Comparison of human cytochrome P450 inhibition by the thienopyridines prasugrel, clopidogrel, and ticlopidine

Development and Characterization of pFluor50, a Fluorogenic-Based Kinetic Assay System for High-Throughput Inhibition Screening and Characterization of Time-Dependent Inhibition and Inhibition Type for Six Human CYPs

Cytochrome P450s (CYPs) play an integral role in drug and xenobiotic metabolism in humans, and thus, understanding CYP inhibition and/or activation by new therapeutic candidates is an important step in the drug development process. Ideally, CYP inhibition/activation assays should be high-throughput, use commercially available components, allow for analysis of metabolism by the majority of human CYPs, and allow for kinetic analysis of inhibition type and time-dependent inhibition. Here, we developed pFluor50, a 384-well microtiter plate-based fluorogenic kinetic enzyme assay system using substrates metabolized by six human CYPs to generate fluorescent products and determined the Michaelis-Menten kinetics constants (KM) and product formation rates (Vmax) for each substrate-CYP pair. The pFluor50 assay was also used to elucidate inhibition type and time-dependent inhibition for some inhibitors, demonstrating its utility for characterizing the observed inhibition, even mechanism-based inhibition. The pFluor50 assay system developed in this study using commercially available components should be very useful for high-throughput screening and further characterization of potential therapeutic candidates for inhibition/activation with the most prevalent human CYPs.

Early Preclinical Studies of Ergosterol Peroxide and Biological Evaluation of Its Derivatives

Ganoderma lucidum is a medicinal mushroom that produces various pharmacological compounds, including triterpenoids. A major bioactive component of G. lucidum is ergosterol peroxide (EP), which is attributed to its anticancer effects. The current study focuses on the in vitro ADME (absorption, distribution, metabolism, and elimination), in vivo efficacy and toxicity of EP, and the synthesis of new EP derivatives to improve aqueous solubility. It was found that EP is metabolically stable in liver microsomes and plasma. In vivo studies showed that EP inhibits tumor growth in murine cancer models, and it is well tolerated by mice. The maximum tolerated dose was investigated in mice at escalating doses with a defined maximum amount of 500 mg/kg, which indicated no signs of toxicity, confirmed by plasma chemistry and analysis of harvested tissues. Complementary organ toxicity assays including cardio and hepatotoxicity assays of EP demonstrated no inhibitory effects. Next, a focused library of EP derivatives was developed to investigate the iterative addition of heteroatoms to improve the aqueous solubility properties of EP. Significant solubility improvement was observed by the introduction of hydrogen bonding promoting groups, particularly the sulfate group. Superior aqueous solubility properties are directly correlated with the biological activity of the compound against triple-negative breast cancer cellular (TNBC) models. The EP derivatives maintain ample therapeutic index at the tested concentrations, indicating they engage with the same biological target(s) as the parental compound (EP). The combined studies indicate that EP and its derivatives are selective TNBC cell death inducers, while sparing noncancerous tissue.

Mechanism-based inactivation of cytochromes P450: implications in drug interactions and pharmacotherapy

Cytochrome P40 (CYP) enzymes dominate the metabolism of numerous endogenous and xenobiotic substances. While it is commonly believed that CYP-catalysed reactions result in the detoxication of foreign substances, these reactions can also yield reactive intermediates that can bind to cellular macromolecules to cause cytotoxicity or irreversibly inactivate CYPs that create them.Mechanism-based inactivation (MBI) produces either irreversible or quasi-irreversible inactivation and is commonly caused by CYP metabolic bioactivation to an electrophilic reactive intermediate. Many drugs that have been known to cause MBI in CYPs have been discovered as perpetrators in drug-drug interactions throughout the last 20-30 years.This review will highlight the key findings from the recent literature about the mechanisms of CYP enzyme inhibition, with a focus on the broad mechanistic elements of MBI for widely used drugs linked to the phenomenon. There will also be a brief discussion of the clinical or pharmacokinetic consequences of CYP inactivation with regard to drug interaction and toxicity risk.Gaining knowledge about the selective inactivation of CYPs by common therapeutic drugs helps with the assessment of factors that affect the systemic clearance of co-administered drugs and improves comprehension of anticipated interactions with other drugs or xenobiotics.

The effects of CYP2B6 inactivators on the metabolism of ciprofol

Ciprofol is a novel short-acting intravenous anaesthetic developed in China that is mainly metabolized by cytochrome P450 2B6 (CYP2B6) and uridine diphosphate glucuronosyltransferase 1A9 (UGT1A9). Currently, insufficient evidence is available to support drug‒drug interactions between ciprofol and CYP2B6 inactivators. Here, we established a high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) method to assess the concentration of ciprofol and investigated the effects of psoralen and clopidogrel on the metabolism of ciprofol in liver microsomes and rats. In rat and human liver microsomes, the median inhibitory concentration (IC50) values of psoralen were 63.31 μmol·L-1 and 34.05 μmol·L-1, respectively, showing mild inhibitory effects on ciprofol metabolism, whereas the IC50 values of clopidogrel were 6.380 μmol·L-1 and 2.565 μmol·L-1, respectively, with moderate inhibitory effects. SD rats were randomly divided into three groups: psoralen (27 mg·kg-1), clopidogrel (7.5 mg·kg-1), and the same volume of 0.5% carboxy methyl cellulose. After 7 days, all rats were injected with 2.4 mg·kg-1 ciprofol. Compared with the control group, the AUC and MRT values of ciprofol in the psoralen and clopidogrel groups were significantly greater, whereas the CL values were significantly lower. In addition, the durations of loss of righting reflex (LORR) in the psoralen and clopidogrel groups were 16.1% and 23.0% longer than that in the control group, respectively. In conclusion, psoralen and clopidogrel inhibit ciprofol metabolism to different degrees and prolong the duration of LORR in rats.

Rottlerin renders a selective and highly potent CYP2C8 inhibition to impede EET formation for implication in cancer therapy

CYP2C8 is a crucial CYP isoform responsible for the metabolism of xenobiotics and endogenous molecules. CYP2C8 converts arachidonic acid to epoxyeicosatrienoic acids (EETs) that cause cancer progression. Rottlerin possess significant anticancer actions. However, information on its CYP inhibitory action is lacking in the literature and therefore, we aimed to explore the same using in silico, in vitro, and in vivo approaches. Rottlerin showed highly potent and selective CYP2C8 inhibition (IC₅₀ < 0.1 μM) compared to negligible inhibition (IC₅₀ > 10 μM) for seven other experimental CYPs in human liver microsomes (HLM) (in vitro) using USFDA recommended index reactions. Mechanistic studies reveal that rottlerin could reversibly (mixed-type) block CYP2C8. Molecular docking (in silico) results indicate a strong interaction could occur between rottlerin and the active site of human CYP2C8. Rottlerin boosted the plasma exposure of repaglinide and paclitaxel (CYP2C8 substrates) by delaying their metabolism using the rat model (in vivo). Multiple-dose treatment of rottlerin with CYP2C8 substrates lowered the CYP2C8 protein expression and up-regulated & down-regulated the mRNA for CYP2C12 and CYP2C11 (rat homologs), respectively, in rat liver tissue. Rottlerin substantially hindered the EET formation in HLM. Overall results of rottlerin on CYP2C8 inhibition and EET formation insinuate further exploration for targeted cancer therapy.

Inhibition of Nicotine Metabolism by Cannabidiol (CBD) and 7-Hydroxycannabidiol (7-OH-CBD)

Cannabis-based products have experienced notable increases in co-usage alongside tobacco products. Several cannabinoids exhibit inhibition of a number of cytochrome P450 (CYP) and UDP glucuronosyltransferase (UGT) enzymes, but few studies have examined their inhibition of enzymes involved in nicotine metabolism. The goal of the present study was to examine potential drug-drug interactions occurring in the nicotine metabolism pathway perpetrated by cannabidiol (CBD) and its active metabolite, 7-hydroxy-CBD (7-OH-CBD). The inhibitory effects of CBD and 7-OH-CBD were tested in microsomes from HEK293 cells overexpressing individual metabolizing enzymes and from human liver tissue. Assays with overexpressing microsomes demonstrated that CBD and 7-OH-CBD inhibited CYP-mediated nicotine metabolism. Binding-corrected IC50,u values for CBD inhibition of nicotine metabolism to cotinine and nornicotine, and cotinine metabolism to trans-3'-hydroxycotinine (3HC), were 0.27 ± 0.060, 0.23 ± 0.14, and 0.21 ± 0.14 μM, respectively, for CYP2A6; and 0.26 ± 0.17 and 0.029 ± 0.0050 μM for cotinine and nornicotine formation, respectively, for CYP2B6. 7-OH-CBD IC50,u values were 0.45 ± 0.18, 0.16 ± 0.08, and 0.78 ± 0.23 μM for cotinine, nornicotine, and 3HC formation, respectively, for CYP2A6, and 1.2 ± 0.44 and 0.11 ± 0.030 μM for cotinine and nornicotine formation, respectively, for CYP2B6. Similar IC50,u values were observed in HLM. Inhibition (IC50,u = 0.37 ± 0.06 μM) of 3HC to 3HC-glucuronide formation by UGT1A9 was demonstrated by CBD. Significant inhibition of nicotine metabolism pathways by CBD and 7-OH-CBD suggests that cannabinoids may inhibit nicotine metabolism, potentially impacting tobacco addiction and cessation.

Construction of a fused grid-based CYP2C19-Template system and the application

A ligand-accessible space in the CYP2C19 active site was reconstituted as a fused grid-based Template with the use of structural data of the ligands. An evaluation system of CYP2C19-mediated metabolism has been developed on Template with the introduction of the idea of Trigger-residue initiated ligand-movement and fastening. Reciprocal comparison of the data of simulation on Template with experimental results suggested a unified way of the interaction of CYP2C19 and its ligands through the simultaneous plural-contact with Rear-wall of Template. CYP2C19 was expected to have a room for ligands between vertically standing parallel walls termed Facial-wall and Rear-wall, which were separated by a distance corresponding to 1.5-Ring (grid) diameter size. The ligand sittings were stabilized through contacts with Facial-wall and the left-side borders of Template including specific Position 29 or Left-end after Trigger-residue initiated ligand-movement. Trigger-residue movement is suggested to force ligands to stay firmly in the active site and then to initiate CYP2C19 reactions. Simulation experiments for over 450 reactions of CYP2C19 ligands supported the system established.

Physiologically Based Pharmacokinetic (PBPK) Modeling of Clopidogrel and Its Four Relevant Metabolites for CYP2B6, CYP2C8, CYP2C19, and CYP3A4 Drug–Drug–Gene Interaction Predictions

The antiplatelet agent clopidogrel is listed by the FDA as a strong clinical index inhibitor of cytochrome P450 (CYP) 2C8 and weak clinical inhibitor of CYP2B6. Moreover, clopidogrel is a substrate of—among others—CYP2C19 and CYP3A4. This work presents the development of a whole-body physiologically based pharmacokinetic (PBPK) model of clopidogrel including the relevant metabolites, clopidogrel carboxylic acid, clopidogrel acyl glucuronide, 2-oxo-clopidogrel, and the active thiol metabolite, with subsequent application for drug–gene interaction (DGI) and drug–drug interaction (DDI) predictions. Model building was performed in PK-Sim^® using 66 plasma concentration-time profiles of clopidogrel and its metabolites. The comprehensive parent-metabolite model covers biotransformation via carboxylesterase (CES) 1, CES2, CYP2C19, CYP3A4, and uridine 5′-diphospho-glucuronosyltransferase 2B7. Moreover, CYP2C19 was incorporated for normal, intermediate, and poor metabolizer phenotypes. Good predictive performance of the model was demonstrated for the DGI involving CYP2C19, with 17/19 predicted DGI AUC_last and 19/19 predicted DGI C_max ratios within 2-fold of their observed values. Furthermore, DDIs involving bupropion, omeprazole, montelukast, pioglitazone, repaglinide, and rifampicin showed 13/13 predicted DDI AUC_last and 13/13 predicted DDI C_max ratios within 2-fold of their observed ratios. After publication, the model will be made publicly accessible in the Open Systems Pharmacology repository.

Pirfenidone 5-hydroxylation is mainly catalysed by CYP1A2 and partly catalysed by CYP2C19 and CYP2D6 in the human liver

Pirfenidone is a first-line drug for the treatment of idiopathic pulmonary fibrosis. The primary metabolic pathways of pirfenidone in humans are 5-hydroxylation and subsequent oxidation to 5-carboxylpirfenidone. The aims of this study were to determine the cytochrome P450 isoforms responsible for pirfenidone 5-hydroxylation and to evaluate their contributions in human liver microsomes (HLM).Among the recombinant P450 isoforms, CYP1A2, CYP2D6, CYP2C19, CYP2A6, and CYP2B6 were shown to catalyse the 5-hydroxylation of pirfenidone. Pirfenidone 5-hydroxylase activity by HLM was inhibited by α-naphthoflavone (by 45%), 8-methoxypsolaren (by 84%), tranylcypromine (by 53%), and quinidine (by 15%), which are CYP1A2, CYP1A2/CYP2A6/CYP2C19, CYP2A6/CYP2C19, and CYP2D6 inhibitors, respectively.In 17 individual HLM donors, pirfenidone 5-hydroxylase activity was significantly correlated with phenacetin O-deethylase (r = 0.89, P < 0.001) and S-mephenytoin 4'-hydroxylase activities (r = 0.51, P < 0.05), which are CYP1A2 and CYP2C19 marker activities, respectively.By using the relative activity factors, the contributions of CYP1A2, CYP2C19, and CYP2D6 to pirfenidone 5-hydroxylation in the human liver were 72.8%, 11.8%, and 8.9%, respectively.In conclusion, we clearly demonstrated the predominant P450 involved in pirfenidone 5-hydroxylation in the human liver is CYP1A2, with CYP2C19 and CYP2D6 playing a minor role.

An automated cocktail method for in vitro assessment of direct and time-dependent inhibition of nine major cytochrome P450 enzymes - application to establishing CYP2C8 inhibitor selectivity

We developed an in vitro high-throughput cocktail assay with nine major drug-metabolizing CYP enzymes, optimized for screening of time-dependent inhibition. The method was applied to determine the selectivity of the time-dependent CYP2C8 inhibitors gemfibrozil 1-O-β-glucuronide and clopidogrel acyl-β-D-glucuronide. In vitro incubations with CYP selective probe substrates and pooled human liver microsomes were conducted in 96-well plates with automated liquid handler techniques and metabolite concentrations were measured with quantitative UHPLC-MS/MS analysis. After determination of inter-substrate interactions and K_m values for each reaction, probe substrates were divided into cocktails I (tacrine/CYP1A2, bupropion/CYP2B6, amodiaquine/CYP2C8, tolbutamide/CYP2C9 and midazolam/CYP3A4/5) and II (coumarin/CYP2A6, S-mephenytoin/CYP2C19, dextromethorphan/CYP2D6 and astemizole/CYP2J2). Time-dependent inhibitors (furafylline/CYP1A2, selegiline/CYP2A6, clopidogrel/CYP2B6, gemfibrozil 1-O-β-glucuronide/CYP2C8, tienilic acid/CYP2C9, ticlopidine/CYP2C19, paroxetine/CYP2D6 and ritonavir/CYP3A) and direct inhibitor (terfenadine/CYP2J2) showed similar inhibition with single substrate and cocktail methods. Established time-dependent inhibitors caused IC₅₀ fold shifts ranging from 2.2 to 30 with the cocktail method. Under time-dependent inhibition conditions, gemfibrozil 1-O-β-glucuronide was a strong (>90% inhibition) and selective (<< 20% inhibition of other CYPs) inhibitor of CYP2C8 at concentrations ranging from 60 to 300 μM, while the selectivity of clopidogrel acyl-β-D-glucuronide was limited at concentrations above its IC₈₀ for CYP2C8. The time-dependent IC₅₀ values of these glucuronides for CYP2C8 were 8.1 and 38 µM, respectively. In conclusion, a reliable cocktail method including the nine most important drug-metabolizing CYP enzymes was developed, optimized and validated for detecting time-dependent inhibition. Moreover, gemfibrozil 1-O-β-glucuronide was established as a selective inhibitor of CYP2C8 for use as a diagnostic inhibitor in in vitro studies.

Investigation of CYP2B6, 3A4 and β-esterase interactions of Withania somnifera (L.) dunal in human liver microsomes and HepG2 cells

ETHNOPHARMACOLOGICAL RELEVANCE

Withania somnifera (L.) Dunal (Solanaceae) is a traditional herb, used in African indigenous systems of medicine for the treatment of various diseases (including HIV/AIDS and tuberculosis). The relevance of clinically significant interactions of Withania with ARVs and anti-TB drugs needs to be investigated.

AIM OF THE STUDY

This study evaluated the effects of its roots on cytochromes P450 (CYPs) 2B6, 3A4, and rifampicin metabolism pathway, using methanol, ethanol, aqueous, and ethyl acetate solvent extractions.

MATERIALS AND METHODS

The extracts were tested on human liver microsomes (HLM) for CYP inhibition, mRNA expression in HepG2 cells for CYP induction. Biochemical qualitative tests and LC-MS/MS methodology were used to determine active phytoconstituents.

RESULTS

The methanolic and ethyl acetate extracts inhibited CYP2B6 with IC₅₀s 79.16 and 57.96 μg/ml respectively, while none of the extracts had any effect on rifampicin metabolism or showed time-dependant inhibition (TDI). All extracts were moderate inducers of CYP3A4; the aqueous extract exhibited 38%-fold shift induction of CYP3A4 compared to the control. The methanolic extract had the lowest CTC₅₀ (50% of cytotoxicity inhibition) (67.13 ± 0.83 μg/ml). LC-MS/MS-PDA full scans were consistent with the presence of flavone salvigenin (m/z 327), alkaloid isopelletierine (m/z 133), steroidal lactone 2,3-dihydrowithaferin-A (m/z 472), and other withanolides including withaperuvin I (m/z 533), withaferin derivative (m/z 567), some of these compounds likely being responsible for the observed CYP2B6 inhibition and CYP3A4 induction. The putative gastrointestinal tract (GIT) concentration for the active extracts was 1800 μg/ml and the hepatic circulation concentrations were estimated at about 220 μg/ml and 13.5 μg/ml for the methanolic and ethyl acetate extracts, respectively. The extrapolated in vivo percentage of inhibition was at 85% for the methanolic extract against CYP2B6.

CONCLUSIONS

The findings reported in this study suggest that W. somnifera extracts have the potential of causing clinically significant herb-drug interactions (HDI) as moderate inducer of CYP3A4 and inhibitor of CYP2B6 metabolism pathway (methanol and ethyl acetate extracts).

Highly productive and scalable approach to synthesize ticlopidine: A potent thienopyridine anti-platelet aggregation drug

Ticlopidine (trade name Ticlid), an acidic thienopyridine derivative, is an effective, well-known and long-acting inhibitor of platelet aggregation. Because of its potent inhibitory activity for treating a variety of diseases, the development of efficient approaches for accessing ticlopidine represents an important endeavour. Therefore, in this research work, we developed a promising novel five-step synthetic approach for synthesizing ticlopidine. This method provides ticlopidine in 60% overall yield from readily available starting material viz. thiophene. In this methodology, all steps afforded excellent yields and are operationally simple and environmentally acceptable. This approach also offers various attractive advantages, for example, it's applicable for large-scale synthesis, has simple work-up procedures and short reaction times, and uses inexpensive and readily available reagents. Furthermore, 4,5,6,7-tetrahydrothieno[3,2-c]pyridine is a key precursor for the synthesis of numerous bioactive compounds such as prasugrel and clopidogrel. This protocol provides 4,5,6,7-tetrahydrothieno[3,2-c]pyridine in 62% overall yield via a 4-step synthetic approach.

In Vitro Assessment of the Interaction Potential of Ocimum basilicum (L.) Extracts on CYP2B6, 3A4, and Rifampicin Metabolism

Ocimum basilicum L. or basilicum is a common culinary herb, used as a traditional medicine for various medical conditions including HIV/AIDS and tuberculosis, in Africa. The objective of this study was to evaluate the effect of methanol, ethanol, aqueous and ethyl acetate extracts of the dried leaves and inflorescence of O. basilicum, on the activity of cytochrome P450 enzymes (CYPs) CYP2B6 and 3A4, as well as esterase-mediated metabolism of rifampicin to 25-O-desacetyl rifampicin (25ODESRIF). Human liver microsomes (HLM) were used to evaluate inhibition and CYP2B6/3A4 mRNA expression HepG2 assays were used to measure induction. Furthermore, the phytoconstituents likely involved in causing the observed effect were analyzed using biochemical tests and LC-MS. The aqueous and methanolic extracts showed reversible and time-dependent inhibition (TDI) of CYP2B6 with TDI-IC₅₀s 33.35 μg/ml (IC₅₀ shift-fold >1.5) and 4.93 μg/ml (IC₅₀ shift-fold >7) respectively, while the methanolic and ethanolic extracts inhibited 25ODESRIF formation (IC₅₀s 31 μg/ml, 8.94 μg/ml). In HepG2 assays, the methanolic and ethanolic extracts moderately induced CYP2B6, 3A4 mRNA with 38%-, 28%-fold shift, and 22%-, 44%-fold shift respectively. LC-MS full scans identified phenols rosmarinic acid [m/z 359 (M-H)^-, approximately 2298 mg/L in aqueous extract] and caftaric acid along with flavones salvigenin [m/z 329 (M+H)⁺, approximately 1855 mg/L in ethanolic extract], eupatorin [m/z 345 (M+H)⁺, 668.772 mg/L in ethanolic extract], rutin [m/z 609 (M-H)^-] and isoquercetin [m/z 463 (M-H)^-] and other compounds—linalool [m/z 153 (M-H)^-], hydroxyjasmonic acid [m/z 225 (M-H)^-], eucommiol [m/z 187 (M-H)^-] and trihydroxy octadecenoic acid [m/z 329 (M-H)^-, 530 mg/L in ethanolic extract]. The putative gastrointestinal tract (GIT) concentration for all extracts was calculated as 2,400 μg/ml and hepatic circulation concentrations were estimated at 805.68 μg/ml for the aqueous extract, and 226.56 μg/ml for methanolic extract. Based on the putative GIT concentration, estimated hepatic circulation concentration [I] and inhibition constant K_i, the predicted percentile of inhibition in vivo was highest for the aqueous extract on CYP2B6 (96.7%). The observations indicated that O. basilicum extracts may have the potential to cause clinically relevant herb-drug interactions (HDI) with CYP2B6 and rifampicin metabolism in vivo, if sufficient hepatic concentrations are reached in humans.

Pharmacokinetics and pharmacodynamics analysis of XQ-1H and its combination therapy with clopidogrel on cerebral ischemic reperfusion injury in rats

Ischemic stroke is the main cause of disability and mortality worldwide. 10-O-(N N-dimethylaminoethyl)-ginkgolide B methane-sulfonate (XQ-1 H) is a novel drug based on the remedial approach for ischemic stroke. Clopidogrel, a widely used anti-platelet drug, is often co-prescribed in the clinic. In this study, we established an UPLC-MS/MS spectrometry method for the determination of XQ-1H and investigated the pharmacokinetic effect of clopidogrel on XQ-1H in rats subjected to middle cerebral artery occlusion (MCAO). Meanwhile, the anti-apoptotic and neuroprotective effects of XQ-1H and its combination with clopidogrel were also studied. The results revealed that XQ-1H and its combination with clopidogrel abridged brain infarct volume, cerebral edema and alleviated neurological dysfunction caused by cerebral ischemic reperfusion injury. Further study demonstrated that XQ-1H combined with clopidogrel lessened TUNEL positive cells, up-regulated bcl-2 expression notably and down-regulated bax expression as compared to both XQ-1H and clopidogrel individually. In addition, a rapid, sensitive UPLC-MS/MS method was developed to quantify the concentration of XQ-1H in MCAO/R rats. Our pharmacokinetic results showed that clopidogrel significantly increased the exposure of XQ-1H, increased the peak plasma concentration (C_max), area under the curve (AUC) and slowed elimination of XQ-1H in the co-administered group. Besides, for further exploring which CYP450 isoforms are involved in the XQ-1H metabolism, XQ-1H was incubated in human liver microsomes (HLMs) system with or without P450 isoform-selective inhibitors. Our results revealed that clopidogrel altered pharmacokinetics of XQ-1H potentially and subsequently enhanced the pharmacological effect of XQ-1H. Moreover, XQ-1H could be applied as an efficacious neuroprotective agent for ischemic stroke because of its considerable effect on averting neuronal apoptosis.

The effect of chronic kidney disease on CYP2B expression and activity in male Wistar rats

Chronic kidney disease (CKD) is characterized by progressive reduction in kidney function over time. CKD affects greater than 10% of the population and its incidence is on the rise due to the growing prevalence of its risk factors. Previous studies demonstrated CKD alters nonrenal clearance of drugs in addition to reducing renal clearance. We assessed the function and expression of hepatic CYP2B enzymes using a rat model of CKD. CKD was induced in Wistar rats by supplementing their chow with adenine and confirmed through the detection of elevated uremic toxins in plasma. Liver enzymes AST and ALT were unchanged by the adenine diet. Bupropion was used as a probe substrate for hepatic CYP2B function using rat liver microsomes. The resulting metabolite, hydroxy-bupropion, and bupropion were quantified by ultra-performance liquid chromatography coupled to time-of-flight mass spectrometry. Level of mRNA and protein were determined by RT-PCR and Western blot, respectively. The results of our study demonstrate that CYP2B1 is downregulated in a rat model of CKD. CYP2B1 mRNA level was significantly decreased (88%, P < 0.001) in CKD relative to control. Similarly, maximal enzymatic velocity (Vmax) for CYP2B was decreased by 46% in CKD relative to control (P < 0.0001). Previous studies involving patients with CKD demonstrated altered bupropion pharmacokinetics compared to control. Hence, our results suggest that these alterations may be mediated by attenuated CYP2B hepatic metabolism. This finding may partially explain the alterations in pharmacokinetics and nonrenal drug clearance frequently observed in patients with CKD.

Percutaneous pleural drainage in patients taking clopidogrel: real danger or phantom fear?

Despite being categorized as a procedure associated with a low risk of iatrogenic hemorrhage, percutaneous pleural drainage in patients taking the ubiquitous antiplatelet agent clopidogrel is still commonly perceived as a risky proposition. There is mounting evidence, however, in support of the safety of percutaneous needle procedures in persons receiving this medication. Establishing that these pleural interventions can be performed safely without clopidogrel interruption would be of great clinical significance, especially in those taking it for recent cardiac stenting and therefore in danger of stent thrombosis should antiplatelet therapy (APT) be withheld. The purpose of the present review is to summarize the available data from published studies and series of thoracentesis and chest tube insertion in patients exposed to clopidogrel. Also incorporated into this review are relevant investigations from the thoracic surgery and interventional radiology experience, which contribute indirect evidence and help shape the context for interpreting the safety data reported in the pleural literature. At the end, an attempt is made to synthesize the current knowledge on this topic into conclusions for guiding practice.

Comparison of antiplatelet treatment in patients with clopidogrel nonresponders with or without carriage of CYP2C19 polymorphism

BACKGROUND/AIMS

Several interventions exist for overcoming high platelet reactivity (HPR) on clopidogrel therapy. The goal of this study was to identify strategies that improve inhibition of platelet reactivity in clopidogrel nonresponders with or without loss of function CYP2C19 genotypes, resulting in platelet reactivity similar to that in responders.

METHODS

A total of 376 patients with stenting for coronary artery disease underwent platelet function testing in three centers. Blinded platelet function tests were performed after 75 mg daily clopidogrel treatment for 28 days. In total, 183 nonresponders were genotyped, were randomized to four treatment groups with each treatment lasting approximately 28 days, and underwent repeated measurements of platelet reactivity after treatment.

RESULTS

With 75 mg of daily clopidogrel, nonresponders had significantly higher HPR than did responders (multiple electrode aggregometry [MEA, arbitrary platelet aggregation unit]: mean, 71.4; 95% confidence intervals [CI], 68.6 to 74.3; and mean, 27.5; 95% CI, 26.0 to 28.9, respectively; p < 0.001). Ticagrelor or ticlopidine treatment in nonresponders resulted in platelet reactivity similar to that in responders in intermediate metabolizers (mean, 24.0; 95% CI, 19.6 to 28.4; p > 0.05; and mean, 30.0; 95% CI, 24.7 to 37.5; p > 0.05, respectively) and poor metabolizers (mean, 23.2; 95% CI, 18.0 to 28.3; p > 0.05; and mean, 30.3; 95% CI, 24.5 to 6.0; p > 0.05, respectively). However, in extensive metabolizers, only ticagrelor treatment showed platelet reactivity similar to that in responders (mean, 26.1; 95% CI, 24.1 to 28.0; p > 0.05).

CONCLUSIONS

Among clopidogrel nonresponders with cardiovascular disease on 75 mg daily clopidogrel, ticagrelor resulted in a comparable degree of platelet inhibition in all nonresponders compared with 150 mg daily clopidogrel or triple therapy with clopidogrel and cilostazol, irrespective of phenotype.

Clarification of the Mechanism of Clopidogrel-Mediated Drug-Drug Interaction in a Clinical Cassette Small-dose Study and Its Prediction Based on In Vitro Information

Clopidogrel is reported to be associated with cerivastatin-induced rhabdomyolysis, and clopidogrel and its metabolites are capable of inhibiting CYP2C8 and OATP 1B1 in vitro. The objective of the present study was to identify the mechanism of clopidogrel-mediated drug-drug interactions (DDIs) on the pharmacokinetics of OATP1B1 and/or CYP2C8 substrates in vivo. A clinical cassette small-dose study using OATPs, CYP2C8, and OATP1B1/CYP2C8 probe drugs (pitavastatin, pioglitazone, and repaglinide, respectively) with or without the coadministration of either 600 mg rifampicin (an inhibitor for OATPs), 200 mg trimethoprim (an inhibitor for CYP2C8), or 300 mg clopidogrel was performed, and the area under the concentration-time curve (AUC) ratios (AUCRs) for probe substrates were predicted using a static model. Clopidogrel increased the AUC of pioglitazone (2.0-fold) and repaglinide (3.1-fold) but did not significantly change the AUC of pitavastatin (1.1-fold). In addition, the AUC of pioglitazone M4, a CYP2C8-mediated metabolite of pioglitazone, was reduced to 70% of the control by coadministration of clopidogrel. The predicted AUCRs using the mechanism-based inhibition of CYP2C8 by clopidogrel acyl-β-glucuronide were similar to the observed AUCRs, and the predicted AUCR (1.1) of repaglinide using only the inhibition of OATP1B1 did not reach the observed AUCR (3.1). In conclusion, a single 300 mg of clopidogrel mainly inhibits CYP2C8-mediated metabolism by clopidogrel acyl-β-glucuronide, but its effect on the pharmacokinetics of OATP1B1 substrates is negligible. Clopidogrel is expected to have an effect not only on CYP2C8 substrates, but also dual CYP2C8/OATP1B1 substrates as seen in the case of repaglinide.

Potent and Orally Bioavailable Antiplatelet Agent, PLD-301, with the Potential of Overcoming Clopidogrel Resistance

PLD-301, a phosphate prodrug of clopidogrel thiolactone discovered by Prelude Pharmaceuticals with the aim to overcome clopidogrel resistance, was evaluated for its in vivo inhibitory effect on ADP-induced platelet aggregation in rats. The potency of PLD-301 was similar to that of prasugrel, but much higher than that of clopidogrel. The results of pharmacokinetic analysis showed that the oral bioavailability of clopidogrel thiolactone converted from PLD-301 was 4- to 5-fold higher than that of the one converted from clopidogrel, suggesting that in comparison with clopidogrel, lower doses of PLD-301 could be used clinically. In summary, PLD-301 presents a potent and orally bioavailable antiplatelet agent that might have some advantages over clopidogrel, such as overcoming clopidogrel resistance for CYP2C19-allele loss-of-function carriers, and lowering dose-related toxicity due to a much lower effective dose.

Effect of antituberculosis treatment on CYP2C19 enzyme activity in genetically polymorphic South Indian Tamilian population

Patients on antituberculosis therapy (ATT) are more prone to drug interactions in the presence of coexisting illnesses which require drug therapy. Rifampicin is a pleiotropic inducer of CYP enzymes, and isoniazid is an enzyme inhibitor. Genetic variations are common in the gene coding for CYP2C19 enzyme. These variations would be important in predicting the individual variations in CYP2C19 activity. The objectives of the study were to find the net effect of 1-month ATT on CYP2C19 enzyme activity and its association with CYP2C19 genetic polymorphisms. Newly diagnosed tuberculosis patients (n = 125) were included in the study. Before commencing ATT, they were given a single dose of omeprazole 20 mg as a probe drug for CYP2C19. Blood sample was collected after 3 h to carry out phenotyping for CYP2C19 enzyme by measuring omeprazole hydroxylation index (OHI) using LC-MS/MS. The phenotyping procedure was repeated after 1 month of ATT. CYP2C19 genotyping was carried out by PCR-RFLP method. Significant reduction in OHI was observed after 1 month of ATT in all the metabolizer groups. The percentage reduction in OHI was maximum with poor metabolizers, 84.1 (IQR - 74.6, 86.6), and minimum with ultra-rapid metabolizers, 39.6 (IQR - 12.7, 54.7). CYP2C19 enzyme induction is predominant in patients after 1 month of antituberculosis treatment (ATT). Genetic variations in the enzyme could not clearly explain the interindividual differences in induction. There is a potential risk of drug failure/adverse effect in poor metabolizers regardless of their genotype after ATT.

Proton Pump Inhibitors in Cardiovascular Disease: Drug Interactions with Antiplatelet Drugs

Aspirin and P2Y₁₂ receptor antagonists are widely used across the spectrum of cardiovascular diseases. Upper gastrointestinal complications, including ulcer and bleeding, are relatively common during antiplatelet treatment and, therefore, concomitant proton pump inhibitor (PPI) treatment is often prescribed.PPIs provide gastroprotection by changing the intragastric milieu, essentially by raising intragastric pH. In recent years, it has been heavily discussed whether PPIs may reduce the cardiovascular protection by aspirin and, even more so, clopidogrel. Pharmacodynamic and pharmacokinetic studies suggested an interaction between PPIs and clopidogrel, and subsequent clinical studies were conducted to evaluate the clinical impact of this interaction. More recently, it was reported that PPIs may also attenuate the antiplatelet effect of aspirin. This may be clinically important, because a fixed combination of aspirin and a PPI (esomeprazole) has recently been approved and because aspirin is the most widely used drug in patients with cardiovascular disease. The antiplatelet effect of the new P2Y₁₂ receptor antagonists, ticagrelor and prasugrel, seems less influenced by PPI co-treatment.Given the large number of patients treated with antithrombotic drugs and PPIs, even a minor reduction of platelet inhibition potentially carries considerable clinical impact. The present book chapter summarizes the evidence regarding the widespread use of platelet inhibitors and PPIs in combination. Moreover, it outlines current evidence supporting or opposing drug interactions between these drugs and discusses clinical implications.

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.

Effect of scutellarin on the metabolism and pharmacokinetics of clopidogrel in rats

Erigeron breviscapus (Vant.) Hand-Mazz, a traditional Chinese medicine, is often co-prescribed with clopidogrel for the treatment of ischemic vascular diseases. Scutellarin is the representative bioactive flavonoid isolated from this herb. The aim of this study was to explore the effect of scutellarin on the metabolism and pharmacokinetics of clopidogrel. The in vitro studies using rat liver microsomes showed that scutellarin significantly inhibited the metabolism of clopidogrel. The IC50 value was 2.1 µM. Ten male rats were employed to investigate the effect of scutellarin on the pharmacokinetics of clopidogrel in vivo. After pretreatment with scutellarin, there were significant increases in the AUC0-∞ (from 0.9 ± 0.4 to 1.7 ± 0.6 ng/ml h; p <0.05) and Cmax (from 0.4 ± 0.1 to 0.9 ± 0.1 ng/ml; p <0.05) of clopidogrel. The pharmacokinetic data for clopidogrel active metabolite showed significant decreases in AUC0-∞ (18.2 ± 5.6 to 11.4 ± 3.7 ng/ml h; p <0.05) and Cmax (from 8.2 ± 1.2 to 4.3 ± 0.3 ng/ml; p <0.05) after pretreatment with scutellarin. Collectively, the metabolism and pharmacokinetics of clopidogrel were significantly affected by scutellarin. This study indicated that potential herb-drug interaction between scutellarin and clopidogrel should be taken into consideration in clinical use.

Development of an LC-MS/MS method for determination of 2-oxo-clopidogrel in human plasma

A sensitive and selective liquid chromatography–tandem mass spectrometric (LC−MS/MS) method was established to determine 2-oxo-clopidogrel, a crucial intermediate metabolite in human plasma. A chromatographic separation was performed on a Sapphire C₁₈ column following a liquid–liquid extraction sample preparation with methyl t-butyl ether. Detection was carried out on a triple quadrupole mass spectrometer operated in multiple reaction monitoring (MRM) with an electrospray ionization (ESI) mode. The method was validated in terms of specificity, accuracy, precision and limit of quantification. The calibration curves ranged from 0.50 to 50.0 ng/mL with good linearity. The stability was fully validated with addition of 1,4-dithio-DL-threitol (DTT) into the plasma sample prior to and in the preparation procedure. The validated method was proved to be suitable for use in pharmacokinetic study after single oral administration of 75 mg clopidogrel tablets in human subjects, which could make contribution to intensive study of the clinical drug–drug interactions of clopidogrel and individual treatment.

Prasugrel but not high dose clopidogrel overcomes the lansoprazole neutralizing effect of P2Y12 inhibition: Results of the randomized DOSAPI study

AIMS

The potential negative metabolic interaction between proton pump inhibitors and clopidogrel is an unsolved issue. We hypothesized that doubling the clopidogrel maintenance dose (150 mg) would be less effective than switching to prasugrel 10 mg maintenance dose (MD) to overcome this negative interaction.

METHOD AND RESULTS

In a randomized study with a factorial design, 82 stable coronary artery disease patients treated with 75 mg clopidogrel MD and aspirin were assigned to receive in a double blind fashion lansoprazole (30 mg/day) or placebo and to receive in an open fashion 150 mg clopidogrel MD or 10 mg prasugrel MD. The primary endpoint was the relative change in residual platelet reactivity over the 14-day study period [(RPA14day-RPAbaseline)/RPAbaseline]. The effect of doubling the clopidogrel MD on relative change in RPA was neutralized by lansoprazole (-53.6±48.4% versus +0.8±53.7% without and with lansoprazole, respectively, p = 0.02) whereas 10 mg of prasugrel MD dramatically reduced RPA irrespective of lansoprazole co-administration (-81.8 %±24.8% vs. -72.9%±32.9% without and with lansoprazole, respectively, p = NS). Lansoprazole exposure was the only parameter with a significant interaction with RPA among subgroups.

CONCLUSION

The higher platelet inhibitory effect obtained by doubling the clopidogrel MD was totally neutralized by the co-administration of lansoprazole. This drug interaction was not observed with prasugrel 10 mg.

Effect of esomeprazole with/without acetylsalicylic acid, omeprazole and lansoprazole on pharmacokinetics and pharmacodynamics of clopidogrel in healthy volunteers

OBJECTIVE

The effect of proton pump inhibitors (PPIs) on the pharmacokinetics and pharmacodynamics of clopidogrel was assessed in two healthy volunteer crossover studies.

SUBJECTS AND METHODS

Study 1: subjects received clopidogrel alone (300-mg loading dose, then 75 mg/day for 28 days) and two of three PPIs (omeprazole 80 mg, esomeprazole 40 mg or lansoprazole 60 mg) plus clopidogrel for 29 days in three treatment periods (randomized treatment sequence assignment). Study 2: subjects received clopidogrel alone (75 mg/day for 9 days) and clopidogrel alone for 4 days followed by clopidogrel plus fixed-combination esomeprazole 20 mg/low-dose acetylsalicylic acid (ASA) 81 mg for 5 days in two treatment periods (randomized treatment sequence assignment). Pharmacokinetic effects were estimated by measuring active metabolite of clopidogrel, and pharmacodynamic effects by inhibition of adenosine diphosphate (ADP)-induced platelet aggregation.

RESULTS

There was a relative decrease of up to 50 % in exposure to the active metabolite of clopidogrel with the different PPIs (study 1), and close to 40 % with esomeprazole/low-dose ASA (study 2), compared with clopidogrel alone. There was an absolute decrease of up to 17 % in inhibition of ADP-induced platelet aggregation with co-administration of different PPIs, compared with clopidogrel alone; however, no differences in platelet inhibition were observed during co-administration with the esomeprazole/low-dose ASA fixed-dose combination.

CONCLUSION

Omeprazole, esomeprazole and lansoprazole decreased systemic exposure to the active metabolite of clopidogrel in healthy volunteers, leading to modest decreases in its antiplatelet effect. However, no apparent differences in platelet inhibition were observed when esomeprazole was co-administered with low-dose ASA as a fixed-dose combination.

Clopidogrel, prasugrel, or ticagrelor? a practical guide to use of antiplatelet agents in patients with acute coronary syndromes

Aspirin is a cornerstone of therapy in the treatment of patients with acute coronary syndromes (ACS). However, dual antiplatelet therapy reduces the risk of stent thrombosis and cardiovascular events compared with aspirin alone in the treatment of patients with ACS. Recently, there has been debate as to which antiplatelet agent should be added to aspirin in the ACS treatment regimen. This review summarizes the pharmacologic and clinical data comparing clopidogrel, prasugrel, and ticagrelor, and provides a practical guide to clinicians for determining which antiplatelet to use for patients with ACS.

Thiolactone sulfoxides as new reactive metabolites acting as bis-electrophiles: implication in clopidogrel and prasugrel bioactivation

The antithrombotics of the tetrahydrothienopyridine series, clopidogrel and prasugrel, are prodrugs that must be metabolized in two steps to become pharmacologically active. The first step is the formation of a thiolactone metabolite. The second step is a cytochrome P450 (P450)-dependent oxidation of this thiolactone resulting in the formation of a sulfenic acid that is eventually reduced into the corresponding active thiol. It has been postulated that the sulfenic acid metabolite resulted from a nucleophilic attack of water on a highly reactive thiolactone sulfoxide derived from P450-dependent oxidation of the thiolactone primary metabolite. The data described in the present article are in complete agreement with this proposition as they show that it was possible to trap these thiolactone sulfoxides by a series of nucleophiles such as amines, thiols, or cyclopentane-1,3-dione (CPDH), an equivalent of dimedone that is used as a sulfenic acid trapping agent. HPLC-MS studies showed that various bis-adducts having incorporated two nucleophile molecules were formed in these reactions. One of them that resulted from the oxidation of 2-oxo-prasugrel by human liver microsomes in the presence of ethanolamine and CPDH was isolated and completely characterized by (1)H and (13)C NMR spectroscopy in addition to MS and MS(2) spectrometry. All metabolites derived from an attack of H2O or an amine at the CO carbon of the intermediate thiolactone sulfoxide existed as a mixture of two diastereomers having a cis configuration of the double bond, whereas those formed in the presence of thiols appeared as a mixture of four diastereomers with a cis or trans configuration of the double bond. This led us to propose tentative mechanisms for the previously reported formation of trans isomers of the active thiol metabolite of clopidogrel upon microsomal metabolism of this antithrombotic in the presence of thiols. The results described in this article showed that thiolactone sulfoxides are formed as reactive metabolites during the metabolism of clopidogrel and prasugrel and are able to react as bis-electrophiles with a variety of nucleophiles. The possible implications of the formation of these reactive metabolites in the pharmacological and/or secondary toxic effects of these drugs remain to be studied.

Genotoxic bioactivation of constituents of a diesel exhaust particle extract by the human lung

The ability of the human lung to catalyze genotoxic bioactivation of constituents of diesel exhaust particle (DEP) extract (DEPE) and the identity of the lung enzymes involved in the bioactivation were investigated using human lung tissues obtained from surgical resections. Genotoxicity was determined by lung S9-catalyzed mutagenicity of DEPE constituents to Salmonella typhimurium TA98NR in the Ames test and by DEPE-induced pneumocyte DNA damage response as determined by γH2Ax expression in ex vivo tissues. S9 was prepared from lung explants treated ex vivo with either DEPE to induce pulmonary enzymes (DEPE-S9) or vehicle only (CON-S9). TA98NR served as the tester strain for the purpose of enhancing and minimizing the contribution of lung S9 and Salmonella, respectively, to DEPE bioactivation. DEPE-S9 was 2.2-fold more active than CON-S9 or rat liver S9 in DEPE bioactivation and the bioactivation was inhibited 58, 45, 22, and 16% by α-naphthoflavone, dicumarol, ketoconazole, and ticlopidine, respectively. Alveolar S9 was less active than bronchioalveolar S9 in DEPE bioactivation. DEPE and diesel exhaust particles (DEP) induced γ-pH2Ax expression in pulmonary cells. Pulmonary CYP1A1 and NQO1 were induced by DEPE treatment, with the constitutive and induced CYP1A1 distributed throughout all peripheral lung regions, whereas NQO1 was limited in distribution to bronchiolar epithelium. The results show that the human lung is highly active in catalyzing genotoxic bioactivation of diesel emission constituents and that CYP1A and NQO1 play major roles in the reaction. The findings underscore the usefulness of human lung tissues in studies of the pneumotoxicity potential of chemicals to humans.

The paraoxonase-1 pathway is not a major bioactivation pathway of clopidogrel in vitro

BACKGROUND AND PURPOSE

Clopidogrel is a prodrug bioactivated by cytochrome P450s (CYPs). More recently, paraoxonase-1 (PON1) has been proposed as a major contributor to clopidogrel metabolism. The purpose of this study was to assess the relative contribution of CYPs and PON1 to clopidogrel metabolism in vitro.

EXPERIMENTAL APPROACH

Clopidogrel metabolism was studied in human serum, recombinant PON1 enzyme (rePON1), pooled human liver microsomes (HLMs), HLMs with the CYP2C19*1/*1 genotype and HLMs with the CYP2C19*2/*2 genotype. Inhibition studies were also performed using specific CYP inhibitors and antibodies. Clopidogrel and its metabolites were measured using LC/MS/MS method.

KEY RESULTS

PON1 activity was highest in the human serum and there was no difference in PON1 activity between any of the HLM groups. The production of clopidogrel's active metabolite (clopidogrel-AM) from 2-oxo-clopidogrel in pooled HLMs was approximately 500 times that in serum. When 2-oxo-clopidogrel was incubated with rePON1, clopidogrel-AM was not detected. Clopidogrel-AM production from 2-oxo-clopidogrel was lower in CYP2C19*2/*2 HLMs compared with CYP2C19*1/*1 HLMs, while PON1 activity in HLMs with both genotypes was similar. Moreover, incubation with inhibitors of CYP3A, CYP2B6 and CYP2C19 significantly reduced clopidogrel bioactivation while a PON1 inhibitor, EDTA, had only a weak inhibitory effect.

CONCLUSION AND IMPLICATIONS

This in vitro study shows that the contribution of PON1 to clopidogrel metabolism is limited at clinically relevant concentrations. Moreover, CYP2C19, CYP2B6 and CYP3A play important roles in the bioactivation of clopidogrel.

Ticlopidine inhibits both O-demethylation and renal clearance of tramadol, increasing the exposure to it, but itraconazole has no marked effect on the ticlopidine-tramadol interaction

PURPOSE

We assessed possible drug interactions of tramadol given concomitantly with the potent CYP2B6 inhibitor ticlopidine, alone or together with the potent CYP3A4 and P-glycoprotein inhibitor itraconazole.

METHODS

In a randomized, placebo-controlled cross-over study, 12 healthy subjects ingested 50 mg of tramadol after 4 days of pretreatment with either placebo, ticlopidine (250 mg twice daily) or ticlopidine plus itraconazole (200 mg once daily). Plasma and urine concentrations of tramadol and its active metabolite O-desmethyltramadol (M1) were monitored over 48 h and 24 h, respectively.

RESULTS

Ticlopidine increased the mean area under the plasma concentration-time curve (AUC0-∞) of tramadol by 2.0-fold (90 % confidence interval (CI) 1.6-2.4; p < 0.001) and Cmax by 1.4-fold (p < 0.001), and reduced its oral and renal clearance (p < 0.01). Ticlopidine reduced the AUC0-3 of M1 (p < 0.001) and the ratio of the AUC0-∞ of M1 to that of tramadol, but did not influence the AUC0-∞ of M1. Tramadol or M1 pharmacokinetics did not differ between the ticlopidine alone and ticlopidine plus itraconazole phases.

CONCLUSIONS

Ticlopidine increased exposure to tramadol, reduced its renal clearance and inhibited the formation of M1, most likely via inhibition of CYP2B6 and/or CYP2D6. The addition of itraconazole to ticlopidine did not modify the outcome of the drug interaction. Concomitant clinical use of ticlopidine and tramadol may enhance the risk of serotonergic effects, especially when higher doses of tramadol are used.

Importance of potent P2Y(12) receptor blockade in acute myocardial infarction: focus on prasugrel

INTRODUCTION

Prasugrel therapy is recommended in patients with acute coronary syndromes (ACS) undergoing percutaneous coronary intervention (PCI).

AREAS COVERED

This article reviews the efficacy and safety profile of prasugrel, cost considerations, and its role in clinical practice based on published data. The authors searched PubMed and Ovid databases for English language clinical trial articles involving the use of prasugrel in human subjects and patients, published through June 2012. The keyword "prasugrel" was used. The review focuses on clinical trials, but other articles including Food and Drug Administration documents are also reviewed.

EXPERT OPINION

Prasugrel has a more rapid and greater pharmacodynamic (PD) effect than clopidogrel. No significant drug - drug interactions have been reported. In a large-scale randomized clinical trial, prasugrel was associated with better protection against ischemic event occurrence compared to clopidogrel, but more bleeding in ACS patients undergoing PCI. Adverse outcomes outweighed the benefit of prasugrel in certain subgroups, including patients over the age of 75, those weighing less than 60 kg, and patients with a prior history of stroke or transient ischemic attack. In subsequent PD studies, prasugrel therapy showed suboptimal platelet inhibition in selected patients. In addition, "hyper-responsiveness" to prasugrel may increase the risk of serious bleeding in high-risk patients. More detailed studies are warranted to explore antiplatelet regimens tailored to optimally limit ischemic and bleeding event occurrences. A Phase-III TRILOGY trial (NCT00699998) will indicate the clinical efficacy and safety of prasugrel in patients with non-ST-segment elevation ACS, who are medically managed without coronary revascularization.

Metabolic activation of prasugrel: nature of the two competitive pathways resulting in the opening of its thiophene ring

The mechanism generally admitted for the bioactivation of the antithrombotic prodrug, prasugrel, 1c, is its two-step enzymatic conversion into a biologically active thiol metabolite. The first step is an esterase-catalyzed hydrolysis of its acetate function leading to a thiolactone metabolite 2c. The second step was described as a cytochrome P450 (P450)-dependent oxidative opening of the thiolactone ring of 2c, with intermediate formation of a reactive sulfenic acid metabolite that is eventually reduced to the corresponding active thiol 3c. This article describes a detailed study of the metabolism of 1c by human liver microsomes and human sera, with an analysis by HPLC-MS under conditions allowing a complete separation of the thiol metabolite isomers, after derivatization with 3'-methoxy phenacyl bromide. It shows that there are two competing metabolic pathways for the opening of the 2c thiolactone ring. The major one, which was previously described, results from a P450- and NADPH-dependent redox bioactivation of 2c and leads to 3c, two previously reported thiol diastereomers bearing an exocyclic double bond. It occurs with NADPH-supplemented human liver microsomes but not with human sera. The second one results from a hydrolysis of 2c and leads to an isomer of 3c, 3c endo, in which the double bond has migrated from an exocyclic to an endocyclic position in the piperidine ring. It occurs both with human liver microsomes and human sera, and does not require NADPH. However, it requires Ca(2+) and is inhibited by paraoxon, which suggests that it is catalyzed by a thioesterase such as PON-1. Chemical experiments have confirmed that hydrolytic opening of thiolactone 2c exclusively leads to derivatives of the endo thiol isomer 3c endo.

Mechanism-based inactivation of human cytochrome P450 2B6 by clopidogrel: involvement of both covalent modification of cysteinyl residue 475 and loss of heme

We have investigated the mechanisms by which clopidogrel inactivates human cytochrome P450 2B6 (CYP2B6) in a reconstituted system. It was found that clopidogrel and its thiolactone metabolite, 2-oxo-clopidogrel, both inactivate CYP2B6 in a time- and concentration-dependent manner. On the basis of k(inact)/K(I) ratios, clopidogrel is approximately 5 times more efficient than 2-oxo-clopidogrel in inactivating CYP2B6. Analysis of the molecular mass of the CYP2B6 wild-type (WT) protein that had been inactivated by either clopidogrel or 2-oxo-clopidogrel showed an increase in the mass of the protein by ∼350 Da. This increase in the protein mass corresponds to the addition of the active metabolite of clopidogrel to CYP2B6. It is noteworthy that this adduct can be cleaved from the protein matrix by incubation with dithiothreitol, confirming that the active metabolite is linked to a cysteinyl residue of CYP2B6 via a disulfide bond. Peptide mapping of tryptic digests of the inactivated CYP2B6 using electrospray ionization liquid chromatography-tandem mass spectrometry identified Cys475 as the site of covalent modification by the active metabolite. This was further confirmed by the observation that mutation of Cys475 to a serine residue eliminates the formation of the protein adduct and prevents the C475S variant from mechanism-based inactivation by 2-oxo-clopidogrel. However, this mutation did not prevent the C475S variant from being inactivated by clopidogrel. Furthermore, inactivation of both CYP2B6 WT and C475S by clopidogrel, but not by 2-oxo-clopidogrel, led to the loss of the heme, which accounts for most of the loss of the catalytic activity. Collectively, these results suggest that clopidogrel inactivates CYP2B6 primarily through destruction of the heme, whereas 2-oxo-clopidogrel inactivates CYP2B6 through covalent modification of Cys475.

Clopidogrel and proton pump inhibitors: influence of pharmacological interactions on clinical outcomes and mechanistic explanations

Dual antiplatelet therapy with aspirin and clopidogrel is associated with a significant reduction in vascular ischemic events; however, gastrointestinal bleeding events are a major concern in high-risk and older patients. Clinical practice guidelines recommend combination therapy with proton pump inhibitors (PPI) and dual antiplatelet therapy to attenuate gastrointestinal bleeding risk. In addition, high on-treatment platelet reactivity has been associated with recurrent ischemic events. Whether or not the pharmacological interaction between clopidogrel and PPI, which results in diminished antiplatelet effect, adversely influences clinical efficacy is highly controversial and the subject of debate. Based on largely anecdotal post-hoc analyses, the U.S. Federal Drug Administration's and European Medicines Agency's recommendations discourage PPI use (particularly omeprazole) in patients treated with clopidogrel. However, many American College of Cardiology/American Heart Association/Society for Cardiovascular Angiography and Interventions experts do not support change in clinical practice guidelines recommendations without adequately powered, prospective, randomized clinical trial data.

Structure and function of cytochromes P450 2B: from mechanism-based inactivators to X-ray crystal structures and back

This article reviews work from the author dating back to 1978 and focuses on the structural basis of cytochrome P450 (P450) function using available contemporary techniques. Early studies used mechanism-based inactivators that bound to the protein moiety of hepatic P450s to try to localize the active site. Subsequent studies used cDNA cloning, heterologous expression, site-directed mutagenesis, and homology modeling based on multiple bacterial P450 X-ray crystal structures to predict the active sites of CYP2B enzymes with considerable accuracy. Breakthroughs in engineering and expression of mammalian P450s enabled us to determine our first X-ray crystal structure of ligand-free rabbit CYP2B4. To date, we have solved 11 CYP2B4 and three human CYP2B6 structures, which represent four significantly different conformations. The plasticity of CYP2B4 has been confirmed by deuterium exchange mass spectrometry and is substantiated by molecular dynamics simulations. In addition to major movement of secondary structure elements, more subtle reorientation of active site side chains, especially Phe206, Phe297, and Glu301, contributes to the ability of CYP2B enzymes to bind various ligands. Isothermal titration calorimetry has proven to be a useful tool for studying the thermodynamics of ligand binding to CYP2B4 and CYP2B6, and NMR has enabled study of ligand binding orientation in solution as an adjunct to X-ray crystallography. A major challenge remains to harness the power of the various approaches to facilitate prediction of CYP2B specificity and inhibition.

Effects of clopidogrel on the pharmacokinetics of sibutramine and its active metabolites

Sibutramine is metabolized by the enzymes CYP2B6 and CYP2C19 into 2 active metabolites, M1 (mono-desmethyl sibutramine) and M2 (di-desmethyl sibutramine). Clopidogrel is a mechanism-based inhibitor of CYP2B6 and CYP2C19. In this study, 13 extensive metabolizers of CYP2B6 and CYP2C19 were evaluated to clarify whether clopidogrel inhibits the formation of the active metabolites of sibutramine. In the control phase, each subject received a 15-mg oral dose of sibutramine. After a washout period of 2 weeks, in the clopidogrel phase, the subjects received 300 mg of clopidogrel on the first day and then 75-mg once daily for 6 days. One hour after the last dosing of clopidogrel, all subjects received 15-mg of sibutramine. Compared with the control phase, the mean sibutramine and M1 plasma concentrations were higher after clopidogrel treatment. Clopidogrel significantly increased the half-life (242% of control phase) and area under the plasma concentration-time curve from 0 to infinity (AUC(inf)) (227% of control phase) of sibutramine and decreased the apparent oral clearance (31% of control phase) of sibutramine. Pharmacokinetic analysis showed significant increases in the AUC(inf) (162% of control phase) of M1. The CYP2B6 and CYP2C19 inhibitor clopidogrel significantly inhibited the formations of M1 from sibutramine and M2 from sibutramine by 37% and 64%, respectively. Therefore, CYP2B6 and CYP2C19 are in vivo catalysts for the formation of the 2 active metabolites of sibutramine.

Effects of drug interactions on biotransformation and antiplatelet effect of clopidogrel in vitro

BACKGROUND AND PURPOSE

The conversion of clopidogrel to its active metabolite, R-130964, is a two-step cytochrome P450 (CYP)-dependent process. The current investigations were performed to characterize in vitro the effects of different CYP inhibitors on the biotransformation and on the antiplatelet effect of clopidogrel.

EXPERIMENTAL APPROACH

Clopidogrel biotransformation was studied using human liver microsomes (HLM) or specific CYPs and platelet aggregation using human platelets activated with ADP.

KEY RESULTS

Experiments using HLM or specific CYPs (3A4, 2C19) revealed that at clopidogrel concentrations >10 microM, CYP3A4 was primarily responsible for clopidogrel biotransformation. At a clopidogrel concentration of 40 microM, ketoconazole showed the strongest inhibitory effect on clopidogrel biotransformation and clopidogrel-associated inhibition of platelet aggregation with IC(50) values of 0.03 +/- 0.07 microM and 0.55 +/- 0.06 microM respectively. Clarithromycin, another CYP3A4 inhibitor, impaired clopidogrel biotransformation and antiplatelet activity almost as effectively as ketoconazole. The CYP3A4 substrates atorvastatin and simvastatin both inhibited clopidogrel biotransformation and antiplatelet activity, less potently than ketoconazole. In contrast, pravastatin showed no inhibitory effect. As clopidogrel itself inhibited CYP2C19 at concentrations >10 microM, the CYP2C19 inhibitor lansozprazole affected clopidogrel biotransformation only at clopidogrel concentrations < or =10 microM. The carboxylate metabolite of clopidogrel was not a CYP substrate and did not affect platelet aggregation.

CONCLUSIONS AND IMPLICATIONS

At clopidogrel concentrations >10 microM, CYP3A4 is mainly responsible for clopidogrel biotransformation, whereas CYP2C19 contributes only at clopidogrel concentrations < or =10 microM. CYP2C19 inhibition by clopidogrel at concentrations >10 microM may explain the conflicting results between in vitro and in vivo investigations regarding drug interactions with clopidogrel.