FIBRIC ACID DERIVATIVES (FIBRATES)
Gemfibrozil and fenofibrate decrease levels of VLDL and, insome patients, LDL as well. Another fibrate, bezafibrate, is not yet available in the USA.
Gemfibrozil is absorbed quantitatively from the intestine and is tightly bound to plasma proteins. It undergoes enterohepatic circulation and readily passes the placenta. The plasma half-life is 1.5 hours. Seventy percent is eliminated through the kidneys, mostly unmodified. The liver modifies some of the drug to hydroxymethyl, carboxyl, or quinol derivatives. Fenofibrate is an isopropyl ester that is hydrolyzed completely in the intestine. Its plasma half-life is 20 hours. Sixty percent is excreted in the urine as the glucuronide, and about 25% in feces.
Fibrates function primarily as ligands for the nuclear transcription receptor, PPAR-α. They transcriptionally up-regulate LPL, apo A-I and apo A-II, and down-regulate apo C-III, an inhibitor of lipolysis. A major effect is an increase in oxidation of fatty acids in liver and striated muscle (Figure 35–4). They increase lipolysis of lipoprotein triglyceride via LPL. Intracellular lipolysis in adipose tissue is decreased. Levels of VLDL decrease, in part as a result of decreased secretion by the liver.
Only modest reductions of LDL occur in most patients. In others, especially those with combined hyperlipidemia, LDL often increases as triglycerides are reduced. HDL cholesterol increases moderately. Part of this apparent increase is a consequence of decreasing triglycerides in plasma, with reduction in exchange of triglycerides into HDL in place of cholesteryl esters.
Fibrates are useful drugs in hypertriglyceridemias in which VLDL predominate and in dysbetalipoproteinemia. They also may be of benefit in treating the hypertriglyceridemia that results from treat-ment with viral protease inhibitors. The usual dose of gemfibrozil is 600 mg orally once or twice daily. The dosage of fenofibrate (as Tricor) is one to three 48 mg tablets (or a single 145 mg tablet) daily. Absorption of gemfibrozil is improved when the drug is taken with food.
Rare adverse effects of fibrates include rashes, gastrointestinal symp-toms, myopathy, arrhythmias, hypokalemia, and high blood levels of aminotransferases or alkaline phosphatase. A few patients show decreases in white blood count or hematocrit. Both agents potenti-ate the action of coumarin and indanedione anticoagulants, and doses of these agents should be adjusted. Rhabdomyolysis has occurred rarely. Risk of myopathy increases when fibrates are given with reductase inhibitors. Fenofibrate is the fibrate of choice for use in combination with a statin. Fibrates should be avoided in patients with hepatic or renal dysfunction. There appears to be a modest increase in the risk of cholesterol gallstones, reflecting an increase in the cholesterol content of bile. Therefore, fibrates should be used with caution in patients with biliary tract disease or in those at high risk such as women, obese patients, and Native Americans.