Development of cannabinoid subtype-1 (CB1) receptor ligands for PET

Abstract

Introduction: Abnormalities in brain cannabinoid subtype-1 (CB1) receptor concentrations and/or signaling pathways may be involved in a variety of psychiatric and neurodegenerative disorders. There is a strong need to image and quantify brain CB1 receptor concentrations in living humans under baseline and diseased conditions. When this project began there were no CB1 receptor radioligands suitable for in vivo imaging with PET. The overall aim of this project was the development of CB1 receptor PET radioligands as tools for neuropsychiatric research and drug development.

Methods: 1,5-Diarylpyrazole and 3,4-diarylpyrazoline CB1 receptor ligands were prepared and assayed in vitro for affinity/potency and selectivity at CB1 receptors. A facile and selective approach to the synthesis of 1,5-diarylpyrazoles was identified to assist in advancing SAR in this class. Three high-potency [11C]1,5-diarylpyrazoles ([11C]12, [11C]14 and [11C]17) were synthesized from their O-desmethyl precursors (23 25) and [11C]iodomethane and purified with HPLC. PipISB (26) was radiolabeled in either of two positions, with [11C]carbon monoxide or with [18F]4-fluoro-benzyl bromide as labeling agent. Candidate radioligands ([11C]27 and [18F]28 30) from the 1,5-diphenyl-pyrrolidin-2-one platform of [11C]MePPEP ([11C]16) were prepared from O-desmethyl precursor (31 or 32) with [11C]iodomethane, [18F]FCH2Br, [18F]FCD2Br or [18F]F(CH2)2Br as labeling agents. A promising racemic [11C]3,4-diarylpyrazoline ([11C](±)-33), its eutomer ([11C]( )-33) and distomer ([11C](+)-33) were radiolabeled with [11C]HCN using a custom-made remotely-controlled apparatus. With the exceptions of [11C]27 and [18F]28 30, each candidate radioligand was injected into monkey and investigated with PET imaging.

Results and Discussion: Ligands (12 14, 17, (±)-33) and their O-desmethyl precursors (23 25, 34) were synthesized efficiently and in useful chemical yields. The achieved RCYs, SRs and purities of each radioligand were adequate for future investigation in vivo with PET imaging. After injection into monkey, two 1,5-diarylpyrazoles, [11C]12 and [11C]17, gave some receptor-specific signal in brain but were likely contaminated with brain-penetrating radiometabolites and were therefore unsuitable for PET imaging. The third, [11C]14, gave an appreciable receptor-specific signal and fast brain washout. The success of [11C]14 is likely related to the favorable metabolism conferred by the nitrile substituent in the 4-position of the pyrazole ring. [11C]26 and [18F]26 showed high CB1 receptor-specific binding in monkey brain in vivo and merit further investigation as prospective PET radioligands in humans. [11C](±)-33 showed an appropriate regional brain distribution and its binding was selectively blocked by pre- or post treatment with a high dose of CB1-selective ligand. After injection of [11C](−)-33, there was high uptake and retention of radioactivity across brain according to the rank order of CB1 receptor densities. The distomer, [11C](+)-33 failed to give a sustained CB1 receptor-specific distribution.

Conclusion: From the work produced in this thesis, four radioligands ([11C]14, [11C]26, [18F]26 and [11C](−)-33) appear promising for in vivo imaging of brain CB1 receptors with PET in monkey. [11C](−)-33 is currently being considered for use in human subjects under a clinical protocol.