Synthesis and Pharmacological Evaluation of Novel Heteroaromatic Ligands of the A1 Adenosine Receptor
2017-02-08T04:51:52Z (GMT) by
Selective adenosine receptor (AR) ligands have potential therapeutic applications in cardiovascular, inflammatory and neuro-degenerative diseases. Many potent and selective adenosine agonists have been reported, although their development as therapeutic agents has been limited by side effects. A promising alternative are allosteric modulators, which can either enhance or inhibit the action of endogenous adenosine and selectively tune receptor signalling in both a site- and event-specific fashion. A pharmacophore-based screening approach identified ethyl 5-amino-3-(tert-butylphenyl)-4-oxo-3,4-dihydrothieno[3,4-d]pyridazine-1-carboxylate (207) as an interesting lead compound of allosteric modulation of the A1AR. On the basis of this lead, various derivatives were synthesised and evaluated for activity at the human A1AR. All of the 5-aminothieno[3,4-d]pyridazinones showed allosteric modulation of A1AR in dissociation kinetic assays but in subsequent functional assays the predominant pharmacological effect observed was antagonism of the A1AR. This study identified a new class of potent A1AR antagonists, with ethyl 5-amino-3-(3-chlorophenyl)-4-oxo-3,4-dihydrothieno[3,4-d]pyridazine-1-carboxylate (213b) evaluated in more detail and defined as a potent inverse agonist with a pA2 value of 8.7. Since the majority of allosteric enhancers to date have demonstrated antagonism at high concentrations, this project investigated the structural relationships that govern antagonism versus allosteric modulation for aminothienopyridazinones. A series of N-derivatized aminothienopyridazinones were synthesized, including an unsubstituted compound 232, and N-benzyl derivatives with varying substituents around the aromatic ring (237a-k and 237m). These compounds were evaluated in functional assay and it was found that removal or replacement of the N-phenyl ring significantly reduced antagonism, indicating that the N-phenyl ring is an important motif for antagonist activity in this class of compounds. A series of compounds with various modifications to substituents at the 1-position of the thienopyridazinone core, found that modification at this position retains antagonist activity, 5-amino-3-(4-methoxyphenyl)-4-oxo-N-phenyl-3,4-dihydrothieno[3,4-d]pyridazine-1-carboxamide (241d) was the most potent antagonist in this series with a pA2 of 7.4. Overall the N-(3-chlorophenyl) thienopyridazinone 1-carboxamides were more potent antagonists than the corresponding N-(4-methoxyphenyl) analogues, apart from 241d. Synthesis and evaluation of 5-amidothienopyridazines (242a-j) observed that substitution at the 5-position of thienopyridazinones is tolerated, with only a slight reduction in antagonist activity. Straight chain alkyl amido derivatives produce the most potent compounds pA2 values in a range of 7.2-7.9; with the two-carbon chain optimum for antagonist activity, ethyl 3-(3-chlorophenyl)-4-oxo-5-propionamido-3,4-dihydrothieno[3,4-d]pyridazine-1-carboxylate (242b) the most potent compound in the 5-amido series with a pA2 of 7.9. A series of 2-aminothiophenes were synthesized with varying 3-carboxylates and substitution at the 4-position to produce ring opened mimics of the aminothienopyridazinones. The majority of compounds display negligible effect on EC50 concentration response of R-PIA. Only diethyl 2-amino-4-methylthiophene-3,5-carboxylate (254) was found to inhibit agonist function with pA2 of 6.4.