Pathway-Directed High Throughput Drug Screen Identifies PI3K Inhibitors That Synergistically Potentiate Anti-Tumor Activity of HDAC Inhibitors in Mycosis Fungoides and Sezary Syndrome

Introduction and Objectives: Mycosis fungoides and Sezary syndrome (MF/SS) represent a group of heterogeneous diseases.  Recent studies demonstrated dysregulation of several signaling pathways in MF/SS, including PI3K/AKT, JAK/STAT, RAS and NFkB pathways. We performed a high throughput drug screen to determine the potential of novel agents targeting these pathways for the treatment of MF/SS.

Materials and Methods:  We compiled a library of 94 compounds targeting pathways known to be relevant in cancer biology. These included kinases involved in growth factor receptor signaling, HDACs, proteasome, DNA repair and regulators of apoptosis. The compounds were screened for anti-proliferative activity against four MF/SS cell lines in high throughput proliferation assays. Selected hits were further studied in xenograft models of MF/SS and in primary T cell lymphomas.  Promising candidates from different classes were also tested in combination therapy assays using a matrix block method across dose gradients of each compound designed to detect synergistic activities.

Results:  From the high throughput screen, we identified 14 compounds with anti-proliferative activity in MF/SS, including multiple inhibitors of the PI3K pathway.  PI3K inhibitors emerged as preliminary hits in this screen and secondary validation assays confirmed the class effect of PI3K inhibitors. From this class, the PI3K inhibitor BKM120 was selected for in vivo studies. In a xenograft model of MF, BKM120 exhibited striking anti-tumor activity measured by a marked suppression of tumor growth and prolonged survival of tumor-bearing mice compared with vehicle control. In a search for even more effective combination therapies, we identified that BKM120 and the HDAC inhibitor class of compounds exhibit synergistic anti-proliferative effects in MF/SS tumor cells. Each of three HDAC inhibitors including LBH, Romidepsin and Vorinosat showed synergistic activity with BKM120, most evident at the GI50 concentrations of each drug, and apparent in both growth inhibition and apoptotic assays.

Conclusion:  BKM120 is highly active in preclinical models of MF/SS.  Furthermore, it synergistically potentiates the effect of HDAC inhibitors against MF/SS tumor cells. These are highly promising approaches for the treatment of MF/SS and warrant clinical investigation.