Penicillins consist of a thiazolidine ring (A) connected to a b-lactam ring (B), to which is attached a side chain (R) (Figure 44—1). The penicillin nucleus itself is the chief structural requirement for biological activity. The discovery that 6-aminopenicillanic acid could be obtained from cultures that were depleted of side-chain precursors led to the development of the semisynthetic penicillins, which have side chains that alter the susceptibility of the resulting compounds to inactivating enzymes (b-lactamases) and that change the antibacterial activity and pharmacological properties of the drug (Table 44—1).

The international unit of penicillin is the specific penicillin activity contained in 0.6 mg of the crystalline sodium salt of penicillin G. Doses of semisynthetic penicillins are expressed by weight.

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