Abstract 2657: Elevated level of BCL-2 is the primary target for inhibition during duvelisib (IPI-145) therapy: ABT-199 neutralizes the resistance mechanism in chronic lymphocytic leukemia
Authors: Patel Viralkumar M. , Balakrishnan Kumudha , Guerrieri Renato , Wierda William , O'Brien Susan , Gandhi Varsha 
Published: August 2015

Abstract
Inhibition of BTK by ibrutinib, PI3K-delta with idelalisib, or PI3K-delta and gamma with duvelisib (IPI-145), all components of B-cell receptor (BCR) pathway, has significantly improved the treatment outcome in chronic lymphocytic leukemia (CLL) via disrupting the interactions with microenvironment. Though BCR network inhibitors induce durable remission in the majority of CLL patients, a proportion of patients that initially respond to treatment develop resistance or some show maintained lymphocytosis in the blood. Identifying and overcoming resistance mechanisms will be crucial for the most effective combinatorial use of these agents. Duvelisib is an orally bioavailable, highly potent small molecule inhibitor of p110δ and p110γ with KD values of 0.023 nM and 0.24 nM, respectively. Preclinical investigations with duvelisib overcame signals from PI3K/AKT/S6 pathway and promoted apoptosis in primary CLL cells (Balakrishnan et al, ASH 2013). Duvelisib is currently in a phase III trial in CLL. During first phase I study, we performed molecular investigations. PBMCs collected from CLL patients from Phase I study of duvelisib treatment (Pre/Day 0 and Post/Day 28) were subjected to RPPA analysis (n = 7). Interestingly, of the 141 proteins analyzed, Bcl-2 was maximally and significantly elevated in Day 28 samples (mean fold + SEM: 1.7 + 0.2; p = 0.015), suggesting that this could be in part the mechanism involved in resistance during therapy. Further analysis of protein expression by immunoblotting confirmed that Bcl-2 protein was elevated in post-treatment samples (1.3 + 0.1; p = 0.086; n = 7). This was in conjunction with elevated levels of Bcl-2 transcripts analyzed by mRNA array (TaqMan Human Apoptosis - 93 genes) assay (3.0 + 0.4; p = 0.002) and RT-PCR (1.9 + 0.2; p = 0.003), while other anti-apoptotic genes (Bfl-1, Mcl-1, Bcl-w Bcl-g, Bcl-b, Bcl-xL) were unchanged. Importantly, ex-vivo incubations of pre- and post- duvelisib samples with 3 nM ABT-199, a highly selective clinically promising Bcl-2 protein antagonist, induced significantly greater apoptosis in post-therapy samples (79%) in comparison to pre-treatment (58%) samples suggesting that Bcl-2 is the primary target for inhibition during duvelisib intake (n = 5; p = 0.041). In addition, duvelisib induced sensitivity on ex-vivo post- duvelisib clinical samples (n = 15) was specifically towards ABT-199 and produced significantly more apoptosis in the presence of ABT-199 (45%, p&lt;0.0001; 3 nM) compared to other clinically-relevant agents such as ibrutinib (1%, p = 0.10; 10 μM), idelalisib (4% p = 0.05; 10 μM) or ABT-737 (16%, p = 0.0002; 10 nM). We report that elevated Bcl-2 level is the primary target for inhibition during duvelisib therapy and thereby combination with ABT-199 could be a rational approach to overcome the resistance mechanism.
Citation Format: Viralkumar M. Patel, Kumudha Balakrishnan, Renato Guerrieri, William Wierda, Susan O'Brien, Varsha Gandhi. Elevated level of BCL-2 is the primary target for inhibition during duvelisib (IPI-145) therapy: ABT-199 neutralizes the resistance mechanism in chronic lymphocytic leukemia. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2657. doi:10.1158/1538-7445.AM2015-2657