The disposition kinetics of diazepam in pregnant women at parturition

Mechanistic Modeling of Placental Drug Transfer in Humans: How Do Differences in Maternal/Fetal Fraction of Unbound Drug and Placental Influx/Efflux Transfer Rates Affect Fetal Pharmacokinetics?

Background: While physiologically based pharmacokinetic (PBPK) models generally predict pharmacokinetics in pregnant women successfully, the confidence in predicting fetal pharmacokinetics is limited because many parameters affecting placental drug transfer have not been mechanistically accounted for. Objectives: The objectives of this study were to implement different maternal and fetal unbound drug fractions in a PBPK framework; to predict fetal pharmacokinetics of eight drugs in the third trimester; and to quantitatively investigate how alterations in various model parameters affect predicted fetal pharmacokinetics. Methods: The ordinary differential equations of previously developed pregnancy PBPK models for eight drugs (acyclovir, cefuroxime, diazepam, dolutegravir, emtricitabine, metronidazole, ondansetron, and raltegravir) were amended to account for different unbound drug fractions in mother and fetus. Local sensitivity analyses were conducted for various parameters relevant to placental drug transfer, including influx/efflux transfer clearances across the apical and basolateral membrane of the trophoblasts. Results: For the highly-protein bound drugs diazepam, dolutegravir and ondansetron, the lower fraction unbound in the fetus vs. mother affected predicted pharmacokinetics in the umbilical vein by ≥10%. Metronidazole displayed blood flow-limited distribution across the placenta. For all drugs, umbilical vein concentrations were highly sensitive to changes in the apical influx/efflux transfer clearance ratio. Additionally, transfer clearance across the basolateral membrane was a critical parameter for cefuroxime and ondansetron. Conclusion: In healthy pregnancies, differential protein binding characteristics in mother and fetus give rise to minor differences in maternal-fetal drug exposure. Further studies are needed to differentiate passive and active transfer processes across the apical and basolateral trophoblast membrane.

Assessing the impacts on fetal dosimetry of the modelling of the placental transfers of xenobiotics in a pregnancy physiologically based pharmacokinetic model

The developmental origin of health and diseases theory supports the critical role of the fetal exposure to children's health. We developed a physiologically based pharmacokinetic model for human pregnancy (pPBPK) to simulate the maternal and fetal dosimetry throughout pregnancy. Four models of the placental exchanges of chemicals were assessed on ten chemicals for which maternal and fetal data were available. These models were calibrated using non-animal methods: in vitro (InV) or ex vivo (ExV) data, a semi-empirical relationship (SE), or the limitation by the placental perfusion (PL). They did not impact the maternal pharmacokinetics but provided different profiles in the fetus. The PL and InV models performed well even if the PL model overpredicted the fetal exposure for some substances. The SE and ExV models showed the lowest global performance and the SE model a tendency to underprediction. The comparison of the profiles showed that the PL model predicted an increase in the fetal exposure with the pregnancy age, whereas the ExV model predicted a decrease. For the SE and InV models, a small decrease was predicted during the second trimester. All models but the ExV one, presented the highest fetal exposure at the end of the third trimester. Global sensitivity analyses highlighted the predominant influence of the placental transfers on the fetal exposure, as well as the metabolic clearance and the fraction unbound. Finally, the four transfer models could be considered depending on the framework of the use of the pPBPK model and the availability of data or resources to inform their parametrization.

Drug dosing during pregnancy-opportunities for physiologically based pharmacokinetic models

Drugs can have harmful effects on the embryo or the fetus at any point during pregnancy. Not all the damaging effects of intrauterine exposure to drugs are obvious at birth, some may only manifest later in life. Thus, drugs should be prescribed in pregnancy only if the expected benefit to the mother is thought to be greater than the risk to the fetus. Dosing of drugs during pregnancy is often empirically determined and based upon evidence from studies of non-pregnant subjects, which may lead to suboptimal dosing, particularly during the third trimester. This review collates examples of drugs with known recommendations for dose adjustment during pregnancy, in addition to providing an example of the potential use of PBPK models in dose adjustment recommendation during pregnancy within the context of drug-drug interactions. For many drugs, such as antidepressants and antiretroviral drugs, dose adjustment has been recommended based on pharmacokinetic studies demonstrating a reduction in drug concentrations. However, there is relatively limited (and sometimes inconsistent) information regarding the clinical impact of these pharmacokinetic changes during pregnancy and the effect of subsequent dose adjustments. Examples of using pregnancy PBPK models to predict feto-maternal drug exposures and their applications to facilitate and guide dose assessment throughout gestation are discussed.

A Physiologically Based Pharmacokinetic Model for Pregnant Women to Predict the Pharmacokinetics of Drugs Metabolized Via Several Enzymatic Pathways

BACKGROUND

Physiologically based pharmacokinetic modeling is considered a valuable tool for predicting pharmacokinetic changes in pregnancy to subsequently guide in-vivo pharmacokinetic trials in pregnant women. The objective of this study was to extend and verify a previously developed physiologically based pharmacokinetic model for pregnant women for the prediction of pharmacokinetics of drugs metabolized via several cytochrome P450 enzymes.

METHODS

Quantitative information on gestation-specific changes in enzyme activity available in the literature was incorporated in a pregnancy physiologically based pharmacokinetic model and the pharmacokinetics of eight drugs metabolized via one or multiple cytochrome P450 enzymes was predicted. The tested drugs were caffeine, midazolam, nifedipine, metoprolol, ondansetron, granisetron, diazepam, and metronidazole. Pharmacokinetic predictions were evaluated by comparison with in-vivo pharmacokinetic data obtained from the literature.

RESULTS

The pregnancy physiologically based pharmacokinetic model successfully predicted the pharmacokinetics of all tested drugs. The observed pregnancy-induced pharmacokinetic changes were qualitatively and quantitatively reasonably well predicted for all drugs. Ninety-seven percent of the mean plasma concentrations predicted in pregnant women fell within a twofold error range and 63% within a 1.25-fold error range. For all drugs, the predicted area under the concentration-time curve was within a 1.25-fold error range.

CONCLUSION

The presented pregnancy physiologically based pharmacokinetic model can quantitatively predict the pharmacokinetics of drugs that are metabolized via one or multiple cytochrome P450 enzymes by integrating prior knowledge of the pregnancy-related effect on these enzymes. This pregnancy physiologically based pharmacokinetic model may thus be used to identify potential exposure changes in pregnant women a priori and to eventually support informed decision making when clinical trials are designed in this special population.

Risk-benefit assessment of anaesthetic agents in the puerperium

A critical evaluation of anaesthetic agents in the puerperium is difficult because systematic, relevant studies are still lacking. Current knowledge of the effects of different agents used in labour and caesarean section indicates that significant residual effects on the mother and newborn are limited. In the early puerperium, based on physiological and/or hormonal changes, the mother could be more sensitive to inhalational anaesthetic agents and local analgesics. To date there is no evidence that any anaesthetic agent is excreted in breast milk in clinically significant amounts when given as a single dose. The only exception is perhaps in the case of very premature neonates whose mothers have had multidrug therapy before labour. Even then the importance of breast milk should be carefully assessed against possible adverse drug effect. However, repeated administration of long-acting benzodiazepines and continuous epidural administration of pethidine (meperidine) can have adverse effects on the neonate. The essential conclusion of this review is that breast-feeding is best. The different anaesthetic agents are excreted in the milk in amounts so low that detrimental effects on the neonate should not be expected.

Pharmacokinetic implications for the clinical use of propofol

Propofol, the recently marketed intravenous induction agent for anaesthesia, is chemically unrelated to earlier anaesthetic drugs. This highly lipophilic agent has a fast onset and short, predictable duration of action due to its rapid penetration of the blood-brain barrier and distribution to the CNS, followed by redistribution to inactive tissue depots such as muscle and fat. On the basis of pharmacokinetic-pharmacodynamic modelling, a mean blood-brain equilibration half-life of only 2.9 minutes has been calculated. In most studies, the blood concentration curve of propofol has been best fitted to a 3-compartment open model, although in some patients only 2 exponential phases can be defined. The first exponential phase half-life of 2 to 3 minutes mirrors the rapid onset of action, the second (34 to 56 minutes) that of the high metabolic clearance, whereas the long third exponential phase half-life of 184 to 480 minutes describes the slow elimination of a small proportion of the drug remaining in poorly perfused tissues. Thus, after both a single intravenous injection and a continuous intravenous infusion, the blood concentrations rapidly decrease below those necessary to maintain sleep (around 1 mg/L), based on both the rapid distribution, redistribution and metabolism during the first and second exponential phases (more than 70% of the drug is eliminated during these 2 phases). During long term intravenous infusions cumulative drug concentrations and effects might be expected, but even then the recovery times do not appear to be much delayed. The liver is probably the main eliminating organ, and renal clearance appears to play little part in the total clearance of propofol. On the other hand, because the total body clearance may exceed liver blood flow, an extrahepatic metabolism or extrarenal elimination (e.g. via the lungs) has been suggested. Approximately 60% of a radiolabelled dose of propofol is excreted in the urine as 1- and 4-glucuronide and 4-sulphate conjugates of 2.6-diisopropyl 1,4-quinol, and the remainder consists of the propofol glucuronide. Thus for hepatic and renal diseases, co-medication, surgical procedure, gender and obesity do not appear to cause clinically significant changes in the pharmacokinetic profile of propofol, but the decrease in the clearance value in the elderly might produce higher concentrations during a long term infusion, with an increased drug effect. In addition, the lower induction dose observed in relation to increased age might be partly explained by a smaller central volume of distribution.(ABSTRACT TRUNCATED AT 400 WORDS)

Obstetric analgesia. Clinical pharmacokinetic considerations

All drugs used in obstetric analgesia are more or less lipophilic, their site of action is in the central nervous system, and they have good membrane penetrability in the fetomaternal unit. Thus the dose and method of administration as well as the duration of treatment are important clinical determinants of drug effects in the fetus and newborn. In the past, too much emphasis has been placed on fetomaternal blood concentration ratios of different agents; it is now appreciated that the extent of fetal tissue distribution and the neonatal elimination rate are pharmacokinetically much more important. Extensive fetal tissue distribution is reflected in a low fetomaternal drug concentration ratio, which may be followed by prolonged neonatal elimination of the drug. Currently, the most effective and safest method for obstetric analgesia is regional epidural administration of bupivacaine or lignocaine (lidocaine); only low doses are needed and the newborn is able to handle these agents efficiently. On the basis of pharmacokinetic and neurobehavioural assessments, inhalational anaesthetic agents appear to be more attractive than pethidine (meperidine) or benzodiazepines. Intermittent administration and fast pulmonary elimination of inhalational agents ensure that long-lasting residual effects are unlikely to occur. The kinetics of epidural and intrathecal opiates explain the problems associated with their use in obstetrics. Among the newer drugs used in obstetric analgesia, the properties of meptazinol and isoflurane appear interesting and these agents warrant further study. All drugs used in obstetric analgesia have a potentially detrimental effect on the neonate and, therefore, knowledge of fetal and neonatal pharmacokinetics is of importance to the clinician.

Hepatic drug metabolism in pregnancy

The results of both isolated tissue and whole animal experimentation, whilst showing some unexplored inconsistencies, suggest that late pregnancy is associated with a reduced ability of the liver to metabolise foreign compounds. The mechanism of this reduced capacity and the physiological reason for it are unclear but such change does have implication for therapeutic response in pregnancy. Available results from the limited and often poorly structured studies of drug levels in pregnant women neither prove nor disprove the existence of similar changes in hepatic monooxygenase activity during human pregnancy.

Differential transplacental binding of diazepam: causes and implications

Diazepam plasma binding was determined in 17 matched pairs of maternal and foetal plasma, collected at delivery. Diazepam % free was higher (p less than 0.001) in maternal (mean 3.24%) than in either umbilical venous (mean 1.50%) or umbilical arterial (mean 1.24%) plasma. The data from in vitro dialysis studies were consistent with the reported higher diazepam concentrations in infants than in mothers at delivery. Plasma nonesterified fatty acids (NEFA) concentrations were higher (p less than 0.001) in maternal (mean = 643 microM) than in matched umbilical venous plasma (means = 211 microM) and there was a significant correlation (p less than 0.01) between diazepam % free and corresponding plasma NEFA concentration for pooled data (r = 0.871, n = 34). Multiple and partial regression analysis indicates that transplacental differences in albumin, bilirubin and total protein concentrations made a minimal contribution to diazepam binding differences between mother and foetus and that approximately 76% of the variability in diazepam % free was accounted for by plasma NEFA concentration. The binding of diazepam to human serum albumin (HSA) was markedly perturbed by the presence of NEFA but not by bilirubin and there was no apparent cooperativity between bilirubin and NEFA on diazepam-HSA binding. Moreover, our findings provide further evidence that substantial differences in binding affinities exist between foetal and maternal plasma albumins.

Intravenous benzodiazepines as anaesthetic agents: pharmacokinetics and clinical consequences

Despite extensive and numerous pharmacokinetic studies on benzodiazepines, the published pharmacokinetic data do not adequately explain the clinical differences found between different benzodiazepine derivatives after intravenous administration. Especially, correlations between initial drug responses and distributional changes of the benzodiazepines are limited. However, during the elimination phase some relationships exist between the kinetic and dynamic phenomena. Age, sex, diseases and concomitantly given drugs cause clinically important alterations in the pharmacokinetics of benzodiazepines. Generally these anxiolytics and sedatives should be considered as adjuvants to general anaesthesia, but not primarily as routine induction agents. The major reasons for this limitation are a high variability in drug response, a relatively slow onset of action and long-lasting residual effects. However, benzodiazepines have many important advantages (see Table 5) when used as intravenous inducing agents of general anaesthesia.

Use of benzodiazepines during pregnancy, labour and lactation, with particular reference to pharmacokinetic considerations

Knowledge of the pharmacokinetic properties of the benzodiazepines is playing an increasingly important role in their use during pregnancy, labour and lactation. All of the benzodiazepine derivatives are lipophilic, undissociated agents which readily penetrate membranes. Thus, they exhibit rapid placental transfer with significant fetal uptake of the drug. In the first trimester of pregnancy there is seldom a clear indication for the use of benzodiazepines. In late pregnancy and at parturition there may be more clear indications for their use. During delivery, the lowest effective dose should be used, since after high doses the so-called 'floppy infant syndrome' may occur, and the slow elimination of these agents by the newborn should be considered. Oxazepam, lorazepam, nitrazepam and, especially, flunitrazepam, appear to penetrate the human placenta more slowly than diazepam, but the clinical significance of this phenomenon remains uncertain. All of these derivatives appear in human milk, but only high clinical doses might be expected to exert a possible effect on the nursing newborn.

Diazepam plasma binding in the perinatal period: influence of nonesterified fatty acids

The plasma binding of diazepam was determined serially in 24 women undergoing either elective induction of labour (vaginal or emergency caesarean delivery) or elective caesarean section at term and in 5 nonpregnant women requiring abdominal surgery. In the majority of pregnant patients, a marked increase in diazepam percentage free was observed during labour or prior to caesarean section, reaching a maximum, 1.6 to 3.2 fold increase at delivery or within 4 h postpartum; by the fifth day postpartum, diazepam percentage free was lower than on admission to hospital. In contrast, little change in diazepam percentage free was observed during the perisurgical period in nonpregnant patients. In parturient and surgical patients, the time courses of diazepam percentage free and plasma nonesterified fatty acid (NEFA) concentration were parallel. Bivariate regression analyses of pooled data demonstrated a strong correlation (r = 0.642, p = less than 0.010 between diazepam percentage free and corresponding NEFA concentration and a weaker correlation between diazepam percentage free and both albumin (r = 0.319, p less than 0.02) or total protein (r = 0.438, p less than 0.01). From multiple linear regression it was demonstrated that 54% of the variability in diazepam percentage free could be attributed to plasma NEFA and albumin concentrations. NEFA displacement of plasma bound diazepam was substantiated using crystalline human serum albumin. An approximate 65% increase in plasma alpha 1 acid glycoprotein levels was observed posttrauma in both parturient and surgical patients but was unrelated to diazepam binding events. A relationship between diazepam plasma binding changes and concurrently altered disposition of diazepam during parturition is postulated.

Comparison of the placental transfer of thiopental and diazepam in caesarean section

Drug concentrations were measured in whole blood obtained from mother and child after induction of general anaesthesia with thiopental or diazepam and delivery by Caesarean section. In 27 cases given thiopental 3 mg/kg intravenously the 5-min child/mother concentration ratio rose with increasing injection-delivery (I-D) interval up to 8-10 min. The concentration in the newborn at 2 h showed a similar trend. In 30 cases given diazepam 0.3 mg/kg for sleep induction, there were some low values in cases delivered within 4 min after the injection. However, higher neonatal concentrations and child/mother ratios were observed when the operation lasted 4-5 min, and there was no further increase in cases with longer I-D intervals. There is evidence to suggest that net transfer to the fetus proceeds at a slower rate with thiopental than with diazepam. However, in the present series of low risk elective Caesarean sections, there was no appreciable difference between the induction agents with regard to their effect on the newborn infant.

Drugs in obstetric anaesthesia

From the anaesthetist's viewpoint, pharmacokinetics of pregnancy, drugs affecting uterine muscle tension and foetal effects of anaesthetic drugs are reviewed.