OBJECTIVE EVIDENCE FOR TOLERANCE, AGAINST A BACKGROUND OF IMPROVEMENT, DURING MAINTENANCE THERAPY WITH CONTROLLED RELEASE LEVODOPA CARBIDOPA

We have investigated whether the potential benefits of a controlled release formulation of levodopa (200 mg)/carbidopa (50 mg), Sinemet CR, are realised during maintenance therapy Eight sufferers from idiopathic Parkinsonism, mean age 69.9 y, were studied: all exhibited "end of dose" effect within 4 h of a dose of their maintenance therapy with levodopa (100 mg)/carbidopa (25 mg) in a conventional release formulation, Sinemet Plus. They received, in random order, initial single dose challenges with one tablet of Sinemet Plus, one and two tablets of Sinemet CR and placebo alone, each on a separate day. After a mean of 21 weeks on maintenance therapy with Sinemet CR, subsequent single dose challenges with Sinemet CR and placebo were made. Objective measures of performance and blood sampling for assay of plasma concentrations of levodopa and the major peripheral metabolite, 3-0-methyldopa (30MD) were carried out immediately before (10.00 h) and serially until 6 h after each challenge. The overall mean stride length was significantly greater in relation to the subsequent (679 mm) than the initial (517 mm) placebo challenge. Moreover, stride length immediately before the challenges was significantly greater on the subsequent occasions. Improved performance, also seen for free walking speed, was not explained by plasma levodopa or 30MD concentrations. In the initial challenges, the mean increment in stride length achieved by active treatment, as compared with placebo, did not differ significantly between the one (210 mm) and two (235 mm) tablet doses of Sinemet CR: a maximal response had been obtained. The increment over placebo values following two active tablets did not differ significantly between initial and subsequent (192 mm) challenges. However, the increment produced by one active tablet in the subsequent (79 mm) challenge was significantly less than that produced by two, and than that which had been produced by one tablet in the initial challenge. Findings for free coalting speed mirrored those for stride length. These differences could, in part, be accounted for by attenuation of the effect of levodopa by 30MD. The improvement in baseline performance, in relation to the subsequent challenges, possibly resulted from striatal dopamine stores being more replete during maintenance therapy with the controlled release formulation. It was seen despite evidence of tolerance to an acute challenge with the lower dose.