Lean body weight dosing avoids excessive systemic exposure to proton pump inhibitors for children with obesity.

BACKGROUND: Children with obesity are more likely to suffer gastroesophageal reflux disease, requiring acid-suppression therapy with proton pump inhibitors (PPIs) and no guidelines regarding dosing. OBJECTIVE: To prospectively evaluate lean-body-weight-based (LBW) dosing of the PPI pantoprazole for children with and without obesity. METHODS: Methods: Sixty-two children (6-17 years) received a one-time oral dose of liquid pantoprazole (1.2 mg kg-1 LBW). Plasma pantoprazole concentrations were measured at 10 time points over 8 h and pharmacokinetic (PK) profiles generated using non-compartmental techniques, in order to compare PK parameters of interest between children with and without obesity, while accounting for CYP2C19 genotype. RESULTS: Adjusted for milligram-per-kilogram total body weight (TBW) pantoprazole received, apparent drug clearance (CL/F) was reduced 50% in children with vs. without obesity (p=0.03). LBW-based dosing compensated for this reduction in CL/F (p = 0.15). CONCLUSION: To achieve comparable systemic PPI exposures for children with and without obesity, we recommend using LBW, rather than TBW-based dosing for pantoprazole.

Review article: the physiological effects and safety of peppermint oil and its efficacy in irritable bowel syndrome and other functional disorders.

BACKGROUND: Peppermint oil has been used for centuries as a treatment for gastrointestinal ailments. It has been shown to have several effects on gastrointestinal physiology relevant to clinical care and management. AIM: To review the literature on peppermint oil regarding its metabolism, effects on gastrointestinal physiology, clinical use and efficacy, and safety. METHODS: We performed a PubMed literature search using the following terms individually or in combination: peppermint, peppermint oil, pharmacokinetics, menthol, oesophagus, stomach, small intestine, gallbladder, colon, transit, dyspepsia, nausea, abdominal pain, and irritable bowel syndrome. Full manuscripts evaluating peppermint oil that were published through 15 July 2017 were reviewed. When evaluating therapeutic indications, only randomised clinical trials were included. References from selected manuscripts were used if relevant. RESULTS: It appears that peppermint oil may have several mechanisms of action including: smooth muscle relaxation (via calcium channel blockade or direct enteric nervous system effects); visceral sensitivity modulation (via transient receptor potential cation channels); anti-microbial effects; anti-inflammatory activity; modulation of psychosocial distress. Peppermint oil has been found to affect oesophageal, gastric, small bowel, gall-bladder, and colonic physiology. It has been used to facilitate completion of colonoscopy and endoscopic retrograde cholangiopancreatography. Placebo controlled studies support its use in irritable bowel syndrome, functional dyspepsia, childhood functional abdominal pain, and post-operative nausea. Few adverse effects have been reported in peppermint oil trials. CONCLUSION: Peppermint oil is a natural product which affects physiology throughout the gastrointestinal tract, has been used successfully for several clinical disorders, and appears to have a good safety profile.

Considerations in the rational design and conduct of phase I/II pediatric clinical trials: avoiding the problems and pitfalls.

Over the past decade, there has been a heightened awareness of the need to include children in the drug development process. With this awareness has come an expansion of the infrastructure for conducting studies in children and an increase in the sponsorship of pediatric clinical trials. However, the growth in pediatric research has, in many cases, not been accompanied by an increase in the involvement of trained pediatric investigators when it comes to trial design and/or interpretation. Pediatric phase I/II protocols continue to span a spectrum from those that are carefully constructed to those that are poorly designed. This paper highlights the basic elements of phase I/II protocols that merit unique consideration when the clinical trial involves children. Illustrations are provided from our experience, which highlight problems that may arise when trials are not designed with the pediatric patient in mind.

Ontogeny of dextromethorphan O- and N-demethylation in the first year of life.

The exponential increase in the number of drugs used to treat infant and childhood illnesses necessitates an understanding of the ontogeny of drug biotransformation for the development of safe and effective therapies. Healthy infants received an oral dose (0.3 mg/kg) of dextromethorphan (DM) at 0.5, 1, 2, 4, 6, and 12 months of age. DM and its major metabolites were measured in urine. CYP2D6 genotype was determined by polymerase chain reaction-restriction fragment length polymorphism. Genotyping data indicated a strong correlation between CYP2D6 genotype and DM O-demethylation (beta=-0.638; 95% CI: -0.745, -0.532; P<0.001). CYP2D6 activity was detectable and concordant with genotype by 2 weeks of age, showed no relationship with gestational age, and did not change with post natal age up to 1 year. In contrast, DM N-demethylation developed significantly more slowly over the first year of life. Genotype and the temporal acquisition of drug biotransformation are critical determinants of a drug response in infants.

Alteration of phenytoin binding by glycosylation of albumin in IDDM.

We measured glycosylated albumin and hemoglobin and serum protein binding of phenytoin in 57 children and adolescents with insulin-dependent diabetes mellitus (IDDM). Serum was incubated with phenytoin to yield concentrations of 15 and 25 mg/L, and a serum ultrafiltrate was prepared from an aliquot of each sample. We observed a linear correlation between glycosylated albumin and the free fraction of phenytoin at serum phenytoin concentrations of 15 mg/L (r = .35, P = .03) and 25 mg/L (r = .40, P = .003). A better correlation existed between the free fraction of phenytoin and total albumin concentrations for both serum concentrations (r = .45, P = .005 for 15 mg/L; r = .56, P = 10-5) for 25 mg/L), whereas the best linear correlation resided between the free fraction of phenytoin and the concentration of nonglycosylated albumin (r = .54, P = .0005 for 15 mg/L; r = .63, P less than 10(-6) for 25 mg/L). There was no correlation between the free fraction of phenytoin and the concentration of glycosylated albumin. Incubation of solutions of glycosylated and nonglycosylated albumin demonstrated significantly lower binding to the glycosylated fraction (P = 8.1 X 10(-6)). These results indicate that glycosylation of albumin diminishes the affinity of the phenytoin binding site on albumin. This alteration may have clinical significance in that it may alter the disposition of phenytoin in patients with IDDM and produce free phenytoin serum concentrations that are not accurately reflected by total serum phenytoin concentrations.

Selecting the proper pediatric dose: It is more than size that matters.

Appropriate pediatric dose selection remains one of the most vexing clinical problems faced by healthcare professionals who are charged to provide medical care to infants and children. While body size does reflect the ontogeny of processes that govern drug disposition, there are extremes of disease that perturb the expected relationships.

Single and multiple dose pharmacokinetics of methionyl growth hormone in children with idiopathic growth hormone deficiency.

The pharmacokinetics (PK) of methionyl GH (metGH) were characterized in 20 newly diagnosed GH-deficient children (19 males; 12.9 +/- 3.3 yr old; initial height, 138.8 +/- 16.2 cm; weight, 32.1 +/- 13.1 kg) after the first dose (FD) of metGH and again after 4-5 weeks of multiple dosing (MD). All subjects received a total metGH dose of 0.3 mg/kg.week by sc administration, but were randomized to receive the drug daily (D; n = 12; dose, 0.043 mg/kg) or three times per week (TIW; n = 8; dose, 0.1 mg/kg). After drug administration, repeated blood samples (n = 14) were obtained over a 10-h period. Concentrations of metGH from each sample were determined using a monoclonal antibody radiometric assay (range of linearity, 0.5-40.0 ng/ml; coefficient of variation, less than 4%). Plasma concentration vs. time data were curve fit using a nonlinear weighted least squares algorithm which permitted calculation of the following PK parameters (mean +/- SEM; FD vs. MD group): elimination rate constant (0.23 +/- 0.04 vs. 0.25 +/- 0.04 h-1), absorption rate constant (0.43 +/- 0.05 vs. 0.48 +/- 0.04 h-1), elimination half-life (t1/2; 3.01 vs. 2.77 h), total plasma clearance (CL/F; 0.32 +/- 0.02 vs. 0.54 +/- 0.09 L/h.kg), and apparent volume of distribution (VDss/F; 2.2 +/- 0.14 vs. 3.15 +/- 0.28 L/kg). Both the CL/F and VDss/F of metGH were significantly greater when data from the entire study population were compared on the basis of FD vs. MD administration. With the exception of a larger VDss/F in subjects who received daily (3.6 +/- 0.4 L/kg) vs. TIW metGH (2.4 +/- 0.2 L/kg), no significant differences were found for the PK parameters between the D and TIW dosing groups. In all subjects, absorption of metGH was slow, with an average time to reach maximum concentration (Tmax) of 4.4 h and an absorption t1/2 that ranged from 1.4-1.8 h. Proportionality was also found between the dose and the area under the plasma concentration vs. time curve, suggesting dose-independent PK of metGH. Our data demonstrate that the PK of metGH after sc administration to children are markedly different from those previously reported in adults and, also, do not vary as a consequence of dosing schedule (i.e. D vs. TIW). The apparent increase in CL/F and VDss/F for met GH with multiple dosing may reflect concentration-dependent changes in plasma binding of the drug or, alternatively, represent the effect of increased body mass on the pharmacokinetics of GH.

Pharmacokinetics and pharmacodynamics of growth hormone-releasing peptide-2: a phase I study in children.

Administration of GH-releasing peptide-2 (GHRP-2) represents a potential mode of therapy for children of short stature with inadequate secretion of GH. Requisite information to determine the dosing route and frequency for GHRP-2 consists of the pharmacokinetics (PK) and pharmacodynamics (PD) for this compound, neither of which have been previously evaluated in children. The purpose of this study was to characterize the PK and PD of GHRP-2 in children with short stature. Ten prepubertal children (nine boys and one girl; 7.7 +/- 2.4 yr old) received a single 1 microg/kg i.v. dose of GHRP-2 over 1 min, followed by repeated (n = 9) blood sampling over 2 h. GHRP-2 and GH were quantitated by specific RIA methods. PK parameters were calculated from curve fitting of GHRP-2 and GH vs. time data. Posttreatment plasma GH concentrations (normalized for pretreatment values) were used as the effect measurement. PD parameters were generated using the sigmoid Emax model. Disposition of GHRP-2 best fit a biexponential function. GHRP-2 PK parameters (mean +/- SD) were: alpha = 13.4 +/- 9.7 h(-1), beta = 1.3 +/- 0.3 h(-1), t(1/2beta) = 0.55 +/- 0.14 h, AUC(0-infinity) = 2.02 +/- 1.37 ng/mL x h, Cmax = 7.4 +/- 3.8 ng/mL, plasma clearance = 0.66 +/- 0.32 L/h x kg, and apparent volume of distribution = 0.32 +/- 0.14 L/kg. PK parameters for GH were: appearance rate constant = 5.9 +/- 3.1 h(-1), elimination t(1/2) = 0.37 +/- 0.15 h, lag time = 0.05 +/- 0.01 h, Cmax = 50.7 +/- 17.2 ng/mL, Tmax = 0.42 +/- 0.16 h, and AUC(0-infinity) = 47.9 +/- 26.1 ng/mL x h. PD parameters for GHRP-2 were: Ke0 = 1.13 +/- 0.94 h(-1), gamma = 13.15 +/- 9.44, E0 = 6.63 +/- 4.86 ng/mL (GH), Emax = 67.5 +/- 23.5 ng/mL (GH), and EC50 = 1.09 +/- 0.59 ng/mL. We concluded that 1) GHRP-2 produced a predictable and significant (i.e. compared to pretreatment values) increase in plasma GH concentrations; 2) the PK-PD link model enabled quantitative assessment of GHRP-2 modulation of serum GH levels; and 3) definition of the EC50 for GHRP-2 will enable PD and PK evaluations of extravascular dosing regimens for children.

Quantification of intraindividual variability and the influence of menstrual cycle phase on CYP2D6 activity as measured by dextromethorphan phenotyping.

Intraindividual variability and the effects of menstrual cycle phase on CYP2D6 activity were evaluated by dextromethorphan phenotyping in 20 Caucasian normal volunteers. Dextromethorphan 30 mg was administered to 10 men every 14 days for 3 months, and to 10 premenopausal women during the mid-follicular and mid-luteal phases of each menstrual cycle for three complete cycles. Urinary dextromethorphan/dextrorphan molar ratios were obtained after an overnight urine collection. Ten women and nine men were extensive metabolizer phenotypes, and one man was a poor metabolizer phenotype (confirmed by genotyping). There was no difference in dextromethorphan metabolic ratios between the mid-follicular (mean +/- SD: 0.00728+/-0.00717) and mid-luteal (0.00745+/-0.00815) phases of the menstrual cycle (P = 0.88). Also, no significant difference was found in the intraindividual variability of the metabolic ratios between the two phases (P = 0.80). No statistically significant sex difference in CYP2D6 activity was found between men (0.00537+/-0.00431) and women (0.00737+/-0.00983) extensive metabolizers (P = 0.84). For all individuals, intraindividual variability in dextromethorphan ratios ranged from 12.1-136.6% with a median of 36.7%. Because hormonal fluctuations within the mid-follicular and mid-luteal phases of the menstrual cycle do not appear to affect CYP2D6 activity, pharmacokinetic or clinical investigations of CYP2D6 substrate activity may not require menstrual cycle phase stratification. Because baseline metabolic ratios may fluctuate an average of 37%, repeat baseline and treatment phenotyping assessments should be obtained for accurate determination of a given drug's effect on CYP2D6 activity when measured by dextromethorphan.

Optimization of cytochrome P4502D6 (CYP2D6) phenotype assignment using a genotyping algorithm based on allele frequency data.

Cytochrome P4502D6 (CYP2D6) is a highly polymorphic gene locus with > 50 variant alleles which lead to a wide range in enzymatic activity. So called poor metabolizers are carriers of any two non-functional alleles of the CYP2D6 gene. CYP2D6 genotyping is cumbersome and the question of how much genotyping is necessary for an accurate phenotype prediction is still debated. The goal of this study was to determine the optimum amount of genotyping required to accurately predict the phenotype at a reasonable cost in a white North American population. To address this issue, we designed a polymerase chain reaction (PCR)/restriction fragment length polymorphism-based genotyping strategy to detect 'key' mutations linked to extensive metabolizer or poor metabolizer associated alleles in combination with extra-long PCR (XL-PCR). All mutations with the exception of gene deletions and duplications are detectable by simple restriction digestion analysis and agarose gel electrophoresis. In addition, we utilized a genotyping algorithm based on our own and published allele frequency data and phenotype analysis to calculate the probability of a correct genotype (and thus, phenotype) assignment. As little as one XL-PCR reaction followed by a maximum of six reamplification reactions allows an accurate prediction of an individual's genotype to 99.15%. As few as four reamplification reactions identify 97.9% of poor metabolizer individuals. We evaluated our model in 208 white North Americans by testing for the presence of 'key' mutations linked to CYP2D6*2, *3, *4, *6, *7, *8, *9, *10, *11, *12, *15, *17 and *18 alleles and the *5, *13 and *16 gene deletions. For all individuals, the correct phenotype has been predicted. Discordant phenotype assignment occurred in only two individuals which subsequently was attributed to CYP2D6 inhibition by concomitant drug therapy.

Limitations of dextromethorphan N-demethylation as a measure of CYP3A activity.

We evaluated the utility of the 3-methoxymorphinan/dextromethorphan (3MM/DM) urinary ratio to reflect baseline CYP3A activity, and its ability to discriminate moderate CYP3A inhibition during fluvoxamine therapy. For 4 months, oral dextromethorphan 30 mg and intravenous midazolam 0.025 mg/kg were administered to nine men every 14 days, and to 10 premenopausal women during the follicular and luteal phases of their menstrual cycles. Phenotyping during the first 3 months or cycles established baseline CYP3A activity. During the fourth month, individuals were given fluvoxamine 150 mg/day. CYP3A activity was expressed as both the urinary 3MM/DM molar ratio and midazolam plasma clearance (MDZ CL). 3MM/DM ratios were independent of dextromethorphan CYP2D6 phenotype (r = 0.13, P = 0.6). Intraindividual variability in baseline CYP3A activity (median, 25-75th percentile), as determined by coefficients of variation, was 48.3% (36.8-68.8%) for 3MM/DM and 10.3% (8.3-11.8%) for MDZ CL. No significant correlation between 3MM/DM and MDZ CL either at baseline (r = -0.22, P = 0.4) or during fluvoxamine therapy (r = -0.15, P = 0.6) was noted. With fluvoxamine 150 mg/day, median percentage change in the 3MM/DM ratios was -50.0% (-105.6-6.0%; P = 0.7), and median percentage change in MDZ CL was -33.7% (-27.0-39.3%; P < 0.0001). Only MDZ CL consistently indicated moderate inhibition of hepatic CYP3A activity. In addition, there was a lack of correlation between the magnitudes of fluvoxamine-induced change in 3MM/DM and MDZ CL (r = 0.41, P = 0.1). The large intraindividual variability of the 3MM/DM urinary ratio, as well as the inability to discriminate moderate CYP3A inhibition, makes this a suboptimal method for accurately assessing CYP3A activity.

Developmental pharmacodynamics of cyclosporine.

OBJECTIVE: To determine the relationship between human development and in vitro cyclosporine (INN, ciclosporin) pharmacodynamics. METHODS: Fifty-six subjects ranging in age from 3 months to 39 years were studied in this prospective laboratory investigation at a university children's hospital and clinical pharmacology laboratory. Peripheral blood monocytes were separated from whole blood and cultured with phytohemagglutinin A (5 microg/mL) and cyclosporine (0, 6.25, 12.5, 25, 50, 100, 500, 1000, 2500, and 5000 ng/mL). Peripheral blood monocytes cultures were assayed for cell proliferation and supernatant interleukin-2 concentration with use of radionuclide DNA tagging and enzyme-linked immosorbent assay, respectively. After concentration-effect modeling, summary pharmacodynamic parameters, including the maximal drug effect (Emax) and cyclosporine concentration at which 50% of maximal effect (IC50) and 90% of maximal effect (IC90), were determined. These parameters were compared between four consecutive subject age groups: infants (0-1 years), children (>1-4 years), preadolescents (>4-12 years), and adults (>12 years). RESULTS: The peripheral blood monocytes of the infants showed a twofold lower mean IC50 (peripheral blood monocyte proliferation) and sevenfold lower mean IC90 (interleukin-2 expression) than peripheral blood monocytes from older subjects. The three older age groups were similar with respect to mean IC50 and Emax (peripheral blood monocyte proliferation). Lymphocyte subtype proportions measured in peripheral blood monocytes preparations from each age group were generally similar. The experimental conditions (eg, general anesthesia and cyclosporine solvents) did not affect peripheral blood monocytes proliferation, but the highest experimental cyclosporine concentration (ie, 5000 ng/mL) was associated with decreased peripheral blood monocytes viability. CONCLUSIONS: Cyclosporine pharmacodynamics in vitro are related to age. This factor, if neglected, may be a source of iatrogenic risk during pediatric immunosuppressive therapy.

Serum sickness-like reaction to cefaclor: lack of in vitro cross-reactivity with loracarbef.

OBJECTIVES: A lymphocyte-based in vitro rechallenge technique was used to examine the potential for cross-reactivity between loracarbef and cefaclor in children who had had adverse reactions to cefaclor. STUDY DESIGN: The study cohort included 10 patients (2.2 +/- 1.1 years old) with a serum sickness-like reaction to cefaclor, five patients (4.1 +/- 4.6 years old) with immediate-type hypersensitivity reactions to the drug, and five patients (1.5 +/- 0.9 years old) who had a delayed hypersensitivity reaction to cefaclor without joint involvement (i.e., not a serum sickness-like reaction). Peripheral blood mononuclear cells were isolated from each patient and were exposed to cefaclor, loracarbef, and the metabolites of each generated with phenobarbital-induced murine hepatic microsomes. RESULTS: Among patients with cefaclor-associated serum sickness-like reactions, lymphocyte killing (expressed as percentage cell kill above baseline) in the presence of cefaclor metabolites (83.6% +/- 42.2%) was significantly (p < 0.02) higher than that observed among the patients with either immediate-type (1.1% +/- 2.4%) or delayed hypersensitivity (0%) reactions. In contrast, loracarbef did not produce significant in vitro cytotoxicity among any of the patient subgroups. Our in vitro evidence was supported at therapeutic rechallenge with loracarbef among three children with cefaclor-associated serum sickness-like reactions who tolerated a full course of therapy without adverse reactions. CONCLUSION: The metabolite-mediated cytotoxicity associated with cefaclor among patients who had had serum sickness-like reactions after therapeutic administration of the drug was not shared with loracarbef. The extent to which the apparent lack of in vitro cross-reactivity between cefaclor and loracarbef may be predictive of clinical cross-reactivity is not known.

Ibuprofen pharmacokinetics in preterm infants with patent ductus arteriosus.

OBJECTIVE: Our objective was to study the pharmacokinetics of ibuprofen in premature infants with patent ductus arteriosus on day 3 and day 5 after birth. METHODS: Ibuprofen was administered on days 3, 4, and 5 by a 15-minute intravenous infusion of 10, 5, and 5 mg/kg, respectively, with the aim of closing the ductus arteriosus. Blood samples were drawn at time zero and at 0.5, 1, 2, 4, 12, and 24 hours after the first and third doses. Ibuprofen plasma concentrations were assayed by HPLC. RESULTS: A total of 27 premature infants were included (gestational age, 28.6 +/- 1.9 weeks; birth weight, 1250 +/- 460 g; values are mean +/- standard deviation). Ibuprofen pharmacokinetics followed a 2-compartment open model. Between the first and third doses (day 3 and day 5) there was a significant decrease of the volume of distribution of the central compartment (Vd(c)) (0.244 versus 0.171 L/kg; P =.03) and area under the plasma concentration-time curve (524 versus 447 mg. h/L; P =.01). The decrease in Vd(c) was most pronounced in patients with a closing ductus. Total body clearance and plasma half-life did not change significantly. No significant differences were observed in ibuprofen peak plasma concentrations after the first and third doses in relation to ductal status after treatment. CONCLUSION: Ibuprofen pharmacokinetics showed a large interindividual variation in premature infants during treatment for patent ductus arteriosus, and significant changes may occur between day 3 and day 5 after birth in those infants with a closing ductus. These findings may have implications for the treatment schedule with ibuprofen in patients with patent ductus arteriosus.

Hepatic drug clearance in patients with mild cystic fibrosis.

The plasma disposition of three model substrates (lorazepam, indocyanine green, and antipyrine) and the formation clearance of antipyrine metabolites (3-hydroxymethylantipyrine, norantipyrine, and 4-hydroxyantipyrine) were evaluated in 15 subjects with mild cystic fibrosis and in 15 healthy control subjects. Plasma clearance was significantly greater in patients with cystic fibrosis for both lorazepam (1.7 +/- 0.4 versus 1.2 +/- 0.5 ml/min/kg) and indocyanine green (14.2 +/- 6.1 versus 9.1 +/- 3.0 ml/min/kg). In contrast, the clearance of antipyrine was not significantly different (1.0 +/- 0.7 versus 0.8 +/- 0.3 ml/min/kg), but the formation clearance for 3-hydroxymethylantipyrine was significantly greater in patients with cystic fibrosis. Lorazepam and antipyrine apparent steady-state volume of distribution were not different between groups. These results suggest that clearance of drugs that undergo conjugation (e.g., lorazepam) or biliary excretion (e.g., indocyanine green) is increased in patients with mild cystic fibrosis. In contrast, the increased formation clearance of only one antipyrine metabolite suggests that alterations in clearance of drugs metabolized by cytochrome P450 enzymes are substrate specific and isoform specific in patients with cystic fibrosis.

Investigation of terbinafine as a CYP2D6 inhibitor in vivo.

BACKGROUND: Terbinafine is an orally active antifungal used in the treatment of dermatophytoses. To date, studies evaluating the effect of terbinafine on the cytochromes P450 have failed to show any significant interactions. This prospective open-label study was designed to confirm our previous finding that terbinafine may inhibit CYP2D6. METHODS: Nine healthy volunteers were enrolled in this study-6 genotypically consistent with an extensive metabolizer phenotype and 3 genotypic poor metabolizers for CYP2D6. The change in CYP2D6 enzyme activity before (x 3) and after (monthly x 6 months) administration of terbinafine (250 mg once daily x 14 days) was evaluated with the dextromethorphan to dextrorphan urinary metabolite ratios. On each study day a predose urine sample was collected, 0.3 mg/kg dextromethorphan was administered, and urine was collected for 24 hours. Dextromethorphan and its metabolites were quantified from urine by HPLC. RESULTS: Baseline phenotype values were concordant with individual genotype. In all extensive metabolizers, the administration of terbinafine resulted in a dramatic increase in the dextromethorphan/dextrorphan ratio, converting 4 of the 6 extensive metabolizers into phenotypic poor metabolizers. On average, a 97-fold increase in ratio (range, 35 to 265) was observed for extensive metabolizers after the administration of terbinafine. No significant change was observed in the metabolite ratios of poor metabolizers during the course of the study. CONCLUSIONS: Terbinafine inhibits CYP2D6 sufficiently to produce a discordance between genotype and phenotype for the enzyme. The dextromethorphan/dextrorphan metabolite ratios increased in all individuals, with otherwise functional CYP2D6 activity. The disposition of CYP2D6 substrates coadministered with terbinafine may be significantly altered in extensive metabolizers for this cytochrome P450 isoform, who comprise approximately 93% of the population.

Pharmacokinetics and metabolism of intravenous midazolam in preterm infants.

BACKGROUND: Midazolam, a benzodiazepine, is finding expanded use in neonatal intensive care units. We studied the pharmacokinetics and metabolism of midazolam after a single intravenous dose in preterm infants. METHODS: The pharmacokinetics of midazolam and its hydroxylated metabolite (1-OH-midazolam) after a single 0.1 mg/kg intravenous dose of midazolam were determined in 24 preterm infants (gestational age, 26 to 34 weeks; postnatal age, 3 to 11 days). Blood samples were obtained before drug administration and at 0.5, 1, 2, 4, 6, 12, and 24 hours after the start of the infusion. Midazolam and 1-OH-midazolam concentrations were determined by use of gas chromatography-mass spectrometry. RESULTS: Total body clearance, apparent volume of distribution, and plasma half-life of midazolam were (median [range]): 1.8 (0.7-6.7) ml/kg per min, 1.1 (0.4-4.2) L/kg, and 6.3 (2.6-17.7) h, respectively. In 19 of 24 preterm infants, 1-OH-midazolam concentrations could be detected: 1-OH-midazolam (1-OH-M) maximal concentration of drug in plasma (C(max)), time to reach C(max) (T(max)), and 1-OH-M/M area under the concentration-time curve from time zero to the last sampling time point (AUC(0-t)) ratio were [median (range)]: 8.2 (<0.5-68.2) ng/ml, 6 (1-12) h, and 0.09 (<0.001-1), respectively. Midazolam plasma clearance was increased in those infants who had indomethacin (INN, indometacin) exposure. DISCUSSION: Consequent to immature hepatic cytochrome P450 3A4 (CYP3A4) activity, midazolam clearance and 1-OH-midazolam concentrations are reduced markedly in preterm infants as compared to concentrations in previous reports from studies in older children and adults. Indomethacin exposure and its apparent impact on midazolam clearance support alteration of drug disposition produced by a patent ductus arteriosus or by the direct effects of indomethacin on hemodynamic or renal function.

Combined phenotypic assessment of CYP1A2, CYP2C19, CYP2D6, CYP3A, N-acetyltransferase-2, and xanthine oxidase with the "Cooperstown cocktail".

BACKGROUND: Simultaneous administration of several probes enhances the utility of phenotyping, but poor specificity, side effects, and use of drugs not approved by the Food and Drug Administration limit the usefulness of prior phenotyping cocktails. OBJECTIVES: To evaluate potential drug-drug interactions associated with use of a cocktail of caffeine, omeprazole, dextromethorphan, and midazolam for simultaneous phenotyping of CYP1A2, CYP2C19, CYP2D6, CYP3A, N-acetyltransferase-2, and xanthine oxidase. METHODS: Twelve subjects received caffeine + dextromethorphan, omeprazole, and midazolam (each alone), and a cocktail of caffeine + dextromethorphan + omeprazole + midazolam. Blood samples were collected at 120 minutes for omeprazole and 5/-hydroxyomeprazole, and at 0, 5, 30, 60, 120, 240, 300, and 360 minutes for midazolam. Twelve-hour urine samples were collected for analysis of dextromethorphan, caffeine, and metabolites. RESULTS: The median CYP1A2 metabolic ratio after administration of caffeine + dextromethorphan was not significantly different from that obtained with the cocktail (P = .84). Likewise, the median N-acetyltransferase-2, xanthine oxidase, and CYP2D6 metabolic ratios were not significantly different after cocktail administration (P = .977 for each N-acetyltransferase-2; P = .795 for xanthine oxidase; P = .75 for CYP2D6). The median CYP2C19 metabolic ratio after cocktail administration was not significantly different from that obtained after omeprazole administered alone (P = 1.000). Also, midazolam plasma clearance was not significantly different after cocktail administration compared with that after administration of midazolam alone (P = .708). The only side effect was sedation, which was associated with intravenous midazolam and occurred to a similar extent after both individual and cocktail phenotyping. CONCLUSIONS: These results indicate no pharmacokinetic or pharmacodynamic interactions that would limit the utility of this phenotyping cocktail for simultaneous measurement of the activity of multiple drug-metabolizing enzymes.

Quantitation of three-month intraindividual variability and influence of sex and menstrual cycle phase on CYP1A2, N-acetyltransferase-2, and xanthine oxidase activity determined with caffeine phenotyping.

OBJECTIVE: To evaluate intraindividual variability and the effects of sex and menstrual cycle phase on the activity of cytochrome P450 1A2 (CYP1A2), N-acetyltransferase 2 (NAT2), and xanthine oxidase. METHODS: Ten white men were given 2 mg/kg caffeine orally every 14 days for 3 months. The same dosage of caffeine was given to 10 premenopausal white women during the midfollicular and midluteal phases of three complete menstrual cycles. Phenotype was determined with urinary caffeine metabolite ratios. RESULTS: For CYP1A2, mean metabolic ratio (+/- SD) was 5.97 +/- 2.78 during the midfollicular phase and 5.32 +/- 1.99 during the midluteal phase (p = 0.2). For extensive and poor metabolizer of NAT2. Mean midfollicular phase metabolite ratios were 0.71 +/- 0.060 and 0.37 +/- 0.030, and mean midluteal phase metabolite ratios were 0.69 +/- 0.076 and 0.39 +/- 0.053 (p = 0.9). For xanthine oxidase, mean midfollicular phase metabolite ratio was 0.63 +/- 0.06 and mean midluteal phase metabolite ratio was 0.63 +/- 0.05 (p = 0.3). Among the men, mean CYP1A2, NAT2 rapid and slow acetylator, and xanthine oxidase indices were 9.42 +/- 10.18, 0.66 +/- 0.021, 0.31 +/- 0.056, and 0.64 +/- 0.03. There were no differences in metabolite ratios between men and women for CYP1A2, NAT2 extensive metabolizers, or xanthine oxidase. A statistically significant sex difference was found for poor metabolizers of NAT2 (p < 0.05). Median coefficients of variation for CYP1A2, NAT2 extensive and poor metabolizers, and xanthine oxidase ratios were 16.8% (range, 4.5% to 49.3%), 2.9% (range, 2.2% to 4.7%), 13.4% (range, 7.5% to 27.2%), and 4.5% (range, 2.3% to 13.0%). CONCLUSION: Stratification by menstrual cycle phase or sex need not be performed for pharmacokinetic or clinical investigations of substrates for CYP1A2, NAT2, or xanthine oxidase in which the subject are adults.

Dose dependency of dextromethorphan for cytochrome P450 2D6 (CYP2D6) phenotyping.

Most dextromethorphan CYP2D6 phenotyping studies use a 30-mg dose, but data that show superiority of any particular dose are lacking. We compared metabolic ratios from six different dextromethorphan phenotyping doses to ascertain whether linearity existed over a dosage range. Forty subjects were enrolled in the study. Each subject received 0.05 mg/kg, 0.15 mg/kg, 0.3 mg/kg, 30 mg, 0.8 mg/kg, and 1.2 mg/kg dextromethorphan in a randomized crossover fashion. Urinary dextromethorphan to dextrorphan molar ratios were used to measure CYP2D6 activity. Single blood samples were obtained for CYP2D6 genotyping, which revealed one poor metabolizer and 39 extensive metabolizers. A statistical difference was found for the molar ratio between the 0.8 mg/kg and the 1.2 mg/kg dose compared with the other four doses. None of the 39 genotypic extensive metabolizers were incorrectly phenotyped with any of these doses. These data support the use of moderate doses of dextromethorphan for phenotyping to avoid dose dependency.