Preclinical evaluation of a promising C-11 labeled PET tracer for imaging phosphodiesterase 10A in the brain of living subject

Abstract

Phosphodiesterase 10A (PDE10A) plays a key role in the regulation of brain striatal signaling. A PET tracer for PDE10A may serve as a tool to evaluate PDE10A expression in vivo in central nervous system disorders with striatal pathology. Here, we further characterized the binding properties of a previously reported radioligand we developed for PDE10A, [11C]TZ1964B, in rodents and nonhuman primates (NHPs). The tritiated counterpart [3H]TZ1964B was used for in vitro binding characterizations in rat striatum homogenates and in vitro autoradiographic studies in rat brain slices. The carbon-11 labeled [11C]TZ1964B was utilized in the ex vivo autoradiography studies for the brain of rats and microPET imaging studies for the brain of NHPs. MicroPET scans of [11C]TZ1964B in NHPs were conducted at baseline, as well as with using a selective PDE10A inhibitor MP-10 for either pretreatment or displacement. The in vivo regional target occupancy (Occ) was obtained by pretreating with different doses of MP-10 (0.05 - 2.00 mg/kg). Both in vitro binding assays and in vitro autoradiographic studies revealed a nanomolar binding affinity of [3H]TZ1964B to the rat striatum. The striatal binding of [3H]TZ1964B and [11C]TZ1964B was either displaced or blocked by MP-10 in rats and NHPs. Autoradiography and microPET imaging confirmed that the specific binding of the radioligand was found in the striatum but not in the cerebellum. Blocking studies also confirmed the suitability of the cerebellum as an appropriate reference region. The binding potentials (BPND) of [11C]TZ1964B in the NHP striatum that were calculated using either the Logan reference model (LoganREF, 3.96 ± 0.17) or the simplified reference tissue model (SRTM, 4.64 ± 0.47), with the cerebellum as the reference region, was high and had good reproducibility. The occupancy studies indicated a MP-10 dose of 0.31 ± 0.09 mg/kg (LoganREF) / 0.45 ± 0.17 mg/kg (SRTM) occupies 50% striatal PDE10A binding sites. Studies in rats and NHPs demonstrated radiolabelled TZ1964B has a high binding affinity and good specificity for PDE10A, as well as favorable in vivo pharmacokinetic properties and binding profiles. Our data suggests that [11C]TZ1964B is a promising radioligand for in vivo imaging PDE10A in the brain of living subject.

Comparison of [11C]TZ1964B and [18F]MNI659 for PET imaging brain PDE10A in nonhuman primates

“…; Liu et al. ). According to the LoganREF model, the distribution volume ratio (DVR) of each ROI was calculated using a MATLAB (Mathworks Inc., Natick, MA).…”

“…Animals were prepared for microPET studies as previously reported (Liu et al. ). About 1.5–2.5% isoflurane inhalation anesthesia was maintained throughout the microPET imaging sessions.…”

“…The PET images were processed according to the previous report (Liu et al. ). Briefly, emission scans were corrected and reconstructed using filtered back projection as described previously (Miller et al.…”

“…; Liu et al. ) and simplified reference tissue model (SRTM) (Lammertsma and Hume ; Gunn et al. ; Innis et al.…”

“…; Liu et al. ). In comparison, MNI659 has subnanomolar binding potency for PDE10A (0.097 nM) (Barret et al.…”