The 23rd Congress of the European Hematology Association (EHA) took place in Stockholm from 14-17 June 2018. On Sunday 17 June, an oral session took place during which Esperanza M. Algarín from the University Hospital of Salamanca , Salamanca, Spain, presented results from an in-vitro and in-vivo preclinical study on the effects of targeting pro-survival proteins BCL-2 and MCL-1.
BCL-2 and MCL-1 belong to the BCL-2 protein family, which regulates the intrinsic apoptotic pathway through complex interactions between apoptotic and anti-apoptotic members. Binding of the pro-apoptotic protein BIM to either BCL-2 or MCL-1 inhibits apoptosis. Venetoclax, a BCL-2 inhibitor, sequesters BIM and induces apoptosis. Venetoclax is currently used to treat chronic lymphocytic leukemia (CLL) and is being studied as a possible therapy for multiple myeloma (MM) patients in several clinical trials.
It is known that MM cells specifically overexpress MCL-1, rather than BCL-2. Inactivation of MCL-1 via gene editing in MM cell lines led to reduced tumor cell viability, demonstrating a dependence on MCL-1 for survival. Based on these results, it is argued that specific MCL-1 inhibition, using a compound called S63845, could be an effective way to induce MM cell apoptosis.
In this study, BCL-2 was targeted with venetoclax, and MCL-1 was targeted with S63845. Using both in-vitro assays and in-vivo models the study set out to define the efficacy and mechanism of action of S63845 and venetoclax in myeloma, both as monotherapies, and in combination.
Efficacy of monotherapies:
- Both S63845 and venetoclax show strong anti-myeloma dose-dependent effects in-vitro (S63845 in the nM range and venetoclax in the µM range) on nine representative MM cell lines) and ex-vivo treating plasma cells (PCs) and lymphocytes of three MM patients (both in the nM range)
- S63845 and venetoclax seem to inhibit the interaction of the pro-apoptotic protein BIM with MCL-1 and BCL-2, respectively
- Immunoprecipitation (IP) studies with BIM reveal that treatment with S63845 and venetoclax individually does not significantly decrease expression of MCL-1 or BCL-2, but do disrupt MCL-1 or BCL-2/BIM complexes, leading to a shift in MM cell dependence as a potential escape mechanism
Efficacy of combination therapies:
- Pronounced synergistic effect of both S63845 and venetoclax, reflected by:
- increased apoptosis in an in-vitro assay (four MM cell lines)
- increased apoptosis in an ex-vivo assay (PCs and lymphocytes of three MM patients)
- increased elimination of luciferase-labelled myeloma cells injected into an immunodeficient mouse strain (BALB/Ca-Rag2null;Il2rgnull = BRG mouse)
- S63845 and venetoclax simultaneously impair the interactions of MCL-1 and BCL-2 with BIM, thus preventing escape; this was shown by IP studies with BIM revealing that dual administration of S63845 and venetoclax lead to a reduction in both MCL-1 and BCL-2 as well as disrupting both BCL-2/BIM and MCL-1/BIM complexes
This study shows that S63845 and venetoclax have anti-myeloma activity as single agents and show a strong synergy when used in combination. Targeting the pro-survival protein MCL-1, which is expressed at high levels in myeloma cells, together with BCL-2, could provide an effective future treatment for MM patients.
Questions & Answers
The following questions were asked after the presentation:
Q1. Do you think this type of inhibition will be effective in particular subtypes of patients?
A1. The levels of the different proteins belonging to the BCL-2 family should be firstly studied in the different populations of patients to establish if there is a strong association between expression levels and patient subgroups.
Q2. Did you look at all at the expression of BCL-XL in any of your cell lines?
A2. Targeting BCL-XL has been shown to lead to thrombocytopenia. There is a compensation of these three proteins but I think that in myeloma MCL-1 and BCL-2 play a major role. There is some kind of compensation, but it doesn’t appear to be that important.
Q3. You showed toxicity with regards to the effects on lymphocytes but potentially targeting both MCL-1 and BCL-2 will have effects on hematopoiesis. Did you check this?
A3. Ex-vivo we have not seen a high percentage of apoptotic lymphocytes and in-vivo the mice do not present any symptoms, although it is true that MCL-1 inhibitor does not bind the same way in mice as in humans. It is necessary to explore the combination in clinical trials to understand the toxicity of the combination.
Q4a. You showed data from three primary samples from three patients. Patient 2 seemed more sensitive to the combination. Was there a difference in clinical features? Was this a relapsed patient or newly diagnosed compared to the others?
A4a. The second patient was the one who had the translocation t(11;14) and responded to venetoclax in monotherapy and I think because venetoclax has also been effective in this patient the potentiation was higher with the double combination.
Q4b. So do you see potentiation in the context of t(11;14) as well?
A4b. In one patient with t(11;14) the two drugs were effective So, the combination is going to be better than if there was only one drug acting. However, even in the non t(11;14) patient or in the one with t(11;14) but non-responsive to venetoclax, we saw a potentiation, although less than in the first patient. So we think that the combination might be effective regardless of the presence of this abnormality.