Effects of oral contraceptive steroids on acetaminophen metabolism and elimination

Pharmacological interactions and menopausal hormone therapy: a review

IMPORTANCE AND OBJECTIVE

Menopausal hormone therapy (HT) is widely used, and there are several statements of international scientific societies to guide prescribers; however, a summary of existing literature about possible drug interactions with HT does not exist, although many midlife women take medications for other conditions. Therefore, our objective was to create a document that presents and synthesizes the most relevant interactions. The impact of the interaction itself and the number of candidates for HT who are likely to use other treatments are considered based on the best available evidence.

METHODS

A systematic review was performed to determine the best evidence of interaction effects on relevant outcomes of interest for decision making. A working framework was developed to formulate explicit and reasoned recommendations according to four predefined categories for coadministration: (1) can be used without expected risks, (2) acceptable use (no evidence of negative interaction), (3) alternative treatment should be considered, and (4) nonuse without express justification. The project protocol was registered in the Open Science Framework platform (doi: 10.17605/OSF.IO/J6WBC) and in PROSPERO (registration number CRD42020166658).

RESULTS

Studies targeting our objective are scarce, but 23 pharmacological groups were assigned to one of the predefined categories of recommendation for concomitant use of HT. Vaginal HT was assigned to category 1 for 21 of the analyzed pharmacological groups. For oral and transdermal HT (estrogen-only or combined) and tibolone, there were 12 pharmacological groups assigned to category 1, 12 to category 2, 5 to category 3, and 4 to category 4. Results are shown in crossed-tables that are useful for counseling and prescription.

DISCUSSION AND CONCLUSIONS

Available evidence of HT interactions with other drugs is scarce and mainly indirect. It comes from biological plausibility, knowledge of extensive concomitant use without reported incidents, and/or extrapolation from hormonal contraception, but there are pharmacological groups in all categories showing that information is useful. These eligibility criteria summarize it and can help in the decision process of HT coadministration with other drugs. Decisions should be taken based on these recommendations but also individualized risk/benefit evaluation, according to underlying pathology, patient's clinical requirements, and the existence or nonexistence of alternatives.

Drug-drug interactions that alter the exposure of glucuronidated drugs: Scope, UDP-glucuronosyltransferase (UGT) enzyme selectivity, mechanisms (inhibition and induction), and clinical significance

Drug-drug interactions (DDIs) arising from the perturbation of drug metabolising enzyme activities represent both a clinical problem and a potential economic loss for the pharmaceutical industry. DDIs involving glucuronidated drugs have historically attracted little attention and there is a perception that interactions are of minor clinical relevance. This review critically examines the scope and aetiology of DDIs that result in altered exposure of glucuronidated drugs. Interaction mechanisms, namely inhibition and induction of UDP-glucuronosyltransferase (UGT) enzymes and the potential interplay with drug transporters, are reviewed in detail, as is the clinical significance of known DDIs. Altered victim drug exposure arising from modulation of UGT enzyme activities is relatively common and, notably, the incidence and importance of UGT induction as a DDI mechanism is greater than generally believed. Numerous DDIs are clinically relevant, resulting in either loss of efficacy or an increased risk of adverse effects, necessitating dose individualisation. Several generalisations relating to the likelihood of DDIs can be drawn from the known substrate and inhibitor selectivities of UGT enzymes, highlighting the importance of comprehensive reaction phenotyping studies at an early stage of drug development. Further, rigorous assessment of the DDI liability of new chemical entities that undergo glucuronidation to a significant extent has been recommended recently by regulatory guidance. Although evidence-based approaches exist for the in vitro characterisation of UGT enzyme inhibition and induction, the availability of drugs considered appropriate for use as 'probe' substrates in clinical DDI studies is limited and this should be research priority.

Eligibility criteria for Menopausal Hormone Therapy (MHT): a position statement from a consortium of scientific societies for the use of MHT in women with medical conditions. MHT Eligibility Criteria Group

This project aims to develop eligibility criteria for menopausal hormone therapy (MHT). The tool should be similar to those already established for contraception A consortium of scientific societies coordinated by the Spanish Menopause Society met to formulate recommendations for the use of MHT by women with medical conditions based on the best available evidence. The project was developed in two phases. As a first step, we conducted 14 systematic reviews and 32 metanalyses on the safety of MHT (in nine areas: age, time of menopause onset, treatment duration, women with thrombotic risk, women with a personal history of cardiovascular disease, women with metabolic syndrome, women with gastrointestinal diseases, survivors of breast cancer or of other cancers, and women who smoke) and on the most relevant pharmacological interactions with MHT. These systematic reviews and metanalyses helped inform a structured process in which a panel of experts defined the eligibility criteria according to a specific framework, which facilitated the discussion and development process. To unify the proposal, the following eligibility criteria have been defined in accordance with the WHO international nomenclature for the different alternatives for MHT (category 1, no restriction on the use of MHT; category 2, the benefits outweigh the risks; category 3, the risks generally outweigh the benefits; category 4, MHT should not be used). Quality was classified as high, moderate, low or very low, based on several factors (including risk of bias, inaccuracy, inconsistency, lack of directionality and publication bias). When no direct evidence was identified, but plausibility, clinical experience or indirect evidence were available, "Expert opinion" was categorized. For the first time, a set of eligibility criteria, based on clinical evidence and developed according to the most rigorous methodological tools, has been defined. This will provide health professionals with a powerful decision-making tool that can be used to manage menopausal symptoms.

Modelling Tools to Characterize Acetaminophen Pharmacokinetics in the Pregnant Population

This review describes acetaminophen pharmacokinetics (PK) throughout pregnancy, as analyzed by three methods (non-compartmental analyses (NCA), population PK, and physiologically based PK (PBPK) modelling). Eighteen studies using NCA were reported in the scientific literature. These studies reported an increase in the volume of distribution (3.5-60.7%) and an increase in the clearance (36.8-84.4%) of acetaminophen in pregnant women compared to non-pregnant women. Only two studies using population PK modelling as a technique were available in the literature. The largest difference in acetaminophen clearance (203%) was observed in women at delivery compared to non-pregnant women. One study using the PBPK technique was found in the literature. This study focused on the formation of metabolites, and the toxic metabolite N-acetyl-p-benzoquinone imine was the highest in the first trimester, followed by the second and third trimester, compared with non-pregnant women. In conclusion, this review gave an overview on acetaminophen PK changes in pregnancy. Also, knowledge gaps, such as fetal and placenta PK parameters, have been identified, which should be explored further before dosing adjustments can be suggested on an evidence-based basis.

Physiologically Based Pharmacokinetic Modeling to Characterize Acetaminophen Pharmacokinetics and N-Acetyl-p-Benzoquinone Imine (NAPQI) Formation in Non-Pregnant and Pregnant Women

Background and Objective

Little is known about acetaminophen (paracetamol) pharmacokinetics during pregnancy. The aim of this study was to develop a physiologically based pharmacokinetic (PBPK) model to predict acetaminophen pharmacokinetics throughout pregnancy.

Methods

PBPK models for acetaminophen and its metabolites were developed in non-pregnant and pregnant women. Physiological and enzymatic changes in pregnant women expected to impact acetaminophen pharmacokinetics were considered. Models were evaluated using goodness-of-fit plots and by comparing predicted pharmacokinetic profiles with in vivo pharmacokinetic data. Predictions were performed to illustrate the average concentration at steady state (C_ss,avg) values, used as an indicator for efficacy, of acetaminophen achieved following administration of 1000 mg every 6 h. Furthermore, as a measurement of potential hepatotoxicity, the molar dose fraction of acetaminophen converted to N-acetyl-p-benzoquinone imine (NAPQI) was estimated.

Results

PBPK models successfully predicted the pharmacokinetics of acetaminophen and its metabolites in non-pregnant and pregnant women. Predictions resulted in the lowest C_ss,avg in the third trimester (median [interquartile range]: 4.5 [3.8–5.1] mg/L), while C_ss,avg was 6.7 [5.9–7.4], 5.6 [4.7–6.3], and 4.9 [4.1–5.5] mg/L in non-pregnant, first trimester, and second trimester populations, respectively. Assuming a constant raised cytochrome P450 2E1 activity throughout pregnancy, the molar dose fraction of acetaminophen converted to NAPQI was highest during the first trimester (median [interquartile range]: 11.0% [9.1–13.4%]), followed by the second (9.0% [7.5–11.0%]) and third trimester (8.2% [6.8–10.1%]), compared with non-pregnant women (7.7% [6.4–9.4%]).

Conclusion

Acetaminophen exposure is lower in pregnant than in non-pregnant women, and is related to pregnancy duration. Despite these findings, higher dose adjustments cannot be advised yet as it is unknown whether pregnancy affects the toxicodynamics of NAPQI. Information on glutathione abundance during pregnancy and NAPQI in vivo data are required to further refine the presented model.

Electronic supplementary material

The online version of this article (10.1007/s40262-019-00799-5) contains supplementary material, which is available to authorized users.

Integration of Placental Transfer in a Fetal-Maternal Physiologically Based Pharmacokinetic Model to Characterize Acetaminophen Exposure and Metabolic Clearance in the Fetus

BACKGROUND AND OBJECTIVE

Although acetaminophen is frequently used during pregnancy, little is known about fetal acetaminophen pharmacokinetics. Acetaminophen safety evaluation has typically focused on hepatotoxicity, while other events (fetal ductal closure/constriction) are also relevant. We aimed to develop a fetal-maternal physiologically based pharmacokinetic (PBPK) model (f-m PBPK) to quantitatively predict placental acetaminophen transfer, characterize fetal acetaminophen exposure, and quantify the contributions of specific clearance pathways in the term fetus.

METHODS

An acetaminophen pregnancy PBPK model was extended with a compartment representing the fetal liver, which included maturation of relevant enzymes. Different approaches to describe placental transfer were evaluated (ex vivo cotyledon perfusion experiments, placental transfer prediction based on Caco-2 cell permeability or physicochemical properties [MoBi^®]). Predicted maternal and fetal acetaminophen profiles were compared with in vivo observations.

RESULTS

Tested approaches to predict placental transfer showed comparable performance, although the ex vivo approach showed highest prediction accuracy. Acetaminophen exposure in maternal venous blood was similar to fetal venous umbilical cord blood. Prediction of fetal acetaminophen clearance indicated that the median molar dose fraction converted to acetaminophen-sulphate and N-acetyl-p-benzoquinone imine was 0.8% and 0.06%, respectively. The predicted mean acetaminophen concentration in the arterial umbilical cord blood was 3.6 mg/L.

CONCLUSION

The median dose fraction of acetaminophen converted to its metabolites in the term fetus was predicted. The various placental transfer approaches supported the development of a generic f-m PBPK model incorporating in vivo placental drug transfer. The predicted arterial umbilical cord acetaminophen concentration was far below the suggested postnatal threshold (24.47 mg/L) for ductal closure.

Acetaminophen pharmacokinetics in severely obese adolescents and young adults

BACKGROUND

Intravenous acetaminophen is commonly administered as an adjunctive to opioids during major surgical procedures, but neither the correct pharmacokinetic size descriptor nor the dose is certain in severely obese adolescents undergoing bariatric surgery.

METHODS

Adolescents, 14-20 years of age, with a body mass index (BMI) ≥95th percentile for age and sex or BMI ≥40 kg·m^-2 , presenting for laparoscopic or robotic assisted or vertical sleeve gastrectomy were administered intravenous acetaminophen (1000 mg) following completion of the surgical procedure. Venous blood was drawn for acetaminophen assay at eight time points, starting 15 minutes after completion of the infusion and up to 12 hours afterward. Time-concentration data profiles were analyzed using nonlinear mixed effects models. Parameter estimates were scaled to a 70-kg person using allometry. Normal fat mass was used to assess the impact of obesity on pharmacokinetic parameters.

RESULTS

The study cohort comprised 11 female patients, age 17 SD 2 years with a weight of 125 SD 19 kg and a mean BMI of 46 SD 5 kg·m^-2 . The plasma acetaminophen serum concentration was 17 (SD 4) μg·mL^-1 at 10-20 minutes after completion of the infusion and 5 (SD 6) μg·mL^-1 at 80-100 minutes. A two-compartment model, used to investigate pharmacokinetics, estimated clearance 10.6 (CV 72%) L·h·70 kg^-1 , intercompartment clearance 37.3 (CV 63%) L·h·70 kg^-1 , central volume of distribution 20.4 (CV 46%) L·70 kg^-1 , and peripheral volume of distribution 16.8 (CV 42%) L·70 kg^-1 . Clearance was best described using total body weight. Normal fat mass with a parameter that accounts for fat mass contribution (Ffat) of 0.88 best described volumes.

CONCLUSION

Current recommendations of acetaminophen to a maximum dose of 1000 mg resulted in serum concentrations below detection limits in all patients within 2 hours after administration. Dose is better predicted using total body mass with allometric scaling.

Covariates of intravenous paracetamol pharmacokinetics in adults

BACKGROUND

Pharmacokinetic estimates for intravenous paracetamol in individual adult cohorts are different to a certain extent, and understanding the covariates of these differences may guide dose individualization. In order to assess covariate effects of intravenous paracetamol disposition in adults, pharmacokinetic data on discrete studies were pooled.

METHODS

This pooled analysis was based on 7 studies, resulting in 2755 time-concentration observations in 189 adults (mean age 46 SD 23 years; weight 73 SD 13 kg) given intravenous paracetamol. The effects of size, age, pregnancy and other clinical settings (intensive care, high dependency, orthopaedic or abdominal surgery) on clearance and volume of distribution were explored using non-linear mixed effects models.

RESULTS

Paracetamol disposition was best described using normal fat mass (NFM) with allometric scaling as a size descriptor. A three-compartment linear disposition model revealed that the population parameter estimates (between subject variability,%) were central volume (V1) 24.6 (55.5%) L/70 kg with peripheral volumes of distribution V2 23.1 (49.6%) L/70 kg and V3 30.6 (78.9%) L/70 kg. Clearance (CL) was 16.7 (24.6%) L/h/70 kg and inter-compartment clearances were Q2 67.3 (25.7%) L/h/70 kg and Q3 2.04 (71.3%) L/h/70 kg. Clearance and V2 decreased only slightly with age. Sex differences in clearance were minor and of no significance. Clearance, relative to median values, was increased during pregnancy (F(PREG) = 1.14) and decreased during abdominal surgery (F(ABDCL) = 0.715). Patients undergoing orthopaedic surgery had a reduced V2 (F(ORTHOV) = 0.649), while those in intensive care had increased V2 (F(ICV) = 1.51).

CONCLUSIONS

Size and age are important covariates for paracetamol pharmacokinetics explaining approximately 40% of clearance and V2 variability. Dose individualization in adult subpopulations would achieve little benefit in the scenarios explored.

High accuracy in silico sulfotransferase models

Predicting enzymatic behavior in silico is an integral part of our efforts to understand biology. Hundreds of millions of compounds lie in targeted in silico libraries waiting for their metabolic potential to be discovered. In silico "enzymes" capable of accurately determining whether compounds can inhibit or react is often the missing piece in this endeavor. This problem has now been solved for the cytosolic sulfotransferases (SULTs). SULTs regulate the bioactivities of thousands of compounds--endogenous metabolites, drugs and other xenobiotics--by transferring the sulfuryl moiety (SO3) from 3'-phosphoadenosine 5'-phosphosulfate to the hydroxyls and primary amines of these acceptors. SULT1A1 and 2A1 catalyze the majority of sulfation that occurs during human Phase II metabolism. Here, recent insights into the structure and dynamics of SULT binding and reactivity are incorporated into in silico models of 1A1 and 2A1 that are used to identify substrates and inhibitors in a structurally diverse set of 1,455 high value compounds: the FDA-approved small molecule drugs. The SULT1A1 models predict 76 substrates. Of these, 53 were known substrates. Of the remaining 23, 21 were tested, and all were sulfated. The SULT2A1 models predict 22 substrates, 14 of which are known substrates. Of the remaining 8, 4 were tested, and all are substrates. The models proved to be 100% accurate in identifying substrates and made no false predictions at Kd thresholds of 100 μM. In total, 23 "new" drug substrates were identified, and new linkages to drug inhibitors are predicted. It now appears to be possible to accurately predict Phase II sulfonation in silico.

Gender-dependent effects of gonadectomy on lung carcinogenesis by 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) in female and male A/J mice

The present study was conducted to investigate the effects of gonadectomy on lung carcinogenesis in female and male mice, and to determine an association between sex hormone and lung carcinogenesis. Female and male A/J mice were divided into gonadectomized and unoperated control groups and all animals were treated intraperitoneally with 1 or 2 injections of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) at the dose of 2 mg/mouse. The mice were sacrificed 18 or 56 weeks after surgery. Serum levels of estradiol in females and testosterone in males were confirmed to be decreased by gonadectomy. Lung white nodules were detected in all mice of all groups. In the control groups of 18- and 56-week studies, the multiplicities of lung nodules in females were significantly greater than in males. In males in the 56-week study, the multiplicity of macroscopical lung nodules, bronchiolo-alveolar hyperplasias, adenomas and tumors (adenomas and adenocarcinomas) showed significant increase with castration. In females in the 18-week study, the multiplicity of adenomas decreased significantly by ovariectomy. Based on the results of the present study, female A/J mice were confirmed to be more susceptible to NNK-induced lung carcinogenesis than males. Furthermore, it was suggested that the process is inhibited by testosterone and accelerated by estradiol. These findings indicate the possibility that sex hormones play important roles in determining sex differences in lung carcinogenesis in the A/J mice initiated by NNK.

Metabolism and disposition of acetaminophen: recent advances in relation to hepatotoxicity and diagnosis

Acetaminophen (APAP) is one of the most widely used drugs. Though safe at therapeutic doses, overdose causes mitochondrial dysfunction and centrilobular necrosis in the liver. The first studies of APAP metabolism and activation were published more than 40 years ago. Most of the drug is eliminated by glucuronidation and sulfation. These reactions are catalyzed by UDP-glucuronosyltransferases (UGT1A1 and 1A6) and sulfotransferases (SULT1A1, 1A3/4, and 1E1), respectively. However, some is converted by CYP2E1 and other cytochrome P450 enzymes to a reactive intermediate that can bind to sulfhydryl groups. The metabolite can deplete liver glutathione (GSH) and modify cellular proteins. GSH binding occurs spontaneously, but may also involve GSH-S-transferases. Protein binding leads to oxidative stress and mitochondrial damage. The glucuronide, sulfate, and GSH conjugates are excreted by transporters in the canalicular (Mrp2 and Bcrp) and basolateral (Mrp3 and Mrp4) hepatocyte membranes. Conditions that interfere with metabolism and metabolic activation can alter the hepatotoxicity of the drug. Recent data providing novel insights into these processes, particularly in humans, are reviewed in the context of earlier work, and the effects of altered metabolism and reactive metabolite formation are discussed. Recent advances in the diagnostic use of serum adducts are covered.

Adverse events caused by drug interactions involving glucuronoconjugates of zidovudine, valproic acid and lamotrigine, and analysis of how such potential events are discussed in package inserts of Japan, UK and USA

BACKGROUND AND OBJECTIVE

As pharmacokinetic drug interactions frequently cause adverse events, it is important that the relevant information is given in package inserts (PIs). We previously analysed the provision of PIs for HMG-CoA reductase inhibitors and Ca antagonists, for which metabolism by cytochrome P450 could be a major interaction mechanism. In this article, we focus on interactions involving glucuronoconjugates because many drugs and their metabolites undergo this conjugation.

METHODS

We reviewed clinical drug interactions related to glucuronoconjugates, focusing on reports of adverse events. Then, we picked out three important drugs (zidovudine, valproic acid and lamotrigine), and examined how the literature information is reflected in the relevant PIs in Japan, UK and USA.

RESULTS AND DISCUSSION

Pharmacokinetic interactions related to glucuronoconjugates were found with 33 drug combinations. Of these, five combinations induced clear adverse events: (i) severe anaemia due to zidovudine and caused by interaction with valproic acid, (ii) recurrence/increased frequency of seizure or increased manic states from a reduction in therapeutic effects of valproic acid caused by panipenem, (iii) meropenem or (iv) ritonavir and (v) of lamotrigine caused by oral contraceptives. Analysis of PIs showed a lack of description of the interaction of zidovudine with valproic acid in the Japanese PI. The UK PI mentioned this interaction without quantitative data, whereas full information was given in the US PI. A lack of description was also present on the interaction between valproic acid with ritonavir, reported in 2006, in the PIs of all three countries. For the interactions involving valproic acid and panipenem or meropenem, even though marked reduction of blood valproic acid level has been reported, no quantitative data were provided in any of the PIs.

CONCLUSION

Five combinations were identified to cause severe adverse events because of interactions related to glucuronoconjugates. This information, including quantitative data, is not always properly provided in the relevant PIs in Japan, UK or USA. PIs should be improved to better inform healthcare providers and thereby help them and their patients.

Pediatric intravenous paracetamol (propacetamol) pharmacokinetics: a population analysis

BACKGROUND

The aim of this study was to describe propacetamol pharmacokinetics in children in order to predict concentrations after a standard dosing regimen of propacetamol 30 mg x kg(-1) (15 mg x kg(-1) paracetamol) 6 h.

METHODS

A population pharmacokinetic analysis of paracetamol time-concentration profiles (846 observations) from 144 children [postconception age (PCA) 27 weeks-14 years] was undertaken using nonlinear mixed effects models (NONMEM). These data were taken from seven separate studies involving children given intravenous propacetamol. Time-concentration profiles (503 observations) from a further 86 children (PCA: 37 weeks-14 years) given paracetamol elixir orally were included in the analysis to assess relative bioavailability of intravenous propacetamol.

RESULTS

A three-compartment (depot, central and peripheral) linear disposition model fitted data better than a two-compartment (depot and central) model. Population parameter estimates (between subject variability, %) were central volume (V2/F(oral)) 24 (55%) l x 70 kg(-1), peripheral volume of distribution (V3/F(oral)) 30 (32%) l x 70 kg(-1), clearance (CL/F(oral)) 16 (40%) l x h(-1) x 70 kg(-1) and intercompartment clearance (Q/F(oral)) 55 (116%) l x h(-1) x 70 kg(-1). Clearance increased from 27 weeks PCA (1.87 l x h(-1) 70 kg(-1)) to reach 84% of the mature value by 1 year of age (standardized to a 70 kg person using allometric '1/4 power' models). Peripheral volume of distribution decreased from 27 weeks PCA (45.0 l x 70 kg(-1)) to reach 110% of its mature value by 6 months of age. Central volume of distribution and intercompartment clearance did not change with age. Between occasions variability for the peripheral volume of distribution (V3/F(oral)) and clearance (CL/F(oral)) were 18.5 and 19.3%, respectively. A rate constant representing hydrolysis of propacetamol to paracetamol (K(a) 96 h(-1)) was size related, but not age related. The relative bioavailability of intravenous propacetamol compared with an oral elixir was 0.5.

CONCLUSIONS

A mean paracetamol serum concentration of 10 mg x l(-1) is achieved in children 2-15 years given a standard dose of propacetamol 30 mg x kg(-1) 6 h. This concentration in the effect compartment is associated with a pain reduction of 2.6/10 after tonsillectomy and provides satisfactory analgesia for mild to moderate pain. Clearance is reduced in children less than 1 year of age and the target concentration of 10 mg x l(-1) may be achieved by scaling this standard dose regimen using predicted clearance in this younger age group.

AN ASSESSMENT OF UDP-GLUCURONOSYLTRANSFERASE INDUCTION USING PRIMARY HUMAN HEPATOCYTES

Uridine diphosphate glucuronosyltransferases (UGTs) catalyze the glucuronidation of a wide range of xenobiotics and endogenous substrates. However, there is a lack of information concerning the response of human UGTs to inducers, and this observation prompted the current investigation. The glucuronidation of estradiol (3- and 17-positions), naphthol, propofol, and morphine (3- and 6-positions) was assessed against a battery of recombinant human UGTs to determine selective glucuronidation reactions for induction studies. The potential induction of the glucuronidation of estradiol at the 3-position, naphthol, propofol, and morphine at the 3-position was subsequently investigated in cultured primary human hepatocytes against a range of prototypic inducers including dexamethasone, 3-methylcholanthrene (3-MC), phenobarbital, rifampicin, and omeprazole. Treatment with 3-MC induced estradiol-3-glucuronidation (up to 2.5-fold) in four of five donors investigated. Statistically significant increases in naphthol glucuronidation (up to 1.7-fold) were observed following treatment with carbamazepine. UGT1A9-mediated propofol glucuronidation was induced by phenobarbital (up to 2.2-fold) and rifampicin (up to 1.7-fold). However, treatment with alpha-naphthoflavone and tangeretin resulted in a decrease in propofol glucuronidation (30% of control values). Statistically significant induction of morphine-3-glucuronidation was observed in at least three donors following treatment with phenobarbital, rifampicin, and carbamazepine. Each UGT isoform investigated displayed a distinct induction profile. Although statistically significant increases in glucuronidation were observed for each reaction studied, the level of induction was less than that observed for CYP1A2 or CYP3A4 and exhibited a large interdonor variability. The clinical relevance of the induction responses obtained in this study is unclear.

Paracetamol and metabolite pharmacokinetics in infants

BACKGROUND

Data concerning metabolism of paracetamol in infants are scant. Previous studies have examined urinary metabolite recovery rates after a single dose of paracetamol in either neonates (<6 weeks) or children (3-9 years). There are no studies investigating infants.

METHODS

Infants ( n=47) undergoing major craniofacial surgery were given paracetamol 19-45 mg/kg 6-, 8-, or 12-hourly as either elixir or suppository formulation for postoperative analgesia, after a loading dose of 33-59 mg/kg rectally during the operation. Serum was assayed for paracetamol concentration in 40 of these infants at 5, 8, 11, 14, 17 and 20 h postoperatively. Urine samples were collected every 3 h for 24 h in 15 of these infants. The clearances of paracetamol to glucuronide and sulphate metabolites as well as the urinary clearance of unmetabolised paracetamol were estimated using non-linear, mixed-effects models.

RESULTS

Mean (+/-SD) age and weight of the patients were 11.8+/-2.5 months and 9.1+/-1.9 kg. Clearances of paracetamol to paracetamol-glucuronide (%CV) and to paracetamol-sulphate were 6.6 (11.5) l/h and 7.5 (11.5) l/h respectively, standardised to a 70-kg person using allometric "1/4 power" models. Glucuronide formation clearance, but not sulphate formation, was related to age and increased with age from a predicted value in a neonate of 2.73 l/h/70 kg to a mature value of 6.6 l/h/70 kg with a maturation half-life of 8.09 months. Urine clearance of paracetamol-glucuronide, paracetamol-sulphate and unchanged paracetamol (%CV) were, respectively, 2.65, 3.03 and 0.55 (28) l/h/70 kg. The urine clearance of unchanged paracetamol and metabolites was related to urine volume flow rate. Clearance attributable to pathways other than these measured in urine was not identifiable. The glucuronide/sulphate formation clearance ratio was 0.69 at 12 months of age. Sulphate metabolism contributed 50% towards paracetamol clearance.

CONCLUSION

Glucuronide formation clearance increases with age in the infant age range but sulphate formation does not. Renal clearance of paracetamol and its metabolites increases with urine flow rate. This and other studies show that paracetamol metabolism to glucuronide appears to be similar in infants and children, but in adults is increased in comparison with children. Oxidative pathways were undetectable in this infant study and may explain, in part, the reduced incidence of hepatotoxicity in infants.

Concomitant overdosing of other drugs in patients with paracetamol poisoning

AIMS

Paracetamol is frequently involved in intended self-poisoning, and concomitant overdosing of other drugs is commonly reported. The purpose of the study was to investigate further concomitant drug overdose in patients with paracetamol poisoning and to evaluate its effects on the outcome of the paracetamol intoxication.

METHODS

Six hundred and seventy-one consecutive patients admitted with paracetamol poisoning were studied and concomitant drug intake was recorded. The relative risk of hepatic encephalopathy, death or liver transplantation, hepatic dysfunction, liver cell damage, and renal dysfunction associated with concomitant overdosing of other drugs was evaluated by multivariate analysis.

RESULTS

Concomitant drug overdose was found in 207 patients (31%, 95% confidence interval [CI] 27, 34%). Concomitant overdosing of benzodiazepines (99 cases), opioid analgesics (38 cases), acetylsalicylic acid (33 cases), and NSAID (32 cases) predominated. Concomitant benzodiazepine overdose was an independent risk factor in the development of hepatic encephalopathy (odds ratio [OR] 1.91; CI 1.00, 3.65) and renal dysfunction (OR 1.81; CI 1.00, 3.22). Concomitant overdosing of opioid analgesics was a protective factor in the development of hepatic encephalopathy (OR 0.26; CI 0.07, 0.96). Concomitant acetylsalicylic acid overdose was a risk factor in the development of hepatic encephalopathy (OR 4.87; CI 1.52, 15.7) and death or liver transplantation (OR 6.04; CI 1.69, 21.6). A tendency towards a more favourable outcome was observed in patients with concomitant NSAID overdose.

CONCLUSIONS

Concomitant overdosing of benzodiazepines or analgesics is frequent in patients admitted with paracetamol poisoning. Concomitant benzodiazepine or acetylsalicylic acid overdose was associated with more severe toxicity, whereas concomitant overdosing of opioid analgesics was associated with less toxicity.

Influence of oral contraceptive use and cigarette smoking, alone and together, on antipyrine pharmacokinetics

The pharmacokinetics of antipyrine following a single 1-g intravenous dose was determined in 63 healthy women. Subjects were divided into 4 groups as follows: 1) cigarette smokers using low-dose oral contraceptives (n = 15); 2) nonsmokers using low-dose oral contraceptives (n = 12); 3) cigarette smokers not using oral contraceptives (n = 10); and 4) controls, neither cigarette smokers nor oral contraceptive users. Plasma antipyrine concentrations during 24 to 48 hours after dosage were measured by high-performance liquid chromatography. Mean kinetic variables in the nonsmoking, non-oral contraceptive using control group were: volume of distribution, 37.7 L; elimination half-life, 13.2 hours; and clearance, 34.4 mL/min. In cigarette smoking, non-oral contraceptive users versus controls, elimination half-life was reduced (8.0 vs. 13.2 hours, P < 0.05) and clearance increased (56.0 vs. 34.4 mL/min, P < 0.05). In nonsmoker oral contraceptive users, the reverse was true (elimination half-life was significantly increased: 16.6 vs. 13.2 hours, P < 0.05; and clearance was significantly decreased: 24.8 vs. 34.4 mL/min, P < 0.05). In smokers who were using oral contraceptives, values were not significantly different from controls (elimination half-life, 11.2 hours; clearance, 39.5 mL/min). Volume of distribution did not differ among the four groups. Thus the opposing effects on antipyrine clearance of the induction of metabolism by cigarette smoking and the inhibition due to low dose oral contraceptive use in effect negate each other when combined in humans.

Oral contraceptive effects on methylprednisolone pharmacokinetics and pharmacodynamics

OBJECTIVE

Oral contraceptive (OC) steroids alter the disposition of numerous drugs, including corticosteroids. We investigated the pharmacokinetics and pharmacodynamics of methylprednisolone.

METHODS

Twelve women (six women used OC steroids and six women did not) received intravenous methylprednisolone (0.6 mg/kg ideal body weight). Methylprednisolone disposition was assessed from plasma concentrations. Pharmacodynamic parameters measured were plasma cortisol, whole blood histamine (reflecting basophils), and blood helper T lymphocytes.

RESULTS

Methylprednisolone clearance was significantly decreased in the women who used OC steroids (0.298 versus 0.447 L/hr/kg), resulting in a longer elimination half-life (2.20 versus 1.72 hours). With use of indirect response models, significant differences were observed with the cortisol and basophil responses. A larger value for the concentration that inhibits the zero-order production rate by 50% (0.37 versus 0.11 ng/ml) was observed in the women who used OC steroids for suppression of cortisol secretion, indicating less sensitivity to the suppressive effects of methylprednisolone. Greater net suppression of basophils was observed in the users of OC steroids (area under the response curve, 694 versus 401 ng x hr/ml). No differences were observed for helper T-cell responses.

CONCLUSION

OC steroids appear to inhibit methylprednisolone metabolism. However, mixed changes in several responses occur, indicating that women can probably receive similar doses of methylprednisolone irrespective of OC steroid use.

[13C]Aminopyrine breath test detects altered liver metabolism caused by low-dose oral contraceptives

The [13C]aminopyrine breath test measures hepatic mixed function oxidase activity. The cumulative percent dose recovered over 2 hr is a sensitive indicator of hepatic dysfunction; values < or = 7.0% have been shown to indicate severe liver disease. Previous studies have suggested that the test results may be influenced by the use of oral contraceptives steroids. We compared the results from five non-oral contraceptive-using women with those from 31 women whose duration of oral contraceptive steroid usage ranged from 4 to 204 months. The women were taking one of four oral contraceptive formulations that differed in the amounts of estrogen (20, 35, or 50 micrograms with 1 mg progesterone) and progesterone (35 micrograms estrogen with stepped levels of progesterone of 0.5, 0.75, and 1.0 mg). The [13C]aminopyrine breath test was performed on days 21 and 28 of the menstrual cycle. Cumulative percent dose recovery values among the normal menstrual cycle of non-oral contraceptive steroid-using women were 12.1 +/- 1.6 and 11.8 +/- 1.5% (mean +/- SD). In contrast, oral contraceptive steroid users showed a marked reduction in cumulative percent dose recovery at 21 days, averaging 6.1 +/- 2.3% (P < 0.001), and returned to normal values (10.2 +/- 3.5%) at 28 days in most women(seven days after oral contraceptive steroid usage was paused). The adverse impact on hepatic mixed function oxidase by oral contraceptive formulations did not differ on the basis of estrogen or progesterone content. The adverse impact of oral contraceptive usage on the mixed function oxidase activity measured by the [13C]aminopyrine breath test must be considered for women of childbearing potential.

Pharmacokinetic and pharmacologic variation between different estrogen products

Due to the complex nature of endogenous and exogenous hormone concentration, formation, and metabolism and assay complexity, the pharmacokinetics of estrogen are difficult to study. Oral estrogens have minimal systemic bioavailability (2% to 10%) due to gut and liver (first-pass) metabolism. High concentrations of estrone are achieved with oral administration, whereas higher concentrations of estradiol are generally achieved after percutaneous absorption. Although vaginal products (such as gel, rings, etc.) are administered locally, they achieve high serum concentrations. Estradiol and estrone concentrations and estradiol-to-estrone ratios vary with different estrogen therapies. Approximately 95% to 98% of estradiol is bound loosely to albumin or tightly to sex hormone binding globulin, the major binding protein. The terminal half lives for the different estrogen compounds (after oral or intravenous administration) vary from 1-12 hours. Some conversion rates have been calculated between estrogen and its metabolites. Smoking decreases achievable estrogen concentrations, and has a greater effect on oral products. Oral contraceptives have been found to decrease antipyrine clearance. In the one study evaluating conjugated estrogens, antipyrine clearance was not altered. Oral contraceptives have a variable effect on the elimination of medications. Acetaminophen clearance is increased, whereas clearance of some benzodiazepines, caffeine,and prednisolone is decreased. Phenytoin increases the metabolism of conjugated estrogens. The various estrogen products may produce different clinical effects based on composition. The metabolites (minor components) of conjugated estrogens have been found to have significant effects on lipid concentrations, uterine weight, liver generated compounds, and bone resorption. Because transdermal products bypass the first-pass effect, delayed or decreased effects on lipid profiles and liver generated compounds have been observed.

Pharmacokinetic considerations in the adolescent: non-cytochrome P450 metabolic pathways

The non-oxidative metabolic reactions of methylation, acetylation, sulfation, glucuronidation, conjugation with amino acids (primarily with glycine), glutathione conjugation and esterase hydrolysis function to detoxify and enhance the elimination of drugs and pro-drugs, many of them as phase II reactions subsequent to oxidation. While the influence of maturation has been recognized in the development of hepatic oxidative capacity and specific pathways studied, considerably less attention has focused on the maturational effect in non-oxidative metabolism despite its role in drug deactivation, detoxification, and elimination. Consequently phase II metabolic capacities during adolescence remain primarily speculative. Conflicting data about one of these pathways, glucuronidation, suggest unexplored puberty- or gender-related phenomena influencing function in this system.

Paracetamol glucuronidation by recombinant rat and human phenol UDP-glucuronosyltransferases

Stably expressed human and rat phenol UDP-glucuronosyltransferases (UGTs) of the UGT1 complex (HlugP1, HlugP4 and 3-methylcholanthrene-inducible rat UGT1A1, the latter considered to be an orthologous enzyme to HlugP1) have been used to investigate the role of UGTs in paracetamol glucuronidation. Kinetic analysis of recombinant UGTs was compared to that of total UGT activities in liver microsomes. Paracetamol was found to be an overlapping substrate of several UGTs. It shows higher affinity for HlugP1 and rat UGT1A1 (apparent Km values of 2 and 3 mM, respectively) than for HlugP4 (Km = 50 mM) and other UGTs present in liver microsomes (Km values of > 12 mM). Glucuronidation of paracetamol with HlugP1 contrasts with that of 6-hydroxychrysene and of 4-methylumbelliferone, which are conjugated with higher affinity by HlugP4 than by HlugP1. Due to the wide tissue distribution of rat UGT1A1, paracetamol glucuronidation was also investigated in extrahepatic rat and human tissues. Paracetamol UGT activity was present and inducible by 2,3,7,8-tetrachlorodibenzo-p-dioxin in rat kidney, lung and spleen. It was also detected in human kidney. A selective cDNA probe for exon 1 of HlugP1 cross-reacted with mRNA from both human liver and kidney. The results demonstrate that paracetamol is conjugated by HlugP1 and its rat orthologue UGT1A1 with higher affinity than by HlugP4 and other UGTs.

Drug glucuronidation in humans

Glucuronidation is a major metabolic pathway for a large number of drugs in humans. Conjugation of drugs and other chemicals with glucuronic acid is catalyzed by the multigene UDP-glucuronosyltransferase family. It is believed that a number (unspecified at present) of glucuronosyltransferase isozymes, which probably differ in terms of substrate specificity and regulation, contribute to drug glucuronidation. Factors known to influence the pharmacokinetics of glucuronidated drugs in man, presumably via an effect on specific glucuronosyltransferases, include age (especially the neonatal period), cigarette smoking, diet, certain disease states, coadministered drugs, ethnicity, genetics and hormonal effects.

Oral contraceptives stimulate the excretion of clofibric acid glucuronide in women and female rats

1. Glucuronidation of clofibric acid, the pharmacologically active form of the hypolipidemic drug clofibrate was investigated in a human population, either in vitro with liver homogenates from biopsies, or after ingestion of the drug and determination of the urinary metabolite. No difference in the glucuronidation rate according to age of the patients was observed. Bilirubin but not clofibric acid glucuronidation was significantly higher in women (106% increase), when expressed per gram of tissue. 2. The excretion of clofibryl glucuronide in women who took oral contraceptives was significantly enhanced by 25%. 3. In female rats, treatment with the contraceptive agent norethindrone also stimulated by 48% the formation of clofibrylglucuronide in liver microsomes.

Influence of oral contraceptives on drug therapy

Interferences between drugs and oral contraceptives are considered to alter pharmacokinetics and thus the efficacy of steroidal hormones. It should be noted, however, that steroids can also modify the metabolism and pharmacodynamic effects of various substances. To the present knowledge, phase I (i.e., oxidation, demethylation) and phase II reactions (conjugation) are concerned. Drugs sharing those enzymatic systems with oral contraceptives experience either an increase in bioavailability by inhibition of oxidative metabolism or undergo accelerated elimination by induced conjugation. Such interaction may be of practical interest in subjects who take oral contraceptives and are simultaneously treated with antidepressants, antihypertensives, insulin, synthetic glucocorticoids, theophylline, and caffeine.

Effect of oral contraceptive steroids on the pharmacokinetics of phenprocoumon

1. The effect of chronic administration of oral contraceptive steroids (OCS) on the pharmacokinetics of the oral anticoagulant phenprocoumon was investigated in seven healthy females. 2. Plasma concentrations of phenprocoumon and the urinary recovery of unchanged as well as conjugated drug were measured following a single oral dose of 0.22 mg kg-1. A group of seven non-smoking, drug-free women matched for age and body weight served as controls. 3. Administration of OCS was associated with a significant increase in the clearance of phenprocoumon from 1.6 +/- 0.7 to 2.0 +/- 0.7 ml min-1 kg-1 (P less than 0.05). The urinary recovery of phenprocoumon glucuronide was significantly higher in OCS users (21.0 +/- 16 vs 14.0 +/- 10 (% of dose); P less than 0.05). No difference in plasma protein binding of phenprocoumon was observed, being 99.2 +/- 0.07 in both groups. 4. The accelerated glucuronidation of phenprocoumon in OCS users suggests the need for careful monitoring of the anticoagulatory response in these subjects, especially when the OCS are withdrawn.

Pharmacokinetic drug interactions with oral contraceptives

Oral contraceptive steroids are used by an estimated 60 to 70 million women world-wide. Over the past 20 years there have been both case reports and clinical studies on the topic of drug interactions with these agents. Some of the interactions are of definite therapeutic relevance, whereas others can be discounted as being of no clinical significance. Pharmacological interactions between oral contraceptive steroids and other compounds may be of 2 kinds: (a) drugs may impair the efficacy of oral contraceptive steroids, leading to breakthrough bleeding and pregnancy (in a few cases, the activity of the contraceptive is enhanced); (b) oral contraceptive steroids may interfere with the metabolism of other drugs. A number of anticonvulsants (phenobarbital, phenytoin, carbamazepine) are enzyme-inducing agents and thereby increase the clearance of the oral contraceptive steroids. Valproic acid has no enzyme-inducing properties, and thus women on this anticonvulsant can rely on their low dose oral contraceptive steroids for contraceptive protection. Researchers are now beginning to unravel the molecular basis of this interaction, with evidence of specific forms of cytochrome P450 (P450IIC and IIIA gene families) being induced by phenobarbital. Rifampicin, the antituberculous drug, also induces a cytochrome P450 which is a product of the P450IIIA gene subfamily. This isozyme is one of the major forms involved in 2-hydroxylation of ethinylestradiol. Broad spectrum antibiotics have been implicated in causing pill failure; case reports document the interaction, and general practitioners are convinced that it is real. The problem remains that there is still no firm clinical pharmacokinetic evidence which indicates that blood concentrations of oral contraceptive steroids are altered by antibiotics. However, perhaps this should not be a surprise, given that the incidence of the interaction may be very low. It is suggested that an individual at risk will have a low bioavailability of ethinylestradiol, a large enterohepatic recirculation and gut flora particularly susceptible to the antibiotic being used. Two drugs, ascorbic acid (vitamin C) and paracetamol (acetaminophen), give rise to increased blood concentrations of ethinylestradiol due to competition for sulphation. The interactions could have some significance to women on oral contraceptive steroids who regularly take high doses of either drug. Although on theoretical grounds adsorbents (e.g. magnesium trisilicate, aLuminium hydroxide, activated charcoal and kaolin) could be expected to interfere with oral contraceptive efficacy, there is no firm evidence that this is the case. Similarly, there is no evidence that smoking alters the pharmacokinetics of oral contraceptive steroids. These agents are now well documented as being able to alter the pharmacokinetics of other concomitantly administered drugs.(ABSTRACT TRUNCATED AT 400 WORDS)

The effect of propranolol on paracetamol metabolism in man

Ten healthy volunteers were treated for 4 days with 160 mg propranolol HCl and placebo in random order. At the end of each treatment salivary antipyrine kinetics and the plasma kinetics and urinary excretion of paracetamol and its major metabolites were measured following a 1500 mg oral dose. Propranolol prolonged the half-life of antipyrine by 11 +/- 5% (mean +/- s.e. mean) and lowered its clearance by 14 +/- 3% (P less than 0.05). Propranolol increased the half-life of paracetamol by 25 +/- 12% (P less than 0.05) and lowered its clearance by 14 +/- 3% (P less than 0.05). Propranolol decreased the partial clearance of paracetamol to its cysteine and mercapturate derivatives by 16 +/- 3% (P less than 0.05) and 32 +/- 7% (P less than 0.05), respectively. The partial clearance to the glucuronide conjugate was decreased by 27 +/- 6% (P less than 0.05), whereas that to sulphate was not changed significantly. Propranolol inhibits paracetamol metabolism predominantly through inhibition of the oxidation and glucuronidation pathways.

Zonal distribution of paracetamol glucuronidation in the isolated perfused rat liver

1. The lobular distribution of paracetamol conjugation was studied using antegrade and retrograde perfusion of isolated rat liver. After addition of 25 mg of paracetamol, recovery of sulphate from antegrade perfusions was greater than from retrograde perfusions indicating periportal predominance of this metabolic pathway. 2. Recovery of sulphate was greater than recovery of glucuronide after addition of 25 mg of paracetamol during both antegrade and retrograde perfusions. Recovery of sulphate was similar after addition of 25 or 100 mg of paracetamol, whereas recovery of the glucuronide was increased at the higher dose, indicating a higher capacity for glucuronidation than for sulphation, but a higher affinity for sulphation. 3. Recovery of glucuronide from antegrade and retrograde perfusions was similar after addition of 25 mg of paracetamol, but was greater from antegrade perfusions after addition of 100 mg. This indicates that drug concentration may be an important determinant of the zonal distribution of drug metabolism. Factors other than localization of drug-metabolizing enzymes, such as competition with other metabolic pathways or availability of cofactors, may also influence the apparent zonal distribution of drug metabolism.

Influence of chronic diflunisal treatment on the plasma levels, metabolism and excretion of indomethacin

The single-dose pharmacokinetics of indomethacin following 100 mg rectally was measured in two groups of 8 healthy subjects before and after diflunisal 500 mg p.o. once daily, or 500 mg in the morning and 1000 mg in the evening, until steady state conditions were reached. A further group of 8 healthy subjects was given 50 mg indomethacin rectally before and after diflunisal 500 mg p.o. twice daily. High dose diflunisal (1500 mg/day) decreased the renal clearance of indomethacin from 21.9 to 1.8 ml/min (92%) and reduced the renal excretion of both unchanged (63%) and conjugated (82%) indomethacin. The apparent total body clearance (0.12 l/h/kg), apparent volume of distribution (0.98 l/kg), and volume of distribution at steady state (0.80 l/kg) were decreased by 47%, 35% and 30%. The maximum plasma concentration (2.4 micrograms/ml) and total area under the curve (13.0 micrograms x h/ml) were increased by 40% and 119%, respectively. The terminal elimination half-life (5.7 h) and mean residence time (6.7 h) were slightly prolonged (7.0 h and 8.8 h) in the presence of diflunisal. The contribution of metabolism to the overall elimination of indomethacin was increased by only 2%. Similar results were obtained when the subjects were challenged with the low dose of diflunisal (500 mg/day), although the magnitude of the changes were smaller. The interaction between indomethacin and diflunisal may be due to competition both at the metabolic (conjugation) and the excretory (tubular secretion) levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential induction of human liver UDP-glucuronosyltransferase activities by phenobarbital-type inducers

(1) UDP-glucuronosyltransferase (UDP-GT) activities and their inducibility were investigated in human liver microsomes from a "liver bank". (2) UDP-GT activities were differentially induced in liver microsomes from patients treated with the phenobarbital-type inducers phenytoin or pentobarbital. UDP-GT activity towards bilirubin was induced 3-fold. Enzyme activities towards paracetamol, benzo(a)pyrene-3,6-quinol, 4-methylumbelliferone and 1-naphthol were moderately induced and to similar extents (2-fold). In contrast, morphine and 4-hydroxybiphenyl glucuronidation were not significantly affected. Cytochrome P-450 dependent 7-ethoxycoumarin O-deethylase was increased 5-fold. (3) A human hepatoma cell line (Hep G2) was studied to obtain information on the inducibility of human UDP-GT activities by 3-methylcholanthrene-type inducers. UDP-GT activities towards benzo(a)pyrene-3,6-quinol and 1-naphthol were moderately but significantly induced by 3-methylcholanthrene-treatment of the cells (2-fold), whereas 7-ethoxyresorufin O-deethylase and 7-ethoxycoumarin O-deethylase were increased over 100- and 10-fold, respectively. (4) The results suggest the existence of differentially inducible UDP-GT isoenzymes in human liver. The findings may be useful as a guide to characterize human liver UDP-GT isoenzymes.

Intestinal metabolism of ethinyloestradiol and paracetamol in vitro: studies using Ussing chambers

The intestinal mucosal metabolism of ethinyloestradiol (EE2) and paracetamol (P) has been studied in vitro in Ussing chambers. Histologically normal jejunum or ileum was obtained from 19 patients undergoing various resections. The muscularis externa was stripped off the mucosa and the mucosal sheets mounted between two perspex chambers. Tissue viability was routinely assessed by measurement of the transmural potential difference. The percentage of steroid in the serosal chamber, 2 h after addition of EE2 (2 microCi; 80 ng) to the mucosal chamber was 2.3 +/- 0.8% (mean +/- s.d.) which comprised unconjugated drug (0.4 +/- 0.3%), sulphate conjugates (0.7 +/- 0.5%) and glucuronides (0.9 +/- 0.8%). In the mucosal chamber, 56.6 +/- 11.4% was unconjugated steroid, 33.3 +/- 12.4% sulphate conjugates and 2.1 +/- 2.3% glucuronides. Small amounts of the oxidation products 2-hydroxy and 16-hydroxy-EE2 were present. At 2 h, the percentage of paracetamol in the serosal chamber was 3.2 +/- 1.4% (of added P; 2 microCi; 50 ng) of which 0.5 +/- 0.3% was paracetamol sulphate (PS) and 0.1% was paracetamol glucuronide (PG). In the mucosal chamber 2.4 +/- 0.8% and 1.0 +/- 0.2% was present as PS and PG respectively. The total amount of paracetamol conjugated was approximately 4.0%. When paracetamol in the mucosal chamber was increased to 50 micrograms (i.e. by a factor of 1000) there was a decrease in the percentage of added drug metabolized to PS and an increase in formation of PG. The glucuronide:sulphate ratio was increased from 0.34 to 3.56. Competition for sulphation was evident when both paracetamol and EE2 were presented to the intestinal mucosa.(ABSTRACT TRUNCATED AT 250 WORDS)

Single dose primaquine has no effect on paracetamol clearance

The kinetics of paracetamol and the formation of metabolites were evaluated in 6 healthy volunteers before and during concomitant administration of a single dose (45 mg) of primaquine. There was no effect of the antimalarial drug on either conjugation (to paracetamol glucuronide and paracetamol sulphate) or oxidation (as judged by the presence of paracetamol cysteine and paracetamol mercapturate) pathways. Although primaquine inhibits certain oxidative metabolism (e.g. of antipyrine) it has no effect, in therapeutic doses, on paracetamol metabolism.

Acetaminophen pharmacokinetics: comparison between pregnant and nonpregnant women

Acetaminophen is the most commonly taken drug during pregnancy, but knowledge about its absorption and disposition is lacking. Six healthy women volunteered to ingest a standard 1000 mg dose at 36 weeks' gestation and 6 weeks post partum. Mean maternal serum concentrations of acetaminophen were consistently less than but not significantly different from the postpartum values. The mean half-life of acetaminophen during pregnancy (3.7 hours) was not significantly different from the nonpregnant value (3.1 hours). The maximum plasma concentration occurred at 0.8 hours and was 20.8 +/- 6.9 micrograms/ml during pregnancy and 23.7 +/- 6.0 micrograms/ml in the nonpregnant state. The absorption, metabolism, and renal clearance of acetaminophen were unchanged. The decrease in the mean area under the curve during pregnancy may be explained by the increase in volume of distribution of acetaminophen. Potentially hepatotoxic metabolites were not measurable in the maternal serum. We conclude that the absorption and disposition of acetaminophen, when used in a standard oral dose, are not affected by pregnancy.

Is biliary excretion of paracetamol significant in man?

The extent of biliary excretion of paracetamol and its principal metabolites was studied in 10 healthy adults. Duodenal intubation was performed approximately 12 h after an oral dose of 1 g paracetamol. Duodenal fluid was aspirated following intravenous cholecystokinin, and urine was collected for 12 h after dosing. The mean concentration of the cysteine and glucuronide conjugates in the bile was 253.7 +/- 39.0 micrograms ml-1 (s.e. mean) and 69.3 +/- 15.3 micrograms ml-1, respectively. The mean recoveries of the cysteine and glucuronide conjugates in the bile were 6.0 +/- 1.5 mg and 1.2 +/- 0.41 mg, respectively. The bile is an important route of elimination for the cysteine conjugate and accounts for 19.4 +/- 4.4% of the total excretion of the metabolite. The fate of the biliary excreted conjugates is not known, and needs further investigation.

Influence of gender and oral contraceptive steroids on the metabolism of salicylic acid and acetylsalicylic acid

Salicylic acid and acetylsalicylic acid (aspirin) disposition after an oral dose of aspirin, 900 mg (equivalent to 689.7 mg of salicylic acid) was studied in eight males, eight females and eight females receiving oral contraceptive steroids (OCS). Salicylic acid clearance was 61% higher in males compared to the control female group, an effect due largely to enhanced activity of the glycine conjugation pathway (salicyluric acid formation) in males. Salicylic acid clearance was 41% higher in OCS-users compared to the control female group due to increases in both the glycine and glucuronic acid conjugation pathways in the pill users. There was no difference in any salicylic acid disposition parameter between males and OCS-users. Area under the plasma concentration-time curve (AUC) and elimination half-life of aspirin was significantly greater and aspirin plasma hydrolysis rate was significantly lower in both female groups compared to males. There was no difference between OCS-users and the control female group in any of these parameters. Aspirin AUC and elimination half-life were significantly correlated with aspirin plasma hydrolysis rate. These data confirm the importance of hormonal factors in the regulation of drug conjugation reactions in humans and suggest that sex-related differences in salicylic acid and aspirin disposition may be of clinical importance.

Drug interactions with oral contraceptives

In the very rare cases where a pregnancy occurs during oral contraceptive use, the blame is usually laid against the patient for having forgotten to take the pill. Evidence has started to accumulate to suggest that neither the patient nor the pill is at fault in some contraceptive failures. It may be because the patient is taking other medicines and these may be preventing the pill from suppressing ovulation. Most drug interactions reducing or negating contraceptive activity are due to concomitant use of drugs having microsomal enzyme-inducing activity (e.g., some antibiotics, especially rifampicin, and anticonvulsants, including phenobarbital, phenytoin, and primidone. Other antibiotics (e.g., tetracycline) may also interact by interruption of the enterohepatic circulation of contraceptive steroids. Less well appreciated, oral contraceptive steroids may themselves modify the metabolism and pharmacological activity of various other drugs (e.g., anticoagulants, benzodiazepines, beta-blockers, caffeine, corticosteroids, and tricyclic antidepressants); in this respect the oral contraceptives are acting as enzyme inhibitors. Contraceptive steroids may also interact with drugs that cause enzyme inhibition and this delays the metabolism of the hormonal agents. Interactions of this type would be expected to potentiate the action of the contraceptive steroids. It is suggested that the effects of such interaction might be presented in terms of increased incidence of side effects, including water retention, diabetogenic effects, hypertension, and an increased risk of thromboembolic disorders. The spectrum of interactions with oral contraceptives is presented in three tables.

Impairment of caffeine clearance by chronic use of low-dose oestrogen-containing oral contraceptives

The effect of chronic (greater than 3 months) administration of low-dose oestrogen-containing (less than 50 micrograms oestrogen) oral contraceptives (OCS) on the pharmacokinetics of caffeine has been examined in a treated females matched with 9 non-smoking, drug-free, healthy control females of similar age, weight and ethnic origin. Each subject received 162 mg caffeine base orally after an overnight fast. OCS subjects had a prolonged elimination half-life of caffeine, (mean 7.88 h vs 5.37 h in the controls). This was the result of marked impairment of the plasma clearance of caffeine (1.05 vs 1.75 ml/min/kg, respectively) with no change in apparent volume of distribution (0.685 in OCS vs 0.7501/kg in the control group). The absorption parameters determined were peak plasma caffeine concentration (3.99 vs 4.09 micrograms/ml) and time to peak concentration after drug administration (1.52 vs 0.79), which was moderately prolonged in OCS users. Thus, caffeine clearance, previously reported to be a specific marker of cytochrome P-448 activity in man, is decreased by chronic OCS use. This suggests that OCS may cause significant impairment of this enzyme activity as assessed in vivo. With chronic caffeine consumption, OCS users are predicted to have an increased steady-state plasma caffeine concentration as compared to non-OCS users.

Gender and oral contraceptive steroids as determinants of drug glucuronidation: effects on clofibric acid elimination

The disposition of clofibric acid, a drug metabolised largely by glucuronidation, was studied in eight males, eight females and eight females receiving oral contraceptive steroids (OCS). Clofibric acid plasma clearance was not significantly different in males compared to the control female group but was 48% greater (P less than 0.01) in women receiving OCS compared to non-pill using females. This difference was due to an alteration in clofibric acid metabolic clearance as there were no differences between the groups in clofibric acid free fraction. Along with previous data the results suggest that induction of drug glucuronidation by OCS may be of clinical importance, although any sex-related differences are unlikely to be of clinical significance.

Selective inhibition of acetaminophen oxidation and toxicity by cimetidine and other histamine H2-receptor antagonists in vivo and in vitro in the rat and in man

Acetaminophen-induced hepatotoxicity results from hepatic enzymatic oxidation of acetaminophen to a toxic, electrophilic intermediate. Acetaminophen is ordinarily eliminated after conjugation with glucuronic acid and sulfate to nontoxic derivatives. Cimetidine has been shown to inhibit the hepatic oxidation of a number of drugs and to protect rats from acetaminophen-induced hepatic necrosis. The aim of this study was to define the mechanism by which cimetidine reduced acetaminophen-induced hepatic necrosis and to determine whether inhibition of formation of the reactive metabolite(s) of acetaminophen occurred also in man. In vivo cimetidine pretreatment decreased covalent binding of [3H]acetaminophen to the liver from 552 +/- 23.8 to 170 +/- 31.6 nmol/g protein 2 h after a toxic dose of acetaminophen in 3-methylcholanthrene pretreated rats (P less than 0.05). Cimetidine pretreatment also significantly reduced the rate of hepatic glutathione depletion. Both cimetidine and metiamide produced dose-dependent inhibition of acetaminophen oxidation in vitro, whereas inhibition by ranitidine and cimetidine sulfoxide was quantitatively less. Inhibition of acetaminophen oxidation by cimetidine and metiamide was primarily competitive with an inhibition constant (Ki) of 130 +/- 16 and 200 +/- 50 microM, respectively. By contrast, cimetidine inhibited acetaminophen glucuronidation minimally with a Ki of 1.39 +/- 0.23 mM. Similar results were obtained using human liver microsomes as a source of enzymes. In a dose-related fashion, cimetidine also reduced acetaminophen-induced toxicity to human lymphocytes when incubated with microsomes and NADPH. Pharmacokinetics of acetaminophen elimination were studied in normal volunteers with and without co-administration of cimetidine 300 mg every 6 h. In normal volunteers, cimetidine decreased the fractional clearance of the oxidized (potentially toxic) metabolites of acetaminophen more than the conjugated metabolites. This finding confirmed the hypothesis that cimetidine is a relatively selective inhibitor of the oxidation of acetaminophen to reactive metabolites in man as well as in animals. When considered together with the results of previous studies showing improved survival and decreased hepatoxicity in acetaminophen-poisoned animals, the present results provide a rational basis for assessing possible benefits of cimetidine treatment of acetaminophen overdoses in man.