A method for treating cancer comprising administering to a patient in need thereof an effective amount of a compound having the formula:wherein R1 and R2 are independently alkyl of 1 to 6 carbon atoms or cycloalkyl of 3 to 6 carbon atoms, Xa is CH or N, Ya is hydrogen or a substituent, each of which is independently selected； n is an integer from 1 to 5, J is a counter ion.
