Vitamin C may inhibit cancer fighting drugs: study
According to results of an in vitro study, American researchers found that the antioxidant vitamin may reduce the number of cell deaths following chemotherapy by up to 70 per cent.
Results of the study are published in the journal Cancer Research.
The study adds to the ongoing controversy over antioxidant supplementation for people receiving chemotherapy, with some studies reporting potential benefits while others report that the supplements may have a detrimental effect.
Commenting independently on the research, Dr Joanna Owens from the British charity Cancer Research UK, said “these are interesting but early results”.
"As yet, there is not enough evidence to know whether antioxidants such as vitamin C are helpful or harmful during cancer treatment.
"It is possible that high doses of antioxidants can make treatment less effective, but until we know for sure our advice is to try and get the vitamins you need through a balanced and varied diet rather than through vitamin supplements."
Study details
Using lymphoma and leukemia cell lines with and without pre-exposure to dehydroascorbic acid (DHA), Heaney and his co-workers investigated the effects of the anti-cancer drugs doxorubicin, cisplatin, vincristine, methotrexate, and imatinib.
DHA is the form of vitamin C that gets into cells, explained the researchers.
They found that the efficacy of the chemotherapy drugs tested was greatly reduced if the cells were pre-treated with vitamin C, compared to untreated cancer cells. Indeed, the reduction ranged from 30 to 70 per cent, report the researchers.
When tested in mice, similar results were observed, with rapid tumour growth reported in mice that were given cancer pre-treated with vitamin C.
The role of antioxidants
Antioxidants, such as vitamins A, C and E and carotenoids like beta-carotene, are believed to exert a protective effect on cells. They scavenge toxic molecules called free-radicals, which cause oxidative stress and can lead to DNA cell damage.
However, the aim of some classes of chemotherapy drugs is to produce free radicals to promote death of the cancer cells. By mopping up the free radicals, vitamin C may protect cancer cells from the cancer drugs.
This theory was disproved as the researchers report that this was not occurring in their experiments. They found instead that DHA was restoring viability to the cancer cell’s damaged mitochondria – the cell’s power plant, which, when injured, sends signals to force a cell to die.
“Vitamin C appears to protect the mitochondria from extensive damage, thus saving the cell,” said Mark Heaney, lead author of the new report. “And whether directly or not, all anticancer drugs work to disrupt the mitochondria to push cell death.”
The amount of DHA used in the experiments resulted in a build-up in the cells that could be achieved by cancer patients using large supplemental doses of vitamin C, said Heaney.
“We recognized that DHA is the form of vitamin C that gets into cells, and that the tumour microenvironment allows cancer cells to convert more vitamin C into DHA,” he said. “Inside the cell, DHA is converted back into ascorbic acid, and it gets trapped there and so is available to safeguard the cell.”
Only preliminary findings
Pamela Mason, scientific advisor to the Health Supplements Information Service, is quoted by the BBC as saying no conclusions could be drawn from the study until similar findings are achieved in human studies.
Source: Cancer Research Volume 68, Pages 8031-8038, doi: 10.1158/0008-5472.CAN-08-1490“Vitamin C Antagonizes the Cytotoxic Effects of Antineoplastic Drugs”Authors: M.L. Heaney, J.R. Gardner, N. Karasavvas, D.W. Golde, D.A. Scheinberg, E.A. Smith, O.A. O'Connor
