Synthesis and Structure Activity Relationship Studies of Benzimidazole-based P2X3 Receptor Antagonists as Novel Anti-nociceptive Agents

Abstract

P2X3 receptors (P2X3R) are ATP-gated ion channels, that targeted by many drug discovery programs, predominantly expressed in C- and Aδ- fiber primary afferent neurons and have been actively investigated for several neurological disorders, including neuropathic pain and chronic cough, etc. Because of its localized distribution, antagonism of P2X3R has been thoroughly considered, and the avoidance of issues related to CNS side effects has been proven in clinical trials. Furthermore, avoiding the loss of taste, has been currently issued for the the unmet need of P2X3R targeted drugs, that could be decreased by high P2X3R selectivity versus P2X2/3R. In this dissertation, researches to discovery and development of benzimidazole-based P2X3R antagonist were performed as follows. Firstly, benzimidazole-4,7-dione-based derivatives were introduced as a new chemical entity for the development of P2X3R antagonists. As a start of the research, the hit compound discovery from the screening of 8364 random library compounds in the Korea Chemical Bank, the major hit showed an IC50 value of 1030 nM. For studies of structure–activity and structure–property relationships were investigated for further optimization toward improving the antagonistic activities as well as the drug’s physicochemical properties, including metabolic stability. For the results, the final optimized compound 14h (LDD-4962) was developed with an IC50 value of 375 nM at P2X3R with more than 23-fold selectivity versus P2X2/3R, with stabilities of metabolic and improved solubility. Defining the anti-nociceptive effect, studies of neuropathic pain animal models were proceeded. The result showed anti-nociceptive effects through an increase in the mechanical withdrawal threshold as measured by von Frey filament following intravenous administration which for the rodent models that evoked by either nerve ligation or chemotherapeutics. Secondly, the design and synthesis of 5-methyl-1H-benzo[d]imidazole derivatives were investigated by the analysis of key pharmacophores of BLU-5937 and Eliapixant. As a result of the optimization, the most potent compounds 29a and 29b (LDD-5046, 5047) for the P2X3R antagonists were discovered by the structure-activity relationship study. 29a and 29b showed the not-competitive antagonism as IC50 values of 145 and 206 nM, and selectivity index of 60 and 41, respectively. In addition, for the docking study, poor binding scores at the known allosteric binding site of which Gefapixant were identified, indicating that the other allosteric binding site for the novel P2X3R antagonists.