Lecture 4 -Antimicrobial agents PDF

MICROBIOLOGY AND IMMUNOLOGY DEP. Fall 2022 - 2023 Pharmaceutical Microbiology PM 502 Lecture 4 Control of microbial growth  Lecture Outline Drug combinations Antitubercular drugs Antiprotozoal Antifungal agents Antiviral agents  I. D...

MICROBIOLOGY AND IMMUNOLOGY DEP. Fall 2022 - 2023 Pharmaceutical Microbiology PM 502 Lecture 4 Control of microbial growth  Lecture Outline Drug combinations Antitubercular drugs Antiprotozoal Antifungal agents Antiviral agents  I. DRUG COMBINATIONS Synergistic Antagonistic Log Log A+B cfu/ml B cfu/ml A A B A+B hours hours For antimicrobials “A” and “B” used in combination: Actual killing rate = A + B → Additive Actual killing rate > A + B → Synergistic Actual killing rate < A + B → Antagonistic  1. Synergism ▪ The combined effect may be greater than that which either agent alone could achieve a. One drug may potentiate the activity of the other. e.g. Trimethoprim / sulphonamides b. One drug may assist the other to penetrate into the bacterial cell. e.g. β-lactam antibiotics and aminoglycosides c. One drug may protect the other from destruction. e.g. Clavulanic acid / amoxycillin 1. Antagonism The overall effect may be reduced. i.e. there is a lesser effect of the mixture than that of the more potent drug acting alone. I. Mutual antagonism e.g. Fusidic acid and some penicillins when combined show mutual antagonism against staphylococci while each of fusidic acid is bacteriostatic and penicillin require actively growing bacteria II. Dissociated resistance One antibiotic induce the resistance to another antibiotic. e.g. Erythromycin and Clindamycin AB1 AB1 AB2 or AB2  ❑ Drug combinations: a) Bactericidal + Bactericidal: may be synergistic b) Bactericidal + Bacteriostatic: may be antagonistic ❑ Determination of Antibiotic Synergism and Antagonism: ▪ Antimicrobial synergy and antagonism can be demonstrated in agar diffusion test using discs. ▪ When this method is employed, it is common to use two strips of antibiotic impregnated filter paper instead of discs, which are then laid at right angles on the inoculated agar plate. ▪ Results: Increase in inhibition at the junction of the two strips indicate synergism, while decrease in inhibition at junction indicate antagonism. Results: Increase or reduction in inhibition can be seen at the junction of the two strips. Anti-tubercular compounds Primary (First-line) antitubercular agents: combination of the four following drugs to shorten the clinical course of TB, prevent complications, prevent the development of latency and/or subsequent recurrences: Rifampicin – Isoniazid – Pyrazinamide – Ethambutol Therapy extends over several months (6 months), so resistance development is a significant possibility. This risk is minimized by the use of two or three antibiotics in combination. e.g. rifampin and isoniazid Pyrazinamide is especially effective against dormant, non-replicating M. tuberculosis cells, which are typically resistant to most antibiotics. Ethambutol targets and inhibits an enzyme called arabinosyl transferase, which is involved in the synthesis of arabinogalactan (an essential polysaccharide in the mycobacterial cell wall), linking peptidoglycan to mycolic acids. ▪ When bacterial resistance to the primary agents develops→ treatment with the secondary (Second-line) anti-tubercular agents has to be considered. e.g. Kanamycin, Amikacin, Cycloserine and Fluoroquinolones (Ciprofloxacin, levofloxacin). Antitubercular drugs The ideal antitubercular drug should also have the potential to kill (cidal) rather than merely inhibit (static) the growth of the infecting organism. WHY? Mycobacteria can survive, and even reproduce, within macrophages, relying on the immune system to eradicate an infection following the use of a bacteriostatic drug is unlikely to be an effective strategy. The current approach is to treat tuberculosis in two phases: ▫ Initial phase: ▫ 2 months; A combination of three or four drugs: isoniazid, rifampicin and pyrizinamide (with or without ethambutol) ▫ Continuation phase: 4 months; 2 drugs are used: isoniazid and rifampicin. II. Antiprotozoal drugs Metronidazole (Flagyl®) ▪ It inhibits the growth of pathogenic protozoa ▪ Activity: ▪ It is effective against Entamoeba histolytica, Giardia lamblia and Trichomonas vaginalis. ▪ It is also effective against anaerobic bacteria. ▪ Mechanism of action: Strand breakage of DNA ▪ Side effects: metallic taste, nausea , vomiting.  III. Antifungal (Antimycotic) drugs Infectious diseases caused by fungi are called mycoses, and they are often chronic in nature. Mycotic infections may be: Superficial and involve only the skin (Cutaneous mycoses= epidermis). Others may penetrate the skin, causing (Subcutaneous mycosis). Others may cause (Systemic mycosis). Unlike bacteria, fungi are eukaryotic, with rigid cell walls composed largely of chitin and glucan rather than peptidoglycan. In addition, the fungal cell membrane contains ergosterol rather than the cholesterol found in mammalian membranes. Common pathogenic organisms of kingdom Fungi Examples of some fungal infections Candidiasis (thrush or moniliasis) the most common problems are skin, mouth and vaginal infections. It also is a common cause of diaper rash. Candidiasis may also infect the blood stream or internal organs such as the liver or spleen (candidemea). Tinea (Fungal skin infection) refers to a skin infection with a dermatophytes (40 species of fungi which obtain nutrients from keratinized tissues). They cause fungal infections of skin, nails and hair. (Tinea capiti, tinea versicolour,……………..) Cryptococcal meningitis is now the most common cause of meningitis and it accounts for 20–25% of AIDS-related deaths in Africa. Histoplasmosis: is a disease caused by the fungus Histoplasma capsulatum. Symptoms of this infection vary greatly, but the disease affects primarily the lungs. Black Fungus Infection: severe respiratory fungal infection which affects also eyes and brain. III. Antifungal drugs Griseofulvin A few agents with special Superficial infection antifungal properties have been Polyenes developed for treating Amphotericin (systemic) systemic & superficial fungal and Nystatin (superficial) infections. Azoles (Imidazole) Systemic and superficial Antifungal drugs Flucytosin (Mechanisms & classification) (Systemic) https://youtu.be/59aJJ6N2a3c Echinocandins (systemic) Targets of antifungal drugs  CLASSIFICATION OF ANTIFUNGAL DRUGS Drugs for systemic fungal infections Polyene antibiotics -Amphotericin B Pyrimidine analogue Flucytosin Azoles - Ketoconazole - Fluconazole - Itraconazole Echinocandins Drugs for superficial fungal infections Systemic drugs (oral for skin infections) -Griseofulvin Topical drugs - Polyenes (Nystatin) - Azoles (miconazole, econazole, clotrimazole, etc.) - Terbinafen 1. Griseofulvin (Natural Antifungal) Mechanism of action: The drug inhibits mitosis in fungi. It is a keratophilic drug (binds to keratin in newly formed cells of skin, hair and nails and makes them resistant to fungal infections). ▪ Activity: Against the dermatophytic fungi of hair, skin and nails, but usually only when topical therapy has failed. NOT effective against Candida albicans. ▪ Route of administration: Administered orally in the form of tablets. Selective toxicity: uptake is more in fungal cell than mammalian cells. ▪ Side effect: It can increase the rate of metabolism of a number of drugs, reduce the effectiveness, including oral contraceptives as it is a cytochrome p450 enzyme inducer. 2. Polyenes (Nystatin, Amphotericin) Mechanism of action: It combines with ergosterol and change the cell membrane integrity and form channels causing leakage of cytoplasm. Selectivetoxicity: Good: Amphotericin B is selectively toxic to fungi because it targets ergosterol, which is not found in human cells. This is the basis of its selectivity, as it primarily disrupts fungal cell membranes while leaving human cell membranes relatively intact. Bad: Although Amphotericin B prefers ergosterol, it can still interact with cholesterol (found in human cell membranes) to some extent. This off-target effect can damage human cell membranes, particularly in kidney cells, leading to side effects like nephrotoxicity (kidney toxicity).. 2. Polyenes (Nystatin, Amphotericin) 1-Nystatin Activity: has a specific action on Candida albicans, used as a topical agent. (oral, and vaginal candidiasis) 2-Amphotericin B Activity: against systemic (I.V.) infections caused by Candida albicans and Cryptococcus neoformans (Cryptococcal meningitis). III. Antifungal drugs 3. Imidazole and triazole Derivatives ❑ Mechanism of action: ▪ Selective inhibition of a step in ergosterol biosynthesis (Inhibits conversion of lanosterol to ergosterol through inhibition of 14 - α – demethylase enzyme ). ▪ They all possess a broad antimycotic spectrum. ❖ Miconazole, Econazole and Clotrimazole Route of administration: they are used topically. ❖ Ketoconazole, Fluconazole and Itraconazole Route of administration: administered orally. ❖ Posaconazole and Voriconazole much more recent antifungals. They tend to be reserved for severe, possibly life- threatening infections, in which other antifungals have failed or are inappropriate.  Azoles: Interfere with ergosterol biosynthesis by binding to the cytochrome P - 450 mediated enzyme known as 14 - α - demethylase (P - 450 DM ). This blocks the formation of ergosterol by preventing the methylation of lanosterol (a precursor of ergosterol). This result in a reduction in the amount of ergosterol in the fungal cell membrane which leads to membrane instability, growth inhibition and cell death. 4. Terbinafine (allylamine antifungal): A member of the synthetic allylamine. Similar to azoles, allylamines inhibit ergosterol biosynthesis by inhibiting the formation of squalene epoxide, a precursor of lanosterol in the ergosterol biosynthetic pathway. It is orally active, fungicidal, effective against a broad range of dermatophytes and yeasts and is the drug of choice for fungal nail infections. The mode of action of squalene epoxidase inhibitors III. Antifungal drugs 4. Pyrimidine analogue e.g. Flucytosine Mechanism of action: Inhibit fungal DNA and RNA synthesis through inhibition of thymidine synthase enzyme. Uses: Deep candida infections, cryptococcal meningitidis. Contraindications: Pregnancy ( 5-fluorouracil is teratogenic) Selective toxicity: - It is converted into active form (5-FU) in fungal cells. - Human cells have a much lower level or absence of cytosine deaminase enzyme. Synergistic combination - The same mechanism (inhibition of with amphotericin which thymidine synthase enzyme) occurs in both permits amphotericin dose fungi and humans but much higher in fungal reduction and a lower risk cells. of toxicity.  5. Echinocandins (I.V.) Echinocandins are antifugal drugs that specifically target fungal cell wall synthesis by inhibiting the synthesis of glucan in the cell wall, via inhibition of the enzyme 1,3 β glucan synthase, which serve as essential cross-linking structural components of the cell wall and are thus called "penicillin of antifungals”. They are used for Aspergillus (systemic invasive Aspergillus infection ) and Candida species. Selective toxicity: Inhibit cell wall which is not in human cells. e.g. Caspofungin, Anidulafungin  Pyrimidine Polyenes Griseofulvin Imidazoles Echinocandins analogue Bind to ergosterol Inhibit Beta Inhibit Inhibit DNA and change Inhibit mitosis glucan ergosterol and RNA integrity of cell of fungal cell formation of biosynthesis synthesis membrane cell wall Nystatin : topical Treat Miconazole flucytosine candidiasis dermatophyte (topical) Treat deep Amphotericin B: Intravenous Oral Ketoconazole candida systemic candida preparation (oral) infection infection Ergosterol is Uptake by Converted Ergosterol is found found in fungi fungal cell to active Human has no in fungi and not in and not in more than form in cell wall human human human cells fungal cell  IV. Antiviral drugs Unlike most antibiotics, antiviral drugs do not destroy their target pathogen; instead they inhibit their development. Challenges: 1- Lack of selective toxicity lead to limited amount of antiviral drugs 2- Use of host cell metabolism Antiviral drugs (Mechanism & classification) https://youtu.be/QgZNfuxP0wM Targets for Antiviral drugs 1- Attachment Antibodies 2- Penetration and uncoating Amantadine 3- RNA synthesis (nucleotide analogue) Targets for Antiviral drugs (cont.) 4- Genome replication (nucleotide analogue) 5- Protein synthesis (interferon) 6- Virion Assembly and release (zanamivir and oseltamivir) 7- Stimulators of host innate immune protective response (interferon and antibodies) IV. Antiviral drugs a) Anti-influenza drugs 1. Amantadine Uses: Prophylactic against infection with influenza A virus Mechanism of action: Inhibits viral penetration into host cell 2. Zanamivir and Oseltamivir Zanamivir (Relenza ® ) and Oseltamivir (Tamiflu®) ▪ Uses: Prophylactic against infection with influenza A and B virus or to reduce length of illness ▪ Mechanism of action: inhibit neuroamindase enzyme responsible for viral release from host cell. Nucleoside vs. Nucleotide  IV. Antiviral drugs Nucleoside analogues 1. Vidarabin (Vira-A): Interferes with the synthesis of viral DNA in herpes. When used as a substrate for viral DNA polymerase, leading to the formation of 'faulty' DNA. 2. Ribavirin (Virazole): Used to stop viral RNA synthesis and viral mRNA capping. In this form, it interferes with RNA metabolism required for viral replication. - It is indicated for the treatment of hospitalized infants and young children with severe lower respiratory tract infections caused by respiratory syncytial virus (RSV), HIV and Hepatitis C infections. IV. Antiviral drugs Nucleotide analogues 3. Zidovudine (structural analogue of thymidine) which stops the action of reverse transcriptase in HIV infection, blocking viral DNA production. Uses: to treat AIDS patients 4. Acyclovir (structural analogue of guanosine) Terminates DNA replication in herpes viruses via inhibition of viral DNA polymerase. (It incorporates into and terminates the growing viral DNA chain) Its selective toxicity towards herpes infected cells contributes to presense of thymidine kinase only in virus infected cells. Immunological products Interferon Is a low molecular weight protein, produced by virus-infected cells, that itself induces the formation of a second protein inhibiting the transcription of viral mRNA in neighboring cells. Interferon is produced by host cell in response to the virus particle. ▫ Antiviral ▫ Antitumour ▫ Immunomodulators Recombinant interferon ▫ Interferon alfacon-1 ▫ Interferon alfacon-2 Uses: Treatment of Chronic HCV, HBV and HAV Antiviral Drugs for hepatitis C virus (HCV) Sofosbuvir (Sovaldi) Sofosbuvir and Ledipasvir (Harvoni) Sofosbuvir is a nucleotide analogue used in combination with other drugs for the treatment of hepatitis C virus (HCV) infection. It has been marketed since 2013. Sofosbuvir inhibits the RNA polymerase that the hepatitis C virus uses to replicate its RNA. Sofosbuvir allows most patients to be treated successfully without the use of interferon an injectable drug with severe side effects, that is a key component of older drug combinations for the treatment of HCV). In 2013, the FDA approved sofosbuvir in combination with ribavirin (RBV) for oral dual therapy of HCV genotypes 2 and 3. and for triple therapy with RBV and injected interferon for treatment of HCV genotypes 1 and 4. Read Only: Sofosbuvir was discovered at Pharmasset and developed by Gilead Sciences (American companies). In September 2014, Gilead announced that it would permit generic manufacturers to sell sofosbuvir in 91 developing countries (involving Egypt). Heart Treatment Combination Dangers: In March 2015, Gilead sciences e-mailed warnings to health care providers about 9 patients that began taking its hepatitis C drugs along with the heart treatments amiodarone developed abnormally slow heartbeats and one died of cardiac arrest. Three required a pacemaker to be inserted. Gilead said the combinations aren't recommended and product labels will be updated. THANK YOU