Vitamin C Antagonizes the Cytotoxic Effects of Antineoplastic Drugs.

Vitamin C is an antioxidant vitamin that has been hypothesized to antagonize the effects of antineoplastic drugs that generate reactive oxygen species. Leukemia (K562) and lymphoma (RL) cells pre-treated with dehydroascorbic acid, the commonly transported form of vitamin C, demonstrated a dose-dependent attenuation of cytotoxicity after treatment with the widely-used antineoplastic drugs, doxorubicin, cisplatin, vincristine, methotrexate and imatinib that ranged from 30–80% as measured by trypan blue exclusion and colony formation in methylcellulose. While treatment with vitamin C alone did not appreciably alter tumor growth compared with untreated mice with xenogeneic tumors (RL) (p=0.114), mice treated with vitamin C prior to doxorubicin administration had tumors that were, on average, four times larger than tumors in mice treated with doxorubicin alone (p=0.01) at day 32 of treatment. This result indicates that vitamin C treatment can attenuate antineoplastic therapy in vivo. Pretreatment of cells with vitamin C led to a dose-dependent decrease in apoptosis with all agents tested. At the highest concentrations of intracellular vitamin C, the decrease in apoptosis ranged from 37–82% as measured by TUNEL. There was no change in P-glycoprotein expression in vitamin C-treated cells and no difference in doxorubicin concentrations in the tumors of vitamin C-treated mice suggesting that the inhibitory effects of vitamin C are not due to generalized effects of antineoplastic drug uptake or efflux. When K562 and RL cells were treated with antineoplastic agents, we found that, as has been previously reported, doxorubicin and, to a lesser extent, cisplatin, increased the intracellular levels of ROS, while other agents had little discernible effect on ROS levels. In cells treated with chemotherapeutic drugs, as well as in untreated cells, pretreatment of cells with vitamin C caused a small (<15%) reduction in intracellular levels of ROS. This modest reduction in intracellular ROS levels appeared to be minor in comparison to the reduction in cytotoxicity. While treatment with N-acetylcysteine only attenuated the cytotoxic effects of cisplatin, vitamin C pretreatment attenuated the effects of all drugs tested, suggesting a distinct mechanism of action. We found that all of the antineoplastic agents tested caused rapid mitochondrial membrane depolarization. Treatment with vitamin C prior to exposure to antineoplastic agents prevented mitochondrial membrane depolarization. Mitochondrial membrane potentials in vitamin C-treated cells 6 hr after treatment with chemotherapeutic drugs were similar to untreated control cells (p<0.003). A mitochondrial site of action was further supported by the finding that vitamin C antagonizes the cytotoxic effects of actinonin, an inhibitor of peptide deformylase. These findings suggest that vitamin C supplementation during cancer treatment may detrimentally affect therapeutic response to a broad range of anti-cancer drugs and point to preserved mitochondrial membrane potential as a mechanism of action.