Design, synthesis, and evaluation of isoquinoline ureas as trpv1 antagonists

Background: The inhibition of transient receptor potential vanilloid receptor 1 (TRPV1) has emerged as a novel approach for the treatment of various pain states. Pyrrolidinyl urea, SB 705498 with pK_b = 7.3 in guinea pig TRPV1 receptor has been investigated in Phase II clinical trials for pain and chronic cough. Another heteroaryl urea derivative, A-425619 1, has been reported to be a potent and selective TRPV1 antagonist of capsaicin-evoked receptor activation with an IC₅₀ value of 4 nM in hTRPV1. Objective: A series of thirteen A-425619 1 analogues with modifications centered around the C-region were synthesized to understand the binding site characteristics of TRPV1 receptors. Method: We synthesized a series of isoquinoline ureas and evaluated their antagonist potency using smooth muscle assay using guinea pig trachea along with the evaluation of the molecular properties and molecular modeling using CoMFA studies. Results: p-Chloro 4, p-bromo 5, m-isothiocyanate 15, and p-isothiocyanate 16 derivatives were found to be the most potent members of the series with pK_b values in the range of 7.3-7.4 in the functional assay using guinea pig trachea. The lead compound A-425619 1 exhibited a pK_b value of 8.1 in this assay. Conclusion: The para-substituted analogues were found to be more potent than the ortho-and meta-analogues in the biological assay. This observation was further supported by molecular modeling studies using CoMFA.