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

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.

List of scientific papers

I. Donohue SR, Halldin C, Pike VW (2006). Synthesis and structure-activity relationships (SARs) of 1,5-diarylpyrazole cannabinoid type-1 (CB(1)) receptor ligands for potential use in molecular imaging. Bioorg Med Chem. 14(11): 3712-20. Epub 2006 Feb 8.
https://pubmed.ncbi.nlm.nih.gov/16466922

II. Donohue SR, Halldin C, Pike VW (2008). A facile and regioselective synthesis of rimonabant through an enamine-directed 1,3-dipolar cycloaddition. Tetrahedron Lett. 49: 2789 2791.

III. Donohue, SR, Zoghbi SS, Yasuno F, Gourley JP, Innis RB, Halldin C, Pike VW (2008). Radiosynthesis and evaluation in monkey of three 11C-labelled 1,5-diarylpyrazoles as high potency candidate PET radioligands for cannabinoid subtype-1 (CB1) receptors in brain. Current Radiopharmaceuticals. 1: 93-102.

IV. Donohue SR, Halldin C, Schou M, Hong J, Phebus L, Chernet E, Hitchcock SA, Gardinier KM, Ruley KM, Krushinski JH, Schaus J, Pike VW (2008). Radiolabeling of a high potency cannabinoid subtype-1 receptor ligand, N-(4-fluoro-benzyl)-4-(3-(piperidin-1-yl)-indole-1-sulfonyl)benzamide (PipISB), with carbon-11 or fluorine-18. J Label Compd Radiopharm. 51: 146-152.

V. Finnema SJ, Donohue SR, Zoghbi SS, Brown AK, Gulyás B, Innis RB, Halldin C, Pike VW (2008). Evaluation in monkey of [11C]PipISB and [18F]PipISB as candidate radioligands for imaging brain cannabinoid type-1 (CB1) receptors in vivo. [Submitted]

VI. Donohue SR, Krushinski JH, Pike VW, Chernet E, Chesterfield AK, Felder CC, Halldin C, Schaus JM (2008). Synthesis, ex vivo evaluation and radiolabeling of potent 1,5-diphenyl-pyrrolidin-2-one cannabinoid subtype-1 (CB1) receptor ligands as candidates for in vivo imaging. [Submitted]

VII. Donohue SR, Pike VW, Finnema SJ, Truong P, Andersson J, Gulyás B, Halldin C (2008). Discovery and labeling of high affinity 3,4-diarylpyrazolines as candidate radioligands for in vivo imaging of cannabinoid subtype-1 (CB1) receptors. [Submitted]