Plasma cholesterol is derived from intestinal absorption and endogenous synthesis. Ezetimibe and simvastatin are two lipid-lowering compounds with complementary mechanisms of action. Ezetimibe/simvastatin combination reduces elevated total cholesterol (total-C), LDL-C, apolipoprotein B (Apo B), triglycerides (TG), and non-high-density lipoprotein cholesterol (non-HDL-C), and increases high-density lipoprotein cholesterol (HDL-C) through dual inhibition of cholesterol absorption and synthesis.
Ezetimibe inhibits the intestinal absorption of cholesterol. Ezetimibe is orally active and has a mechanism of action that differs from other classes of cholesterol‑reducing compounds (e.g. statins, bile acid sequestrants [resins], fibric acid derivatives, and plant stanols). The molecular target of ezetimibe is the sterol transporter, Niemann-Pick C1-Like 1 (NPC1L1), which is responsible for the intestinal uptake of cholesterol and phytosterols.
Ezetimibe localises at the brush border of the small intestine and inhibits the absorption of cholesterol, leading to a decrease in the delivery of intestinal cholesterol to the liver; statins reduce cholesterol synthesis in the liver and together these distinct mechanisms provide complementary cholesterol reduction. In a 2‑week clinical study in 18 hypercholesterolaemic patients, ezetimibe inhibited intestinal cholesterol absorption by 54%, compared with placebo.
A series of preclinical studies was performed to determine the selectivity of ezetimibe for inhibiting cholesterol absorption. Ezetimibe inhibited the absorption of [14C]‑cholesterol with no effect on the absorption of triglycerides, fatty acids, bile acids, progesterone, ethinyl estradiol, or fat soluble vitamins A and D.
After oral ingestion, simvastatin, which is an inactive lactone, is hydrolysed in the liver to the corresponding active b‑hydroxyacid form which has a potent activity in inhibiting HMG‑CoA reductase (3 hydroxy‑3 methylglutaryl CoA reductase). This enzyme catalyses the conversion of HMG-CoA to mevalonate, an early and rate-limiting step in the biosynthesis of cholesterol.
Simvastatin has been shown to reduce both normal and elevated LDL-C concentrations. LDL is formed from very-low-density protein (VLDL) and is catabolized predominantly by the high affinity LDL receptor. The mechanism of the LDL-lowering effect of simvastatin may involve both reduction of VLDL-cholesterol (VLDL-C) concentration and induction of the LDL receptor, leading to reduced production and increased catabolism of LDL-C. Apolipoprotein B also falls substantially during treatment with simvastatin. In addition, simvastatin moderately increases HDL-C and reduces plasma TG. As a result of these changes, the ratios of total- to HDL-C and LDL- to HDL-C are reduced.
No clinically significant pharmacokinetic interaction was seen when ezetimibe was co-administered with simvastatin.
Ezetimibe/simvastatin fixed-dose combination is bioequivalent to co-administered ezetimibe and simvastatin.
After oral administration, ezetimibe is rapidly absorbed and extensively conjugated to a pharmacologically active phenolic glucuronide (ezetimibe-glucuronide). Mean maximum plasma concentrations (Cmax) occur within 1 to 2 hours for ezetimibe-glucuronide and 4 to 12 hours for ezetimibe. The absolute bioavailability of ezetimibe cannot be determined as the compound is virtually insoluble in aqueous media suitable for injection.
Concomitant food administration (high-fat or non-fat meals) had no effect on the oral bioavailability of ezetimibe when administered as 10‑mg tablets.
The availability of the active b-hydroxyacid to the systemic circulation following an oral dose of simvastatin was found to be less than 5% of the dose, consistent with extensive hepatic first-pass extraction. The major metabolites of simvastatin present in human plasma are the b-hydroxyacid and four additional active metabolites.
Relative to the fasting state, the plasma profiles of both active and total inhibitors were not affected when simvastatin was administered immediately before a test meal.
Ezetimibe and ezetimibe‑glucuronide are bound 99.7% and 88 to 92% to human plasma proteins, respectively.
Both simvastatin and the b-hydroxyacid are bound to human plasma proteins (95%).
The pharmacokinetics of single and multiple doses of simvastatin showed that no accumulation of drug occurred after multiple dosing. In all of the above pharmacokinetic studies, the maximum plasma concentration of inhibitors occurred 1.3 to 2.4 hours post-dose.
Ezetimibe is metabolised primarily in the small intestine and liver via glucuronide conjugation (a phase II reaction) with subsequent biliary excretion. Minimal oxidative metabolism (a phase I reaction) has been observed in all species evaluated. Ezetimibe and ezetimibe-glucuronide are the major drug-derived compounds detected in plasma, constituting approximately 10 to 20% and 80 to 90% of the total drug in plasma, respectively. Both ezetimibe and ezetimibe-glucuronide are slowly eliminated from plasma with evidence of significant enterohepatic recycling. The half-life for ezetimibe and ezetimibe-glucuronide is approximately 22 hours.
Simvastatin is an inactive lactone which is readily hydrolysed in vivo to the corresponding b‑hydroxyacid, a potent inhibitor of HMG-CoA reductase. Hydrolysis takes place mainly in the liver; the rate of hydrolysis in human plasma is very slow.
In man simvastatin is well absorbed and undergoes extensive hepatic first-pass extraction. The extraction in the liver is dependent on the hepatic blood flow. The liver is its primary site of action, with subsequent excretion of drug equivalents in the bile. Consequently, availability of active drug to the systemic circulation is low.
Following an intravenous injection of the b-hydroxyacid metabolite, its half-life averaged 1.9 hours.
Following oral administration of 14C‑ezetimibe (20 mg) to human subjects, total ezetimibe accounted for approximately 93% of the total radioactivity in plasma. Approximately 78% and 11% of the administered radioactivity were recovered in the faeces and urine, respectively, over a 10‑day collection period. After 48 hours, there were no detectable levels of radioactivity in the plasma.
Simvastatin acid is taken up actively into the hepatocytes by the transporter OATP1B1.
Simvastatin is a substrate of the efflux transporter BCRP.
Following an oral dose of radioactive simvastatin to man, 13% of the radioactivity was excreted in the urine and 60% in the faeces within 96 hours. The amount recovered in the faeces represents absorbed drug equivalents excreted in bile as well as unabsorbed drug. Following an intravenous injection of the b-hydroxyacid metabolite, an average of only 0.3% of the IV dose was excreted in urine as inhibitors.
The absorption and metabolism of ezetimibe are similar between children and adolescents (10 to 18 years) and adults. Based on total ezetimibe, there are no pharmacokinetic differences between adolescents and adults. Pharmacokinetic data in the paediatric population <10 years of age are not available. Clinical experience in paediatric and adolescent patients includes patients with HoFH, HeFH, or sitosterolaemia.
Plasma concentrations for total ezetimibe are about 2‑fold higher in the elderly (≥65 years) than in the young (18 to 45 years). LDL‑C reduction and safety profile are comparable between elderly and younger subjects treated with ezetimibe.
After a single 10‑mg dose of ezetimibe, the mean AUC for total ezetimibe was increased approximately 1.7‑fold in patients with mild hepatic impairment (Child-Pugh score 5 or 6), compared to healthy subjects. In a 14‑day, multiple-dose study (10 mg daily) in patients with moderate hepatic impairment (Child-Pugh score 7 to 9), the mean AUC for total ezetimibe was increased approximately 4‑fold on Day 1 and Day 14 compared to healthy subjects. No dosage adjustment is necessary for patients with mild hepatic impairment. Due to the unknown effects of the increased exposure to ezetimibe in patients with moderate or severe (Child-Pugh score >9) hepatic impairment, ezetimibe is not recommended in these patients.
After a single 10‑mg dose of ezetimibe in patients with severe renal disease (n=8; mean CrCl <30 mL/min), the mean AUC for total ezetimibe was increased approximately 1.5‑fold, compared to healthy subjects (n=9).
An additional patient in this study (post-renal transplant and receiving multiple medications, including ciclosporin) had a 12‑fold greater exposure to total ezetimibe.
In a study of patients with severe renal impairment (creatinine clearance <30 mL/min), the plasma concentrations of total inhibitors after a single dose of a related HMG-CoA reductase inhibitor were approximately two-fold higher than those in healthy volunteers.
Plasma concentrations for total ezetimibe are slightly higher (approximately 20%) in women than in men. LDL‑C reduction and safety profile are comparable between men and women treated with ezetimibe.
Carriers of the SLCO1B1 gene c.521T > C allele have lower OATP1B1 activity. The mean exposure (AUC) of the main active metabolite, simvastatin acid is 120% in heterozygote carriers (CT) of the C allele and 221% in homozygote (CC) carriers relative to that of patients who have the most common genotype (TT). The C allele has a frequency of 18% in the European population. In patients with SLCO1B1 polymorphism there is a risk of increased exposure of simvastatin acid, which may lead to an increased risk of rhabdomyolysis.
In co-administration studies with ezetimibe and simvastatin, the toxic effects observed were essentially those typically associated with statins. Some of the toxic effects were more pronounced than observed during treatment with statins alone. This is attributed to pharmacokinetic and/or pharmacodynamic interactions following co-administration. No such interactions occurred in the clinical studies. Myopathies occurred in rats only after exposure to doses that were several times higher than the human therapeutic dose (approximately 20 times the AUC level for simvastatin and 1,800 times the AUC level for the active metabolite). There was no evidence that co-administration of ezetimibe affected the myotoxic potential of simvastatin.
In dogs co-administered ezetimibe and statins, some liver effects were observed at low exposures (≤1 times human AUC). Marked increases in liver enzymes (ALT, AST) in the absence of tissue necrosis were seen. Histopathologic liver findings (bile duct hyperplasia, pigment accumulation, mononuclear cell infiltration and small hepatocytes) were observed in dogs co-administered ezetimibe and simvastatin. These changes did not progress with longer duration of dosing up to 14 months. General recovery of the liver findings was observed upon discontinuation of dosing. These findings were consistent with those described with HMG-CoA inhibitors or attributed to the very low cholesterol levels achieved in the affected dogs.
The co-administration of ezetimibe and simvastatin was not teratogenic in rats. In pregnant rabbits a small number of skeletal deformities (fused caudal vertebrae, reduced number of caudal vertebrae) were observed.
In a series of in vivo and in vitro assays, ezetimibe, given alone or co-administered with simvastatin, exhibited no genotoxic potential.
Animal studies on the chronic toxicity of ezetimibe identified no target organs for toxic effects. In dogs treated for four weeks with ezetimibe (≥0.03 mg/kg/day) the cholesterol concentration in the cystic bile was increased by a factor of 2.5 to 3.5. However, in a one year study on dogs given doses of up to 300 mg/kg/day no increased incidence of cholelithiasis or other hepatobiliary effects were observed. The significance of these data for humans is not known. A lithogenic risk associated with the therapeutic use of ezetimibe cannot be ruled out.
Long-term carcinogenicity tests on ezetimibe were negative.
Ezetimibe had no effect on the fertility of male or female rats, nor was it found to be teratogenic in rats or rabbits, nor did it affect prenatal or post-natal development. Ezetimibe crossed the placental barrier in pregnant rats and rabbits given multiple doses of 1,000 mg/kg/day.
Based on conventional animal studies regarding pharmacodynamics, repeated dose toxicity, genotoxicity and carcinogenicity, there are no other risks for the patient than may be expected on account of the pharmacological mechanism. At maximally tolerated doses in both the rat and the rabbit, simvastatin produced no foetal malformations, and had no effects on fertility, reproductive function or neonatal development.
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