Methods for treating respiratory and gastrointestinal diseases mediated by a P2X3 and/or a P2X2/3 receptor antagonist, the methods comprising administering to a subject in need thereof an effective amount of a compound of formula (I) or a pharmaceutically acceptable salt thereof, wherein D, X, Y, R1, R2, R3, R4, R5, R6, R7 and R8 are as defined herein.