Effect of Lonidamine on the cytotoxicity of four alkylating agents in vitro

We examined the ability of lonidamine, which has been described as an inhibitor of cellular respiration and glycolysis, to enhance the cytotoxicity of alkylating agents to MCF-7 human breast-carcinoma cells. Lonidamine was increasingly cytotoxic to MCF-7 cells with increasing time of exposure. With a 12-h exposure, the IC50 for lonidamine was about 365 microM, and with a 24-h exposure it was about 170 microM. A drug concentration of 250 microM was chosen for use in the drug combination studies. Lonidamine appeared to have a dose-modifying effect on cisplatin (CDDP), producing increasingly supra-additive cell kill with increasing CDDP concentration. When simultaneously incubated with lonidamine for 1 h, 500 microM CDDP yielded a cell kill that was 2 log greater than additive cytotoxicity. Extending the exposure to lonidamine for 12 h after CDDP treatment led to a small, additional aliquot of cell kill of about 2.5-fold over the CDDP concentration range. Lonidamine also appeared to have a dose-modifying effect on melphalan cytotoxicity in the melphalan concentration range of 100-500 microM. Between concentrations of 10 and 100 microM melphalan, the drug combination survival after 1 h exposure fell within the envelope of additivity for the two agents. However, maintaining the presence of lonidamine for an additional 12 h increased the effect such that the combination was supra-additive over the entire concentration range of melphalan. Simultaneous exposure to 4-hydroperoxycyclophosphamide (4-HC) and lonidamine for 1 h resulted in greater than additive cell kill, and extending the lonidamine exposure period such that lonidamine was present during and 12 h after 4-HC treatment further increased this effect. Lonidamine had a moderate effect on the cytotoxicity of carmustine (BCNU) with a 1 h simultaneous exposure; however, this treatment combination reached greater than additive cytotoxicity only at the highest concentration of BCNU tested. Extending the lonidamine exposure time for an additional 12 h resulted in supra-additive cell kill over the BCNU concentration range. Therefore, when lonidamine was present during exposure to the alkylating agent and its presence was then extended for an additional 12 h, a synergistic cell kill was produced with all four alkylating agents tested.