Targeting Dependence on the B-cell lymphoma 2 (Bcl-2) Family in Multiple Myeloma

Resisting programmed cell death (apoptosis) is one of the many necessary events for the development of cancer.¹ It is required to overcome the pro-death signals triggered by other tumorigenic events such as inappropriate proliferation through dysregulation of oncogenes or loss of tumor suppressor genes.² The primary mechanism by which oncogenic stress induces apoptosis is through the induction/activation of pro-apoptotic members of the B-cell lymphoma (Bcl)-2 family.³ For cells to survive such signals, these pro-apoptotic molecules must be neutralized through upregulation and binding of the anti-apoptotic Bcl-2 proteins, e.g. Bcl-2, B cell lymphoma-extra-large (BCL-XL), and myeloid cell leukemia 1 (Mcl-1).³ This neutralization comes at a cost, however, leaving cancer cells more dependent on anti-apoptotic BCL2 family members than normal cells.⁴ This makes the anti-apoptotic Bcl-2 family members attractive therapeutic targets since normal adult tissues do not experience similar stress.

Inhibiting the Bcl-2 proteins, however, proved to be a major challenge that resulted in a nearly 30-year journey from the discovery of Bcl-2 in the 1980s to regulatory approval of the first Bcl-2 inhibitor, venetoclax (formerly ABT-199 ), for treatment of relapsed chronic lymphocytic leukemia with 17p deletion, by the US Food and Drug Administration (FDA) in 2016.⁵ Venetoclax was derived from a Bcl-2/Bcl-xL inhibitor, navitoclax (ABT-263), an orally available version of the initial tool compound ABT-737.⁶ These drugs function similarly by binding to a groove in the anti-apoptotic proteins, freeing the pro-apoptotic Bcl-2 family members, thus allowing them to induce apoptosis. In myeloma, both Bcl-2 inhibitors and Mcl-1 inhibitors have emerged as promising therapeutic agents.

Martine Amiot, PhD and colleagues at the University of Nantes were the first to demonstrate that a subset of myeloma cell lines harboring the t(11;14) translocation was exquisitely sensitive to ABT-737.⁷ Based on gene expression, they concluded that the sensitivity was more likely due to Bcl-2 dependence and went on to demonstrate this, using both BH3-profiling and venetoclax sensitivity.^8,9 These studies provided the rationale for clinical testing of venetoclax in myeloma.

Clinical development of venetoclax in multiple myeloma

While the initial phase 1 study of venetoclax monotherapy study was not designed to specifically test t(11;14) myeloma, nearly all the responses were in t(11;14)-positive patients, where the response rate was 40% compared with only 6% of the t(11;14)-negative population.¹⁰ Importantly, responses were associated with high Bcl-2/Bcl-xL (BCL2L1) ratios, providing a potential prognostic biomarker. Toxicity was minimal in this study. Based on a preclinical study that demonstrated that dexamethasone could synergize with venetoclax in cell lines and patient samples,¹¹ an expansion cohort of this trial was opened with t(11;14)-positive myeloma using this combination. The response rate increased to 60%, although in the subsequent phase 2 study, consisting of more heavily pretreated and daratumumab-exposed patients, the response rate was lower.¹²

In parallel with the venetoclax monotherapy phase 1 study, a second phase 1 trial was completed in patients with relapsed/refractory multiple myeloma who received the combination of venetoclax with bortezomib and dexamethasone.¹³ The rationale for this combination was data demonstrating that bortezomib-induced expression of the pro-apoptotic protein NOXA, which functions by specifically inhibiting Mcl-1. This suggested that inhibiting two anti-apoptotic Bcl-2 family members could have synergistic activity. Indeed, in this trial, the response rate was 67%¹³ and this led to a phase 3 study (BELLINI) that compared venetoclax-bortezomib-dexamethasone with bortezomib-dexamethasone.¹⁴

In the BELLINI trial there was a marked difference in progression-free survival (PFS, 22.4 vs 11.5 months, respectively). Unfortunately, this did not translate into an increase in overall survival (OS), which was initially lower in the venetoclax-containing arm due to early deaths associated with infections and disease progression. While it remains unclear why this happened, the addition of antibiotic prophylaxis and the requirement for non-venetoclax-containing control arms for combination studies were put in place. Combinations with daratumumab and venetoclax-dexamethasone and daratumumab with venetoclax-bortezomib-dexamethasone were recently shown to be highly effective in daratumumab-naïve t(11;14)-positive patients and unselected respectively.¹⁵ Importantly, there was only 1 treatment-emergent death in this trial.

Predictive biomarkers of sensitivity to venetoclax

Together, these studies demonstrate the potential for use of venetoclax in multiple myeloma therapy, but they also point to the need for predictive biomarkers. t(11;14) remains a potential marker, although only about half of these patients responded to monotherapy and both preclinical and clinical studies demonstrated that others may also benefit. BCL2/BCL2L1 ratios also have value, although these are most predictive when the ratio is high and determining the optimal cutoff will require additional studies. Moreover, both protein and mRNA quantification have been used in the clinical trials and each of these will have its challenges for use outside a clinical trial.

Functional profiling has also proven to be an accurate measure of sensitivity, either through BH3-profiling⁹ or ex vivo sensitivity testing,^16,17 but these also have limitations as they are not easily performed in clinical laboratories. A recent report demonstrated that venetoclax-sensitive cell lines and patient samples express B-cell markers, consistent with the CD2 subset of myeloma initially identified in the UAMS classification.¹⁸ This opens up the potential of using a flow cytometry panel to identify venetoclax-sensitive patients.

Mcl-1 as potential target in multiple myeloma

While targeting Bcl-2 will likely benefit a specific population of myeloma patients, Mcl-1 appears to be the primary family member on which myeloma cells are dependent and they are therefore a potentially more productive target.19 Additionally, the MCL1 locus is found at chromosome 1q21, a region of frequent gain or amplification in multiple myeloma, resulting in increased Mcl-1 expression.20 Several potent and selective Mcl-1 inhibitors have been developed and are currently in clinical trials,21 although early genetic studies in mice suggested that on-target cardiotoxicity could limit use.22 For instance, a phase 1 trial with oral MCL-1 inhibitor murizatoclax (AMG 397) was paused because one patient had high serum troponin C, a marker of cardiotoxicity.²³ Thus, considerations for dosing and patient selection (e.g. 1q gain)²⁴ may be necessary for Mcl-1 inhibitor development.

Targeting key proteins – future directions

Targeting key proteins that myeloma cells are dependent on for survival, such as Bcl-2 and Mcl-1, holds great promise, even in a disease where there are many active therapeutic options. However, understanding how Bcl-2 family proteins function in multiple myeloma, as well as finding accurate predictive biomarkers, holds the key to how to overcome challenges with combination therapies and on-target toxicities.

Vikas A. Gupta, MD, PhD is an Instructor in the Department of Hematology and Medical Oncology at Emory University School of Medicine and a practicing physician at Winship Cancer Institute of Emory University.

Lawrence Boise, PhD is the R. Randall Rollins Chair of Oncology and Professor in the Department of Hematology/ and Medical Oncology in the Emory University School of Medicine.  He also serves as the Associate Director of Education and Training at the Winship Cancer Institute of Emory University. 

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