Chromen-4-ones as novel potent and selective ligands for purinoceptor-related class A δ-branch <em>orphan</em> G protein-coupled receptors

Abstract

bout 100 of the roughly 800 members of the G protein-coupled receptor (GPCR) family are so-called <i>orphan</i> GPCRs whose endogenous ligand is not known or not confirmed yet. As the <i>orphan</i> receptors might qualify as drug targets there is an enormous potential in this poorly studied group of receptors. In this study synthetic ligands for four promising <i>orphan</i> receptors of the delta-branch of class A GPCRs were developed and optimized. Although GPR17, GPR35, GPR55 and GPR84, on which we focused, are poorly studied so far, there are strong indications that these receptors may be involved in pathological processes. For the development of new potent and selective ligands we synthesized a large library of chromen-4-one derivatives. <br /> 8-(2,6-Difluoro-4-methoxybenzamido)-4-oxo-6-phenyl-4<i>H</i>-chromene-2-carboxylic (hGPR35, EC₅₀ = 1.1 nM) is the most potent agonist at the human GPR35 known to date, whereas 6-butyl-8-(carboxyformamido)-4-oxo-4<i>H</i>-chromene-2-carboxylic acid (rGPR35, EC₅₀ = 20.9 nM) and 6-bromo-8-(carboxyformamido)-4-thioxo-4<i>H</i>-chromene-2-carboxylic acid (mGPR35, EC₅₀ = 174 nM) also showed good agonistic activity at the rodent orthologues of GPR35. Some of our compounds were antagonists at GPR84 in the low nanomolar range. The most potent compound of the series 6-(4-chlorophenyl)-8-(4-((4-fluorobenzyl)oxy)benzamido)-4-oxo-4<i>H</i>-chromene-2-carboxylic acid (IC₅₀ = 7.3 nM) is able to block GPR84 selectively in the low nanomolar range. We also identified potent agonists and antagonists for the cannabinoid-like GPR55, which were based on the same scaffold. Interestingly also some potent antagonists for the <i>orphan</i> receptor GPR17 could be developed which were able to block the receptor in the nanomolar range. <br /> The new chromen-4-one derivatives are important pharmacological tools for further investigation of the receptors in vitro and in vivo, and can be used as lead compounds for the development of new drugs.<br /> In a second part of the study potent ecto-5’-nucleotidase (eN/CD73) inhibitors, which were synthesized in our lab, were evaluated in different biological systems. Ecto-5’-nucleotidase mediates hydrolysis of adenosine-5’-monophosphate (AMP) to adenosine and subsequently activates adenosine receptors. It is therefore involved in several physiological and pathological processes such as immunity, inflammation, neurotransmission and tumorigenesis. The most potent CD73 inhibitor of the series was able to block AMP hydrolysis not only in recombinant human eN, (K_i = 0.318 nM), but also in different cell lines, tissue samples, and human serum and can therefore be used in future in vivo studies.