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Drug efficacy

Exposure to light at night may reduce effectiveness of breast cancer drug tamoxifen

Artificial light suppresses melatonin causing cancer cells to ‘wake up’ and ignore tamoxifen.

Melatonin suppressed when breast cancer patients sleep with light on

Source: Alfred Pasieka / Science Photo Library

 Light micrograph in polarised light of crystals of the hormone melatonin, which has been shown to influence the effectiveness of tamoxifen

Keeping the lights on during the night may cause breast cancer patients to become resistant to a commonly-prescribed treatment.

A study published in the journal  found that even low levels of light at night-time could affect resistance to the breast cancer drug tamoxifen by suppressing levels of melatonin needed for the treatment to work.

This is the first study to highlight the importance of melatonin in the success of tamoxifen as a breast cancer treatment. Tamoxifen, an oestrogen antagonist, decreases cell division in oestrogen-dependent tissues.

The findings, from a study conducted by Tulane University School of Medicine, show that exposure to light at night shuts off night-time production of the hormone melatonin. On its own, melatonin has been shown to delay tumour formation and significantly slow tumour growth.

David E. Blask, an investigator in the study and a member of Tulane’s Circadian Cancer Biology Group, can explain the melatonin-tamoxifen link.

“High melatonin levels at night put breast cancer cells ‘to sleep’ by turning off key growth mechanisms. These cells are vulnerable to tamoxifen,” says Blask, a professor in the department of structural and cellular biology at Tulane. “But when the lights are on and melatonin is suppressed, breast cancer cells ‘wake up’ and ignore tamoxifen.”

The findings may have implications for women being treated with tamoxifen who are also regularly exposed to light at night for various reasons, including night-shift work, sleep problems or looking at TV screens, mobile phones or computers.

The research could lead to a focus on night-time light as a new, serious risk factor tied to the development of resistance to tamoxifen and other anti-cancer drugs.

At this stage, the researchers cannot speculate on the safety of using melatonin supplements to improve the effectiveness of tamoxifen.

“Although melatonin supplements are readily available over the counter at most drug and health-food stores, our research is not at a point where we can make a general recommendation that breast cancer patients taking tamoxifen should go out and buy melatonin,” says Steven M. Hill, the lead author of the study and a member of Tulane’s Circadian Cancer Biology Group.

“Melatonin is produced by our bodies at a very specific time of day, exclusively during darkness at night, and taking melatonin supplements at the wrong time of day would potentially disrupt the circadian system, particularly the natural melatonin cycle, which may, in itself, paradoxically impair breast cancer responsiveness to tamoxifen,” says Hill, a professor of structural and cellular biology at Tulane.

In the study, Tulane’s Circadian Cancer Biology Group used rats implanted with human cancer cells. During the first phase, animals were kept in a daily cycle of 12 hours of light, followed by 12 hours of total darkness when melatonin production is elevated. In the second phase, during the 12-hour dark cycle, the animals were exposed to extremely dim light – roughly equivalent to faint light coming through under a door – which suppressed melatonin production. The negative effects of dim light exposure on tamoxifen treatment were overcome by giving rats a melatonin supplement.



Citation: Electronicjuice DOI: 10.1211/PJ.2014.20065973

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