanthone and its metabolite hycanthone were the first used clinically, but serious neurological, gastrointestinal, cardiovascular, and hepatic side effects led to their discontinuation. Oxamniquine, a less toxic derivative of hycanthone, is only effective against S. mansoni, one of the three species known to cause schistosomiasis in humans. Praziquantel was developed to target the other two schistosome species, but concerns about increasing resistance have renewed interest in developing additional derivatives of oxamniquine to target all three clinically important schistosome species.

Table (PageIndex{3}): Common Antihelminthic Drugs
Mechanism of ActionDrug ClassSpecific DrugsClinical Uses
Inhibit microtubule formation, reducing glucose uptakeBenzimidazolesMebendazole, albendazoleVariety of helminth infections
Block neuronal transmission, causing paralysis and starvationAvermectinsIvermectinRoundworm diseases, including river blindness and strongyloidiasis, and treatment of parasitic insects
Inhibit ATP productionNot applicableNiclosamideIntestinal tapeworm infections
Induce calcium influxNot applicablePraziquantelSchistosomiasis (blood flukes)
Inhibit RNA synthesisThioxanthenonesLucanthone, hycanthone, oxamniquineSchistosomiasis (blood flukes)

Exercise (PageIndex{3})

Why are antihelminthic drugs difficult to develop?

Unlike the complex structure of fungi, protozoa, and helminths, viral structure is simple, consisting of nucleic acid, a protein coat, viral enzymes, and, sometimes, a lipid envelope. Furthermore, viruses are obligate intracellular pathogens that use the host’s cellular machinery to replicate. These characteristics make it difficult to develop drugs with selective toxicity against viruses.

Many antiviral drugs are nucleoside analogs and function by inhibiting nucleic acid biosynthesis. For example, acyclovir(marketed as Zovirax) is a synthetic analog of the nucleoside guanosine (Figure (PageIndex{4})). It is activated by the herpes simplex viral enzyme thymidine kinase and, when added to a growing DNA strand during replication, causes chain termination. Its specificity for virus-infected cells comes from both the need for a viral enzyme to activate it and the increased affinity of the activated form for viral DNA polymerase compared to host cell DNA polymerase. Acyclovir and its derivatives are frequently used for the treatment of herpes virus infections, including genital herpes, chickenpox, shingles, Epstein-Barr virus infections, and cytomegalovirus infections. Acyclovir can be administered either topically or systemically, depending on the infection. One possible side effect of its use includes nephrotoxicity. The drug adenine-arabinoside, marketed as vidarabine, is a synthetic analog to deoxyadenosine that has a mechanism of action similar to that of acyclovir. It is also effective for the treatment of various human herpes viruses. However, because of possible side effects involving low white blood cell counts and neurotoxicity, treatment with acyclovir is now preferred.

Ribavirin, another synthetic guanosine analog, works by a mechanism of action that is not entirely clear. It appears to interfere with both DNA and RNA synthesis, perhaps by reducing intracellular pools of guanosine triphosphate (GTP). Ribavarin also appears to inhibit the RNA polymerase of hepatitis C virus. It is primarily used for the treatment of the RNA viruses like hepatitis C (in combination therapy with interferon) and respiratory syncytial virus. Possible side effects of ribavirin use include anemia and developmental effects on unborn children in pregnant patients. In recent years, another nucleotide analog, sofosbuvir (Solvaldi), has also been developed for the treatment of hepatitis C. Sofosbuvir is a uridine analog that interferes with viral polymerase activity. It is commonly coadministered with ribavirin, with and without interferon.

Inhibition of nucleic acid synthesis is not the only target of synthetic antivirals. Although the mode of action of amantadine and its relative rimantadine are not entirely clear, these drugs appear to bind to a transmembrane protein that is involved in the escape of the influenza virus from endosomes. Blocking escape of the virus also prevents viral RNA release into host cells and subsequent viral replication. Increasing resistance has limited the use of amantadine and rimantadine in the treatment of influenza A. Use of amantadine can result in neurological side effects, but the side effects of rimantadine seem less severe. Interestingly, because of their effects on brain chemicals such as dopamine and NMDA (N-methyl D-aspartate), amantadine and rimantadine are also used for the treatment of Parkinson’s disease.

Neuraminidase inhibitors, including olsetamivir (Tamiflu), zanamivir (Relenza), and peramivir (Rapivab), specifically target influenza viruses by blocking the activity of influenza virus neuraminidase, preventing the release of the virus from infected cells. These three antivirals can decrease flu symptoms and shorten the duration of illness, but they differ in their modes of administration: olsetamivir is administered orally, zanamivir is inhaled, and peramivir is administered intravenously. Resistance to these neuraminidase inhibitors still seems to be minimal.

Pleconaril is a synthetic antiviral under development that showed promise for the treatment of picornaviruses. Use of pleconaril for the treatment of the common cold caused by rhinoviruses was not approved by the FDA in 2002 because of lack of proven effectiveness, lack of stability, and association with irregular menstruation. Its further development for this purpose was halted in 2007. However, pleconaril is still being investigated for use in the treatment of life-threatening complications of enteroviruses, such as meningitis and sepsis. It is also being investigated for use in the global eradication of a specific enterovirus, polio.6 Pleconaril seems to work by binding to the viral capsid and preventing the uncoating of viral particles inside host cells during viral infection.

Viruses with complex life cycles, such as HIV, can be more difficult to treat. First, HIV targets CD4-positive white blood cells, which are necessary for a normal immune response to infection. Second, HIV is a retrovirus, meaning that it converts its RNA genome into a DNA copy that integrates into the host cell’s genome, thus hiding within host cell DNA. Third, the HIV reverse transcriptase lacks proofreading activity and introduces mutations that allow for rapid development of antiviral drug resistance. To help prevent the emergence of resistance, a combination of specific synthetic antiviral drugs is typically used in ART for HIV (Figure).

The reverse transcriptase inhibitors block the early step of converting viral RNA genome into DNA, and can include competitive nucleoside analog inhibitors (e.g., azidothymidine/zidovudine, or AZT) and non-nucleoside noncompetitive inhibitors (e.g., etravirine) that bind reverse transcriptase and cause an inactivating conformational change. Drugs called protease inhibitors (e.g., ritonavir) block the processing of viral proteins and prevent viral maturation. Protease inhibitors are also being developed for the treatment of other viral types.7 For example, simeprevir (Olysio) has been approved for the treatment of hepatitis C and is administered with ribavirin and interferon in combination therapy. The integrase inhibitors (e.g., raltegravir), block the activity of the HIV integrase responsible for the recombination of a DNA copy of the viral genome into the host cell chromosome. Additional drug classes for HIV treatment include the CCR5 antagonists and the fusion inhibitors (e.g., enfuviritide), which prevent the binding of HIV to the host cell coreceptor (chemokine receptor type 5 [CCR5]) and the merging of the viral envelope with the host cell membrane, respectively. Table (PageIndex{4}) shows the various therapeutic classes of antiviral drugs, categorized by mode of action, with examples of each.

Figure (PageIndex{4}): Acyclovir is a structural analog of guanosine. It is specifically activated by the viral enzyme thymidine kinase and then preferentially binds to viral DNA polymerase, leading to chain termination during DNA replication.

Figure (PageIndex{5}): Antiretroviral therapy (ART) is typically used for the treatment of HIV. The targets of drug classes currently in use are shown here. (credit: modification of work by Thomas Splettstoesser)

Table (PageIndex{4}): Common Antiviral Drugs
Mechanism of ActionDrugClinical Uses
Nucleoside analog inhibition of nucleic acid synthesisAcyclovirHerpes virus infections
Azidothymidine/zidovudine (AZT)HIV infections
RibavirinHepatitis C virus and respiratory syncytial virus infections
VidarabineHerpes virus infections
SofosbuvirHepatitis C virus infections
Non-nucleoside noncompetitive inhibitionEtravirineHIV infections
Inhibit escape of virus from endosomesAmantadine, rimantadineInfections with influenza virus
Inhibit neuraminadaseOlsetamivir, zanamivir, peramivirInfections with influenza virus
Inhibit viral uncoatingPleconarilSerious enterovirus infections
Inhibition of proteaseRitonavirHIV infections
SimeprevirHepatitis C virus infections
Inhibition of integraseRaltegravirHIV infections
Inhibition of membrane fusionEnfuviritideHIV infections

Exercise (PageIndex{4})

Why is HIV difficult to treat with antivirals?

To learn more about the various classes of antiretroviral drugs used in the ART of HIV infection, explore each of the drugs in the HIV drug classes provided by US Department of Health and Human Services at this website.

Key Concepts and Summary

  • Because fungi, protozoans, and helminths are eukaryotic organisms like human cells, it is more challenging to develop antimicrobial drugs that specifically target them. Similarly, it is hard to target viruses because human viruses replicate inside of human cells.
  • Antifungal drugs interfere with ergosterol synthesis, bind to ergosterol to disrupt fungal cell membrane integrity, or target cell wall-specific components or other cellular proteins.
  • Antiprotozoan drugs increase cellular levels of reactive oxygen species, interfere with protozoal DNA replication (nuclear versus kDNA, respectively), and disrupt heme detoxification.
  • Antihelminthic drugs disrupt helminthic and protozoan microtubule formation; block neuronal transmissions; inhibit anaerobic ATP formation and/or oxidative phosphorylation; induce a calcium influx in tapeworms, leading to spasms and paralysis; and interfere with RNA synthesis in schistosomes.
  • Antiviral drugs inhibit viral entry, inhibit viral uncoating, inhibit nucleic acid biosynthesis, prevent viral escape from endosomes in host cells, and prevent viral release from infected cells.
  • Because it can easily mutate to become drug resistant, HIV is typically treated with a combination of several antiretroviral drugs, which may include reverse transcriptase inhibitors, protease inhibitors, integrase inhibitors, and drugs that interfere with viral binding and fusion to initiate infection.

Footnotes

  1. 1 Centers for Disease Control and Prevention. “Valley Fever: Awareness Is Key.” http://www.cdc.gov/features/valleyfever/. Accessed June 1, 2016.
  2. 2 B. Chu et al. “A Benzimidazole Derivative Exhibiting Antitumor Activity Blocks EGFR and HER2 Activity and Upregulates DR5 in Breast Cancer Cells.” Cell Death and Disease 6 (2015):e1686
  3. 3 J.-X. Pan et al. “Niclosamide, An Old Antihelminthic Agent, Demonstrates Antitumor Activity by Blocking Multiple Signaling Pathways of Cancer Stem Cells.” Chinese Journal of Cancer 31 no. 4 (2012):178–184.
  4. 4 F. Imperi et al. “New Life for an Old Drug: The Anthelmintic Drug Niclosamide Inhibits Pseudomonas aeruginosa Quorum Sensing.” Antimicrobial Agents and Chemotherapy 57 no. 2 (2013):996-1005.
  5. 5 A. Jurgeit et al. “Niclosamide Is a Proton Carrier and Targets Acidic Endosomes with Broad Antiviral Effects.” PLoS Pathogens 8 no. 10 (2012):e1002976.
  6. 6 M.J. Abzug. “The Enteroviruses: Problems in Need of Treatments.” Journal of Infection 68 no. S1 (2014):108–14.
  7. 7 B.L. Pearlman. “Protease Inhibitors for the Treatment of Chronic Hepatitis C Genotype-1 Infection: The New Standard of Care.” Lancet Infectious Diseases 12 no. 9 (2012):717–728.

Contributor

  • Nina Parker, (Shenandoah University), Mark Schneegurt (Wichita State University), Anh-Hue Thi Tu (Georgia Southwestern State University), Philip Lister (Central New Mexico Community College), and Brian M. Forster (Saint Joseph’s University) with many contributing authors. Original content via Openstax (CC BY 4.0; Access for free at https://openstax.org/books/microbiology/pages/1-introduction)


17.4: Drugs Targeting Other Microorganisms - Biology

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Editor’s Choice articles are based on recommendations by the scientific editors of MDPI journals from around the world. Editors select a small number of articles recently published in the journal that they believe will be particularly interesting to authors, or important in this field. The aim is to provide a snapshot of some of the most exciting work published in the various research areas of the journal.


17.4: Drugs Targeting Other Microorganisms - Biology

All articles published by MDPI are made immediately available worldwide under an open access license. No special permission is required to reuse all or part of the article published by MDPI, including figures and tables. For articles published under an open access Creative Common CC BY license, any part of the article may be reused without permission provided that the original article is clearly cited.

Feature Papers represent the most advanced research with significant potential for high impact in the field. Feature Papers are submitted upon individual invitation or recommendation by the scientific editors and undergo peer review prior to publication.

The Feature Paper can be either an original research article, a substantial novel research study that often involves several techniques or approaches, or a comprehensive review paper with concise and precise updates on the latest progress in the field that systematically reviews the most exciting advances in scientific literature. This type of paper provides an outlook on future directions of research or possible applications.

Editor’s Choice articles are based on recommendations by the scientific editors of MDPI journals from around the world. Editors select a small number of articles recently published in the journal that they believe will be particularly interesting to authors, or important in this field. The aim is to provide a snapshot of some of the most exciting work published in the various research areas of the journal.


Designing microorganisms for heterologous biosynthesis of cannabinoids

During the last decade, the use of medical Cannabis has expanded globally and legislation is getting more liberal in many countries, facilitating the research on cannabinoids. The unique interaction of cannabinoids with the human endocannabinoid system makes these compounds an interesting target to be studied as therapeutic agents for the treatment of several medical conditions. However, currently there are important limitations in the study, production and use of cannabinoids as pharmaceutical drugs. Besides the main constituent tetrahydrocannabinolic acid, the structurally related compound cannabidiol is of high interest as drug candidate. From the more than 100 known cannabinoids reported, most can only be extracted in very low amounts and their pharmacological profile has not been determined. Today, cannabinoids are isolated from the strictly regulated Cannabis plant, and the supply of compounds with sufficient quality is a major problem. Biotechnological production could be an attractive alternative mode of production. Herein, we explore the potential use of synthetic biology as an alternative strategy for synthesis of cannabinoids in heterologous hosts. We summarize the current knowledge surrounding cannabinoids biosynthesis and present a comprehensive description of the key steps of the genuine and artificial pathway, systems biotechnology needs and platform optimization.

Keywords: Cannabis sativa Saccharomyces cerevisiae biotechnology cannabinoids synthetic biology.

Figures

Isoprenoid formation in S. cerevisiae…

Isoprenoid formation in S. cerevisiae . The isoprenoid biosynthesis starts with acetyl-CoA, which…

Biosynthetic pathway of cannabinoids in…

Biosynthetic pathway of cannabinoids in C. sativa . Highlighted enzymes have to be…

THCA metabolization products in humans.…

THCA metabolization products in humans. The C 11 position is the major attacked…


Chemotherapeutic Agents: Definition and History | Microbiology

In this article we will discuss about:- 1. Introduction to Chemotherapeutic Agents 2. Definition of Chemotherapeutic Agents 3. Short History 4. Groups.

Introduction to Chemotherapeutic Agents:

For centuries, physicians believed that heroic measures were necessary to save patients from the ravages of infectious diseases. They prescribed frightening courses of purges and bloodlettings, enormous doses of strange chemical concoctions, ice water baths, starvation, and other drastic remedies. These treatments probably complicated an already bad situation by reducing the natural body defenses to the point of exhaustion.

But a revolution in medicine took place about 1825 when a group of doctors in Boston and London experimented to see what would happen if such treatments were withheld from diseases patients. Surprisingly, they found the survival rate essentially the same and, in some cases, better.

Over the next few decades, the lessons from their experiments spread, and as the worst features of heroic therapy disappeared, doctors adopted a conservative, non-meddling approach to disease. It became the doctor’s job to diagnose the illness, explain it to the family, predict what would happen in the next several days, and then stand by the care for the patient within the limits of what was known.

When the germ theory of disease emerged in the late 1800s, the information about microorganisms added considered to the understanding of disease and increased the storehouse of knowledge available to the doctor. However, it did not change the fact that little, if anything, could be done for the infected patient from all causes and Stereptococcal disease was a fatal experience, as were Pneumococcal pneumonia and Meningococcal meningitis.

Then, in the 1940s, the chemotherapeutic agents burst on the scene, and another revolution in medicine began. Doctors were astonished to learn that they could kill bacteria in the body without doing substantial harm to the body itself.

Medicine experienced a period of powerful, decisive therapy for infectious disease, and doctors found they could successfully alter the course of disease. The chemotherapeutic agents effected a radical change in medicine and charted a new course that has followed through to the present day.

The antimicrobial drugs that have become mainstays of our health-care delivery system. We shall explore their discovery and examine their uses, while noting the important side effects attributed to many of them.

When Pasteur performed his experiments a hundred years ago, he implied that microorganisms could be destroyed and that someday, a way would be found to successfully treat many diseases. Only since the 1940s has Pasteur’s prophecy become reality.

Definition of Chemotherapeutic Agents:

Chemotherapeutic agents are chemical substances used within the body for therapeutic purposes. The term generally implies a chemical that has been synthesized by chemists or produced by a modification of a preexisting chemical. By contrast, an antibiotic is a product of the metabolism of a microorganism.

Although many antibiotics are currently produced by synthetic or semisynthetic means and are more correctly “chemotherapeutic agents.” Our discussion of chemotherapeutic agents will begin with a brief review of their developments.

Short History of Chemotherapy:

In the drive to control and cure disease, the efforts of early microbiologists were primarily directed toward enhancing the body’s natural defenses. Sera containing antibodies lessened the impact of diphtheria, typhoid fever and tetanus and effective vaccines for smallpox and rabies (and later, tuberculosis, diphtheria, and tetanus) reduced the incidence of these diseases.

Among the leaders in the effort to control disease was an imaginative investigator named Paul Ehrlich. Ehrlich envisioned antibody molecules as “magic bullets” that seek out and destroy disease organisms in the tissues without harming the tissues. His experiments in stain technology indicated that certain dyes also had antimicrobial qualities, and by the early 1900s, his attention had turned to magic bullets of a purely chemical nature.

One of Ehrlich’s collaborators was the Japanese investigator Sahachiro Hata. Hata wished to perform research on the chemical control of the syphilis spirochete Treponema pallidum, and Ehrlich was happy to oblige. Previously, Enrich and his staff had synthesized hundreds of arsenic-phenol compounds, and Hata set to work testing them for antimicrobial qualities.

After months of painstaking study, Hata’s attention focused on arsphenamine, compound #606 in the series. Hata and Ehrlich successfully tested arsphenamine against syphilis in animals and human subjects, and in 1910, they made a derivative of the drug available to doctors for use against the disease. Arsphenamine, the first modern chemotherapeutic agent, was given the common name Salvarsan because it offered salvation from syphilis and contained arsenic.

Salvarsan met with mixed success during the ensuring years. Its value against syphilis was without question, but local reactions at the injection site and indiscriminate use by some physicians brought adverse publicity. Moreover, some church officials used the threat of syphilis as a deterrent to immoral behaviour, and they were less than enthusiastic about Salvarsan’s therapeutic effect.

Ehrlich’s death in 1915 together with the general ignorance of organic chemistry and the emerging world war further eroded enthusiasm for chemotherapy. Instead, interest strengthened in serum and vaccine therapy for war-related wounds and diseases.

Significant advances in chemotherapy would not be made for another 20 years. During this interval, German chemists continued to synthesize and manufacture dyes for fabrics and other industries, and they routinely tested their new products for antimicrobial qualities. Among these products was a red dye, prontosil, synthesized in 1932. Prontosil had no apparent effect on bacteria in culture.

But things were different in animals. When the German chemist Gerhard Domagk tested prontosil in animals he found a pronounced inhibitory effect on staphylococci, streptococci, and other Gram-positive bacteria. In February 1935, Domagk injected the dye to his daughter Hildegarde, who was gravely ill with septicemia. She had pricked her finger with a needle, and blood infection had followed rapidly.

Hildegarde’s condition gradually improved, and, to many historinas, her recovery set into motion the age of modern chemotherapy. For his discovery, Gerhand Domagk was awarded the 1939 Nobel Prize in Physiology or Medicine (in absentia, however, because Hitler forbade him to accept it).

Later in 1935, a group at the Pasteur Institute headed by Jacques and Therese Trefouel isolated the active principle in prontosil. They found it to be sulfanilamide, a substance first synthesized by Paul Gelmo in 1908. Sulfanilamide quickly became a mainstay for the treatment of wound-related infections sustained during World War II.

Groups of Chemotherapeutic Agents:

Sulfanilamide and Other Sulfonamides:

Sulfanilamide was the first of a group of chemotherapeutic agents known as sulfonamides. In 1940, the British investigators D.D. Woods and E.M. Fildes proposed a mechanism of action for sulfanilamide and other sulfonamides, and gave insight on how they interfere with the metabolism of bacteria without damaging body tissues. The mechanism came to be known as competitive inhibition.

According to the mechanism of competitive inhibition, certain bacteria synthesize an important molecule called folic acid for use in nucleic acid production. Humans cannot synthesize folic acid for use in nucleic acid production. Humans cannot synthesize folic acid and must consume it in foods or vitamin capsules. However, bacteria possess the necessary enzyme to manufacture folic acid and are incapable of absorbing folic acid from the surrounding environment.

In the production of folic acid, the bacterial enzyme joins together three important components, one of which is para-aminobenzoic acid (PABA). This molecule is similar to sulfanilamide in chemical structure. Therefore if the environment contains large amounts of sulfanilamide, the enzyme selects the sulfanilamide molecule instead of the PABA molecule for use in folic acid production.

Once combined with the enzyme, the molecule binds tightly and effectively inhibits the enzyme, thus making it unavailable for folic acid synthesis. As the production of folic acid is reduced, nucleic acid synthesis ceases, and the bacteria die.

Modern sulfonamides are typified by sulfamethoxazole. Doctors prescribe this drug for urinary tract infections due to Gram-negative rods, and for meningococcal meningitis. Frequently the drug is combined with trimethoprim, a drug that inhibits another step in folic acid synthetis.

Commercially the drug combination is known as Bactrim. It is frequently used to treat Pneumocystis pneumonia. Another common sulfonamide, sulfisoxazole, is marketed as Gantrisin cream for vaginal infections due to Gram-negative bacteria. In some patients, a drug allergy to sulfonamides develops, with a skin rash, gastrointestinal distress, or type II cytotoxic hypersensitivity.

Other Chemotherapeutic Agents:

The discovery and development of sulfanilamide led to the development of numerous other chemotherapeutic agents, many of which are currently in wide use. One example is the antituberculosis drug isoniazid (isonicotinic acid hydrazide, INH). Biochemists believe that isoniazid interferes with cell-wall synthesis in Mycobacterium species by inhibiting the production of mycolic acid, a component of the wall.

Isoniazid is often combined in therapy, with such drugs as rifampin and ethambutol. Ethambutol is a synthetic, well-absorbed drug that is tuberculocidal. Visual disturbances limit its use to treatment of tuberculosis.

Another chemotherapeutic agent, a quinolone called nalidixic acid, blocks DNA synthesis in certain Gram-negative bacteria that cause urinary tract infections. Synthetic derivatives of nalidixic acid called fluoroquinolones are also used in urinary tract infections as well as for gonorrhea and chlamydia and for intestinal tract infections due to Gram-negative bacteria. Examples of the fluoroquinolone drugs are ciprofloxacin (Cipro), enoxacin, and norfloxacin.

Nitorfurantoin is a drug actively excreted in the urine for urogenital infections. Metronidazole (Flagyl) has been used for decades against trichomoniasis, amebiasis and giardiasis. However, evidence that the drug causes tumors in mice has prompted physicians to prescribe it with caution. The treatment of malaria has long depended upon the consumption of quinine.

When the tree bark used in its production became unavailable during World War II, researchers quickly set to work to develop two alternatives – chloroquine and primaquine. Chloroquine is effective for terminating malaria attacks primaquine destroys the malaria parasites outside red blood cells.

Two other chemotherapeutic drugs, both inhibitory to Mycobacterium species, bear brief mention. The first is para-aminosalicylic acid (PAS), a drug that closely resembles sulfonamides and is used for tuberculosis. The second agent is diaminodiphenylsulfone, or dapsone, used to treat leprosy.


Antiviral Drugs

Unlike the complex structure of fungi, protozoa, and helminths, viral structure is simple, consisting of nucleic acid, a protein coat, viral enzymes, and, sometimes, a lipid envelope. Furthermore, viruses are obligate intracellular pathogens that use the host’s cellular machinery to replicate. These characteristics make it difficult to develop drugs with selective toxicity against viruses.

Many antiviral drugs are nucleoside analogs and function by inhibiting nucleic acid biosynthesis. For example, acyclovir (marketed as Zovirax ) is a synthetic analog of the nucleoside guanosine (Figure 15.18). It is activated by the herpes simplex viral enzyme thymidine kinase and, when added to a growing DNA strand during replication, causes chain termination. Its specificity for virus-infected cells comes from both the need for a viral enzyme to activate it and the increased affinity of the activated form for viral DNA polymerase compared to host cell DNA polymerase. Acyclovir and its derivatives are frequently used for the treatment of herpes virus infections, including genital herpes , chickenpox , shingles , Epstein-Barr virus infections, and cytomegalovirus infections. Acyclovir can be administered either topically or systemically, depending on the infection. One possible side effect of its use includes nephrotoxicity . The drug adenine-arabinoside , marketed as vidarabine , is a synthetic analog to deoxyadenosine that has a mechanism of action similar to that of acyclovir. It is also effective for the treatment of various human herpes viruses. However, because of possible side effects involving low white blood cell counts and neurotoxicity , treatment with acyclovir is now preferred.

Ribavirin, another synthetic guanosine analog, works by a mechanism of action that is not entirely clear. It appears to interfere with both DNA and RNA synthesis, perhaps by reducing intracellular pools of guanosine triphosphate (GTP). Ribavarin also appears to inhibit the RNA polymerase of hepatitis C virus. It is primarily used for the treatment of the RNA viruses like hepatitis C (in combination therapy with interferon) and respiratory syncytial virus . Possible side effects of ribavirin use include anaemia and developmental effects on unborn children in pregnant patients. In recent years, another nucleotide analog, sofosbuvir ( Solvaldi ), has also been developed for the treatment of hepatitis C. Sofosbuvir is a uridine analog that interferes with viral polymerase activity. It is commonly coadministered with ribavirin, with and without interferon .

Inhibition of nucleic acid synthesis is not the only target of synthetic antivirals. Although the mode of action of amantadine and its relative rimantadine are not entirely clear, these drugs appear to bind to a transmembrane protein that is involved in the escape of the influenza virus from endosomes. Blocking escape of the virus also prevents viral RNA release into host cells and subsequent viral replication. Increasing resistance has limited the use of amantadine and rimantadine in the treatment of influenza A. Use of amantadine can result in neurological side effects, but the side effects of rimantadine seem less severe. Interestingly, because of their effects on brain chemicals such as dopamine and NMDA (N-methyl D-aspartate), amantadine and rimantadine are also used for the treatment of Parkinson’s disease .

Neuraminidase inhibitors, including olsetamivir ( Tamiflu ), zanamivir ( Relenza ), and peramivir ( Rapivab ), specifically target influenza viruses by blocking the activity of influenza virus neuraminidase , preventing the release of the virus from infected cells. These three antivirals can decrease flu symptoms and shorten the duration of illness, but they differ in their modes of administration: olsetamivir is administered orally, zanamivir is inhaled, and peramivir is administered intravenously. Resistance to these neuraminidase inhibitors still seems to be minimal.

Pleconaril is a synthetic antiviral under development that showed promise for the treatment of picornaviruses . Use of pleconaril for the treatment of the common cold caused by rhinoviruses was not approved by the FDA in 2002 because of lack of proven effectiveness, lack of stability, and association with irregular menstruation. Its further development for this purpose was halted in 2007. However, pleconaril is still being investigated for use in the treatment of life-threatening complications of enteroviruses , such as meningitis and sepsis . It is also being investigated for use in the global eradication of a specific enterovirus, polio . [2] Pleconaril seems to work by binding to the viral capsid and preventing the uncoating of viral particles inside host cells during viral infection.

Viruses with complex life cycles, such as HIV , can be more difficult to treat. First, HIV targets CD4-positive white blood cells, which are necessary for a normal immune response to infection. Second, HIV is a retrovirus , meaning that it converts its RNA genome into a DNA copy that integrates into the host cell’s genome, thus hiding within host cell DNA. Third, the HIV reverse transcriptase lacks proofreading activity and introduces mutations that allow for rapid development of antiviral drug resistance. To help prevent the emergence of resistance, a combination of specific synthetic antiviral drugs is typically used in ART for HIV (Figure 15.17).

The reverse transcriptase inhibitors block the early step of converting viral RNA genome into DNA, and can include competitive nucleoside analog inhibitors (e.g., azidothymidine/zidovudine , or AZT) and non-nucleoside noncompetitive inhibitors (e.g., etravirine ) that bind reverse transcriptase and cause an inactivating conformational change. Drugs called protease inhibitors (e.g., ritonavir ) block the processing of viral proteins and prevent viral maturation. Protease inhibitors are also being developed for the treatment of other viral types. [3] For example, simeprevir ( Olysio ) has been approved for the treatment of hepatitis C and is administered with ribavirin and interferon in combination therapy. The integrase inhibitors (e.g., raltegravir ), block the activity of the HIV integrase responsible for the recombination of a DNA copy of the viral genome into the host cell chromosome. Additional drug classes for HIV treatment include the CCR5 antagonists and the fusion inhibitors (e.g., enfuviritide ), which prevent the binding of HIV to the host cell coreceptor (chemokine receptor type 5 [CCR5]) and the merging of the viral envelope with the host cell membrane, respectively. Table 3 shows the various therapeutic classes of antiviral drugs, categorized by mode of action, with examples of each.

Figure 15.16. Acyclovir is a structural analog of guanosine. It is specifically activated by the viral enzyme thymidine kinase and then preferentially binds to viral DNA polymerase, leading to chain termination during DNA replication. Figure 15.17. Antiretroviral therapy (ART) is typically used for the treatment of HIV. The targets of drug classes currently in use are shown here. [Credit: modification of work by Thomas Splettstoesser]


Principles of pharmacodynamics and their applications in veterinary pharmacology

Pharmacodynamics (PDs) is the science of drug action on the body or on microorganisms and other parasites within or on the body. It may be studied at many organizational levels – sub-molecular, molecular, cellular, tissue/organ and whole body – using in vivo, ex vivo and in vitro methods and utilizing a wide range of techniques. A few drugs owe their PD properties to some physico-chemical property or action and, in such cases, detailed molecular drug structure plays little or no role in the response elicited. For the great majority of drugs, however, action on the body is crucially dependent on chemical structure, so that a very small change, e.g. substitution of a proton by a methyl group, can markedly alter the potency of the drug, even to the point of loss of activity. In the late 19th century and first half of the 20th century recognition of these facts by Langley, Ehrlich, Dale, Clarke and others provided the foundation for the receptor site hypothesis of drug action. According to these early ideas the drug, in order to elicit its effect, had to first combine with a specific ‘target molecule’ on either the cell surface or an intracellular organelle. It was soon realized that the ‘right’ chemical structure was required for drug–target site interaction (and the subsequent pharmacological response). In addition, from this requirement, for specificity of chemical structure requirement, developed not only the modern science of pharmacology but also that of toxicology. In relation to drug actions on microbes and parasites, for example, the early work of Ehrlich led to the introduction of molecules selectively toxic for them and relatively safe for the animal host.

In the whole animal drugs may act on many target molecules in many tissues. These actions may lead to primary responses which, in turn, may induce secondary responses, that may either enhance or diminish the primary response. Therefore, it is common to investigate drug pharmacodynamics (PDs) in the first instance at molecular, cellular and tissue levels in vitro, so that the primary effects can be better understood without interference from the complexities involved in whole animal studies.

When a drug, hormone or neurotransmitter combines with a target molecule, it is described as a ligand. Ligands are classified into two groups, agonists (which initiate a chain of reactions leading, usually via the release or formation of secondary messengers, to the response) and antagonists (which fail to initiate the transduction pathways but nevertheless compete with agonists for occupancy of receptor sites and thereby inhibit their actions). The parameters which characterize drug receptor interaction are affinity, efficacy, potency and sensitivity, each of which can be elucidated quantitatively for a particular drug acting on a particular receptor in a particular tissue. The most fundamental objective of PDs is to use the derived numerical values for these parameters to classify and sub-classify receptors and to compare and classify drugs on the basis of their affinity, efficacy, potency and sensitivity.

This review introduces and summarizes the principles of PDs and illustrates them with examples drawn from both basic and veterinary pharmacology. Drugs acting on adrenoceptors and cardiovascular, non-steroidal anti-inflammatory and antimicrobial drugs are considered briefly to provide a foundation for subsequent reviews in this issue which deal with pharmacokinetic (PK)–PD modelling and integration of these drug classes. Drug action on receptors has many features in common with enzyme kinetics and gas adsorption onto surfaces, as defined by Michaelis–Menten and Langmuir absorption equations, respectively. These and other derived equations are outlined in this review. There is, however, no single theory which adequately explains all aspects of drug–receptor interaction. The early ‘occupation’ and ‘rate’ theories each explain some, but not all, experimental observations. From these basic theories the operational model and the two-state theory have been developed. For a discussion of more advanced theories see Kenakin (1997) .


Contents

The majority of drugs either

  1. mimic or inhibit normal physiological/biochemical processes or inhibit pathological processes in animals or
  2. inhibit vital processes of endo- or ectoparasites and microbial organisms.

There are 7 main drug actions: [3]

    through direct receptor agonism and downstream effects through direct receptor agonism and downstream effects (ex.: inverse agonist)
  • blocking/antagonizing action (as with silent antagonists), the drug binds the receptor but does not activate it
  • stabilizing action, the drug seems to act neither as a stimulant or as a depressant (ex.: some drugs possess receptor activity that allows them to stabilize general receptor activation, like buprenorphine in opioid dependent individuals or aripiprazole in schizophrenia, all depending on the dose and the recipient)
  • exchanging/replacing substances or accumulating them to form a reserve (ex.: glycogen storage)
  • direct beneficial chemical reaction as in free radical scavenging
  • direct harmful chemical reaction which might result in damage or destruction of the cells, through induced toxic or lethal damage (cytotoxicity or irritation)

Desired activity Edit

The desired activity of a drug is mainly due to successful targeting of one of the following:

    disruption with downstream effects
  • Interaction with enzyme proteins
  • Interaction with structural proteins
  • Interaction with carrier proteins
  • Interaction with ion channels to receptors:
      receptors receptors receptors
  • General anesthetics were once thought to work by disordering the neural membranes, thereby altering the Na + influx. Antacids and chelating agents combine chemically in the body. Enzyme-substrate binding is a way to alter the production or metabolism of key endogenous chemicals, for example aspirin irreversibly inhibits the enzyme prostaglandin synthetase (cyclooxygenase) thereby preventing inflammatory response. Colchicine, a drug for gout, interferes with the function of the structural protein tubulin, while Digitalis, a drug still used in heart failure, inhibits the activity of the carrier molecule, Na-K-ATPase pump. The widest class of drugs act as ligands that bind to receptors that determine cellular effects. Upon drug binding, receptors can elicit their normal action (agonist), blocked action (antagonist), or even action opposite to normal (inverse agonist).

    In principle, a pharmacologist would aim for a target plasma concentration of the drug for a desired level of response. In reality, there are many factors affecting this goal. Pharmacokinetic factors determine peak concentrations, and concentrations cannot be maintained with absolute consistency because of metabolic breakdown and excretory clearance. Genetic factors may exist which would alter metabolism or drug action itself, and a patient's immediate status may also affect indicated dosage.

    Undesirable effects Edit

    Undesirable effects of a drug include:

    • Increased probability of cell mutation (carcinogenic activity)
    • A multitude of simultaneous assorted actions which may be deleterious
    • Interaction (additive, multiplicative, or metabolic)
    • Induced physiological damage, or abnormal chronic conditions

    Therapeutic window Edit

    The therapeutic window is the amount of a medication between the amount that gives an effect (effective dose) and the amount that gives more adverse effects than desired effects. For instance, medication with a small pharmaceutical window must be administered with care and control, e.g. by frequently measuring blood concentration of the drug, since it easily loses effects or gives adverse effects.

    Duration of action Edit

    The duration of action of a drug is the length of time that particular drug is effective. [4] Duration of action is a function of several parameters including plasma half-life, the time to equilibrate between plasma and target compartments, and the off rate of the drug from its biological target. [5]

    The binding of ligands (drug) to receptors is governed by the law of mass action which relates the large-scale status to the rate of numerous molecular processes. The rates of formation and un-formation can be used to determine the equilibrium concentration of bound receptors. The equilibrium dissociation constant is defined by:

    where L=ligand, R=receptor, square brackets [] denote concentration. The fraction of bound receptors is

    This expression is one way to consider the effect of a drug, in which the response is related to the fraction of bound receptors (see: Hill equation). The fraction of bound receptors is known as occupancy. The relationship between occupancy and pharmacological response is usually non-linear. This explains the so-called receptor reserve phenomenon i.e. the concentration producing 50% occupancy is typically higher than the concentration producing 50% of maximum response. More precisely, receptor reserve refers to a phenomenon whereby stimulation of only a fraction of the whole receptor population apparently elicits the maximal effect achievable in a particular tissue.

    The simplest interpretation of receptor reserve is that it is a model that states there are excess receptors on the cell surface than what is necessary for full effect. Taking a more sophisticated approach, receptor reserve is an integrative measure of the response-inducing capacity of an agonist (in some receptor models it is termed intrinsic efficacy or intrinsic activity) and of the signal amplification capacity of the corresponding receptor (and its downstream signaling pathways). Thus, the existence (and magnitude) of receptor reserve depends on the agonist (efficacy), tissue (signal amplification ability) and measured effect (pathways activated to cause signal amplification). As receptor reserve is very sensitive to agonist's intrinsic efficacy, it is usually defined only for full (high-efficacy) agonists. [6] [7] [8]

    Often the response is determined as a function of log[L] to consider many orders of magnitude of concentration. However, there is no biological or physical theory that relates effects to the log of concentration. It is just convenient for graphing purposes. It is useful to note that 50% of the receptors are bound when [L]=Kd .

    The graph shown represents the conc-response for two hypothetical receptor agonists, plotted in a semi-log fashion. The curve toward the left represents a higher potency (potency arrow does not indicate direction of increase) since lower concentrations are needed for a given response. The effect increases as a function of concentration.

    The concept of pharmacodynamics has been expanded to include Multicellular Pharmacodynamics (MCPD). MCPD is the study of the static and dynamic properties and relationships between a set of drugs and a dynamic and diverse multicellular four-dimensional organization. It is the study of the workings of a drug on a minimal multicellular system (mMCS), both in vivo and in silico. Networked Multicellular Pharmacodynamics (Net-MCPD) further extends the concept of MCPD to model regulatory genomic networks together with signal transduction pathways, as part of a complex of interacting components in the cell. [9]

    Pharmacokinetics and pharmacodynamics are termed toxicokinetics and toxicodynamics in the field of ecotoxicology. Here, the focus is on toxic effects on a wide range of organisms. The corresponding models are called toxicokinetic-toxicodynamic models. [10]


    Near Infrared Photo-Antimicrobial Targeting Therapy for Candida albicans

    Advanced Analytical and Diagnostic Imaging Center (AADIC)/Medical Engineering Unit (MEU), B3 Unit, Nagoya University Institute for Advanced Research, 65 Tsuumai-cho, Showa-ku, Nagoya, Aichi, 466-8550 Japan

    Advanced Analytical and Diagnostic Imaging Center (AADIC)/Medical Engineering Unit (MEU), B3 Unit, Nagoya University Institute for Advanced Research, 65 Tsuumai-cho, Showa-ku, Nagoya, Aichi, 466-8550 Japan

    EW Nutrition Japan, Immunology Research Institute in Gifu, 839-7, Gifu-City, Sano, Gifu, 501-1101 Japan

    EW Nutrition Japan, Immunology Research Institute in Gifu, 839-7, Gifu-City, Sano, Gifu, 501-1101 Japan

    Division of OMICS Analysis, Nagoya University Graduate School of Medicine, 65 Tsuumai-cho, Showa-ku, Nagoya, Aichi, 466-8550 Japan

    Division of Systems Biology, Nagoya University Graduate School of Medicine, 65 Tsuumai-cho, Showa-ku, Nagoya, Aichi, 466-8550 Japan

    S-YLC, Nagoya University Institute for Advanced Research, Furo-cho, Chikusa-ku, Nagoya, Aichi, 464-8601 Japan

    EW Nutrition Japan, Immunology Research Institute in Gifu, 839-7, Gifu-City, Sano, Gifu, 501-1101 Japan

    Division of OMICS Analysis, Nagoya University Graduate School of Medicine, 65 Tsuumai-cho, Showa-ku, Nagoya, Aichi, 466-8550 Japan

    Respiratory Medicine, Nagoya University Graduate School of Medicine, 65 Tsuumai-cho, Showa-ku, Nagoya, Aichi, 466-8550 Japan

    CREST, JST, Honcho Kawaguchi, Saitama, 332-0012 Japan

    Respiratory Medicine, Nagoya University Graduate School of Medicine, 65 Tsuumai-cho, Showa-ku, Nagoya, Aichi, 466-8550 Japan

    Respiratory Medicine, Nagoya University Graduate School of Medicine, 65 Tsuumai-cho, Showa-ku, Nagoya, Aichi, 466-8550 Japan

    Respiratory Medicine, Nagoya University Graduate School of Medicine, 65 Tsuumai-cho, Showa-ku, Nagoya, Aichi, 466-8550 Japan

    Advanced Analytical and Diagnostic Imaging Center (AADIC)/Medical Engineering Unit (MEU), B3 Unit, Nagoya University Institute for Advanced Research, 65 Tsuumai-cho, Showa-ku, Nagoya, Aichi, 466-8550 Japan

    Advanced Analytical and Diagnostic Imaging Center (AADIC)/Medical Engineering Unit (MEU), B3 Unit, Nagoya University Institute for Advanced Research, 65 Tsuumai-cho, Showa-ku, Nagoya, Aichi, 466-8550 Japan

    EW Nutrition Japan, Immunology Research Institute in Gifu, 839-7, Gifu-City, Sano, Gifu, 501-1101 Japan

    EW Nutrition Japan, Immunology Research Institute in Gifu, 839-7, Gifu-City, Sano, Gifu, 501-1101 Japan

    Division of OMICS Analysis, Nagoya University Graduate School of Medicine, 65 Tsuumai-cho, Showa-ku, Nagoya, Aichi, 466-8550 Japan

    Division of Systems Biology, Nagoya University Graduate School of Medicine, 65 Tsuumai-cho, Showa-ku, Nagoya, Aichi, 466-8550 Japan

    S-YLC, Nagoya University Institute for Advanced Research, Furo-cho, Chikusa-ku, Nagoya, Aichi, 464-8601 Japan

    EW Nutrition Japan, Immunology Research Institute in Gifu, 839-7, Gifu-City, Sano, Gifu, 501-1101 Japan

    Division of OMICS Analysis, Nagoya University Graduate School of Medicine, 65 Tsuumai-cho, Showa-ku, Nagoya, Aichi, 466-8550 Japan

    Respiratory Medicine, Nagoya University Graduate School of Medicine, 65 Tsuumai-cho, Showa-ku, Nagoya, Aichi, 466-8550 Japan

    CREST, JST, Honcho Kawaguchi, Saitama, 332-0012 Japan

    Abstract

    Drug-resistant microorganisms are a pressing issue, and innovative antimicrobial therapies are required antibodies targeting antigens on pathogen surfaces have emerged as one such modality. IgY, abundant in chicken egg yolk, confers passive immunity, and is effective against various pathogens however, its antimicrobial effects remain limited. Near-infrared photoimmunotherapy (NIR-PIT), originally developed as a cancer treatment, specifically kills cancer cells via a photosensitizing phthalocyanine dye (IRDye700Dx IR700)-conjugated monoclonal antibody, and irradiating NIR light. IgY-photo-antimicrobial targeting therapy (IgY-PAT 2 ), exploiting NIR-PIT, is investigated to destroy only microorganisms. IR700 is conjugated with anti-Candida albicans IgY (CA-IgY) to generate CA-IgY-IR700, which specifically binds various Candida spp. (and not human skin cells). The antimicrobial effect of CA-IgY-PAT 2 is dependent on the NIR-light dose (p < 0.0001). CA-IgY-PAT 2 significantly reduces the area of ulcers in a mouse model of CA-infected cutaneous ulcers (p < 0.0001), indicating that CA-IgY-PAT 2 is a new promising therapeutic method for CA infection.

    Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.


    14.5 Drug Resistance

    Antimicrobial resistance is not a new phenomenon. In nature, microbes are constantly evolving in order to overcome the antimicrobial compounds produced by other microorganisms. Human development of antimicrobial drugs and their widespread clinical use has simply provided another selective pressure that promotes further evolution. Several important factors can accelerate the evolution of drug resistance . These include the overuse and misuse of antimicrobials, inappropriate use of antimicrobials, subtherapeutic dosing, and patient noncompliance with the recommended course of treatment.

    Exposure of a pathogen to an antimicrobial compound can select for chromosomal mutations conferring resistance, which can be transferred vertically to subsequent microbial generations and eventually become predominant in a microbial population that is repeatedly exposed to the antimicrobial. Alternatively, many genes responsible for drug resistance are found on plasmids or in transposons that can be transferred easily between microbes through horizontal gene transfer (see How Asexual Prokaryotes Achieve Genetic Diversity). Transposons also have the ability to move resistance genes between plasmids and chromosomes to further promote the spread of resistance.

    Mechanisms for Drug Resistance

    There are several common mechanisms for drug resistance, which are summarized in Figure 14.18. These mechanisms include enzymatic modification of the drug, modification of the antimicrobial target, and prevention of drug penetration or accumulation.

    Drug Modification or Inactivation

    Resistance genes may code for enzymes that chemically modify an antimicrobial, thereby inactivating it, or destroy an antimicrobial through hydrolysis. Resistance to many types of antimicrobials occurs through this mechanism. For example, aminoglycoside resistance can occur through enzymatic transfer of chemical groups to the drug molecule, impairing the binding of the drug to its bacterial target. For β-lactams , bacterial resistance can involve the enzymatic hydrolysis of the β-lactam bond within the β-lactam ring of the drug molecule. Once the β-lactam bond is broken, the drug loses its antibacterial activity. This mechanism of resistance is mediated by β-lactamases , which are the most common mechanism of β-lactam resistance. Inactivation of rifampin commonly occurs through glycosylation , phosphorylation , or adenosine diphosphate (ADP) ribosylation, and resistance to macrolides and lincosamides can also occur due to enzymatic inactivation of the drug or modification.

    Prevention of Cellular Uptake or Efflux

    Microbes may develop resistance mechanisms that involve inhibiting the accumulation of an antimicrobial drug, which then prevents the drug from reaching its cellular target. This strategy is common among gram-negative pathogens and can involve changes in outer membrane lipid composition, porin channel selectivity, and/or porin channel concentrations. For example, a common mechanism of carbapenem resistance among Pseudomonas aeruginosa is to decrease the amount of its OprD porin, which is the primary portal of entry for carbapenems through the outer membrane of this pathogen. Additionally, many gram-positive and gram-negative pathogenic bacteria produce efflux pump s that actively transport an antimicrobial drug out of the cell and prevent the accumulation of drug to a level that would be antibacterial. For example, resistance to β-lactams, tetracyclines , and fluoroquinolones commonly occurs through active efflux out of the cell, and it is rather common for a single efflux pump to have the ability to translocate multiple types of antimicrobials.

    Target Modification

    Because antimicrobial drugs have very specific targets, structural changes to those targets can prevent drug binding, rendering the drug ineffective. Through spontaneous mutations in the genes encoding antibacterial drug targets, bacteria have an evolutionary advantage that allows them to develop resistance to drugs. This mechanism of resistance development is quite common. Genetic changes impacting the active site of penicillin-binding proteins (PBPs) can inhibit the binding of β-lactam drugs and provide resistance to multiple drugs within this class. This mechanism is very common among strains of Streptococcus pneumoniae , which alter their own PBPs through genetic mechanisms. In contrast, strains of Staphylococcus aureus develop resistance to methicillin ( MRSA ) through the acquisition of a new low-affinity PBP, rather than structurally alter their existing PBPs. Not only does this new low-affinity PBP provide resistance to methicillin but it provides resistance to virtually all β-lactam drugs, with the exception of the newer fifth-generation cephalosporins designed specifically to kill MRSA. Other examples of this resistance strategy include alterations in

    • ribosome subunits, providing resistance to macrolides, tetracyclines, and aminoglycosides
    • lipopolysaccharide (LPS) structure, providing resistance to polymyxins
    • RNA polymerase, providing resistance to rifampin
    • DNA gyrase, providing resistance to fluoroquinolones
    • metabolic enzymes, providing resistance to sulfa drugs , sulfones , and trimethoprim and
    • peptidoglycan subunit peptide chains, providing resistance to glycopeptides .

    Target Overproduction or Enzymatic Bypass

    When an antimicrobial drug functions as an antimetabolite, targeting a specific enzyme to inhibit its activity, there are additional ways that microbial resistance may occur. First, the microbe may overproduce the target enzyme such that there is a sufficient amount of antimicrobial-free enzyme to carry out the proper enzymatic reaction. Second, the bacterial cell may develop a bypass that circumvents the need for the functional target enzyme. Both of these strategies have been found as mechanisms of sulfonamide resistance . Vancomycin resistance among S. aureus has been shown to involve the decreased cross-linkage of peptide chains in the bacterial cell wall, which provides an increase in targets for vancomycin to bind to in the outer cell wall. Increased binding of vancomycin in the outer cell wall provides a blockage that prevents free drug molecules from penetrating to where they can block new cell wall synthesis.

    Target Mimicry

    A recently discovered mechanism of resistance called target mimicry involves the production of proteins that prevent drugs from binding to their bacterial cellular targets. For example, fluoroquinolone resistance by Mycobacterium tuberculosis can involve the production of a protein that resembles DNA. This protein is called MfpA (Mycobacterium fluoroquinolone resistance protein A). The mimicry of DNA by MfpA results in DNA gyrase binding to MfpA, preventing the binding of fluoroquinolones to DNA gyrase.

    Check Your Understanding

    Multidrug-Resistant Microbes and Cross Resistance

    From a clinical perspective, our greatest concerns are multidrug-resistant microbes (MDRs) and cross resistance. MDRs are colloquially known as “ superbugs ” and carry one or more resistance mechanism(s), making them resistant to multiple antimicrobials. In cross-resistance , a single resistance mechanism confers resistance to multiple antimicrobial drugs. For example, having an efflux pump that can export multiple antimicrobial drugs is a common way for microbes to be resistant to multiple drugs by using a single resistance mechanism. In recent years, several clinically important superbugs have emerged, and the CDC reports that superbugs are responsible for more than 2 million infections in the US annually, resulting in at least 23,000 fatalities. 19 Several of the superbugs discussed in the following sections have been dubbed the ESKAPE pathogens . This acronym refers to the names of the pathogens ( Enterococcus faecium , Staphylococcus aureus , Klebsiella pneumoniae , Acinetobacter baumannii , Pseudomonas aeruginosa and Enterobacter spp. ) but it is also fitting in that these pathogens are able to “escape” many conventional forms of antimicrobial therapy. As such, infections by ESKAPE pathogens can be difficult to treat and they cause a large number of nosocomial infections.

    Methicillin-Resistant Staphylococcus aureus (MRSA)

    Methicillin, a semisynthetic penicillin, was designed to resist inactivation by β-lactamases. Unfortunately, soon after the introduction of methicillin to clinical practice, methicillin-resistant strains of S. aureus appeared and started to spread. The mechanism of resistance, acquisition of a new low-affinity PBP, provided S. aureus with resistance to all available β-lactams. Strains of methicillin-resistant S. aureus (MRSA) are widespread opportunistic pathogens and a particular concern for skin and other wound infections, but may also cause pneumonia and septicemia . Although originally a problem in health-care settings (hospital-acquired MRSA [HA-MRSA]), MRSA infections are now also acquired through contact with contaminated members of the general public, called community-associated MRSA (CA-MRSA). Approximately one-third of the population carries S. aureus as a member of their normal nasal microbiota without illness, and about 6% of these strains are methicillin resistant. 20 21

    Micro Connections

    Clavulanic Acid: Penicillin’s Little Helper

    With the introduction of penicillin in the early 1940s, and its subsequent mass production, society began to think of antibiotics as miracle cures for a wide range of infectious diseases. Unfortunately, as early as 1945, penicillin resistance was first documented and started to spread. Greater than 90% of current S. aureus clinical isolates are resistant to penicillin. 22

    Although developing new antimicrobial drugs is one solution to this problem, scientists have explored new approaches, including the development of compounds that inactivate resistance mechanisms. The development of clavulanic acid represents an early example of this strategy. Clavulanic acid is a molecule produced by the bacterium Streptococcus clavuligerus . It contains a β-lactam ring , making it structurally similar to penicillin and other β-lactams , but shows no clinical effectiveness when administered on its own. Instead, clavulanic acid binds irreversibly within the active site of β-lactamases and prevents them from inactivating a coadministered penicillin.

    Clavulanic acid was first developed in the 1970s and was mass marketed in combination with amoxicillin beginning in the 1980s under the brand name Augmentin. As is typically the case, resistance to the amoxicillin-clavulanic acid combination soon appeared. Resistance most commonly results from bacteria increasing production of their β-lactamase and overwhelming the inhibitory effects of clavulanic acid, mutating their β-lactamase so it is no longer inhibited by clavulanic acid, or from acquiring a new β-lactamase that is not inhibited by clavulanic acid. Despite increasing resistance concerns, clavulanic acid and related β-lactamase inhibitors (sulbactam and tazobactam) represent an important new strategy: the development of compounds that directly inhibit antimicrobial resistance-conferring enzymes.

    Vancomycin-Resistant Enterococci and Staphylococcus aureus

    Vancomycin is only effective against gram-positive organisms, and it is used to treat wound infections, septic infections, endocarditis, and meningitis that are caused by pathogens resistant to other antibiotics. It is considered one of the last lines of defense against such resistant infections, including MRSA. With the rise of antibiotic resistance in the 1970s and 1980s, vancomycin use increased, and it is not surprising that we saw the emergence and spread of vancomycin-resistant enterococci (VRE) , vancomycin-resistant S. aureus (VRSA) , and vancomycin-intermediate S. aureus (VISA) . The mechanism of vancomycin resistance among enterococci is target modification involving a structural change to the peptide component of the peptidoglycan subunits, preventing vancomycin from binding. These strains are typically spread among patients in clinical settings by contact with health-care workers and contaminated surfaces and medical equipment.

    VISA and VRSA strains differ from each other in the mechanism of resistance and the degree of resistance each mechanism confers. VISA strains exhibit intermediate resistance, with a minimum inhibitory concentration (MIC) of 4–8 μg/mL, and the mechanism involves an increase in vancomycin targets. VISA strains decrease the crosslinking of peptide chains in the cell wall, providing an increase in vancomycin targets that trap vancomycin in the outer cell wall. In contrast, VRSA strains acquire vancomycin resistance through horizontal transfer of resistance genes from VRE, an opportunity provided in individuals coinfected with both VRE and MRSA. VRSA exhibit a higher level of resistance, with MICs of 16 μg/mL or higher. 23 In the case of all three types of vancomycin-resistant bacteria, rapid clinical identification is necessary so proper procedures to limit spread can be implemented. The oxazolidinones like linezolid are useful for the treatment of these vancomycin-resistant, opportunistic pathogens, as well as MRSA.

    Extended-Spectrum β-Lactamase–Producing Gram-Negative Pathogens

    Gram-negative pathogens that produce extended-spectrum β-lactamases (ESBLs) show resistance well beyond just penicillins. The spectrum of β-lactams inactivated by ESBLs provides for resistance to all penicillins , cephalosporins , monobactams , and the β-lactamase-inhibitor combinations, but not the carbapenems . An even greater concern is that the genes encoding for ESBLs are usually found on mobile plasmids that also contain genes for resistance to other drug classes (e.g., fluoroquinolones , aminoglycosides , tetracyclines ), and may be readily spread to other bacteria by horizontal gene transfer . These multidrug-resistant bacteria are members of the intestinal microbiota of some individuals, but they are also important causes of opportunistic infections in hospitalized patients, from whom they can be spread to other people.

    Carbapenem-Resistant Gram-Negative Bacteria

    The occurrence of carbapenem-resistant Enterobacteriaceae (CRE) and carbapenem resistance among other gram-negative bacteria (e.g., P. aeruginosa, Acinetobacter baumannii , Stenotrophomonas maltophila ) is a growing health-care concern. These pathogens develop resistance to carbapenems through a variety of mechanisms, including production of carbapenemases (broad-spectrum β-lactamases that inactivate all β-lactams, including carbapenems), active efflux of carbapenems out of the cell, and/or prevention of carbapenem entry through porin channels . Similar to concerns with ESBLs, carbapenem-resistant, gram-negative pathogens are usually resistant to multiple classes of antibacterials, and some have even developed pan-resistance (resistance to all available antibacterials). Infections with carbapenem-resistant, gram-negative pathogens commonly occur in health-care settings through interaction with contaminated individuals or medical devices, or as a result of surgery.

    Multidrug-Resistant Mycobacterium tuberculosis

    The emergence of multidrug-resistant Mycobacterium tuberculosis (MDR-TB) and extensively drug-resistant Mycobacterium tuberculosis ( XDR-TB ) is also of significant global concern. MDR-TB strains are resistant to both rifampin and isoniazid , the drug combination typically prescribed for treatment of tuberculosis. XDR-TB strains are additionally resistant to any fluoroquinolone and at least one of three other drugs ( amikacin , kanamycin , or capreomycin ) used as a second line of treatment, leaving these patients very few treatment options. Both types of pathogens are particularly problematic in immunocompromised persons, including those suffering from HIV infection. The development of resistance in these strains often results from the incorrect use of antimicrobials for tuberculosis treatment, selecting for resistance.


    Drug Discovery and Development via Synthetic Biology

    Ryan E. Cobb , . Huimin Zhao , in Synthetic Biology , 2013

    Combinatorial Approaches for Novel Drug Generation

    In the past few decades, more than 20 000 chemically diverse and biologically active compounds have been discovered from microbial sources by traditional screening efforts. 103 The successful cloning of the first antibiotic biosynthetic gene cluster 104 prompted researchers to investigate whether recombinant bacteria containing one or more genes from different organisms that make biosynthetically related metabolites could produce novel ones. The resulting combinatorial biosynthesis method, which was developed independently by Houghten and Geysen in the 1980s, allows interchanging of secondary metabolism genes between antibiotic-producing microorganisms to generate a large library of pathways for high-throughput screening.

    Combinatorial biosynthesis has been particularly successful with PKS genes. Novel combinations of type I and type II PKS genes produced numerous derivatives of medically important macrolide antibiotics and unusual polycyclic aromatic compounds. 103,105–108 The initial demonstration was comprised of novel polycyclic aromatic metabolites produced by hybrid forms of the actinorhodin producing genes and tetracenomycin type II PKS genes, 109–113 resulting in rational design of new analogues. 114 A large number of novel compounds were produced through such a strategy, 115 including tetracenomycin M resulting from the combination of mithramycin and tetracenomycin genes, 116 and a novel 18-carbon polyketide made by hybrid forms of tetracenomycin and griseusin cyclases. 117 The modular PKS fosters the real excitement of using combinatorial biosynthesis for drug discovery and development. One comprehensively studied example is the 6-deoxyerythronolide B synthase (DEBS), which will be discussed in detail in the following section. Deletion, inactivation, or shuffling of domains or modules within or outside the system generated many novel compounds. 118,119 Another strategy to generate novel compounds is to change the starter or extension units. 120–133 This approach has been extended to other drugs. 134–136

    The potential of combinatorial biosynthesis was further expanded by the addition of the deoxysugar (DOS) biosynthesis genes. Usually the formation of DOSs as glycosides, which are made by the glucose-1-phosphate key metabolic intermediate, thymidine diphospho 4-keto-6-deoxyglucose or one of its derivatives, 137 generates biological activities. Since the genes involved in these DOS-producing pathways have been identified, attempts have been made to use combinatorial biosynthesis to make analogues of known antibiotic glycosides or novel metabolites. 138–141


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