Optimization of pharmacokinetic properties by modification of a carbazole-based cannabinoid receptor subtype 2 (CB2) ligand

Abstract

Recently, the development of the fluorinated PET tracer [¹⁸F]1a for imaging of CB₂ receptors in the central nervous system was reported. [¹⁸F]1a showed high CB₂ affinity and selectivity over the CB₁ subtype, but rapid biotransformation in mice. In addition to the amide hydrolysis, oxidative N-dealkylation and carbazole oxidation were postulated as main metabolic pathways. Based on these results, novel carbazole derivatives with additional 6-substituents (23a, 24a), modified hydrogenation state (26a) and enlarged fluoroalkyl substituent (13a, 13b) were synthesized and pharmacologically evaluated. The key step in the synthesis of substituted carbazoles 23a, 24a and 26a was a Fischer indole synthesis. Nucleophilic substitution of tosylated lactate 5 by carbazole anion provided the fluoroisopropyl derivatives 13a and 13b. Partial hydrogenation of the aromatic carbazole system (26a) was not tolerated by the CB₂ receptor. A methylsulfonyl moiety in 6-position (24a) led to considerably reduced CB₂ affinity, whereas a 6-methoxy moiety (23a) was well tolerated. An additional methyl moiety in the fluoroethyl side chain of 1a resulted in fluoroisopropyl derivatives 13 with unchanged high CB₂ affinity and CB₂: CB₁ selectivity. Compared with the fluoroethyl derivative 1a, the carbazole N-atom of the fluoroisopropyl derivative 13a (K_i(CB₂) = 2.9 nM) is better shielded against the attack of CYP enzymes as formation of N-oxides was not observed and N-dealkylation took place to a less amount.

Keywords: Cannabinoid CB(2) receptor ligands; Fischer indole synthesis; Fluoroisopropyl side chain; Identification of metabolites; Metabolic stability; PET; Selectivity; Structure affinity relationships; carbazole.