Benchmarking effects of mTOR, PI3K, and dual PI3K/mTOR inhibitors in hepatocellular and renal cell carcinoma models developing resistance to sunitinib and sorafenib.

PURPOSE: To evaluate first-generation rapamycin analogs (everolimus, temsirolimus, and rapamycin) and second-generation drugs inhibiting mTOR kinase (AZD-8055), PI3K (BKM-120) or both (BEZ-235 and GDC-0980) in hepatocellular carcinoma (HCC) and renal cell carcinoma (RCC) cells characterized for acquired resistance to sorafenib or sunitinib.

METHODS: Anti-proliferative (MTT assay) and cell signaling (Western blot) effects of rapamycin analogs (1-20 μM) and second-generation drugs (0.03-20.0 μM) were assessed in human HCC SK-HEP1, RCC 786-0, and sorafenib- (SK-Sora) or sunitinib-resistant (786-Suni) cells.

RESULTS: In SK-HEP1 cells displaying high PTEN and Bcl2 expression, rapamycin analogs had poor anti-proliferative effects. However, SK-Sora cells were more sensitive to rapamycin analogs (≥1 μM) than SK-HEP1 cells. In 786-0 cells, lacking PTEN and Bcl2 expression, ≥1 μM rapamycin analogs blocked mTORC1 signaling, transiently activated Akt, and inhibited cell proliferation. Protracted sunitinib exposure in 786-Suni cells yielded an increase in p27 expression and a decreased sensitivity to rapamycin analogs, although mTORC1 function could be inhibited with rapamycin analogs. Second-generation drugs induced more potent growth inhibition than rapamycin analogs at concentrations >0.03 μM in parental cells, SK-Sora, and 786-Suni cells. Growth inhibitory concentrations of these new drugs also blocked mTORC1 downstream targets.

CONCLUSIONS: Rapamycin analogs inhibited mTORC1 downstream targets and yielded anti-proliferative effects in HCC and RCC cells. Second-generation drugs also appeared to be potent inhibitors of mTORC1 signaling; however, they appeared to be far more potent in inhibiting cellular proliferation in parental HCC and RCC cells and in cells developing resistance to sorafenib or sunitinib.