The mTOR Inhibitor RAD001 (Everolimus) Improves Survival in Preclinical Models of Primary Human ALL.

The Akt/mammalian target of rapamycin (mTOR) signaling pathway is a potential target in acute lymphoblastic leukemia (ALL) due to its constitutive expression and critical role in regulating leukemic cell growth and survival. In this study, we investigated whether RAD001 (Everolimus, Novartis), is effective as a therapeutic agent in a murine model of human ALL. Leukemic cells from 5 separate patient samples were injected into sub-lethally irradiated NOD/SCID mice and treatment was started after leukemia was clearly detectable in the blood (>5% peripheral blasts) and continued for 4 weeks. In 2 of 3 cases examined, animals treated with RAD001 as a single agent (5mg/kg, 3 times weekly) showed 2 to12-fold reduction in the number of circulating blasts and 2.5 to 30-fold decrease in splenomegaly after only 7 days of treatment (p<0.01). In 1 case, blasts completely disappeared by 14 days of treatment accompanied by recovery of normal hematopoiesis. In contrast vehicle treated mice showed progressive leukemia. In all 5 cases examined, decreased leukemic burden was associated with increased survival, with a median survival of 52 days in the RAD001 treated mice compared to 24 days in the control group (p<0.02 for all samples). Combination treatment with RAD001 and Vincristine (0.15–0.25 mg/kg, weekly) was more effective than either agent alone in reducing leukemic cell count and prolonging survival (p<0.02 compared to Vincristine alone). In one case, all mice treated with either RAD001 alone or RAD001 plus Vincristine survived for greater than 10 weeks following the completion of treatment and only one animal (RAD001 treated) showed signs of disease on elective sacrifice. Cell cycle analysis of cells recovered from the spleens of RAD001 treated animals revealed an arrest in the G₀/G₁ phase. Furthermore, electron microscopy demonstrated the predominant appearance of autophagy as well as limited apoptosis in cells within the sternums of treated animals. In conclusion, we have demonstrated the efficacy of RAD001 alone or with a subtherapeutic dose of Vincristine in an in vivo leukemia model. These preclinical results support further clinical development of mTOR inhibitors for the treatment of ALL patients. RAD001 may provide significant therapeutic benefit when used alone, or in combination with standard chemotherapy agents such as Vincristine.