Selinexor Prolongs Survival in Patient-Derived Xenograft Models of Down Syndrome Myeloid Leukemia

Background: Selinexor, an inhibitor of the nuclear exportin protein XPO1, has been shown to promote cell cycle arrest and induce apoptosis in acute myeloid leukemia (AML) cells. By inhibiting protein export from the nucleus to the cytoplasm, Selinexor causes nuclear accumulation of proteins including tumor suppressors and translation initiation factor eIF4E, resulting in reduced translation of anti-apoptotic proteins BCL2 and MCL1. Venetoclax, a BCL2 inhibitor, has been considered to synergize with Selinexor in preclinical studies of multiple myeloma (Nguyen et a., Blood, 138:2270, 2021), lymphoma and AML (Fischer et al., Blood Adv, 4:586, 2020). A clinical trial evaluating the combination of Selinexor and venetoclax in relapsed/refractory pediatric AML is currently underway (NCT04898894). However, children with Down syndrome, who have a 500-fold increased risk of developing acute leukemia are excluded from this trial.

Methods: The viability of mouse passaged Down syndrome myeloid leukemia (DS-ML) samples following exposure to Selinexor and/or venetoclax was evaluated ex vivo by flow cytometry using annexin V and propidium iodide staining. BLISS synergy scores were calculated using SynergyFinder. For the PDX models, 2-3 million cells were injected intravenously in NSG-SGM3 mice. The percentage of human cells in mouse peripheral blood was determined at periodic intervals by flow cytometry. Mice were randomly recruited to treatment groups (n=5 each) when the percentage of human CD45+ cells were first detected in blood. Selinexor (10 mg/Kg) and venetoclax (100 mg/Kg) were administered by oral gavage for 2 weeks and 4 weeks respectively. Mice were carefully monitored and euthanized at the first instance of pre-determined experimental endpoints. Kaplan-Meier survival estimates were generated using GraphPad Prism.

Results: We have generated and extensively characterized patient-derived xenograft models of DS-ML, and used for preclinical evaluation of epigenetic drug combination (azacitidine and panobinostat) (Barwe et al., Blood, 134:2683, 2019), and a combination of azacitidine and venetoclax (Barwe et al., Blood, 138:3452, 2021). Ex vivo studies showed a dose-dependent decrease in the viability of mouse passaged DS-ML cells treated with Selinexor or venetoclax. The estimated EC50 concentrations for Selinexor were 0.31 uM (NTPL-60), 0.53 uM (NTPL-386), and 1.01 uM (15384-029). Venetoclax reduced cell viability at EC50 concentrations ranging from 0.37 uM to 1.54 uM. A combination of these drugs was either additive (NTPL-60), mildly synergistic (NTPL-386) or antagonistic (15384-029) (Figure 1A).

We tested the efficacy of Selinexor and venetoclax alone or in combination in vivo in three distinct DS-ML PDX models. Selinexor treatment significantly increased median survival in all three models, while venetoclax was effective in two of three models (Figure 1B, *P<0.05, **P<0.01). However, unlike prior reports in multiple myeloma and AML, addition of venetoclax to the treatment regimen did not provide any survival advantage compared to Selinexor alone. This effect was also observed in the percentage of human CD45+ in blood over time, where human CD45+ blood counts were significantly higher in the combination than Selinexor treatment alone, indicating that Selinexor monotherapy was much more efficient in reducing leukemia burden.

Conclusion: We evaluated a combination of Selinexor and venetoclax in DS-ML PDX models ex vivo and in vivo. The combination did not show significant synergy in any of the models ex vivo. Selinexor showed significant improvement in median survival in each of the models and performed better than venetoclax alone. Selinexor venetoclax combination was not beneficial than Selinexor alone in DS-ML PDX models. Studies are in progress to identify the determinants of combinatorial response to Selinexor and venetoclax in DS-ML.

No relevant conflicts of interest to declare.