Vol. 299, Issue 3, 1112-1119, December 2001

Benzylidene Analogs of Anabaseine Display Partial Agonist and Antagonist Properties at the Mouse 5-Hydroxytryptamine_3A Receptor

Tina K. Machu , Margaret E. Hamilton, Tonia F. Frye, Christopher L. Shanklin, Michael C. Harris, Hongwei Sun, Thomas E. Tenner, Jr., Ferenc S. Soti and William R. Kem

Department of Pharmacology (T.K.M., M.E.H., T.F.F, C.L.S., M.C.H., H.S., T.E.T.), and Department of Anesthesiology (T.K.M.), Texas Tech University Health Sciences Center, Lubbock, Texas and Department of Pharmacology and Therapeutics (F.S.C., W.R.K.), University of Florida Health Sciences Center, Gainesville, Florida

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.