Circadian rhythms can influence the effectiveness of drugs

Giving drugs at different times of the day could significantly affect how they are metabolized in the liver, according to a new MIT study.

Using tiny, engineered livers derived from cells from human donors, the researchers found that many genes involved in drug metabolism are under circadian control. These circadian variations affect how much drug is available and how effectively the body can break it down. For example, they found that the enzymes that break down Tylenol and other drugs are more abundant at certain times of the day.

Overall, the researchers identified more than 300 liver genes that follow a circadian clock, including many involved in drug metabolism as well as other functions such as inflammation. Analyzing these rhythms could help researchers develop better dosing schedules for existing drugs.

“One of the first applications of this method could be to adjust drug regimens of already approved drugs to maximize their effectiveness and minimize their toxicity,” says Sangeeta Bhatia, professor of Health Sciences and Technology and of Electrical Engineering and John and Dorothy Wilson. Computer Science at MIT and member of the Koch Institute for Integrative Cancer Research and the Institute of Medical Engineering and Science (IMES).

The study also revealed that the liver is more susceptible to infections such as malaria at certain times of the circadian cycle, when fewer inflammatory proteins are produced.

Bhatia is the lead author of the new study, which appears today in Advances in Science. The main author of the paper is Sandra March, scientific researcher at IMES.

Metabolic cycles

It is estimated that about 50% of human genes follow a circadian cycle, and many of these genes are active in the liver. However, exploring how circadian cycles affect liver function has been difficult because many of these genes are not identical in mice and humans, so mouse models cannot be used to study them.

Bhatia’s lab had previously developed a way to grow miniaturized livers using liver cells called hepatocytes, from human donors. In this new study, she and her colleagues set out to investigate whether these engineered livers have their own circadian clocks.

Working with Charles Rice’s group at Rockefeller University, they identified the culture conditions that support circadian expression of a clock gene called Bmal1. This gene, which regulates the cyclic expression of a wide range of genes, allowed liver cells to develop synchronized circadian oscillations. The researchers then measured gene expression in these cells every three hours for 48 hours, which allowed them to identify more than 300 genes that were expressed in waves.

Most of these genes clustered into two groups: about 70% of the genes peaked together, while the remaining 30% were at their lowest point when the others peaked. These include genes involved in a variety of functions, including drug metabolism, glucose and lipid metabolism, and various immune processes.

Once the engineered livers established these circadian cycles, researchers could use them to explore how circadian cycles affect liver function. First, they set out to study how the time of day would affect drug metabolism by looking at two different drugs: acetaminophen (Tylenol) and atorvastatin, a drug used to treat high cholesterol.

When Tylenol breaks down in the liver, a small fraction of the drug becomes a toxic byproduct known as NAPQI. The researchers found that the amount of NAPQI produced can vary by up to 50%, depending on the time of day the drug is administered. They also found that atorvastatin generates higher toxicity at certain times of the day.

Both drugs are metabolized in part by an enzyme called CYP3A4, which has a circadian cycle. CYP3A4 is involved in the processing of approximately 50% of all drugs, so the researchers now plan to test more of these drugs using their liver models.

“In this set of drugs, it will be helpful to identify the time of day to administer the drug to achieve maximum drug effectiveness and minimize adverse effects,” says March.

MIT researchers are now working with collaborators to analyze a cancer drug they suspect may be affected by circadian cycles, and hope to investigate whether this may also be true for drugs used to treat pain.

Susceptibility to infection

Many of the genes in the liver that show circadian behavior are involved in immune responses such as inflammation, so the researchers wondered if this variation could influence susceptibility to infection. To answer this question, they exposed the modified livers to Plasmodium falciparum, a parasite that causes malaria, at different times of the circadian cycle.

These studies revealed that livers were more likely to become infected after exposure at different times of the day. This is due to variations in the expression of genes called interferon-stimulated genes, which help suppress infections.

“Inflammatory signals are much stronger at certain times of the day than at others,” says Bhatia. “This means that a virus like hepatitis or a parasite like the one that causes malaria might be better at taking on the liver at certain times of the day.”

The researchers believe that this cyclical variation may occur because the liver dampens its response to pathogens after meals, when it is normally exposed to an influx of microorganisms that can trigger inflammation even if they are not actually harmful.

Bhatia’s lab is taking advantage of these cycles to study infections that are usually difficult to establish in modified livers, including malaria infections caused by parasites other than Plasmodium falciparum.

“This is quite important for the field, because just by setting up the system and choosing the right time of infection, we can increase the infection rate of our culture by 25%, allowing for drug screens that otherwise wouldn’t they were practices,” says March.

This story is republished courtesy of MIT News (web.mit.edu/newsoffice/), a popular site covering news about MIT research, innovation, and teaching.