Rapamycin Promote the Inhibition Rate of Proliferation on SHI-1 Cells and Enhance the Efficacy of Chemotherapy in Npg Leukemia Mice

Acute myeloid leukemia (AML) is a malignant hematologic disease. The remission rate of traditional chemotherapy is about 70-80%. In order to improve the efficacy of chemotherapy, some new drug which could enhance the efficacy of chemotherapy should be explored. Rapamycin is an immunosuppressive agent which can regulate cell proliferation, autophagy and other activities by inhibiting mTOR signal pathway. The studies of rapamycin as an anti-leukemia agent were limited. We had successfully establish a leukemia model in NPG mice with systemic leukemia infiltration using the human acute monocytic leukemia cell line SHI-1 cells. So in this study, the rapamycin and/or chemotherapy were used to treat the SHI-1 cells in vitro and NPG leukemia mice to investigate the efficacy of rapamycin in the treatment of AML. Rapamycin alone could effectively inhibit the proliferation of SHI-1 cells in vitro, and the inhibition rate were 35.4% which lower than the inhibition rate chemotherapy group (49.9%) using cytarabine and adriamycin. The combination with rapamycin and chemotherapy could significantly enhanced the inhibition rate to 60.3% (P<0.05) (Fig A). At day 19 after SHI-1 were inoculated to the NPG mice from tail vein, nearly 1% CD45 and CD33 positive cells were found in the peripheral blood. Then the NPG mice were divided to 4 groups to receive treatment. Control group without any treatment, rapamycin group were intraperitoneally injected with rapamycin (10mg/kg) every day for 5 days, chemotherapy group were intraperitoneally injected with cytarabine (100mg/kg for 5 days) and adriamycin (1mg/kg every other day for 3 times), combined group were treated with rapamycin, cytarabine and adriamycin. At the first day after treatment, one NPG mice were sacrificed randomly in four groups. The weight of the spleen in control group, rapamycin group, chemotherapy group and combination group were 78.6 mg, 48.9 mg, 16.1 mg, 11.7 mg respectively (Fig B). The ratio of human CD33 and CD45 positive cells in peripheral blood, bone marrow and spleen of NPG mice were 20.2%, 23.2% and 16.8% in control group, 2.02%, 2.68% and 3.31% in rapamycin group, 13.6%, 14.2% and 5.50% in chemotherapy group, 0.31%, 2.49% and 1.22% in combined group respectively (Fig C). Histopathological results showed that the degree of leukemia infiltration in the organs of NPG in the rapamycin group, chemotherapy group, and the combined group was significantly less than that of the control group. The median survival time of NPG mice in control group was 30.5 days. The treatment of rapamycin or chemotherapy could significantly prolong the media survival time of NPG mice to 35.0 and 34.0 days (P<0.05). When combined rapamycin with chemotherapy, the life time of NPG mice were significantly extended to 39 days (P<0.05) (Fig D). When NPG mice were died, more neoplasms were grow in the organs such as kidney, liver, stomach, mesenteric, mediastinum, bladder, spine, skin and neck in the NPG mice of control group, the treatment of rapamycin, chemotherapy and combined treatment would significantly decrease the counts of infiltrative organ with leukemia cells (Fig E). Altogether, our studies verified that rapamycin could inhibit the growth of SHI-1 cells and enhance the efficacy of chemotherapy in vitro. In NPG leukemia mice, the treatment of rapamycin alone could gain the efficacy as in chemotherapy group, when combined with chemotherapy, rapamycin had synergistic effect to significantly enhance the efficacy of chemotherapy to prolong the median survival time and decrease the degree of leukemia infiltration in NPG mice.

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