¹⁸F-Nifrolene: A new PET imaging agent for α₄β₂ nAChR nicotinic receptors

Abstract

76

Objectives: Nicotinic α₄β₂ receptors are implicated in Alzheimer’s disease, schizophrenia, substance abuse and other disorders. We have developed putative antagonists ¹⁸F-nifrolidine, ¹⁸F-nifzetidine and agonist ¹⁸F-nifene as PET imaging agents for the α₄β₂ nicotinic receptors. The in vivo kinetic profile of the agonist, ¹⁸F-nifene was found to be the fastest. Thus, to optimize imaging properties of the antagonist, ¹⁸F-nifrolidine, we introduced the unsaturated ring, 3,4-dehydropyrrolidine (present in nifene) to prepare 5-(3'-¹⁸F-fluoropropyl)-3-[2-((S)-3-pyrrolinyl)methoxy]pyridine (¹⁸F-nifrolene).

Methods: The bromo-3-pyridyl ether derivative was synthesized by the etherification of the 5-bromo-3-hydroxypyridine with (S)-N-tert-butoxycarbonyl-2-hydroxymethyl-3-pyrroline under Mitsunobu conditions. Allylation of bromo-3-pyridyl derivative with allyltributyltin gave 5-allylpyridylether. Hydroboration of allyl group followed by oxidation gave the alcohol. The alcohol was converted into tosylate precursor. The radiosynthesis of ¹⁸F-nifrolene was carried by the reaction of N-BOC-protected tosylate with K[¹⁸F]F/kryptofix222 in acetonitrile at 96 ^oC for 30 mins and deprotection of the N-BOC group with TFA at 80 ^oC. Product mixture was then purified by HPLC (C₁₈ Alltech column) with 0.1 M ammonium formate(40%) and acetonitrile (60%) with a flow rate of 1.5 mL/min; retention time of ¹⁸F-nifrolene was found to be 31 mins. Rat brain slices were incubated with ¹⁸F-nifrolene (1.5-2 μCi/cc) final concentration of either radioligand in Tris buffer at 37 ^oC for 1 hour in the absence and presence of 300 μM nicotine. Slices were washed and exposed to phosphor films and read by the Optiquant Imaging System.

Results: The radiosynthesis was accomplished in 2-2.5 hrs with an overall radiochemical yield 30-40% decay corrected. The specific activity of the radiotracer was estimated to be above 4000 Ci/mmol. In vitro study with ¹⁸F-nifrolene indicates selective high binding in the thalamus (Th) as well as binding in extrathalamus regions such as cortex, (Cor), striatum (Str), subiculum (Sub), and other regions. Cerebellum showed lower binding. Thalamus to cerebellum ratio was the highest for ¹⁸F-nifrolene with several other regions showing selective binding (Th/Cer = 10.5; Sub/Cer = 6.5; Str/Cer = 4.2; Cor/Cer = 3.8). The ratios of ¹⁸F-nifrolene are higher than measured for ¹⁸F-nifrolidine. Nicotine (300 µM) was able to displace >90% of specific binding.

Conclusions: ¹⁸F-Nifrolene is capable of identifying the α₄β₂ nAChR rich regions, similar to that previously reported for ¹⁸F-nifrolidine (Chattopadhyay et al, 2005) and ¹⁸F-nifene (Pichika et al., 2006). The unique ring structure of ¹⁸F-nifrolene may allow faster in vivo binding kinetics similar to that abserved for the agonist ¹⁸F-nifene.