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B.Z. Carter et al.
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Figure 6. Combined inhibition of BCL-2 and MCL-1 has strong anti-leukemia activity and prolongs survival in a patient-derived xenograft model of acquired venetoclax/decitabine-resistant acute myeloid leukemia. (A) Patient-derived xenograft (PDX) model and experimental scheme. (B) Circulating human CD45+ (hCD45+) cells in mouse peripheral blood (PB) samples obtained from each treatment group on treatment day (d) 18. (C) hCD45+ cells in mouse bone marrow (BM) samples obtained from each treatment group on treatment d 25. (D) hCD45+ cells in mouse spleens and spleen sizes of each treatment group on treatment d 25. hCD45+ cells were measured by flow cytom- etry. (E) Survival curves. (F) BM leukemia cell clusters based on the expression of cell surface markers and viable leukemic cells in various cell populations on treatment d 25 in each treatment group as determined by cytometry by time of flight (CyTOF). *P≤0.05; **P≤0.01; ***P≤0.001; ****P≤0.0001. Results are expressed as the mean ± stan- dard error of the mean. Survival data were analyzed using the log-rank test. Con: control; VEN: venetoclax; 5991: AZD5991; 4573: AZD4573.
WT1- or BCORL1-mutated cells was too small to draw definitive conclusions. Additional studies are needed to confirm our finding, which may help guide patient selec- tion.
In contrast to a recent study showing that venetoclax- resistant cells are more sensitive to the MCL-1 inhibitor VU0661013,30 we found that intrinsic or acquired veneto- clax resistant AML cells were less responsive to the MCL- 1 inhibitor AZD5991, but the combination of venetoclax and AZD5991 or AZD4573 synergistically induced cell death regardless of the single-agent response, both in vitro and in vivo. These data suggest that patients whose disease is resistant to, or relapses from, venetoclax therapy may
potentially benefit from co-targeting MCL-1 and BCL-2. In order to understand the mechanisms of synergy, especially in cells insensitive to BCL-2 or MCL-1 inhibi- tion alone, we examined the interactions of anti- and pro- apoptotic BCL-2 proteins in OCI-AML3 cells treated with venetoclax, AZD5991, or both. Targeting either BCL-2 or MCL-1 decreased the interactions of one of them with pro-apoptotic BCL-2 proteins, but increased the interac- tions of the other with pro-apoptotic proteins. Only when both are inhibited, maximal amounts of unbound activa- tor protein BIM and effector proteins BAX/BAK become
available, and more effectively induce apoptosis. Interestingly, Pollyea et al. recently attributed the effec-
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haematologica | 2022; 107(1)