Specific radioimmunoassays for codeine and morphine. Metabolism of codeine to morphine in the rat

Specific antisera to morphine have been raised in response to immunization with a conjugate of N-carboxypropylnormorphine with bovine serum albumin (BSA). These antisera effectively distinguish changes in substituents at the 3 and 6 positions of the alkaloid, thus reducing cross-reactivity with codeine and morphine-3-glucuronide to negligible levels. The utility of these antisera has been illustrated by their application in radioimmunoassay procedures, along with similarly specific anti-codeine sera (Findlay et al., 1976) to a study of the biotransformation of codeine to morphine in the rat. After oral administration of codeine, serum levels of morphine were low, but significantly higher than codeine levels after 15 min., indicating rapid metabolism of codeine to morphine in this species.