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)-anabase ine (also known as GTS-21; DMXBA), its hydroxy metabolites, and some relate d 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 m etabolites and related analogs were partial agonists, displaying the follow ing rank order of potency (EC,,) and apparent efficacy: 5-HT, 0.9 +/- 0.06 muM (100% efficacy) > 3-(2-hydroxy,4-methoxybenzylidene)-anabaseine (2-OH-M BA), 2.0 +/- 0.3 muM (63% efficacy) > 3-(2,4-dihydroxybenzylidene)-anabasei ne, 2.6 +/- 0.3 muM (63% efficacy) > 3-(2-methoxy,4-hydroxybenzylidene)-ana baseine, 17.2 +/- 1.0 muM (30% efficacy). To examine the influence of a ben zylidene ring hydroxy substituent, the agonist actions of the three possibl e monohydroxy isomers were examined. The rank order of potency, based on EC 50 determinations, and apparent efficacy was: 3-(2-hydroxybenzylidene)-anab aseine, 20.3 +/- 2.6 muM (63% efficacy) > 3-(4-hydroxybenzylidene)-anabasei ne, 32.3 +/- 5.9 muM (14% efficacy) > 3-(3-hydroxybenzylidene)-anabaseine ( 3-OH-BA) (no agonist activity). Both DMXBA and 3-OH-BA antagonized 5-HT-med iated currents, with IC50 values of 15.7 +/- 0.9 and 27.5 +/- 4.7 muM, resp ectively. DMXBA demonstrated both competitive and noncompetitive forms of a ntagonism over the range of concentrations tested. These results suggest th at a hydroxy substituent at the 2' position of the benzene ring is necessar y and sufficient for partial agonist activity; substitution at the 4' posit ion with a hydroxy or methoxy group further enhances agonist potency. Becau se 2-OH-MBA is a primary metabolite of DMXBA, it may contribute to the phys iological, biochemical, and behavioral effects of the parent compound when administered in vivo.