Joseph S. Bertino

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.

Pharmacogenetics affects dosing, efficacy, and toxicity of cytochrome P450-metabolized drugs.

Drug-metabolizing enzyme activity is one of many factors affecting patient response to medications. The objective of this review is to highlight the potential for genetic variability in cytochrome P450 enzyme activity that can lead to interperson differences in response to drugs. Awareness and application of this knowledge will improve drug use in clinical practice and provide the physician with further appreciation that standard drug dosing may not be appropriate in all patients.

Back to the Future: Using Aminoglycosides Again and How to Dose Them Optimally

Gram-negative organisms have become increasingly resistant to both beta-lactam antibiotics and fluoroquinolones. Consequently, aminoglycoside antibiotics have undergone a resurgence in use. Because of the known toxicities of aminoglycoside antibiotics, clinicians have avoided their use, unless no other alternatives were extant. Over the past 2 decades, we have learned much about the relationship between aminoglycoside exposure and the likelihood of a good clinical outcome or the occurrence of nephrotoxicity. For example, minimum inhibitory concentration values > or = 2.0 mg/L lead to unacceptably low probabilities of a good clinical outcome, and infrequent administration of doses (i.e., intervals of 24 h and longer intervals for patients with compromised renal function) plays a central role in minimizing the likelihood of toxicity. Using these new insights, we suggest ways of evaluating the dose and schedule of administration of aminoglycosides in empirical therapy to obtain the highest likelihood of an efficacious and nontoxic 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.

Combined phenotypic assessment of cytochrome p450 1A2, 2C9, 2C19, 2D6, and 3A, N-acetyltransferase-2, and xanthine oxidase activities with the "Cooperstown 5+1 cocktail".

Previously, we have validated a 4‐drug phenotyping cocktail, the “Cooperstown cocktail,” using caffeine (cytochrome P450 [CYP] 1A2, N‐acetyltransferase‐2 [NAT2], and xanthine oxidase [XO]), dextromethorphan (CYP2D6), omeprazole (CYP2C19), and intravenous midazolam (hepatic CYP3A). Data suggest that warfarin can be used as a safe and accurate biomarker for CYP2C9, and if warfarin is administered with vitamin K, the pharmacodynamic effect is ablated. Twelve subjects received the Cooperstown cocktail, warfarin plus vitamin K, and both sets of biomarkers (Cooperstown 5+1 cocktail) in a randomized crossover fashion. On the basis of log‐transformed data and a paired t test, no significant difference was seen for S‐warfarin area under the serum concentration–time curve from time 0 to infinity (P = .09), omeprazole metabolic ratio (P = .374), caffeine metabolic ratio (P = .169 for CYP1A2 activity), midazolam plasma clearance (P = .573), or dextromethorphan metabolic ratio (P = .747) with the Cooperstown cocktail, warfarin plus vitamin K alone, or the Cooperstown 5+1 cocktail. During drug administration, the only side effect was mild and short‐lived sedation after intravenous midazolam administration. Phenotypic measurements were in concordance with the subjects CYP2C9, CYP2C19, and CYP2D6 genotypes. The Cooperstown 5+1 cocktail may be used to simultaneously assess the activities of CYP1A2, CYP2C9, CYP2C19, CYP2D6, CYP3A, NAT2, and XO.

Quantification of 3‐month intraindividual variability and the influence of sex and menstrual cycle phase on CYP3A activity as measured by phenotyping with intravenous midazolam

Intraindividual variability and the effects of sex and menstrual cycle phase on CYP3A activity were evaluated by phenotyping with use of midazolam as the probe drug.

Aminoglycoside dosing weight correction factors for patients of various body sizes.

Prior investigations have suggested the use of a dosing weight correction factor of ideal body weight (IBW) plus 40% excess body weight (EBW, where EBW = total body weight [TBW] - IBW) to determine the weight to use for aminoglycoside dosing in morbidly obese (TBW/IBW ratio, > 2) patients. Little data are available to provide dosing information for underweight or moderately obese patients. We investigated aminoglycoside pharmacokinetics in 1,708 patients receiving gentamicin and tobramycin. Patients were stratified into underaverage-weight or overweight weight categories based on both TBW/IBW ratio and body mass index (weight/height2 ratio), which has been shown to correlate with physiologic estimates of body fat. Regression analyses revealed that the TBW/IBW ratio predicts the volume of distribution. Dosing weight correction factors to give equivalent predicted peak aminoglycoside concentrations with a 2-mg/kg loading dose are 1.13 times the TBW for underweight patients and 0.43 times the EBW plus IBW for overweight patients. There were no large differences between the dosing weight correction factors derived from IBW- and body mass index-based classification systems. These data generate useful aminoglycoside dosing weight equations for both underweight and overweight patients.

Comparative Pharmacokinetics and Pharmacodynamic Target Attainment of Ertapenem in Normal-Weight, Obese, and Extremely Obese Adults

ABSTRACT 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, ≥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 (P ≤ 0.05). Achieving a bacteriostatic target of fT>MIC of 20% with a 90% probability was attained at MICs of ≤0.5 μg/ml for normal-weight subjects. Class I-II and class III obese subjects were able to achieve this target only at a MIC of ≤0.25 μg/ml. For maximal bactericidal activity (fT>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 μg/ml.

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

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.

An evaluation of the suitability of intravenous midazolam as an in vivo marker for hepatic cytochrome P4503A activity

Clinical Pharmacology & Therapeutics (2003) 73, 153–158; doi: 10.1067/mcp.2003.23