Functional properties of the high-affinity TRPV1 (VR1) vanilloid receptor antagonist (4-hydroxy-5-iodo-3-methoxyphenylacetate ester) iodo-resiniferatoxin

J Pharmacol Exp Ther. 2002 Dec;303(3):1052-60. doi: 10.1124/jpet.102.040394.


We have synthesized iodinated resiniferatoxin bearing a 4-hydroxy-5-iodo-3-methoxyphenylacetate ester (I-RTX) and have characterized its activity on rat and human TRPV1 (VR1) receptors, as well as in behavioral assays of nociception. In whole cell patch-clamp recordings from transfected cells the functional activity of I-RTX was determined. Currents activated by capsaicin exhibited characteristic outward rectification and were antagonized by capsazepine and I-RTX. On rat TRPV1 the affinity of I-RTX was 800-fold higher than that of capsazepine (IC50 = 0.7 and 562 nM, respectively) and 10-fold higher on rat versus human receptors (IC50 = 0.7 and 5.4 nM, respectively). The same difference was observed when comparing the inhibition of [3H]RTX binding to rat and human TRPV1 membranes for both RTX and I-RTX. Additional pharmacological differences were revealed using protons as the stimulus. Under these conditions capsazepine only partly blocked currents through rat TRPV1 receptors (by 70 to 80% block), yet was a full antagonist on human receptors. In contrast, I-RTX completely blocked proton-induced currents in both species and that activated by noxious heat. I-RTX also blocked capsaicin-induced firing of C-fibers in a rat in vitro skin-nerve assay. Despite this activity and the high affinity of I-RTX for rat TRPV1, only capsazepine proved to be an effective antagonist of capsaicin-induced paw flinching in rats. Thus, although I-RTX has limited utility for in vivo behavioral studies it is a high-affinity TRPV1 receptor antagonist that will be useful to characterize the functional properties of cloned and native vanilloid receptor subtypes in vitro.

Publication types

  • Comparative Study

MeSH terms

  • Action Potentials / drug effects*
  • Action Potentials / physiology
  • Animals
  • CHO Cells
  • Cannabinoids / antagonists & inhibitors
  • Cannabinoids / genetics
  • Capsaicin / metabolism*
  • Capsaicin / pharmacology
  • Cricetinae
  • Diterpenes / pharmacology*
  • Dose-Response Relationship, Drug
  • Humans
  • Male
  • Mice
  • Mice, Inbred C57BL
  • Mice, Knockout
  • Rats
  • Rats, Sprague-Dawley
  • Receptors, Drug / antagonists & inhibitors*
  • Receptors, Drug / deficiency*
  • Receptors, Drug / genetics


  • Cannabinoids
  • Diterpenes
  • Receptors, Drug
  • iodoresiniferatoxin
  • Capsaicin