Biotransformation of lovastatin--III. Effect of cimetidine and famotidine on in vitro metabolism of lovastatin by rat and human liver microsomes

Pharmacogenomics of lipid-lowering agents: the impact on efficacy and safety

Hyperlipidemia is a significant risk factor for cardiovascular disease morbidity and mortality. The lipid-lowering drugs are considered the cornerstone of primary and secondary prevention of atherosclerotic cardiovascular disease. Unfortunately, the lack of efficacy and associated adverse effects, ranging from mild-to-moderate to potentially life-threatening, lead to therapy discontinuation. Numerous reports support the role of gene polymorphisms in drugs' pharmacokinetic parameters and their associated adverse reactions. Therefore, this study aims to understand the pharmacogenomics of lipid-lowering drugs and the impact of genetic variants of key genes on the drugs' efficacy and toxicity. Indeed, genetically guided lipid-lowering therapy enhances overall safety, improves drug adherence and achieves long-term therapy.

Pharmacogenetics of Statin-Induced Myotoxicity

Statins, a class of lipid-lowering medications, have been a keystone treatment in cardiovascular health. However, adverse effects associated with statin use impact patient adherence, leading to statin discontinuation. Statin-induced myotoxicity (SIM) is one of the most common adverse effects, prevalent across all ages, genders, and ethnicities. Although certain demographic cohorts carry a higher risk, the impaired quality of life attributed to SIM is significant. The pathogenesis of SIM remains to be fully elucidated, but it is clear that SIM is multifactorial. These factors include drug-drug interactions, renal or liver dysfunction, and genetics. Genetic-inferred risk for SIM was first reported by a landmark genome-wide association study, which reported a higher risk of SIM with a polymorphism in the SLCO1B1 gene. Since then, research associating genetic factors with SIM has expanded widely and has become one of the foci in the field of pharmacogenomics. This review provides an update on the genetic risk factors associated with SIM.

Novel vitamin D3 antipsoriatic antedrugs: 16-En-22-oxa-1α,25-(OH)2D3 analogs

A series of 16-en-22-oxa-derivatives of vitamin D3 based on the structure of maxacalcitol (2) were prepared. Maxacalcitol is currently used topically for the treatment of psoriasis and is recognized as the most successful antedrug of natural vitamin D(3) because it retains the original antiproliferative activity of calcitriol without increased calcemic activity. We introduced 16-olefinic functionality to accelerate the oxidative metabolism of the drug in liver, presumed to be essential for the reduction of calcemic activity, and modified the side-chain moiety by placing the 22-oxygen on the more labile allylic carbon center. Novel 22-oxa analogs (7a-i), carrying either the 24-alkynyl bond or 24-hydroxy functionality in addition to the 16-double bond were synthesized and their pharmacokinetics were evaluated.

Genetic polymorphisms in cytochrome P450 enzymes: effect on efficacy and tolerability of HMG-CoA reductase inhibitors

Adverse drug reactions are common; they are responsible for a number of debilitating side effects and are a significant cause of death following drug therapy. It is now clear that a significant proportion of these adverse drug reactions, as well as therapeutic failures, are caused by genetic polymorphism, genetically based interindividual differences in drug absorption, disposition, metabolism, or excretion. HMG-CoA reductase inhibitors are generally very well tolerated and easy to administer with good patient acceptance. There are only two uncommon but potentially serious adverse effects related to HMG-CoA reductase inhibitor therapy: hepatotoxicity and myopathy. The occurrence of lethal rhabdomyolysis in patients treated with cerivastatin has prompted concern on the part of physicians and patients regarding the tolerability of HMG-CoA reductase inhibitors. Apart from pravastatin and rosuvastatin, HMG-CoA reductase inhibitors are metabolized by the phase I cytochrome P450 (CYP) superfamily of drug metabolizing enzymes. The best-characterized pharmacogenetic polymorphisms are those within this enzyme family. One of these enzymes, CYP2D6, plays an important role in the metabolism of simvastatin. It has been shown that the cholesterol-lowering effect as well as the efficacy and tolerability of simvastatin is influenced by CYP2D6 genetic polymorphism. Because the different HMG-CoA reductase inhibitors differ, with respect to the degree of metabolism by the different CYP enzymes, genotyping may help to select the appropriate HMG-CoA reductase inhibitor and the optimal dosage during the start of the treatment and will allow for more efficient individual therapy. A detailed knowledge of the genetic basis of individual drug response is potentially of major clinical and economic importance.

Pharmacokinetic-pharmacodynamic drug interactions with HMG-CoA reductase inhibitors

The HMG-CoA reductase inhibitors (statins) are effective in both the primary and secondary prevention of ischaemic heart disease. As a group, these drugs are well tolerated apart from two uncommon but potentially serious adverse effects: elevation of liver enzymes and skeletal muscle abnormalities, which range from benign myalgias to life-threatening rhabdomyolysis. Adverse effects with statins are frequently associated with drug interactions because of their long-term use in older patients who are likely to be exposed to polypharmacy. The recent withdrawal of cerivastatin as a result of deaths from rhabdomyolysis illustrates the clinical importance of such interactions. Drug interactions involving the statins may have either a pharmacodynamic or pharmacokinetic basis, or both. As these drugs are highly extracted by the liver, displacement interactions are of limited importance. The cytochrome P450 (CYP) enzyme system plays an important part in the metabolism of the statins, leading to clinically relevant interactions with other agents, particularly cyclosporin, erythromycin, itraconazole, ketoconazole and HIV protease inhibitors, that are also metabolised by this enzyme system. An additional complicating feature is that individual statins are metabolised to differing degrees, in some cases producing active metabolites. The CYP3A family metabolises lovastatin, simvastatin, atorvastatin and cerivastatin, whereas CYP2C9 metabolises fluvastatin. Cerivastatin is also metabolised by CYP2C8. Pravastatin is not significantly metabolised by the CYP system. In addition, the statins are substrates for P-glycoprotein, a drug transporter present in the small intestine that may influence their oral bioavailability. In clinical practice, the risk of a serious interaction causing myopathy is enhanced when statin metabolism is markedly inhibited. Thus, rhabdomyolysis has occurred following the coadministration of cyclosporin, a potent CYP3A4 and P-glycoprotein inhibitor, and lovastatin. Itraconazole has been shown to increase exposure to simvastatin and its active metabolite by at least 10-fold. Pharmacodynamically, there is an increased risk of myopathy when statins are coprescribed with fibrates or nicotinic acid. This occurs relatively infrequently, but is particularly associated with the combination of cerivastatin and gemfibrozil. Statins may also alter the concentrations of other drugs, such as warfarin or digoxin, leading to alterations in effect or a requirement for clinical monitoring. Knowledge of the pharmacokinetic properties of the statins should allow the avoidance of the majority of drug interactions. If concurrent therapy with known inhibitors of statin metabolism is necessary, the patient should be monitored for signs and symptoms of myopathy or rhabdomyolysis and the statin should be discontinued if necessary.

Metabolism and drug interactions of 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors in transplant patients: are the statins mechanistically similar?

3-Hydroxy-3-methylglutaryl coenzyme A reductase (EC 1.1.1.88) inhibitors are the most effective drugs to lower cholesterol in transplant patients. However, immunosuppressants and several other drugs used after organ transplantation are cytochrome P4503A (CYP3A, EC 1.14.14.1) substrates. Pharmacokinetic interaction with some of the 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors, specifically lovastatin and simvastatin, leads to an increased incidence of muscle skeletal toxicity in transplant patients. It is our objective to review the role of drug metabolism and drug interactions of lovastatin, simvastatin, pravastatin, fluvastatin, atorvastatin, and cerivastatin. In the treatment of transplant patients, from a drug interaction perspective, pravastatin, which is not significantly metabolized by CYP enzymes, and fluvastatin, presumably a CYP2C9 substrate, compare favorably with the other statins for which the major metabolic pathways are catalyzed by CYP3A.

The interaction of diltiazem with lovastatin and pravastatin

BACKGROUND

Lovastatin is oxidized by cytochrome P4503A to active metabolites but pravastatin is active alone and is not metabolized by cytochrome P450. Diltiazem, a substrate and a potent inhibitor of cytochrome P4503A enzymes, is commonly coadministered with cholesterol-lowering agents.

METHODS

This was a balanced, randomized, open-label, 4-way crossover study in 10 healthy volunteers, with a 2-week washout period between the phases. Study arms were (1) administration of a single dose of 20 mg lovastatin, (2) administration of a single dose of 20 mg pravastatin, (3) administration of a single dose of lovastatin after administration of 120 mg diltiazem twice a day for 2 weeks, and (4) administration of a single dose of pravastatin after administration of 120 mg diltiazem twice a day for 2 weeks.

RESULTS

Diltiazem significantly (P < .05) increased the oral area under the serum concentration-time curve (AUC) of lovastatin from 3607 +/- 1525 ng/ml/min (mean +/- SD) to 12886 +/- 6558 ng/ml/min and maximum serum concentration (Cmax) from 6 +/- 2 to 26 +/- 9 ng/ml but did not influence the elimination half-life. Diltiazem did not affect the oral AUC, Cmax, or half-life of pravastatin. The average steady-state serum concentrations of diltiazem were not significantly different between the lovastatin (130 +/- 58 ng/ml) and pravastatin (110 +/- 30 ng/ml) study arms.

CONCLUSION

Diltiazem greatly increased the plasma concentration of lovastatin, but the magnitude of this effect was much greater than that predicted by the systemic serum concentration, suggesting that this interaction is a first-pass rather than a systemic event. The magnitude of this effect and the frequency of coadministration suggest that caution is necessary when administering diltiazem and lovastatin together. Further studies should explore whether this interaction abrogates the efficacy of lovastatin or enhances toxicity and whether it occurs with other cytochrome P4503A4-metabolized 3-hydroxy-3-methylglutaryl-coenzyme A reductase inhibitors, such as simvastatin, fluvastatin, and atorvastatin.

Clinical pharmacokinetics of 3-hydroxy-3-methylglutaryl-coenzyme A reductase inhibitors

3-Hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase is the key enzyme of cholesterol synthesis. HMG-CoA reductase inhibitors are potent reversible inhibitors of this enzyme, which act by competing for the substrate HMG-CoA. This review is mainly devoted to the 4 main HMG-CoA reductase inhibitors used today: lovastatin, simvastatin, pravastatin and fluvastatin. Depending upon the dosage, these drugs are able to reduce plasma cholesterol levels by more than 40%. After absorption, each undergoes extensive hepatic first-pass metabolism. Up to 5 primary metabolites are formed, some of which are active inhibitors. The elimination half-lives vary from 0.5 to 3.5 hours and excretion is mainly via the faeces. A limited number of drug interactions has been reported. Increases in liver enzymes and muscle creatine kinase activity are among the most severe adverse effects. These powerful drugs should be reserved for patients with high plasma cholesterol levels and/or those with cardiovascular disease. New therapeutic approaches to atherosclerosis are currently under investigation. HMG-CoA reductase inhibitors are the cornerstone of this research.

Biotransformation of lovastatin. IV. Identification of cytochrome P450 3A proteins as the major enzymes responsible for the oxidative metabolism of lovastatin in rat and human liver microsomes

Previous studies from our laboratories have shown that the metabolism of the cholesterol-lowering drug lovastatin by rat and human liver microsomes occurs primarily at the 6'-position, giving 6' beta-hydroxy- and 6'-exomethylene-lovastatin and that these oxidations are catalyzed by cytochrome P450-dependent monooxygenases. In the present study, the specific cytochrome P450 form involved in lovastatin oxidation was identified through immunoinhibition studies. Among several antibodies prepared against various cytochrome P450s, only anti-rat P450 3A IgG inhibited lovastatin metabolism in liver microsomes from untreated, phenobarbital-treated, and pregnenolone-16 alpha-carbonitrile-treated rats. Lovastatin metabolism at the 6'-position was markedly inhibited (6' beta-hydroxy, greater than 95%; 6'-exomethylene, 70-80%) by this antibody whereas the effect of anti-rat P450 3A on the 3"-hydroxylation was variable depending on the source of the microsomes. With human liver microsomes, both anti-rat P450 3A and anti-human P450 3A inhibited lovastatin metabolism. Correlation between lovastatin oxidation and the P450 3A content in human liver microsomes (measured by immunoblot analysis) was excellent (r2 = 0.97). In addition, preincubation of human liver microsomes with troleandomycin and NADPH inhibited metabolism by 60%. These results clearly indicate that cytochrome P450 3A enzymes are primarily responsible for the metabolism of lovastatin in rat and human liver microsomes.