Stereochemical selectivity of methanandamides for the CB1 and CB2 cannabinoid receptors and their metabolic stability

Several chiral, analogues of the endogenous cannabinoid receptor ligand, ar achidonylethanolamide (anandamide)methylated at the 2,1' and 2' positions u sing asymmetric synthesis were evaluated in order to study (a) stereoselect ivity of binding to CB1 and CB2 cannabinoid receptors, and (b) metabolic st ability with regard to anandamide amidase. Enantiomerically pure 2-methyl a rachidonic acids were synthesized through diastereoselective methylation of the respective chiral 2-oxazolidinone enolate derivatives and CBI and CB2 receptor affinities of the resulting chiral anandamides were evaluated usin g a standard receptor binding assay. Introduction of a single 2-methyl grou p increased affinity for CBI, led to limited enantioselectivity and only mo destly improved metabolic stability. However. a high degree of enantio- and diastereoselectivity was observed for the 2,1'-dimethyl analogues. (R)-N-( 1- methyl-2-hydroxyethyl)-2-(R)-methyl-arachidonamide (4) exhibited the hig hest CB1 receptor affinity in this series with a K-i of 7.42 nM, an at leas t 10-fold improvement on anandamide (K-i = 78.2 nM), The introduction of tw o methyl groups at the 2-position of anandamide led to no change in affinit y for CB1 but somewhat enhanced metabolic stability. Conversely, chiral hea dgroup methylation in the: 2-gem-dimethyl series led to chiral analogues po ssessing a wide range of CB1 affinities. Of these the (S)-2,2,2'-trimethyl analogue (12) had the highest affinity for CBI almost equal to that of anan damide. In agreement with our previous anandamide structure-activity relati onship work, the analogues in this study showed high selectivity for the CB I receptor over CB2. The results are evaluated in terms of stereochemical f actors affecting the ligands affinity for CB1 using receptor-essential volu me mapping as an aid. Based on the results, a partial CBI receptor site mod el is proposed. that bears two hydrophobic pockets capable of accommodating 1'- and 2-methyl groups (C) 2001 Elsevier Science Ltd. All rights reserved .