Evaluation of Five Candidate Radioligands for PET Imaging of Phosphodiesterase-4D in Monkey Brain

Introduction: Phosphodiesterase-4 (PDE4) is a class of enzyme that hydrolyzes the important secondary messenger, cyclic adenosine monophosphate. PDE4 exist as four subtypes (A, B, C, and D) that are highly conserved in their catalytic domains. However, they differ in biodistribution and function. Inhibition of PDE4 is therapeutically beneficial in major depression and neurodegeneration. PDE4D-selective inhibitors are being developed as cognition enhancers and for treatment of Fragile X Syndrome with reduced side-effect liability. A PET radioligand capable of quantifying brain PDE4D would be valuable for drug development and biomedical research. Recently, [¹¹C]T1650 proved promising in monkey but could not quantify PDE4D in human brain, likely because of metabolically vulnerable nitro and methylene moieties that lead to brain radiometabolite accumulation (1). Therefore, we pursued further elaboration of T1650 structure to meet our goal of producing an effective PDE4D PET radioligand. Here we report an evaluation of new candidate radioligands in monkey.

Methods: Five high-affinity PDE4D-selective inhibitors (IC₅₀ = 1.1–2.7 nM; 193–93-fold selective versus PDE4B) having no nitro group (1) were each labeled by treating a phenolic precursor with [¹¹C]iodomethane (Figure 1A). Each radioligand was administered to a pair of monkeys for PET imaging of brain at baseline and after treatment with the pan-PDE4 inhibitor, rolipram. Arterial blood was sampled concurrently to provide radiometabolite-corrected input functions. Brain region total distribution volumes (V_T) were derived from a two-tissue compartmental model or Logan plot and assessed for time-stability. PET scans at baseline and after treatment with either PDE4D-selective BPN14770 (3.0 mg/kg, i.v.) or PDE4B-selective T1666 (0.3 mg/kg, i. v.) were performed for [¹¹C]JMJ-81 and [¹¹C]JMJ-129. The in vitro and ex vivo stabilities of [¹¹C]JMJ-81 and [¹¹C]JMJ-129 in fresh rat brain homogenates were also assessed.

Results: Each radioligand was obtained in good radiochemical yield with high molar and high radiochemical purity. All five radioligands showed sizeable PDE4D specific signal (baseline V_T = 6.3–10.3, BP_ND = 0.5–1.7) in whole monkey brain. Among them, [¹¹C]JMJ-81 [¹¹C]JMJ-129 also showed time-stable V_T after 90 min (Figure 1B). [¹¹C]JMJ-129 showed higher PDE4D-selective binding than [¹¹C]JMJ-81 (Figure 1C and1D). [¹¹C]JMJ-81 and [¹¹C]JMJ-129 were stable in fresh rat brain homogenates for 60 min (unlike [¹¹C]T1650 (1)) (Figure 1E). A moderately lipophilic radiometabolite fraction found in rat and monkey plasma for [¹¹C]T1650 was negligible in monkey plasma over the whole scan time for both [¹¹C]JMJ-81 and [¹¹C]JMJ-129 (Figure F and G). All the radioligands showed radiometabolites in rat brain after 30 min when measured ex vivo. However, these results in rat may not predict those in monkey brain because of faster metabolism in rat, absence of primate PDE4D-specific binding, and negligible presence of moderately lipophilic radiometabolites in monkey plasma.

Conclusions: Among the tested radioligands, [¹¹C]JMJ-81 and [¹¹C]JMJ-129 robustly quantified PDE4D in monkey brain and are worthy of further development.

Acknowledgements: The Intramural Research Program of NIH (NIMH), the NIH Clinical PET Center for C-11 production, and Tetra Therapeutics for in vitro assays.

Reference: (1). Y. Wakabayashi et al., ACS Chem, Neurosci, 11, 1311, 2020.

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