BCL2 blockade overcomes MCL1 resistance in multiple myeloma

Abstract

Disruption of the intrinsic apoptotic pathway by the aberrant expression of the BCL2 family members are frequent events in multiple myeloma (MM). In particular, the anti-apoptotic protein myeloid cell leukemia-1 (MCL1) is highly expressed in MM and plays a crucial role in disease progression [1, 2]. Using an unbiased approach to analyze cell death clustering, Gomez-Bougie and colleagues recently identified a group of MM patients insensitive to all the three classes of BH3 mimetics targeting MCL1, BCL2, and BCLxL. These BH3 mimetic-resistant patients were mostly found at diagnosis, and they often do not possess any recurrent chromosomal translocations. BCL2 dependency is mainly found in patients with t (11;14) CCND1 translocation. BCLxL dependency is rare in MM as they are often co-dependent on either BCL2 or MCL1. MCL1 dependency was strikingly predominant at relapse and in patients lacking common translocations and in the CCND1 subgroup. These findings suggested a shift of cellular plasticity towards MCL1 dependence during disease progression as a result of prior treatments or clonal selection [3]. A majority of well-established human MM cell lines and low-passage patient-derived myeloma cell lines have been shown to be MCL1 dependent using pharmacological inhibitors or gene editing approaches that specifically target MCL1 [4]. Clinically, overexpression of MCL1 is observed in 52% of MM patients at diagnosis and 81% at relapse. The level of MCL1 expression correlates with disease progression, and a higher MCL1 expression is associated with shorter survival [5]. Since MM is heavily reliant on MCL1, MM patients, particularly those at relapse, would benefit from an MCL1-targeted therapy. However, there is no FDA-approved drug with the ability to selectively target MCL1. To address this unmet medical need, a few selective MCL1 inhibitors are currently being developed in preclinical phase or clinical trials and have thus far shown promising results as single agents or used in combination with established therapies in various cancers, particularly in hematologic malignancies [6–8]. To further explore MCL1 biology in MM, we use a clinical-grade small-molecule MCL1 inhibitor, AZD5991, to investigate the mechanistic underpinning of MCL1 inhibition in MM. AZD5991 is a potent and selective macrocyclic inhibitor of MCL1 [9] that is currently in phase I clinical trial in patients with relapsed or refractory MM and other hematologic malignancies (ClinicalTrials.gov Identifier: NCT03218683). Using AZD5991 as a test compound, we aimed to determine the survival dependency of human MM cells on the antiapoptotic protein MCL1. First, we evaluated the cytotoxicity of AZD5991 on a panel of MM cell lines. MM cell lines showed a heterogeneous response to MCL1 inhibition. AZD5991 treatment resulted in dose-dependent cytotoxicity with EC50 values (Table S1) ranging from 64 nM to 417 nM at 24 h for AZD5991-sensitive cell lines (Fig. 1a). We next assessed the effect of AZD5991 in MM patient-derived CD138 cells. AZD5991 treatment led to 40–82% decrease in viability of primary cells isolated from relapsed and refractory MM patients at a dose of 300 nM at 24 h (Fig. 1b). AZD5991 also induces potent anti-MM activity in vivo [9]. Together, these results indicate that AZD5991 has promising singleagent activity, but it would be prudent to study it in combination with other anti-MM therapies. To understand the mechanism of cytotoxicity, we treated the AZD5991-sensitive MM.1S and H929 cells with 50 nM of AZD5991 for 24 h. The decrease in cell viability upon MCL1 inhibition is due to an increase in apoptosis as shown by an increase in Annexin V signals after MCL1 inhibition. * Noopur Raje [email protected]