Radiolabeled neuronal nitric oxide synthase inhibitors: Synthesis, in vivo evaluation, and primate PET studies

The objectives of this study were to synthesize neuronal nitric oxide synthase (NOS-I)-selective imaging agents based on the 2 potent, selective inhibitors AR-R 17443 [N-(4-((2-((phenylmethyl) (methyl)amino)ethyl)phenyl)-2-thiophenecarboximidamide)] and AR-R 18512 [(N-(2-methyl-1,2,3,4-tetrahydroisoquinoline-7-yl)-2-thiophenecar boximidamide)] in positron-emitting form and to evaluate regional brain uptake in rodents and primates. Methods: [¹¹C]AR-R 17443 and [¹¹C]AR-R 18512 were produced by N-alkylation of the corresponding desmethyl precursors using [¹¹C]iodomethane. Regional brain uptake of [¹¹C]AR-R 17443 and [¹¹C]AR-R 18512 was assayed in rats and NOS-I knockout mice, and PET was performed in baboons. Tracer kinetic modeling used a 2-compartment plasma and brain tissue model. Results: Yields of [¹¹C]AR-R 17443 and [¹¹C]AR-R 18512 ranged from 8% to 16% at the end of synthesis, with specific activities of 50-178 GBq/μmol (1350-4800 Ci/mmol) at the end of synthesis. In rat cerebellum and cortex at 30 min after injection, [¹¹C]AR-R 17443 showed 1.01 ± 0.01 and 1.63 ± 0.12 percentage injected dose per gram (%lD/g) uptake, respectively, whereas [¹¹C]AR-R 18512 showed 0.88 ± 0.01 and 1.30 ± 0.07 %ID/g uptake, respectively. Attempts to block tracer uptake by pretreatment with the NOS-I-selective inhibitor 7-nitroindazole or the corresponding unlabeled inhibitor (or desmethyl precursor to AR-R 17443 of similar potency) were unsuccessful. A small but significant (20%) decrease in cerebellar uptake of [¹¹C]AR-R 18512 was present in NOS-I knockout mice compared with control mice. PET of [¹¹C]AR-R 18512 in baboons with concurrent regional cerebral blood flow (rCBF) determination before and after administration of blocker showed dose-related decreases in cerebellar uptake that were greater than or equal to decreases in rCBF. Plasma metabolites accounted for 27% of total activity at 30 min after injection. Kinetic modeling of binding potentials revealed a distribution volume of 334 in cerebral blood that dropped 51% after blocker administration. Conclusion: Rodent studies for [¹¹C]AR-R 17443 and [¹¹C]AR-R 18512 showed little evidence of specific NOS-I binding. In baboons, we detected a higher uptake of [¹¹C]AR-R 18512 in the cerebellum than in the cortex (approximately 5%, accounting for decreased rCBF because of blockade), indicating minimal specific binding. Analogs of higher affinity are likely required if this class of agents is to prove viable for PET.