Introduction and objectives Effective therapies for chronic cough are a significant unmet need. Recently a P2X3 antagonist (AF-219), markedly reduced cough frequency in two phase 2 trials, but the role of P2X3 receptors and their ligand (adenosine triphosphate, ATP) in chronic cough is not well-understood. This study assessed the effect of P2X3 antagonism on cough evoked by capsaicin and ATP in healthy volunteers and chronic cough patients.

Method We performed a double-blind, placebo-controlled, randomised, 4-period crossover study. During each period (≥48 h apart), cough challenges were performed 2h after single doses of study medication [capsaicin (0.48–1000 μM) periods 1/2 and ATP (0.227–929 μmol/mL) periods 3/4]. Two cohorts were enrolled; cohort (1) 14 healthy volunteers (HV, mean age 37.5 yrs, 100% male) and 12 chronic cough (CC, mean age 60.3 yrs, 17% male) received AF-219 300mg/placebo and cohort (2) 12 HV (mean age 34.8 yrs, 100% male) and 12 CC (mean age 57.8 yrs, 25% male) received AF-219 50 mg/placebo. Cough challenges consisted of four inhalations of each doubling concentration of tussive agent, from a dosimeter 30s apart. Coughs in the first 15s were counted and challenges continued to the maximum tolerated dose. The concentrations evoking at least 2 and 5 coughs, C2 and C5 (from inhalation 1) were analysed using mixed effect models; pharmacodynamic modelling was used to estimate Emax/ED50.

Results AF-219 had no effect on capsaicin C2 or C5 at 300 mg or 50 mg in HV or CC (all p > 0.05). For ATP challenges, AF-219 300 mg significantly increased C2 in CC (AF-219 10.8 μmol/mL vs. placebo 2.3 μmol/mL, p = 0.005) but not HV (p = 0.135), whereas AF-219 50mg significantly increased C2 in both groups (CC 40.4 μmol/mL vs. 2.8 μmol/mL, p = 0.002 and HV 114.4 μmol/mL vs. 20.7 μmol/mL, p = 0.046). AF-219 had no significant effect on ATP C5 at either dose in HV (all p > 0.05), but in CC patients 50mg AF-219 (but not 300mg) increased C5 (70.1 μmol/mL vs. 17.1 μmol/mL, p = 0.027). Of note, ATP inhalation evoked less coughing than capsaicin, limiting the utility of the C5 endpoint.

Conclusions P2X3 antagonism reduced cough responses to ATP, particularly in patients with CC, but did not alter cough responses to an off-target tussive agent.