The body's ability to break down medicines may be closely related to exposure to sunlight, according to a study from the Swedish medical university Karolinska Institutet.   This could mean drug metabolism is influenced by seasons and might explain individual differences in the effects of drugs, and how the surroundings can influence the body's ability to deal with toxins.

The connection between sunlight, vitamin D and variations in drug concentration is believed to arise from the activation by vitamin D of the detoxification system of the liver by increasing the amount of an enzyme known as CYP3A4. This enzyme, in turn, is responsible for the breakdown of tacrolimus and sirolimus, which are used to prevent rejection following transplantation.

The effects of vitamin D on CYP3A4 have previously been demonstrated in experiments in cell cultures but the new study seeks to show for the first time that the mechanism can play an important role in the pharmacological treatment of patients, and that variation in exposure to sunlight may affect the sensitivity of individuals to drugs.

The study will be published in Drug Metabolism&Disposition and is based on nearly 70,000 analyses from patients who have undergone regular monitoring of the levels of drugs in their blood. The acrolimus and sirolimus taken by these patients are used to suppress the immune system in association with organ transplants. Samples taken during the winter months were compared with those taken late in the summer.

A more detailed analysis showed that the concentrations of the drugs varied throughout the year in a manner that closely reflects changes in the level of vitamin D in the body. The ability of the body to form vitamin D depends on sunlight, and the highest levels in the patients taking part in the study were reached during that part of the year when the levels of the drugs were lowest.

"If the breakdown capacity increases, then higher doses of a drug are normally required in order to achieve the same effect. More research will be needed to confirm the results, but CYP3A4 is considered to be the most important enzyme in drug turnover in the body, and the results may have significance for many drugs", says Jonatan Lindh at the Department of Laboratory Medicine and one of the scientists who carried out the study.