We do not usually report the results of test tube experiments, but found this research so fascinating that we decided to make an exception.

WICHITA, KANSAS. Several studies have shown that intravenous infusions of vitamin C (ascorbic acid) can increase survival times among terminal cancer patients. Researchers at the Bio-Communications Research Institute now report on a test tube experiment designed to see if vitamin C actually kills cancer cells in vitro. SW620 human colon cancer cells were grown in cell cultures and then exposed to various concentrations of vitamin C solutions. A two-day treatment with vitamin-C at a concentration of 33.7 mM killed 75 per cent of all cancer cells while a concentration of about 10 mM (200 mg/dL) killed about 50 per cent. Unfortunately, even infusions of up to 60 grams of vitamin C solution in a human volunteer did not result in a comparable level (10 mM) in blood plasma.
The researchers conclude that it may not be possible to kill enough cancer cells by vitamin C infusion. However, they discovered that vitamin-K and, in particular, lipoic acid were highly efficient in killing colon cancer cells. Using a 10:1 ratio of vitamin C to lipoic acid had a synergistic effect and this mixture killed 50 per cent of all cancer cells at a concentration of only 4.5 mM. The researchers conclude that it would be feasible to obtain this concentration by intravenous infusion and urge further work to investigate the use of a combination of vitamin C and lipoic acid as an anti-cancer agent in humans.
Casciari, J.J., et al. Cytotoxicity of ascorbate, lipoic acid, and other antioxidants in hollow fibre in vitro tumours. British Journal of Cancer, Vol. 84, No. 11, June 1, 2001, pp. 1544-50