The proton pump inhibitor, omeprazole, but not lansoprazole or pantoprazole, is a metabolism-dependent inhibitor of CYP2C19: implications for coadministration with clopidogrel

First-in-Human Phase 1 Study to Evaluate the Clinical Pharmacology Properties of RBN-3143, a Novel Inhibitor of Mono-Adenosine Diphosphate Ribosyltransferase-PARP14

RBN-3143 is an inhibitor of PARP14 in development for inflammatory diseases. Multiple assessments were conducted to evaluate the clinical pharmacology properties of RBN-3134. A randomized, double-blind, placebo-controlled study assigned healthy volunteers (HVs) to single ascending doses (SADs) (25-1000 mg) or multiple ascending doses (MADs) (150, 300, and 500 mg twice daily [BID] for 14 days) of RBN-3143 or placebo. An open-label, randomized, 3-period, cross-over study evaluated the effects of food and pantoprazole (40 mg once daily [QD]) on the pharmacokinetics of RBN-3143 (500 mg), and a pharmacokinetic drug-drug interaction study with oral midazolam (2 mg) determined whether RBN-3143 (300 mg BID for 14 days) is an inducer of cytochrome P4503A4 (CYP3A4). The most common treatment-related treatment-emergent adverse events in subjects taking RBN-3143 were headache, nausea, vomiting, and elevated serum creatinine. In the SAD, RBN-3143 Cmax and AUCinf increased with dose, and Tmax was 2 hours. RBN-3143 was cleared from plasma with an apparent terminal half-life ranging from 3 to 11 hours. In the MAD, Cmax and AUCinf increased 1.5- and 1.6-fold, respectively, following 14 days of 150, 300, and 500 mg BID dosing. Dosing of RBN-3143 with food resulted in higher Cmax and AUCinf ratios of 1.74 and 1.42, respectively. Coadministration with pantoprazole did not impact RBN-3143 exposure. RBN-3143 was an inducer of CYP3A4 in most but not all subjects, with mean midazolam Cmax and AUCinf ratios of 0.92 and 0.88, respectively. The clinical pharmacology properties of RBN-3143 in HVs support further development for inflammatory diseases.

Comparative effectiveness of potassium-competitive acid blockers and proton pump inhibitors in dual antiplatelet therapy patients: a nationwide cohort study

BACKGROUND

Proton pump inhibitors (PPIs) are widely used in combination with dual antiplatelet therapy (DAPT) to reduce the risk of gastrointestinal bleeding; however, some PPIs may interfere with clopidogrel metabolism through CYP2C19 inhibition. Potassium-competitive acid blockers (P-CABs) have emerged as alternatives to PPIs, although their effectiveness in patients receiving DAPT have not been sufficiently explored.

AIM

This study aimed to compare the effectiveness of PPIs and P-CABs in preventing cardiovascular events and gastrointestinal bleeding among patients receiving DAPT after coronary stent implantation.

METHOD

A retrospective cohort design was employed using nationwide claim data from South Korea. The study included patients who received aspirin-clopidogrel DAPT with PPI or P-CAB for a minimum of 60 days. After applying propensity score matching (PSM) at a 1:1 ratio, survival analysis was conducted. The primary outcome was a composite of acute myocardial infarction, ischemic stroke, revascularization, and in-hospital mortality. Secondary outcomes involved the individual components of the composite outcome and gastrointestinal bleeding.

RESULTS

The initial cohort included 39,234 patients in the PPI group and 3,434 in the P-CAB group. After 1:1 PSM, 3,434 patients were included in each group. No statistically significant differences were observed between the PPI and P-CAB groups for the primary outcome (hazard ratio [HR], 0.85; 95% confidence interval [CI], 0.68-1.07; p = 0.167) and gastrointestinal bleeding (HR, 1.36; 95% CI, 0.76-2.42; p = 0.3).

CONCLUSION

Both PPIs and P-CABs appear to be viable options for acid suppression in patients undergoing DAPT, with no significant differences in cardiovascular or gastrointestinal outcomes.

Intestinal Drug Absorption After Subarachnoid Hemorrhage and Elective Neurosurgery: Insights From Esomeprazole Pharmacokinetics

OBJECTIVES

Subarachnoid hemorrhage (SAH) may critically impair cardiovascular, metabolic, and gastrointestinal function. Previous research has demonstrated compromised drug absorption in this group of patients. This study aimed to examine the impact of SAH on gastrointestinal function and its subsequent effect on the absorption of enterally administered drugs, using esomeprazole as a probe drug.

DESIGN

Prospective observational cohort study.

SETTING

Academic hospital in Germany.

PATIENTS

We included 17 patients with high-grade SAH and 17 controls, comparable in age, sex, body weight, and renal function, who underwent elective cranial surgery.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

Both groups received esomeprazole per standard protocol to prevent acid-associated mucosal damage, either orally or through a nasogastric tube. On day 4, esomeprazole was administered IV to estimate oral bioavailability. Esomeprazole serum concentrations were measured on days 1, 3, and 4 in both groups and on day 7 in the SAH group. Patients with high-grade SAH exhibited severely impaired drug absorption. Most patients showed no improvement in intestinal drug absorption even a week after hemorrhage.

CONCLUSIONS

Following SAH, significantly reduced drug absorption may be attributed to decreased intestinal motility and compromised intestinal mucosal function. Clinicians should anticipate the reduced effectiveness of enterally administered medications for at least seven days after high-grade SAH.

Impact of fresh grape juice on the pharmacokinetics of omeprazole: results of a food-drug interaction study

Grapes have been widely used for dietary ailments due to their attributed pharmacological activities. Resveratrol, the chief constituent of grapes, is responsible for their pharmacological benefits. However, apart from their beneficial effects, grapes have also recently been considered in drug interaction studies. This study investigated the pharmacokinetic profile of omeprazole administered alone compared to omeprazole administered with grape juice, with a prior intake of grape juice, for 1 continuous week. The study was conducted on two groups of healthy male volunteers [n = 12]. One group was orally administered 40 mg of omeprazole alone, while the other group was administered omeprazole with grape juice. Blood samples were analyzed for omeprazole concentration by a reverse-phase HPLC method. Co-administration of 40 mg omeprazole with grape juice significantly decreased the AUC0-t and Cmax by 32% and 34%, respectively, suggesting a role being played by grapes in the activation of P-glycoprotein and omeprazole metabolizing enzymes, including CYP3A4 and CYP2C19. In conclusion, the addition of grapes as a dietary supplement in patients taking omeprazole for the management of peptic ulcer symptoms may lead to a higher required dose of omeprazole.

An in vitro evaluation on metabolism of mitragynine to 9-O-demethylmitragynine

Kratom (Mitragyna Speciosa Korth.) is an indigenous tree native to Southeast Asia whose leaves have been traditionally ingested as a tea and has seen its popularity increase in the United States. Although kratom and its constituents presently have no approved uses by the Food and Drug Administration, its major alkaloids (e.g., mitragynine) have psychoactive properties that may hold promise for the treatment of opioid cessation, pain management, and other indications. 9-O-demethylmitragynine is a major metabolite formed from mitragynine metabolism (36 % total metabolism) and displays similar pharmacologic activity. Cytochrome P450 (CYP) 3A4 has been identified as a major enzyme involved in mitragynine metabolism; however, the in vitro metabolism parameters of 9-O-demethylmitragynine formation are not well defined and a risk of potential drug interactions exists. Using human liver S9 fractions, 9-O-demethylmitragynine formation was generally linear for enzyme concentrations of 0-0.25 mg/mL and incubation times of 5-20 min. 9-O-demethylmitragynine displayed a Km 1.37 μM and Vmax of 0.0931 nmol/min/mg protein. Known CYP inhibitors and compounds that might be concomitantly used with kratom were assessed for inhibition of 9-O-demethylmitragynine formation. Ketoconazole, a CYP3A index inhibitor, demonstrated a significant effect on 9-O-demethylmitragynine formation, further implicating CYP3A4 as a major metabolic pathway. Major cannabinoids (10 μg/mL) displayed minor inhibition of 9-O-demethylmitragynine formation, while all other compounds had minimal effects. Mixtures of physiological achievable cannabinoid concentrations also displayed minor effects on 9-O-demethylmitragynine formation, making a metabolic drug interaction unlikely; however, further in vitro, in vivo, and clinical studies are necessary to fully exclude any risk.

Potential of pharmacogenetics in minimizing drug therapy problems in cystic fibrosis

BACKGROUND

With advancements in CF drug development, people with cystic fibrosis (PwCF) now take a median of seven medications daily, increasing treatment complexity, risk of drug therapy problems (DTPs), and interference with treatment goals. Given that some of these DTPs can be prevented with preemptive pharmacogenetic testing, the overall goal of this study was to test the clinical utility of a multi-gene pharmacogenetics (PGx) panel in potentially reducing DTPs in PwCF.

METHODS

A population based retrospective study of patients with CF was conducted at the University of Utah Health Care System. The patients were genotyped for CYP450 enzymes using a pharmacogenomic assay, and their drug utilization information was obtained retrospectively. This pharmacogenomic information was combined with clinical guidelines to predict the number of actionable PGx interventions in this patient cohort.

RESULTS

A total of 52 patients were included in this study. In the patient sample, a minimum of one order of actionable PGx medication was observed in 75 % of the cases. Results revealed that 4.2 treatment modifications per 10 patients can be enabled with the help of a PGx intervention in this patient population. Additionally, our findings suggest that polymorphisms in CYP2D6 and CYP2C19 are most likely to be the primary contributors to DTP's within PwCF.

CONCLUSION

This study provides evidence that the PGx panel has the potential to help alleviate the clinical burden of DTPs in PwCF and can assist in informing pharmacotherapy recommendations. Future research should validate these findings and evaluate which subgroups of PwCF would most benefit from pharmacogenetic testing.

Impact of obesity and roux-en-Y gastric bypass on the pharmacokinetics of (R)- and (S)-omeprazole and intragastric pH

This study employed physiologically-based pharmacokinetic-pharmacodynamics (PBPK/PD) modeling to predict the effect of obesity and gastric bypass surgery on the pharmacokinetics and intragastric pH following omeprazole treatment. The simulated plasma concentrations closely matched the observed data from non-obese, morbidly obese, and post-gastric bypass populations. Obesity significantly reduces CYP3A4 and CYP2C19 activities, as reflected by the metabolic ratio [omeprazole sulphone]/[omeprazole] and [5-hydroxy-omeprazole]/[omeprazole]. The morbidly obese model accounted for the down-regulation of CYP2C19 and CYP3A4 to recapitulate the observed data. Sensitivity analysis showed that intestinal CYP3A4, gastric pH, small intestine bypass, and the delay in bile release do not have a major influence on omeprazole exposure. Hepatic CYP3A4 had a significant impact on the AUC of (S)-omeprazole, while hepatic CYP2C19 affected both (R)- and (S)-omeprazole AUC. After gastric bypass surgery, the activity of CYP3A4 and CYP2C19 is restored. The PBPK model was linked to a mechanism-based PD model to assess the effect of omeprazole on intragastric pH. Following 40 mg omeprazole, the mean intragastric pH was 4.3, 4.6, and 6.6 in non-obese, obese, and post-gastric bypass populations, and the daily time with pH >4 was 14.7, 16.4, and 24 h. Our PBPK/PD approach provides a comprehensive understating of the impact of obesity and weight loss on CYP3A4 and CYP2C19 activity and omeprazole pharmacokinetics. Given that simulated intragastric pH is relatively high in post-RYGB patients, irrespective of the dose of omeprazole, additional clinical outcomes are imperative to assess the effect of proton pump inhibitor in preventing marginal ulcers in this population.

A network meta-analysis: evaluating the efficacy and safety of concurrent proton pump inhibitors and clopidogrel therapy in post-PCI patients

Introduction

The objective of this research was to evaluate the risk of major adverse cardiovascular events (MACEs) associated with the use of various proton pump inhibitors (PPIs) in combination with clopidogrel in patients who underwent percutaneous coronary intervention (PCI).

Methods

To accomplish this, we analyzed data from randomized controlled trials and retrospective cohort studies sourced from key electronic databases. These studies specifically examined the effects of different PPIs, such as lansoprazole, esomeprazole, omeprazole, rabeprazole, and pantoprazole, when used in conjunction with clopidogrel on MACEs. The primary focus was on the differential impact of these PPIs, while the secondary focus was on the comparison of gastrointestinal (GI) bleeding events in groups receiving different PPIs with clopidogrel vs. a placebo group. This study's protocol was officially registered with INPLASY (INPLASY2024-2-0009).

Results

We conducted a network meta-analysis involving 16 studies with a total of 145,999 patients. Our findings indicated that rabeprazole when combined with clopidogrel, had the lowest increase in MACE risk (effect size, 1.05, 95% CI: 0.66-1.66), while lansoprazole was associated with the highest risk increase (effect size, 1.48, 95% CI: 1.22-1.80). Esomeprazole (effect size, 1.28, 95% CI: 1.09-1.51), omeprazole (effect size, 1.23, 95% CI: 1.07-1.43), and pantoprazole (effect size, 1.38, 95% CI: 1.18-1.60) also significantly increased MACE risk. For the secondary outcome, esomeprazole (effect size, 0.30, 95% CI: 0.09-0.94), omeprazole (effect size, 0.34, 95% CI: 0.14-0.81), and pantoprazole (effect size, 0.33, 95% CI: 0.13-0.84) demonstrated an increased potential for GI bleeding prevention.

Conclusions

In conclusion, the combination of lansoprazole and clopidogrel was found to significantly elevate the risk of MACEs without offering GI protection in post-PCI patients. This study is the first network meta-analysis to identify the most effective regimen for the concurrent use of clopidogrel with individual PPIs.

Systematic Review Registration

https://inplasy.com/inplasy-2024-2-0009/, identifier (INPLASY2024-2-0009).

Physiologically based pharmacokinetic model analysis of the inhibitory effect of vonoprazan on the metabolic activation of proguanil

We previously reported that repeated oral administration of vonoprazan (VPZ) followed by oral administration of proguanil (PG) in healthy adults increased blood concentration of PG and decreased blood concentration of its metabolite cycloguanil (CG) compared with administration of PG alone. In this study, we investigated whether this interaction can be quantitatively explained by VPZ inhibition of PG metabolism. In an in vitro study using human liver microsomes, VPZ inhibited CG formation from PG in a concentration-dependent manner, and the inhibition was enhanced depending on preincubation time. Then, a physiologically based pharmacokinetic (PBPK) model analysis was performed incorporating the obtained inhibition parameters. By fitting the blood concentration profiles of VPZ and PG/CG after VPZ and PG were orally administered alone to our PBPK model, parameters were obtained which can reproduce their concentration profiles. In contrast, when the VPZ inhibition parameters for CG formation from the in vitro study were incorporated, the predicted blood PG and CG concentrations were unchanged; the apparent dissociation constant had to be set to about 1/23 of the obtained in vitro value to reproduce the observed interaction. Further comprehensive evaluation is required, including the possibility that mechanisms other than metabolic inhibition may be involved.

Effects of treatment with clopidogrel with or without proton pump inhibitor omeprazole on the risk of ischemic stroke: a nationwide cohort study

Most proton pump inhibitors (PPIs) inhibit the bioactivation of clopidogrel to its active metabolite. There is controversy concerning whether PPIs alter the effectiveness of clopidogrel in reducing the risk of ischemic stroke (IS). We therefore aimed to examine the risk of IS associated with concomitant use of clopidogrel and omeprazole, a PPI commonly used in clinical settings. We conducted a retrospective cohort study using the National Health Insurance Research Database of Taiwan dated from 2000 to 2013. The study cohorts comprised 407 patients diagnosed with acute coronary syndrome (ACS) and with concomitant use of clopidogrel and omeprazole (the exposed cohort), 814 ACS patients with single use of clopidogrel (the comparison cohort), and 230 ACS patients with concurrent use of clopidogrel and pantoprazole (the reference cohort). The primary outcome was incident IS. The hazard ratios (HRs) and 95% confidence intervals (CIs) derived from the time-dependent Cox regression model were used to assess the association between concomitant use of clopidogrel and omeprazole and the risk of IS. The incidence rate of IS was significantly higher in the exposed cohort (81.67 per 1000 person-years) than in the comparison cohort (57.45 per 1000 person-years), resulting in an adjusted HR of 1.39 (95% CI 1.03-1.74). By contrast, there was no significant difference in the risk of IS between the exposed and reference cohorts (adjusted HR 1.11; 95% CI 0.81-1.52). The present study revealed that patients taking both clopidogrel and omeprazole was associated with an increased risk of IS.

Concomitant Use of Antiplatelet Agents and Proton-Pump Inhibitors Increases the Risk of Adverse Cardiovascular Events: A Nationwide Population-Based Cohort Study Using Balanced Operational Definitions

Antiplatelet agents are commonly used in combination with proton-pump inhibitors (PPIs) in patients with acute coronary syndrome who are at risk of gastrointestinal hemorrhage. However, studies have reported that PPIs can alter the pharmacokinetics of antiplatelet agents and result in adverse cardiovascular events. We enrolled 311 patients who received antiplatelet therapy with PPIs for >30 days and 1244 matched controls following a 1:4 propensity score matching during the index period. Patients were followed up until death, myocardial infarction, coronary revascularization, or the end of the follow-up period. Patients who used antiplatelet therapy with PPIs were found to be at higher risk of mortality than the controls (adjusted hazard ratio (HR): 1.77; 95% confidence interval (CI): 1.30-2.40). The adjusted HR for patients who used antiplatelet agents with PPIs developing myocardial infarction and coronary revascularization events was 3.52 (95% CI: 1.34-9.22) and 4.74 (95% CI: 2.03-11.05), respectively. Additionally, middle-aged patients or those within 3 years of concomitant use showed a higher risk of myocardial infarction and coronary revascularization. Our findings suggest that antiplatelet therapy combined with PPIs has a higher mortality risk in patients with gastrointestinal bleeding and is associated with an increased risk of myocardial infarction and coronary revascularization.

The impact of CYP2C19 genotype on phenoconversion by concomitant medication

Introduction: Pharmacogenetics-informed drug prescribing is increasingly applied in clinical practice. Typically, drug metabolizing phenotypes are determined based on genetic test results, whereupon dosage or drugs are adjusted. Drug-drug-interactions (DDIs) caused by concomitant medication can however cause mismatches between predicted and observed phenotypes (phenoconversion). Here we investigated the impact of CYP2C19 genotype on the outcome of CYP2C19-dependent DDIs in human liver microsomes. Methods: Liver samples from 40 patients were included, and genotyped for CYP2C19*2, *3 and *17 variants. S-mephenytoin metabolism in microsomal fractions was used as proxy for CYP2C19 activity, and concordance between genotype-predicted and observed CYP2C19 phenotype was examined. Individual microsomes were subsequently co-exposed to fluvoxamine, voriconazole, omeprazole or pantoprazole to simulate DDIs. Results: Maximal CYP2C19 activity (V_max) in genotype-predicted intermediate metabolizers (IMs; *1/*2 or *2/*17), rapid metabolizers (RMs; *1/*17) and ultrarapid metabolizers (UMs; *17/*17) was not different from V_max of predicted normal metabolizers (NMs; *1/*1). Conversely, CYP2C19*2/*2 genotyped-donors exhibited V_max rates ∼9% of NMs, confirming the genotype-predicted poor metabolizer (PM) phenotype. Categorizing CYP2C19 activity, we found a 40% concordance between genetically-predicted CYP2C19 phenotypes and measured phenotypes, indicating substantial phenoconversion. Eight patients (20%) exhibited CYP2C19 IM/PM phenotypes that were not predicted by their CYP2C19 genotype, of which six could be linked to the presence of diabetes or liver disease. In subsequent DDI experiments, CYP2C19 activity was inhibited by omeprazole (-37% ± 8%), voriconazole (-59% ± 4%) and fluvoxamine (-85% ± 2%), but not by pantoprazole (-2 ± 4%). The strength of CYP2C19 inhibitors remained unaffected by CYP2C19 genotype, as similar percental declines in CYP2C19 activity and comparable metabolism-dependent inhibitory constants (K_inact/K_I) of omeprazole were observed between CYP2C19 genotypes. However, the consequences of CYP2C19 inhibitor-mediated phenoconversion were different between CYP2C19 genotypes. In example, voriconazole converted 50% of *1/*1 donors to a IM/PM phenotype, but only 14% of *1/*17 donors. Fluvoxamine converted all donors to phenotypic IMs/PMs, but *1/*17 (14%) were less likely to become PMs than *1/*1 (50%) or *1/*2 and *2/*17 (57%). Conclusion: This study suggests that the differential outcome of CYP2C19-mediated DDIs between genotypes are primarily dictated by basal CYP2C19 activity, that may in part be predicted by CYP2C19 genotype but likely also depends on disease-related factors.

Prevalence and Safety of Prescribing PPIs with Clopidogrel in Palestine

BACKGROUND

Proton pump inhibitors (PPIs) are commonly prescribed medications that are thought to increase the risk of cardiovascular events because they reduce the effectiveness of clopidogrel via shared hepatic pathways.

OBJECTIVE

This study examined the prevalence of concomitant prescribing of clopidogrel/PPI among patients diagnosed with acute coronary syndrome and the adverse cardiovascular event associated with this interaction.

METHODS

A retrospective cohort study was conducted by retrieving patient data from the Nat Health Insurance claims processor database in Palestine. Adults diagnosed with Acute Coronary Syndrome (ACS) from 2019 through 2021 who were prescribed clopidogrel or clopidogrel in combination with a PPI were included in the study. Endpoints were adverse cardiac events, including readmission for revascularization during the first year of treatment.

RESULTS

The study included 443 patients; the prevalence of prescribing concomitant clopidogrel with a PPI was 74.7%, whereas 49.2% were prescribed interacting PPI (omeprazole, esomeprazole, and lansoprazole). 59 (13.3%) of participants experienced a cardiovascular event within 1 year of starting therapy, including 27 (12.4%) patients who had a cardiovascular event while taking an interacting PPI. No significant association was found between PPI administration and increased CV event risk in patients receiving concomitant clopidogrel and PPIs therapy (p = 0.579).

CONCLUSION

In this study, we observed a high prevalence of prescribing a PPI in combination with clopidogrel, regardless of the FDA recommendations. No significant increase in cardiovascular events was observed in patients receiving concomitant clopidogrel and PPI therapy.

Pyridyl Methylsulfinyl Benzimidazole Derivatives as Promising Agents against Giardia lamblia and Trichomonas vaginalis

Protozoan parasites, such as Giardia lamblia and Trichomonas vaginalis, cause the most prevalent infections in humans in developing countries and provoke significant morbidity and mortality in endemic countries. Despite its side-effects, metronidazole is still the drug of choice as a giardiacidal and trichomonacidal tissue-active agent. However, the emergence of metronidazole resistance and its evolved strategies of parasites to evade innate host defenses have hindered the identification and development of new therapeutic strategies against these parasites. Here, we tested five synthesized benzimidazole derivatives as possible drugs for treating giardiasis and trichomoniasis, probing the bifunctional enzyme glucose 6-phosphate dehydrogenase::6-phosphogluconolactone from G. lamblia (GlG6PD::6PGL) and T. vaginalis (TvG6PD::6PGL) as a drug target. The investigated benzimidazole derivatives were H-B2M1, H-B2M2, H₂N-BZM6, O₂N-BZM7, and O₂N-BZM9. The recombinant enzymes were used in inhibition assays, and in silico computational predictions and spectroscopic studies were applied to follow the structural alteration of the enzymes and identify the possible mechanism of inhibition. We identified two potent benzimidazole compounds (O₂N-BZM7 and O₂N-BZM9), which are capable of inhibiting both protozoan G6PD::6PGL enzymes and in vitro assays with these parasites, showing that these compounds also affect their viability. These results demonstrate that other therapeutic targets of the compounds are the enzymes GlG6PD::6PGL and TvG6PD::6PGL, which contribute to their antiparasitic effect and their possible use in antigiardial and trichomonacidal therapies.

Defining the Selectivity of Chemical Inhibitors Used for Cytochrome P450 Reaction Phenotyping: Overcoming Selectivity Limitations with a Six-Parameter Inhibition Curve-Fitting Approach

The utility of chemical inhibitors in cytochrome P450 (CYP) reaction phenotyping is highly dependent on their selectivity and potency for their target CYP isoforms. In the present study, seventeen inhibitors of CYP1A2, 2B6, 2C8, 2C9, 2C19, 2D6, and 3A4/5 commonly used in reaction phenotyping were evaluated for their cross-enzyme selectivity in pooled human liver microsomes. The data were evaluated using a statistical desirability analysis to identify (1) inhibitors of superior selectivity for reaction phenotyping and (2) optimal concentrations for each. Among the inhibitors evaluated, α-naphthoflavone, furafylline, sulfaphenazole, tienilic acid, N-benzylnirvanol, and quinidine were most selective, such that their respective target enzymes were inhibited by ~95% without inhibiting any other CYP enzyme by more than 10%. Other commonly employed inhibitors, such as ketoconazole and montelukast, among others, were of insufficient selectivity to yield a concentration that could adequately inhibit their target enzymes without affecting other CYP enzymes. To overcome these shortcomings, an experimental design was developed wherein dose response data from a densely sampled multi-concentration inhibition curve are analyzed by a six-parameter inhibition curve function, allowing accounting of the inhibition of off-target CYP isoforms inhibition and more reliable determination of maximum targeted enzyme inhibition. The approach was exemplified using rosiglitazone N-demethylation, catalyzed by both CYP2C8 and 3A4, and was able to discern the off-target inhibition by ketoconazole and montelukast from the inhibition of the targeted enzyme. This methodology yields more accurate estimates of CYP contributions in reaction phenotyping. Significance Statement Isoform-selective chemical inhibitors are important tools for identifying and quantifying enzyme contributions as part of a CYP reaction phenotyping assessment for projecting drug-drug interactions. However, currently employed practices fail to adequately compensate for shortcomings in inhibitor selectivity and the resulting confounding impact on estimates of the CYP enzyme contribution to drug clearance. In this report, we describe a detailed IC50 study design with 6-parameter modeling approach that yields more accurate estimates of enzyme contribution.

Predictive In Vitro-In Vivo Extrapolation for Time Dependent Inhibition of CYP1A2, CYP2C8, CYP2C9, CYP2C19, and CYP2D6 Using Pooled Human Hepatocytes, Human Liver Microsomes, and a Simple Mechanistic Static Model

Inactivation of Cytochrome P450 (CYP450) enzymes can lead to significant increases in exposure of comedicants. The majority of reported in vitro to in vivo extrapolation (IVIVE) data have historically focused on CYP3A, leaving the assessment of other CYP isoforms insubstantial. To this end, the utility of human hepatocytes (HHEP) and human liver microsomes (HLM) to predict clinically relevant drug-drug interactions was investigated with a focus on CYP1A2, CYP2C8, CYP2C9, CYP2C19, and CYP2D6. Evaluation of IVIVE for CYP2B6 was limited to only weak inhibition. A search of the University of Washington Drug-Drug Interaction Database was conducted to identify a clinically relevant weak, moderate, and strong inhibitor for selective substrates of CYP1A2, CYP2C8, CYP2C9, CYP2C19, and CYP2D6, resulting in 18 inhibitors for in vitro characterization against 119 clinical interaction studies. Pooled human hepatocytes and HLM were preincubated with increasing concentrations of inhibitors for designated timepoints. Time dependent inhibition was detected in HLM for four moderate/strong inhibitors, suggesting that some optimization of incubation conditions (i.e., lower protein concentrations) is needed to capture weak inhibition. Clinical risk assessment was conducted by incorporating the in vitro derived kinetic parameters maximal rate of enzyme inactivation (min-1) (kinact) and concentration of inhibitor resulting in 50% of the maximum enzyme inactivation (KI) into static equations recommended by regulatory authorities. Significant overprediction was observed when applying the basic models recommended by regulatory agencies. Mechanistic static models, which consider the fraction of metabolism through the impacted enzyme, using the unbound hepatic inlet concentration lead to the best overall prediction accuracy with 92% and 85% of data from HHEPs and HLM, respectively, within twofold of the observed value. SIGNIFICANCE STATEMENT: Coupling time-dependent inactivation parameters derived from pooled human hepatocytes and human liver microsomes (HLM) with a mechanistic static model provides an easy and quantitatively accurate means to determine clinical drug-drug interaction risk from in vitro data. Optimization is needed to evaluate time-dependent inhibition (TDI) for weak and moderate inhibitors using HLM. Recommendations are made with respect to input parameters for in vitro to in vivo extrapolation (IVIVE) of TDI with non-CYP3A enzymes using available data from HLM and human hepatocytes.

Treatment of Digital Ulcers and Reflux Oesophagitis in a Patient with Systemic Sclerosis: Increased Risk of Hepatotoxicity due to a Potential Drug-drug Interaction Between Bosentan and Vonoprazan

Abstract is missing (Short communication)

Prediction of cytochromes P450 3A and 2C19 modulation by both inflammation and drug interactions using physiologically based pharmacokinetics

Xenobiotics can interact with cytochromes P450 (CYPs), resulting in drug-drug interactions, but CYPs can also contribute to drug-disease interactions, especially in the case of inflammation, which downregulates CYP activities through pretranscriptional and posttranscriptional mechanisms. Interleukin-6 (IL-6), a key proinflammatory cytokine, is mainly responsible for this effect. The aim of our study was to develop a physiologically based pharmacokinetic (PBPK) model to foresee the impact of elevated IL-6 levels in combination with drug interactions with esomeprazole on CYP3A and CYP2C19. Data from a cohort of elective hip surgery patients whose CYP3A and CYP2C19 activities were measured before and after surgery were used to validate the accurate prediction of the developed models. Successive steps were to fit models for IL-6, esomeprazole, and omeprazole and its metabolite from the literature and to validate them. The models for midazolam and its metabolite were obtained from the literature. When appropriate, a correction factor was applied to convert drug concentrations from whole blood to plasma. Mean ratios between simulated and observed areas under the curve for omeprazole/5-hydroxy omeprazole, esomeprazole, and IL-6 were 1.53, 1.06, and 0.69, respectively, indicating an accurate prediction of the developed models. The impact of IL-6 and esomeprazole on the exposure to CYP3A and CYP2C19 probe substrates and respective metabolites were correctly predicted. Indeed, the ratio between predicted and observed mean concentrations were <2 for all observations (ranging from 0.51 to 1.7). The impact of IL-6 and esomeprazole on CYP3A and CYP2C19 activities after a hip surgery were correctly predicted with the developed PBPK models.

Analysis of Combined Effect of CYP2C19 Genetic Polymorphism and Proton Pump Inhibitors Coadministration on Trough Concentration of Voriconazole

Purpose

To analyze the combined effect of CYP2C19 genetic polymorphism and PPIs coadministration on voriconazole trough concentration (VCZ-C_trough) in Chinese patients with hematological disorders.

Patients and Methods

A prospective observational study involved 250 plasma samples from 114 adult patients receiving voriconazole with or without PPIs were analyzed. Demographics and clinical characteristics were obtained from patient’s records. A validated LC-MS/MS was used to quantify the plasma VCZ-C_trough. Genotyping for CYP2C19*2 and CYP2C19*3 variant alleles was performed by PCR-RFLP followed by DNA sequencing. The combined total score (from 2 to 5) was calculated for each patient. The higher the score, the lesser the metabolism of the patient.

Findings

Fifty percent of patients administered with voriconazole were coadministered with PPIs, predominantly omeprazole or esomeprazole. Patients exhibiting CYP2C19 poor metabolizer phenotype showed a significantly higher median VCZ-C_trough, (4.31µg/mL [IQR, 1.64µg/mL–7.36µg/mL]) than patients with normal metabolizer (1.38µg/mL, [IQR, 0.79µg/mL–2.14µg/mL], p < 0.0001). Similarly, patients co-administration with PPIs had higher median VCZ-C_trough (2.86µg/mL [IQR 1.33µg/mL–4.66µg/mL]), than PPIs non-users (1.71µg/mL, [IQR, 0.86µg/mL–3.48µg/mL], p = 0.001). However, we noted that the median VCZ-C_trough for each factor was ranging within the normal recommended therapeutic range in the Chinese population (0.5µg/mL–5µg/mL). But when the two factors were combined, the median VCZ-C_trough was steadily increasing as the metabolic capacity (reflected by combined total score) was increasing. Importantly, the median VCZ-C_trough in PM/PPIs user (total score 5) was significantly elevated to supra-therapeutic levels compared to NM/PPI non-user group (total score 2) (5.83µg/mL [IQR, 2.19µg/mL–9.51µg/mL] versus 1.13µg/mL [IQR, 0.67µg/mL–1.82µg/mL]), respectively, P < 0.0001. Furthermore, we observed that the elevation of median VCZ-C_trough to supra-therapeutic levels was largely contributed by omeprazole or esomeprazole compared to lansoprazole or pantoprazole.

Conclusion

Coadministration with PPIs significantly increased voriconazole trough concentrations and there was an additive effect in CYP2C19 PMs, who were most likely to have supra-therapeutic levels.

Potential drug-drug interactions in patients with cardiovascular diseases: findings from a prospective observational study

Background

Patients with cardiovascular diseases (CVD) are at high risk of experiencing drug–drug interactions (DDIs). The objective of this study was to evaluate the frequency, level and risk factors associated with potential-DDIs (pDDIs) in hospitalized CVD patients at cardiology departments of two tertiary care hospitals in Quetta, Pakistan.

Methods

In the current prospective observational study, a total of 300 eligible CVD inpatients were evaluated for pDDIs using Lexicomp Interact®. The pDDIs were classified into class A (no known interaction); B (no action needed); C (monitor therapy: it is documented that the benefits of an interaction outweigh the risk, appropriately monitor therapy in order to avoid potential adverse outcomes); D (consider therapy modification: it is documented that proper actions must be taken to reduce the toxicity resulting from an interaction); X (avoid combination: the risk of an interaction outweighs the benefits and are usually contraindicated). Multivariate binary logistic regression analysis was used to find factors associated with the presence of Class-D and/or X pDDIs. A p-value < 0.05 was considered statistically significant.

Results

With a median of 8.50 pDDIs per patient, all patients (100%) had ≥ 1 pDDIs. Out of total 2787 pDDIs observed, 74.06% (n = 2064) were of moderate and (n = 483) 17.33% of major severity. Class C pDDIs were most common (n = 1971, 70.72%) followed by D (n = 582, 20.88%), B (n = 204, 7.32%) and X (n = 30, 1.08%). Suffering from cardiovascular diseases other than myocardial infarction (OR 0.053, p-value < 0.001) and receiving > 12 drugs (OR 4.187, p-value = 0.009) had statistical significant association with the presence of class D and/or X pDDIs.

Conclusion

In the current study, pDDIs were highly prevalent. The inclusion of DDI screening tools, availability of clinical pharmacists and paying special attention to the high-risk patients may reduce the frequency of pDDIs at the study sites.

Interaction between Omeprazole and Gliclazide in Relation to CYP2C19 Phenotype

The antidiabetic drug gliclazide is partly metabolized by CYP2C19, the main enzyme involved in omeprazole metabolism. The aim of the study was to explore the interaction between omeprazole and gliclazide in relation to CYP2C19 phenotype using physiologically based pharmacokinetic (PBPK) modeling approach. Developed PBPK models were verified using in vivo pharmacokinetic profiles obtained from a clinical trial on omeprazole-gliclazide interaction in healthy volunteers, CYP2C19 normal/rapid/ultrarapid metabolizers (NM/RM/UM). In addition, the association of omeprazole cotreatment with gliclazide-induced hypoglycemia was explored in 267 patients with type 2 diabetes (T2D) from the GoDARTS cohort, Scotland. The PBPK simulations predicted 1.4–1.6-fold higher gliclazide area under the curve (AUC) after 5-day treatment with 20 mg omeprazole in all CYP2C19 phenotype groups except in poor metabolizers. The predicted gliclazide AUC increased 2.1 and 2.5-fold in intermediate metabolizers, and 2.6- and 3.8-fold in NM/RM/UM group, after simulated 20-day dosing with 40 mg omeprazole once and twice daily, respectively. The predicted results were corroborated by findings in patients with T2D which demonstrated 3.3-fold higher odds of severe gliclazide-induced hypoglycemia in NM/RM/UM patients concomitantly treated with omeprazole. Our results indicate that omeprazole may increase exposure to gliclazide and thus increase the risk of gliclazide-associated hypoglycemia in the majority of patients.

Preliminary in vitro approach to evaluate the drug-drug interaction potential of EST73502, a dual µ-opioid receptor partial agonist and σ1 receptor antagonist

The potential for drug-drug interactions (DDI) of EST73502 was preliminary explored in vitro. EST73502 is a new chemical entity intended for oral pain treatment with dual sigma-1 receptor (σ1R) antagonism and μ-opioid receptor (MOR) partial agonism, that presents a promising potent analgesic activity.Several enzymes were involved in EST73502 metabolism catalysing the formation of different metabolites, CYP3A4 and CYP2D6 being the main ones.Fraction unbound was determined due to its impact in interactions, a considerable proportion of EST73502 being available.EST73502 showed a low potential for CYP inhibition, except for CYP2D6 that showed time-dependent inhibition.No induction potential was found for CYP1A2 and 3A4, while CYP2B6 was induced at high concentration.EST73502 seemed to be a potential efflux transporter substrate (efflux ratio ≥ 2) but a negligible in vivo impact would be expected due to its high solubility and permeability in Caco-2 cells. P-gp inhibition was observed while no BCRP inhibition was detected.Preliminary in vitro interaction studies suggested that neither CYPs nor efflux transporters interactions would preclude further development of EST73502 to thoroughly assess the clinical relevance of these findings.

Physiologically Based Pharmacokinetic Modeling Approach to Identify the Drug-Drug Interaction Mechanism of Nifedipine and a Proton Pump Inhibitor, Omeprazole

BACKGROUND AND OBJECTIVES

Proton pump inhibitors (PPIs) can affect the intragastric release of other drugs from their dosage forms by elevating the gastric pH. They may also influence drug absorption and metabolism by interacting with P-glycoprotein or with the cytochrome P450 (CYP) enzyme system. Nifedipine is a Biopharmaceutics Classification System (BCS) class II drug with low solubility across physiologic pH and high permeability. Previous studies have demonstrated that drug-drug interaction (DDI) existed between omeprazole and nifedipine with significantly increased systemic exposure of nifedipine in subjects after pre-treatment for 7 days with omeprazole compared to the subjects without omeprazole treatment. It was shown that omeprazole not only induced an increase in intragastric pH, but also inhibited the CYP3A4 activity, while CYP3A4-mediated oxidation is the main metabolic pathway of nifedipine. The purpose of this study is to apply a physiologically based pharmacokinetic (PBPK) modeling approach to investigate the DDI mechanism for an immediate release formulation of nifedipine with omeprazole.

METHODS

A previously published model for omeprazole was modified to integrate metabolites and to update CYP inhibition based on the most updated published in vitro data. We simulated the nifedipine pharmacokinetics in healthy subjects with or without the multiple-dose pretreatment of omeprazole (20 mg) following oral administrations of immediate-release (IR) (10 mg) nifedipine. Nifedipine solubility at different pHs was used to simulate the nifedipine pharmacokinetics for both clinical arms. Multiple sensitivity analyses were performed to understand the impact of gastric pH and the CYP3A4-mediated gut and liver first pass metabolism on the overall nifedipine pharmacokinetics.

RESULTS

The developed PBPK model properly described the pharmacokinetics of nifedipine and predicted the inhibitory effect of multiple-dose omeprazole on CYP3A4 activity. With the incorporation of the physiologic effect of omeprazole on both gastric pH and CYP3A4 to the PBPK model, the verified PBPK model allows evaluating the impact of the increase in gastric pH and/or CYP3A4 inhibition. The simulated results show that the nifedipine metabolic inhibition by omeprazole may play an important role in the DDI between nifedipine and omeprazole for IR nifedipine formulation.

CONCLUSION

The developed full PBPK model with the capability to simulate DDI by considering gastric pH change and metabolic inhibition provides a mechanistic understanding of the observed DDI of nifedipine with a PPI, omeprazole.

Population pharmacokinetic analysis of esomeprazole in Japanese subjects with various CYP2C19 phenotypes

WHAT IS KNOWN AND OBJECTIVE

Esomeprazole, the S-isomer of omeprazole, is a proton pump inhibitor which has been approved by over 125 countries, also known as NEXIUM^® . Esomeprazole was developed to provide further improvement on efficacy for acid-related diseases with higher systemic bioavailability due to the less first-pass metabolism and lower plasma clearance. Esomeprazole is primarily metabolized by CYP2C19. Approximately <1% of Caucasians and 5%-10% of Asians have absent CYP2C19 enzyme activity. Although the influence of various CYP2C19 phenotypes on esomeprazole pharmacokinetics has been studied, this is the first report in the Japanese population where 27 low CYP2C19 metabolizers were included.

METHODS

In this study, a population PK model describing the PK of esomeprazole was developed to understand the difference of CYP2C19 phenotypes on clearance in the Japanese population. The model quantitatively assessed the influence of CYP2C19 phenotype on esomeprazole PK in healthy Japanese male subjects after receiving repeated oral dosing. The inhibition mechanism of esomeprazole on CYP2C19 activity was also included in the model.

RESULTS AND DISCUSSION

CYP2C19 phenotype and dose were found as statistically significant covariates on esomeprazole clearance. The apparent clearance at 10-mg dose was 17.32, 9.77 and 7.37 (L/h) for homozygous extensive metabolizer, heterozygous extensive metabolizer and poor metabolizer subjects, respectively. And the apparent clearance decreased as dose increased.

WHAT IS NEW AND CONCLUSION

The established population PK model well described the esomeprazole PK and model-predicted esomeprazole PK was in good agreement with external clinical data, suggesting the robustness and applicability of the current model for predicting esomeprazole PK.

Cardiovascular Outcomes Associated With Clinical Use of Citalopram and Omeprazole: A Nationwide Population-Based Cohort Study

Background Recent studies have raised concerns about the reduced efficacy of citalopram when used concurrently with proton pump inhibitors. The aim of this study was to evaluate the associations between clinical use of citalopram and omeprazole and the risk of sudden cardiac arrest (SCA) in an Asian population. Methods and Results A retrospective cohort study was conducted using the National Health Insurance Research Database of Taiwan dated from 2000 to 2013. The study cohorts comprised 3882 patients with citalopram use alone, 31 090 patients with omeprazole use alone, and 405 patients with concomitant use of citalopram and omeprazole (as the exposed cohort), and 141 508 patients received treatment with antidepressants without the risk of SCA and/or proton pump inhibitors other than omeprazole (as the comparison cohort). The primary outcome was the occurrence of SCA. The hazard ratios and 95% CIs derived from the time-dependent Cox regression model were used to assess the association between the proposed drug treatments and risk of SCA. The adjusted hazard ratios of SCA was 1.32 (95% CI, 1.17-1.50) for citalopram use alone, 1.08 (95% CI, 0.98-1.20) for omeprazole use alone, and 2.23 (95% CI, 1.79-2.78) for concomitant use of citalopram and omeprazole. The cumulative incidence of SCA over the Kaplan-Meier curves was more pronounced in patients with concomitant use of citalopram and omeprazole than those treated with citalopram alone and omeprazole alone. Conclusions This cohort study demonstrated use of citalopram and omeprazole either in isolation use or in concomitant use to be at increased risk for SCA.

Development and Validation of a Higher-Throughput Cytochrome P450 Inhibition Assay with the Novel Cofactor-Supplemented Permeabilized Cryopreserved Human Hepatocytes (MetMax Human Hepatocytes)

Here, we report the application of a novel hepatocyte system, the cofactor-supplemented permeabilized cryopreserved human hepatocytes [MetMax human hepatocytes (MMHHs)] in a higher-throughput 384-well plate assay for the evaluation of cytochrome P450 (P450) inhibition. The assay was created to develop physiologically relevant P450 inhibition information, taking advantage of the complete organelle composition and their associated drug-metabolizing enzymes of the MMHH but with the ease of use of human liver microsomes, including storage at -80°C instead of in liquid nitrogen, and thaw and use without centrifugation and microscopic evaluation as required for intact hepatocytes. Nine key P450 isoforms for drug metabolism (CYP1A2, CYP2A6, CYP2B6, CYP2C8, CYP2C9, CYP2C19, CYP2D6, CYP2E1, and CYP3A4) were evaluated using multiple isoform-selective inhibitors. Results with MMHH were found to be comparable to those obtained with intact cryopreserved human hepatocytes (CHHs). Isoform-selective drug-metabolizing enzyme pathways evaluated were phenacetin O-deethylation (CYP1A2), coumarin 7-hydroxylation (CYP2A6), bupropion hydroxylation (CYP2B6), amodiaquine N-deethylation (CYP2C8), diclofenac 4-hydroxylation (CYP2C9), s-mephenytoin 4'-hydroxylation (CYP2C19), dextromethorphan O-demethylation (CYP2D6), chlorzoxazone 6-hydroxylation (CYP2E1), and midazolam 1'-hydroxylation and testosterone 6β-hydroxylation (CYP3A4). The Km values obtained with MMHHs were comparable with those reported in the literature for CHHs. Using substrate concentrations at or near Km values, the IC50 values for the standard inhibitors against the P450 activities were found to be comparable between MMHHs and CHHs, with 73% and 84% of values falling within 2-fold and 3-fold, respectively, from the line of unity. The results indicate that MMHHs can be an efficient experimental system for the evaluation of P450 inhibition in hepatocytes. SIGNIFICANCE STATEMENT: MetMax human hepatocytes (MMHHs) are cofactor-supplemented cryopreserved human hepatocytes with the complete drug-metabolizing enzyme pathways of the conventional hepatocytes but with the convenience of human liver microsomes, including storage at -80°C instead of in liquid nitrogen, and direct thaw and use without a need for centrifugation and microscopic examination. Here, we report the application of MMHH in a high-throughput assay in a 384-well plate format for the evaluation of cytochrome P450 (P450) inhibition. Our results show that data obtained with MMHH are similar to those with conventional hepatocytes, suggesting that the MMHH 384-well P450 inhibition assay can be used routinely for the evaluation of drug-drug interaction potential of new chemical entities in drug development.

Effects of the Proton Pump Inhibitors Omeprazole and Pantoprazole on the Cytochrome P450-Mediated Metabolism of Venlafaxine

BACKGROUND AND OBJECTIVE: An increasing trend in prescribing proton pump inhibitors (PPIs) inevitably increases the risk of unwanted drug-drug interactions (DDIs). The aim of this study was to uncover pharmacokinetic interactions between two PPIs-omeprazole and pantoprazole-and venlafaxine.

METHODS

A therapeutic drug monitoring database contained plasma concentrations of venlafaxine and its active metabolite O-desmethylvenlafaxine. We considered three groups: a group of patients who received venlafaxine without confounding medications (non-PPI group, n = 906); a group of patients who were comedicated with omeprazole (n = 40); and a group of patients comedicated with pantoprazole (n = 40). Plasma concentrations of venlafaxine, O-desmethylvenlafaxine and active moiety (venlafaxine + O-desmethylvenlafaxine), as well as dose-adjusted plasma concentrations, were compared using non-parametrical tests.

RESULTS

Daily doses of venlafaxine did not differ between groups (p = 0.949). The Mann-Whitney U test showed significantly higher plasma concentrations of active moiety, as well as venlafaxine and O-desmethylvenlafaxine, in both PPI groups [p = 0.023, p = 0.011, p = 0.026, +29% active moiety, +27% venlafaxine, +36% O-desmethylvenlafaxine (pantoprazole); p = 0.003, p = 0.039 and p < 0.001, +36% active moiety, +27% venlafaxine, +55% O-desmethylvenlafaxine (omeprazole)]. Significantly higher concentration-by-dose (C/D) values for venlafaxine and active moiety were detected in the pantoprazole group (p = 0.013, p = 0.006, respectively), while in the omeprazole group, C/D ratios for all three parameters-venlafaxine, O-desmethylvenlafaxine and active moiety-were significantly higher (p = 0.021, p < 0.001 and p < 0.001, respectively).

CONCLUSIONS

Significantly higher plasma concentrations for all parameters (venlafaxine, O-desmethylvenlafaxine, active moiety) suggest clinically relevant inhibitory effects of both PPIs, most likely on the cytochrome P450 (CYP) 2C19-mediated metabolism of venlafaxine. The findings might be the result of different degrees of CYP2C19 involvement, therefore the inhibition of CYP2C19 by both PPIs may lead to an increased metabolism via CYP2D6 to O-desmethylvenlafaxine.

Different Clopidogrel Response Elicited by Lansoprazole or Esomeprazole in Patients Undergoing Neurointervention with Dual Antiplatelet Therapy

BACKGROUND

Aspirin-clopidogrel dual antiplatelet therapy and a proton-pump inhibitor are used worldwide to prevent thromboembolism and peptic ulceration in patients undergoing neurointervention. We performed VerifyNow assays (Accumetrics, San Diego, CA, USA) to retrospectively examine the relationship between the effectiveness of antiplatelet agents and different proton-pump inhibitor types.

METHODS

Sixty-four patients with unruptured intracranial aneurysm scheduled for neurointervention received aspirin-clopidogrel dual antiplatelet therapy plus the proton-pump inhibitor lansoprazole (n = 34) or esomeprazole (n = 30). A low response to aspirin and clopidogrel was defined in terms of aspirin reaction units > 550 and P2Y12 reaction units ≥ 230, respectively, by VerifyNow assay. The characteristics, response to antiplatelet therapy, and clinical outcomes were compared in patients treated with lansoprazole or esomeprazole.

RESULTS

The preoperative mean VerifyNow aspirin reaction units and P2Y12 reaction units were 466.0 ± 67.3 and 205.0 ± 67.6, respectively. The mean aspirin reaction unit value was 482.0 ± 64.1 in the lansoprazole group, and 461.5 ± 70.9 in the esomeprazole group (p = 0.77). The mean P2Y12 reaction unit was 220.0 ± 64.4 in the lansoprazole group, and 174.5 ± 65.0 in the esomeprazole group; there was a significant difference in the clopidogrel response of patient treated with lansoprazole or esomeprazole (p = 0.005).

CONCLUSIONS

Our VerifyNow assay results suggest that when on lansoprazole fewer patients achieved the therapeutic goal and required extra therapy before neurointervention.

Effects of proton pump inhibitors, esomeprazole and vonoprazan, on the disposition of proguanil, a CYP2C19 substrate, in healthy volunteers

AIMS

Vonoprazan, a new class of potassium-competitive proton pump inhibitors has been found to attenuate the antiplatelet function of clopidogrel in a recent clinical study, despite weak in vitro activity against CYP2C19. To elucidate the mechanism of this interaction, the present study investigated the effects of esomeprazole and vonoprazan on the pharmacokinetics of proguanil, a CYP2C19 substrate.

METHODS

Seven healthy male volunteers (CYP2C19 extensive metabolizers) received a single oral administration of 100 mg proguanil/250 mg atovaquone (control phase), oral esomeprazole (20 mg) for 5 days followed by proguanil/atovaquone (esomeprazole phase) and oral vonoprazan (20 mg) for 5 days followed by proguanil/atovaquone (vonoprazan phase). Concentrations of proguanil and its metabolite, cycloguanil, in plasma and urine in each phase were determined using liquid chromatography-tandem mass spectrometry.

RESULTS

Coadministration with proton pump inhibitors resulted in increase and decrease in the area under the plasma concentration-time curve (AUC) of proguanil and cycloguanil, respectively, significantly reducing their AUC ratio (cycloguanil/proguanil) to 0.317-fold (95% confidence interval [CI] 0.256-0.379) and 0.507-fold (95% CI 0.409-0.605) in esomeprazole phase and vonoprazan phase, respectively. Esomeprazole and vonoprazan also significantly reduced the apparent formation clearance (cumulative amount of cycloguanil in urine divided by AUC of proguanil) to 0.324-fold (95% CI 0.212-0.436) and 0.433-fold (95% CI 0.355-0.511), respectively, without significant changes in renal clearance of proguanil and cycloguanil.

CONCLUSIONS

Although further studies are needed, both esomeprazole and vonoprazan potentially inhibit CYP2C19 at clinical doses, suggesting caution in the coadministration of these drugs with CYP2C19 substrates.

Assessing CYP2C19 Ontogeny in Neonates and Infants Using Physiologically Based Pharmacokinetic Models: Impact of Enzyme Maturation Versus Inhibition

The objective of this study was to develop pediatric physiologically based pharmacokinetic (PBPK) models for pantoprazole and esomeprazole. Pediatric PBPK models were developed by Simcyp version 15 by incorporating cytochrome P450 (CYP)2C19 maturation and auto-inhibition. The predicted-to-observed pantoprazole clearance (CL) ratio ranged from 0.96-1.35 in children 1-17 years of age and 0.43-0.70 in term infants. The predicted-to-observed esomeprazole CL ratio ranged from 1.08-1.50 for children 6-17 years of age, and 0.15-0.33 for infants. The prediction was markedly improved by assuming no auto-inhibition of esomeprazole in infants in the PBPK model. Our results suggested that the CYP2C19 auto-inhibition model was appropriate for esomeprazole in adults and older children but could not be directly extended to infants. A better understanding of the complex interplay of enzyme maturation, inhibition, and compensatory mechanisms for CYP2C19 is necessary for PBPK modeling in infants.

Asia-Pacific working group consensus on non-variceal upper gastrointestinal bleeding: an update 2018

Non-variceal upper gastrointestinal bleeding remains an important emergency condition, leading to significant morbidity and mortality. As endoscopic therapy is the 'gold standard' of management, treatment of these patients can be considered in three stages: pre-endoscopic treatment, endoscopic haemostasis and post-endoscopic management. Since publication of the Asia-Pacific consensus on non-variceal upper gastrointestinal bleeding (NVUGIB) 7 years ago, there have been significant advancements in the clinical management of patients in all three stages. These include pre-endoscopy risk stratification scores, blood and platelet transfusion, use of proton pump inhibitors; during endoscopy new haemostasis techniques (haemostatic powder spray and over-the-scope clips); and post-endoscopy management by second-look endoscopy and medication strategies. Emerging techniques, including capsule endoscopy and Doppler endoscopic probe in assessing adequacy of endoscopic therapy, and the pre-emptive use of angiographic embolisation, are attracting new attention. An emerging problem is the increasing use of dual antiplatelet agents and direct oral anticoagulants in patients with cardiac and cerebrovascular diseases. Guidelines on the discontinuation and then resumption of these agents in patients presenting with NVUGIB are very much needed. The Asia-Pacific Working Group examined recent evidence and recommends practical management guidelines in this updated consensus statement.

The impact of proton pump inhibitors on the pharmacokinetics of voriconazole in vitro and in vivo

Voriconazole (VRC) and proton pump inhibitors (PPIs) have similar metabolic pathways. The objectives of the study are to evaluate the impact of PPIs on the pharmacokinetics of VRC. Human liver microsomes model was applied to assess the inhibitory effects of PPIs on the metabolism of VRC in vitro. A retrospective study was also carried out to explore the relationship between the plasma VRC trough concentrations and PPIs uses. Patients were divided into six groups: control (n = 166), lansoprazole (LAN, n = 38), esomeprazole (ESO, n = 19), omeprazole (OME, n = 45), pantoprazole (PAN, n = 43), and ilaprazole (ILA, n = 38) groups. All five PPIs showed concentration-dependent inhibitory effects on the VRC metabolism in human liver microsomes, among which LAN, OME and ESO were three of the most potent inhibitors. Consistently, co-administered with LAN, OME and ESO significantly increased the plasma VRC trough levels (p <  0.05), whereas there was no significant association between VRC concentrations and PAN or ILA use. Interestingly, patients in the PPIs groups were more likely to reach the therapeutic VRC range of 1-5.5 μg/mL in steady state when compared with control patients (75-81% VS 69%). In conclusion, although all PPIs showed inhibitory effects on the VRC metabolism in vitro, only LAN, OME and ESO significantly increased VRC plasma concentrations. This study should be helpful for choice of the type of PPIs for patients administered with VRC.

Assessment of Pharmacokinetic Drug-Drug Interactions in Humans: In Vivo Probe Substrates for Drug Metabolism and Drug Transport Revisited

Pharmacokinetic parameters of selective probe substrates are used to quantify the activity of an individual pharmacokinetic process (PKP) and the effect of perpetrator drugs thereon in clinical drug-drug interaction (DDI) studies. For instance, oral caffeine is used to quantify hepatic CYP1A2 activity, and oral dagibatran etexilate for intestinal P-glycoprotein (P-gp) activity. However, no probe substrate depends exclusively on the PKP it is meant to quantify. Lack of selectivity for a given enzyme/transporter and expression of the respective enzyme/transporter at several sites in the human body are the main challenges. Thus, a detailed understanding of the role of individual PKPs for the pharmacokinetics of any probe substrate is essential to allocate the effect of a perpetrator drug to a specific PKP; this is a prerequisite for reliably informed pharmacokinetic models that will allow for the quantitative prediction of perpetrator effects on therapeutic drugs, also in respective patient populations not included in DDI studies.

An Assessment of the In Vitro Inhibition of Cytochrome P450 Enzymes, UDP-Glucuronosyltransferases, and Transporters by Phosphodiester- or Phosphorothioate-Linked Oligonucleotides

Oligonucleotides represent an expanding class of pharmacotherapeutics in development for various indications. Typically, oligonucleotides are developed with phosphorothioate linkages for the improvement of biologic stability; however, limited data are available on the potential of these molecules to cause drug-drug interactions (DDIs). In this study, four nontherapeutic oligonucleotides with either a phosphodiester or phosphorothioate linkage and partial sequences towards glutathione peroxidase or β-actin (PD-GP and PD-Ac or PT-GP and PT-Ac, respectively) were evaluated in vitro for their potential to inhibit cytochrome P450 (P450) enzymes and UGP-glucuronosyltransferases (UGTs) in both human liver microsomes (HLMs) and cryopreserved human hepatocytes (CHHs) and to inhibit select transporters in expression systems. PD-GP and PD-Ac had little to no inhibitory effect on any P450 or UGT enzymes in HLMs and CHHs, except for PD-Ac in HLMs for CYP2C19 (IC50 = 29 μM). Conversely, PT-GP and PT-Ac caused direct inhibition of almost all P450 and UGT enzymes, with CYP1A2 (IC50 values of 0.8-4.2 μM), CYP2C8 (IC50 values of 1.1-12 μM), and UGT1A1 (IC50 values of 4.5-5.4 μM) inhibited to the greatest extent. There was evidence of possible time-dependent inhibition (TDI) of P450 enzymes with PT-GP and PT-Ac for CYP2B6, CYP2C8, CYP2C19, CYP2C9, CYP2D6, and CYP3A4/5; however, this TDI was reversible. In contrast to HLMs, there was little to no direct P450 inhibition by any oligonucleotide in CHHs [except for PD-Ac with CYP2C19 (IC50 = 36 μM) and TDI by PT-GP with CYP2C8], demonstrating test system-dependent outcomes. Inhibition was observed for the organic anion uptake transporters, including organic anion-transporting polypeptide OATP1B1 and OATP1B3, organic anion transporters OAT1 and OAT3, and organic cation transporter OCT2 (IC50 values of 12-29 μM), but not OCT1 or the efflux transporters breast cancer resistance protein and P-glycoprotein by the phosphorothioate oligonucleotides.

Decorporation potential of lansoprazole against radiothallium (201Tl) in internally contaminated BALB/c mice and New Zealand White rabbits

INTRODUCTION:

This study examined the decorporation potential of lansoprazole (LNP) as radioactivity decorporation agent for radiothallium (²⁰¹Tl) in internally contaminated BALB/c mice and New Zealand White rabbits using radiometry and gamma scintigraphy.

METHODS:

Animals were divided into three groups, that is, control, pretreatment-1 (1X LNP), and pretreatment-2 (2X LNP). Mice received LNP intraperitoneally, while in rabbits LNP was given through oral route 0.5 and 1.5 h before radiothallium administration, respectively. Mice and rabbits used in the experiment were administered 1.48 and 11.1 MBq radiothallium (²⁰¹TlCl) through intravenous and oral route, respectively. Once started as prophylactic, LNP was continued as therapeutic twice a day till the end of study period. Radiometry and gamma scintigraphy were used to monitor radiothallium retention and uptake patterns in animals. Gamma scintigraphic images of rabbits were taken at different time intervals up to 72 h and were analyzed for comparative uptake pattern of ²⁰¹TlCl in all the groups.

RESULTS:

LNP treatment significantly increased the ²⁰¹Tl elimination over untreated control and considerably reduced the retention of ²⁰¹Tl in various tissues and organs. Decrease in radiothallium uptake up to 40% was observed in LNP-treated mice as compared to untreated control. While in rabbits, whole-body radioactivity burden at 72 h was found to be 31.24%, 26%, and 18.54% in untreated control, 9 and 18 mg/kg LNP-treated groups, respectively.

CONCLUSION:

LNP exhibited dose-dependent decorporation potential to effectively enhance the elimination of ²⁰¹Tl in mice and rabbits experimentally contaminated with ²⁰¹TlCl.

Comparative Study of Effects of Vonoprazan and Esomeprazole on Antiplatelet Function of Clopidogrel or Prasugrel in Relation to CYP2C19 Genotype

Drug-drug interaction between antiacid and antiplatelet agents has not been fully elucidated. Vonoprazan, a new potassium competitive acid blocker, has been available in Japan. CYP2C19 and CYP3A4 are involved in the metabolism of clopidogrel, prasugrel, esomeprazole, and vonoprazan. Using a P2Y12 assay, we compared the effects of vonoprazan and esomeprazole on the antiplatelet functions of clopidogrel or prasugrel in 31 healthy Japanese volunteers (14 CYP2C19 homo-extensive (homo-EMs), nine hetero-extensive (hetero-EMs), and eight poor metabolizers (PMs)). Vonoprazan decreased the median inhibition of platelet aggregation (IPA) values of clopidogrel and prasugrel more potently than esomeprazole (P < 0.001 for clopidogrel and P = 0.011 for prasugrel). The same tendencies were observed when stratified by CYP2C19 genotype groups (P = 0.004 in homo-EMs, 0.033 in hetero-EMs, and 0.043 in PMs). Vonoprazan attenuated the antiplatelet function of clopidogrel more potently than esomeprazole. Esomeprazole did not affect that of prasugrel irrespective of CYP2C19 genotype.

Predictive Performance of Physiologically Based Pharmacokinetic (PBPK) Modeling of Drugs Extensively Metabolized by Major Cytochrome P450s in Children

The accuracy of physiologically based pharmacokinetic (PBPK) model prediction in children, especially those younger than 2 years old, has not been systematically evaluated. The aim of this study was to characterize the pediatric predictive performance of the PBPK approach for 10 drugs extensively metabolized by CYP1A2 (theophylline), CYP2C8 (desloratidine, montelukast), CYP2C9 (diclofenac), CYP2C19 (esomeprazole, lansoprazole), CYP2D6 (tramadol), and CYP3A4 (itraconazole, ondansetron, sufentanil). Model performance in children was evaluated by comparing simulated plasma concentration-time profiles with observed clinical results for each drug and age group. PBPK models reasonably predicted the pharmacokinetics of desloratadine, diclofenac, itraconazole, lansoprazole, montelukast, ondansetron, sufentanil, theophylline, and tramadol across all age groups. Collectively, 58 out of 67 predictions were within 2-fold and 43 out of 67 predictions within 1.5-fold of observed values. Developed PBPK models can reasonably predict exposure in children age 1 month and older for an array of predominantly CYP metabolized drugs.

Acid-Suppressive Therapy and Risk of Infections: Pros and Cons

This narrative review summarises the benefits, risks and appropriate use of acid-suppressing drugs (ASDs), proton pump inhibitors and histamine-2 receptor antagonists, advocating a rationale balanced and individualised approach aimed to minimise any serious adverse consequences. It focuses on current controversies on the potential of ASDs to contribute to infections-bacterial, parasitic, fungal, protozoan and viral, particularly in the elderly, comprehensively and critically discusses the growing body of observational literature linking ASD use to a variety of enteric, respiratory, skin and systemic infectious diseases and complications (Clostridium difficile diarrhoea, pneumonia, spontaneous bacterial peritonitis, septicaemia and other). The proposed pathogenic mechanisms of ASD-associated infections (related and unrelated to the inhibition of gastric acid secretion, alterations of the gut microbiome and immunity), and drug-drug interactions are also described. Both probiotics use and correcting vitamin D status may have a significant protective effect decreasing the incidence of ASD-associated infections, especially in the elderly. Despite the limitations of the existing data, the importance of individualised therapy and caution in long-term ASD use considering the balance of benefits and potential harms, factors that may predispose to and actions that may prevent/attenuate adverse effects is evident. A six-step practical algorithm for ASD therapy based on the best available evidence is presented.

A strategy for early-risk predictions of clinical drug-drug interactions involving the GastroPlusTM DDI module for time-dependent CYP inhibitors

1. A set of reference compounds for time-dependent inhibition (TDI) of cytochrome P450 with available literature data for k_inact and K_I was used to predict clinical implications using the GastroPlus^TM software. Comparisons were made to in vivo literature interaction data. 2. The predicted AUC ratios (AUC_+inhibitor/AUC_control) could be compared with the observed ratios from literature for all compounds with detailed information about in vivo administration, pharmacokinetics and in vivo interactions (N = 21). For this dataset, the difference between predicted and observed AUC ratios for interactions with midazolam was within twofold for all compounds except one (telaprevir, for which non-CYP-mediated metabolism likely plays a role after multiple dosing). 3. The sensitivity, specificity and accuracy of the GastroPlus^TM predictions using a binary classification as no-to-weak interaction versus moderate-to-strong interaction for all compounds with available in vivo interaction data, were 80%, 82% and 81%, respectively (N = 31). 4. As a result of our evaluations of the DDI module in GastroPlus^TM, we have implemented an early TDI risk assessment decision tree for our drug discovery projects involving in vitro screening and early GastroPlus^TM predictions. Shifted IC₅₀ values are determined and k_inact/K_I estimated (by using a regression line established with in house-shifted IC₅₀ values and literature k_inact/K_I ratios), followed by GastroPlus^TM predictions.

Serum clomipramine and desmethylclomipramine levels in a CYP2C19 and CYP2D6 intermediate metabolizer

Pharmacogenetics within psychiatry has the potential to aid in the dose and selection of medications. A substantial number of psychiatric medications are metabolized through either of the highly polymorphic drug-metabolizing enzymes CYP2D6 and CYP2C19. Of these, clomipramine is subject to metabolism by both CYP2C19 and CYP2D6, leaving individuals with deficiencies of these drug-metabolizing enzymes at risk of higher concentrations of the parent molecule. Herein, we present the case of a 29-year-old male with diagnoses of depression and obsessive compulsive disorder who had trialed and failed a dozen psychiatric medications, many of which are subject to metabolism by CYP2D6 and/or CYP2C19, and had most recently been taking clomipramine for approximately 2.5 years. Pharmacogenetic testing revealed this patient to be an intermediate metabolizer for both CYP2C19 (*1/*2) and CYP2D6 (*4/*41), which resulted in considerably elevated serum trough concentrations of clomipramine and its active metabolite desmethylclomipramine. This case provides a retrospective view of how the knowledge of an individual's pharmacogenetic test results can aid in their clinical care.

Influence of different proton pump inhibitors on the pharmacokinetics of voriconazole

This study aimed to determine the influence of proton pump inhibitors (PPIs) on the pharmacokinetics of voriconazole and to characterise potential drug-drug interactions (DDIs) between voriconazole and various PPIs (omeprazole, esomeprazole, lansoprazole and rabeprazole). Using adjusted physicochemical data and the pharmacokinetic (PK) parameters of voriconazole and PPIs, physiologically based pharmacokinetic (PBPK) models were built and were verified in healthy subjects using GastroPlus^TM to predict the plasma concentration-time profiles of voriconazole and PPIs. These models were then used to assess potential DDIs for voriconazole when administered with PPIs. The results indicated the PBPK model-simulated plasma concentration-time profiles of both voriconazole and PPIs were consistent with the observed profiles. In addition, the DDI simulations suggested that the PK values of voriconazole increased to various degrees when combined with several PPIs. The area under the plasma concentration-time curve for the time of the simulation (AUC_0-t) of voriconazole was increased by 39%, 18%, 12% and 1% when co-administered with omeprazole, esomeprazole, lansoprazole and rabeprazole, respectively. Omeprazole was the most potent CYP2C19 inhibitor tested, whereas rabeprazole had no influence on voriconazole (omeprazole > esomeprazole > lansoprazole > rabeprazole). However, in consideration of the therapeutic concentration range, dosage adjustment of voriconazole is unnecessary regardless of which PPI was co-administered.

Neurology of cardiopulmonary resuscitation

This chapter aims to provide an up-to-date review of the science and clinical practice pertaining to neurologic injury after successful cardiopulmonary resuscitation. The past two decades have seen a major shift in the science and practice of cardiopulmonary resuscitation, with a major emphasis on postresuscitation neurologic care. This chapter provides a nuanced and thoughtful historic and bench-to-bedside overview of the neurologic aspects of cardiopulmonary resuscitation. A particular emphasis is made on the anatomy and pathophysiology of hypoxic-ischemic encephalopathy, up-to-date management of survivors of cardiopulmonary resuscitation, and a careful discussion on neurologic outcome prediction. Guidance to practice evidence-based clinical care when able and thoughtful, pragmatic suggestions for care where evidence is lacking are also provided. This chapter serves as both a useful clinical guide and an updated, thorough, and state-of-the-art reference on the topic for advanced students and experienced practitioners in the field.

Use of a cocktail probe to assess potential drug interactions with cytochrome P450 after administration of belatacept, a costimulatory immunomodulator

Aim

This open‐label study investigated the effect of belatacept on cytokine levels and on the pharmacokinetics of caffeine, losartan, omeprazole, dextromethorphan and midazolam, as CYP probe substrates after oral administration of the Inje cocktail in healthy volunteers.

Methods

Twenty‐two evaluable subjects received the Inje cocktail on Days 1, 4, 7 and 11 and belatacept infusion on Day 4.

Results

Since belatacept caused no major alterations to cytokine levels, there were no major effects on CYP‐substrate pharmacokinetics, except for a slight (16–30%) increase in omeprazole exposure, which was probably due to omeprazole‐mediated, time‐dependent CYP inhibition. Belatacept did not cause major alterations in the pharmacokinetics, as measured by the geometric mean ratios and associated 90% confidence interval for area under the plasma concentration ‐time curve from time zero to infinity on Day 7 comparing administration with and without belatacept for caffeine (1.002 [0.914, 1.098]), dextromethorphan (1.031 [0.885, 1.200]), losartan (1.016 [0.938, 1.101)], midazolam (0.968 [0.892, 1.049]) or their respective metabolites.

Conclusions

Therefore, no dose adjustments of CYP substrates are indicated with belatacept coadministration.

How Informative Are Drug-Drug Interactions of Gene-Drug Interactions?

FDA recommendations to manage polymorphic CYP-mediated drug-drug interactions (DDIs) and gene-drug interactions (GDIs) are typically similar. However, DDIs may not always reliably predict GDIs because the victim drug may have multiple metabolic pathways and the perpetrator drug may affect multiple enzymes or transporters. Consequently, it is of great interest to both the pharmaceutical industry and regulatory agencies to determine if DDI studies can be leveraged to inform GDIs or vice versa for dose adjustment and labeling. The objective of this study was to investigate under what circumstances DDIs can be used to predict GDIs for prototypical CYP2C9, CYP2C19, and CYP2D6 substrates. We investigated model substrates for CYP2D6 (metoprolol, dextromethorphan, atomoxetine, and vortioxetine), CYP2C9 (warfarin, flurbiprofen, and celecoxib), and CYP2C19 (omeprazole and clopidogrel). Data on drug exposure for poor metabolizers (GDI) and for DDIs mediated by strong/moderate inhibitors in extensive metabolizers were collected. The impact of DDIs and GDIs on drug exposure was compared using: (1) a descriptive and (2) a physiologically based pharmacokinetic convergence analysis. Results from both approaches indicate that information on DDIs can be used to reliably predict GDIs for CYP2D6 substrates. The situation is more complex for CYP2C9 and CYP2C19 substrates because dose of the inhibitor (CYP2C9) and potency of the inhibitor (CYP2C19) impact the extent to which perpetrator drugs phenotypically convert extensive metabolizers to poor(er) metabolizers.

Drug-Drug Interaction of Omeprazole With the HCV Direct-Acting Antiviral Agents Paritaprevir/Ritonavir and Ombitasvir With and Without Dasabuvir

Paritaprevir (administered with low-dose ritonavir), ombitasvir, and dasabuvir are direct-acting antiviral agents administered as combination regimens for the treatment of chronic hepatitis C virus infection. Drug-drug interactions between 2D (ombitasvir/paritaprevir/ritonavir) or 3D (ombitasvir/paritaprevir/ritonavir and dasabuvir) regimens and omeprazole, a CYP2C19 substrate and acid-reducing agent, were evaluated in 24 healthy volunteers. Subjects received omeprazole (40 mg once daily) on day 1 and days 20-24 and the 2D or 3D regimen (ombitasvir/paritaprevir/ritonavir 25/150/100 mg once daily ± dasabuvir 250 mg twice daily) on days 6-24. Compared with omeprazole alone, coadministration with the 2D or 3D regimen decreased omeprazole geometric mean Cmax and AUCt values by 40% to 50%. Ombitasvir, dasabuvir, and ritonavir mean exposures showed <10% change, and paritaprevir mean exposures showed <20% change when the 2D or 3D regimen was administered with omeprazole compared with administration without omeprazole. Although no a priori dose adjustment is needed, a higher omeprazole dose should be considered if clinically indicated when coadministered with the 2D or 3D regimen. No dose adjustment is required for the 2D or 3D regimen when administered with omeprazole, other acid-reducing agents, or CYP2C19 inhibitors.

The Reliability of Estimating Ki Values for Direct, Reversible Inhibition of Cytochrome P450 Enzymes from Corresponding IC50 Values: A Retrospective Analysis of 343 Experiments

In the present study, we conducted a retrospective analysis of 343 in vitro experiments to ascertain whether observed (experimentally determined) values of Ki for reversible cytochrome P450 (P450) inhibition could be reliably predicted by dividing the corresponding IC₅₀ values by two, based on the relationship (for competitive inhibition) in which Ki = IC₅₀/2 when [S] (substrate concentration) = Km (Michaelis-Menten constant). Values of Ki and IC₅₀ were determined under the following conditions: 1) the concentration of P450 marker substrate, [S], was equal to Km (for IC₅₀ determinations) and spanned Km (for Ki determinations); 2) the substrate incubation time was short (5 minutes) to minimize metabolism-dependent inhibition and inhibitor depletion; and 3) the concentration of human liver microsomes was low (0.1 mg/ml or less) to maximize the unbound fraction of inhibitor. Under these conditions, predicted Ki values, based on IC₅₀/2, correlated strongly with experimentally observed Ki determinations [r = 0.940; average fold error (AFE) = 1.10]. Of the 343 predicted Ki values, 316 (92%) were within a factor of 2 of the experimentally determined Ki values, and only one value fell outside a 3-fold range. In the case of noncompetitive inhibitors, Ki values predicted from IC₅₀/2 values were overestimated by a factor of nearly 2 (AFE = 1.85; n = 13), which is to be expected because, for noncompetitive inhibition, Ki = IC₅₀ (not IC₅₀/2). The results suggest that, under appropriate experimental conditions with the substrate concentration equal to Km, values of Ki for direct, reversible inhibition can be reliably estimated from values of IC₅₀/2.