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The expression of cytochrome P450 (CYP) enzymes is highly variable and associated with factors, such as age, genotype, sex, and disease states. In this study, quantification of metronidazole metabolizing CYP isoforms (CYP2A6, CYP2E1, CYP3A4, CYP3A5, and CYP3A7) in human liver microsomes from 115 children and 35 adults was performed using a quantitative proteomics method. The data confirmed age‐dependent increase in CYP2A6, CYP2E1, and CYP3A4 abundance, whereas, as expected, CYP3A7 abundance showed postnatal decrease with age. In particular, the fold difference (neonatal to adulthood levels) in the protein abundance of CYP2A6, CYP2E1, and CYP3A4 was 14, 11, and 20, respectively. In contrast, protein abundance of CYP3A7 was > 125‐fold higher in the liver microsomes of neonates than of adults. The abundance of CYP2A6 and CYP3A5 was associated with genotypes, rs4803381 and rs776746, respectively. A proteomics‐informed physiologically based pharmacokinetic (PBPK) model was developed to describe the pharmacokinetics of metronidazole and its primary metabolite, 2‐hydroxymethylmetronidazole. The model revealed an increase in the metabolite‐to‐parent ratio with age and showed a strong correlation between CYP2A6 abundance and metabolite formation (r2 = 0.75). Notably, the estimated contribution of CYP3A7 was ~ 75% in metronidazole clearance in neonates. These data suggest that variability in CYP2A6 and CYP3A7 in younger children poses the risk of variable pharmacokinetics of metronidazole and its active metabolite with a potential impact on drug efficacy and safety. No sex‐dependent difference was observed in the protein abundance of the studied CYPs. The successful integration of hepatic CYP ontogeny data derived from a large liver bank into the pediatric PBPK model of metronidazole can be extended to other drugs metabolized by the studied CYPs. [ABSTRACT FROM AUTHOR]

In this brief report, we provide an analysis of the influence of a novel CYP2C haplotype (CYP2C:TG) on proton pump inhibitor (PPI) pharmacokinetics (PK) in children. The CYP2C:TG haplotype has been proposed to be associated with increased CYP2C19 activity. We sought to determine if this CYP2C:TG haplotype resulted in similar alterations in metabolism for proton pump inhibitors, which are primarily metabolized by CYP2C19. In a cohort of 41 children aged 6–21 participating in a PPI pharmacokinetic study, effects of the CYP2C:TG allele were assessed by fitting two linear regression models for each of the six PK outcomes assessed, the second of which accounted for the presence of the CYP2C:TG allele. The difference in R2 values between the two models was computed to quantify the variability in the outcome that could be accounted for by the CYP2C:TG allele after adjustment for the CYP2C19 genotype. We found the CYP2C:TG haplotype to have no measurable additive impact on CYP2C19‐mediated metabolism of PPIs in vivo in older children and adolescents. The findings of this study do not support the clinical utility of routine testing for the CYP2C:TG haplotype to guide PPI dose adjustments in children. [ABSTRACT FROM AUTHOR]

CYP2D6 substrates are among the most highly prescribed medications in teenagers and also commonly associated with serious adverse events. To investigate the relative contributions of genetic variation, growth, and development on CYP2D6 activity during puberty, healthy children and adolescents 7–15 years of age at enrollment participated in a longitudinal phenotyping study involving administration of 0.3 mg/kg dextromethorphan (DM) and 4‐h urine collection every 6 months for 3 years (7 total visits). At each visit, height, weight, and sexual maturity were recorded, and CYP2D6 activity was determined as the urinary molar ratio of DM to its metabolite dextrorphan (DX). A total of 188 participants completed at least one visit, and 102 completed all seven study visits. Following univariate analysis, only CYP2D6 activity score (p < 0.001), urinary pH (p < 0.001), weight (p = 0.018), and attention‐deficit/hyperactivity disorder (ADHD) diagnosis (p < 0.001) were significantly correlated with log(DM/DX). Results of linear mixed model analysis with random intercept, random slope covariance structure revealed that CYP2D6 activity score had the strongest effect on log(DM/DX), with model‐estimated average log(DM/DX) being 3.8 SDs higher for poor metabolizers than for patients with activity score 3. A moderate effect on log(DM/DX) was observed for sex, and smaller effects were observed for ADHD diagnosis and urinary pH. The log(DM/DX) did not change meaningfully with age or pubertal development. CYP2D6 genotype remains the single, largest determinant of variability in CYP2D6 activity during puberty. Incorporation of genotype‐based dosing guidelines should be considered for CYP2D6 substrates given the prevalent use of these agents in this pediatric age group. [ABSTRACT FROM AUTHOR]

Byline: J. Steven Leeder, PharmD, PhD (Section of Developmental Pharmacology and Experimental Therapeutics, Department of Pediatrics, Children's Mercy Hospitals and Clinics, and University of Missouri-Kansas City, Kansas City, Missouri, sleeder@cmh.edu); [...]

The 13C‐pantoprazole breath test (PAN‐BT) is a safe, noninvasive, in vivo CYP2C19 phenotyping probe for adults. Our objective was to evaluate PAN‐BT performance in children, with a focus on discriminating individuals who, according to guidelines from the Clinical Pharmacology Implementation Consortium (CPIC), would benefit from starting dose escalation versus reduction for proton pump inhibitors (PPIs). Children (n = 65, 6–17 years) genotyped for CYP2C19 variants *2, *3, *4, and *17 received a single oral dose of 13C‐pantoprazole. Plasma concentrations of pantoprazole and its metabolites, and changes in exhaled 13CO2 (termed delta‐over‐baseline or DOB), were measured 10 times over 8 h using high performance liquid chromatography with ultraviolet detection and spectrophotometry, respectively. Pharmacokinetic parameters of interest were generated and DOB features derived using feature engineering for the first 180 min postadministration. DOB features, age, sex, and obesity status were used to run bootstrap analysis at each timepoint (Ti) independently. For each iteration, stratified samples were drawn based on genotype prevalence in the original cohort. A random forest was trained, and predictive performance of PAN‐BT was evaluated. Strong discriminating ability for CYP2C19 intermediate versus normal/rapid metabolizer phenotype was noted at DOBT30 min (mean sensitivity: 0.522, specificity: 0.784), with consistent model outperformance over a random or a stratified classifier approach at each timepoint (p < 0.001). With additional refinement and investigation, the test could become a useful and convenient dosing tool in clinic to help identify children who would benefit most from PPI dose escalation versus dose reduction, in accordance with CPIC guidelines. [ABSTRACT FROM AUTHOR]

rs5758550 has been associated with enhanced transcription and suggested to be a useful marker of CYP2D6 activity. As there are limited and inconsistent data regarding the utility of this distant "enhancer" single nucleotide polymorphism (SNP), our goal was to further assess the impact of rs5758550 on CYP2D6 activity toward two probe substrates, atomoxetine (ATX) and dextromethorphan (DM), using in vivo urinary metabolite (DM; n = 188) and pharmacokinetic (ATX; n = 70) and in vitro metabolite formation (ATX and DM; n = 166) data. All subjects and tissues were extensively genotyped, the "enhancer" SNP phased with established CYP2D6 haplotypes either computationally or experimentally, and the impact on CYP2D6 activity investigated using several linear models of varying complexity to determine the proportion of variability in CYP2D6 activity captured by each model. For all datasets and models, the "enhancer" SNP had no or only a modest impact on CYP2D6 activity prediction. An increased effect, when present, was more pronounced for ATX than DM suggesting potential substate‐dependency. In addition, CYP2D6*2 alleles with the "enhancer" SNP were associated with modestly higher metabolite formation rates in vitro, but not in vivo; no effect was detected for CYP2D6*1 alleles with "enhancer" SNP. In summary, it remains inconclusive whether the small effects detected in this investigation are indeed caused by the "enhancer" SNP or are rather due to its incomplete linkage with other variants within the gene. Taken together, there does not appear to be sufficient evidence to warrant the "enhancer" SNP be included in clinical CYP2D6 pharmacogenetic testing. [ABSTRACT FROM AUTHOR]

Background: WHO recommends simplified antibiotics for young infants with sepsis in countries where hospitalisation is not feasible. Amoxicillin provides safe, Gram-positive coverage. This study was done to determine pharmacokinetics, drug disposition and interpopulation variability of oral amoxicillin in this demographic.Methods: Young infants with signs of sepsis enrolled in an oral amoxicillin/intramuscular gentamicin treatment arm of a sepsis trial in Karachi, Pakistan, were studied. Limited pharmacokinetic (PK) sampling was performed at 0, 2-3 and 6-8 hours following an index dose of oral amoxicillin. Plasma concentrations were determined by high-performance liquid chromatography/mass spectrometry. Values of ≥2 mg/L were considered as the effect threshold, given the regional minimal inhibitory concentration (MIC) of resistant Streptococcus pneumoniae. RESULTS: Amoxicillin concentrations were determined in 129 samples from 60 young infants. Six of 44 infants had positive blood cultures with predominant Gram-positive organisms. Forty-four infants contributing blood at ≥2 of 3 specified timepoints were included in the analysis. Mean amoxicillin levels at 2-3 hours (11.6±9.5 mg/L, n=44) and 6-8 hours (16.4±9.3 mg/L, n=20) following the index dose exceeded the MIC for amoxicillin (2.0 mg/L) against resistant S. pneumoniae strains. Of 20 infants with three serum levels, 7 showed a classic dose-exposure profile and 13 showed increasing concentrations with time, implying delayed absorption or excretion.Conclusion: Amoxicillin concentrations in sera of young infants following oral administration at 75-100 mg/kg/day daily divided doses exceeds the susceptibility breakpoint for >50% of a 12-hour dosing interval.Oral amoxicillin may hold potential as a safe replacement of parenteral ampicillin in newborn sepsis regimens, including aminoglycosides, where hospitalisation is not feasible.Trial Registration Number: NCT01027429. [ABSTRACT FROM AUTHOR]

Aims: CYP2A6 is a genetically polymorphic enzyme resulting in differential substrate metabolism and health behaviours. Current phenotyping probes for CYP2A6 exhibit limitations related to procurement (deuterated cotinine), toxicity (coumarin), specificity (caffeine) and age‐appropriate administration (nicotine, NIC). In vitro, CYP2A6 selectively forms 2‐hydroxymetronidazole (2HM) from metronidazole (MTZ). The purpose of this study was to evaluate MTZ as a CYP2A6 phenotyping probe drug in healthy adults against the well‐established method of measuring trans‐3‐hydroxycotinine (3HC)/cotinine (COT). Methods: A randomized, cross‐over, pharmacokinetic study was completed in 16 healthy, nonsmoking adults. Separated by a washout period of at least 2 weeks, MTZ 500 mg and NIC gum 2 mg were administered and plasma was sampled over 48 hours and 8 hours, respectively. Correlations of plasma metabolite/parent ratios (2HM/MTZ; 3HC/COT) were assessed by Pearson coefficient. CYP2A6 genotyping was conducted and incorporated as a variable of plasma ratio response. Results: Correlations between the plasma ratio 2HM/MTZ and 3HC/COT were ≥ 0.9 at multiple time points (P < 0.001), demonstrating a wide window during which 2HM/MTZ can be queried post‐MTZ dose. CYP2A6 genotype had significant impacts on both MTZ and NIC phenotyping ratios with decreased activity predicted phenotypes demonstrating 2HM/MTZ ratios ≤58% and 3HC/COT ratios ≤56% compared with extensive activity predicted phenotypes at all time points examined in the study (P < 0.05). No adverse events were reported in the MTZ arm while 38% (n = 6) of participants reported mild adverse events in the NIC arm. Conclusions: Metronidazole via 2HM/MTZ performed well as a novel, safe phenotyping probe for CYP2A6 in healthy adults. [ABSTRACT FROM AUTHOR]

The ontogeny of hepatic uridine diphosphate‐glucuronosyltransferases (UGTs) was investigated by determining their protein abundance in human liver microsomes isolated from 136 pediatric (0–18 years) and 35 adult (age >18 years) donors using liquid chromatography / tandem mass spectrometry (LC‐MS/MS) proteomics. Microsomal protein abundances of UGT1A1, UGT1A4, UGT1A6, UGT1A9, UGT2B7, and UGT2B15 increased by ∼8, 55, 35, 33, 8, and 3‐fold from neonates to adults, respectively. The estimated age at which 50% of the adult protein abundance is observed for these UGT isoforms was between 2.6–10.3 years. Measured in vitro activity was generally consistent with the protein data. UGT1A1 protein abundance was associated with multiple single nucleotide polymorphisms exhibiting noticeable ontogeny–genotype interplay. UGT2B15 rs1902023 (*2) was associated with decreased protein activity without any change in protein abundance. Taken together, these data are invaluable to facilitate the prediction of drug disposition in children using physiologically based pharmacokinetic modeling as demonstrated here for zidovudine and morphine. [ABSTRACT FROM AUTHOR]

CYP2B6*6 and CYP2B6*18 are the most clinically important variants causing reduced CYP2B6 protein expression and activity. However, these variants do not account for all variability in CYP2B6 activity. Emerging evidence has shown that genetic variants in the 3′UTR may explain variable drug response by altering microRNA regulation. Five 3′UTR variants were associated with significantly altered efavirenz AUC0‐48 (8‐OH‐EFV/EFV) ratios in healthy human volunteers. The rs70950385 (AG>CA) variant, predicted to create a microRNA binding site for miR‐1275, was associated with a 33% decreased CYP2B6 activity among normal metabolizers (AG/AG vs. CA/CA (P < 0.05)). In vitro luciferase assays were used to confirm that the CA on the variant allele created a microRNA binding site causing an 11.3% decrease in activity compared to the AG allele when treated with miR‐1275 (P = 0.0035). Our results show that a 3′UTR variant contributes to variability in CYP2B6 activity. [ABSTRACT FROM AUTHOR]

A [13C]-dextromethorphan ([13C]-DM) breath test was evaluated to assess its feasibility as a rapid, phenotyping assay for CYP2D6 activity. [13C]-DM (0.5 mg/kg) was administered orally with water or potassium bicarbonate-sodium bicarbonate to 30 adult Caucasian volunteers (n = 1 each): CYP2D6 poor metabolizers (2 null alleles; PM-0) and extensive metabolizers with 1 (EM-1) or 2 functional alleles (EM- 2). CYP2D6 phenotype was determined by 13CO2 enrichment measured by infrared spectrometry (delta-over-baseline [DOB] value) in expired breath samples collected before and up to 240minutes after [13C]-DMingestion and by 4-hour urinary metabolite ratio. The PM-0 group was readily distinguishable from either EM group by both the breath test and urinary metabolite ratio. Using a single point determination of phenotype at 40 minutes and defining PMs as subjects with a DOB ≤ 0.5, the sensitivity of the method was 100%; specificity was 95% with 95% accuracy and resulted in the misclassification of 1 EM-1 individual as a PM. Modification of the initial protocol (timing of potassium bicarbonate-sodium bicarbonate administration relative to dose) yielded comparable results, but there was a tendency toward increased DOB values. Although further development is required, these studies suggest that the [13C]-DM breath test offers promise as a rapid, minimally invasive phenotyping assay for CYP2D6 activity. [ABSTRACT FROM AUTHOR]