Absorption, Metabolism, and Excretion of the Semisynthetic Penicillin 6-(2-Ethoxy-l-naphthamido)penicillanic Acid (Nafcillin)

Nafcillin, 6-(2-ethoxy-1-naphthamido)penicillanic acid, a semisynthetic penicillin effective against penicillin G-resistant staphylococci, was labeled with carbon-14, and its absorption, metabolism, and excretion studied in dogs and rats after oral and intramuscular administration. Maximum plasma levels were attained within 15 minutes of an intramuscular dose and within 2 hours of an oral dose, although detectable levels appeared in plasma within 30 minutes in the latter case. Low levels of activity persisted in plasma up to 8 days, although about one-third was excreted in feces and about one-sixth in urine within 48 hours. Thirty minutes after injection, the drug was found in all major organs and tissues examined. Within the first 6 hours, about half the carbon-14 excreted in urine was due to nafcillin, about 30 per cent was due to a metabolite of similar Rƒ to unchanged drug, and the remaining 20 per cent was divided equally between two other metabolites.