Assessing dopaminergic function in Parkinson's disease: Levodopa kinetic-dynamic modeling and SPECT

Levodopa pharmacokinetic-phamacodynamic (PK-PD) modeling may be able to test the functional integrity of the nigrostriatal dopaminergic system in Parkinson's disease (PD). [¹²³I]-FP-CIT SPECT imaging of striatal dopamine transporters has also been introduced for the evaluation of presynaptic dopaminergic homeostasis. We aimed to assess the intrapatient relation between levodopa PK-PD and SPECT measures of dopaminergic function in PD. Thirty-five PD patients, 1 to 4 on the Hoehn and Yahr (H&Y) scale, enrolled in the study. Each patient was examined by levodopa PK-PD modeling and SPECT imaging. Primary measure outcomes were the levodopa half-life in the effect compartment (t1/2_eq) for PK-PD modeling and the ratio of specific to non specific (SP/NSP) tracer striatal uptake for SPECT. Levodopa t1/2_eq was highly significantly correlated with H&Y scale (r = -0.815, p <0.0001), Unified Parkinson's disease Rating Scale (UPDRS) (r = -0.691, p <0.0001) and PD symptom duration (r = -0.647, p <0.0001). SPECT contralateral putamen SP/NSP ratio showed the most significant correlations with clinical indicators of disease severity: H&Y, r = -0.526, p <0.002; UPDRS, r = -0.523, p <0.002; symptom duration, r = -0.513, p <0.002. Significant correlations were observed between levodopa t1/2 _eq and putamen SP/NSP ratios, yielding the closest correlation for the contralateral region (r = 0.522, p <0.002). An indirect PK-PD dopaminergic functional variable and direct SPECT measures of presynaptic dopaminergic system homeostasis were in close agreement with clinical data and correlated to each other. Levodopa PK-PD modeling can be a practical clinical tool indirectly assessing the functional integrity of the nigrostriatal dopaminergic system in PD patients.