The Combination of the Autophagy Inhibitor Chloroquine and Recombinant Methioninase Has Selective Synergistic Efficacy on Human Colon Cancer Cells But Not on Normal Human Fibroblasts

Abstract

Background/aim: Metastatic colon cancer is a recalcitrant disease with a 5-year survival rate of 15.7-26.0%, and more effective treatments are necessary. Methionine addiction is a fundamental and general hallmark of cancer. Targeting methionine addiction with recombinant methioninase (rMETase) is effective against colon-cancer cells. Combining the autophagy inhibitor chloroquine with rMETase has shown efficacy on breast cancer and osteosarcoma cells. The present study aimed to determine whether the combination of chloroquine and rMETase is selectively synergistic against colon-cancer cells in contrast to normal fibroblasts.

Materials and methods: The present study used the human colon-cancer cell line HCT-116 and Hs27 normal human fibroblasts. Cells (1×10³) were seeded in each well of 96-well plates and cultured in Dulbecco's modified Eagle's medium (DMEM) with 10% fetal bovine serum at 37°C in 5% CO₂ for 24 h. Cells were then treated with rMETase at concentrations ranging from 0.0625 U/ml to 8 U/ml or chloroquine at concentrations ranging from 0.5 μM to 128 μM for 72 h. Drug-sensitivity curves were generated using a cell viability assay with the WST-8 reagent. The half-maximal inhibitory concentration (IC₅₀) of rMETase and chloroquine against HCT-116 and Hs27 cells was calculated. Cell viability of cells treated with rMETase alone, chloroquine alone, and the combination of rMETase and chloroquine at IC₅₀ concentrations was assessed.

Results: The IC₅₀ concentrations of rMETase on HCT-116 and Hs27 were 0.61 μM and 0.67 μM, respectively. The IC₅₀ concentrations of chloroquine on HCT-116 and Hs27 were 7.52 U/ml and 10.85 U/ml, respectively. Cell viability assays using these IC₅₀ concentrations showed that the combination of rMETase and chloroquine had synergistic efficacy on HCT-116 cells, but not on Hs27 cells.

Conclusion: The combination of chloroquine and rMETase had synergistic efficacy on colon-cancer cells, but not on normal fibroblasts. rMETase is used clinically as an oral supplement, and chloroquine is a drug approved for clinical use. Therefore, the combination of rMETase and chloroquine has the potential for safe clinical application.

Keywords: Chloroquine; Hoffman effect; colon cancer; methionine addiction; normal fibroblasts; recombinant methioninase (rMETase); synergy.