Spirocyclic Iodonium Ylide (SCIDY)-mediated radiofluorination of a novel PDE10A PET Ligand for neuroimaging

150 Objectives: Phosphodiesterase-10 (PDE10A), one of the dual-substrate specific enzymes of PDE superfamily, represents a potential biomarker for neurodegenerative and neuropsychiatric diseases. However, a number of outcomes in clinical trials with PDE10A inhibitors so far are disappointing. Imaging of PDE10A in vivo with effective PET tracers would considerably accelerate drug discovery and clinical application for disease diagnosis. Here we report a novel PDE10A radioligand 18F-P10A1910, including the radiolabeling, ex vivo biodistribution (bioD) and metabolism, and preliminary PET imaging on rats and rhesus by dose-dependent blocking study for evaluating specificity and receptor occupancy. Methods: The PDE10A inhibitor P10A1910 and the SCIDY precursor Pre-1 were synthesized according to the literature [1,2,3].The binding affinity and selectivity of P10A1910 to PDE10A and other fourteen PDE family enzymes were examined in vitro. The radiolabeling of 18F-P10A1910 was realized by one-step 18F incorporation via the commercially automatic GE FX2 N module. Pharmacokinetic properties were tested via bioD studies in ICR mice. In addition, dynamic PET imaging studies of 18F-P10A1910 were conducted on SD rats and rhesus for evaluating in vivo brain penetration, specificity, and target engagement. Results: The inhibitor P10A1910 and SCIDY precursor Pre-1 were synthesized in 4-5 steps with 11% and 8% overall yields, respectively. P10A1910 exhibited good binding affinity for PDE10A (IC50 = 2.05 nM) and excellent selectivity over other PDE subtypes.18F-P10A1910 was produced from Pre-1 in 15 ± 5% n.d.c RCYs (n = 30) in less than 70 min from EOB with more than 99% radiochemical purity and around 3 Ci/μmol molar radioactivity. The radioactivity at 5 min post-injection was mainly accumulated in the liver and small intestine (more than 50 %ID/g), which indicated that the excretion of radiotracer occurred primarily through the hepatobiliary clearance. Blood clearance of 18F-P10A1910 was fairly fast (activity ratio is more than 20). No obvious bone uptake was observed. PET imaging on rats showed a high brain uptake (SUVpeak = 1.5 SUV at 2 min post-injection) and a rapid washout (SUV1.4min/60 min = 3). Furthermore, the pre-blocking and chase-blocking PET study on rats demonstrated the binding of 18F-P10A1910 to PDE10A was reversible and specific. Metabolite analysis demonstrated the first-class stability of 18F-P10A-1910 in rat brain (more than 96% of intact tracer retained at 60 min post injection). Furthermore, blocking experiments with escalating doses of PF-02545920 revealed a dose-dependent decrease of striatal tracer uptake in the rhesus brain. Target engagement studies unveiled that dose of 2.09 μmol/kg PF-02545920 occupied 50% of the target receptor. In addition, our results showed that the recently reported therapeutically active PF-02545920 doses of 0.21 mg/kg and 1.33 mg/kg occupy 22% and 63% of PDE10A in the monkey brain, respectively. Accordingly, our results imply that a relatively low receptor occupancy already elicits neuroprotective activity. Further, we hypothesize that higher doses might lead to improved therapeutic efficacy by occupying a higher degree of PDE10A. These results underline the potential utility of 18F-P10A1910 for in vivo visualization and quantification of PDE10A-based target engagement. Conclusion: Based on the designed SPIAd precursor Pre-1, we successfully synthesized a novel PDE10A radioligand 18F-P10A1910 in good radiochemical yields with high purities and molar activities. In vitro binding affinity and selectivity testing, ex vivo bioD studies in mice, as well as dynamic neuroPET imaging on rats and rhesus were performed, and the results demonstrated 18F-P10A1910 is a promising candidate with adequate brain penetration, good stability and excellent specificity.