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

The objectives of this study were to synthesize neuronal nitric oxide synth ase (NOS-l)-selective imaging agents based on the 2 potent, selective inhib itors AR-R 17443 [N-(4-((2-((phenylmethyl) (methyl)-amino)ethyl)phenyl)-2-t hiophenecarboximidamide)] and AR-R 18512 [(N-(2-methyl-1,2,3,4-tetrahydrois oquinoline-7-yl)-2-thiophenecarboximidamide)] in positron-emitting form and to evaluate regional brain uptake in rodents and primates. Methods: [C-11] AR-R 17443 and [C-11]AR-R 18512 were produced by N-alkylation of the corres ponding desmethyl precursors using [C-11]iodomethane. Regional brain uptake of [C-11]AR-R 17443 and [C-11]AR-R 18512 was assayed in rats and NOS-I kno ckout mice, and PET was performed in baboons. Tracer kinetic modeling used a 2-compartment plasma and brain tissue model. Results: Yields of [C-11]AR- R 17443 and [C-11]AR-R 18512 ranged from 8% to 16% at the end of synthesis, with specific activities of 50-178 GBq/mu mol (1350-4800 Ci/mmol) at the e nd of synthesis. In rat cerebellum and cortex at 30 min after injection, [C -11]AR-R 17443 showed 1.01 +/- 0.01 and 1.63 +/- 0.12 percentage injected d ose per gram (%ID/g) uptake, respectively, whereas [C-11]AR-R 18512 showed 0.88 +/- 0.01 and 1.30 +/- 0.07 %ID/g uptake, respectively. Attempts to blo ck tracer uptake by pretreatment with the NOS-I-selective inhibitor 7-nitro indazole or the corresponding unlabeled inhibitor (or desmethyl precursor t o AR-R 17443 of similar potency) were unsuccessful. A small but significant (20%) decrease in cerebellar uptake of [C-11]AR-R 18512 was present in NOS -I knockout mice compared with control mice. PET of [C-11]AR-R 18512 in bab oons with concurrent regional cerebral blood flow (rCBF) determination befo re and after administration of blocker showed dose-related decreases in cer ebellar 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 3 34 in cerebral blood that dropped 51% after blocker administration. Conclus ion: Rodent studies for [C-11]AR-R 17443 and [C-11]AR-R 18512 showed little evidence of specific NOS-I binding. In baboons, we detected a higher uptak e of [C-11]AR-R 18512 in the cerebellum than in the cortex (approximately 5 %, accounting for decreased rCBF because of blockade), indicating minimal s pecific binding. Analogs of higher affinity are likely required if this cla ss of agents is to prove viable for PET.