12. CHEMOTHERAPEUTIC AGENTS

Chemotherapeutic Agents: A Comprehensive Guide to Antibiotics, Antivirals, and More

While we often associate the word “chemotherapy” exclusively with cancer, its simplest definition is the treatment of any ailment using chemicals, specifically to kill microorganisms or parasites. From the first modern agent, Arsphenamine (1909), to today’s advanced antivirals, chemotherapy remains the cornerstone of infectious disease management.

---

1. Core Principles of Chemotherapy

To understand how these drugs work, we must look at five key principles:

• Selectivity: The drug must be “selectively toxic,” meaning it kills the pathogen (bacteria/virus) without harming the human host.
• Therapeutic Index: This is the ratio of the toxic dose to the effective dose. A higher therapeutic index means the drug is safer.
• Cidal vs. Static:
  • Bactericidal: Kills the bacteria (e.g., Penicillin).
  • Bacteriostatic: Reversibly inhibits growth (e.g., Tetracycline).
• Susceptibility (MIC & MBC):
  • MIC (Minimum Inhibitory Concentration): The lowest dose that stops visible growth.
  • MBC (Minimum Bactericidal Concentration): The lowest dose that kills 99.9% of the bacteria.
• Synergism: When combining two drugs produces an effect greater than the sum of their individual parts.

---

2. Classification of Major Antibiotics

A. Penicillins (Cell Wall Inhibitors)

The first clinical antibiotic (1941), Penicillins are derived from fungi and work by rupturing bacterial cell walls.

• Natural: Penicillin G.
• Beta-Lactamase Resistant: Methicillin, Cloxacillin (used for MRSA).
• Extended Spectrum: Amoxicillin, Ampicillin.
• Side Effects: Diarrhoea, rashes, and rare anaphylactic shocks.

B. Cephalosporins

Classified into “Generations” based on their spectrum of activity:

• 1st Gen: Cefazolin (Active against E. coli).
• 2nd Gen: Cefuroxime (Broader gram-negative coverage).
• 3rd Gen: Ceftriaxone (Drug of choice for Meningitis).
• 4th Gen: Cefepime (Highly resistant to beta-lactamase).

C. Aminoglycosides

Used primarily for serious systemic infections.

• Examples: Gentamicin, Streptomycin, Amikacin.
• Warning: These can be Ototoxic (hearing loss) and Nephrotoxic (kidney damage).

---

3. Specialty Chemotherapy: TB, Fungi, and Viruses

Anti-Tubercular Drugs (Tuberculosis)

TB treatment is long-term (8 months to 3 years).

• First-Line (The Essentials): Isoniazid (INH), Rifampicin, Pyrazinamide, Ethambutol.
• Second-Line: Used for resistant strains (e.g., Kanamycin, Ethionamide).

Anti-Fungal Agents

• Polyenes: Amphotericin B (for systemic infections).
• Azoles: Fluconazole, Ketoconazole (for yeast and skin infections).
• Allylamines: Terbinafine (highly effective for nail infections).

Anti-Viral Agents

• Anti-Herpes: Acyclovir (inhibits DNA polymerase).
• Anti-Retrovirus (HIV): Managed via HAART (Highly Active Antiretroviral Therapy), utilizing NRTIs like Zidovudine (AZT).
• Anti-Influenza: Amantadine (prevents viral uncoating).

---

4. Understanding Neoplastic (Cancer) Diseases

When chemotherapy is used for Neoplasms (abnormal tissue growth), it targets different types of cancers:

1. Carcinomas: Originate in epithelial tissue (skin, organ linings).
2. Sarcomas: Connective tissue (bones, muscle, fat).
3. Leukemias: Blood cancers originating in the bone marrow.
4. Lymphomas: Cancers of the lymphatic system.