Synergy of Rapamycin and Methioninase on Colorectal Cancer Cells Requires Simultaneous and Not Sequential Administration: Implications for mTOR Inhibition.

Background/aim: Rapamycin inhibits the mTOR protein kinase. Methioninase (rMETase), by degrading methionine, targets the methionine addiction of cancer cells and has been shown to improve the efficacy of chemotherapy drugs, reducing their effective doses. Our previous study demonstrated that rapamycin and rMETase work synergistically against colorectal-cancer cells, but not on normal cells, when administered simultaneously in vitro. In the present study, we aimed to further our previous findings by exploring whether  synergy exists between rapamycin and rMETase when used sequentially against HCT-116 colorectal-carcinoma cells, compared to simultaneous administration, in vitro.

Materials and methods: The half-maximal inhibitory concentrations (IC₅₀) of rapamycin alone and rMETase alone against the HCT-116 human colorectal-cancer cell line were previously determined using the CCK-8 cell viability assay (11). We then examined the efficacy of rapamycin and rMETase, both at their IC₅₀, administered simultaneously or sequentially on the HCT-116 cell line, with rapamycin administered before rMETase and vice versa.

Results: The IC₅₀ for rapamycin and rMETase, determined from previous experiments (11), was 1.38 nM and 0.39 U/ml, respectively, of HCT-116 cells. When rMETase was administered four days before rapamycin, both at the IC₅₀, there was a 30.46% inhibition of HCT-116 cells. When rapamycin was administered four days before rMETase, both at the IC₅₀, there was an inhibition of 41.13%. When both rapamycin and rMETase were simultaneously administered, both at the IC₅₀, there was a 71.03% inhibition.

Conclusion: Rapamycin and rMETase have synergistic efficacy against colorectal-cancer cells in vitro when administered simultaneously, but not sequentially.