Ezetimibe

Absorption

Administration of a single 10-mg dose of ezetimibe in fasted adults resulted in peak plasma concentrations (Cmax) of 3.4-5.5 ng/mL within 4-12 hours (Tmax). The Cmax of the major pharmacologically-active metabolite, ezetimibe-glucuronide, was 45-71 ng/mL and its Tmax was 1-2 hours. Food consumption has minimal effect on ezetimibe absorption, but the Cmax is increased by 38% when administered alongside a high-fat meal. The true bioavailability of ezetimibe cannot be determined, as it is insoluble in aqueous media suitable for intravenous injection.

Toxicity

Oral LD50 and intraperitoneal LD50 in rat were >2000 mg/kg. Estimated oral LD50 values in mouse and dog are >5000 mg/kg and >3000 mg/kg, respectively. One case of accidental overdose occurred in clinical studies in one female patient with homozygous sitosterolemia receiving 120 mg/day for 28 days with no reported clinical or laboratory adverse events. In case of overdose, symptomatic treatment is recommended.

Description

Ezetimibe is a once-daily orally active cholesterol absorption inhibitor, launched as a hypolipidemic agent. The one-step diastereo- and enantioselective formation of β-lactams starting from commercially available (3S)-hydroxy-y-lactone is the key point of the asymmetric synthesis of ezetimibe. The 2-azetidinone class was initially designed as acylcoenzyme A: cholesterol acyltransferase (ACAT) inhibitors but experimental data suggest that this compound acts in the intestinal wall to inhibit cholesterol through a novel mechanism with an as yet undiscovered target. Orally administered ezetimibe inhibited increases in plasma cholesterol in four cholesterol-fed animals species (hamster, rats, dogs and rhesus monkeys). In rats cannulated in the intestine and bile duct, [³H]-ezetimibe inhibited cholesterol absorption by more than 95%. In cholesterol-fed LDL receptor+apoE knockout mice, treatment with ezetimibe reduced atherosclerotic lesion cross sectional area by 48% in the aorta and 20% in the carotid artery. Moreover, the plasma cholesterol levels were reduced and the progression of lesions was inhibited. Ezetimibe is highly protein bound and is metabolized by the liver to its glucuronide metabolite, which represents 80-90% of circulating ezetimibe. About 90% of ezetimibe and/or the glucuronide metabolite are excreted in the feces and 10% in the urine. The parent compound and its glucuronide metabolite undergo enterohepatic recirculation; in consequence, the drug is slowly eliminated. In hypercholesterolemic patients, ezetimibe (10 mglday, 12 weeks) reduced LDL cholesterol by 18% and total cholesterol by 12%, with a similar safety profile to placebo. Co-administration of ezetimibe with statins or fenofibrate lowered LDL cholesterol levels more than either monotherapy. Ezetimibe was well tolerated and interaction studies provided evidence that ezetimibe had no significant effect on the activity of major CYP450 drug-metabolizing enzymes. Moreover, no pharmacokinetic/pharmacodynamic interactions were seen between ezetimibe and statins and others frequently administered drugs. .

The Uses of Ezetimibe

Ezetimibe (9) was approved as the first hypolipidemic drug to act by blocking the absorption of dietary cholesterol. This drug was discovered by Schering-Plough and is codeveloped and co-marketed by Merck and Schering-Plough for the treatment of hypercholesterolemia and also two less common forms of hyperlipidemia: homozygous familial hypercholesterolemia and homozygous sitosterolemia.

Indications

Ezetimibe is indicated to reduce elevated total-C, LDL-C, Apo B, and non-HDL-C in patients with primary hyperlipidemia, alone or in combination with an HMG-CoA reductase inhibitor (statin). It is also indicated to reduce elevated total-C, LDL-C, Apo B, and non-HDL-C in patients with mixed hyperlipidemia in combination with fenofibrate, and to reduce elevated total-C and LDL-C in patients with homozygous familial hypercholesterolemia (HoFH), in combination with atorvastatin or simvastatin. Ezetimibe may also be used to reduce elevated sitosterol and campesterol in patients with homozygous sitosterolemia (phytosterolemia).

Background

Ezetimibe is a lipid-lowering compound that inhibits intestinal cholesterol and phytosterol absorption. The discovery and research of this drug began in the early 1990s, after the intravenous administration of radiolabelled ezetimibe in rats revealed that it was being localized within enterocytes of the intestinal villi - this prompted studies investigating the effect of ezetimibe on intestinal cholesterol absorption. Ezetimibe is used as an adjunctive therapy to a healthy diet to lower cholesterol levels in primary hyperlipidemia, mixed hyperlipidemia, homozygous familial hypercholesterolemia (HoFH), and homozygous sitosterolemia (phytosterolemia).

Pharmacokinetics

Ezetimibe was shown to reduce the levels of total cholesterol (total-C), low-density lipoprotein cholesterol (LDL-C), apoprotein B (Apo B), non-high-density lipoprotein cholesterol (non-HDL-C), and triglycerides (TG), and increase high-density lipoprotein cholesterol (HDL-C) in patients with hyperlipidemia. This therapeutic effect was more profound when ezetimibe was co-administered with a statin or fenofibrate compared to either treatment alone.
In clinical trials involving patients with homozygous and heterozygous familial hypercholesterolemia and in those with sitosterolemia, a recommended therapeutic dose of ezetimibe was effective in reducing the LDL levels by 15-20% while increasing HDL-C by 2.5-5%.

Metabolism

In humans, ezetimibe is rapidly and extensively metabolized via a phase II glucuronide conjugation reaction in the small intestine and liver to form its main phenolic metabolite, ezetimibe glucuronide. The main human liver and/or intestinal uridine 5′-diphosphate (UDP)-glucuronosyltransferase (UGT) enzymes responsible for the glucuronidation of ezetimibe were shown to be UGT1A1, 1A3, and 2B15 in vitro. Minimal phase I reaction involving oxidation of ezetimibe also occurs to form SCH 57871, and human jejunum microsomes also produced trace levels of a benzylic glucuronide (SCH 488128). Ezetimibe glucuronide accounts for 80-90% of the total circulating compound in plasma, and retains some pharmacological activity in inhibiting intestinal cholesterol uptake. In humans, ezetimibe and ezetimibe-glucuronide constitutes approximately 93% of the total drug in plasma. Plasma concentration-time profiles exhibit multiple peaks, suggestive of enterohepatic recycling, and about 20% of the drug distributed is reabsorbed due to enterohepatic recirculation.

Side Effects

Ezetimibe may increase the risk of liver damage (when taken with a "statin") or muscle damage when taken with a fibrate or "statin". Tell your doctor right away if you experience any of the following symptoms: signs of liver problems (such as nausea/vomiting that doesn't stop, severe stomach/abdominal pain, yellowing eyes/skin, dark urine), muscle pain/tenderness/weakness (especially with fever or unusual tiredness).