Fig. 7. Synergistic activity of venetoclax and S63845 in patient-derived xenograft ALL.

Combination effects of venetoclax and S63845 were evaluated in primary BCP-ALL xenograft samples. A Cell death rates of patient-derived xenograft ALL cells were analyzed by propidium iodide staining following exposure of cells to venetoclax and/or S63845 at increasing concentrations for 24 h in triplicates. B Synergyfinder was used to visualize combination effects (red indicating synergism, white additive effect and green antagonism) and to calculate δ-scores based on the Bliss independence model. The average synergy scores over the dose-response matrix for each sample are shown. C Experimental treatment schematic and human CD45⁺ (huCD45⁺) cells in mouse peripheral blood. NOD/SCID mice were transplanted with a high-risk leukemia (PDX-18) derived from an infant ALL and treatment was started upon manifestation of ≥ 5% human leukemia cells in the peripheral blood (huCD45⁺). Engrafted mice (N = 3–5 per group) were treated with vehicle, venetoclax (25 mg/kg per day), S63845 (25 mg/kg per day) or the combination of both compounds for five days per week for two consecutive weeks as indicated in the scheme. Leukemia burden was analyzed by (D) spleen weight and (E) absolute huCD45⁺ cell count in spleen, (F) bone marrow and (G) central nervous system of the mice. Violin plots show individual data points of single mice and median (solid) and quartile (dotted) lines for each group. Unpaired two-tailed Student’s t test was used to calculate p values. H When ALL cells are exposed to venetoclax (VEN), pro-apoptotic BIM is displaced from BCL-2 to MCL-1. Conversely, increased BCL-2/BIM binding is present upon exposure to S63845. Combined BCL-2 and MCL-1 blocking proficiently induces release of the apoptosis activator BIM leading to activation of BAX and BAK and triggering of downstream apoptosis signaling.