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-thiophenecarboxim idamide)] 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/micromol (1,350-4,800 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 (%ID/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.


CONCLUSIONS
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.
Pomper
M G
MG
Department of Radiology, Johns Hopkins University School of Medicine, Baltimore, Maryland 21287-2182, USA.
Musachio
J L
JL
Scheffel
U
U
Macdonald
J E
JE
McCarthy
D J
DJ
Reif
D W
DW
Villemagne
V L
VL
Yokoi
F
F
Dannals
R F
RF
Wong
D F
DF
D000818:Animals; D001812:Blood-Brain Barrier; D001921:Brain / Q000000981:diagnostic imaging* / Q000201:enzymology*; D002250:Carbon Radioisotopes / Q000493:pharmacokinetics; D004791:Enzyme Inhibitors / Q000138:chemical synthesis / Q000493:pharmacokinetics*; D007546:Isoquinolines / Q000138:chemical synthesis / Q000493:pharmacokinetics*; D008297:Male; D051379:Mice; D018345:Mice, Knockout; D008954:Models, Biological; D019001:Nitric Oxide Synthase / Q000032:analysis* / Q000172:deficiency; D052248:Nitric Oxide Synthase Type I; D009928:Organ Specificity; D010215:Papio; D051381:Rats; D017207:Rats, Sprague-Dawley; D044005:Tetrahydroisoquinolines; D013876:Thiophenes / Q000138:chemical synthesis / Q000493:pharmacokinetics*; D014018:Tissue Distribution; D014055:Tomography, Emission-Computed
C412689:AR-R 17443; C412691:AR-R 18512; D002250:Carbon Radioisotopes; D004791:Enzyme Inhibitors; D007546:Isoquinolines; D044005:Tetrahydroisoquinolines; D013876:Thiophenes; D019001:Nitric Oxide Synthase; D052248:Nitric Oxide Synthase Type I; C496321:Nos1 protein, mouse; C496322:Nos1 protein, rat