Intestinal decarboxylation of orally administered L-dopa. Influence of pharmacological preparations, dose magnitude, dose sequence and food intake.

The intestinal decarboxylation and the absorption of orally administered L-dopa have been studied in 20 parkinsonian patients in different conditions. The decarboxylation generally amounted to 50-70% of the dose given, i.e. only 30-50% of orally administered L-dopa reached the general circulation. The shape of the plasma concentration curve varied individually and with the resolvation time of the type of pharmacological preparation given. No obvious difference in the degree of intestinal decarboxylation was observed when tablets with different resolvation times were given but the net absorption of L-dopa was somewhat greater when it was given as a solution. The decarboxylation in the intestinal organs was found to be rather constant at different times of the day. Isocaloric meals, whether rich in protein or carbohydrate, caused a delay in absorption of L-dopa, but did not change the degree of decarboxylation. Diurnal variations in the clinical response to L-dopa, could, thus, not be related to diurnal variations in the decarboxylation in the intestinal organs. The delay in L-dopa absorption after meals may be misinterpreted as lack of response. At dosage with short intervals, the delay may cause an additive effect of two doses.