The antinociceptive triterpene β-amyrin inhibits 2-arachidonoylglycerol (2-AG) hydrolysis without directly targeting cannabinoid receptors

BACKGROUND AND PURPOSE Pharmacological activation of cannabinoid CB1 and CB2 receptors is a therapeutic strategy to treat chronic and inflammatory pain. It was recently reported that a mixture of natural triterpenes alpha- and beta-amyrin bound selectively to CB1 receptors with a subnanomolar Ki value (133 pM). Orally administered alpha/beta-amyrin inhibited inflammatory and persistent neuropathic pain in mice through both CB1 and CB2 receptors. Here, we investigated effects of amyrins on the major components of the endocannabinoid system. EXPERIMENTAL APPROACH We measured CB receptor binding interactions of alpha- and beta-amyrin in validated binding assays using hCB1 and hCB2 transfected CHO-K1 cells. Effects on endocannabinoid transport in U937 cells and breakdown using homogenates of BV2 cells and pig brain, as well as purified enzymes, were also studied. KEY RESULTS There was no binding of either alpha- or beta-amyrin to hCB receptors in our assays (Ki > 10 mu M). The triterpene beta-amyrin potently inhibited 2-arachidonoyl glycerol (2-AG) hydrolysis in pig brain homogenates, but not that of anandamide. Although beta-amyrin only weakly inhibited purified human monoacylglycerol lipase (MAGL), it also inhibited alpha,beta-hydrolases and more potently inhibited 2-AG breakdown than alpha-amyrin and the MAGL inhibitor pristimerin in BV2 cell and pig brain homogenates. CONCLUSIONS AND IMPLICATIONS We propose that beta-amyrin exerts its analgesic and anti-inflammatory pharmacological effects via indirect cannabimimetic mechanisms by inhibiting the degradation of the endocannabinoid 2-AG without interacting directly with CB receptors. Triterpenoids appear to offer a very broad and largely unexplored scaffold for inhibitors of the enzymic degradation of 2-AG. LINKED ARTICLES This article is part of a themed section on Cannabinoids. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2012.167.issue-8