Age and diet effects on drug action

Clinical and experimental research in antituberculosis drug-induced hepatotoxicity: a review

Drug-induced liver injury is the common adverse effect seen in patients receiving antituberculosis drugs (ATDs). There are several risk factors associated with the development of hepatotoxicity in such patients. Though there have been appreciable efforts taken by carrying out studies investigating the efficacy of several natural and synthetic compounds in minimising this effect, the only choice available for clinicians is withdrawal of drugs. This review would give a precise idea of ATD-induced hepatotoxicity, its underlying mechanisms and alternative therapies for the same.

Molecular mechanisms of the microsomal mixed function oxidases and biological and pathological implications

The cytochrome P450 mixed function oxidase enzymes play a major role in the metabolism of important endogenous substrates as well as in the biotransformation of xenobiotics. The liver P450 system is the most active in metabolism of exogenous substrates. This review briefly describes the liver P450 (CYP) mixed function oxidase system with respect to its enzymatic components and functions. Electron transfer by the NADPH-P450 oxidoreductase is required for reduction of the heme of P450, necessary for binding of molecular oxygen. Binding of substrates to P450 produce substrate binding spectra. The P450 catalytic cycle is complex and rate-limiting steps are not clear. Many types of chemical reactions can be catalyzed by P450 enzymes, making this family among the most diverse catalysts known. There are multiple forms of P450s arranged into families based on structural homology. The major drug metabolizing CYPs are discussed with respect to typical substrates, inducers and inhibitors and their polymorphic forms. The composition of CYPs in humans varies considerably among individuals because of sex and age differences, the influence of diet, liver disease, presence of potential inducers and/or inhibitors. Because of such factors and CYP polymorphisms, and overlapping drug specificity, there is a large variability in the content and composition of P450 enzymes among individuals. This can result in large variations in drug metabolism by humans and often can contribute to drug–drug interactions and adverse drug reactions. Because of many of the above factors, especially CYP polymorphisms, there has been much interest in personalized medicine especially with respect to which CYPs and which of their polymorphic forms are present in order to attempt to determine what drug therapy and what dosage would reflect the best therapeutic strategy in treating individual patients.

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The CYP P450 system is important in metabolism of endogenous substrates and drugs.

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About 150 forms of CYPs have been identified and they are grouped into families.

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CYPs catalyze a wide variety of reactions and are among the most diverse catalysts known.

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Electrons are passed to the CYP via NADPH+NADPH-cytochrome P450 reductase.

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Metabolism of certain compounds by CYPs generate reactive intermediates which are toxic.

A predictive model for exemestane pharmacokinetics/pharmacodynamics incorporating the effect of food and formulation

AIMS

Exemestane (Aromasin) is an irreversible aromatase inactivator used for the treatment of postmenopausal women with advanced breast cancer. The objective of this study was to evaluate the effect of formulation comparing a sugar-coated tablet (SCT) with a suspension and food on the pharmacokinetics (PK) and pharmacodynamics (PD) with respect to plasma estrone sulphate (E1S) concentrations of exemestane, using a PK/PD approach.

METHODS

This was an open, three-period, randomized, crossover study. Twelve healthy postmenopausal women received single oral doses of 25 mg exemestane as a SCT after fasting or food and as a suspension after fasting. Exemestane and E1S concentrations were determined before and up to 14 days after drug administration. Population analysis was performed in two steps: (i) a compartmental PK model was selected incorporating the effect of food and formulation; (ii) conditional on the PK model, a PD model was developed employing indirect response models. Model selection was performed using standard statistical tests. Validation and assessment of the predictive capability of the selected model was performed using real test data sets obtained from the literature.

RESULTS

A three-compartment model with first-order elimination rate best described exemestane disposition (k12 0.454, k21 0.158, k13 0.174, k31 0.016 and k 0.738 h(-1)). Absorption was described by a mono-exponential function [ka 2.3 (SCT after fasting), 1.1 (SCT after food) and 7.6 h(-1) (suspension); lag time 0.2 h]. The PD model assumed that E1S plasma concentrations are determined by a zero-order synthesis rate (6.5 pg ml(-1) h(-1)) and a first-order elimination constant (0.032 h(-1)). Exemestane inhibited E1S synthesis with a C50 value of 22.1 pg ml(-1). The mean population estimates were used to simulate the administration of different doses of the drug (0.5, 1, 2.5, 5 and 25 mg day(-1)). The model predictions were in agreement with historical data.

CONCLUSIONS

Exemestane absorption is influenced by the formulation of the drug and by food, but its disposition is independent of both. PK differences do no translate into clinically important differences in the PD. The PK/PD model developed was able to predict successfully the response to different doses and administration schedules with respect to oestrogen suppression.

Pharmacokinetics and pharmacodynamics of FK143, a nonsteroidal inhibitor of steroid 5 alpha-reductase, in healthy volunteers

The pharmacokinetics and pharmacodynamics of FK143, a new nonsteroidal inhibitor of steroid 5 alpha-reductase, were investigated in healthy volunteers, with use of plasma FK143 concentrations and serum dihydrotestosterone levels as an index for pharmacologic effects. The area under the plasma concentration-time curve from zero to infinity [AUC(0-infinity)] and maximum plasma concentration [Cmax] were increased dose proportionally after oral administration (100 to 500 mg) while subjects were in the fed state. The AUC(0-infinity) and Cmax after 500 mg oral administration during fed conditions were significantly larger than those during the fasted state, suggesting an increase of the absorption of FK143. Dihydrotestosterone concentrations after a single administration of FK143 (100 to 500 mg) during fed conditions decreased to about 65% of predose values and thereafter slowly recovered to the same levels as predose values at 168 hours. A combined pharmacokinetic-pharmacodynamic model was constructed with use of changes in dihydrotestosterone concentrations. The pharmacokinetic-pharmacodynamic profiles of FK143 after repeated administration were predictable with use of the pharmacokinetic-pharmacodynamic parameters obtained after a single administration of FK143.

The influence of food on the absorption and metabolism of drugs: an update

Food-drug interactions can lead to a loss of therapeutic efficacy or toxic effects of drug therapy. Generally, the effect of food on drugs results in a reduction in the drug's bioavailability; however, food can also alter drug clearance. The benefits of considering metabolism and pharmacokinetic information in the drug discovery process have been highlighted by Humphrey and Smith (79) and the process of rational drug design should include considerations of the chemistry, pharmacology and pharmacokinetics of the drug (80) and the impact of diet on these parameters.

Effect of food on the relative bioavailability of a hypolipidemic agent (CGP 43371) in healthy subjects

The relative bioavailability of a capsule formulation and the effects of food on the pharmacokinetics of a hypolipidemic agent (CGP 43371) in 12 healthy subjects were examined. Each subject randomly received a single dose of 800 mg of CGP 43371, either as a dispersion formulation under fasting conditions or as a capsule formulation under fasting and fed conditions in a three-way crossover design with a washout period of 2 weeks between each treatment. Serial blood samples were collected at frequent intervals up to 96 h after each treatment. The concentrations of CGP 43371 in plasma were determined by a normal-phase HPLC method. Similar mean pharmacokinetic data (peak plasma drug concentration, 0.16 versus 0.18 micrograms/mL; area under the plasma drug concentration-time curve from time zero to infinity, 4.56 versus 4.22 micrograms.h/mL; time to the peak plasma drug concentration, 10.3 versus 10.2 h; lag time, 3.7 versus 3.8 h; and terminal elimination half-life, 17.8 versus 15.0 h) for the dispersion and capsule formulations under fasting conditions indicated that both formulations were bioequivalent with respect to the rate and extent of absorption. In contrast, the mean peak plasma drug concentration (2.01 micrograms/mL) and area under the curve from time zero to infinity (57.35 micrograms.h/mL) for the capsule formulation with food were enhanced approximately 11- and 14-fold, respectively, when compared with that without food. The corresponding mean lag time (2.1 h) was decreased approximately 50%. These differences in pharmacokinetic parameters were statistically significant, on the basis of an analysis of variance.(ABSTRACT TRUNCATED AT 250 WORDS)

The influence of food on the absorption and metabolism of drugs

Food-drug interactions can lead to a loss of therapeutic efficacy or toxic effects of drug therapy. Generally, the effect of food on drugs results from a reduction in the drug's bioavailability; however, an alteration in drug clearance can occur due to the effect of certain foods on drug metabolism. The proportion of adverse drug reactions due to food-drug interactions is not known and unfortunately only when a serious adverse drug reaction follows a food-drug interaction does the matter usually receive any significant attention. In order to improve therapeutic efficacy and to help prevent adverse drug reactions, it is necessary that clinicians be knowledgeable of the important food-drug incompatibilities and risk factors related to the increased likelihood of developing an adverse drug reaction due to food-drug interactions.