Synthesis in vivo evaluation and PET study of a carbon-11-labeled neuronal nitric oxide synthase (nNOS) inhibitor S-Methyl-L-thiocitrulline

Reports have implicated neuronal nitric oxide synthetase (nNOS) in the pathological effects of neurodegenerative diseases. S-Methyl-L-thiocitrulline (MTICU), a potent and selective nNOS inhibitor (K-i=1.2 nM), was chosen as our initial target molecule for positron emitter labeling as a potential nNOS tracer. We report the synthesis, biological evaluation and primate brain images of S-[C-11]methyl-L-thiocitrulline ([C-11]MTICU). Methods: The two-step synthesis of [C-11]MTICU consisted of the S-alkylation of alpha-N-Boc-L-thiocitrulline t-butyl ester with [C-11]Mel followed by TFA hydrolysis and HPLC purification. The final product was obtained within 50 min (yield = 9.1%-12.5%, based on [C-11]Mel S.A.=27-680 Ci/mmol at end of synthesis). The lipophilicity of [C-11]MTICU was determined by octanol/water partition coefficient (LogP). Blood stability of this tracer in vitro and in vivo was measured by HPLC analysis. Biodistribution using female Sprague-Dawley rats was performed, including examination of uptake in cerebellum and olfactory bulb (high nNOS) as well as cortex and brain stem (low nNOS). Carbon-11-MTICU was administered to a female baboon and brain images were obtained using a Siemens ECAT EXACT scanner for determination of brain regional uptake and blood-brain barrier permeability. Results: At 30 min postinjection, [C-11]MTICU remained 64% intact in vivo and 95% intact in vitro. Lipophilicity estimation gave Log p = 1.08 +/- 0.08 (n = 6). The brain (0.11% ID/g)-to-blood (0.20% ID/g) ratio was 1:2 at 30 min postinjection. Uptake in the cerebellum was 20% higher than in either the cortex or the brain stem (p < 0.05). Blockage using 1 mg/kg MTICU reduced uptake in the cerebellum and the cortex by 22%, but did not affect the brain stem. PET imaging showed that [C-11]MTICU brain uptake, corrected for blood volume, was stable from 10 min to 1 hr at approximately 0.4% ID/organ. PET images of a baboon brain showed increased uptake in the region of the olfactory bulb compared to uniform biodistribution in the rest of the brain. Conclusion: The [C-11]MTICU is a tracer that is potentially useful in determining nNOS levels in vivo.