Pharmacokinetic analysis of the novel glycogen synthase kinase-3 radioligand [11C]PF367 in non-human primates

295 Objectives We recently developed [11C]PF367 as a radiotracer for positron emission tomography (PET) imaging of the glycogen synthase kinase-3 (GSK-3), a critical enzyme involved in numerous intracellular signaling pathways and implicated in several pathological conditions including Alzheimer9s disease, diabetes, cancer, and bipolar disorder. The goal of this work was to provide an initial preclinical characterization of [11C]PF367 pharmacokinetics using arterial input functions. Methods A total of 9 [11C]PF367 PET scans were conducted in two male rhesus macaques using a NeuroPET/CT scanner (PhotoDiagnostic Systems). Each monkey underwent 2 paired baseline-blocking studies using each of 2 different doses of cold PF367 (0.135 and 1 mg/kg of body weight). Each baseline scan was performed followed 4 hours later by the blocking scan wherein unlabeled PF367 was co-injected with the radiotracer. One chase study was performed in which a cold dose of PF367 (1 mg/kg i.v.) was administered 45 min after injection of [11C]PF367. Each scan commenced with infusion of [11C]PF367 as a slow bolus over 3 min and dynamic PET data were measured for 120 min. Arterial samples were drawn and radioactivity concentration measured in whole blood and plasma. Selected plasma samples were analyzed on a column-switching radio-HPLC system to generate metabolite-corrected arterial input functions. CT images were aligned to a standardized space and the transformation matrix was applied to the dynamic PET images. Compartmental analyses with one- (1T) and two- (2T) tissue models were performed on regional time activity curves. The Patlak and Logan graphical methods were also applied to region-based data and for voxelwise analysis to generate parametric maps. Results Plasma:whole blood ratio was 1.30±0.09 after 1 minute. Plasma parent fraction was best fitted with a sum of two-exponentials (T1/2s = 5.9±1.8 min, 108.7±58.3 min) and revealed relatively rapid metabolism with ~40% of plasma activity attributable to unmetabolized [11C]PF367 at 15 min. [11C]PF367 peaked quickly in brain with SUV of 1.00±0.18 at ~4 min. For all brain regions a 1T model gave poor fits and therefore was found to be insufficient. According to Akaike information criterion (AIC) and visual inspection, best fits were obtained using a 2T reversible binding model (4 kinetic parameters plus vascular correction, 2T4k1v) but the total volume of distribution (VT) calculated from the micro-parameters was unstable and failed to correlate with VT estimated by the Logan graphical method. A 2T irreversible model (3 kinetic parameters plus vascular correction, 2T3k1v) gave reasonable fits for most datasets and Ki calculated from the micro-parameters was found to be robust. Ki obtained from the Patlak graphical method was in excellent agreement with Ki calculated from the 2T3k1v compartmental model outcomes (KiPatlak=1.069 x Ki2T3k1v-0.048, R2= 0.979; mean difference: -4.3±7.5%). Blocking studies showed a 29.7±6.5% reduction in Ki as compared to baseline with similar results at the 0.135 and 1 mg/kg doses. The chase study showed a displacement of activity starting shortly after the introduction of cold PF367. Conclusions [11C]PF367 binding was blockable and displaceable as demonstrated by competition studies with unlabeled ligand. The dose levels employed to date gave similar outcomes suggesting near maximal effect at 0.135 mg/kg. Additional studies at lower injected mass will elucidate a more detailed dose-response profile to help confirm the optimal analysis methods and outcome parameters for this novel radiotracer.