In Vitro and In Vivo Antileukemic Effects of the mTOR-Targeting Drug Rapamycin in Patients with Imatinib-Resistant CML.

The mammalian target of rapamycin (mTOR) has been described to be activated in BCR/ABL-transformed cells and to mediate rapamycin-induced inhibition of cell growth. We have recently shown that rapamycin downregulates expression of vascular endothelial growth factor (VEGF), a mediator of leukemia-associated angiogenesis, in primary CML cells. In the current study, we analyzed growth-inhibitory in vitro- and in vivo effects of rapamycin in leukemic cells in patients with imatinib-resistant CML, and asked, whether the rapamycin-induced suppression of VEGF in these cells is related to growth-inhibition. Rapamycin dose-dependently inhibited the in vitro growth of primary CML cells in all pts with imatinib-resistant CML tested (rapamycin, 10 nM: 50±17% of control; p<0.05). Moreover, rapamycin was found to downregulate growth of Ba/F3 cells exhibiting wild type (wt) BCR/ABL or various imatinib-resistant mutants of BCR/ABL including the T315I-mutation, which renders BCR/ABL resistant against tyrosine kinase inhibitors. We also found that rapamycin and imatinib exert synergistic inhibitory effects on growth of Ba/F3p210^WT cells as well as on Ba/F3 cells containing imatinib-resistant mutants of BCR/ABL. In all cells tested including imatinib-resistant primary cells, rapamycin was found to downregulate expression of VEGF mRNA and secretion of the VEGF protein. However, exogenously added VEGF did not counteract the rapamycin-induced decrease in proliferation suggesting that these two effects of rapamycin are separable. In a consecutive phase of the project, we treated 7 pts with imatinib-resistant CML with rapamycin at 2 mg per os daily for 14 consecutive days, with dose-adjustment allowed to reach a target serum rapamycin concentration of 10–20 ng/ml. A major leukocyte response with decrease from >20,000 to normal values (<10,000/μL) was obtained in 3 pts, and a minor transient response in 2 other pts, whereas 2 pts did not respond. No molecular or cytogenetic responses were seen. No severe side effects of rapamycin were recorded. In aggregate, rapamycin may be an interesting targeted drug in imatinib-resistant CML. However, combinations with other targeted (or conventional) drugs may be required to obtain sufficient antileukemic activity in vivo.