Two drugs are better than one

A two-pronged approach to increasing cholesterol elimination

“Cholesterol: We can’t live with it, can’t live without it,” quips Gregory Graf of the University of Kentucky, whose research team wrote in the Journal of Lipid Research about a potentially powerful two-drug combination that appears to be pretty good at removing the sometimes pesky molecule from the body.

Chemical structure of ursodiol

The body regulates cholesterol in a complex manner. It makes cholesterol and absorbs it from food — achieving a balance that is not fully understood. The process of getting rid of excess cholesterol is even more complex. The liver is central to cholesterol synthesis and elimination. In addition to directly secreting cholesterol, the liver turns cholesterol into detergentlike bile acids. The gallbladder stores the bile — a mix of bile acids, unmodified cholesterol and many other waste products, such as bilirubin — before secreting it into the small intestine to help dissolve and absorb dietary fats. Much of the cholesterol secreted into the small intestine is then reabsorbed. The built-in redundancies of the cholesterol recycling process make designing drugs to target cholesterol elimination particularly difficult. The two-drug combination therapy developed by Graf’s team stimulates cholesterol secretion from the liver and reduces cholesterol absorption in the intestine, leading to an increase in cholesterol elimination in a mouse model. Both of the drugs in the cocktail are already approved by the Food and Drug Administration.

Chemical structure of ezetimibe

One, ursodiol — originally found in bear bile — treats gallstones by making the bile a little bit more water-soluble to dissolve and flush excess cholesterol out of the gallbladder. The other, ezetimibe, blocks cholesterol absorption in the intestine and is modestly effective in the treatment of high cholesterol. Graf said he is hopeful “that these two agents may in fact be effective in working cooperatively to promote cholesterol elimination in human subjects” but cautions that there are differences between mice and humans.

Graf and co-workers tested ursodiol and determined that it increased the levels of a key cholesterol transport protein (the ABCG5-ABCG8 heterodimer, or G5G8). Ursodiol also increased the amount of cholesterol secreted into the gallbladder and eliminated in feces in a dose-dependent manner. Unfortunately, although cholesterol was flushed out of the body, the level of plasma cholesterol did not decrease, which is the clinical goal. Graf admits this puzzled them at first, but then they realized that the secreted cholesterol was being reabsorbed in the small intestine. So they added ezetimibe, a potent inhibitor of cholesterol absorption.

The combination led to no increase in G5G8 protein levels compared with the ursodiol-alone treatment, but mice treated with both drugs did excrete significantly more cholesterol in their feces. Compellingly, treatment with moderate amounts of ursodiol and high amounts of ezetimibe led to a modest but significant reduction in plasma cholesterol level. In addition, the researchers concluded that the marked decrease in the intestinal level of ABCA1 — a cholesterol transporter involved in making high-density lipoproteins  — was the main cause for the lowered blood cholesterol level.

Graf and co-workers didn’t stop there; they also wanted to see if the treatment was dependent upon the G5G8 transport pathway. They tested the ursodiol alone and the ursodiol-ezetimibe combination on mice lacking the G5G8 cholesterol transporter. The G5G8 knockout mice had significantly lower fecal cholesterol levels than the wild-type mice, but for both strains, the ursodiol-alone treatment dramatically increased the amount of cholesterol eliminated in feces by the same proportion — 700 percent. On top of that, the combination therapy doubled the fecal cholesterol levels in both mice strains.

While the results from this work are promising, there are significant species differences, and it is unknown whether this combination will be effective in human subjects. Therefore, Graf and co-workers are conducting a small clinical study in human subjects. Not only did Graf’s team discover a promising one-two punch against cholesterol, but they also uncovered evidence for a cholesterol-elimination pathway not dependent upon the G5G8 transport protein. Characterization of this novel pathway could uncover new fundamental knowledge and innovative cholesterol-reduction therapeutics.

Mollie Rappe Mollie Rappe is an intern at ASBMB Today and a Ph.D. candidate in biophysics at Johns Hopkins University.