Structure–activity studies of a novel series of 5,6-fused heteroaromatic ureas as TRPV1 antagonists

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Abstract

Novel 5,6-fused heteroaromatic ureas were synthesized and evaluated for their activity as TRPV1 antagonists. It was found that 4-aminoindoles and indazoles are the preferential cores for the attachment of ureas. Bulky electron-withdrawing groups in the para-position of the aromatic ring of the urea substituents imparted the best in vitro potency at TRPV1. The most potent derivatives were assessed in in vivo inflammatory and neuropathic pain models. Compound 46, containing the indazole core and a 3,4-dichlorophenyl group appended to it via a urea linker, demonstrated in vivo analgesic activity upon oral administration. This derivative also showed selectivity versus other receptors in the CEREP screen and exhibited acceptable cardiovascular safety at levels exceeding the therapeutic dose.

Introduction

The capsaicin-sensitive TRPV1 receptor is a member of the mammalian transient receptor potential (TRP) channel family and is highly expressed on small diameter (C-fiber) nociceptive sensory neurons. It is also expressed at lower levels in other non-neuronal tissues such as skin and bladder.1 This receptor has been called a polymodal detector of noxious stimuli since it can be activated in several ways. Low pH, heat,2, 3 and vanilloid ligands such as capsaicin and resiniferatoxin4 activate TRPV1 causing burning pain sensation. Endogenous mediators of inflammation, such as the cannabinoid anandamide5 and arachidonic acid metabolites6 can also activate TRPV1.

Interest in the TRPV1 receptor as a therapeutic target has been driven in part by the observation that ‘knock-out’ mice lacking this receptor exhibited deficits chemically or inflammatory-evoked thermal hyperalgesia.7, 8 Until recently, agonists were the major focus of research, but the therapeutic effect of such agents was compromised by an initial burning effect, possibly due to receptor desensitization.9 Moreover, attempts to separate analgesic and excitatory effect were not successful.10 The alternative approach of developing competitive TRPV1 antagonists, which do not cause initial painful sensation, is being extensively pursued,11 as evidenced by the burgeoning number of publications on the subject.12, 13, 14, 15 A recently published article16 detailed our efforts in identifying TRPV1 antagonists within a series of 6,6-fused heteroaromatic ureas. Herein we examine the transition to 5,6-fused heteroaromatic rings, such as indole, indazole, and benzimidazole (Chart 1), and discuss the structure–activity relationships associated with this group of novel chemotypes.

Section snippets

Chemistry

The synthesis of urea derivatives could be accomplished via Method A (Scheme 1) where the aminoindole 1, aminoindazoles 25, or benzimidazole 6 was reacted with the various commercially available isocyanates 7. Reaction of 4-aminoindazoles 3 with isocyanates 7 resulted in the mixture of products generated by the addition of the nitrogen of the ring to the isocyanate. To circumvent this, the urea formation was carried out on the protected indazole 2.

Method B (Scheme 1) entailed the formation of

Results and discussion

In our earlier publication16 we have described a new series of TRPV1 antagonists bearing various 6,6-bicyclic core structures. The lead compound of that series, A-425619.0, demonstrated in vivo activity in several chronic pain models. Our goal was to expand this series to include 5,6-heterocyclic cores and to study the SAR (Chart 2) that could be broken into three parts: core replacement (P1 region), modification of the linker (P2 region), and benzyl fragment (P3 region). Compounds were

Conclusion

To gain increased understanding of the structural requirements for TRPV1 antagonists, we studied a series of 5,6-fused heteroaromatic benzyl ureas. It was discovered that both indole and indazole ureas have the best potency when urea moiety is attached at the 4-position. Potency of 4-aminoindazole derivatives exceeded the one of comparable indole analogs. Methyl groups were tolerated only in the 1-position of indazole core. Large electron-withdrawing groups, such as CF3, OCF3, and 3,4-diCl, in

In vitro assays

The functional activity of compounds at the TRPV1 receptor was determined with a Ca2+ influx assay and measurement of intracellular Ca2+ levels ([Ca2+]i). All compounds were tested over an 11-point half-log concentration range. Compound solutions were prepared in D-PBS (4× final concentration) and diluted serially across 96-well v-bottomed tissue culture plates using a Biomek 2000 robotic automation workstation (Beckman–Coulter, Inc., Fullerton, CA). A 0.2 μM solution of the TRPV1 agonist

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