Benzylidene analogs of anabaseine display partial agonist and antagonist properties at the mouse 5-hydroxytryptamine_3A receptor

The nicotinic receptor drug candidate, 3-(2,4-dimethoxybenzylidene)-anabaseine (also known as GTS-21; DMXBA), its hydroxy metabolites, and some related analogs were evaluated with the two-electrode voltage-clamp technique in mouse 5-hydroxytryptamine (5-HT)_3A receptors expressed in Xenopus oocytes. Although DMXBA lacked partial agonist activity, its hydroxy-benzylidene metabolites and related analogs were partial agonists, displaying the following rank order of potency (EC₅₀) and apparent efficacy: 5-HT, 0.9 ± 0.06 μM (100% efficacy) > 3-(2-hydroxy,4-methoxybenzylidene)-anabaseine (2-OH-MBA), 2.0 ± 0.3 μM (63% efficacy) > 3-(2,4-dihydroxybenzylidene)-anabaseine, 2.6 ± 0.3 μM (63% efficacy) > 3-(2-methoxy,4-hydroxybenzylidene)-anabaseine, 17.2 ± 1.0 μM (30% efficacy). To examine the influence of a benzylidene ring hydroxy substituent, the agonist actions of the three possible monohydroxy isomers were examined. The rank order of potency, based on EC₅₀ determinations, and apparent efficacy was: 3-(2-hydroxybenzylidene)-anabaseine, 20.3 ± 2.6 μM (63% efficacy) > 3-(4-hydroxybenzylidene)-anabaseine, 32.3 ± 5.9 μM (14% efficacy) > 3-(3-hydroxybenzylidene)-anabaseine (3-OH-BA) (no agonist activity). Both DMXBA and 3-OH-BA antagonized 5-HT-mediated currents, with IC₅₀ values of 15.7 ± 0.9 and 27.5 ± 4.7 μM, respectively. DMXBA demonstrated both competitive and noncompetitive forms of antagonism over the range of concentrations tested. These results suggest that a hydroxy substituent at the 2′ position of the benzene ring is necessary and sufficient for partial agonist activity; substitution at the 4′ position with a hydroxy or methoxy group further enhances agonist potency. Because 2-OH-MBA is a primary metabolite of DMXBA, it may contribute to the physiological, biochemical, and behavioral effects of the parent compound when administered in vivo.