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NEUROPHARMACOLOGY

[³H]A-778317 [1-((R)-5-tert-Butyl-indan-1-yl)-3-isoquinolin-5-yl-urea]: a Novel, Stereoselective, High-Affinity Antagonist Is a Useful Radioligand for the Human Transient Receptor Potential Vanilloid-1 (TRPV1) Receptor

Departments of Neuroscience Research (B.R.B., R.E.K., T.R.N., C.-H.L., A.G., H.A.M., C.S.S., C.R.F., M.F.J., R.B.M., P.S.P.) and Radiochemistry (S.N.R., S.N.V., B.S.), Global Pharmaceutical Research and Development, Abbott Laboratories, Abbott Park, Illinois

1-((R)-5-tert-Butyl-indan-1-yl)-3-isoquinolin-5-yl-urea (A-778317) is a novel, stereoselective, competitive antagonist that potently blocks transient receptor potential vanilloid-1 (TRPV1) receptor-mediated changes in intracellular calcium concentrations (pIC₅₀ = 8.31 ± 0.13). The (S)-stereoisomer, 1-((S)-5-tert-butyl-indan-1-yl)-3-isoquinolin-5-yl-urea (A-778316), is 6.8-fold less potent (pIC₅₀ = 7.47 ± 0.07). A-778317 also potently blocks capsaicin and acid activation of native rat TRPV1 receptors in dorsal root ganglion neurons. A-778317 was tritiated ([³H]A-778317; 29.3 Ci/mmol) and used to study recombinant human TRPV1 (hTRPV1) receptors expressed in Chinese ovary cells (CHO) cells. [³H]A-778317 labeled a single class of binding sites in hTRPV1-expressing CHO cell membranes with high affinity (K_D = 3.4 nM; B_max = 4.0 pmol/mg protein). Specific binding of 2 nM [³H]A-778317 to hTRPV1-expressing CHO cell membranes was reversible. The rank-order potency of TRPV1 receptor antagonists to inhibit binding of 2 nM [³H]A-778317 correlated well with their functional potencies in blocking TRPV1 receptor activation. The present data demonstrate that A-778317 blocks polymodal activation of the TRPV1 receptor by binding to a single high-affinity binding site and that [³H]A-778317 possesses favorable binding properties to facilitate further studies of hTRPV1 receptor pharmacology.

Address correspondence to: Pamela S. Puttfarcken, Abbott Laboratories, R4PM, AP9A/2, 100 Abbott Park Rd, Abbott Park, IL 60064-6123. E-mail: pamela.puttfarcken{at}abbott.com