Short-chain alkyl esters of L-dopa as prodrugs for rectal absorption

The bioavailability of L-dopa following rectal administration of a series of short-chain alkyl esters of L-dopa was determined in rats and dogs. The esters were stable (greater than 360 min) to hydrolysis in physiological buffer. In vitro enzymatic hydrolysis of the esters in plasma was species dependent, with the hydrolytic rate being faster in rat plasma (t 1/2 less than 5 min) than dog plasma (t 1/2 = 68-181 min) or human plasma (t 1/2 = 96-238 min). In vivo hydrolysis in dogs, as indicated by the L-dopa plasma profile following intravenous administration of the esters, was very rapid (high extravascular esterase activity). Significant L-dopa bioavailability was observed in rats following rectal administration of the methyl (46%), ethyl (14%), isopropyl (48%), butyl (100%), and 4-hydroxybutyl (13%) esters of L-dopa (rectal L-dopa absorption, less than 5%). In dogs, significant L-dopa bioavailability was also observed for the methyl (28%), isopropyl (30%), butyl (32%), and 4-hydroxybutyl (34%) esters of L-dopa in the presence of carbidopa. The data indicate that these highly water-soluble (greater than 600 mg/ml) esters of L-dopa are potential candidates for controlled-release rectal delivery systems designed to provide more constant plasma L-dopa levels.