THE ROLE OF THE GASTRO-INTESTINAL TRACT IN THE EXCRETION OF C¹⁴-LABELED METHADONE BY RATS

¹ Division of Pharmacology and Experimental Therapeutics, University of California Medical School, San Francisco 22

Based on the determination of radioactivity after a subcutaneous injection of C¹⁴-labeled methadone*·HBr into rats, it was found that gastric excretion does not play a significant role in the elimination of the drug. The accumulation of radioactivity in the stomach is a result of regurgitation from the small intestine. The presence of radioactivity in the small intestine itself is a result of the inflow of radioactive bile.

Radioactivity makes its appearance in the bile within ten minutes after a subcutaneous injection and becomes increasingly concentrated resulting in an increasing minute output as time progresses. As much as 17 per cent of an injected dose of 5 mgm./kgm. can be recovered from the bile during a period of three hours. An amount representing 1.5 per cent of a subcutaneous dose is readily reabsorbed from the small intestine within this period.

In one pregnant rat, considerable amounts of radioactivity were found in the fetuses and placentae.