PF159 COMBINATION OF SELECTIVE GAMMA‐SECRETASE INHIBITOR MRK‐560 WITH OTHER TARGETED THERAPY IS AN EFFECTIVE AND SAFE TREATMENT FOR T‐ALL
Authors: I. Govaerts, O. Gielen, R. Habets, C. de Bock, K. De Keersmaecker, H. Segers, A. Uyttebroeck, J. Maertens, N. Boeckx, B. De Strooper, J. Cools
Journal: HemaSphere (2019)

Background: T‐cell acute lymphoblastic leukemia (T‐ALL) is an aggressive malignancy of the lymphocytes that mainly affects children. High cure rates can be achieved in pediatric T‐ALL at the cost of long‐term side‐effects of conventional chemotherapy. Despite progress in the past years, cure rates remain below 50% in adults and for refractory or relapsed patients the prognosis remains dismal. Given that over 60% of T‐ALL patients carry activating mutations in NOTCH1, and that mutant NOTCH1 requires cleavage by the g‐secretase complex for its oncogenic activity, gamma‐secretase inhibitors have been investigated for the treatment of T‐ALL. MRK‐560 is a selective inhibitor of Presenilin 1 containing gamma‐secretase complexes for which we have recently shown that it has potent anti‐leukemia efficacy with minimal effects on normal cells and hence little gastro‐intestinal toxicity compared to broad gamma‐secretase inhibitors. Aims: We investigated whether MRK‐560 could act synergistically with other targeted inhibitors for the treatment of T‐ALL patients with NOTCH1 mutations. We tested the combination of MRK‐560 with a general antineoplastic inhibitor such as the exportin1‐inhibitor KPT‐8602 or a kinase inhibitor specifically chosen based on the mutational landscape of the T‐ALL patient. Methods: We cultured T‐ALL cell lines in vitro in the presence of MRK‐560, ruxolitinib (JAK inhibitor), imatinib (ABL1 inhibitor) or KPT‐8602 (XPO‐1 inhibitor) alone or in combination to assess cell growth and viability. We selected 3 patient‐derived T‐ALL xenograft samples with NOTCH1 or FBXW7 mutation and a mutation in JAK1/3 or an ABL1 fusion. These samples were treated in vivo with MRK‐560, ruxolitinib, imatinib and/or KPT‐8602. We monitored disease progression by measuring the percentage of human CD45+ cells in the blood of the mice and bio luminescence imaging of Luciferase‐positive PDX samples. Endpoint analysis included survival as well as leukemic infiltration in bone marrow and spleen. To evaluate gastro‐intestinal toxicity we weighed the mice on a weekly basis and performed histological analysis of the small intestine with Periodic acid‐Schiff staining. Results: Treatment of T‐ALL cell lines with MRK‐560 in combination with a second inhibitor resulted in a strong inhibition of proliferation that was more profound than the effect of either drug alone in cell lines that depend on NOTCH1 signaling. In PDX samples carrying a mutation in NOTCH1 or FBXW7 we observed a significantly stronger reduction of disease burden when MRK‐560 was combined with ruxolitinib, imatinib or KPT‐8602 compared to vehicle or single drug treatment. Moreover, the mice that were treated with a combination regimen (MRK‐560 + KPT‐8602, ruxolitinib or imatinib) had a significantly longer survival than with single drug treatment. There was no difference in weight or abundance of goblet cells in the gut of the mice from the different treatment groups, indicating that MRK‐560 was not toxic as single drug and also not in the combinations. Summary/Conclusion: Combination of the selective gamma‐secretase inhibitor MRK‐560 with other targeted therapies effectively inhibits T‐ALL cell growth in vitro as well as in vivo. Selective targeting of the gamma‐secretase complex is well tolerated and toxicity is not increased when MRK‐560 is combined with ruxolitinib, imatinib or KPT‐8602. These findings create new targeted treatment options for a large cohort of T‐ALL patients.