Question

why do penicillins and cephalosporins have a much higher therapeutic index than most other antibiotics?

Answer 1

First of all, the therapeutic index can be defined as ratio of toxic dose/therapeutic dose. Where, therapeutic dose is the drug level required for clinical treatment of a particular infection. On the other hand, toxic dose is the drug level at which the agent becomes too toxic for the host.

Penicillin and cephalosporins have a high therapeutic index than other antibiotics because they inhibit bacterial cell wall peptidoglycan synthesis but have little effect on host cells because they lack cell walls. Cephalosporins are the most commonly prescribed class of antibiotics, and its structure and pharmacology are similar to that of penicillin. It's a bactericidal, and its structure contains beta-lactam ring, as like of penicillin, which intervenes in bacterial cell wall synthesis.