Inhibition of Decarboxylase and Levels of Dopa and 3-O-Methyldopa: A Comparative Study of Benserazide versus Carbidopa in Rodents and of Madopar Standard versus Madopar HBS in Volunteers

The combinations of benserazide and levodopa (1:4, Madopar) and of carbidopa and levodopa (1:10 and 1:4, Sinemet) are currently the most effective treatment of Parkinson's disease. In the present comparative study some effects of the peripheral aromatic L-amino acid decarboxylase (AADC) inhibitors benserazide and carbidopa administered alone or in combination with levodopa by the oral route were investigated in two animal species (rat and mouse) and in healthy volunteers. Benserazide is about 10 times more potent than carbidopa as inhibitor of peripheral AADC both in animals and man. Even at relatively high doses (up to 60 mumol/kg p.o.) benserazide is shown in animals to inhibit the decarboxylation of levodopa only in the extracerebral tissues, thus permitting the formation of dopamine in the striatum and in the hypothalamus. As benserazide is the most potent peripheral AADC inhibitor presently available, is well tolerated and relatively nontoxic even when used chronically, it appears to be the peripheral AADC inhibitor of choice for the development of controlled-release formulations in which Dopa is combined with a peripheral AADC inhibitor. When administered to healthy subjects the pharmacokinetics of the new drug delivery system named Madopar HBS (hydrodynamically balanced system) was characterized by lower and delayed plasma peak concentrations but a longer-lasting concentration of Dopa than after Madopar standard. Therefore, this new controlled-release system may reduce the clinical fluctuations occurring in patients with 'wearing-off' and 'on-off' phenomena.