BLU-5937 is a small molecule that was shown to be a potent, selective and non-competitive P2X3 homotrimeric receptor antagonist. P2X3 receptors are ATP ion-gated channels located on primary afferent neurons. ATP released from damaged or inflamed tissues in the airways acts on P2X3 receptors of primary afferent neurons, triggering depolarization and action potentials that are transmitted centrally and interpreted as urge to cough. There are strong preclinical and clinical evidence supporting the role of P2X3 receptors in hypersensitization of the cough reflex, leading to chronic cough. By inhibiting P2X3 receptors on the primary sensory neurons, BLU-5937 would inhibit the hypersensitization of the cough reflex and, hence, the exaggerated cough experienced in chronic cough patients. BLU-5937 is being developed for the treatment of unexplained, refractory chronic cough. The high potency and selectivity of BLU-5937 for P2X3 homotrimeric receptors was demonstrated in vitro by inhibiting αβ-meATP-evoked P2X3 or P2X2/3 receptor activity in cloned human hP2X3 and hP2X2/3 channels expressed in mammalian cells. The IC of BLU-5937 for hP2X3 homotrimeric and hP2X2/3 heterotrimeric receptors was established at 25 nM and >24 μM, respectively. Furthermore, BLU-5937 (500 nM) was able to block αβ-meATP-induced sensitization and firing activity of isolated primary nociceptors in rat dorsal root ganglions (DRGs), through P2X3 homotrimeric receptor antagonism. In a guinea pig cough model, BLU-5937 (0.3, 3 and 30 mg/kg, oral) significantly reduced, in a dose-dependent fashion, the histamine-induced enhancement in the number of citric acid-induced coughs. BLU-5937 (3 and 30 mg/kg, oral) was also shown to reduce significantly and dose-dependently the ATP-induced enhancement of citric acid-induced coughs in the guinea pig. These anti-tussive effects were obtained at a plasma concentration known to block P2X3 homotrimeric receptors, but at concentration 50-fold lower than that required to block P2X2/3 heterotrimeric receptors. These results indicate that the anti-tussive effect of BLU-5937 is primarily mediated through the inhibition of P2X3 homotrimeric receptors. In a rat behavioral taste model, BLU-5937 (10-20 mg/kg, IP) did not alter taste perception as compared to control animals. In the same experiment, N-00588 (10-20 mg/kg, IP), a weakly selective antagonist for P2X3 versus P2X2/3 receptors, had a significant inhibitory effect on taste perception. Pharmacokinetic analysis of drug plasma concentrations showed that BLU-5937 did not affect taste function at concentrations up to 30 times the IC for P2X3. These results suggest that N-00588 achieved systemic concentration that blocked P2X3 and P2X2/3 receptors expressed on gustatory nerve ending innervating taste buds. The lack of effect of BLU-5937, even at high doses, on taste perception may be attributed to its higher selectivity for the P2X3 versus P2X2/3 receptors on the taste buds. The safety, tolerability and pharmacokinetic profile of BLU-5937 was assessed in a battery of preclinical studies and have revealed that BLU-5937 exhibits excellent drug-like characteristics, including good oral bioavailability, low predicted clearance in human, no blood-brain barrier permeability and high safety margin versus human predicted efficacious exposure. BLU-5937 is currently in clinical phase I development stage. In conclusion, BLU-5937 was selected as a drug candidate for the treatment of chronic cough due to its high potency and selectivity for P2X3 homotrimeric receptors, strong anti-tussive effects, excellent tolerability and predicted pharmacokinetic properties in humans.