Effect of Supplemental Carbidopa on Bioavailability of l-Dopa

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The effect of doubling carbidopa intake on single dose bioavailability of l-Dopa was examined in five parkinsonian patients. Increasing carbidopa from a mean of 145 to 290 mg/day caused a mean increase in peak plasma Dopa concentrations from 1,893 ± 476 to 2,733 ± 607 ng/ml and area under the Dopa plasma concentration versus time curve of 24.6 ± 10.0%, as well as a decrease in the time to peak plasma concentration to 0.7 ± 0.2 versus 1.2 ± 0.3 h. Increasing the within-dose ratio of carbidopa to l-Dopa, even in patients receiving “maximally effective” doses of carbidopa, further increases bioavailability of l-Dopa, presumably by inhibiting “first pass” metabolism in the gut.

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