Structural requirements for binding of anandamide-type compounds to the brain cannabinoid receptor

In order to establish the structural requirements for binding to the brain cannabinoid receptor (CB₁), we have synthesized numerous fatty acid amides, ethanolamides, and some related simple derivatives and have determined their K(i) values. A few α-methyl- or α,α- dimethylarachidonoylalkylamides were also examined. In the 20:4, n-6 series, the unsubstituted amide is inactive; N-monoalkylation, at least up to a branched pentyl group, leads to significant binding. N,N-Dialkylation, with or without hydroxylation on one of the alkyl groups, leads to elimination of activity. Hydroxylation of the N-monoalkyl group at the ω carbon atom retains activity. In the 20:x, n-6 series, x has to be either 3 or 4; the presence of only two double bonds leads to inactivation. In the n-3 series, the limited data reported suggest that the derived ethanolamides are either inactive or less active than comparable compounds in the n-6 series. Alkylation or dialkylation of the α carbon adjacent to the carbonyl group retains the level of binding in the case of anandamide (compounds 48, 49); however, α-monomethylation or α,α-dimethylation of N-propyl derivatives (50-53) potentiates binding and leads to the most active compounds seen in the present work (K(i) values of 6.9 ± 0.7 to 8.4 ± 1.1 nM). We have confirmed that the presence of a chiral center on the N-alkyl substituent may lead to enantiomers which differ in their levels of binding (compounds 54, 57 and 55, 56).