MPRINT’s Modeling Support (MMS) will focus on enhancing the synergies between the MPRINT Hub Knowledgebase and the NIH Common Fund Data Ecosystem (CFDE). The key objective of the MMS effort is to build and populate a Modeling Knowledge Repository, which will centralize and extend MPRINT's knowledge resources on quantitative pharmacology models relevant to maternal and pediatric health. In addition, the project addresses the two-way interoperability between resources in MPRINT the CFDE data to enhance findability and access to biomedical data, such as tissue expression or biomarker data. Furthermore, the MMS effort will promote collaboration between MPRINT and CF communities through joint training workshops, knowledge dissemination, and the creation of a shared roadmap for training.

Aortic root enlargement, a common manifestation of the connective tissue disorder, Marfan syndrome, is associated with life-threatening aortic root dissection. To mitigate this risk, initiation of prophylactic anti-hypertensive drug therapy is recommended. Atenolol, a beta blocker, is commonly utilized. Unfortunately, patients continue to have suboptimal response to drug therapy. The human organic cation uptake transporters (OCT) 1 and 2 are influx transporters involved in the uptake of atenolol. This objective of this study, Maximizing Atenolol Response through Functional Assessment iNvestigations (MARFAN), is to determine the role of genetic variants on the cellular uptake and clearance of atenolol, which may thereby affect drug disposition and clinical effect. This project will characterize the role of OCT1 and OCT2 on atenolol cellular uptake and test the hypothesis that OCT1 and OCT2 proteins with genetic polymorphisms, when expressed in vitro, will result in diminished cellular uptake of atenolol relative to the reference genotype. This approach will utilize transiently transfected OCT1 and OCT2 cell assays to perform time-dependent atenolol uptake experiments to generate kinetics parameters. The rationale for this study is to provide a mechanistic understanding of polymorphisms associated with OCT1 and OCT2 expression, and function as a means to individualize pharmacologic therapy for not only atenolol, but other OCT1 and OCT2-dependent drugs utilized in cardiovascular patients. Dr. Walton is supported by the mentorship of Jon Wagner, DO (Children’s Mercy Hospital-Kansas City) and Bruno Hagenbuch, PhD (University of Kansas Medical Center).

Understanding drug disposition in the brain is critical to 1) uncovering the link between brain exposure and clinical response, tolerability, and safety, 2) enabling inquiry into mechanistic changes during dynamic periods of life such as childhood and pregnancy, and 3) improving pharmacometric precision. At present, brain disposition is generally inferred from experimentally derived drug distribution characteristics, such as lipophilicity and blood-brain barrier efflux. Positron emission topography (PET) imaging can be used to elucidate preclinical brain concentrations and the pharmacodynamic, target receptor binding properties of a drug, but does not directly measure brain concentrations following clinically-relevant dosing in humans. Further, due to radiation exposure, PET poses ethical challenges in children and pregnant people, who are traditionally not candidates for this higher risk method, despite their being end-users of the candidate medication. Alternative feasible and minimal risk strategies are needed to ensure all populations are included in research that promotes safe and effective use of medications. 19F-magnetic resonance spectroscopy (19F-MRS) is a non-invasive, non-radioactive MRI method that detects naturally-occurring, non-radioactive fluorine. Nearly 20% of FDA-approved medications include fluorine to improve solubility and cellular uptake, including many drugs targeting the central nervous system (CNS). We leverage the existing fluorine(s) in each drug molecule to detect their MRS signal in living human brain without ionizing radiation. The number of fluorines in the drug molecule impacts the signal strength, with stronger signal expected with 3 fluorines per molecule (e.g., fluoxetine, C17H18F3NO) compared with two (pantoprazole, C16H15F2N3O4S), for example. Our pilot work with fluoxetine successfully quantified brain concentrations in adolescents across a range of doses and demonstrated 3-fold variability in brain concentration at any given dose. The goal of this project is to further develop 19F-MRS as safe, non-invasive, and highly sensitive tool to detect drug concentrations in the brain. We aim to 1) increase the sensitivity of 19F-MRS, 2) broaden the detectability of 19F-MRS to various fluorine-containing medicines, and 3) determine the feasibility of using 19F-MRS in youth of different ages and sizes. Our translational approach pairs in vitro experiments using phantom standards with in vivo optimization in humans. We will identify the most sensitive technique, comparing 3 distinct head coil designs (i.e., single loop, birdcage and helmet), then optimize and validate 19F-MRS parameters in youth taking fluoxetine. The enhanced 19F-MRS scan protocol will then be adapted for other drugs with varying fluorine signal strength (e.g., pantoprazole and lansoprazole). Successful completion of this work with demonstrate analytical validity and feasibility of 19F-MRS neuroimaging in pediatrics. The downstream applications of this technique are vast, including changing the landscape of drug development by generating data from a physiologic compartment not otherwise accessible (i.e., the brain), and enabling safer, more precise use of therapeutics.

Vitamin D and its metabolites are crucial in early life, including bone growth, metabolic regulation, and immune system development. Children with cancer may be particularly susceptible to fluctuating vitamin D levels because of their illness, younger age, extensive chemotherapy, reduced outdoor activities, and limited sun exposure. Consistent with other reports, our preliminary study demonstrated more than one-third of previously healthy children exhibited notable vitamin D deficiency upon diagnosis with cancer across diverse malignancies. However, health literacy on how vitamin D deficiency impacts pediatric oncology patients and their diseases is very limited. Immaturity of the gut microbiome has been shown to contribute to the development of childhood acute lymphoblastic leukemia (ALL), the most commonly diagnosed malignancy among children.

Preeclampsia is a major complication that affects ~5-8% of pregnancies. When preeclampsia is diagnosed prior to 34 weeks gestation, continuation of pregnancy is recommended to reduce the risks of infant morbidity and mortality associated with prematurity. It is possible to prolong pregnancy in this case by controlling severely elevated blood pressures with medications such as Nifedipine, a calcium channel blocker that is first-line in pregnancy. Little is known about the pharmacokinetics of this drug in pregnancy despite its widespread use, nor its optimal dosing. ​ This is a randomized controlled unblinded trial comparing the rates of elevated blood pressure between individuals on Nifedipine XL 60mg daily to 30mg twice daily in patients admitted for expectant management of severe preeclampsia. Secondarily, we are assessing the pharmacokinetic measures of Nifedipine in pregnancy by analyzing nifedipine concentations in plasma at different timepoints after Nifedipine XL administration.

Babies exposed to opioids before birth often develop neonatal opioid withdrawal syndrome (NOWS) after birth. Buprenorphine is an emerging therapy for NOWS; however, establishing optimal dosing regimens in this population remains an unmet clinical need. This pilot project will employ a physiologically-based pharmacokinetic (PBPK) modeling approach to predict buprenorphine disposition in pregnant women and fetuses to evaluate the association of buprenorphine prenatal exposure with postnatal NOWS severity. We will also investigate the effect of neonatal genetic variants on postnatal response to buprenorphine treatment. Our overarching goal is to inform postnatal buprenorphine treatment based on prenatally predicted NOWS severity and enable PK/PD and pharmacogenetics-guided buprenorphine dose tailoring.

Preeclampsia (PE) occurs in 5-8% of pregnancies and is associated with significant maternal and neonatal morbidity and mortality. For over decades, the role of aspirin in the primary or secondary prevention of preeclampsia has been the subject of numerous trials using a variety doses. To date, professional societies recommend use of low-dose aspirin for preeclampsia prevention in pregnant individuals at high-risk of developing the disease, but the optimal dose is unknown. Furthermore, there are limited biomarkers available as candidates for monitoring therapeutic response to aspirin treatment in pregnant individuals at high-risk for PE, which ultimately may be used to generate desirable data to optimize aspirin dosing and improve pregnancy outcomes. Recent advancement in isolating specific extracellular vesicles (exosomes; 40–160 nm) from maternal blood and characterizing their cargo content have helped to better understand the response to therapeutic interventions. Although diagnostic potential of exosomes has been reported, no studies have examined the response to therapeutic interventions such as different doses of aspirin on exosomes. Therefore, we are proposing this proof-of-concept, exosome profiling study to characterize the response to aspirin therapy in patients at high-risk of PE. Using previously collected samples from over 100 high-risk patients assigned to 81 or 162mg of aspirin daily, we plan to characterize the maternal plasma proteome profile in high-risk individuals receiving different doses of daily aspirin and demonstrate that the proteome profile from patients receiving the higher dose of aspirin is more likely to mimic what is seen in low-risk control group, compared with the profile in patients receiving the lower dose of aspirin.

Opioid use disorder continues to be a significant contributor of both maternal morbidity and mortality as well as adverse neonatal and early childhood outcomes. Methadone remains a mainstay of therapy. Despite long documented alteration of methadone pharmacokinetics in pregnancy, there is no guideline for modified dosing interval for methadone in pregnancy. Suboptimal or subtherapeutic dosing in pregnancy due to use of dosing regimens for non-pregnant adults likely contributes to relapse and loss to follow up. This project has two aims, first to evaluate whether a methadone:metabolite ratio during the course of methadone induction in pregnancy is a helpful pharmacodynamic end point to guide dosing interval/frequency in pregnancy to help develop trimester specific dosing guidelines. Second, to evaluate the impact of pharmacogenomics on methadone induction in pregnancy to determine the utility of genotyping in order to guide dosing.

Current weight-based dosing algorithms (mg/kg) in the Neonatal ICU lead to varying drug exposures and require multiple dose modifications for optimal efficacy. This imprecision leads to highly variable drug concentrations, therapeutic failure, and significant toxicity. Sub-optimal drug efficacy in neonates can lead to lifelong impairments and overdosing can lead to life-threatening complications requiring additional tests and intensive treatments. Two commonly prescribed drugs with narrow therapeutic windows, and variable and unpredictable pharmacokinetics are the low molecular weight heparin enoxaparin (anticoagulation) and fosphenytoin (seizure treatment). These drugs require dose escalations for efficacy or, when given as a standard loading dose for seizure control, fail to achieve efficacy. There are pediatric pharmacokinetic (PK) models, which if implemented clinically, have the potential to more accurately dose these medications through a process called “Model Informed Precision Dosing”, or MIPD. Using an already established and validated PK model, individual patient-specific covariates can be used to devise a custom dose for an individual child, based on their age, stage of development, renal function and other variables relevant to the specific drug’s PK profile. A novel EPIC-embedded MIPD tool called Lyv was developed by the University of Maryland Baltimore and PUMAS-AI (http://lyv.ai/). It places the PK model within the EPIC electronic health record, so that real-time clinical decision support for individualized dosing is facilitated at the bedside. To date, this tool has not been used in the Neonatal ICU. The lack of research utilizing such a tool leaves neonates behind in the progress towards precision medicine. In the funded research project, a feasibility trial of MIPD in neonates less than 44 weeks PMA will be conducted in the Neonatal ICU, focusing on two drugs: enoxaparin and fosphenytoin. Prospectively, infants at high risk for requiring these medications during their inpatient hospitalization will be recruited to participate. If the infant clinically requires the start of one of these drugs, the research team will assist the clinical team to determine a precision dose for the patient using the novel tool. A historical cohort of infants who received the drug with standard of care dosing in the previous three years will serve as control group. The primary outcome will be time to drug efficacy from the start of dosing to the point when the pharmacodynamic endpoint is achieved. Secondary outcomes feasibility and end-user experience of this MIPD tool in the ICU will be evaluated through an end-user semi-structured interview. After completion of this study, we expect to understand the feasibility and early efficacy of the MIPD tool in sick neonates, and will be able to proceed with optimal power and design of large-scale randomized controlled trials to assess safety and benefit of MIPD in neonates.

HNF1A-MODY and HNF4A-MODY are autosomal dominant forms of diabetes comprising ~3% of young-onset diabetes. Experimental studies in affected adult populations show hypersensitivity to sulfonylureas, which act downstream of the molecular defect. Sulfonylureas, along with the closely related meglitinides which bind the same receptor, are first line precision therapy for HNF1A-MODY and HNF4A-MODY. However, durability of effect is variable and weight gain leading to insulin resistance is a recognized risk factor for inadequate glycemic control with sulfonylurea therapy. Case studies have demonstrated efficacy of sulfonylureas and meglitinides in pediatric populations. However, there are several important knowledge gaps in precision therapeutics in children with HNF1A-MODY and HNF4A-MODY. There is no standard for deciding on the starting dose of sulfonylureas or meglitinides for drug-naïve patients, nor for dose escalation or augmentative therapy to address sulfonylurea failure related to the numerous factors impacting insulin sensitivity, including negative fetal metabolic programming. This study aims to assess the impact of in utero exposure to maternal hyperglycemia on metabolic programming and therapy outcomes in children with HNF1A-MODY and HNF4A-MODY. We will characterize the association of maternal glycemia and therapeutic interventions on the in-utero course, childhood growth trajectory, pubertal timing, insulin sensitivity and appearance of dysglycemia in children with heterozygous pathogenic variants in the HNF1A and HNF4A gene. We also aim to inform treatment dose recommendations for adolescents with HNF1A-MODY and HNF4A-MODY, extrapolating from the limited available adult literature and supplemented by preliminary studies in affected patients and to inform guidance on augmentative therapies to address pubertal insulin resistance-related glycemic deterioration. Regression modeling will help to determine the relationship between maternal factors and child anthropometric, metabolic, and pubertal health as well as treatment dosage. This data will inform the design of a pilot study in which pharmacokinetic and pharmacodynamic data will be characterized.

Fetal loss via miscarriages and stillbirth are catastrophic outcomes of pregnancy. Despite fetal loss in over 15% of pregnancies, the mechanisms are unknown in a majority of cases. Recently, it has been reported that the risk of fetal loss increases eight-fold in the presence of a maternal congenital disease, Long-QT Syndrome (LQTS). The most common forms of LQTS prolongs the QT interval on a surface ECG through inhibition of repolarizing potassium channels. In fact, multiple investigations have implicated the congenital loss-of-function of these potassium channels in cases of miscarriages and fetal death. Interestingly, paternal LQTS does not confer the same risk to the unborn fetus. Thus, it is widely believed that the increased risk of fetal loss from LQTS is due to the channelopathy prompting maternal myometrial dysfunction, as these channels are important during labor and delivery. It has been estimated that inherited LQTS accounts for approximately 8% of stillbirths. However, acquired channelopathies could play an important role in unknown cases since several exogenous and endogenous compounds inhibit these potassium channels. For example, sex hormones are known to be important regulators of cardiac repolarization, but the effects on ion channels during pregnancy are largely unstudied. We have recently reported that exogenous progesterone is transformed to metabolites that inhibit repolarizing potassium currents at physiological concentrations during pregnancy. Similarly, the QT interval is lengthened in pregnant women to the greatest extent during second and third trimesters, when progesterone concentrations are highest. The primary objective of this research direction is to characterize the effects of genetics and progesterone metabolite exposure on maternal cardiac repolarization and fetal loss. In the first aim, we will establish the impact of genetic variants on the variability in progesterone metabolite exposure and cardiac repolarization. Progesterone is metabolized by the polymorphically expressed CYP 3A enzymes. Therefore, we hypothesize that CYP3A genotype will predict metabolite exposure and maternal QT interval length. This hypothesis will be tested in pregnant women in a prospective pilot clinical trial. The association of genetics on progesterone metabolite exposure and cardiac repolarization will be assessed. In the second aim, we will determine the impact of the genetics of sex hormone metabolism on the risk of fetal loss using a multi-level targeted genome-wide association study with a case control design.

Moxifloxacin is a fluoroquinolone antibiotic with borad-spectrum activity against Gram-negative, Gram-positive, and acid-fast bacteria. Despite many indications in adults, moxifloxacin use in patients under 18 years of age is entirely off-label and is generally limited to rare or last resort scenarios. Pediatric data on pharmacokinetics and safety are very limited lacking and dosing is unestablished. In the proposed study, we aim to leverage existing knowledge of moxifloxacin population pharmacokinetics in children to determine the optimal sampling times for moxifloxacin concentration evaluation and individualized dosing to support safe and effective clinical use. Additionally, we aim to use real-world data of moxifloxacin and other fluroquinolone use to generate real-world evidence of its cardiac safety. Our methodology includes a combination of machine learning, pharmacometrics, and standard statistical approaches applied in novel ways to learn the most from existing pharmacokinetic and outcomes data. Our real-world data sources include electronic health records from the Unversity of California Health System and The Ohio State University - the MPRINT real-world evidence core. We hypothesize that drug levels measured 6 hours after dose will predict AUC over 24 hours and that pediatric moxifloxacin use is safe from severe cardiac adverse events. The outputs of this proposed work will be (i) establishing a limited sampling strategy for moxifloxacin target assessment, (ii) a pediatric-data-driven tool for precision dosing of moxifloxacin, and (iii) real-world evidence of comparative safety of fluoroqinolones in children.

SSRIs are effective in children and adolescents with psychiatric disorders and their use is increasing, yet, pediatric patients often endure multiple antidepressant trials before finding an efficacious and well-tolerated medication. SSRI blood concentrations vary substantially and accumulating evidence suggests that side effects are associated with blood concentrations. In adults, variability in SSRI concentration is partially explained by pharmacogenetics. While there are dosing recommendations based on CYP2C19 and CYP2D6, studies in pediatric patients are lacking and it’s unclear whether the adult guidelines should be applied to pediatric patients since their CYP2C19 activity may be 30% higher than adults. Sertraline is an SSRI indicated for obsessive compulsive disorder in pediatric patients but is often prescribed off-label for depression and anxiety. It is metabolized by several enzymes in the liver, including CYP2C19, CYP2D6, and CYP2B6. Our first goal is to determine the influence of CYP2B6, CYP2D6 and CYP2C19 on sertraline exposure in children and adolescents. Our second goal is to determine the influence of sertraline exposure on tolerability in children and adolescents. The proposed study addresses the gaps in knowledge of the influence of pharmacogenetics on sertraline tolerability and exposure in pediatric patients. This population has been excluded from prior pharmacogenetic studies but thousands of pediatric patients are prescribed sertraline each year in the US. Thus, this project will provide much-needed data for improving the care of children and adolescents with psychiatric disorders.

Extracorporeal life support (ECLS) is a life-saving technology in critically ill children with congenital heart disease and low cardiac output state (LCOS). Milrinone is the mainstay of treatment for this condition. Unfortunately, milrinone for children on ECLS lacks dosing information. Our preliminary data demonstrate that dosing is different in this population because the ECLS circuit components. The lack of appropriate dosing information is an urgent, unmet public health need that can result in therapeutic failure and death. Our team uses sophisticated physiologically-based pharmacokinetic (PBPK) mathematical models to translate results from benchside ECLS experiments into bedside dosing recommendations. The overall objective of this research is to evaluate ECLS circuit extraction of milrinone by extracorporeal membrane oxygenation (ECMO) and continuous renal replacement therapy (CRRT) circuits in an ex vivo system (AIM 1). These data will be used to initially develop adult ECLS-PBPK models for milrinone using rich adult PK data from literature. We can then scale to children and predict dosing in children supported with ECMO and CRRT (AIM 2). The models and milrinone dosing recommendations will be validated with data collected in a prospective PK study (AIM 3). This research will directly improve the health of children by determining optimal milrinone dosing in ECLS. In addition, the modeling developed from working with PBPK with a complex system such as ECLS, will create a platform to design and conduct early-phase clinical trials investigating the pharmacology of drugs in critically ill children.

Breast milk is recommended as the sole source of nutrition for infants to 6 months of age. However, only 25% of infants in the US meet this goal. Although infant formula is critical when breastfeeding is not possible, it lacks bioactive components of breastmilk. Milk is a rich source of lipids, and growing evidence supports, PPAR mediated metabolic pathways must be initated shortly after birth so neonates can catabolize fats. Our studies of swine found suboptimal intake of maternal milk impairs PPAR activation and peroxisome function, as indicated by accumulation of lipid substrates metabolized in peroxisomes. We hypothesize that components of maternal milk fat stimulate fatty acid oxidation through PPAR and peroxisomal biogenesis which is needed to initiate β-oxidation, and that if intake of milk is not adequate, eicosanoids, cholesterol esters, and very long chain fatty acids accumulate in tissues. Accumulation of these lipids likely elicit a proinflammatory signaling environment that can affect developmental pathways. Extreme cases of peroxisome dysfunction result in leukodystrophies like Zellweger’s syndrome. The overall goal of the proposed project is to determine if the preclinical data collected is translatable to humans. A longitudinal observational study of maternal-infant dyads beginning at birth to three weeks postnatal will be conducted to: SA1. Determine if neonate’s buccal cell lipidome is related to perinatal feeding, maternal buccal cell lipidome, and maternal milk or formula lipid profiles. This aim will test the working hypothesis that low or no intake (i.e., formula feeding) of maternal milk the first 48 h to 3 weeks postnatal will result in a greater abundance of peroxisome substrates (e.g., very long chain fatty acids) in buccal cells than those of exclusively breastfed counterparts. SA2. Assess the efficacy of isolating cellular DNA from buccal swabs to measure changes in DNA methylation in response to postnatal feeding. In rodents, postnatal intake of milk resulted in the demethylation of PPAR DNA-binding regions. This aim will generate preliminary data to determine the efficacy of using DNA isolated from neonatal buccal swabs to study changes in methylation patterns of the whole genome, which is necessary for applications for external funding. Preliminary data analysis aims to test the working hypothesis that infant feeding practices affect postnatal chromatin remodeling and that formula feeding will be associated with higher levels of methylation on PPAR targets. A primary goal of the National Institute of Child Health and Human Development is to optimize infant survival and health by optimizing formulas to more closely match the composition of human milk. Data collected will provide fundamental information regarding the ontogeny of neonatal metabolism and establish targets for development of breast milk alternatives to better mimic mothers’ milk bioactivity.

Current Centers for Disease Control and Prevention (CDC) guidelines recommend testing all immigrant infants and children 0-16 years for elevated blood lead level, and CDC and The American College of Obstetricians and Gynecologists (ACOG) also recommend that immigrant females who are pregnant and/or breastfeeding should be tested as well. However, the adoption of these screening guidelines by medical providers is sporadic, at best. A novel approach to identifying potential risk of increased lead and other heavy metal exposure in fetuses and children is to test the parent’s breast milk. We will evaluate the prevalence of lead and other heavy metals in a cross-sectional sample of breast milk samples previously collected across the United States. We will also describe the distribution of detectable levels and quantity of each heavy metal by the breastfeeding/chest feeding parent’s race/ethnicity, socioeconomic status, geographic location of reported residence at the time of sample collection, and by selected self-reported recent exposure to cannabis, tobacco, herbal products and dietary intake. Detectable levels of specific heavy metals can potentially inform public health education and intervention strategies, improve health outcomes for patients and families, as well as used as sentinel events, alerting local public health to perform home visits to determine sources of lead and testing of infants and other children in the household.

This project aims to better understanding the causes of pediatric inflammatory bowel disease (IBD) to enable more effective, personalized treatments. For most patients with IBD, the peak age at onset is between 20-40 years. However, a small but growing percentage of IBD patients have very early onset IBD (VEOIBD) at age 6 or younger, and disease in these children is especially severe and often refractory to treatment. Various immunosuppressive drugs are available for VEOIBD treatment, but these therapies are not equally effective in all children and overall efficacy remains low (between 30-50%). The timely selection of an optimal treatment plan is critical for these children, since VEOIBD is associated with poor long-term outcomes including surgical resection, increased risk of colorectal cancer, and even death. Our central hypothesis is that a greater understanding of inter-individual disease determinants through identification of genetic drivers and the impact of these genetic variants on the gut microbiome will lead to more effective, personalized treatments for VEOIBD patients. To this end, our project focuses on identifying genetic influencers of VEOIBD and how human genetic factors shape the gut microbiome in VEOIBD patients.

The benefits of breastfeeding are well documented yet only ~50% of US infants are breastfed at 6 months of age. This goes against recommendations from the American Academy of Pediatrics and the American College of Obstetricians and Gynecologists. A common barrier to breastfeeding is the perceived and unquantified risk of infant exposure to maternal prescription drugs through breastmilk. Up to 1.5 million lactating women in the US take prescription drugs yet

Despite significant improvements in post-operative survival for children born with functional single ventricle congenital heart disease (SVCHD), there is increasing recognition of end-organ damage, specifically liver fibrosis, in these patients. The Fontan operation creates passive systemic venous return of deoxygenated blood to the lungs. Over time, this passive system leads to elevated central venous pressure and hepatic venous stasis, congestion, and subsequent liver fibrosis/cirrhosis, known as Fontan-Associated Liver Disease (FALD). While advanced liver imaging can screen for and quantify degrees of liver stiffness, there remains a persistent disconnect between the degree of liver congestion/fibrosis and its impact on intrinsic hepatic function, making clinical decisions, such as drug dosing, difficult. Although it is known that moderate to severe hepatic damage in adults impairs drug disposition, there is a critical information gap in how liver congestion and fibrosis in patients with Fontan circulation affects hepatocellular processes, such as hepatic drug metabolism and transport. The consequence of this unmet need is suboptimal medical management of Fontan-related comorbidities, which is imperative given the known gradual decline in survival following Fontan palliation and the challenges related to and limited availability of solid organ transplantation. The objective here is to determine the role FALD has on two independent drug disposition pathways through a single dose pharmacokinetic study with administered liver metabolism and transport probe substrates (CYP3A4: sildenafil; OATP1B1: pravastatin). These data will subsequently be utilized in the future to develop and validate the first physiologically based pharmacokinetic (PBPK) model based in the Fontan population that will serve as an in silico foundation to develop future model-informed precision dosing (MIPD) in this vulnerable population. The central hypothesis is that increased liver congestion and fibrosis leads to cellular dysfunction resulting in altered drug disposition. Therefore, the primary goal in this proposal is to address the effect of FALD, specifically liver congestion and fibrosis as measured on magnetic resonance imaging (MRI) 4-dimensional flow and elastography, on drug disposition in SVCHD by addressing these two aims: Aim 1) Determine the contribution of liver congestion and fibrosis on hepatocellular drug metabolism in patients with Fontan circulation, Aim 2) Evaluate the role of liver congestion and fibrosis on hepatocellular drug transport in patients with Fontan circulation. IMProving DRug dosing and Outcomes for single VEntricle patients with Fontan Associated Liver Disease (IMPROVE FALD) study will develop a drug pharmacokinetic evaluation as an end-organ phenotypic biomarker that establishes the liver congestion and fibrosis thresholds where intrinsic hepatic function begins to decline as a way to enhance prediction, prevention, and treatment of FALD. Collectively, the IMPROVE FALD study will serve as a guide to refine not only current CV drug dosing strategies, but also dosing for other drugs that may be used in those with Fontan circulation that have comparable drug disposition pathways.