Idelalisib and bendamustine combination is synergistic and increases DNA damage response in chronic lymphocytic leukemia cells

Abstract

Idelalisib is a targeted agent that potently inhibits PI3Kδ which is exclusively expressed in hematological cells. Bendamustine is a well-tolerated cytotoxic alkylating agent which has been extensively used for treatment of chronic lymphocytic leukemia (CLL). Both these agents are FDA-approved for CLL. To increase the potency of idelalisib and bendamustine, we tested their combination in primary CLL lymphocytes. While each compound alone produced a moderate response, combination at several concentrations resulted in synergistic cytotoxicity. Idelalisib enhanced the bendamustine-mediated DNA damage/repair response, indicated by the phosphorylation of ATM, Chk2, and p53. Each drug alone activated γH2AX but combination treatment further increased the expression of this DNA damage marker. Compared with the control, idelalisib treatment decreased global RNA synthesis, resulting in a decline of early-response and short-lived MCL1 transcripts. In concert, there was a decline in total Mcl-1 protein in CLL lymphocytes. Isogenic mouse embryonic fibroblasts lacking MCL1 had higher sensitivity to bendamustine alone or in combination compared to MCL1 proficient cells. Collectively, these data indicate that bendamustine and idelalisib combination therapy should be investigated for treating patients with CLL.

Transcriptional Modulation by Idelalisib Synergizes with Bendamustine in Chronic Lymphocytic Leukemia

“…Previous studies have demonstrated synergy between IDE and BEN [17], IBR and BEN [18], or MK2206 (an inhibitor of AKT) and BEN [27]. Similarly, we have confirmed that IDE is synergistic with BEN, CLB and FLU in primary CLL cells occurring even in patients who were resistant to one agent.…”

“…To determine if synergy was observed between the BCR pathway inhibitors and chemotherapy, as previously suggested [17], a matrix of dose combinations was created and observed cell death was compared to predicted cell death [19]. In 26 unique primary CLL samples, significant synergy was observed when combining BEN with IDE, using clinically relevant doses of each agent (5 µM for IDE and 10–20 µM for BEN; Figure 2A,B, red box) [20].…”

“…It has been reported that IDE can induce γH2AX formation, and we have evaluated whether this is related to the cytotoxicity of this agent [17]. We integrated γH2AX analysis, an indicator for DNA double-stranded breaks, with AV/7AAD after treating cells with the drugs for 18 h. IDE produced a small but significant increase in γH2AX, which increased with higher drug concentrations (Figure S2A,B), but no breaks by comet assay (data not shown).…”

“…A placebo-controlled Phase III clinical trial of IDE with BEN/rituximab (BR) showed higher clinical efficacy than with BR alone, but with significantly more infections and marrow suppression, demonstrating a lack of tumor specificity [16]. In vitro studies have also suggested synergy between IDE and BEN, although the mechanism remains unclear [17,18].…”

Recent therapeutic advances in chronic lymphocytic leukemia

“…We have recently shown that in mantle cell lymphoma cell lines and patient-derived samples, idelalisib inhibits protein synthesis, which correlates with reductions in AKT (the immediate downstream effector of PI3K) and mitogen-activated protein kinase kinase (MEK) phosphorylation 85 . Idelalisib synergizes with bendamustine in primary CLL cells, increasing DNA damage and suppressing transcription of the anti-apoptotic protein myeloid cell leukemia 1 (MCL-1) 86 . Indeed, in a recently reported phase 3 trial (n = 416), the combination of idelalisib with BR markedly enhanced PFS in patients with R/R CLL (median of 20.8 versus 11.1 months for BR plus placebo after a median follow-up of 14 months) 87 .…”