Selective toxicity of a drug refers to
a) damage to pathogenic organisms only
b) damage to prokaryotic cell membranes
c) damage to the target organisms but not the host cell
d) easily broken down by the host and pathogen.
The correct answer and explanation is :
Correct Answer:
c) damage to the target organisms but not the host cell
Explanation:
Selective toxicity refers to the ability of a drug to specifically target and kill or inhibit the growth of harmful microorganisms while causing minimal or no harm to the host’s cells. This concept is fundamental in antimicrobial therapy and is based on differences between microbial and host cell structures and metabolism.
Mechanism of Selective Toxicity:
Drugs achieve selective toxicity by targeting unique features of microbial cells that are absent or significantly different in host cells. Some key mechanisms include:
- Cell Wall Inhibition:
Many bacteria have peptidoglycan cell walls, which are absent in human cells. Antibiotics like penicillins and cephalosporins inhibit peptidoglycan synthesis, leading to bacterial death without harming human cells. - Protein Synthesis Inhibition:
Bacterial ribosomes (70S) differ from human ribosomes (80S). Drugs like tetracyclines, aminoglycosides, and macrolides target bacterial ribosomes, selectively blocking protein synthesis without affecting host cells. - Nucleic Acid Synthesis Inhibition:
Some drugs selectively inhibit bacterial DNA or RNA synthesis. For example, quinolones (ciprofloxacin) target bacterial DNA gyrase, and rifampin inhibits bacterial RNA polymerase, both of which are structurally different from human enzymes. - Metabolic Pathway Inhibition:
Bacteria synthesize folic acid, which is essential for DNA synthesis, whereas humans obtain folic acid from their diet. Sulfonamides selectively block folic acid synthesis in bacteria, leaving human cells unaffected.
Importance of Selective Toxicity:
- Minimizes Host Damage: Ensures that therapeutic drugs do not harm the patient.
- Enhances Treatment Effectiveness: Directly targets the pathogen, reducing side effects.
- Prevents Drug Resistance: Reducing harm to beneficial host flora helps prevent drug-resistant infections.
Selective toxicity is a key principle in designing effective antimicrobial drugs, ensuring successful treatment with minimal side effects.