Clinical pharmacokinetics of cerivastatin

Biphasic effects of statins on neuron cell functions under oxygen-glucose deprivation and normal culturing conditions via different mechanisms

While there is a growing interest in the use of statins, HMG‐CoA reductase inhibitors, to treat neurodegenerative diseases, statins are associated with conflicting effects within the central nervous system (CNS) without clear evidence of the underlying mechanisms. This study systematically investigated effects of four statins (atorvastatin, pitavastatin, cerivastatin, and lovastatin) on neuronal cells under pathological condition using an in vitro model depicting ischemic injury, as well as tested under physiological condition. All four statins at micromolar concentrations display toxic effects on neuron cells under physiological condition. Atorvastatin and cerivastatin but not pitavastatin or lovastatin at nanomolar concentrations display protective effects on neuron cells under ischemic injury condition, via decreased ischemic injury‐induced oxidative stress, oxidative damage, and inflammation. Mechanistically, atorvastatin, pitavastatin, and lovastatin induces neuron cell apoptosis via prenylation‐independent manner. Other mechanisms are involved in the pro‐apoptotic effect of cerivastatin. Prenylation is not involved in the protective effects of statins under ischemic injury condition. Our work provides better understanding on the multiple differential effects of statins on neuron cells under physiological condition and ischemic injury, and elucidate their underlying mechanisms, which may be of relevance to the influence of statins in CNS.

Improvement of Platelet Respiration by Cell-Permeable Succinate in Diabetic Patients Treated with Statins

Diabetes mellitus (DM) is the most severe metabolic disease that reached the level of a global pandemic and is associated with high cardiovascular morbidity. Statins are the first-line lipid-lowering therapy in diabetic patients with or without a history of atherosclerotic disease. Although well tolerated, chronic treatment may result in side effects that lead to treatment interruption. Mitochondrial dysfunction has emerged as a central pathomechanism in DM- and statin-induced side effects. Assessment of mitochondrial respiration in peripheral platelets has been increasingly used as a mirror of organ mitochondrial dysfunction. The present study aimed to assess the: (i) changes in mitochondrial respiration elicited by statins in patients with type 2 DM and (ii) the effects of cell-permeable succinate (NV118) on respiratory parameters in platelets harvested from these patients. No significant changes were found in global mitochondrial respiration of intact platelets isolated from diabetic patients treated with either atorvastatin or rosuvastatin. Similarly, no significant changes in mitochondrial respiration of permeabilized platelets were found between diabetic patients treated with atorvastatin and healthy controls. Acute ex vivo administration of NV118 significantly improved respiration in isolated platelets. These results prompt further research on the role of permeable succinate as a therapeutic alternative for improving mitochondrial function in metabolic pathologies and point to the role of peripheral platelets as a potential biomarker of treatment response.

Cell-Permeable Succinate Rescues Mitochondrial Respiration in Cellular Models of Statin Toxicity

Statins are the cornerstone of lipid-lowering therapy. Although generally well tolerated, statin-associated muscle symptoms (SAMS) represent the main reason for treatment discontinuation. Mitochondrial dysfunction of complex I has been implicated in the pathophysiology of SAMS. The present study proposed to assess the concentration-dependent ex vivo effects of three statins on mitochondrial respiration in viable human platelets and to investigate whether a cell-permeable prodrug of succinate (complex II substrate) can compensate for statin-induced mitochondrial dysfunction. Mitochondrial respiration was assessed by high-resolution respirometry in human platelets, acutely exposed to statins in the presence/absence of the prodrug NV118. Statins concentration-dependently inhibited mitochondrial respiration in both intact and permeabilized cells. Further, statins caused an increase in non-ATP generating oxygen consumption (uncoupling), severely limiting the OXPHOS coupling efficiency, a measure of the ATP generating capacity. Cerivastatin (commercially withdrawn due to muscle toxicity) displayed a similar inhibitory capacity compared with the widely prescribed and tolerable atorvastatin, but did not elicit direct complex I inhibition. NV118 increased succinate-supported mitochondrial oxygen consumption in atorvastatin/cerivastatin-exposed platelets leading to normalization of coupled (ATP generating) respiration. The results acquired in isolated human platelets were validated in a limited set of experiments using atorvastatin in HepG2 cells, reinforcing the generalizability of the findings.

The Role of Structure and Biophysical Properties in the Pleiotropic Effects of Statins

Statins are a class of drugs used to lower low-density lipoprotein cholesterol and are amongst the most prescribed medications worldwide. Most statins work as a competitive inhibitor of 3-hydroxy-3-methyl-glutaryl-coenzyme A reductase (HMGR), but statin intolerance from pleiotropic effects have been proposed to arise from non-specific binding due to poor enzyme-ligand sensitivity. Yet, research into the physicochemical properties of statins, and their interactions with off-target sites, has not progressed much over the past few decades. Here, we present a concise perspective on the role of statins in lowering serum cholesterol levels, and how their reported interactions with phospholipid membranes offer a crucial insight into the mechanism of some of the more commonly observed pleiotropic effects of statin administration. Lipophilicity, which governs hepatoselectivity, is directly related to the molecular structure of statins, which dictates interaction with and transport through membranes. The structure of statins is therefore a clinically important consideration in the treatment of hypercholesterolaemia. This review integrates the recent biophysical studies of statins with the literature on the physiological effects and provides new insights into the mechanistic cause of statin pleiotropy, and prospective means of understanding the cholesterol-independent effects of statins.

Prediction of Cyclosporin-Mediated Drug Interaction Using Physiologically Based Pharmacokinetic Model Characterizing Interplay of Drug Transporters and Enzymes

Uptake transporter organic anion transporting polypeptides (OATPs), efflux transporters (P-gp, BCRP and MRP2) and cytochrome P450 enzymes (CYP450s) are widely expressed in the liver, intestine or kidney. They coordinately work to control drug disposition, termed as "interplay of transporters and enzymes". Cyclosporine A (CsA) is an inhibitor of OATPs, P-gp, MRP2, BCRP and CYP3As. Drug-drug interaction (DDI) of CsA with victim drugs occurs via disordering interplay of transporters and enzymes. We aimed to establish a whole-body physiologically-based pharmacokinetic (PBPK) model which predicts disposition of CsA and nine victim drugs including atorvastatin, cerivastatin, pravastatin, rosuvastatin, fluvastatin, simvastatin, lovastatin, repaglinide and bosentan, as well as drug-drug interactions (DDIs) of CsA with nine victim drugs to investigate the integrated effect of enzymes and transporters in liver, intestinal and kidney on drug disposition. Predictions were compared with observations. Most of the predictions were within 0.5-2.0 folds of observations. Atorvastatin was represented to investigate individual contributions of transporters and CYP3As to atorvastatin disposition and their integrated effect. The contributions to atorvastatin disposition were hepatic OATPs >> hepatic CYP3A > intestinal CYP3As ≈ efflux transporters (P-gp/BCRP/MRP2). The results got the conclusion that the developed PBPK model characterizing the interplay of enzymes and transporters was successfully applied to predict the pharmacokinetics of 10 OATP substrates and DDIs of CsA with 9 victim drugs.

Cerivastatin for lowering lipids

BACKGROUND

Cerivastatin was the most potent statin until it was withdrawn from the market due to a number of fatalities due to rhabdomyolysis, however, the dose-related magnitude of effect of cerivastatin on blood lipids is not known.

OBJECTIVES

Primary objective To quantify the effects of various doses of cerivastatin on the surrogate markers: LDL cholesterol, total cholesterol, HDL cholesterol and triglycerides in children and adults with and without cardiovascular disease. The aim of this review is to examine the pharmacology of cerivastatin by characterizing the dose-related effect and variability of the effect of cerivastatin on surrogate markers. Secondary objectives To quantify the effect of various doses of cerivastatin compared to placebo on withdrawals due to adverse effects. To compare the relative potency of cerivastatin with respect to fluvastatin, atorvastatin and rosuvastatin for LDL cholesterol, total cholesterol, HDL cholesterol and triglycerides.

SEARCH METHODS

The Cochrane Hypertension Information Specialist searched the following databases for RCTs up to March 2019: CENTRAL (2019, Issue 3), Ovid MEDLINE, Ovid Embase, the WHO International Clinical Trials Registry Platform, and ClinicalTrials.gov.We also searched the European Patent Office, FDA.gov, and ProQuest Dissertations & Theses, and contacted authors of relevant papers regarding further published and unpublished work. The searches had no language restrictions.

SELECTION CRITERIA

RCTs and controlled before-and-after studies evaluating the dose response of different fixed doses of cerivastatin on blood lipids over a duration of three to 12 weeks in participants of any age with and without cardiovascular disease.

DATA COLLECTION AND ANALYSIS

Two review authors independently assessed eligibility criteria for trials to be included and extracted data. We entered data from RCTs and controlled before-and-after studies into Review Manager 5 as continuous and generic inverse variance data respectively. We collected information on withdrawals due to adverse effects from the RCTs. We assessed all trials using the 'Risk of bias' tool under the categories of sequence generation, allocation concealment, blinding, incomplete outcome data, selective reporting, and other potential biases.

MAIN RESULTS

Fifty trials (19 RCTs and 31 before-and-after studies) evaluated the dose-related efficacy of cerivastatin in 12,877 participants who had their LDL cholesterol measured. The participants were of any age with and without cardiovascular disease and the trials studied cerivastatin effects within a treatment period of three to 12 weeks. Cerivastatin 0.025 mg/day to 0.8 mg/day caused LDL cholesterol decreases of 11.0% to 40.8%, total cholesterol decreases of 8.0% to 28.8% and triglyceride decreases of 9.0% to 21.4%. We judged the certainty of evidence for these effects to be high. Log dose-response data over doses of 2.5 mg to 80 mg revealed strong linear dose-related effects on LDL cholesterol, total cholesterol and triglycerides. When compared to fluvastatin, atorvastatin and rosuvastatin, cerivastatin was about 250-fold more potent than fluvastatin, 20-fold more potent than atorvastatin and 5.5-fold more potent than rosuvastatin at reducing LDL cholesterol; 233-fold more potent than fluvastatin, 18-fold more potent than atorvastatin and six-fold more potent than rosuvastatin at reducing total cholesterol; and 125-fold more potent than fluvastatin, 11-fold more potent than atorvastatin and 13-fold more potent than rosuvastatin at reducing triglycerides. There was no dose-related effect of cerivastatin on HDL cholesterol, but overall cerivastatin increased HDL cholesterol by 5%. There was a high risk of bias for the outcome withdrawals due to adverse effects, but a low risk of bias for the lipid measurements. Withdrawals due to adverse effects were not different between cerivastatin and placebo in 11 of 19 of these short-term trials (risk ratio 1.09, 95% confidence interval 0.68 to 1.74).

AUTHORS' CONCLUSIONS

The LDL cholesterol, total cholesterol, and triglyceride lowering effect of cerivastatin was linearly dependent on dose. Cerivastatin log dose-response data were linear over the commonly prescribed dose range. Based on an informal comparison with fluvastatin, atorvastatin and rosuvastatin, cerivastatin was about 250-fold more potent than fluvastatin, 20-fold more potent than atorvastatin and 5.5-fold more potent than rosuvastatin in reducing LDL cholesterol, and 233-fold greater potency than fluvastatin, 18-fold greater potency than atorvastatin and six-fold greater potency than rosuvastatin at reducing total cholesterol. This review did not provide a good estimate of the incidence of harms associated with cerivastatin because of the short duration of the trials and the lack of reporting of adverse effects in 42% of the RCTs.

Statin-Related Myotoxicity: A Comprehensive Review of Pharmacokinetic, Pharmacogenomic and Muscle Components

Statins are a cornerstone in the pharmacological prevention of cardiovascular disease. Although generally well tolerated, a small subset of patients experience statin-related myotoxicity (SRM). SRM is heterogeneous in presentation; phenotypes include the relatively more common myalgias, infrequent myopathies, and rare rhabdomyolysis. Very rarely, statins induce an anti-HMGCR positive immune-mediated necrotizing myopathy. Diagnosing SRM in clinical practice can be challenging, particularly for mild SRM that is frequently due to alternative aetiologies and the nocebo effect. Nevertheless, SRM can directly harm patients and lead to statin discontinuation/non-adherence, which increases the risk of cardiovascular events. Several factors increase systemic statin exposure and predispose to SRM, including advanced age, concomitant medications, and the nonsynonymous variant, rs4149056, in SLCO1B1, which encodes the hepatic sinusoidal transporter, OATP1B1. Increased exposure of skeletal muscle to statins increases the risk of mitochondrial dysfunction, calcium signalling disruption, reduced prenylation, atrogin-1 mediated atrophy and pro-apoptotic signalling. Rare variants in several metabolic myopathy genes including CACNA1S, CPT2, LPIN1, PYGM and RYR1 increase myopathy/rhabdomyolysis risk following statin exposure. The immune system is implicated in both conventional statin intolerance/myotoxicity via LILRB5 rs12975366, and a strong association exists between HLA-DRB1*11:01 and anti-HMGCR positive myopathy. Epigenetic factors (miR-499-5p, miR-145) have also been implicated in statin myotoxicity. SRM remains a challenge to the safe and effective use of statins, although consensus strategies to manage SRM have been proposed. Further research is required, including stringent phenotyping of mild SRM through N-of-1 trials coupled to systems pharmacology omics- approaches to identify novel risk factors and provide mechanistic insight.

Comparative and quantitative assessment on statin efficacy and safety: insights into inter-statin and inter-individual variability via dose- and exposure-response relationships

Introduction: Statins are prescribed widely for cholesterol-lowering therapy, but it is known that their efficacy and safety profiles vary, despite the shared pharmacophore and pharmacological target. The immense body of related clinical and preclinical data offers a unique opportunity to explore the possible factors underlying inter-statin and inter-individual variabilities.Area covered: Clinical and preclinical data from various statins were compiled with regard to the efficacy (cholesterol-lowering effect) and safety (muscle toxicity). Based on the compiled data, dose- and exposure-response relationships were explored to obtain mechanistic and quantitative insights into the variations in the efficacy and safety profiles of statins.Expert opinion: Our analyses indicated that the inter-statin variability in the cholesterol-lowering effect may be mainly attributable to variations in potency of inhibition of the pharmacological target, rather than variations in drug exposure at the site of drug action. However, the drug exposure at the sites of drug action (i.e., the liver for efficacy and the muscle for safety) may contribute to the differences in the efficacy and safety observed in individual patients.

Simvastatin causes pulmonary artery relaxation by blocking smooth muscle ROCK and calcium channels: Evidence for an endothelium-independent mechanism

Simvastatin reduces pulmonary arterial pressure and right ventricular hypertrophy in animal models of pulmonary arterial hypertension (PAH) and is thought to restore endothelial dysfunction. In vivo effects of drugs are complicated by several factors and little is known of the direct effects of statins on pulmonary arteries. This study investigated the direct effects of simvastatin on pulmonary arteries isolated from rats with or without monocrotaline-induced PAH. Simvastatin suppressed contractions evoked by the thromboxane A2 receptor agonist U46619 (30 nM), the α₁–adrenergic agonist phenylephrine (5 μM) and KCl (50 mM) by ~50% in healthy and diseased arteries, but did not reduce contraction evoked by sarco/endoplasmic reticulum ATPase blockers. It relaxed hypertensive arteries in the absence of stimulation. Removing the endothelium or inhibiting eNOS did not prevent the inhibition by simvastatin. Inhibiting RhoA/rho kinase (ROCK) with Y27632 (10 μM) suppressed contractions to U46619 and phenylephrine by ~80% and prevented their inhibition by simvastatin. Y27632 reduced KCl-induced contraction by ~30%, but did not prevent simvastatin inhibition. Simvastatin suppressed Ca²⁺ entry into smooth muscle cells, as detected by Mn²⁺ quench of fura-2 fluorescence. The calcium antagonist, nifedipine (1 μM), almost abolished K⁺-induced contraction with less effect against U46619 and phenylephrine. We conclude that simvastatin relaxes pulmonary arteries by acting on smooth muscle to interfere with signalling through G-protein coupled receptors and voltage-dependent Ca²⁺ entry. Its actions likely include inhibition of ROCK-dependent Ca²⁺ sensitisation and voltage-gated Ca²⁺ channels. These are likely to contribute to the beneficial effects of simvastatin in animal models of PAH.

Quantitative Analysis of Complex Drug-Drug Interactions between Cerivastatin and Metabolism/Transport Inhibitors Using Physiologically Based Pharmacokinetic Modeling

Cerivastatin (CER) was withdrawn from the world market because of lethal rhabdomyolysis. Coadministrations of CER and cyclosporine A (CsA) or gemfibrozil (GEM) have been reported to increase the CER blood concentration. CsA is an inhibitor of organic anion transporting polypeptide (OATP)1B1 and CYP3A4, and GEM and its glucuronide (GEM-glu) inhibit OATP1B1 and CYP2C8. The purpose of this study was to describe the transporter-/enzyme-mediated drug-drug interactions (DDIs) of CER with CsA or GEM based on unified physiologically based pharmacokinetic (PBPK) models and to investigate whether the DDIs can be quantitatively analyzed by a bottom-up approach. Initially, the PBPK models for CER and GEM/GEM-glu were constructed based on the previously reported standard protocols. Next, the drug-dependent parameters were optimized by Cluster Newton Method. Thus, described concentration-time profiles for CER and GEM/GEM-glu agreed well with the clinically observed data. The DDIs were then simulated using the established PBPK models with previously obtained in vitro inhibition constants of CsA or GEM/GEM-glu against the OATP1B1 and cytochrome P450s. DDIs with the inhibitors were underestimated compared with observed data using the geometric means of reported values. To search for better described parameters within the range of in vitro values, sensitivity analyses were performed for DDIs of CER. Using the in vitro parameter sets selected by sensitivity analyses, these DDIs were well reproduced, indicating that the present PBPK models were able to describe adequately the clinical DDIs based on a bottom-up approach. The approaches in this study would be applicable to the prediction of other DDIs involving both transporters and metabolic enzymes.

Genetic Polymorphisms and in Vitro Functional Characterization of CYP2C8, CYP2C9, and CYP2C19 Allelic Variants

Genetic variations in CYP 2C (CYP2C) subfamily, CYP2C8, CYP2C9, and CYP2C19 contribute to interindividual variability in the metabolism of clinically used drugs. Changes in the drug metabolizing activity of CYP2C members may cause unexpected and serious adverse drug reactions and inadequate therapeutic effects. Therefore, CYP2C gene polymorphism is used as a genome biomarker for predicting responsiveness to administered drugs. The most direct method for understanding the extent of the effects of CYP2C gene polymorphism on drug pharmacokinetics is by evaluating the blood and urine concentrations of the drug in subjects. However, in vivo tests are highly invasive, and considering the risk of adverse drug reactions, the burden on the patient may be significant. In addition, examining the functions of rare variant enzymes with an allele frequency of ≤1% requires at least several hundred subjects. Furthermore, it is extremely difficult to evaluate the functions of all variant enzymes in an in vivo test. On the other hand, in vitro enzyme activity can be evaluated using a heterologous expression system to avoid the aforementioned problems. In vitro tests are extremely important as they complement in vivo information. This review focuses on recent findings of in vitro studies on 3 highly polymorphic CYP2C members: CYP2C8, CYP2C9, and CYP2C19.

Role of Cytochrome P450 2C8 in Drug Metabolism and Interactions

During the last 10-15 years, cytochrome P450 (CYP) 2C8 has emerged as an important drug-metabolizing enzyme. CYP2C8 is highly expressed in human liver and is known to metabolize more than 100 drugs. CYP2C8 substrate drugs include amodiaquine, cerivastatin, dasabuvir, enzalutamide, imatinib, loperamide, montelukast, paclitaxel, pioglitazone, repaglinide, and rosiglitazone, and the number is increasing. Similarly, many drugs have been identified as CYP2C8 inhibitors or inducers. In vivo, already a small dose of gemfibrozil, i.e., 10% of its therapeutic dose, is a strong, irreversible inhibitor of CYP2C8. Interestingly, recent findings indicate that the acyl-β-glucuronides of gemfibrozil and clopidogrel cause metabolism-dependent inactivation of CYP2C8, leading to a strong potential for drug interactions. Also several other glucuronide metabolites interact with CYP2C8 as substrates or inhibitors, suggesting that an interplay between CYP2C8 and glucuronides is common. Lack of fully selective and safe probe substrates, inhibitors, and inducers challenges execution and interpretation of drug-drug interaction studies in humans. Apart from drug-drug interactions, some CYP2C8 genetic variants are associated with altered CYP2C8 activity and exhibit significant interethnic frequency differences. Herein, we review the current knowledge on substrates, inhibitors, inducers, and pharmacogenetics of CYP2C8, as well as its role in clinically relevant drug interactions. In addition, implications for selection of CYP2C8 marker and perpetrator drugs to investigate CYP2C8-mediated drug metabolism and interactions in preclinical and clinical studies are discussed.

Clopidogrel Markedly Increases Plasma Concentrations of CYP2C8 Substrate Pioglitazone

The glucose-lowering drug pioglitazone undergoes hepatic CYP2C8-mediated biotransformation to its main metabolites. The antiplatelet drug clopidogrel is metabolized to clopidogrel acyl-β-d-glucuronide, which was recently found to be a strong time-dependent inhibitor of CYP2C8 in humans. Therefore, we studied the effect of clopidogrel on the pharmacokinetics of pioglitazone. In a randomized crossover study, 10 healthy volunteers ingested either 300 mg of clopidogrel on day 1, and 75 mg on days 2 and 3, or placebo. Pioglitazone 15 mg was administered 1 hour after placebo and clopidogrel on day 1. Plasma concentrations of pioglitazone, clopidogrel, and their main metabolites were measured up to 72 hours. Clopidogrel increased the area under the plasma concentration-time curve (AUC0-∞) of pioglitazone 2.1-fold [P < 0.001, 90% confidence interval (CI) 1.8-2.6] and prolonged its half-life from 6.7 to 11 hours (P = 0.002). The peak concentration of pioglitazone was unaffected but the concentration at 24 hours was increased 4.5-fold (range 1.6-9.8; P < 0.001, 90% CI 3.17-6.45) by clopidogrel. The M-IV-to-pioglitazone AUC0-∞ ratio was 49% (P < 0.001, 90% CI 0.40-0.59) of that during the control phase, indicating that clopidogrel inhibited the CYP2C8-mediated biotransformation of pioglitazone. Clopidogrel increases the exposure to pioglitazone by inhibiting its CYP2C8-mediated biotransformation. In consequence, use of clopidogrel may increase the risk of fluid retention and other concentration-related adverse effects of pioglitazone.

Mechanism of Drug-Drug Interactions Between Warfarin and Statins

The anticoagulant drug warfarin and the lipid-lowering statin drugs are commonly co-administered to patients with cardiovascular diseases. Clinically significant drug-drug interactions (DDIs) between these drugs have been recognized through case studies for many years, but the biochemical mechanisms causing these interactions have not been explained fully. Previous theories include kinetic alterations in cytochrome P-450-mediated drug metabolism or disturbances of drug-protein binding, leading to anticoagulant activity of warfarin; however, neither the enantioselective effects on warfarin metabolism nor the potential disruption of drug transporter function have been well investigated. This study investigated the etiology of the DDIs between warfarin and statins. Liquid chromatography-mass spectrometry methods were developed and validated to quantify racemic warfarin, 6 of its hydroxylated metabolites, and pure enantiomers of warfarin; these methods were applied to study the role of different absorption, distribution, metabolism, and excretion properties, leading to DDIs. Plasma protein binding displacement of warfarin was performed in the presence of statins using equilibrium dialysis method. Substrate kinetics of warfarin and pure enantiomers were performed with human liver microsomes to determine the kinetic parameters (Km and Vmax) for the formation of all 6 hydroxywarfarin metabolites, inhibition of warfarin metabolism in the presence of statins, was determined. Uptake transport studies of warfarin were performed using overexpressing HEK cell lines and efflux transport using human adenocarcinoma colonic cell line cells. Fluvastatin significantly displaced plasma protein binding of warfarin and pure enantiomers; no other statin resulted in significant displacement of warfarin. All the statins that inhibited the formation of 10-hydroxywarfarin, atorvastatin, pitavastatin, and simvastatin were highly potent compared to other statins; in contrast, only fluvastatin was found to be a potent inhibitor of formation of 7-hydroxy warfarin. Uptake and efflux drug transporters do not play any role in these DDIs. The results showed that DDIs between warfarin and statins are primarily caused by cytochrome P-450 inhibition.

Current understanding of hepatic and intestinal OATP-mediated drug-drug interactions

At present, many patients are medicated with various drugs, which are, at the same time, associated with an increased risk of drug-drug interactions (DDIs). Detailed analysis of mechanisms underlying DDIs is the basis of a better prediction of adverse drug events caused by drug interactions. In the last few decades, an involvement of transporters in such processes has been more and more recognized. Indeed, uptake transporters belonging to the organic anion-transporting polypeptide (OATP) family have been shown to interact with a variety of drugs in clinical use. Particularly, the subfamily of OATP1B transporters has been extensively studied, identifying several clinical significant DDIs based on those hepatic uptake transporters. By contrast, the role of OATP2B1 in this context is rather underestimated. Therefore, in addition to known interactions based on OATP1B transporters, we have focused on DDIs probably based on OATP2B1 inhibition in the liver and those possibly owing to the inhibition of OATP2B1-mediated drug absorption in the intestine.

Pharmacogenetics of drugs withdrawn from the market

The safety and efficacy of candidate compounds are critical factors during the development of drugs, and most drugs have been withdrawn from the market owing to severe adverse reactions. Individuals/populations with different genetic backgrounds may show significant differences in drug metabolism and efficacy, which can sometimes manifest as severe adverse drug reactions. With an emphasis on the mechanisms underlying abnormal drug effects caused by genetic mutations, pharmacogenetic studies may enhance the safety and effectiveness of drug use, provide more comprehensive delineations of the scope of usage, and change the fates of drugs withdrawn from the market.

A screening study of drug-drug interactions in cerivastatin users: an adverse effect of clopidogrel

An analysis of a case-control study of rhabdomyolysis was conducted to screen for previously unrecognized cytochrome P450 enzyme (CYP) 2C8 inhibitors that may cause other clinically important drug-drug interactions. Medication use in cases of rhabdomyolysis using cerivastatin (n = 72) was compared with that in controls using atorvastatin (n = 287) for the period 1998-2001. The use of clopidogrel was strongly associated with rhabdomyolysis (odds ratio (OR) 29.6; 95% confidence interval (CI), 6.1-143). In a replication effort that used the US Food and Drug Administration (FDA) Adverse Event Reporting System (AERS), it was found that clopidogrel was used more commonly in patients with rhabdomyolysis receiving cerivastatin (17%) than in those receiving atorvastatin (0%, OR infinity; 95% CI = 5.2-infinity). Several medications were tested in vitro for their potential to cause drug-drug interactions. Clopidogrel, rosiglitazone, and montelukast were the most potent inhibitors of cerivastatin metabolism. Clopidogrel and its metabolites also inhibited cerivastatin metabolism in human hepatocytes. These epidemiological and in vitro findings suggest that clopidogrel may cause clinically important, dose-dependent drug-drug interactions with other medications metabolized by CYP2C8.

Cerivastatin, genetic variants, and the risk of rhabdomyolysis

OBJECTIVE

The withdrawal of cerivastatin involved an uncommon but serious adverse reaction, rhabdomyolysis. The bimodal response, rhabdomyolysis in a small proportion of users, points to genetic factors as a potential cause. We conducted a case-control study to evaluate genetic markers for cerivastatin-associated rhabdomyolysis.

METHODS

This study had two components: a candidate gene study to evaluate variants in CYP2C8, UGT1A1, UGT1A3, and SLCO1B1; and a genome-wide association study to identify risk factors in other regions of the genome. A total of 185 rhabdomyolysis cases were frequency matched to statin-using controls from the Cardiovascular Health Study (n=374) and the Heart and Vascular Health Study (n=358). Validation relied on functional studies.

RESULTS

Permutation test results suggested an association between cerivastatin-associated rhabdomyolysis and variants in SLCO1B1 (P=0.002), but not variants in CYP2C8 (P=0.073) or UGTs (P=0.523). An additional copy of the minor allele of SLCO1B1 rs4149056 (p.Val174Ala) was associated with the risk of rhabdomyolysis (odds ratio: 1.89; 95% confidence interval: 1.40-2.56). In transfected cells, this variant reduced cerivastatin transport by 40% compared with the reference transporter (P<0.001). The genome-wide association study identified an intronic variant (rs2819742) in the ryanodine receptor 2 gene (RYR2) as significant (P=1.74E-07). An additional copy of the minor allele of the RYR2 variant was associated with a reduced risk of rhabdomyolysis (odds ratio: 0.48; 95% confidence interval: 0.36-0.63).

CONCLUSION

We identified modest genetic risk factors for an extreme response to cerivastatin. Disabling genetic variants in the candidate genes were not responsible for the bimodal response to cerivastatin.

Statins therapy: a review on conventional and novel formulation approaches

BACKGROUND AND OBJECTIVE

High levels of cholesterol lead to atherosclerosis, a factor predisposing to the development of coronary artery disease. Statin drugs, i.e. HMG-CoA reductase inhibitors, have been known since the end of the last century for their benefits against cardio- and cerebrovascular diseases and are widely used clinically. This review aims at compiling the research inputs being made for developing therapeutically efficacious dosage forms that have the potential to surmount the limitations of conventional dosage forms of statins.

KEY FINDINGS

Statin drugs can reduce the endogenous synthesis of cholesterol and prevent the onset and development of atherosclerosis, and are therefore used as an effective treatment against primary hypercholesterolemia. At present, statin drugs are most often administered orally, on a daily basis. After administration, the bioavailability and the general circulation of statin drugs is fairly low due to the first-pass metabolism in the liver and clearance by the digestive system. Extensive pharmaceutical research in understanding the causes of low oral bioavailability has led to the development of novel technologies to address these challenges.

SUMMARY

These technologies vary from conventional dosage forms to nanoparticulate drug-delivery systems, and have the potential to cause improvements in bioavailability and consequently therapeutic efficacy.

Cerivastatin in vitro metabolism by CYP2C8 variants found in patients experiencing rhabdomyolysis

OBJECTIVES

Cerivastatin, a 3-hydroxy-3-methylglutaryl-coenzyme A reductase inhibitor withdrawn from the market because of serious adverse effects, is metabolized primarily by CYP2C8. The occurrence of associated myotoxicity and rhabdomyolysis were attributed to altered cerivastatin pharmacokinetics on account of gemfibrozil-inhibition or genetic variations in CYP2C8 and drug transporters involved in cerivastatin clearance. However, the effect of CYP2C8 genetic variation on cerivastatin metabolism has not been fully elucidated.

METHODS

In this study, patients (n=126) with confirmed cases of rhabdomyolysis after cerivastatin administration had their CYP2C8 gene resequenced and the metabolism of cerivastatin by the discovered CYP2C8 variants was assessed in proteins expressed in Escherichia coli.

RESULTS

In this unique patient population, 12 novel single nucleotide polymorphisms were discovered of which six were exclusively found in patients not using gemfibrozil. Three rare exonic variants resulted in amino acid substitutions and a frame shift deletion (V472fsL494 generating a defective mostly heme-free CYP2C8 protein). A particular promoter located deletion (-635_-634delTA) was tightly linked to CYP2C8*3. Heterologously expressed CYP2C8.3 and CYP2C8.4 displayed an increase in cerivastatin metabolic clearance of up to six-fold compared with the wild-type enzyme. Similarly, an independent sample of microsomes from human livers carrying the CYP2C8*3 and CYP2C8*4 alleles exhibited a 2-fold to 14-fold increase in normalized cerivastatin intrinsic clearance, compared with microsomes from livers carrying only the wild type allele.

CONCLUSION

Gain or loss of catalytic function found in the CYP2C8 gene could certainly alter cerivastatin pharmacokinetics and may influence, at least in part, susceptibility to the development of myotoxicity.

Genetic determinants of mycophenolate-related anemia and leukopenia after transplantation

BACKGROUND

Mycophenolate-related anemia and leukopenia are well-known toxicities after transplantation. Toxicity leads to dose reduction, addition of colony-stimulating factors or erythropoietin, or discontinuation of immunosuppressive therapy. The causes of and risk factors associated with toxicity are unclear.

METHODS

We studied the association between mycophenolate-related anemia and leukopenia and 2724 single nucleotide polymorphisms (SNP) in 978 patients undergoing living or deceased donor kidney transplant. Patients were followed up to time of first anemia (hemoglobin<10 gm/dL or hematocrit<30%) or first leukopenia (white blood cell [WBC] count <3000 cells/mm), which required clinical intervention in the first 6 months after transplant.

RESULTS

Anemia occurred in 87 (9.5%) subjects and leukopenia in 224 (22.9%). In single SNP analyses, none of the SNPs were associated with time to leukopenia at a false discovery rate (FDR) of 20%. However, SNPs from the IL12A, HUS, CYP2C8 genes were associated with time to anemia, allowing for an FDR of 20%. To assess the independence of these SNPs as predictors of anemia, we conducted a multi-SNP analysis including one SNP from each of the three genes. All three SNPs were associated with time to anemia, after adjusting for recipient age, weight, posttransplant dialysis and antiviral drug use, and stratifying by clinical center.

CONCLUSION

Although these SNPs require validation in an independent population, our results suggest that genetics may play a role in risk of mycophenolate-related hematologic toxicity. This may ultimately provide for better management of maintenance immunosuppression and gives insights into potential mechanism(s) by which toxicity occurs.

Human liver expression of CYP2C8: gender, age, and genotype effects

Research investigating CYP2C8 as a drug-metabolizing enzyme has gained momentum over the past few years. CYP2C8 is estimated to oxidatively metabolize approximately 5% of therapeutically prescribed drugs. It is polymorphically expressed, and several single nucleotide polymorphisms have been identified with varying effects on the clearance of CYP2C8 substrates. However, the human liver expression of CYP2C8 and effects of genetic variation, age, and gender on mRNA and protein levels have not been fully explored. In this report, interindividual variation in CYP2C8 mRNA and protein expression in 60 livers from white individuals was examined. The livers were genotyped for CYP2C8*3 and CYP2C8*4 polymorphisms. The effects of genotype, age, and gender on hepatic CYP2C8 expression and the correlation of CYP2C8 mRNA expression with CYP3A4 and other CYP2C members were evaluated. The mean +/- S.D. protein levels in CYP2C8*1/*1 livers was 30.8 +/- 17.5 pmol/mg protein, and a trend for decreased protein levels was observed for CYP2C8*1/*4 livers (15.8 +/- 9.7 pmol/mg, p = 0.07). The mean expression levels of CYP2C8 was comparable in males and females (p = 0.18). The mRNA expression of CYP2C8, CYP2C9, CYP2C19, and CYP3A4, but not CYP2C18, was highly correlated (p < 0.0001). Moreover, the hepatic CYP2C8 and CYP3A4 protein levels were strongly correlated (r = 0.76, p < 0.0001). This correlation is most likely due to common regulation factors for both genes. CYP2C8 mRNA or protein expression levels were not significantly affected by CYP2C8*3 or *4 genotype, gender, or age, and variation observed clinically in CYP2C8 activity warrants further investigation.

Cytochrome P450 2C8 pharmacogenetics: a review of clinical studies

Cytochrome P450 (CYP) 2C8 is responsible for the oxidative metabolism of many clinically available drugs from a diverse number of drug classes (e.g., thiazolidinediones, meglitinides, NSAIDs, antimalarials and chemotherapeutic taxanes). The CYP2C8 enzyme is encoded by the CYP2C8 gene, and several common nonsynonymous polymorphisms (e.g., CYP2C8*2 and CYP2C8*3) exist in this gene. The CYP2C8*2 and *3 alleles have been associated in vitro with decreased metabolism of paclitaxel and arachidonic acid. Recently, the influence of CYP2C8 polymorphisms on substrate disposition in humans has been investigated in a number of clinical pharmacogenetic studies. Contrary to in vitro data, clinical data suggest that the CYP2C8*3 allele is associated with increased metabolism of the CYP2C8 substrates, rosiglitazone, pioglitazone and repaglinide. However, the CYP2C8*3 allele has not been associated with paclitaxel pharmacokinetics in most clinical studies. Furthermore, clinical data regarding the impact of the CYP2C8*3 allele on the disposition of NSAIDs are conflicting and no definitive conclusions can be made at this time. The purpose of this review is to highlight these clinical studies that have investigated the association between CYP2C8 polymorphisms and CYP2C8 substrate pharmacokinetics and/or pharmacodynamics in humans. In this review, CYP2C8 clinical pharmacogenetic data are provided by drug class, followed by a discussion of the future of CYP2C8 clinical pharmacogenetic research.

Treatment responses in antiretroviral treatment-naive premenopausal and postmenopausal HIV-1-infected women: an analysis from AIDS Clinical Trials Group Studies

Menopause may affect antiretroviral treatment (ART) response. Immunologic and virologic responses to ART were compared in 220 premenopausal and 47 postmenopausal women enrolled in 2 studies involving ART-naive persons. Changes in CD4 counts or human immunodeficiency virus type 1 RNA levels were similar at 24, 48, and 96 weeks after treatment initiation. ART-naive women should respond to ART regardless of menopausal status.

Interplay of conjugating enzymes with OATP uptake transporters and ABCC/MRP efflux pumps in the elimination of drugs

BACKGROUND

Biliary excretion is a major elimination route of many drugs and their metabolites. Hepatobiliary elimination is a vectorial process involving uptake transporters in the basolateral hepatocyte membrane, possibly Phase I and Phase II metabolizing enzymes, and ATP-dependent efflux pumps in the apical hepatocyte membrane.

OBJECTIVES

Because many drugs and their metabolites are anions, this review focuses on transporters involved in their hepatocellular uptake (members of the organic anion transporting polypeptide (OATP) family) and biliary elimination (apical conjugate efflux pump ABCC2/MRP2).

METHODS

The molecular and functional characteristics of the human OATP and ABCC/MRP transporters are presented, including a detailed overview of endogenous and drug substrates. Examples illustrate the interplay of transporters with Phase II conjugating enzymes. Model systems to study the vectorial transport of organic anions are also discussed.

RESULTS/CONCLUSIONS

OATP uptake transporters, conjugating enzymes, and ABCC2/MRP2 work in concert to enable the hepatobiliary elimination of anionic drugs and their metabolites. It is increasingly important to understand how genetic variants of these transporters and enzymes influence the interindividual variability of drug elimination.

Metabolic properties of the acid and lactone forms of HMG-CoA reductase inhibitors

To gain a better understanding of the metabolic properties between the open acid and lactone form of HMG-CoA reductase inhibitors (statins), the paper focused primarily on characterizing the metabolic properties of statins. We compared the metabolism of the acid and lactone forms of several statins, including atrovastatin, simvastatin, cerivastatin fluvastatin, pitavastatin and rosuvastatin with respect to metabolic clearance, CYP enzymes involved and drug-drug interactions. A remarkable increase in metabolic clearance was noted for all lactones compared with all acids except for pitavastatin lactone. The metabolic clearances of the atrovastatin, simvastatin, cerivastatin, fluvastatin and rosuvastatin lactones were 73-, 70-, 30-, 7- and 64-fold higher, respectively, than those of the corresponding acids. CYP2Cs were critically involved in the metabolism of cerivastatin, fluvastatin and pitavastatin acids. In contrast, CYP2Cs were not involved in the metabolism of the corresponding lactones and CYP3A4 was mainly involved. Moreover, a substantial difference in the metabolic inhibition of statins was found between acids and lactones. Overall, the study demonstrates that CYP-mediated metabolism of lactones is also a common metabolic pathway for statins and that the CYP3A4-mediated metabolism of the lactone forms clearly will need to be taken into account in assessing mechanistic aspects of drug-drug interaction involving statins.

In vitro evaluation of reversible and irreversible cytochrome P450 inhibition: current status on methodologies and their utility for predicting drug-drug interactions

It is widely accepted that today's practice of polypharmacy inevitably increases the incidence of drug-drug interactions (DDIs). Serious DDI is a major liability for any new chemical entity (NCE) entering the pharmaceutical market. As such, pharmaceutical companies employ various strategies to avoid problematic compounds for clinical development. A key cause for DDIs is the inhibition of cytochrome P450 enzymes (CYPs) that are responsible for metabolic clearance of many drugs. Screening for inhibition potency of CYPs by NCEs has therefore become a routine practice during the drug discovery stage. However, in order to make proper use of DDI data, an understanding of the strengths and weaknesses of the various experimental systems in current use is required. An illustrated review of experimental practices is presented with discussion of likely future developments. The combination of high quality in vitro data generation and the application of in vivo CYP inhibition modelling approaches should allow more informed decisions to be made in the search for drug molecules with acceptable DDI characteristics.

Determination of two HMG-CoA reductase inhibitors, pravastatin and pitavastatin, in plasma samples using liquid chromatography-tandem mass spectrometry for pharmaceutical study

We developed a method for determining pravastatin or pitavastatin, 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors, in plasma using liquid chromatography and tandem mass spectrometry (LC-MS/MS). Pravastatin, pitavastatin and the internal standard fluvastatin were extracted from plasma with solid-phase extraction columns and eluted with methanol. After drying the organic layer, the residue was reconstituted in mobile phase (acetonitrile:water, 90:10, v/v) and injected onto a reversed-phase C(18) column. The isocratic mobile phase was eluted at 0.2 mL/min. The ion transitions recorded in multiple reaction monitoring mode were m/z 423 --> 101, 420 --> 290 and 410 --> 348 for pravastatin, pitavastatin and fluvastatin, respectively. The coefficient of variation of the assay precision was less than 12.4%, the accuracy exceeded 89%. The limit of detection was 1 ng/mL for all analytes. This method was used to measure the plasma concentration of pitavastatin or pravastatin from healthy subjects after a single 4 mg oral dose of pitavastatin or 40 mg oral dose of pravastatin. This is a very simple, sensitive and accurate analytic method to determine the pharmacokinetic profiles of pitavastatin or pravastatiny.

Pharmacogenetics and pharmacogenomics of cholesterol-lowering therapy

PURPOSE OF REVIEW

To summarize recent findings on pharmacokinetics, pharmacodynamics, drug-drug interactions and influence of lifestyle heterogeneity on adverse events in cholesterol-lowering therapy

RECENT FINDINGS

The prevention of cardiovascular disease is critically dependent on lipid-lowery therapy, including statins, cholesterol absorption inhibitors, fibrates and nicotinic acid. Statins are the most prescribed drugs in lipid lowering therapy with variability in response and almost one third of the patients do not meet their treatment goals. The severe adverse effects of treatment with cerivastatin stimulated the search for new genes and gene variations affecting pharmacokinetics, drug-drug interactions and pharmacodynamics. Moreover, instead of monotherapy, combined therapy of statins with ezetemibe and niacin was considered. This led to the identification of CD13, NPC1L1 and HM74A as new targets and CYP2C8 and glucuronidation enzymes as potential targets for drug-drug interactions. Moreover multiple polymorphic sites and pleiotrophic gene targets were reinvestigated in larger cohorts and the relevant pathogenetic factors start to evolve.

SUMMARY

Statin therapy is widely used and well tolerated by the majority of patients. To further reduce potential adverse effects and to increase efficacy, combined therapy concepts with ezetimibe or niacin are underway.

Mechanistic Approaches to Volume of Distribution Predictions: Understanding the Processes

PURPOSE

To use recently developed mechanistic equations to predict tissue-to-plasma water partition coefficients (Kpus), apply these predictions to whole body unbound volume of distribution at steady state (Vu(ss)) determinations, and explain the differences in the extent of drug distribution both within and across the various compound classes.

MATERIALS AND METHODS

Vu(ss) values were predicted for 92 structurally diverse compounds in rats and 140 in humans by two approaches. The first approach incorporated Kpu values predicted for 13 tissues whereas the second was restricted to muscle.

RESULTS

The prediction accuracy was good for both approaches in rats and humans, with 64-78% and 82-92% of the predicted Vu(ss) values agreeing with in vivo data to within factors of +/-2 and 3, respectively.

CONCLUSIONS

Generic distribution processes were identified as lipid partitioning and dissolution where the former is higher for lipophilic unionised drugs. In addition, electrostatic interactions with acidic phospholipids can predominate for ionised bases when affinities (reflected by binding to constituents within blood) are high. For acidic drugs albumin binding dominates when plasma protein binding is high. This ability to explain drug distribution and link it to physicochemical properties can help guide the compound selection process.

Lipid Lowering Inefficacy of High-dose Statin Therapy Due to Concurrent Use of Phenytoin

HMG-CoA reductase inhibitors ("statins") are the most commonly prescribed lipid lowering agents. Most of the statins are metabolized by the CYP450 cytochrome system. A number of medications either induce or inhibit this system which leads to changes in the bioavailability of the statins and the potential for either an increase in adverse effects or reduction in efficacy. Phenytoin induces the CYP3A4 isoform of the CYP450 system and can reduce the bioavailability, and thus the efficacy of the statins metabolized by this enzyme, including atorvastatin and lovastatin. A case of a patient on multiple lipid-lowering medications, including high-dose atorvastatin whose LDL cholesterol improved significantly after discontinuation of phenytoin is presented, and a review of the literature for similar cases is discussed.

Pharmacogenetics, drug-metabolizing enzymes, and clinical practice

The application of pharmacogenetics holds great promise for individualized therapy. However, it has little clinical reality at present, despite many claims. The main problem is that the evidence base supporting genetic testing before therapy is weak. The pharmacology of the drugs subject to inherited variability in metabolism is often complex. Few have simple or single pathways of elimination. Some have active metabolites or enantiomers with different activities and pathways of elimination. Drug dosing is likely to be influenced only if the aggregate molar activity of all active moieties at the site of action is predictably affected by genotype or phenotype. Variation in drug concentration must be significant enough to provide "signal" over and above normal variation, and there must be a genuine concentration-effect relationship. The therapeutic index of the drug will also influence test utility. After considering all of these factors, the benefits of prospective testing need to be weighed against the costs and against other endpoints of effect. It is not surprising that few drugs satisfy these requirements. Drugs (and enzymes) for which there is a reasonable evidence base supporting genotyping or phenotyping include suxamethonium/mivacurium (butyrylcholinesterase), and azathioprine/6-mercaptopurine (thiopurine methyltransferase). Drugs for which there is a potential case for prospective testing include warfarin (CYP2C9), perhexiline (CYP2D6), and perhaps the proton pump inhibitors (CYP2C19). No other drugs have an evidence base that is sufficient to justify prospective testing at present, although some warrant further evaluation. In this review we summarize the current evidence base for pharmacogenetics in relation to drug-metabolizing enzymes.

Pharmacokinetic and pharmacodynamic alterations of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors: drug-drug interactions and interindividual differences in transporter and metabolic enzyme functions

3-Hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors (statins) are widely used for the treatment of hypercholesterolemia. Their efficacy in preventing cardiovascular events has been shown by a large number of clinical trials. However, myotoxic side effects, sometimes severe, including myopathy or rhabdomyolysis, are associated with the use of statins. In some cases, such toxicity is associated with pharmacokinetic alterations. In this review, the pharmacokinetic aspects and physicochemical properties of statins are reviewed in order to understand the mechanism governing their pharmacokinetic alterations. Among the statins, simvastatin, lovastatin and atorvastatin are metabolized by cytochrome P450 3A4 (CYP3A4) while fluvastatin is metabolized by CYP2C9. Cerivastatin is subjected to 2 metabolic pathways mediated by CYP2C8 and 3A4. Pravastatin, rosuvastatin and pitavastatin undergo little metabolism. Their plasma clearances are governed by the transporters involved in the hepatic uptake and biliary excretion. Also for other statins, which are orally administered as open acid forms (i.e. fluvastatin, cerivastatin and atorvastatin), hepatic uptake transporter(s) play important roles in their clearances. Based on such information, pharmacokinetic alterations of statins can be predicted following coadministration of other drugs or in patients with lowered activities in drug metabolism and/or transport. We also present a quantitative analysis of the effect of some factors on the pharmacokinetics of statins based on a physiologically based pharmacokinetic model. To avoid a pharmacokinetic alteration, we need to have information about the metabolizing enzyme(s) and transporter(s) involved in the pharmacokinetics of statins and, along with such information, model-based prediction is also useful.

Transporter-mediated influx and efflux mechanisms of pitavastatin, a new inhibitor of HMG-CoA reductase

The purpose of this study was to gain a better understanding of the transport mechanism of pitavastatin, a novel synthetic HMG-CoA reductase inhibitor. Experiments were performed using oocytes of Xenopus laevis expressing several solute carrier (SLC) transporters and recombinant membrane vesicles expressing several human ABC transporters. The acid form of pitavastatin was shown to be a substrate for human OATP1, OATP2, OATP8, OAT3 and NTCP, and for rat Oatp1 and Oatp4 with relatively low K(m) values. In contrast, these SLC transporters were not involved in the uptake of the lactone form. A significant stimulatory effect was exhibited by pitavastatin lactone, while the acid form did not exhibit ATPase hydrolysis of P-glycoprotein. In the case of breast cancer resistant protein (BCRP), the acid form of pitavastatin is a substrate, whereas the lactone form is not. Taking these results into consideration, several SLC and ABC transporters were identified as critical to the distribution and excretion of pitavastatin in the body. This study showed, for the first time, that acid and lactone forms of pitavastatin differ in substrate activity towards uptake and efflux transporters. These results will potentially contribute to the differences in the pharmacokinetic profiles of pitavastatin.

Transporters as a determinant of drug clearance and tissue distribution

Transporters play an important role in the processes of drug absorption, distribution and excretion. In this review, we have focused on the involvement of transporters in drug excretion in the liver and kidney. The rate of transporter-mediated uptake and efflux determines the rate of renal and hepatobiliary elimination. Transporters are thus important as a determinant of the clearance in the body. Even when drugs ultimately undergo metabolism, their elimination rate is sometimes determined by the uptake rate mediated by transporters. Transporters regulate the pharmacological and/or toxicological effect of drugs because they limit their distribution to tissues responsible for their effect and/or toxicity. For example, the liver-specific distribution of some statins via organic anion transporters helps them to produce their high pharmacological effect. On the other hand, as in the case of metformin taken up by organic cation transporter 1, drug distribution to the tissue(s) may enhance its toxicity. As transporter-mediated uptake is a determinant of the drug elimination rate, drug-drug interactions involving the process of transporter-mediated uptake can occur. In this review, we have introduced some examples and described their mechanisms. More recently, some methods to analyze such transporter-mediated transport have been reported. The estimation of the contributions of transporters to the net clearance of a drug makes it possible to predict the net clearance from data involving drug transport in transporter-expressing cells. Double transfected cells, where both uptake and efflux transporters are expressed on the same polarized cells, are also helpful for the analysis of the rate of transporter-mediated transcellular transport.

Clearance rates of cerivastatin metabolites in a patient with cerivastatin-induced rhabdomyolysis

We report on a patient who developed acute rhabdomyolysis after taking cerivastatin. A 74-year-old hypercholestrerolaemic woman taking cerivastatin (0.15 mg/day) for 22 days complained of general muscle weakness and muscle pain. Her serum creatinine phosphokinase level was 19,190 IU/L. Serum myoglobin was over 3000 ng/mL. Serum concentration of cerivastatin at 6 h after taking the last dose (0.15 mg) was 8062.5 ng/L, which was almost 5.7 times higher than that of normal persons. The serum concentration of cerivastatin showed that the half-life of cerivastatin in this patient was 22.4 h, compared with 2.4 h for normal controls. Cerivastatin is catabolized by cytochrome P450, 3A4 and 2C8 to M-1, and by 2C8 to M-23. The ratio of M-23 to M-1 in her serum was much lower than that of control persons (0.64 vs. 2.08). She had previously taken simvastatin which is metabolized by CYP3A4, without any sign and symptoms of rhabdomyolysis. These results suggest that the slowed clearance of cerivastatin in this patient might have been compounded by cytochrome P450, 2C8 dysfunction.

Bile salt export pump (BSEP/ABCB11) can transport a nonbile acid substrate, pravastatin

Pravastatin is a well known 3-hydroxy-3-methylglutaryl-CoA reductase inhibitor. Cumulative studies have shown that pravastatin is taken up into hepatocytes by the organic anion transporting polypeptide family transporters and excreted into the bile as an intact form by multidrug resistance-associated protein 2 (MRP2). It is generally accepted that the bile salt export pump (BSEP/ABCB11) mainly transports bile acids and plays an indispensable role in their biliary excretion. Interestingly, we found that BSEP could accept pravastatin as a substrate. Significant ATP-dependent uptake of pravastatin by human BSEP (hBSEP)- and rat BSEP (rBsep)-expressing membrane vesicles was observed, and the ratio of the uptake activity of pravastatin to that of taurocholic acid (TCA) by hBSEP was 3.3-fold higher than that by rBsep. The K(m) value of pravastatin for hBSEP was 124 muM. A mutual inhibition study between TCA and pravastatin revealed that they competitively interact with hBSEP. Several statins inhibited the hBSEP- and rBsep-mediated uptake of TCA; however, the specific uptake of other statins (cerivastatin, fluvastatin, and pitavastatin) by hBSEP and rBSEP was not detected. The inhibitory effects of hydrophilic statins (pravastatin and rosuvastatin) on the uptake of TCA by BSEP were relatively lower than those of lipophilic statins. These data suggest that BSEP may be partly involved in the biliary excretion of pravastatin in both rats and humans.

Evaluation of drug-drug interaction in the hepatobiliary and renal transport of drugs

Recent studies have revealed the import role played by transporters in the renal and hepatobiliary excretion of many drugs. These transporters exhibit a broad substrate specificity with a degree of overlap, suggesting the possibility of transporter-mediated drug-drug interactions with other substrates. This review is an overview of the roles of transporters and the possibility of transporter-mediated drug-drug interactions. Among the large number of transporters, we compare the Ki values of inhibitors for organic anion transporting polypeptides (OATPs) and organic anion transporters (OATs) and their therapeutic unbound concentrations. Among them, cephalosporins and probenecid have the potential to produce clinically relevant OAT-mediated drug-drug interactions, whereas cyclosporin A and rifampicin may trigger OATP-mediated ones. These drugs have been reported to cause drug-drug interactions in vivo with OATs or OATP substrates, suggesting the possibility of transporter-mediated drug-drug interactions. To avoid adverse consequences of such transporter-mediated drug-drug interactions, we need to be more aware of the role played by drug transporters as well as those caused by drug metabolizing enzymes.

Pitavastatin

The growing number of trials that have highlighted the benefit of intensive lowering of total- and low density lipoprotein (LDL)-cholesterol levels especially with statins has created a need for more efficacious agents. Pitavastatin is a new synthetic 3-hydroxy-3-methyl glutaryl coenzyme A reductase inhibitor, which was developed, and has been available in Japan since July 2003. Metabolism of pitavastatin by the cytochrome P450 (CYP) system is minimal, principally through CYP 2C9, with little involvement of the CYP 3A4 isoenzyme, potentially reducing the risk of drug-drug interactions between pitavastatin and other drugs known to inhibit CYP enzymes. To date, human and animal studies have shown pitavastatin to be potentially as effective in lowering LDL-cholesterol levels as rosuvastatin; although, head-to-head studies are yet to be conducted.