Abstract: 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.

Abstract: The present invention provides methods for high level, regulated transgene transcription that is restricted to cell populations of specific types. The process is designed to work with any inducible expression regulation systems, adapting them to a tissue-specific expression pattern while simultaneously delivering maximal achievable expression levels. In particular, the invention utilizes hybrid promoters that contain the DNA elements for both cell type-specific and regulated transcription. By placing the gene of the transcriptional activation factor (TAF) under the control of this tissue-specific/drug-regulated (TSDR) promoter, this invention achieves high expression levels of TAF in specific target cells by first initiating TAF expression using cell-type specific transcription elements, and subsequently amplifying transcriptional activity by establishing an autoregulatory positive feedback loop.