CLINICAL-SIGNIFICANCE OF STRIATAL DOPA DECARBOXYLASE ACTIVITY IN PARKINSONS-DISEASE

We performed dynamic PET studies with fluorodopa (FDOPA) in 9 normal v olunteers and 16 patients with Parkinson's disease to investigate the applicability of dopa decarboxylase (DDC) activity measurements as use ful markers of the parkinsonian disease process. Methods: From the 3-O -methyl-FDOPA (3OMFD)/PET studies, we obtained mean population values of the kinetic rate constants for 3OMFD (K-1(M) = 0.0400 and K-2(M) = 0.0420). We applied these values to calculate striatal DDC activity us ing the FDOPA compartmental model. We estimated k(3)(D) in this group using dynamic FDOPA-PET and population mean K-1(M) and k(2)(M) values. We then applied the mean population K-1(M) and k(2)(M) values to esti mate k(3)(D)(pop) to a new group (6 normal volunteers and 11 patients) studied only with dynamic FDOPA-PET. In all FDOPA/PET studies, we cal culated striatal uptake rate constants (K-i(FD)) using a graphical met hod and also measured the striato-occipital ratio (SOR). Results: Alth ough DDC activity has been postulated as a precise indicator of presyn aptic nigrostriatal dopaminergic function, K-i(FD) and SOR provided be tter between-group discrimination than did estimates of striatal DDC a ctivity. K-i(FD) and k(3)(D)(pop) both correlated significantly with q uantitative disease severity ratings, with a similar degree of accurac y (r = 0.69 and 0.63 for k(3)(D)(pop) and K-i(FD), respectively; p < 0 .01). Conclusion: Although estimated striatal DDC activity correlates with clinical disability, this measure is comparably less effective fo r early diagnosis. We conclude that a simple estimate such as striatal K-i(FD) is superior to k(3)(D) measurements for most clinical and res earch applications.