For everything -- including chemo -- there is a time

Doctors often wonder if there is a best time of day for cancer patients to receive chemotherapy. Past research suggests there probably is an optimal time based on the body's circadian rhythms. Now, a compelling new study offers some biological proof for the idea.

Researchers at the University of North Carolina have discovered that chemotherapy is probably most effective at particular times of day when an enzyme system in the body that can blunt the impact of the drugs is at its lowest levels. The enzyme system is called nucleotide excision repair. It works to fix DNA damage from toxic substances, like chemotherapy agents, but also DNA damage from sun exposure. The study is one of the most convincing pieces of evidence that the body's internal time-keeping system, which resides in every cell, can affect the potency of some drugs. The study is part of a growing interest among scientists to understand the impact of circadian rhythms on human health, as was described in this recent story in the Los Angeles Times Health section.

"Timing is everything, and here we have molecular data showing why this is especially true with regard to cancer," the senior author of the paper, Dr. Aziz Sancar, said in a news release. "By hitting cancer cells with chemo at a time when their ability to repair themselves is minimal, you should be able to maximize effectiveness and minimize side effects of treatment."

The study, which was conducted in mice, found the ability to repair damage was at a minimum in the early morning and reached a maximum in the evening hours. The researchers say their study needs to be replicated in humans and that they plan to explore whether their findings can apply to skin cancer prevention. "If we show the same patterns in humans as we did in mice, then it could tell us when would be the safest time to be in the sun, and when would be the best time to avoid sun exposure," Sancar said. The study will be published this week in the online early edition of the Proceedings of the National Academy of Sciences.

-- Shari Roan

Image credit: Stephen Sedam / Los Angeles Times