Personalized Medicine: Seasonal Variation?

A class of liver enzymes known as cytochrome P450 (CYP450) enzymes are largely responsible for breaking down most prescription drugs. Researchers and physicians are already aware of specific genetic differences that make some individuals metabolize certain drugs more quickly or more slowly than other individuals, and a number of companies are developing diagnostic tests to identify which type of metabolizer a particular patient is: poor, normal, or ultra-metabolizer. Different drugs are metabolized by different sub-categories within this broad CYP450 family, so a patient may be a poor metabolizer for one type of drug, and a normal metabolizer for another type.

A recent publication in the journal Drug Metabolism and Disposition indicates that a person’s ability to metabolize certain drugs may also be influenced by how much sunlight they get. This is because one sub-category of the CYP450 enzymes, referred to as CYP3A4, is activated by higher levels of vitamin D, which in turn is produced by a person’s skin in response to sun exposure. A research team led by Erik Eliasson of the Karolinska Institute found that the levels of certain drugs known to be broken down by the CYP3A4 enzymes varied significantly, depending on the time of year and relative amounts of sunlight exposure, with higher drug levels corresponding to the winter months when the CYP3A4 enzymes were less active. This pattern was not seen in drug levels known to be metabolized by different CYP enzymes.

Of course, this seasonal variation will have the most profound impact in places where differences in sunlight exposure vary significantly throughout the year. It’s not a coincidence that the study’s authors live in Sweden, where the average number of daylight hours in December is six and one-half, and the average number in June is eighteen. In most regions of the world, the variation is not so extreme; still, it does exist, and it will be interesting to see if these less pronounced variations also influence drug metabolism. Physicians may indeed have yet another variable to take into account in order to determine the most accurate drug dosages.