Drug metabolism in pregnancy, infancy and childhood

Variation in the biological half-life of methylmercury in humans: Methods, measurements and meaning

BACKGROUND

Understanding methylmercury (MeHg) toxicity requires a complete understanding of its fundamental toxicokinetic and toxicodynamic characteristics in the human body. The biological half-life (t1/2) of MeHg is a kinetic property that directly influences the body burden of Hg that results from repeated exposures such as can occur with fish and seafood consumption. The t1/2 of MeHg in humans is approximately 50 days, equivalent to an elimination rate (kel) of 0.014 day-1. However, numerous studies report a wide range of half-life values (t1/2 < 30 to >120 days), demonstrating that significant variation in the biological process of MeHg elimination exists. This variation is a source of considerable uncertainty in deriving a meaningful reference dose for MeHg applicable to all individuals in a population.

SCOPE OF REVIEW

First, we summarize fundamentals of MeHg toxicokinetics, emphasizing the central role that biological half-life plays in MeHg dosimetry. We next present important considerations for how kinetic analyses are performed. We provide an example of how MeHg half-life variation directly influences the body burden and, in certain contexts, can result in MeHg levels exceeding the US EPA Reference Dose. We then survey existing studies that report MeHg half-life determinations in people.

MAJOR CONCLUSIONS

Recent advances in methods of determining MeHg kinetics in people have made individualized assessment of MeHg elimination rates more accurate and readily obtainable.

GENERAL SIGNIFICANCE

Characterization of MeHg half-life, particularly in vulnerable individuals, such as pregnant women and children, will diminish the remaining toxicokinetic uncertainty surrounding MeHg exposures and will better inform the risk assessment process.

Therapeutic Drug Monitoring of Antiepileptic Drugs in Epilepsy: A 2018 Update

BACKGROUND

Antiepileptic drugs (AEDs) are the mainstay of epilepsy treatment. Since 1989, 18 new AEDs have been licensed for clinical use and there are now 27 licensed AEDs in total for the treatment of patients with epilepsy. Furthermore, several AEDs are also used for the management of other medical conditions, for example, pain and bipolar disorder. This has led to an increasingly widespread application of therapeutic drug monitoring (TDM) of AEDs, making AEDs among the most common medications for which TDM is performed. The aim of this review is to provide an overview of the indications for AED TDM, to provide key information for each individual AED in terms of the drug's prescribing indications, key pharmacokinetic characteristics, associated drug-drug pharmacokinetic interactions, and the value and the intricacies of TDM for each AED. The concept of the reference range is discussed as well as practical issues such as choice of sample types (total versus free concentrations in blood versus saliva) and sample collection and processing.

METHODS

The present review is based on published articles and searches in PubMed and Google Scholar, last searched in March 2018, in addition to references from relevant articles.

RESULTS

In total, 171 relevant references were identified and used to prepare this review.

CONCLUSIONS

TDM provides a pragmatic approach to epilepsy care, in that bespoke dose adjustments are undertaken based on drug concentrations so as to optimize clinical outcome. For the older first-generation AEDs (carbamazepine, ethosuximide, phenobarbital, phenytoin, primidone, and valproic acid), much data have accumulated in this regard. However, this is occurring increasingly for the new AEDs (brivaracetam, eslicarbazepine acetate, felbamate, gabapentin, lacosamide, lamotrigine, levetiracetam, oxcarbazepine, perampanel, piracetam, pregabalin, rufinamide, stiripentol, sulthiame, tiagabine, topiramate, vigabatrin, and zonisamide).

Clinical pharmacokinetics of new-generation antiepileptic drugs at the extremes of age: an update

Epilepsies occur across the entire age range, and their incidence peaks in the first years of life and in the elderly. Therefore, antiepileptic drugs (AEDs) are commonly used at the extremes of age. Rational prescribing in these age groups requires not only an understanding of the drugs' pharmacodynamic properties, but also careful consideration of potential age-related changes in their pharmacokinetic profile. The present article, which updates a review published in 2006 in this journal, focuses on recent findings on the pharmacokinetics of new-generation AEDs in neonates, infants, children, and the elderly. Significant new information on the pharmacokinetics of new AEDs in the perinatal period has been acquired, particularly for lamotrigine and levetiracetam. As a result of slow maturation of the enzymes involved in glucuronide conjugation, lamotrigine elimination occurs at a particularly slow rate in neonates, and becomes gradually more efficient during the first months of life. In the case of levetiracetam, elimination occurs primarily by renal excretion and is also slow at birth, but drug clearance increases rapidly thereafter and can even double within 1 week. In general, infants older than 2-3 months and children show higher drug clearance (normalized for body weight) than adults. This pattern was confirmed in recent studies that investigated the pediatric pharmacokinetics of several new AEDs, including levetiracetam, rufinamide, stiripentol, and eslicarbazepine acetate. At the other extreme of age, in the elderly, drug clearance is generally reduced compared with younger adults because of less efficient drug-metabolizing activity, decreased renal function, or both. This general pattern, described previously for several AEDs, was confirmed in recent studies on the effect of old age on the clearance of felbamate, levetiracetam, pregabalin, lacosamide, and retigabine. For those drugs which are predominantly eliminated by renal excretion, aging-related pharmacokinetic changes could be predicted by measuring creatinine clearance (CLCR). Overall, most recent findings confirm that age is a major factor influencing the pharmacokinetic profile of AEDs. However, pharmacokinetic variability at any age can be considerable, and the importance of other factors should not be disregarded. These include genetic factors, co-morbidities, and drug interactions, particularly those caused by concomitantly administered AEDs which induce or inhibit drug-metabolizing enzymes.

Therapeutic drug monitoring of antiepileptic drugs by use of saliva

Blood (serum/plasma) antiepileptic drug (AED) therapeutic drug monitoring (TDM) has proven to be an invaluable surrogate marker for individualizing and optimizing the drug management of patients with epilepsy. Since 1989, there has been an exponential increase in AEDs with 23 currently licensed for clinical use, and recently, there has been renewed and extensive interest in the use of saliva as an alternative matrix for AED TDM. The advantages of saliva include the fact that for many AEDs it reflects the free (pharmacologically active) concentration in serum; it is readily sampled, can be sampled repetitively, and sampling is noninvasive; does not require the expertise of a phlebotomist; and is preferred by many patients, particularly children and the elderly. For each AED, this review summarizes the key pharmacokinetic characteristics relevant to the practice of TDM, discusses the use of other biological matrices with particular emphasis on saliva and the evidence that saliva concentration reflects those in serum. Also discussed are the indications for salivary AED TDM, the key factors to consider when saliva sampling is to be undertaken, and finally, a practical protocol is described so as to enable AED TDM to be applied optimally and effectively in the clinical setting. Overall, there is compelling evidence that salivary TDM can be usefully applied so as to optimize the treatment of epilepsy with carbamazepine, clobazam, ethosuximide, gabapentin, lacosamide, lamotrigine, levetiracetam, oxcarbazepine, phenobarbital, phenytoin, primidone, topiramate, and zonisamide. Salivary TDM of valproic acid is probably not helpful, whereas for clonazepam, eslicarbazepine acetate, felbamate, pregabalin, retigabine, rufinamide, stiripentol, tiagabine, and vigabatrin, the data are sparse or nonexistent.

Women and epilepsy

Women with epilepsy face additional challenges when compared to their peers. Hormonal influences may increase seizure activity, alter endocrine function, and affect fertility. In this population, antiepileptic drugs (AEDs) reduce the efficacy of contraception methods and increase the risk of fetal malformations. Other pertinent issues to women with epilepsy include breastfeeding as well as bone mineral health. This article summarizes our current, collective knowledge of these issues and makes specific recommendations with respect to management.

Antiepileptic drugs--best practice guidelines for therapeutic drug monitoring: a position paper by the subcommission on therapeutic drug monitoring, ILAE Commission on Therapeutic Strategies

Although no randomized studies have demonstrated a positive impact of therapeutic drug monitoring (TDM) on clinical outcome in epilepsy, evidence from nonrandomized studies and everyday clinical experience does indicate that measuring serum concentrations of old and new generation antiepileptic drugs (AEDs) can have a valuable role in guiding patient management provided that concentrations are measured with a clear indication and are interpreted critically, taking into account the whole clinical context. Situations in which AED measurements are most likely to be of benefit include (1) when a person has attained the desired clinical outcome, to establish an individual therapeutic concentration which can be used at subsequent times to assess potential causes for a change in drug response; (2) as an aid in the diagnosis of clinical toxicity; (3) to assess compliance, particularly in patients with uncontrolled seizures or breakthrough seizures; (4) to guide dosage adjustment in situations associated with increased pharmacokinetic variability (e.g., children, the elderly, patients with associated diseases, drug formulation changes); (5) when a potentially important pharmacokinetic change is anticipated (e.g., in pregnancy, or when an interacting drug is added or removed); (6) to guide dose adjustments for AEDs with dose-dependent pharmacokinetics, particularly phenytoin.

Pharmacokinetics and Therapeutic Drug Monitoring of Newer Antiepileptic Drugs During Pregnancy and the Puerperium

The treatment of epilepsy in pregnancy is particularly challenging in that the fetal and maternal risks associated with maternal seizures need to be balanced against the potential teratogenic effects of antiepileptic drugs (AEDs). Pregnancy is known to affect the pharmacokinetics of older-generation AEDs. Understanding such alterations is important in the effort to optimise drug therapy since they may affect seizure control as well as fetal drug exposure. Therapeutic drug monitoring has therefore been recommended to control for changes in the disposition of the older-generation AEDs during pregnancy. Much less is known about gestation-induced alterations in the pharmacokinetics of the newer AEDs that have been introduced in the last 15 years. Lamotrigine is by far the most extensively studied of the newer AEDs. Pronounced alterations have been reported in the apparent clearance of lamotrigine, with an increase of >300% from baseline in late pregnancy in some patients on monotherapy, most likely due to enhanced metabolism. The available data suggest that the corresponding decline in plasma concentrations can be associated with loss of seizure control. More limited data indicate that a similar decline in plasma concentrations of the active monohydroxy derivative of oxcarbazepine may occur in late pregnancy. Preliminary experience also suggests that a significant fall in plasma concentrations of levetiracetam may occur during pregnancy. No systematic information is available on the pharmacokinetics during pregnancy of other newer AEDs (e.g. gabapentin, pregabalin, tiagabine, topiramate or zonisamide). Given the importance of maintaining optimal treatment of epilepsy during pregnancy, therapeutic drug monitoring appears to be justified for lamotrigine and oxcarbazepine in particular. Systematic studies of the effects of pregnancy on the pharmacokinetics of the other newer-generation AEDs are urgently needed.

Are there sex and gender differences in acute exposure to chemicals in the same setting?

We have little understanding of the influence that sex and gender may have on exposure to and measurement of occupational chemicals. If men and women are in the same physical environment, whether that be an occupational or an environmental setting, researchers need to question whether their acute exposure, as measured by administered and/or biologically effective dose, is the same. Not doing so may result in incorrect inferences being made about the risks associated with that exposure. Three critical questions arise specifically, do men and women differ in (1) their personal environments (immediate physical environments and personal attributes), (2) their absorption of the substance across the various biological barriers, and (3) the amount of active substance that reaches the target sites? Both contextual (e.g., smoking habits, diet, use of personal care products and jewellery, hobbies, stress, and use of medications) and biological (e.g., endocrine status) factors should be considered in answering these questions. Examples from the literature are provided to show that, depending on the chemical compound, there may be sex and gender differences in exposure to chemicals which can be manifested in sex differences in absorption, distribution, metabolism, storage, and excretion. An argument is developed to support the need to make information available, such as pharmacokinetic modeling studies in both men and women including appropriate age groups representing the spectrum of life stages and reproductive status.

Pharmacokinetic variability of new antiepileptic drugs at different ages

Newer generation antiepileptic drugs (AEDs) are increasingly used to treat epilepsies in infants, children, and the elderly. For rational prescribing in these populations, it is essential to understand the pharmacokinetic changes that occur during development and aging. Data obtained in recent years indicate that the apparent oral clearance (CL/F) of lamotrigine, topiramate, levetiracetam, oxcarbazepine, gabapentin, tiagabine, zonisamide, vigabatrin, and felbamate is considerably higher in children than in adults,the magnitude of the difference being on average in the order of 20%to 120%, depending on the drug and the age distribution of the assessed population. Information on the pharmacokinetics of these drugs in newborns is completely lacking or very sparse. Studies in the elderly have demonstrated that significant pharmacokinetic changes also occur at the other extreme of age. On average, CL/F values of newer generation AEDs have been found to be reduced by 10% to 50% compared with those found in young or middle-aged adults. These pharmacokinetic changes are clinically important and con-tribute to age-related differences in dosage requirements.

Changes in the disposition of oxcarbazepine and its metabolites during pregnancy and the puerperium

PURPOSE

To determine potential changes in the plasma concentrations of oxcarbazepine (OXC) and its metabolites during pregnancy and puerperium.

METHODS

Five women receiving OXC monotherapy were followed prospectively during pregnancy and the puerperium. Four women were enrolled in the first trimester, and one woman, 2 weeks before delivery. Steady-state concentrations of OXC, its active R-(-)- and S-(+)-monohydroxy derivatives (MHD), and the additional metabolite carbamazepine-10,11-trans-dihydrodiol (DHD) were measured at regular intervals by an enantioselective HPLC assay. RESULTS. In all samples, S-(+)-MHD was the most abundant compound in plasma and accounted almost entirely for the amount of active moiety (defined as the molar sum of OXC, R-(-)-MHD, and S-(+)-MHD) found in the circulation. The dose-normalized concentrations of active moiety decreased markedly during gestation and, in four of the five patients, increased strikingly after delivery. Plasma concentrations of S-(+)-MHD mirrored closely the levels of the active moiety. Plasma concentrations of the parent drug and other metabolites also tended to decrease during pregnancy and to increase after delivery.

CONCLUSIONS

During treatment with OXC, S-(+)-MHD is by far the most abundant active compound in plasma. The concentration of this metabolite as well as the active moiety may decrease markedly during pregnancy and may increase severalfold after delivery. Because of these striking pharmacokinetic changes, the clinical response should be monitored closely in OXC-treated women throughout pregnancy and the puerperium.

Clinical Pharmacokinetics of New-Generation Antiepileptic Drugs at the Extremes of Age

In recent years, several new-generation antiepileptic drugs (AEDs) have been introduced in clinical practice. These agents, which include felbamate, gabapentin, lamotrigine, levetiracetam, oxcarbazepine, pregabalin, tiagabine, topiramate, vigabatrin and zonisamide, are being increasingly used in the treatment of epilepsy at the extremes of age. For a rational prescribing of these drugs in specific age groups, major pharmacokinetic changes that occur during development and aging need to be taken into consideration. A review of available evidence indicates that the apparent oral clearance (CL/F) of new-generation AEDs in children is increased by 20-170% (depending on the type of drug and characteristics of the patients studied) compared with adults, with the highest CL/F values usually being observed in the youngest age groups. These findings do not necessarily apply to the first weeks of life, when drug eliminating capacity is still undergoing maturation, as in the case of lamotrigine for which preliminary data suggest that CL/F in neonates aged <2 months can be much lower than in infants aged 2-12 months. At the other extreme of age, in the elderly, CL/F is almost invariably reduced (on average by 10-50%) compared with values found in non-elderly adults. Age-related CL/F changes, together with the large interindividual pharmacokinetic variability, contribute to the need for individualised dosage requirements in these patients. Measurement of serum drug concentrations can be useful as an aid to dosage individualization in these age groups but interpretation of therapeutic drug monitoring data should also take into account the possibility of age-related changes in pharmacodynamic sensitivity and, for neonates and the elderly, alterations in drug binding to serum proteins.

Gender Aspects of Pharmacokinetics of New and Old AEDs

Alterations in drug disposition in pregnancy and drug transfer into breast milk during lactation are the most important gender-related aspects of the pharmacokinetics of antiepileptic drugs(AEDs). The effect of pregnancy on drug disposition has been fairly well characterized for the old AEDs. Total and unbound plasma concentrations of phenobarbital decline up to 50%. Total phenytoin levels may drop to approximately 40% of prepregnancy concentrations,but unbound concentrations considerably less. Unbound valproate levels remain unchanged despite a fairly marked decrease in total concentrations, and changes in unbound and total plasma concentrations of carbamazepine are minor. In contrast, pregnancy has a pronounced effect on lamotrigine, with a 300% increase in apparent clearance in the third trimester. Much less is known about the kinetics of other new AEDs during pregnancy. Umbilical cord/maternal plasma concentration ratios close to unity suggest free transplacental passage of most AEDs,whereas higher ratios indicate fetal accumulation of valproate and gabapentin. Milk/maternal plasma concentration ratios close to 1 demonstrate extensive transfer to breast milk of many AEDs including ethosuximide, gabapentin, lamotrigine, levetiracetam, oxcarbazepine, topiramate, and zonisamide. Plasma levels are, however, in general low in the nursed infant, although pharmacological concentrations have been reported occasionally for carbamazepine, ethosuximide, lamotrigine, and phenobarbital. For most of the new-generation AEDs, there is an urgent need for further kinetic data from pregnancy and breast-feeding.

Concerns regarding lamotrigine and breast-feeding

PURPOSE

Many women with epilepsy who are planning a pregnancy are treated with lamotrigine (LTG), resulting in greater fetal exposure to the drug. Current care guidelines suggest that mothers with epilepsy breast-feed their children. These recommendations are made without regard to how nursing newborns metabolize medication. Lamotrigine is extensively metabolized by glucuronidation, which is immature in neonates and may lead to accumulation of medication. This article reports LTG levels in full-term nursing newborns born to mothers with epilepsy on lamotrigine monotherapy.

METHODS

Serum LTG levels were obtained in nursing mothers and their neonates on Day 10 of life. Maternal LTG clearance during pregnancy and postpartum was determined and correlated with levels.

RESULTS

Four mothers with partial epilepsy on LTG monotherapy were evaluated. Serum LTG levels in nursing newborns ranged from <1.0 to 2.0 microg/mL on Day 10 of life. Three babies had LTG levels >1.0 microg/mL. After excluding one child with an undetectable level, the LTG levels in newborns were on average 30% (range 20-43%) of the maternal drug level. No decline was noted in two children with repeat levels at 2 months.

CONCLUSION

Serum concentrations of LTG in breast-fed children were higher than expected, in some cases reaching "therapeutic" ranges. These high levels may be explained by poor neonatal drug elimination due to inefficient glucuronidation. Our observation that not all newborns had a high LTG level suggests considerable genetic variability in metabolism. Our limited data suggest monitoring blood levels in nursing children and the need for individual counseling for women with epilepsy regarding breast-feeding.

Prenatal expression of N-acetyltransferases in C57Bl/6 mice

Exposure to carcinogens such as 4-aminobiphenyl (4ABP), found in tobacco smoke and other combustion products, results in the formation of detectable levels of 4ABP-hemoglobin adducts in cord blood and 4ABP-DNA adducts in conceptal tissue. The presence of these adducts requires that the parent compound undergo biotransformation. When exposure occurs in utero, the maternal, placental and conceptal tissues are all possible sites for the formation of DNA-reactive products. One step in the activation of 4ABP is catalyzed by N-acetyltransferases (NAT). The expression of NAT was evaluated in gestational day (GD) 10-18 conceptal tissues from C57Bl/6 mice. There was a quantitative increase in NAT1 and NAT2 mRNAs with increasing gestational age that was also reflected in age-related changes in functional protein measured as 4ABP-NAT activity. The ability to acetylate 4ABP increased from GD10 to 18 and was lower in conceptal tissue than in adult liver. The potential toxicologic significance of prenatal NAT expression was assessed by formation of 4ABP-DNA adducts. At GD 15 and 18, 4ABP-DNA adducts were detected by immunohistochemistry 24 h following a single oral dose of 120 mg 4ABP/kg. Based on nuclear fluorescence, conceptual 4ABP-DNA adducts were present at similar levels at GD15 and 18. Levels of 4ABP-DNA adducts were significantly higher in maternal liver compared with the conceptus. Results from this study show that both NAT genes were expressed prenatally and that functional enzymes were present. These data support the possible in situ generation of reactive products by the conceptus. The relative contributions of maternal activation of 4ABP and that by the conceptus remain to be determined.

Lamotrigine clearance during pregnancy

OBJECTIVE

To evaluate changes in lamotrigine (LTG) clearance before, during, and after pregnancy.

METHODS

Twelve pregnancies that had complete steady-state data before, during, and after pregnancy were evaluated. Data included weight, LTG dose, and LTG blood levels at preconception, during pregnancy, and postpartum, and concomitant use of other antiepileptic drugs and their dosages. Apparent clearance (L/[kg.day]) of LTG was calculated by dose/level/weight for time points at preconception; during the first trimester, second trimester, and third trimester; and postpartum. Apparent clearance was compared between preconception and each of the three trimesters. Statistical analysis was performed using one-way analysis of variance, the Student-Newman-Keuls test, and the paired Student's t-test.

RESULTS

An increase in apparent clearance (>65%) was observed between preconception and the second and third trimesters (p < 0.05). Eleven pregnancies required higher doses of LTG to maintain therapeutic levels during pregnancy. There was no significant change in apparent clearance between each trimester. A decrease in apparent clearance was observed between the last two trimesters and postpartum (p < 0.05). In the postpartum period, apparent clearances returned to the preconception baseline, and LTG doses needed to be reduced.

CONCLUSION

Pregnancy increases LTG clearance by >50%. This effect occurs early in pregnancy and reverts quickly after delivery. LTG levels should be monitored before, during, and after pregnancy.

Drug glucuronidation in clinical psychopharmacology

Glucuronidation is a phase II metabolic process and one of the most common pathways in the formation of hydrophilic drug metabolites. At least 33 families of uridine diphosphate-glucuronosyltransferases have been identified in vitro, and specific nomenclature similar to that used to classify the cytochrome (CYP) P450 system has been established. The UGT1 and UGT2 subfamilies represent the most important of these enzymes in human drug metabolism. Factors affecting glucuronidation include the following: cigarette smoking, obesity, age, and gender. In addition, several drugs have been found in vitro to be substrates, inhibitors, or inducers of UGT enzymes. Induction or inhibition of both UGT and CYP isoforms may occur simultaneously. Some important drug interactions involving glucuronidation have been documented and others can be postulated. This review summarizes the relevant literature pertaining to drug glucuronidation and its implications for clinical psychopharmacology.

Uncertainty factors for chemical risk assessment. human variability in the pharmacokinetics of CYP1A2 probe substrates

A 100-fold uncertainty factor is used to derive acceptable daily intakes for compounds causing thresholded toxicity. The 10-fold factor for human variability can be further subdivided into two factors of 10(0.5) (3.16) to allow for toxicokinetics and toxicodynamics. The validity of the human kinetic subfactor has been analysed in relation to CYP1A2 metabolism using published in vivo pharmacokinetic parameters selected to reflect chronic exposure (metabolic and total clearances and area under the plasma concentration-time curve: CLm, CL and AUC) and acute exposure (the peak plasma concentration, C(max)). The variability in CYP1A2 activity in healthy adults, based on data after oral and intravenous dosage (CLm, CL and AUC), ranged from 34 to 42%. The variability in C(max) was 21%. The default kinetic factor of 3.16 would cover at least 99% of the healthy adult population, assuming that the data were log-normally distributed, but would give lower protection for some subgroups (pregnant women at term, healthy elderly, patients with liver disease), and was inadequate for neonates. This analysis of in vivo kinetic data for CYP1A2 substrates illustrates the importance of quantifying human variability in specific metabolic pathways, and of identifying potentially susceptible subgroups of the human population, in order to determine the scientific validity of uncertainty factors.

Management of epilepsy in adolescents and adults

Epilepsy is the most common serious neurological disorder affecting an estimated 50 million people worldwide. Particular focus should be placed on a safe diagnosis, seizure and syndrome classification, and choice of pharmacological and surgical options for a range of patient populations with different health-care requirements. Eight new antiepileptic drugs were licensed in the 1990s with more to come. These new drugs along with earlier resective surgery have led to a better outcome for many more people with this condition.

Pharmacokinetics of lamotrigine in children in the absence of other antiepileptic drugs

We evaluated the pharmacokinetics of lamotrigine in 12 children with epilepsy who were receiving no other antiepileptic drugs. Each patient received a single oral dose of lamotrigine 2 mg/kg. Plasma concentrations of the drug were measured up to 48 hours after dosing. Pharmacokinetic parameters were calculated using noncompartmental methods. After rapid absorption, the lamotrigine concentration declined monoexponentially. Oral clearance was 0.64 +/- 0.26 ml/min/kg. The apparent volume of distribution was 1.50 +/- 0.51 L/kg. Weight-normalized clearance and volume were higher in children than in adults. The mean half-life was 32 hours, similar to that in adults. Should similar plasma lamotrigine concentrations in adults and children be desirable, children will likely require higher weight-normalized doses at the same dosing frequency.

Women's issues in the treatment of epilepsy

BACKGROUND

The management of women with epilepsy involves a number of important issues including conception control, sexual dysfunction and fertility, the effect of seizures on the fetus, possible changes in seizures frequency during pregnancy, potential teratogenic effects of antiepileptic drugs and management issues during pregnancy. The primary goal in the treatment of women with epilepsy remains optimal seizure control in the absence of unacceptable adverse effects. The advantages and disadvantages of the new antiepileptic drugs in women remain to be fully established but these new agents allow a wider choice for improved seizure control.

Pharmacokinetics and clinical effect during continuous epidural infusion with ropivacaine 2.5 mg/ml or bupivacaine 2.5 mg/ml for labour pain relief

BACKGROUND

Ropivacaine has shown less systemic toxicity than bupivacaine, and comparatively low muscle-blocking properties could constitute another advantage when used epidurally for obstetric pain relief. We aimed primarily to compare maternal and foetal drug disposition following continuous epidural infusion of ropivacaine or bupivacaine.

METHODS

Twenty-four full-term, nulliparous women were randomized to continuous epidural infusion (10 ml/h) of ropivacaine 2.5 mg/ml or bupivacaine 2.5 mg/ml for labour pain relief in a double-blind, parallel-group design. Maternal blood samples were collected up to 24 h after the end of infusion as well as taken from the umbilical cord at the time of delivery. Sensory and motor block as well as analgesia were assessed. All the women were monitored by cardiotocography and neonatal assessment was performed.

RESULTS

The sensory block was adequate for both drugs. Higher plasma levels (total and free) were seen with ropivacaine, although the infusion with bupivacaine continued on average for about 2 hours longer. However, the ratios between maternal and umbilical blood concentrations were similar for both drugs. Normal neonatal Apgar and neonatal adaptive capacity scores (NACS) were found in both groups.

CONCLUSION

A continuous epidural infusion of 25 mg/h ropivacaine or bupivacaine both produced good labour pain relief. Higher total and free plasma concentrations were seen for ropivacaine. The ratios between maternal and umbilical plasma levels were similar for both drugs.

Unrecognized pregnancy in the overdosed or poisoned patient

A 22-year-old woman was found in bed unresponsive and hypotensive after an apparent overdose. Subsequent workup in the emergency department identified a ruptured ectopic pregnancy and extensive hemoperitoneum. A significant delay occurred when emergency physicians excluded the possibility of pregnancy because of the patient's normal menstrual history. A qualitative serum or urine pregnancy test is recommended in all women of child-bearing age who present with poisoning or drug overdose.

Pharmacological problems in the management of epilepsy in children

Rational prescribing of anti-epilepsy drugs in children may be complicated by a number of problems, which include: (i) difficulties in arriving rapidly at a syndromic diagnosis, at least in some cases, with consequent uncertainties about therapeutic management; (ii) difficulties in evaluating drug response in young age groups, particularly with respect to subjective side-effects affecting cognitive function; (iii) the vulnerability of infants and children to specific aspects of drug toxicity, such as liver damage induced by valproic acid or behavioural disorders caused by barbiturates; and (iv) the need to adjust dosage to account for age-dependent pharmacokinetic changes. In particular, it is known that the rate of drug metabolism changes markedly during development. Metabolic drug elimination is often reduced at birth, but drug metabolizing enzymes mature rapidly and biotransformation in infants and children usually occurs at a faster rate than in adults. The elimination of drug which are excreted unchanged in urine appears to be less influenced by age in paediatric patients, though impairment in renal drug clearance may be seen in newborns. Monitoring of serum drug concentrations may be helpful for dosage adjustments, but it is not a substitute for careful clinical observation.

Variables affecting nicotine metabolism

Nicotine metabolism is exceedingly sensitive to perturbation by numerous host factors. To reduce the large variations and discrepancies in the literature pertaining to nicotine metabolism, investigators in future studies need to recognize and better control these host factors. Recent advances in the understanding of nicotine metabolism have suggested new approaches to elucidating underlying mechanisms of certain toxic effects associated with cigarette smoking.

Inhibition of phenytoin bioactivation and teratogenicity by dietary n-3 fatty acids in mice

Evidence suggests that the teratogenicity of the anticonvulsant drug phenytoin (DPH) can result from its bioactivation via embryonic prostaglandin synthase and/or maternal cytochromes P450. This study examined whether DPH bioactivation and teratogenicity could be reduced by dietary n-3 fatty acids. Female CD-1 mice were fed diets containing 2 wt% safflower oil and 10 wt% of either hydrogenated coconut oil, safflower oil, or a cod liver oil/linseed oil mixture (CLO/LO) for three weeks prior to impregnation and throughout gestation. DPH (55 or 65 mg/kg) was administered via intraperitoneal injections to pregnant mice at 0900 on gestational days 12 and 13, and on day 19 fetuses were given teratologic assessments. A similar dietary study evaluated in vivo covalent binding of radiolabeled DPH administered on day 12, and dams were killed 24 h later. A reduction in DPH-induced cleft palates and a decrease in DPH covalent binding to embryonic protein was observed in the CLO/LO group. Feeding CLO/LO enhanced incorporation of n-3 fatty acids into embryos and inhibited embryonic prostaglandin synthase activity. No differences in maternal hepatic cytochromes P450 activities were observed among dietary treatments. These data indicate that dietary n-3 fatty acids could reduce DPH teratogenicity via inhibition of embryonic prostaglandin synthase bioactivation of DPH.

Disposition of propofol infusions for caesarean section

The disposition of propofol was studied in women undergoing elective Caesarean section. Indices of maternal recovery and neonatal assessment were correlated with venous concentrations of propofol. After induction of anaesthesia with propofol 2.0 mg.kg-1, ten patients received propofol 6 mg.kg-1.hr-1 with nitrous oxide 50 per cent in oxygen (low group) and nine were given propofol 9 mg.kg-1.hr-1 with oxygen 100 per cent (high group). Pharmacokinetic variables were similar between the groups. The mean +/- SD Vss = 2.38 +/- 1.16 L.kg-1, Cl = 39.2 +/- 9.75 ml.min-1.kg-1 and t1/2 beta = 126 +/- 68.7 min. At the time of delivery (8-16 min), the concentration of propofol ranged from 1.91-3.82 micrograms.ml-1 in the maternal vein (MV), 1.00-2.00 micrograms.ml-1 in the umbilical vein (UV) and 0.53-1.66 micrograms.ml-1 in the umbilical artery (UA). Neonates with high UV concentrations of propofol at delivery had lower neurologic and adaptive capacity scores 15 minutes later. The concentrations of propofol were similar between groups during the infusion but they declined at a faster rate in the low group postoperatively. Maternal recovery times did not depend on the total dose of propofol but the concentration of propofol at the time of eye opening was greater in the high group than the low group (1.74 +/- 0.51 vs 1.24 +/- 0.32 micrograms.ml-1, P less than 0.01). The rapid placental transfer of propofol during Caesarean section requires propofol infusions to be given cautiously, especially when induction to delivery times are long.