Design of a high-affinity competitive antagonist of the vanilloid receptor selective for the calcium entry-linked receptor population

We describe the synthesis and characterization of N-(4-chlorobenzyl) -N'-(4-hydroxy-3-iodo-5-methoxybenzyl) thiourea (IBTU), a novel antagonist of the vanilloid receptor 1 (TRPV1 or VR1). IBTU competitively inhibited Ca-45(2+) uptake into CHO cells heterologously expressing rat TRPV1, whether induced by capsaicin or resiniferatoxin (K-i = 99 +/- 23 and 93 +/- 34 nM, respectively). IBTU was thus somewhat more potent (5-fold) than capsazepine. In contrast to its antagonism of vanilloid-induced calcium uptake, IBTU (30 muM) inhibited [H-3] resiniferatoxin binding to TRPV1 by less than 10%. We hypothesize that these dramatically distinct potencies reflect different fractions of TRPV1 in this system: namely, a minor plasma membrane fraction controlling Ca-45(2+) uptake, and the predominant intracellular fraction that dominates the [H-3] resiniferatoxin binding measurements. Intracellular Ca2+ imaging supports this explanation. IBTU antagonized the elevation in intracellular Ca2+ in response to 50 nM capsaicin with an IC50 of 106 +/- 35 nM. Likewise, 600 nM IBTU was able to antagonize the elevation in intracellular Ca2+ in response to 100 pM resiniferatoxin in the presence of normal (1.8 mM) extracellular Ca2+, where the increase in intracellular calcium reflects calcium influx. In contrast, in the absence of extracellular Ca2+, where in this system resiniferatoxin induces a modest increase in calcium from intracellular stores, IBTU was unable to block the response to resiniferatoxin, although the TRPV1 antagonist 5-iodoresiniferatoxin was able to do so. In summary, IBTU is a novel, potent TRPV1 antagonist with marked selectivity between subpopulations of TRPV1 and may permit the function of these distinct pools to be explored and potentially exploited.