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Absorption by the Rhesus Monkey of Phenylalanine Methyl Ester and Species Differences in Its Metabolism by Blood, Plasma and Intestinal Mucosa

Department of Drug Metabolism, G. D. Searle and Co., Research and Development Division, 4901 Searle Parkway, Skokie, IL 60077

Phenylalanine methyl ester (PM) is a decomposition product of the sweetening agent aspartame. The potential for absorption of PM was examined following intragastric and intraduodenal administration of 20-mg doses of [¹⁴C]PM to rhesus monkeys implanted with hepatic portal vein cannulae. Small amounts of unchanged PM (<0.1 µg/ml) were detected in portal and peripheral blood samples during the first 1–2 hours after administration, but none was detectable (<0.001 µg/ml) at later times. Comparison of the areas under the PM and total ¹⁴C blood concentration-time curves indicated that only 0.2% of the administered PM reached the portal blood unchanged, and 0.1% or less reached the peripheral blood unchanged. Blood and plasma from monkeys and humans hydrolyzed PM in vitro at very similar rates, but plasma PMase activity was much higher in the dog and rat than in the monkey or human. Hydrolysis of PM by intestinal mucosa homogenates was also faster for the rat and dog than for the monkey. The in vitro results suggest that absorption of intact PM in the human would be no greater than that found in the monkey.

KEY WORDS: • phenylalanine methyl ester • monkey • intestinal absorption

Manuscript received 22 March 1984.