The present invention provides a method for the treatment or prevention of conditions which can be ameliorated by Smo antagonism, which method comprises administration to a patient in need thereof of an effective amount of a compound of formula I or a composition comprising a compound of formula I: or a pharmaceutically acceptable salt or solvate thereof; wherein: 2 of X, Y and Z represent nitrogen atoms, and the other represents an oxygen atom; R1 and R2 are taken together with the atom to which they are attached and represent a cyclobutyl ring, optionally substituted with 1-2 fluorine atoms, and R3 represents hydrogen or a fluorine atom; or R1 represents methyl, R2 represents methyl or a fluorine atom and R3 represents a fluorine atom.