Antimicrobial Drugs

History of Chemotherapy

Chemotherapy refers to the use of chemicals to treat diseases, particularly infections caused by microorganisms. The development of antimicrobial drugs revolutionized medicine by enabling the selective targeting of pathogens.

• Selective toxicity: The ability of a drug to destroy harmful microbes without damaging the host.

• Chemotherapy: Treatment of disease using chemical substances.

• Antibiotic: A substance produced by a microbe that inhibits the growth of other microbes.

• Historical milestones:

  • Alexander Fleming discovered penicillin in 1928.

  • Prontosil red dye was used to treat streptococcal infections.

Spectrum of Antimicrobial Activity

Antimicrobial drugs vary in the range of microorganisms they affect.

• Narrow-spectrum drugs: Effective against a limited group of microbes.

• Broad-spectrum antibiotics: Target a wide variety of bacteria.

• Superinfection: Overgrowth of normal microbiota that are resistant to antibiotics, often caused by broad-spectrum drug use.

• Examples of superinfection organisms: Candida albicans (fungus), Clostridium difficile (bacterium).

Actions of Antimicrobial Drugs

Antimicrobial drugs act through various mechanisms to inhibit or kill microbes.

• Bactericidal drugs: Directly kill microorganisms.

• Bacteriostatic drugs: Inhibit microbial growth, allowing the immune system to eliminate the pathogen.

• Major mechanisms of action:

  • Inhibition of cell wall synthesis (e.g., penicillins).

  • Inhibition of protein synthesis (e.g., tetracyclines, chloramphenicol) by targeting bacterial 70S ribosomes.

  • Injury to plasma membrane (e.g., polypeptide antibiotics).

  • Inhibition of nucleic acid synthesis (e.g., rifampin, quinolones).

  • Inhibition of essential metabolite synthesis (e.g., sulfanilamide).

Inhibitors of Cell Wall Synthesis

These drugs prevent the formation of peptidoglycan, weakening bacterial cell walls and causing cell lysis.

• Penicillins: Block peptidoglycan synthesis.

• Isoniazid: Inhibits synthesis of mycolic acid in mycobacteria (e.g., Mycobacterium tuberculosis).

Inhibitors of Protein Synthesis

These drugs target bacterial ribosomes, disrupting protein production.

• Chloramphenicol: Inhibits peptide bond formation.

• Erythromycin: Binds to the 50S subunit of bacterial ribosomes.

• Streptomycin: Interferes with the 30S subunit, causing misreading of mRNA.

Inhibitors of Nucleic Acid Synthesis

These drugs interfere with DNA or RNA synthesis, preventing microbial replication.

• Rifampin: Inhibits RNA synthesis (transcription).

• Quinolones: Inhibit DNA gyrase, blocking DNA replication.

• Ciprofloxacin: A broad-spectrum quinolone antibiotic.

Competitive Inhibition of Essential Metabolites

Some drugs mimic natural substrates, blocking metabolic pathways essential for microbial survival.

• Sulfonamides: Structurally similar to para-aminobenzoic acid (PABA), competitively inhibit folic acid synthesis.

• Trimethoprim: Inhibits production of tetrahydrofolate, a cofactor in nucleotide synthesis.

• TMP-SMZ: Combination of trimethoprim and sulfamethoxazole for synergistic effect.

Antifungal Drugs

Antifungal agents target unique features of fungal cells, such as the cell membrane component ergosterol.

• Nystatin: Used for superficial fungal infections.

• Amphotericin B: Binds to ergosterol, disrupting membrane integrity.

• Azoles: Inhibit ergosterol synthesis, affecting membrane structure.

Antiviral Drugs

Antiviral drugs interfere with various stages of the viral life cycle.

• Entry inhibitors: Block viral attachment or entry into host cells.

• Nucleoside analogs: Mimic nucleotides, inhibiting viral RNA or DNA synthesis.

• Protease inhibitors: Prevent cleavage of viral protein precursors.

• Paxlovid: Used to treat COVID-19.

• Exit inhibitors: Block neuraminidase, preventing viral release from host cells.

Interferons and HIV Treatment

Interferons are host-produced proteins with antiviral properties. HIV, an RNA virus, is treated with specialized drugs.

• Interferons: Produced by virus-infected cells; enhance immune response.

• Antiretroviral drugs: Used to treat HIV infections by targeting viral enzymes.

Antiprotozoan Drugs

These drugs target protozoan parasites responsible for diseases such as malaria and amebiasis.

• Quinine and chloroquine: Treat malaria by interfering with parasite metabolism.

• Artemisinin: Kills Plasmodium species (malaria parasites).

• Metronidazole: Treats giardiasis and amebic dysentery.

• Miltefosine: Used for leishmaniasis and amebic infections.

Antihelminthic Drugs

Antihelminthic agents combat parasitic worms by targeting their metabolism or nervous system.

• Niclosamide: Inhibits ATP production in tapeworms.

• Praziquantel: Alters membrane permeability, causing paralysis.

• Mebendazole: Blocks nutrient absorption in helminths.

• Ivermectin: Causes paralysis of worms by interfering with nervous system function.

Resistance to Antimicrobial Drugs

Microbial resistance to drugs is a growing concern in medicine.

• Persister cells: Survive antimicrobial treatment without genetic resistance.

• Superbugs: Bacteria resistant to multiple antibiotics.

• Spread of resistance: Resistance genes can be transferred via plasmids and transposons.

• Examples: Acinetobacter baumannii, Pseudomonas aeruginosa.

Mechanisms of Resistance

Bacteria employ several strategies to evade antimicrobial drugs.

• Beta-lactamases: Enzymes that destroy the beta-lactam ring of penicillins and related drugs.

• Altered porins: Gram-negative bacteria may prevent drug entry by modifying outer membrane proteins.

• Modified targets: MRSA (Methicillin-resistant Staphylococcus aureus) has altered penicillin-binding proteins.

• Efflux pumps: Actively expel antibiotics from the cell.

Antibiotic Misuse

Improper use of antibiotics accelerates the development of resistance.

• Using antibiotics for viral infections (e.g., common cold) is ineffective and promotes resistance.

• Not completing the prescribed course allows surviving bacteria to develop resistance.

• Using leftover antibiotics is unsafe and contributes to resistance.

Antibiotic Safety and Drug Interactions

Antibiotics must be used judiciously to minimize harm to patients.

• Therapeutic index: Ratio of toxic dose to effective dose; higher values indicate safer drugs.

• Some antibiotics can cause kidney damage (nephrotoxicity) or pose risks to the fetus (teratogenicity).

Effects of Drug Combinations

Combining drugs can enhance or reduce their effectiveness.

• Synergism: Two drugs work better together than alone.

• Antagonism: Two drugs work less effectively together than alone.

Summary Table: Major Classes of Antimicrobial Drugs

Key Equation: Therapeutic Index

The therapeutic index (TI) is a measure of drug safety:

A higher TI indicates a safer drug.

Example: If the toxic dose for 50% of the population is 100 mg and the effective dose for 50% is 10 mg, then TI = 10.

Additional info: The above notes expand on the mechanisms, examples, and clinical relevance of antimicrobial drugs, including resistance and safety considerations, to provide a comprehensive overview suitable for exam preparation.