Gefapixant (AF219) is a P2X3 receptor (P2X3R) antagonist with IC50 of ~30 nM at recombinant hP2X3 homotrimers and 100-250 nM at hP2X2/3 heterotrimeric receptors.

The IC50 of Gefapixant has been reported as ~30 nM versus recombinant hP2X3 homotrimers and 100-250 nM at hP2X2/3 heterotrimeric receptors, potencies very similar to those reported for recombinant rat receptors, and it displays no inhibitory impact on any non-P2X3 subunit-containing receptors (IC50 values>>10,000 nM at recombinant homotrimeric hP2X1, hP2X2, hP2X4, rP2X5, and hP2X7 channels).

In a rat model for knee osteoarthritis (developed 14 days after the intra-articular injection of monoiodoacetate), oral administration of Gefapixant twice daily for seven days resulted in a significant attenuation of weight bearing asymmetry, with a full reversal of observable hyperalgesia at the two higher doses [2].

A rodent model often employs for assessing the potential for drug effect in osteoarthritis (OA) pain is based on the intraarticular injection of monoiodoacetate (mIOA) into one knee joint of the rat. To measure the effect of Gefapixant on the weight bearing laterality and apparent hyperalgesia, Gefapixant is given by intraplantar or oral administration to the rats, with different concentrations (6, 20, and 60 mg/kg) two times a day and continues up to a week[2].