Cancer patients who take tumour-fighting green tea products may unwittingly be reducing the effectiveness of one kind of chemotherapy treatment, new research suggests.
Green tea is known to contain a powerful antioxidant that can help to kill cancer cells and prevent tumours growing.
For this reason many cancer patients drink green tea or take supplements containing green tea extracts.
But according to new evidence those being treated with one common chemotherapy drug could be making a serious mistake.
Laboratory and animal experiments have shown that combining green tea extract and the drug bortezomib cancels out the therapeutic effects of both.
In tests, each ceased to have any significant impact on cancer cells, allowing almost 100% of them to survive.
The key active compound in green tea is an antioxidant polyphenol plant chemical called epigallocatechin gallate (EGCG).
Animal research has shown EGCG to have potent anti-cancer properties, with effects demonstrated against leukaemia as well as lung, prostate, colon and breast cancers.
A key action of the compound is to unfreeze a process of natural cell-suicide called apoptosis, which assists the death of tumours. Bortezomib, marketed as Velcade, also works by inducing tumour cell death.
Researchers in the US set out to see whether combining green tea extract and bortezomib might deliver an extra-effective double-blow to two cancer types - multiple myeloma, which affects blood cells, and the malignant brain tumour glioblastoma.
Unexpectedly, both laboratory and live mouse experiments produced the precise opposite outcome. Instead of killing more cancer cells, the treatment allowed nearly all of them to survive and multiply.
Dr Axel Schonthal, from the University of Southern California Keck School of Medicine, said: "Our surprising results indicate that green tea polyphenols may have the potential to negate the therapeutic efficacy of bortezomib.
"The current evidence is sufficient enough to strongly urge patients undergoing bortezomib therapy to abstain from consuming green tea products, in particular the widely available, highly concentrated green tea and EGCG products that are sold in liquid or capsule form."
Bortezomib belongs to a family of drugs called proteasome inhibitors that prevent the breakdown of certain cellular proteins.
The scientists found that boronic acid in bortezomib allowed EGCG to bind directly onto the drug's molecules, blocking the anti-cancer effects of both the chemotherapy agent and the green tea compound.
The results, reported online today in the journal Blood, were seen at the equivalent of green tea concentrations commonly available in supplements. Just two to three 250 milligram capsules of green tea extract might have the same effect on bortezomib therapy in a human patient, said the researchers.
They added that EGCG may also be expected to reduce some of the usual side effects of bortezomib, causing patients to feel better and encouraging them to up their dose of green tea supplements.
However, the scientists stressed that the same adverse reaction did not occur when EGCG was combined with several other non-boronic acid based proteasome inhibitors, including the HIV treatment nelfinavir (Viracept). For patients taking these drugs, green tea extracts may prove an effective addition to their treatment.
"Although the study has exposed detrimental effects of green tea in specific combination with Velcade, this should not minimise the previously reported potentially beneficial effect of this herb," said Dr Schonthal. "Related studies with other types of cancer therapies are promising and green tea extract may actually improve the anticancer effects of other drugs."