Analysis of pethidine disposition in the pregnant rat by means of a physiological flow model

Salivary Therapeutic Monitoring of Buprenorphine in Neonates After Maternal Sublingual Dosing Guided by Physiologically Based Pharmacokinetic Modeling

BACKGROUND

Opioid use disorder (OUD) during pregnancy is associated with high mortality rates and neonatal opioid withdrawal syndrome (NOWS). Buprenorphine, an opioid, is used to treat OUD and NOWS. Buprenorphine active metabolite (norbuprenorphine) can cross the placenta and cause neonatal respiratory depression (EC 50 = 35 ng/mL) at high brain extracellular fluid (bECF) levels. Neonatal therapeutic drug monitoring using saliva decreases the likelihood of distress and infections associated with frequent blood sampling.

METHODS

An adult physiologically based pharmacokinetic model for buprenorphine and norbuprenorphine after intravenous and sublingual administration was constructed, vetted, and scaled to newborn and pregnant populations. The pregnancy model predicted that buprenorphine and norbuprenorphine doses would be transplacentally transferred to the newborns. The newborn physiologically based pharmacokinetic model was used to estimate the buprenorphine and norbuprenorphine levels in newborn plasma, bECF, and saliva after these doses.

RESULTS

After maternal sublingual administration of buprenorphine (4 mg/d), the estimated plasma concentrations of buprenorphine and norbuprenorphine in newborns exceeded the toxicity thresholds for 8 and 24 hours, respectively. However, the norbuprenorphine bECF levels were lower than the respiratory depression threshold. Furthermore, the salivary buprenorphine threshold levels in newborns for buprenorphine analgesia, norbuprenorphine analgesia, and norbuprenorphine hypoventilation were observed to be 22, 2, and 162 ng/mL.

CONCLUSIONS

Using neonatal saliva for buprenorphine therapeutic drug monitoring can facilitate newborn safety during the maternal treatment of OUD using sublingual buprenorphine. Nevertheless, the suitability of using adult values of respiratory depression EC 50 for newborns must be confirmed.

Evaluating the Pharmacokinetics of Fentanyl in the Brain Extracellular Fluid, Saliva, Urine, and Plasma of Newborns from Transplacental Exposure from Parturient Mothers Dosed with Epidural Fentanyl Utilizing PBPK Modeling

BACKGROUND AND OBJECTIVE

Fentanyl can mitigate the mother and newborn complications resulting from labor pain. However, fentanyl shows a narrow therapeutic index between its respiratory depressive and analgesic effects. Thus, prenatally acquired high fentanyl levels in the newborn brain extracellular fluid (bECF) may induce respiratory depression which requires therapeutic drug monitoring (TDM). TDM using saliva and urine in newborns can reduce the possibility of infections and distress associated with TDM using blood. The objective of this study was to develop  a physiologically based pharmacokinetic (PBPK) model to predict fentanyl concentrations in different newborn tissues due to intrauterine exposure.

METHODS

A fentanyl PBPK model in adults after intravenous and epidural administration was built, validated, and scaled to pregnancy and newborn populations. The dose that the newborn received transplacentally at birth was calculated using the pregnancy model. Then, the newborn bECF, saliva, plasma, and urine concentrations after such a dose were predicted using the newborn PBPK model.

RESULTS

After a maternal epidural dose of fentanyl 245 µg, the predicted newborn plasma and bECF levels were below the toxicity thresholds. Furthermore, the salivary threshold levels in newborns for fentanyl analgesic and respiratory depression effects were estimated to be 0.39 and 14.7-18.2 ng/ml, respectively.

CONCLUSION

The salivary TDM of fentanyl in newborns can be useful in newborns exposed to intrauterine exposure from parturient females dosed with epidural fentanyl. However, newborn-specific values of µ-opioid receptors IC50 for respiratory depression are needed.

The Analysis of Pethidine Pharmacokinetics in Newborn Saliva, Plasma, and Brain Extracellular Fluid After Prenatal Intrauterine Exposure from Pregnant Mothers Receiving Intramuscular Dose Using PBPK Modeling

BACKGROUND AND OBJECTIVE

Pethidine (meperidine) can decrease labor pain-associated mother's hyperventilation and high cortisol-induced newborn complications. However, prenatal transplacentally acquired pethidine can cause side effects in newborns. High pethidine concentrations in the newborn brain extracellular fluid (bECF) can cause a serotonin crisis. Therapeutic drug monitoring (TDM) in newborns' blood distresses them and increases infection incidence, which can be overcome by using salivary TDM. Physiologically based pharmacokinetic (PBPK) modeling can predict drug concentrations in newborn plasma, saliva, and bECF after intrauterine pethidine exposure.

METHODS

A healthy adult PBPK model was constructed, verified, and scaled to newborn and pregnant populations after intravenous and intramuscular pethidine administration. The pregnancy PBPK model was used to predict the newborn dose received transplacentally at birth, which was used as input to the newborn PBPK model to predict newborn plasma, saliva, and bECF pethidine concentrations and set correlation equations between them.

RESULTS

Pethidine can be classified as a Salivary Excretion Classification System class II drug. The developed PBPK model predicted that, after maternal pethidine intramuscular doses of 100 mg and 150 mg, the newborn plasma and bECF concentrations were below the toxicity thresholds. Moreover, it was estimated that newborn saliva concentrations of 4.7 µM, 11.4 µM, and 57.7 µM can be used as salivary threshold concentrations for pethidine analgesic effects, side effects, and the risk for serotonin crisis, respectively, in newborns.

CONCLUSION

It was shown that saliva can be used for pethidine TDM in newborns during the first few days after delivery to mothers receiving pethidine.

Application of the relationship between pharmacokinetics and pharmacodynamics in drug development and therapeutic equivalence: a PEARRL review

Objectives

The objective of this review was to provide an overview of pharmacokinetic/pharmacodynamic (PK/PD) models, focusing on drug-specific PK/PD models and highlighting their value added in drug development and regulatory decision-making.

Key findings

Many PK/PD models, with varying degrees of complexity and physiological understanding have been developed to evaluate the safety and efficacy of drug products. In special populations (e.g. paediatrics), in cases where there is genetic polymorphism and in other instances where therapeutic outcomes are not well described solely by PK metrics, the implementation of PK/PD models is crucial to assure the desired clinical outcome. Since dissociation between the pharmacokinetic and pharmacodynamic profiles is often observed, it is proposed that physiologically based pharmacokinetic and PK/PD models be given more weight by regulatory authorities when assessing the therapeutic equivalence of drug products.

Summary

Modelling and simulation approaches already play an important role in drug development. While slowly moving away from ‘one-size fits all’ PK methodologies to assess therapeutic outcomes, further work is required to increase confidence in PK/PD models in translatability and prediction of various clinical scenarios to encourage more widespread implementation in regulatory decision-making.

Area/moment and compartmental modeling of pharmacokinetics during pregnancy: applications to maternal/fetal exposures to corticosteroids in sheep and rats

PURPOSE

The pharmacokinetics of corticosteroids in pregnancy were analyzed to assess maternal/fetal disposition and factors controlling fetal exposure. Area/Moment equations and compartmental models for estimating pharmacokinetic parameters from single dose data during pregnancy were developed.

METHODS

Betamethasone in the maternal/fetal circulations of sheep was measured by HPLC after maternal intramuscular injection (n = 4) of 170 microg kg(-1) of a depot formulation. Additional data for beta-methasone in sheep and dexamethasone pharmacokinetics in rats were obtained from the literature. Area/Moment equations were derived using mass balance concepts, statistical moments, and Laplace theory. Area/Moment analysis, compartmental modeling, and allometric scaling to man for betamethasone were performed using WinNonlin and ADAPT II programs.

RESULTS

Polyexponential maternal/fetal profiles for corticosteroids were observed. Clearance terms for corticosteroid transfer from fetus to mother were 4-fold higher than the clearance term for transfer in the opposite direction. A placental efflux process may restrict fetal access of corticosteroids which are known PGP substrates. The elimination clearance estimates indicate that fetal metabolism plays a minor role in corticosteroid elimination.

CONCLUSIONS

Generalized and specific models for maternal/fetal pharmacokinetics were developed. An efflux transport mechanism, such as the known placental expression of PGP, could explain the limited fetal exposure of corticosteroids.

Evaluation of physiologically based models of pregnancy and lactation for their application in children's health risk assessments

In today's scientific and regulatory climates, an increased emphasis is placed on the potential health impacts for children exposed either in utero or by nursing to drugs of abuse, pharmaceuticals, and industrial or consumer chemicals. As a result, there is a renewed interest in the development and application of biologically based computational models that can be used to predict the dosimetry (or ultimately response) in a developing embryo, fetus, or newborn. However, fundamental differences between animal and human development can create many unique challenges. For example, unlike models designed for adults,biologically based models of pre-and postnatal development must deal with rapidly changing growth dynamics (maternal embryonic, fetal, and neonatal), changes in the state of differentiation of developing tissues, uniquely expressed or uniquely functioning signal transduction or enzymatic pathways, and unusual routes of exposure (e.g., maternal-mediated placental transfer and lactation). In cases where these challenges are overcome or addressed, biological modeling will likely prove useful in assessments geared toward children's health, given the contributions that this approach has already made in cancer and non-cancer human health risk assessments. Therefore, the purpose of this review is to critically evaluate the current state of the art in physiologically based pharmacokinetic (PBPK) and pharmacodynamic (PD) modeling of the developing embryo, fetus, or neonate and to recommend potential steps that could be taken to improve their use in children's health risk assessments. The intent was not to recommend improvements to individual models per se, but to identify areas of research that could move the entire field forward. This analysis includes a brief summary of current risk assessment practices for developmental toxicity, with an overview of developmental biology as it relates to species-specific dosimetry. This summary should provide a general context for understanding the tension that exists in modeling between describing biological proceses in exquisite detail vs. the simplifications that are necessary due to lack of data (or through a sensitivity analysis, determined to be of little impact) to develop individual PBPK or PD models. For each of the previously published models covered in this review, a description of the underlying assumptions and model structures as well as the data and methods used in model development and validation are highlighted. Although several of the models attempted to describe target tissues in the developing embryo, fetus, or neonate of laboratory animals, extrapolations to humans were largely limited to maternal blood or milk concentrations. Future areas of research therefore are recommended to extend the already significant progress that has been made in this field and perhaps address many of the technical policy, and ethical issues surrounding various approaches for decreasing the uncertainty in extrapolating from animal models to human pregnancies or neonatal exposures.

Comparative physiological pharmacokinetics of fentanyl and alfentanil in rats and humans based on parametric single-tissue models

The objectives of this investigation were to characterize the disposition of fentanyl and alfentanil in 14 tissues in the rat, and to create physiological pharmacokinetic models for these opioids that would be scalable to man. We first created a parametric submodel for the disposition of either drug in each tissue and then assembled these submodels into whole-body models. The disposition of fentanyl and alfentanil in the heart and brain and of fentanyl in the lungs could be described by perfusion-limited 1-compartment models. The disposition of both opioids in all other examined tissues was characterized by 2- or 3-compartment models. From these models, the extraction ratios of the opioids in the various tissues could be calculated, confirming the generally lower extraction of alfentanil as compared to fentanyl. Assembly of the single-tissue models resulted in a wholebody model for fentanyl that accurately described its disposition in the rat. A similar assembly of the tissue models for alfentanil revealed non-first-order elimination kinetics that were not apparent in the blood concentration data. Michaelis-Menten parameters for the hepatic metabolism of alfentanil were determined by iterative optimization of the entire model. The parametric models were finally scaled to describe the disposition of fentanyl and alfentanil in humans.

A physiologically based pharmacokinetic computer model for human pregnancy

A physiologically based pharmacokinetic (PBPK) model for human pregnancy must incorporate many factors that are not usually encountered in PBPK models of mature animals. Models for pregnancy must include the large changes that take place in the mother, the placenta and the embryo/fetus over the period of pregnancy. The embryo/fetal weight change was modeled using the Gompertz equation for growth which gave a good fit to extensive pooled weight data of the human embryo/fetus from 25 to 300 days of gestation. This equation is based on a growth rate that is proportional to the total weight of the organism with the proportionality factor decreasing exponentially with time. Allometric equations, which are widely used to relate organ weights, blood flow rates and other attributes of mature animals to total weight, were adapted to correlate fetal organ weights with total fetal weight. Allometric relationships were also developed for plasma flow rates and other organ-related parameters. The computer model, written in FORTRAN 77, included 27 compartments for the mother and 16 for the fetus; it also accommodates two substances allowing representation of a parent compound and a metabolite (or a second drug or environmental substance). Although this model is large, the inherent sparsity in the equations allow it to be solved numerically in a reasonable time on currently available, reasonably priced desktop computers. A nonlinear regression routine is included to fit key model parameters to experimental data. Concentrations of chemicals administered and measured in the mother may be simulated in both maternal and fetal organs at any day(s) between 25 days and 300 days of gestation. Allometric relationships are also utilized to adopt this human model for use with data obtained from animal experiments.

Pharmacokinetic-pharmacodynamic modelling of meperidine in goats (I): Pharmacokinetics

Plasma and cerebrospinal fluid (CSF) pharmacokinetics of meperidine were investigated after intramuscular (i.m.) or intravenous (i.v.) administration at a dose of 5 mg/kg in adult goats. After i.m. dosing, the plasma profile was best described by a one-compartment open model. In healthy (n = 16) and post-operative (n = 16) goats, the parameters were, respectively: tmax 8.3 +/- 3.9 and 9.2 +/- 5.5 min, Vd 2.763 +/- 1.231 and 3.929 +/- 2.101 l/kg, Clb 0.125 +/- 0.036 and 0.087 +/- 0.025 l/kg/min, Ke 0.0563 +/- 0.0358 and 0.0271 +/- 0.0136 min-1. The plasma profile was best fitted by a two-compartment open model following i.v. injection. In this case, the parameters for healthy (n = 7) and post-operative (n = 13) goats were, respectively: Vd 5.212 +/- 1.992 and 5.085 +/- 2.288 l/kg, Clb 0.096 +/- 0.028 and 0.075 +/- 0.026 l/kg/min, beta 0.0211 +/- 0.0093 and 0.0160 +/- 0.0052 min-1. There were, however, a few individuals with a prolonged elimination phase. Bioavailability of i.m. meperidine was 66.5 +/- 15.8% in healthy (n = 6) goats, but much higher in postoperative (n = 10) ones at 94.6 +/- 30.0%. Meperidine diffused into and out of CSF according to a first-order rate process. The time-course of CSF drug concentration was simulated by a biexponential function. CSF kinetic parameters of i.m. meperidine for healthy (n = 7) and postoperative (n = 13) goats were: elimination rate constant (K(ef)) 0.0269 +/- 0.0131 and 0.0305 +/- 0.0177 min-1, peak CSF concentration time (Tmaxf) 15.9 +/- 5.0 and 17.0 +/- 6.9 min. For the i.v. dosed healthy (n = 6) and postoperative (n = 8) animals, K(ef) was 0.0408 +/- 0.0107, 0.0414 +/- 0.0123 min-1 and Tmaxf was 10.0 +/- 5.0 and 7.7 +/- 2.5 min, respectively. It was demonstrated that an obviously lower peak concentration can be reached significantly later in CSF than in plasma, and the kinetic behaviour of meperidine in plasma is different from that in the CSF, indicating meperidine analgesia might not be predicted by simple extrapolation from the kinetic data.

Tissue distribution of fentanyl and alfentanil in the rat cannot be described by a blood flow limited model

Traditionally, physiological pharmacokinetic models assume that arterial blood flow to tissue is the rate-limiting step in the transfer of drug into tissue parenchyma. When this assumption is made the tissue can be described as a well-stirred single compartment. This study presents the tissue washout concentration curves of the two opioid analgesics fentanyl and alfentanil after simultaneous 1-min iv infusions in the rat and explores the feasibility of characterizing their tissue pharmacokinetics, modeling each of the 12 tissues separately, by means of either a one-compartment model or a unit disposition function. The tissue and blood concentrations of the two opioids were measured by gas-liquid chromatography. The well-stirred one-compartment tissue model could reasonably predict the concentration-time course of fentanyl in the heart, pancreas, testes, muscle, and fat, and of alfentanil in the brain and heart only. In most other tissues, the initial uptake of the opioids was considerably lower than predicted by this model. The unit disposition functions of the opioids in each tissue could be estimated by nonparametric numerical deconvolution, using the arterial concentration times tissue blood flow as the input and measured tissue concentrations as the response function. The observed zero-time intercepts of the unit disposition functions were below the theoretical value of one, and were invariably lower for alfentanil than for fentanyl. These findings can be explained by the existence of diffusion barriers within the tissues and they also indicate that alfentanil is less efficiently extracted by the tissue parenchyma than the more lipophilic compound fentanyl. The individual unit disposition functions obtained for fentanyl and alfentanil in 12 rat tissues provide a starting point for the development of models of intratissue kinetics of these opioids. These submodels can then be assembled into full physiological models of drug disposition.

Pulmonary disposition of pethidine in postoperative patients

1. Two methods of pethidine administration, namely constant-rate infusion and single i.v. injection, were used to assess the pulmonary disposition of the drug in 10 postoperative patients. Using two sites of blood sampling, the pulmonary extraction ratio was determined. 2. Pronounced pulmonary uptake of pethidine was found in all patients (n = 10). On the other hand, there was no significant evidence of pulmonary clearance. 3. The mean total plasma clearance was 810 ml min-1 and the volume of distribution was 3.11 kg-1. 4. A flow model was used to describe the disposition of pethidine in man. The concentration-time profiles calculated by the model were in accordance with observed data. The data showed that both pulmonary uptake and pulmonary release of pethidine were rapid. 5. Constant-rate infusion was found advantageous in the determination of pulmonary extraction, with respect to the accuracy and precision of the results. The extraction obtained after a single injection may be overestimated on account of uptake of the drug by the lungs.