The invention provides the compounds of formula (): and pharmaceutically acceptable salts thereof, wherein R1, R2, R3, R4, A, X, Y, a, b and n are as defined herein. Also disclosed are methods for making the compounds of formula (I) and their use in treating or preventing diseases associated with cell overproliferation and dysfunctional sphingolipid signal transduction. The invention also encompasses the use of the compounds in combination with an apoptosis-signaling ligand, such as Fas ligand. Preferably, the Fas ligand is administered in the form of a gene therapy agent.