Binding of dexetimide and levetimide to [3H](+)pentazocine- and [3H]1,3-di(2-tolyl)guanidine-defined sigma recognition sites.

The potent antimuscarinic benzetimide and its resolved stereoisomers dexetimide and levetimide were tested for their affinities at sigma sites labelled by 3Hpentazocine or [3H]1,3-di(2-tolyl)guanidine. Levetimide was a potent and stereoselective inhibitor of 3Hpentazocine binding, with a Ki of 2.2 nM, while dexetimide was nine-fold less potent (Ki = 19 nM). Dexetimide and levetimide potently inhibited [3H]DTG binding although without stereoselectivity (Ki values of 65 and 103 nM, respectively). Levetimide may be a useful tool with which to investigate sigma recognition sites and sigma subtypes.