Abstract 798: Pathway-directed high-throughput drug screen identifies PI3K inhibitors that synergistically potentiate antitumor activity of HDAC inhibitors in cutaneous T-cell lymphoma
Authors: Wu Chi-Heng , Yang Chen-Yen , Rakhshandhroo Taha , Pincus Laura , Bandyopadhyay Sourav , McCormick Frank , Ai Weiyun 
Published: August 2018

Background: Cutaneous T-cell lymphoma (CTCL) represents a type of malignancy in the skin that is incurable. Recent studies demonstrated dysregulation of several signaling pathways in CTCL, including PI3K/AKT, JAK/STAT, and NFκB pathways. We performed a high-throughput drug screen to determine the potential of novel agents targeting these pathways for the treatment of CTCL.

Methods: We compiled a library of 94 compounds targeting pathways known to be relevant in cancer biology, such as kinases involved in growth factor receptor signaling, HDACs, proteasome, DNA repair and regulators of apoptosis. The compounds were screened for antiproliferative activity against four CTCL cell lines in high-throughput proliferation assays. PI3K inhibitors were selected from the “hits” of the screen and were further studied in xenograft models of CTCL and in primary T-cell lymphomas cells from patients. To examine which PI3K isoform mediates antitumor effect in CTCL, isoform-specific siRNAs were used to knock down each isoforms as well as the combination of doublet isoforms. Based on this experiment, we tested antitumor activity of isoform-specific inhibitors. Promising candidates were also tested in combination assays using a matrix block method designed to detect synergistic activities.

Results: From the high-throughput screen, we identified 14 compounds with antiproliferative activity in CTCL, including multiple inhibitors of the PI3K pathway. Validation assays confirmed the class effect of PI3K inhibitors in growth inhibition. From this class, a pan-PI3K inhibitor BKM120 was selected for in vivo studies. In a xenograft model of CTCL, BKM120 exhibited striking antitumor activity measured by a marked suppression of tumor growth and prolonged survival of tumor-bearing mice compared with vehicle control. To investigate which isoform of the PI3K mediates the antitumor effect in CTCL, we performed isoform-specific siRNA assays and demonstrated that knocking down isoform ð exhibited the most potent growth inhibition. Copanlisib targets α and δ isoforms of PI3K and is newly approved by the FDA for low-grade B-cell lymphoma. We demonstrated that copanlisib exhibits potent antiproliferation activity in CTCL cell lines. Furthermore, combination of copanlisib and LBH, a HDAC inhibitor, display synergistic antiproliferative effects across a broad dose gradient for each agent.

Conclusion: A pan-PI3k inhibitor, BKM120, is highly active in CTCL cell lines and exnograft models. Inhibition of Isoform δ exhibits the most potent growth inhibition. Furthermore, an isoform-specific inhibitor, copanlisib, synergistically potentiates the effect of HDAC inhibitors against CTCL. These are highly promising approaches for the treatment of CTCL and warrant clinical investigation.

Citation Format: Chi-Heng Wu, Chen-Yen Yang, Taha Rakhshandhroo, Laura Pincus, Sourav Bandyopadhyay, Frank McCormick, Weiyun Ai. Pathway-directed high-throughput drug screen identifies PI3K inhibitors that synergistically potentiate antitumor activity of HDAC inhibitors in cutaneous T-cell lymphoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 798.