We have developed a novel and potent chemical series of 5,5-diphenylpentadienamides for targeting TRPV1 in vitro and in vivo. In this investigation, we examined a variety of replacements for the 5-position of dienamides with the goal of addressing issues related to pharmacokinetics. Our data suggest that substitution with alkoxy groups on the phenyl ring at the 5-position increases their ability to penetrate the blood-brain barrier. This investigation culminated in the discovery of compound (R)-36b, which showed a good pharmacokinetic profile. In vivo, compound (R)-36b was found to be effective at reversing mechanical allodynia in rats in a dose-dependent manner, and it reversed thermal hyperalgesia in a model of neuropathic pain induced by sciatic nerve injury.

Abstract. Transient receptor potential vanilloid 1 (TRPV1) is a Ca 2+-permeable non-selective cation channel that transmits pain signals. TRPV1 is activated by multiple stimuli such as capsaicin, acid, and heat. During inflammation, TRPV1 is reported to be sensitized by protein kinase C (PKC) in dorsal root ganglia (DRG) neurons, which leads to reduction in the threshold of the temperature for TRPV1 activation to body temperature. This sensitization is considered to contribute to chronic inflammatory pain. In a previous study, we discovered orally active 5,5-diarylpentadienamide TRPV1 antagonists. To examine the effects of our TRPV1 antagonists on PKC-sensitized TRPV1, we developed an in vitro assay system to monitor the TRPV1 sensitization by PKC. In this assay system, our TRPV1 antagonists, such as (2E,4Z)-N-[(3R)-3-hydroxy-2-oxo-1,2,3,4-tetrahydro-5-quinolyl]-5-(4-isopropoxyphenyl)-5-(4-trifluoromethylphenyl)-2,4-pentadienamide (K-685), inhibited the activation of TRPV1 sensitized by PKC. The potentiation of heat-induced inward currents by PKC was seen in rat DRG neurons, and K-685 attenuated these currents. Furthermore, K-685 reversed the thermal hyperalgesia and mechanical allodynia in a rat complete Freund's adjuvant-induced inflammatory pain model. These results therefore suggest that K-685 has a strong potential as a new analgesic drug for the treatment of inflammatory pain.