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Wednesday, August 20, 2025

Anti-infectives


Introduction

Anti-infectives are a broad class of pharmacological agents used to prevent or treat infections caused by pathogenic microorganisms including bacteria, viruses, fungi, protozoa, and helminths. They work by either killing the pathogen (microbicidal) or inhibiting its growth and replication (microbiostatic), allowing the host’s immune system to clear the infection.

They encompass:

  • Antibacterials (antibiotics)

  • Antivirals

  • Antifungals

  • Antiparasitics (antiprotozoals and anthelmintics)

  • Antituberculosis and antileprosy drugs


Classification of Anti-Infectives

  1. Antibacterials (Antibiotics)

    • Beta-lactams (penicillins, cephalosporins, carbapenems, monobactams)

    • Aminoglycosides

    • Macrolides

    • Tetracyclines

    • Fluoroquinolones

    • Glycopeptides (vancomycin, teicoplanin)

    • Sulfonamides and trimethoprim

    • Oxazolidinones (linezolid)

    • Others: Chloramphenicol, clindamycin, rifamycins

  2. Antivirals

    • Anti-HIV agents (NRTIs, NNRTIs, protease inhibitors, integrase inhibitors)

    • Anti-herpes agents (acyclovir, valacyclovir, ganciclovir)

    • Anti-influenza agents (oseltamivir, zanamivir, baloxavir)

    • Anti-hepatitis agents (entecavir, tenofovir, sofosbuvir)

  3. Antifungals

    • Polyenes (amphotericin B, nystatin)

    • Azoles (fluconazole, itraconazole, voriconazole)

    • Echinocandins (caspofungin, micafungin, anidulafungin)

    • Allylamines (terbinafine)

  4. Antiparasitics

    • Antimalarials (chloroquine, artemisinin derivatives, atovaquone-proguanil)

    • Antiprotozoals (metronidazole, tinidazole, nitazoxanide)

    • Anthelmintics (albendazole, mebendazole, ivermectin, praziquantel)

  5. Antituberculosis and Antileprosy Agents

    • TB: Isoniazid, rifampicin, pyrazinamide, ethambutol

    • Leprosy: Dapsone, clofazimine, rifampicin

Mechanisms of Action

1. Antibacterials

  • Cell wall synthesis inhibitors: Beta-lactams, glycopeptides.

  • Protein synthesis inhibitors: Aminoglycosides (30S), tetracyclines (30S), macrolides (50S), clindamycin (50S), chloramphenicol (50S).

  • DNA/RNA synthesis inhibitors: Fluoroquinolones (DNA gyrase/topoisomerase IV), rifamycins (RNA polymerase).

  • Metabolic pathway inhibitors: Sulfonamides (folate synthesis), trimethoprim (dihydrofolate reductase).

2. Antivirals

  • Nucleoside/nucleotide analogues: Block viral DNA/RNA polymerases (acyclovir, zidovudine, sofosbuvir).

  • Protease inhibitors: Prevent maturation of viral proteins (ritonavir, darunavir).

  • Integrase inhibitors: Block viral genome integration (raltegravir, dolutegravir).

  • Entry/fusion inhibitors: Block viral entry (enfuvirtide, maraviroc).

3. Antifungals

  • Polyenes: Bind ergosterol, form pores (amphotericin B).

  • Azoles: Inhibit ergosterol synthesis (fluconazole).

  • Echinocandins: Inhibit β-1,3-glucan synthase (caspofungin).

  • Allylamines: Inhibit squalene epoxidase (terbinafine).

4. Antiparasitics

  • Antimalarials: Interfere with heme detoxification (chloroquine), generate free radicals (artemisinins).

  • Metronidazole: Disrupts DNA synthesis in anaerobic protozoa.

  • Albendazole/mebendazole: Inhibit microtubule formation in helminths.

  • Ivermectin: Paralyzes worms by enhancing chloride influx via glutamate-gated channels.

5. Antituberculosis Agents

  • Isoniazid: Inhibits mycolic acid synthesis.

  • Rifampicin: Inhibits bacterial RNA polymerase.

  • Pyrazinamide: Disrupts mycobacterial cell membrane energetics.

  • Ethambutol: Inhibits arabinosyl transferases in cell wall synthesis.

Examples of Generic Drugs and Doses

  • Amoxicillin (antibacterial): 500 mg PO every 8 hours.

  • Ciprofloxacin (antibacterial): 500 mg PO every 12 hours.

  • Acyclovir (antiviral): 400 mg PO three times daily.

  • Oseltamivir (antiviral): 75 mg PO twice daily × 5 days (influenza).

  • Fluconazole (antifungal): 150 mg PO single dose (vaginal candidiasis).

  • Amphotericin B (antifungal): 0.5–1 mg/kg IV daily.

  • Metronidazole (antiprotozoal): 500 mg PO every 8 hours × 7 days.

  • Albendazole (anthelmintic): 400 mg PO single dose.

  • Isoniazid (antitubercular): 300 mg PO daily.

  • Rifampicin (antitubercular): 600 mg PO daily.

Adverse Effects

  • Antibacterials: Allergic reactions, diarrhea, Clostridioides difficile infection, nephrotoxicity (aminoglycosides), ototoxicity.

  • Antivirals: Nephrotoxicity (acyclovir), bone marrow suppression (ganciclovir, zidovudine), hepatotoxicity.

  • Antifungals: Nephrotoxicity (amphotericin B), hepatotoxicity (azoles).

  • Antiparasitics: GI upset, neurotoxicity (ivermectin in Loa loa infection), bone marrow suppression (dapsone).

  • Anti-TB drugs: Hepatotoxicity (isoniazid, rifampicin, pyrazinamide), optic neuritis (ethambutol).

Contraindications

  • Severe allergy to the drug class.

  • Renal impairment: Avoid aminoglycosides, adjust dose for antivirals (acyclovir).

  • Hepatic impairment: Avoid or monitor azoles, isoniazid, rifampicin.

  • Pregnancy restrictions for tetracyclines, fluoroquinolones, and some antivirals.

Precautions

  • Monitor liver function for hepatotoxic agents (anti-TB, azoles).

  • Monitor renal function for aminoglycosides, amphotericin, antivirals.

  • Educate patients on adherence, particularly in TB, HIV, and malaria treatment.

  • Prevent resistance by using combination therapy where necessary (e.g., TB, HIV, malaria).

Drug Interactions

  • Rifampicin: Potent CYP450 inducer → reduces levels of many drugs (warfarin, contraceptives, antiretrovirals).

  • Azoles (fluconazole, itraconazole): CYP450 inhibitors → increase toxicity of other drugs.

  • Macrolides (erythromycin, clarithromycin): CYP3A4 inhibitors → increase statin, warfarin toxicity.

  • HIV antiretrovirals: Multiple drug–drug interactions due to CYP effects.

  • Thiazides + antimicrobials (e.g., sulfonamides): Can worsen hyponatremia.

Clinical Efficacy and Limitations

  • Anti-infectives remain cornerstone therapies in medicine.

  • Resistance is a major global problem: MRSA, MDR-TB, HIV drug resistance, artemisinin-resistant malaria.

  • Combination therapy is often essential to prevent resistance and achieve synergistic efficacy.

  • New drug development lags behind resistance emergence, making antimicrobial stewardship critical.




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