Effect of intravenous flumazenil on oral midazolam pharmacokinetics and pharmacodynamics for use as a cytochrome P450 3A probe.

UNLABELLED: Phenotyping intestinal and hepatic cytochrome P450 (CYP) 3A activity with oral midazolam can be limited by midazolam-induced central nervous system (CNS) side effects. Determining methods to minimize CNS side effects optimizes use of midazolam as a CYP3A probe. OBJECTIVE: The objective of this study was to determine the effect of intravenous (i.v.) flumazenil on midazolam apparent oral clearance (a surrogate marker of CYP3A activity). Midazolam pharmacodynamics were also evaluated. METHODS: This was a randomized, double-blind, placebo-controlled, single-dose, two-way crossover study. 16 healthy volunteers (8 women) were concomitantly administered i.v. flumazenil 0.005 mg/kg or i.v. placebo and oral midazolam 0.075 mg/kg. Blood samples were obtained to determine midazolam and flumazenil plasma concentrations. Bioequivalence was assessed by determining geometric mean ratios (GMR) and 90% confidence intervals (90% CI). Baseline and post dose digit symbol substitution tests (DSST), Groton maze learning tests (GMLT), and Stanford sleepiness scales (SSS) were conducted. RESULTS: Apparent oral clearance was 2,030 +/- 651 and 1,939 +/- 658 ml/min for the midazolam plus flumazenil and midazolam plus placebo groups. Equivalence in midazolam apparent oral clearance was observed (%GMR flumazenil/placebo, 90% CI 104.8, 94 - 116.6%). Flumazenil partially attenuated oral midazolam pharmacodynamics. Exploratory post hoc analyses revealed that midazolam exposure was 1.9-fold higher in men compared to women. CONCLUSION: i.v. flumazenil can be used in conjunction with oral midazolam for CYP3A phenotyping.

Comparative pharmacokinetics and pharmacodynamic target attainment of ertapenem in normal-weight, obese, and extremely obese adults.

Little is known of the effects of obesity on ertapenem drug disposition and pharmacodynamics. Thirty healthy volunteers in three body mass index (BMI) groups (10 per group), normal weight (BMI, 18.5 to 24.9 kg/m2), class I-II obesity (BMI, 30 to 39.9 kg/m2), and class III obesity (BMI, >or=40 kg/m2), were administered a 1-g dose of ertapenem. Serum concentrations were obtained over 24 h. Population pharmacokinetic data were obtained using a nonparametric adaptive grid followed by Monte Carlo simulation to determine the probability of obtaining the free drug exposure targets of the time that the free drug concentration remains above the MIC (fT>MIC) of 20% and 40% for bacteriostatic and maximal bactericidal activity, respectively. Compared to the subjects in the obese groups, area under the concentration-time curve from 0 h to infinity was significantly higher in the normal-weight subjects, whereas the total central compartment volume was higher in the class III obese subjects (PMIC of 20% with a 90% probability was attained at MICs of MIC, 40%), no group attained the target at the 90% probability level at any tested MIC. The results suggest that the standard 1-g ertapenem dose may not provide adequate drug exposure for any body mass index classification for MICs in excess of 0.25 to 0.5 microg/ml.

Integrating fluoroquinolones into the hospital formulary.

With the increasing availability of new agents, selection of fluoroquinolones for formulary inclusion can be difficult. Appropriate evaluation of the important characteristics (pharmacokinetic and pharmacodynamic properties, antimicrobial activity, efficacy, tolerability, cost) of these agents should allow selection of the most cost-effective ones. Evidence from in vitro studies and clinical trials indicates differences exist among fluoroquinolones, especially in terms of activity against gram-positive, aerobic organisms. For selected clinical situations, it may be important to choose an agent that is available in both intravenous and oral formulations. Comparative drug costs, as well as costs associated with potential clinical failure and adverse events, should be evaluated carefully. Dosage regimens should be considered, as shorter durations of therapy and less frequent dose administration may lead to reduced labor costs and increased patient compliance, thereby improving effectiveness and economic efficiency.

Use of midazolam urinary metabolic ratios for cytochrome P450 3A (CYP3A) phenotyping.

Midazolam (MDZ) total clearance (ClT) is widely used for cytochrome P450 3A (CYP3A) phenotyping, but requires up to eight blood samples. This study was conducted to compare the use of midazolam ClT to use of a midazolam urinary metabolic ratio for CYP3A phenotyping. Ten male and 10 female subjects received i.v. midazolam 0.025 mg/kg eight times over a 4-month period at approximately 2-week intervals. The first six phenotyping measures were used to estimate baseline CYP3A activity, then subjects received the moderate CYP3A inhibitor fluvoxamine 150 mg/day for the last 4 weeks (two phenotyping visits) of the study. Serial blood samples were obtained for calculation of ClT. Urine was collected for 6 h following each midazolam dose. Midazolam, 1'-hydroxymidazolam (1-OHMDZ), and 4-hydroxymidazolam were measured in plasma and urine by liquid chromatography with tandem mass spectrometry (LC/MS/MS). Analysis of 148 samples from 20 subjects revealed a weak overall correlation between the urinary ratio of 1-OHMDZ/MDZ to midazolam ClT of r(s) = 0.372 (P = 0.0001). There was no correlation when examining either baseline samples or fluvoxamine-inhibited samples alone (r(s) = 0.101, P = 0.289 and r(s) = 0.266, P = 0.123, respectively). The median (range) urinary ratio decreased significantly with fluvoxamine [219 (141-409) versus 127 (50-464); P = 0.005] and to a similar extent to the midazolam ClT (-33.6% versus -42.4%, respectively; P > 0.05). Median urinary recovery of the i.v. midazolam dose varied between 1.4% and 53% and was significantly lower in samples collected while patients were receiving fluvoxamine (34.3% versus 23.1%; P= 0.0004). Based on these results, although this midazolam urinary ratio was not very reflective of baseline CYP3A activity, it may be a useful indicator of CYP3A inhibition.

Effects of oral vitamin K on S- and R-warfarin pharmacokinetics and pharmacodynamics: enhanced safety of warfarin as a CYP2C9 probe.

Evidence for the selectivity of S-warfarin metabolism by CYP2C9 is substantial, suggesting that warfarin may be a potential CYP2C9 phenotyping probe. It is, however, limited by its ability to elevate the international normalized ratio (INR) and potentially cause bleeding. The effect of vitamin K to attenuate the elevation of INR may enable the safe use of warfarin as a probe. The objective of this study was to investigate the pharmacokinetics and pharmacodynamics of S- and R-warfarin in plasma following the administration of warfarin alone versus warfarin and vitamin K in CYP2C9*1 homozygotes. Healthy adults received, in a randomized crossover fashion in a fasted state, warfarin 10 mg orally or warfarin 10 mg plus vitamin K 10 mg orally. Blood samples were obtained over 5 days during each phase. INR measurements were obtained at baseline and day 2 in each phase. INR, AUC0-infinity, and t1/2 of plasma S- and R-warfarin were examined. Eleven CYP2C9*1 homozygotes (3 men, 8 women) were enrolled. INR at day 2 following warfarin 10 mg was 1.18 +/- 0.19, which differed significantly from baseline (INR = 1.00 +/- 0.05) and warfarin with vitamin K (INR = 1.06 +/- 0.07). INR at baseline was not significantly different from warfarin with vitamin K. t1/2 and AUC0-infinity of both enantiomers did not significantly differ between the phases. It was concluded that INR is apparently attenuated by concomitant administration of a single dose of vitamin K without affecting the pharmacokinetics of either warfarin stereoisomer. Warfarin 10 mg may be safely used as a CYP2C9 probe in *1 homozygotes when given concomitantly with 10 mg of oral vitamin K.

A dose-ranging study of gentamicin pharmacokinetics: implications for extended interval aminoglycoside therapy.

Prolonged distribution time has been noted for high-dose (7 mg/kg) gentamicin. Higher doses are used for extended-interval aminoglycoside therapy (EIA). The authors investigated whether the increase in distribution time was proportional to the dose of gentamicin. Twelve healthy volunteers were given low (LD, 2 mg/kg), medium (MD, 4.5 mg/kg), and high (HD, 7 mg/kg) doses of gentamicin in a randomized, crossover fashion. Gentamicin was infused over 30 minutes, with 15 concentrations obtained over 8 hours after each dose. Data were fit to a two-compartment pharmacokinetic model. Distribution half-life for HD (31.1 +/- 5.7 min) differed significantly (p < 0.05) from LD (22.4 +/- 6.1 min) and MD (23.8 +/- 5.1 min) with no significant difference being seen between LD and MD. This study verifies that when using EIA dosing with HD gentamicin, sampling within 90 minutes after the beginning of the infusion provides information that leads to overestimation of peak serum concentration/minimum inhibitory concentration and inaccurate calculation of pharmacokinetic parameters.

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.

Outcomes of weight-based heparin dosing based on literature guidelines and institution individualization.

STUDY OBJECTIVE: To determine whether unfractionated heparin is optimally dosed using published weight-based guidelines. DESIGN: Six-month, prospective study. SETTING: University hospital. PATIENTS: Ninety-six patients in the weight-based unfractionated heparin-dosing group 1 (WBHD1; 37 men; mean age 66.9 +/- 15.1 years; mean weight 80.1 +/- 20.6 kg) and 68 patients in the WBHD2 (25 men; mean age 68.2 +/- 15.6 years; mean weight 82.0 +/- 19.6 kg). INTERVENTIONS: The WBHD1 received a 100-U/kg intravenous bolus followed by an 18-U/kg/hour continuous intravenous infusion. After 3 months, the protocol was modified, and the WBHD2 received a 90-U/kg bolus followed by a 16-U/kg/hour continuous infusion for 3 months. MEASUREMENTS AND MAIN RESULTS: Activated partial thromboplastin times (aPTTs), frequency of bleeding episodes that required blood transfusions, and the number of recurrent thromboembolic events were collected from both groups after 3 months on the study. In the WBHD1, 24 hours after starting heparin, 38.5% of patients had therapeutic aPTTs, and at 48 hours, 54.3% were therapeutic. In the WBHD2, 42.6% and 51.4% of patients had therapeutic aPTTs at 24 and 48 hours, respectively. There was no statistical difference between the WBHD1 and WBHD2 in the percentage of patients with therapeutic aPTTs. CONCLUSIONS: Weight-based heparin dosing resulted in low percentages of patients with therapeutic aPTTs. The use of weight alone to dose heparin may not be adequate to optimize therapy.

The absolute bioavailability of oral melatonin.

The absolute bioavailability of oral melatonin tablets was studied in 12 normal healthy volunteers. Subjects were administered, in a randomized crossover fashion, melatonin 2 mg intravenously and 2 and 4 mg orally. Blood was sampled over approximately eight (estimated) half-lives. Both the 2 and the 4 mg oral dosages showed an absolute bioavailability of approximately 15%. No difference in serum half-life was seen in any of the study phases. Oral melatonin tablets in dosages of 2 and 4 mg show poor absolute bioavailability, either due to poor oral absorption, large first-pass metabolism, or a combination of both. Further studies examining larger doses, in an attempt to saturate first-pass metabolism if it occurs, may be warranted.

Comparison of central nervous system adverse effects of amantadine and rimantadine used as sequential prophylaxis of influenza A in elderly nursing home patients.

BACKGROUND: Amantadine hydrochloride and rimantadine hydrochloride are recommended by the Centers for Disease Control and Prevention for prophylaxis of influenza A. While data suggest that rimantadine is better tolerated, there are no data examining the rate of adverse reactions in elderly patients who receive amantadine vs rimantadine. Our objective was to assess the adverse reaction rate in elderly nursing home patients receiving sequential amantadine and rimantadine for influenza A prophylaxis. METHODS: Data were collected in 156 nursing home patients (70% women; mean+/-SD age, 83.7+/-10.1 years) in a single care setting who received sequential therapy with amantadine and rimantadine during the 1997-1998 influenza season. Patients were assessed for central nervous system adverse effects and therapy discontinuation occurring with each agent. RESULTS: Twenty-nine (18.6%) of the 156 patients experienced an adverse effect when receiving amantadine compared with 3 patients (1.9%) when rimantadine was given (P<.01). Drug use was discontinued due to adverse events in 17.3% (n = 27) of the amantadine courses and 1.9% (n=3) of the rimantadine courses (P<.001). Confusion was the most frequently observed adverse event (amantadine, 10.6%; rimantadine, 0.6%; P<.001). Multivariate logistic regression analysis showed that significant risk factors for central nervous system adverse events included male sex (odds ratio, 3.65), reduced calculated creatinine clearance (odds ratio, 1.78), and use of amantadine (odds ratio, 12.73). CONCLUSIONS: Amantadine use was associated with a significantly higher incidence of central nervous system adverse events than rimantadine use in this elderly population receiving influenza prophylaxis. In addition, the discontinuation rate of amantadine was significantly higher than that with rimantadine.

Phenotyping of drug-metabolizing enzymes in adults: a review of in-vivo cytochrome P450 phenotyping probes.

Cytochrome P450 phenotyping provides valuable information about real-time activity of these important drug-metabolizing enzymes through the use of specific probe drugs. Despite more than 20 years of research, few conclusions regarding optimal phenotyping methods have been reached. Caffeine offers many advantages for CYP1A2 phenotyping, but the widely used caffeine urinary metabolic ratios may not be the optimal method of measuring CYP1A2 activity. Several probes of CYP2C9 activity have been suggested, but little information exists regarding their use, largely due to the narrow therapeutic index of most CYP2C9 probes. Mephenytoin has long been considered the standard CYP2C19 phenotyping probe, but problems such as sample stability and adverse effects have prompted the investigation of potential alternatives, such as omeprazole. Several well-validated CYP2D6 probes are available, including dextromethorphan, debrisoquin and sparteine, but, in most cases, dextromethorphan may be preferred due to its wide safety margin and availability. Chlorzoxazone remains the only CYP2E1 probe that has received much study. However, questions concerning phenotyping method and involvement of other enzymes have impaired its acceptance as a suitable CYP2E1 phenotyping probe. CYP3A phenotyping has been the subject of numerous investigations, reviews and commentaries. Nevertheless, much controversy regarding the selection of an ideal CYP3A probe remains. Of all the proposed methods, midazolam plasma clearance and the erythromycin breath test have been the most rigorously studied and appear to be the most reliable of the available methods. Despite the limitations of many currently available probes, with continued research, phenotyping will become an even more valuable research and clinical resource.

Waste generation of drug product samples versus prescriptions obtained through pharmacy dispensing.

Drug samples are often packaged differently from bulk packaging and thus they may contain a disproportionate amount of waste material. Fifteen drug samples were obtained from seven pharmaceutical companies and the packaging materials were weighed after the samples were removed. The waste produced by the samples was determined for a standard amount of drug and compared with the weight of the waste produced when the same quantity of drug was dispensed through a pharmacy. The average weight of the sample package for a standard course of therapy was significantly greater (p< or =0.05) than that of the pharmacy-dispensed prescription waste weight. The former was 5+/-4.5-fold heavier than the latter. The waste generated by drug samples in the United States was determined to be 5740 metric tons/year.

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.

Incidence of sexual dysfunction in healthy volunteers on fluvoxamine therapy.

BACKGROUND: Current literature suggests that the incidence of sexual dysfunction secondary to fluvoxamine therapy is 1% to 8%, while other selective serotonin reuptake inhibitors may have rates as high as 75%. The objective of this study was to determine the incidence of sexual dysfunction secondary to fluvoxamine in healthy volunteers. METHOD: 20 healthy volunteers (10 men, 10 premenopausal women) had adverse effects assessed at 6 visits while not receiving fluvoxamine, then twice while taking 150 mg fluvoxamine daily. Assessments occurred at 2-week intervals. Incidence rates for sexual dysfunction were calculated. RESULTS: No sexual dysfunction was reported prior to fluvoxamine therapy. After 2 weeks and 4 weeks of therapy respectively, sexual dysfunction occurred in 20% (N = 4) and 35% (N = 7) of the healthy volunteers. CONCLUSION: The incidence of sexual dysfunction during fluvoxamine therapy in healthy volunteers is 35%. This incidence is higher than previously reported and similar to that of other selective serotonin reuptake inhibitors.

Optimizing aminoglycoside therapy for nosocomial pneumonia caused by gram-negative bacteria.

Nosocomial pneumonia is a notable cause of morbidity and mortality and leads to increases in lengths of hospital stays and institutional expenditures. Aminoglycosides are used to treat patients with these infections, but few data on the doses and schedules required to achieve optimal therapeutic outcomes exist. We analyzed aminoglycoside treatment data for 78 patients with nosocomial pneumonia to determine if optimization of aminoglycoside pharmacodynamic parameters results in a more rapid therapeutic response (defined by outcome and days to leukocyte count resolution and temperature resolution). Cox proportional hazards, Classification and Regression Tree (CART), and logistic regression analyses were applied to the data. By all analyses, the first measured maximum concentration of drug in serum (Cmax)/MIC predicted days to temperature resolution and the second measured Cmax/MIC predicted days to leukocyte count resolution. For days to temperature resolution and leukocyte count resolution, CART analyses produced breakpoints, with an 89% success rate at 7 days of therapy for a Cmax/MIC of > 4.7 and an 86% success rate at 7 days of therapy for a Cmax/MIC of > 4.5, respectively. Logistic regression analyses predicted a 90% probability of temperature resolution and leukocyte count resolution by day 7 if a Cmax/MIC of > or = 10 is achieved within the first 48 h of aminoglycoside therapy. Aggressive aminoglycoside dosing immediately followed by individualized pharmacokinetic monitoring would ensure that Cmax/MIC targets are achieved early in therapy. This would increase the probability of a rapid therapeutic response for pneumonia caused by gram-negative bacteria and potentially decreasing durations of parenteral antibiotic therapy, lengths of hospitalization, and institutional expenditures, a situation in which both the patient and the institution benefit.

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.

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.