Objective: The aims of the study were to follow prospectively the intrasubject progression of idiopathic PD in a cohort of patients using levodopa kinetic-dynamic modeling and to assess the relation between the rate of progression of the disease and patients' different clinical characteristics.
Methods: Thirty-four patients (Hoehn and Yahr stages 1 to 3) enrolled in the longitudinal follow-up. Each patient was examined at 1-year intervals over a median 4 years by a standardized oral levodopa test. The primary measure outcome was the computed half-life of levodopa in the "effect compartment" (t1/2eq), a proposed indicator of nigrostriatal dopaminergic functionality and integrity.
Results: Values of levodopa t1/2eq correlated negatively with severity of symptoms (r = -0.652, p < 0.0001) and decreased over the years together with a worsening of patients' clinical stage (p < 0.001). The rate of reduction in drug t1/2eq was more rapid in patients at the earlier stages of the disease compared with the more advanced ones, falling from a median annual reduction of 37 minutes in patients at initial Hoehn and Yahr stage 1 to 6.5 minutes in stage 3 patients (p < 0.001). Patients without tremor at onset, otherwise comparable to patients with tremor for baseline values of levodopa t1/2eq, disease severity, duration, and daily dose of levodopa, tended to show a higher rate of reduction in levodopa t1/2eq than patients with tremor. Overall, patients' annual reduction in levodopa t1/2eq over baseline values averaged 17+/-9%.
Conclusions: These results are in keeping with PET findings on the objective assessment of idiopathic parkinsonism evolution, and they support the suggestion that levodopa pharmacodynamic modeling may offer a practical clinical tool to assess indirectly the functional integrity of the nigrostriatal dopaminergic system over time in parkinsonian patients.