Combined levodopa-anticholinergic therapy in the treatment of Parkinson's disease. Effect on levodopa bioavailability

The effect of chronic intake of the anticholinergic drug orphenadrine on the bioavailability of levodopa was studied in six patients with Parkinson's disease. Plasma levodopa profiles and corresponding motor response to a standard dose of levodopa plus an inhibitor of peripheral L-aromatic aminoacid decarboxylase (benserazide) were followed, over a 5-h period, on two different sessions, with and without anticholinergic cotherapy. Six control patients were also included in the study for assessment of intrasubject variability in levodopa absorption under identical conditions. A considerable delay in levodopa absorption was found in one patient, and decreased absorption in an additional two patients while on anticholinergic. Patients' clinical performance corresponded well to plasma levodopa profiles. A significant impairment of basal clinical state was observed in two cases on anticholinergic withdrawal, probably as a result of pharmacodynamic interactions. Our observations suggest that chronic anticholinergic intake may contribute to a less predictable pattern of levodopa absorption and related therapeutic response in some subjects.