Characterization of neonatal and infant enterostomy fluids

Characterization of luminal contents from the fasted human proximal colon

To treat colonic diseases more effectively, improved therapies are urgently needed. In this respect, delivering drugs locally to the colon is a key strategy to achieve higher local drug concentrations while minimizing systemic side effects. Understanding the luminal environment is crucial to efficiently develop such targeted therapies and to predict drug disposition in the colon. In this clinical study, we collected colonic contents from an undisturbed fasted proximal colon via colonoscopy and characterized their composition with regard to drug disposition. Colonic pH, osmolality, protein content, bile salts, lipids, phospholipids and short-chain fatty acids were investigated in 10 healthy volunteers (8 male and 2 female, age 19-25). The unique environment of the proximal colon was reflected in the composition of the sampled luminal fluids and the effect of the microbiota could be observed on the pH (median 6.55), the composition of bile salts (majority deconjugated and secondary), and the abundance of short-chain fatty acids. At the same time, an increase in phospholipid concentration, osmolality and total protein content compared to reported ileal values was seen, likely resulting from desiccation. Lipids could only be found in low quantities and mainly in the form of cholesterol and free fatty acids, showing almost complete digestion and absorption by the time luminal contents reach the colon. All characteristics also displayed the considerable intersubject variability found in different regions of the gastrointestinal tract. This study contributes to an improved understanding of the luminal conditions in the proximal colon and facilitates the development of new predictive tools to study colonic drug absorption.

Physiologically Based Pharmacokinetic Modeling in Neonates: Current Status and Future Perspectives

Rational drug use in special populations is a clinical problem that doctors and pharma-cists must consider seriously. Neonates are the most physiologically immature and vulnerable to drug dosing. There is a pronounced difference in the anatomical and physiological profiles be-tween neonates and older people, affecting the absorption, distribution, metabolism, and excretion of drugs in vivo, ultimately leading to changes in drug concentration. Thus, dose adjustments in neonates are necessary to achieve adequate therapeutic concentrations and avoid drug toxicity. Over the past few decades, modeling and simulation techniques, especially physiologically based pharmacokinetic (PBPK) modeling, have been increasingly used in pediatric drug development and clinical therapy. This rigorously designed and verified model can effectively compensate for the deficiencies of clinical trials in neonates, provide a valuable reference for clinical research design, and even replace some clinical trials to predict drug plasma concentrations in newborns. This review introduces previous findings regarding age-dependent physiological changes and pathological factors affecting neonatal pharmacokinetics, along with their research means. The application of PBPK modeling in neonatal pharmacokinetic studies of various medications is also reviewed. Based on this, we propose future perspectives on neonatal PBPK modeling and hope for its broader application.

Investigating Tacrolimus Disposition in Paediatric Patients with a Physiologically Based Pharmacokinetic Model Incorporating CYP3A4 Ontogeny, Mechanistic Absorption and Red Blood Cell Binding

Tacrolimus is a crucial immunosuppressant for organ transplant patients, requiring therapeutic drug monitoring due to its variable exposure after oral intake. Physiologically based pharmacokinetic (PBPK) modelling has provided insights into tacrolimus disposition in adults but has limited application in paediatrics. This study investigated age dependency in tacrolimus exposure at the levels of absorption, metabolism, and distribution. Based on the literature data, a PBPK model was developed to predict tacrolimus exposure in adults after intravenous and oral administration. This model was then extrapolated to the paediatric population, using a unique reference dataset of kidney transplant patients. Selecting adequate ontogeny profiles for hepatic and intestinal CYP3A4 appeared critical to using the model in children. The best model performance was achieved by using the Upreti ontogeny in both the liver and intestines. To mechanistically evaluate the impact of absorption on tacrolimus exposure, biorelevant in vitro solubility and dissolution data were obtained. A relatively fast and complete release of tacrolimus from its amorphous formulation was observed when mimicking adult or paediatric dissolution conditions (dose, fluid volume). In both the adult and paediatric PBPK models, the in vitro dissolution profiles could be adequately substituted by diffusion-layer-based dissolution modelling. At the level of distribution, sensitivity analysis suggested that differences in blood plasma partitioning of tacrolimus may contribute to the variability in exposure in paediatric patients.

Characterization of neonatal and infant enterostomy fluids - Part II: Drug solubility

Previous research revealed marked differences in the composition of intestinal fluids between infants and adults. To explore the impact on the solubilization of orally administered drugs, the present study assessed the solubility of five poorly water-soluble, lipophilic drugs in intestinal fluid pools from 19 infant enterostomy patients (infant HIF). For some but not all drugs, the average solubilizing capacity of infant HIF was similar to that of HIF obtained from adults (adult HIF) in fed conditions. Commonly used fed state simulated intestinal fluids (FeSSIF(-V2)) predicted fairly well drug solubility in the aqueous fraction of infant HIF, but did not account for the substantial solubilization by the lipid phase of infant HIF. Despite similarities in the average solubilities of some drugs in infant HIF and adult HIF or SIF, the underlying solubilization mechanisms likely differ, considering important compositional differences (e.g., low bile salt levels). Finally, the huge variability in composition of infant HIF pools resulted in a highly variable solubilizing capacity, potentially causing variations in drug bioavailability. The current study warrants future research focusing on (i) understanding the mechanisms underlying drug solubilization in infant HIF and (ii) evaluating the sensitivity of oral drug products to interpatient variations in drug solubilization.