Physiologically-based Pharmacokinetic (PBPK) Models for Assessing the Kinetics of Xenobiotics during Pregnancy: Achievements and Shortcomings

Physiologically-based pharmacokinetic models to predict drug exposure during pregnancy

As pregnant women are constantly exposed to drugs during pregnancy, either to treat long-term conditions or acute illnesses, drug safety is a major concern for the fetus and the mother. Clinical trials are rarely made in this population due to strict regulation and ethical reasons. However, drug pharmacokinetic (PK) parameters vary during pregnancy with an increase in distribution volume, renal clearance and more. In addition, the fetal distribution should be evaluated with the importance of placental diffusion, both active and passive. Therefore, there is a recent interest in the use of physiologically-based pharmacokinetic (PBPK) modeling to characterize these changes and complete the sparse data available on drug PK during pregnancy. Indeed, PBPK models integrate drug physicochemical and physiological parameters corresponding to each compartment of the body to estimate drug concentrations. This review establishes an overview on the current use of PBPK models in drug dosage determination for the pregnant woman, fetal exposure and drug interactions in the fetal compartment.

Precision dosing of venlafaxine during pregnancy: a pharmacokinetics modelling approach

OBJECTIVES

Venlafaxine exposure through gestation is affected by the longitudinal changes in maternal physiology. Confounding treatment is also the impact of CYP2D6 polymorphisms affecting plasma concentrations of venlafaxine.

METHODS

A pharmacokinetic modelling approach was employed to assess variations in maternal and foetal cord venlafaxine levels throughout gestation and to identify appropriate doses to maintain venlafaxine levels within the therapeutic range.

KEY FINDINGS

Throughout gestation, there was a significant decrease in simulated venlafaxine trough plasma concentrations in both extensive metaboliser (EM) and ultra-rapid metaboliser (UM) phenotypes. Approximately 70%-87% of EM and UM phenotypes exhibited trough venlafaxine plasma concentrations below the therapeutic level (<25 ng/ml), which increased to 96% at week 30. While for poor metabolizer (PM) phenotypes, the percentage was approximately 4%.

CONCLUSION

The standard daily dose of 75 mg required adjustment for all phenotypes examined during gestation. A daily dose of 37.5-112.5 mg is appropriate for PM throughout pregnancy. For EM, a dose of 225 mg daily in the first trimester, 262.5 mg daily in the second trimester, and 375 mg daily in the third trimester is suggested to be optimal. For UM, a dose of 375 mg daily throughout gestation is suggested to be optimal.

Pregnancy-related maternal physiological adaptations and fetal chemical exposure

Prenatal life represents a susceptible window of development during which chemical exposures can permanently alter fetal development, leading to an increased likelihood of disease later in life. Therefore, it is essential to assess exposure in the fetus. However, direct assessment in human fetuses is challenging, so most research measures maternal exposure. Pregnancy induces a range of significant physiological changes in women that may affect chemical metabolism and responses. Moreover, placental function, fetal sex, and pregnancy complications may further modify these exposures. The purpose of this narrative review is to give an overview of major pregnancy-related physiological changes, including placental function and impacts of pregnancy complications, to summarize existing studies assessing chemical exposure in human fetal organs, and to discuss possible interactions between physiological changes and exposures. Our review reveals major knowledge gaps in factors affecting fetal chemical exposure, highlighting the need to develop more sophisticated tools for chemical health risk assessment in fetuses.

Development of a physiologically based toxicokinetic model for lead in pregnant women: The role of bone tissue in the maternal and fetal internal exposure

Epidemiological studies have shown associations between prenatal exposure to lead (Pb) and neurodevelopmental effects in young children. Prenatal exposure is generally characterized by measuring the concentration in the umbilical cord at delivery or in the maternal blood during pregnancy. To assess internal Pb exposure during prenatal life, we developed a pregnancy physiologically based pharmacokinetic (p-PBPK) model that to simulates Pb levels in blood and target tissues in the fetus, especially during critical periods for brain development. An existing Pb PBPK model was adapted to pregnant women and fetuses. Using data from literature, both the additional maternal bone remodeling, that causes Pb release into the blood, and the Pb placental transfers were estimated by Bayesian inference. Additional maternal bone remodeling was estimated to start at 21.6 weeks. Placental transfers were estimated between 4.6 and 283 L.day-1 at delivery with high interindividual variability. Once calibrated, the p-PBPK model was used to simulate fetal exposure to Pb. Internal fetal exposure greatly varies over the pregnancy with two peaks of Pb levels in blood and brain at the end of the 1st and 3rd trimesters. Sensitivity analysis shows that the fetal blood lead levels are affected by the maternal burden of bone Pb via maternal bone remodeling and by fetal bone formation at different pregnancy stages. Coupling the p-PBPK model with an effect model such as an adverse outcome pathway could help to predict the effects on children's neurodevelopment.

Evaluation of a rapid, generic human gestational dose model

Chemical risk assessment considers potentially susceptible populations including pregnant women and developing fetuses. Humans encounter thousands of chemicals in their environments, few of which have been fully characterized. Toxicokinetic (TK) information is needed to relate chemical exposure to potentially bioactive tissue concentrations. Observational data describing human gestational exposures are unavailable for most chemicals, but physiologically based TK (PBTK) models estimate such exposures. Development of chemical-specific PBTK models requires considerable time and resources. As an alternative, generic PBTK approaches describe a standardized physiology and characterize chemicals with a set of standard physical and TK descriptors - primarily plasma protein binding and hepatic clearance. Here we report and evaluate a generic PBTK model of a human mother and developing fetus. We used a published set of formulas describing the major anatomical and physiological changes that occur during pregnancy to augment the High-Throughput Toxicokinetics (httk) software package. We simulated the ratio of concentrations in maternal and fetal plasma and compared to literature in vivo measurements. We evaluated the model with literature in vivo time-course measurements of maternal plasma concentrations in pregnant and non-pregnant women. Finally, we prioritized chemicals measured in maternal serum based on predicted fetal brain concentrations. This new model can be used for TK simulations of 859 chemicals with existing human-specific in vitro TK data as well as any new chemicals for which such data become available. This gestational model may allow for in vitro to in vivo extrapolation of point of departure doses relevant to reproductive and developmental toxicity.

Pharmacokinetics and placental transfer of dolutegravir in pregnancy

Dolutegravir is currently recommended by the WHO as the preferred first-line treatment for all people with HIV, including pregnant women. Estimates indicate that, by 2024, nearly 22 million adults in low- and middle-income countries will have transitioned to dolutegravir-based ART. It is therefore critical that there is a clear appreciation and understanding of the risks that may be associated with in utero exposure to dolutegravir. In this review we consolidate data from studies on dolutegravir and the placenta. The studies have largely focused on the pharmacokinetics and placental transfer of dolutegravir in pregnancy. These include studies on transplacental transfer of dolutegravir, ex vivo placenta perfusion models, physiologically based pharmacokinetic (PBPK) models and animal studies. The data available clearly demonstrate that placental transfer of dolutegravir occurs in moderate to high concentrations. Intracellular placental dolutegravir has been demonstrated in the placental villous tissue. There are limited data suggesting that pregnancy is associated with decreased maternal dolutegravir levels. In addition, PBPK models have great potential in predicting the passage of drugs through the placenta and further contributing towards the elucidation of fetal exposure. The animal studies available demonstrate that in utero dolutegravir exposure can be associated with neural tube defects. Taking into consideration that antiretroviral exposure may be associated with poor placental development or function and increased risk of adverse effects to the fetus, it is crucially important that these risks are evaluated, especially with the rapid scale up of dolutegravir-based ART into national treatment programmes.

Precision dosing of methadone during pregnancy: A pharmacokinetics virtual clinical trials study

BACKGROUND

Methadone use for the management of opioid dependency during pregnancy is commonplace. Methadone levels are altered during pregnancy due to changes in maternal physiology. Despite this, a paucity of data exist regarding the most appropriate optimal dosing regimens during pregnancy.

METHODS

This study applied a pharmacokinetic modeling approach to examine gestational changes in R- and S-methadone concentrations in maternal plasma and fetal (cord) blood. This study did so to derive a theoretical optimal dosing regimen during pregnancy, and to identify the impact of Cytochromes P450 (CYP) 2B6 and 2C19 polymorphisms on methadone maternal and fetal pharmacokinetics.

RESULTS

The study noted significant decreases in maternal R- and S-methadone plasma concentrations during gestation, with concomitant increases in fetal levels. At a dose of 90 mg once daily, 75% (R-) and 94% (S-) of maternal methadone trough levels were below the lower therapeutic window at term (week 40). The developed optimal dosing regimen escalated doses to 110 mg by week 5, followed by 10 mg increments every 5 weeks up to a maximum of 180 mg once daily near term. This increase resulted in 27% (R-) and 11% (S-) of subjects with trough levels below the lower therapeutic window at term. CYP2B6 poor metabolizers (PM) and either CYP2C19 extensive metabolizers (EM), PM, or ultra-rapid (UM) metabolizer phenotypes demonstrated statistically significant increases in concentrations when compared to their matched CYP2B6 EM counterparts.

CONCLUSIONS

Specific and gestation-dependent dose titrations are required during pregnancy to reduce the risks associated with illicit drug use and to maintain fetal safety.

The application of precision dosing in the use of sertraline throughout pregnancy for poor and ultrarapid metabolizer CYP 2C19 subjects: A virtual clinical trial pharmacokinetics study

Sertraline is known to undergo changes in pharmacokinetics during pregnancy. CYP 2C19 has been implicated in the interindividual variation in clinical effect associated with sertraline activity. However, knowledge of suitable dose titrations during pregnancy and within CYP 2C19 phenotypes is lacking. A pharmacokinetic modeling virtual clinical trials approach was implemented to: (i) assess gestational changes in sertraline trough plasma concentrations for CYP 2C19 phenotypes, and (ii) identify appropriate dose titration strategies to stabilize sertraline levels within a defined therapeutic range throughout gestation. Sertraline trough plasma concentrations decreased throughout gestation, with maternal volume expansion and reduction in plasma albumin being identified as possible causative reasons. All CYP 2C19 phenotypes required a dose increase throughout gestation. For extensive metabolizer (EM) and ultrarapid metabolizer (UM) phenotypes, doses of 100-150 mg daily are required throughout gestation. For poor metabolizers (PM), 50 mg daily during trimester 1 followed by a dose of 100 mg daily in trimesters 2 and 3 are required.

Evaluation of Placental Transfer and Tissue Distribution of cis- and Trans-Permethrin in Pregnant Rats and Fetuses Using a Physiological-Based Pharmacokinetic Model

Biomonitoring studies have highlighted the exposure of pregnant women to pyrethroids based on the measurement of their metabolites in urine. Pyrethroids can cross the placental barrier and be distributed in the fetus as some pyrethroids were also measured in the meconium of newborns. Prenatal exposure to pyrethroids is suspected to alter the neurodevelopment of children, and animal studies have shown that early life exposure to permethrin, one of the most commonly used pyrethroid in household applications, can alter the brain development. This study aimed to characterize the fetal permethrin exposure throughout gestation in rats. We developed a pregnancy physiologically based pharmacokinetic (pPBPK) model that describes the maternal and fetal kinetics of the cis- and trans- isomers of permethrin during the whole gestation period. Pregnant Sprague-Dawley rats were exposed daily to permethrin (50 mg/kg) by oral route from the start of gestation to day 20. Permethrin isomers were quantified in the feces, kidney, mammary gland, fat, and placenta in dams and in both maternal and fetal blood, brain, and liver. Cis- and trans-permethrin were quantified in fetal blood and tissues, with higher concentrations for the cis-isomer. The pPBPK model was fitted to the toxicokinetic maternal and fetal data in a Bayesian framework. Several parameters were adjusted, such as hepatic clearances, partition coefficients, and intestinal absorption. Our work allowed to estimate the prenatal exposure to permethrin in rats, especially in the fetal brain, and to quantitatively estimate the placental transfer. These transfers could be extrapolated to humans and be incorporated in a human pPBPK model to estimate the fetal exposure to permethrin from biomonitoring data.

Differences in cytochrome p450 enzyme expression and activity in fetal and adult tissues

INTRODUCTION

Human cytochrome p450 (CYP) enzyme expression and activity is lower in the fetus as compared to the adult; however, limited quantitative data exists regarding the specific differences in magnitude or the degree of inducibility due to environmental factors.

METHODS

We utilized a combination of in silico- and molecular-based approaches to profile and compare CYP expression/activity in human adult liver and fetal tissues. Using public datasets, we evaluated human CYP expression between: 1) placenta vs. adult livers; 2) fetal vs. adult livers; or 3) five compartments of the human placenta. We generated new experimental data, characterizing expression levels of nine CYPs in placenta/fetal liver vs. adult liver. In a subset of samples, we evaluated CYP3A4 activity. Finally, we summarized evidence of human fetal CYP expression/activity and environmental exposures during pregnancy.

RESULTS

In silico, CYPs were predominately expressed at higher levels in the adult liver vs. fetal tissues, with a few noted exceptions. Sixty percent of CYP enzymes were expressed at nominal levels in the placenta. In wet-lab analyses, we observed significant CYP-specific differences in expression/activity between adult and fetal tissues; CYP2E1 and -3A4 were expressed significantly lower in fetal vs. adult livers, while CYP2J2 levels were similar.

DISCUSSION

We provide a qualitative review of the expression of the CYP enzyme family in critical sites of xenobiotic distribution during human pregnancy and novel quantitative data regarding fetal CYP expression and activity during mid-gestation. Data outputs may be a resource for modeling predictions of chemical distribution and sensitivity.

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.

Precision dosing-based optimisation of paroxetine during pregnancy for poor and ultrarapid CYP2D6 metabolisers: a virtual clinical trial pharmacokinetics study

Objective

Paroxetine has been demonstrated to undergo gestation-related reductions in plasma concentrations, to an extent which is dictated by the polymorphic state of CYP 2D6. However, knowledge of appropriate dose titrations is lacking.

Methods

A pharmacokinetic modelling approach was applied to examine gestational changes in trough plasma concentrations for CYP 2D6 phenotypes, followed by necessary dose adjustment strategies to maintain paroxetine levels within a therapeutic range of 20–60 ng/ml.

Key findings

A decrease in trough plasma concentrations was simulated throughout gestation for all phenotypes. A significant number of ultrarapid (UM) phenotype subjects possessed trough levels below 20 ng/ml (73–76%) compared to extensive metabolisers (EM) (51–53%).

Conclusions

For all phenotypes studied, there was a requirement for daily doses in excess of the standard 20 mg dose throughout gestation. For EM, a dose of 30 mg daily in trimester 1 followed by 40 mg daily in trimesters 2 and 3 is suggested to be optimal. For poor metabolisers (PM), a 20 mg daily dose in trimester 1 followed by 30 mg daily in trimesters 2 and 3 is suggested to be optimal. For UM, a 40 mg daily dose throughout gestation is suggested to be optimal.

Methodological Approaches to Evaluate Fetal Drug Exposure

BACKGROUND

Drug prescriptions are usual during pregnancy, however, women and their fetuses still remain an orphan population with regard to drugs efficacy and safety. Most xenobiotics diffuse through the placenta and some of them can alter fetus development resulting in structural abnormalities, growth or functional deficiencies.

METHODS

To summarize the different methodologies developed towards the prediction of fetal drug exposure.

RESULTS

Neonatal cord blood concentration is the most specific measurement of the transplacental drug transfer at the end of pregnancy. Using the cord blood and mother drug concentrations altogether, drug exchanges between the mother and fetus can be modeled and quantified via a population pharmacokinetic analysis. Thereafter, it is possible to estimate the fetus exposure and the fetus-to-mother exposure ratio. However, the prediction of placental transfer before any administration to pregnant women is desirable. Animal studies remain difficult to interpret due to structural and functional inter-species placenta differences. The ex-vivo perfusion of the human placental cotyledon is the method of reference to study the human placental transfer of drugs because it is thought to mimic the functional placental tissue. However, extrapolation of data to in vivo situation remains difficult. Some research groups have extensively worked on physiologically based models (PBPK) to predict fetal drug exposure and showed very encouraging results.

CONCLUSION

PBPK models appeared to be a very promising tool in order to predict fetal drug exposure in-silico. However, these models mainly picture the end of pregnancy and knowledge regarding both, development of the placental permeability and transporters is strongly needed.

Quetiapine dose optimisation during gestation: a pharmacokinetic modelling study

Objectives

The second-generation antipsychotic quetiapine has been demonstrated to undergo gestation-related changes in pharmacokinetics. This study applied pharmacokinetic modelling principles to investigate the mechanism of these changes and to propose new dosing strategies to counteract these changes.

Methods

A pharmacokinetic modelling approach was implemented using virtual population groups. Changes in quetiapine trough plasma concentration during gestation were quantified across all trimesters, and dose adjustment strategies were applied to counteract these changes by targeting a therapeutic range of 50–500 ng/ml throughout gestation.

Key findings

The application of the model during gestation predicted a decrease in trough concentration. A maximum decrease of 58% was predicted during trimester 2, and being associated with a statistically significant decrease in oral clearance at gestation week 25, 204 l/h ± 100.8 l/h compared with non-pregnant subjects, 121.9 l/h ± 51.8 l/h. A dosing optimisation strategy identified that dose increases to 500–700 mg twice daily would result in 32–55% of subjects possessing trough concentration in excess of 50 ng/ml.

Conclusions

Quetiapine doses in pregnancy should be increased to 500–700 mg twice daily to counteract a concomitant increase in metabolic clearance, increase in volume of distribution and decrease in plasma protein binding.

Assessment of Maternal and Fetal Dolutegravir Exposure by Integrating Ex Vivo Placental Perfusion Data and Physiologically-Based Pharmacokinetic Modeling

Antiretroviral therapy during pregnancy reduces the risk of vertical HIV‐1 transmission. However, drug dosing is challenging as pharmacokinetics (PK) may be altered during pregnancy. We combined a pregnancy physiologically‐based pharmacokinetic (PBPK) modeling approach with data on placental drug transfer to simulate maternal and fetal exposure to dolutegravir (DTG). First, a PBPK model for DTG exposure in healthy volunteers was established based on physiological and DTG PK data. Next, the model was extended with a fetoplacental unit using transplacental kinetics obtained by performing ex vivo dual‐side human cotyledon perfusion experiments. Simulations of fetal exposure after maternal dosing in the third trimester were in accordance with clinically observed DTG cord blood data. Furthermore, the predicted fetal trough plasma concentration (C_trough) following 50 mg q.d. dosing remained above the concentration that results in 90% of viral inhibition. Our integrated approach enables simulation of maternal and fetal DTG exposure, illustrating this to be a promising way to assess DTG PK during pregnancy.

Fetal Physiologically Based Pharmacokinetic Models: Systems Information on Fetal Blood Components and Binding Proteins

BACKGROUND

Fetal blood and plasma volume and binding components are critical parameters in a fetal physiologically based pharmacokinetic model. To date, a comprehensive review of their changes during fetal development has not been reported.

OBJECTIVE

The objective of this work was to collate and analyze physiological information on fetal blood and plasma volume and binding component data during development and to provide a mathematical description of these parameters that can be integrated within a fetal physiologically based pharmacokinetic model.

METHODS

A comprehensive literature search was conducted on fetal blood and plasma volume and binding component parameters and their changes during growth from apparently healthy fetuses from uncomplicated pregnancies. Collated data were assessed, integrated, and analyzed to establish continuous mathematical functions describing their growth trends with fetal age and weight.

RESULTS

Data were available from 14 studies for blood, ten studies for hematocrit, 12 studies for albumin, and four studies for alpha-1-acid glycoprotein, while plasma and red blood cell volumes were described based on blood and hematocrit data. Fetal physiologically based pharmacokinetic parameters, including blood, plasma and red blood cell volumes, hematocrit, serum albumin, and acid glycoprotein were quantified as a function of fetal age and weight. Variability around the mean parameters at different fetal ages was also investigated. The growth of each of these parameters was different (with respect to direction and monotonicity).

CONCLUSIONS

Despite the limitations identified in the availability of some values, the collected data presented in this article provide a useful resource for fetal physiologically based pharmacokinetic modeling. Potential applications include predicting xenobiotic exposure and risk assessment in the fetus following maternally administered drugs or unintended exposure to environmental toxicants.

Prediction of Fetal Darunavir Exposure by Integrating Human Ex-Vivo Placental Transfer and Physiologically Based Pharmacokinetic Modeling

Background

Fetal antiretroviral exposure is usually derived from the cord-to-maternal concentration ratio. This static parameter does not provide information on the pharmacokinetics in utero, limiting the assessment of a fetal exposure–effect relationship.

Objective

The aim of this study was to incorporate placental transfer into a pregnancy physiologically based pharmacokinetic model to simulate and evaluate fetal darunavir exposure at term.

Methods

An existing and validated pregnancy physiologically based pharmacokinetic model of maternal darunavir/ritonavir exposure was extended with a feto-placental unit. To parameterize the model, we determined maternal-to-fetal and fetal-to-maternal darunavir/ritonavir placental clearance with an ex-vivo human cotyledon perfusion model. Simulated maternal and fetal pharmacokinetic profiles were compared with observed clinical data to qualify the model for simulation. Next, population fetal pharmacokinetic profiles were simulated for different maternal darunavir/ritonavir dosing regimens.

Results

An average (±standard deviation) maternal-to-fetal cotyledon clearance of 0.91 ± 0.11 mL/min and fetal-to-maternal clearance of 1.6 ± 0.3 mL/min was determined (n = 6 perfusions). Scaled placental transfer was integrated into the pregnancy physiologically based pharmacokinetic model. For darunavir 600/100 mg twice a day, the predicted fetal maximum plasma concentration, trough concentration, time to maximum plasma concentration, and half-life were 1.1, 0.57 mg/L, 3, and 21 h, respectively. This indicates that the fetal population trough concentration is higher or around the half-maximal effective darunavir concentration for a resistant virus (0.55 mg/L).

Conclusions

The results indicate that the population fetal exposure after oral maternal darunavir dosing is therapeutic and this may provide benefits to the prevention of mother-to-child transmission of human immunodeficiency virus. Moreover, this integrated approach provides a tool to prevent fetal toxicity or enhance the development of more selectively targeted fetal drug treatments.

Electronic supplementary material

The online version of this article (doi:10.1007/s40262-017-0583-8) contains supplementary material, which is available to authorized users.

Drug Dosing in Pregnant Women: Challenges and Opportunities in Using Physiologically Based Pharmacokinetic Modeling and Simulations

The unmet medical need of providing evidence‐based pharmacotherapy for pregnant women is recognized by the regulatory bodies. Physiologically based pharmacokinetic (PBPK) modeling offers an attractive platform to quantify anticipated changes in the pharmacokinetics (PKs) of drugs during pregnancy. Recent publications applying a pregnancy PBPK module to the prediction of maternal and fetal exposure of drugs are summarized. Future opportunities to use PBPK models to predict breast milk exposure and assess human fetotoxicity risks are presented.

Humans Vary, So Cardiac Models Should Account for That Too!

The utilization of mathematical modeling and simulation in drug development encompasses multiple mathematical techniques and the location of a drug candidate in the development pipeline. Historically speaking they have been used to analyze experimental data (i.e., Hill equation) and clarify the involved physical and chemical processes (i.e., Fick laws and drug molecule diffusion). In recent years the advanced utilization of mathematical modeling has been an important part of the regulatory review process. Physiologically based pharmacokinetic (PBPK) models identify the need to conduct specific clinical studies, suggest specific study designs and propose appropriate labeling language. Their application allows the evaluation of the influence of intrinsic (e.g., age, gender, genetics, disease) and extrinsic [e.g., dosing schedule, drug-drug interactions (DDIs)] factors, alone or in combinations, on drug exposure and therefore provides accurate population assessment. A similar pathway has been taken for the assessment of drug safety with cardiac safety being one the most advanced examples. Mechanistic mathematical model-informed safety evaluation, with a focus on drug potential for causing arrhythmias, is now discussed as an element of the Comprehensive in vitro Proarrhythmia Assay. One of the pillars of this paradigm is the use of an in silico model of the adult human ventricular cardiomyocyte to integrate in vitro measured data. Existing examples (in vitro—in vivo extrapolation with the use of PBPK models) suggest that deterministic, epidemiological and clinical data based variability models can be merged with the mechanistic models describing human physiology. There are other methods available, based on the stochastic approach and on population of models generated by randomly assigning specific parameter values (ionic current conductance and kinetic) and further pruning. Both approaches are briefly characterized in this manuscript, in parallel with the drug-specific variability.

Use of a probabilistic PBPK/PD model to calculate Data Derived Extrapolation Factors for chlorpyrifos

A physiologically based pharmacokinetic and pharmacodynamic (PBPK/PD) model combined with Monte Carlo analysis of inter-individual variation was used to assess the effects of the insecticide, chlorpyrifos and its active metabolite, chlorpyrifos oxon in humans. The PBPK/PD model has previously been validated and used to describe physiological changes in typical individuals as they grow from birth to adulthood. This model was updated to include physiological and metabolic changes that occur with pregnancy. The model was then used to assess the impact of inter-individual variability in physiology and biochemistry on predictions of internal dose metrics and quantitatively assess the impact of major sources of parameter uncertainty and biological diversity on the pharmacodynamics of red blood cell acetylcholinesterase inhibition. These metrics were determined in potentially sensitive populations of infants, adult women, pregnant women, and a combined population of adult men and women. The parameters primarily responsible for inter-individual variation in RBC acetylcholinesterase inhibition were related to metabolic clearance of CPF and CPF-oxon. Data Derived Extrapolation Factors that address intra-species physiology and biochemistry to replace uncertainty factors with quantitative differences in metrics were developed in these same populations. The DDEFs were less than 4 for all populations. These data and modeling approach will be useful in ongoing and future human health risk assessments for CPF and could be used for other chemicals with potential human exposure.

Population pharmacokinetics of 17α-hydroxyprogesterone caproate in singleton gestation

AIMS

17α-hydroxyprogesterone caproate (17-OHPC) reduces the rate of preterm birth in women with a prior preterm birth. Limited data exist on the pharmacokinetics (PK) of 17-OHPC or the plasma concentrations achieved during therapy. In this study, we evaluated the population PK of 17-OHPC in pregnant subjects with singleton gestation and also evaluated intrinsic and extrinsic factors that may potentially affect 17-OHPC PK in this patient population.

METHODS

Sixty-one women with singleton pregnancies participated in this trial. Subjects received weekly intramuscular injections of 250 mg 17-OHPC in 1 ml castor oil from the time of enrolment (16 0/7 weeks - 20 6/7 weeks) up to 35 weeks gestation or until delivery. Blood samples were obtained between 24 and 28 weeks, between 32 and 35 weeks and over a 28-day period beyond the last injection. Maternal and/or cord blood were obtained at delivery. Data analysis was performed by nonlinear mixed effects modelling (NONMEM(®) ).

RESULTS

The 17-OHPC PK were best described by a model with one maternal compartment and one fetal compartment, with first-order absorption and elimination from the maternal compartment. Maternal body weight was a significant covariate for both clearance (CL/F) and volume of distribution (Vmaternal /F). The final population mean estimates were: CL/F 1797 l/d, Vmaternal /F 32 610 l and mother to cord rate constant 0.005 day(-1) . This report describes for the first time the population PK of 17-OHPC in singleton pregnancy.

CONCLUSIONS

The population PK study reported here represents the initial steps in understanding and optimizing 17-OHPC therapy for preventing preterm birth.

Physiologically Based Pharmacokinetic (PBPK) Modeling and Simulation Approaches: A Systematic Review of Published Models, Applications, and Model Verification

Modeling and simulation of drug disposition has emerged as an important tool in drug development, clinical study design and regulatory review, and the number of physiologically based pharmacokinetic (PBPK) modeling related publications and regulatory submissions have risen dramatically in recent years. However, the extent of use of PBPK modeling by researchers, and the public availability of models has not been systematically evaluated. This review evaluates PBPK-related publications to 1) identify the common applications of PBPK modeling; 2) determine ways in which models are developed; 3) establish how model quality is assessed; and 4) provide a list of publically available PBPK models for sensitive P450 and transporter substrates as well as selective inhibitors and inducers. PubMed searches were conducted using the terms "PBPK" and "physiologically based pharmacokinetic model" to collect published models. Only papers on PBPK modeling of pharmaceutical agents in humans published in English between 2008 and May 2015 were reviewed. A total of 366 PBPK-related articles met the search criteria, with the number of articles published per year rising steadily. Published models were most commonly used for drug-drug interaction predictions (28%), followed by interindividual variability and general clinical pharmacokinetic predictions (23%), formulation or absorption modeling (12%), and predicting age-related changes in pharmacokinetics and disposition (10%). In total, 106 models of sensitive substrates, inhibitors, and inducers were identified. An in-depth analysis of the model development and verification revealed a lack of consistency in model development and quality assessment practices, demonstrating a need for development of best-practice guidelines.

Pharmacometrics in pregnancy: An unmet need

Pregnant women and their fetuses are orphan populations with respect to the safety and efficacy of drugs. Physiological and absorption, distribution, metabolism, and excretion (ADME) changes during pregnancy can significantly affect drug pharmacokinetics (PK) and may necessitate dose adjustment. Here, the specific aspects related to the design, execution, and analysis of clinical studies in pregnant women are discussed, underlining the unmet need for top-down pharmacometrics analyses and bottom-up modeling approaches. The modeling tools that support data analysis for the pregnancy population are reviewed, with a focus on physiologically based pharmacokinetics (PBPK) and population pharmacokinetics (POP-PK). By integrating physiological data, preclinical data, and clinical data (e.g., via POP-PK) to quantify anticipated changes in the PK of drugs during pregnancy, the PBPK approach allows extrapolation beyond the previously studied model drugs to other drugs with well-characterized ADME characteristics. Such a systems pharmacology approach can identify drugs whose PK may be altered during pregnancy, guide rational PK study design, and support dose adjustment for pregnant women.

A pregnancy physiologically based pharmacokinetic (p-PBPK) model for disposition of drugs metabolized by CYP1A2, CYP2D6 and CYP3A4

AIMS

Pregnant women are usually not part of the traditional drug development programme. Pregnancy is associated with major biological and physiological changes that alter the pharmacokinetics (PK) of drugs. Prediction of the changes to drug exposure in this group of patients may help to prevent under- or overtreatment. We have used a pregnancy physiologically based pharmacokinetic (p-PBPK) model to assess the likely impact of pregnancy on three model compounds, namely caffeine, metoprolol and midazolam, based on the knowledge of their disposition in nonpregnant women and information from in vitro studies.

METHODS

A perfusion-limited form of a 13-compartment full-PBPK model (Simcyp® Simulator) was used for the nonpregnant women, and this was extended to the pregnant state by applying known changes to all model components (including the gestational related activity of specific cytochrome P450 enzymes) and through the addition of an extra compartment to represent the fetoplacental unit. The uterus and the mammary glands were grouped into the muscle compartment. The model was implemented in Matlab Simulink and validated using clinical observations.

RESULTS

The p-PBPK model predicted the PK changes of three model compounds (namely caffeine, metoprolol and midazolam) for CYP1A2, CYP2D6 and CYP3A4 during pregnancy within twofold of observed values. The changes during the third trimester were predicted to be a 100% increase, a 30% decrease and a 35% decrease in the exposure of caffeine, metoprolol and midazolam, respectively, compared with the nonpregnant women.

CONCLUSIONS

In the absence of clinical data, the in silico prediction of PK behaviour during pregnancy can provide a valuable aid to dose adjustment in pregnant women. The performance of the model for drugs metabolized by a single enzyme to different degrees (high and low extraction) and for drugs that are eliminated by several different routes warrants further study.

A physiologically based pharmacokinetic model to predict disposition of CYP2D6 and CYP1A2 metabolized drugs in pregnant women

Conducting pharmacokinetic (PK) studies in pregnant women is challenging. Therefore, we asked if a physiologically based pharmacokinetic (PBPK) model could be used to evaluate different dosing regimens for pregnant women. We refined and verified our previously published pregnancy PBPK model by incorporating cytochrome P450 CYP1A2 suppression (based on caffeine PK) and CYP2D6 induction (based on metoprolol PK) into the model. This model accounts for gestational age-dependent changes in maternal physiology and hepatic CYP3A activity. For verification, the disposition of CYP1A2-metabolized drug theophylline (THEO) and CYP2D6-metabolized drugs paroxetine (PAR), dextromethorphan (DEX), and clonidine (CLO) during pregnancy was predicted. Our PBPK model successfully predicted THEO disposition during the third trimester (T3). Predicted mean postpartum to third trimester (PP:T3) ratios of THEO area under the curve (AUC), maximum plasma concentration, and minimum plasma concentration were 0.76, 0.95, and 0.66 versus observed values 0.75, 0.89, and 0.72, respectively. The predicted mean PAR steady-state plasma concentration (Css) ratio (PP:T3) was 7.1 versus the observed value 3.7. Predicted mean DEX urinary ratio (UR) (PP:T3) was 2.9 versus the observed value 1.9. Predicted mean CLO AUC ratio (PP:T3) was 2.2 versus the observed value 1.7. Sensitivity analysis suggested that a 100% induction of CYP2D6 during T3 was required to recover the observed PP:T3 ratios of PAR Css, DEX UR, and CLO AUC. Based on these data, it is prudent to conclude that the magnitude of hepatic CYP2D6 induction during T3 ranges from 100 to 200%. Our PBPK model can predict the disposition of CYP1A2, 2D6, and 3A drugs during pregnancy.

A PBPK Model to Predict Disposition of CYP3A-Metabolized Drugs in Pregnant Women: Verification and Discerning the Site of CYP3A Induction

Besides logistical and ethical concerns, evaluation of safety and efficacy of medications in pregnant women is complicated by marked changes in pharmacokinetics (PK) of drugs. For example, CYP3A activity is induced during the third trimester (T₃). We explored whether a previously published physiologically based pharmacokinetic (PBPK) model could quantitatively predict PK profiles of CYP3A-metabolized drugs during T₃, and discern the site of CYP3A induction (i.e., liver, intestine, or both). The model accounted for gestational age-dependent changes in maternal physiological function and hepatic CYP3A activity. For model verification, mean plasma area under the curve (AUC), peak plasma concentration (C_max), and trough plasma concentration (C_min) of midazolam (MDZ), nifedipine (NIF), and indinavir (IDV) were predicted and compared with published studies. The PBPK model successfully predicted MDZ, NIF, and IDV disposition during T₃. A sensitivity analysis suggested that CYP3A induction in T₃ is most likely hepatic and not intestinal. Our PBPK model is a useful tool to evaluate different dosing regimens during T₃ for drugs cleared primarily via CYP3A metabolism.

Pharmacotherapy in pregnancy; effect of ABC and SLC transporters on drug transport across the placenta and fetal drug exposure

Pharmacotherapy during pregnancy is often inevitable for medical treatment of the mother, the fetus or both. The knowledge of drug transport across placenta is, therefore, an important topic to bear in mind when deciding treatment in pregnant women. Several drug transporters of the ABC and SLC families have been discovered in the placenta, such as P-glycoprotein, breast cancer resistance protein, or organic anion/cation transporters. It is thus evident that the passage of drugs across the placenta can no longer be predicted simply on the basis of their physical-chemical properties. Functional expression of placental drug transporters in the trophoblast and the possibility of drug-drug interactions must be considered to optimize pharmacotherapy during pregnancy. In this review we summarize current knowledge on the expression and function of ABC and SLC transporters in the trophoblast. Furthermore, we put this data into context with medical conditions that require maternal and/or fetal treatment during pregnancy, such as gestational diabetes, HIV infection, fetal arrhythmias and epilepsy. Proper understanding of the role of placental transporters should be of great interest not only to clinicians but also to pharmaceutical industry for future drug design and development to control the degree of fetal exposure.