Gregory T. Knipp

Assessing the performance of amorphous solid dispersions

The characterization and performance of stable amorphous solid dispersion systems were evaluated in 40 research papers reporting active pharmaceutical ingredient (API) dissolution and bioavailability from various systems containing polymers. The results from these studies were broadly placed into three categories: amorphous dispersions that improved bioavailability (∼82% of the cases), amorphous dispersions possessing lower bioavailability than the reference material (∼8% of the cases), and amorphous dispersions demonstrating similar bioavailabilities as the reference material (∼10% of the cases). A comparative analysis of these studies revealed several in vitro and in vivo variables that could have influenced the results. The in vitro factors compared primarily centered on dissolution testing and equipment, content and amount of dissolution media, sink or nonsink conditions, agitation rates, media pH, dissolution characteristics of the polymer, and dispersion particle size. The in vivo factors included reference materials used for bioavailability comparisons, animal species utilized, fasting versus fed conditions, and regional differences in gastrointestinal (GI) content and volume. On the basis of these considerations, a number of recommendations were made on issues ranging from the assessment of physical stability of API-polymer dispersions to in vivo GI physiological factors that require consideration in the performance evaluation of these systems.

Spatial expression patterns of peptide transporters in the human and rat gastrointestinal tracts, Caco-2 In Vitro cell culture model, and multiple human tissues

This study sought to identify the spatial patterns of expression of peptide transporter 1 (PepT1), peptide transporter 3 (PTR3), peptide/histidine transporter 1 (PHT1), and the human peptide transporter 1 (HPT-1) mRNA in complementary DNA (cDNA) libraries of the human and rat gastrointestinal tracts (GIT), Caco-2 in vitro cell culture model, and in a human multiple tissue panel. Human PTR3 and PHT1 are putative peptide transporters recently discovered. Using sequence-specific primers designed to amplify regions of PepT1, PTR3, PHT1, and HPT-1, we were able to identify the expression of mRNA for each of these transporters in human cDNA panels (Clontech, Palo Alto, CA), the rat GIT, and in Caco-2 cDNA libraries by the polymerase chain reaction (PCR) and Southern Blot analysis. These studies suggest that in the human GIT, PepT1 appears to be localized predominantly in the duodenum, with decreasing expression in the jejunum and ileum. In contrast, PTR3 and HPT-1 were widely expressed in the human GIT, with predominant expression in the different regions of the colon. PHT1 appeared to be expressed in low levels throughout the human GI tract. Interestingly, the mRNAs for all 4 peptide transporters were expressed in Caco-2 cells throughout 30 days of culture. PepT1, PTR3, PHT1, and HPT-1 were also widely expressed in the rat GIT. Human tissue cDNA panel screening suggests that PTR3 and PHT1 are more uniformly expressed, whereas PepT1 and HPT-1 demonstrated site-specific expression. These results suggest that PepT1, PTR3, PHT1, and HPT-1 all may act to facilitate the diffusion of peptides and peptide-based pharmaceuticals in the GIT, PTR3, PHT1, and HPT-1 expressions in Caco-2 cell monolayers strongly suggest that their function needs to be further elucidated and their contribution to peptide transport not ignored. Taken together, these results demonstrate the potential for molecular biological characterization in localizing active transporter systems that can potentially be targeted for enhancing the absorption of peptide-based pharmaceuticals.

ABC and SLC transporter expression and POT substrate characterization across the human CMEC/D3 blood-brain barrier cell line

Initial studies indicate that the newly developed hCMEC/D3 cell line may prove to be a useful model for studying the physiology of the human blood-brain barrier (BBB) endothelium. The purpose of this study was to assess the mRNA expression of several ABC and SLC transporters, with an emphasis on the proton-coupled oligopeptide transporter superfamily (POT) transporters in this immortalized BBB cell model. The transport kinetics of POT-substrates was also evaluated. The hCMEC/D3 cell line was maintained in a modified EGM-2 medium in collagenated culture flasks and passaged every 3-4 days at approximately 85%-95% confluence. Messenger RNA (mRNA) expression of a variety of ABC and SLC transporters was evaluated using qRT-PCR arrays, while additional qRT-PCR primers were designed to assess the expression of POT members. The transport kinetics of mannitol and urea were utilized to quantitatively estimate the intercellular pore radius, while POT substrate transport was also determined to assess the suitability of the cell model from a drug screening perspective. Optimization of the cell line was attempted by culturing with on laminin and fibronectin enhanced collagen and in the presence of excess Ca(2+). hCMEC/D3 cells express both hPHT1 and hPHT2, while little to no expression of either hPepT1 or hPepT2 was observed. The relative expression of other ABC and SLC transporters is discussed. While POT substrate transport does suggest suitability for BBB drug permeation screening, the relative intercellular pore radius was estimated at 19 A, significantly larger than that approximated in vivo. Culturing with extracellular matrix proteins did not alter mannitol permeability. These studies characterized this relevant human hCMEC/D3 BBB cell line with respect to both the relative mRNA expression of various ABC and SLC transporters and its potential utility as an in vitro screening tool for brain permeation. Additional studies are required to adequately determine the potential to establish an in vivo correlation.Initial studies indicate that the newly developed hCMEC/D3 cell line may prove to be a useful model for studying the physiology of the human blood-brain barrier (BBB) endothelium. The purpose of this study was to assess the mRNA expression of several ABC and SLC transporters, with an emphasis on the proton-coupled oligopeptide transporter superfamily (POT) transporters in this immortalized BBB cell model. The transport kinetics of POT-substrates was also evaluated. The hCMEC/D3 cell line was maintained in a modified EGM-2 medium in collagenated culture flasks and passaged every 3-4 days at approximately 85%-95% confluence. Messenger RNA (mRNA) expression of a variety of ABC and SLC transporters was evaluated using qRT-PCR arrays, while additional qRT-PCR primers were designed to assess the expression of POT members. The transport kinetics of mannitol and urea were utilized to quantitatively estimate the intercellular pore radius, while POT substrate transport was also determined to assess the suitability of the cell model from a drug screening perspective. Optimization of the cell line was attempted by culturing with on laminin and fibronectin enhanced collagen and in the presence of excess Ca2+. hCMEC/D3 cells express both hPHT1 and hPHT2, while little to no expression of either hPepT1 or hPepT2 was observed. The relative expression of other ABC and SLC transporters is discussed. While POT substrate transport does suggest suitability for BBB drug permeation screening, the relative intercellular pore radius was estimated at 19 Å, significantly larger than that approximated in vivo. Culturing with extracellular matrix proteins did not alter mannitol permeability. These studies characterized this relevant human hCMEC/D3 BBB cell line with respect to both the relative mRNA expression of various ABC and SLC transporters and its potential utility as an in vitro screening tool for brain permeation. Additional studies are required to adequately determine the potential to establish an in vivo correlation.

Human Ontogeny of Drug Transporters: Review and Recommendations of the Pediatric Transporter Working Group

The critical importance of membrane‐bound transporters in pharmacotherapy is widely recognized, but little is known about drug transporter activity in children. In this white paper, the Pediatric Transporter Working Group presents a systematic review of the ontogeny of clinically relevant membrane transporters (e.g., SLC, ABC superfamilies) in intestine, liver, and kidney. Different developmental patterns for individual transporters emerge, but much remains unknown. Recommendations to increase our understanding of membrane transporters in pediatric pharmacotherapy are presented.

Maternal Di-(2-ethylhexyl)-phthalate Exposure Influences Essential Fatty Acid Homeostasis in Rat Placenta

Maintaining essential fatty acid (EFA) homeostasis during pregnancy is critical for fetal development. As the organ that controls the maternal-to-fetal supply of nutrients, the placenta plays a significant role in guiding EFA transfer to the fetus. Many EFA homeostasis proteins are regulated by peroxisome proliferator-activated receptors (PPARs). The metabolites of di-(2-ethylhexyl)-phthalate (DEHP), a ubiquitous environmental contaminant, might influence EFA homeostasis via trans-activation of PPARs with subsequent downstream effects on EFA transporters and enzymes. To investigate DEHPs effect on placental/fetal EFA homeostasis, female Sprague-Dawley rats were orally gavaged with either vehicle or DEHP at 750 or 1500 mg/kg/day from gestational day (GD) 0 to GD 19. Changes in the expression of several EFA homeostasis regulating proteins were determined in the junctional (JXN) and labyrinthine (LAB) zones of the placenta, including PPAR isoforms (alpha, beta and gamma), fatty acid translocase (FAT/CD36), fatty acid transport protein 1 (FATP1), plasma membrane fatty acid binding protein (FABPpm), heart cytoplasmic fatty acid binding protein (HFABP), cytochrome P450 (CYP) 4A1, and cyclooxygenase (COX)-1 and -2. Additionally, effects of DEHP maternal exposure on the placental transfer and fetal distribution of representative EFAs, arachidonic acid (AA) and docosahexaenoic acid (DHA), and the placental production of prostaglandins (PGs) were investigated. Expression of PPARalpha, PPARgamma, FAT/CD36, FATP1, HFABP and CYP4A1 was up-regulated in JXN and/or LAB while COX-2 was down-regulated in JXN. PPARbeta, FABPpm, and COX-1 demonstrated variable expression. Reduced directional maternal-to-fetal placental transfer and altered fetal distribution of AA and DHA were observed in concordance with a decreased total placental PG production. These results correlate with previous in vitro data, suggesting that DEHP could influence placental EFA homeostasis with potential downstream effects in the developing fetus.

Summary of the National Institute of Child Health and Human Development-Best Pharmaceuticals for Children Act Pediatric Formulation Initiatives Workshop-Pediatric Biopharmaceutics Classification System Working Group

BACKGROUND The Biopharmaceutics Classification System (BCS) allows compounds to be classified based on their in vitro solubility and intestinal permeability. The BCS has found widespread use in the pharmaceutical community to be an enabling guide for the rational selection of compounds, formulation for clinical advancement, and generic biowaivers. The Pediatric Biopharmaceutics Classification System (PBCS) Working Group was convened to consider the possibility of developing an analogous pediatric-based classification system. Because there are distinct developmental differences that can alter intestinal contents, volumes, permeability, and potentially biorelevant solubilities at different ages, the PBCS Working Group focused on identifying age-specific issues that need to be considered in establishing a flexible, yet rigorous PBCS. OBJECTIVE We summarized the findings of the PBCS Working Group and provided insights into considerations required for the development of a PBCS. METHODS Through several meetings conducted both at The Eunice Kennedy Shriver National Institute of Child Health, Human Development-US Pediatric Formulation Initiative Workshop (November 2011) and via teleconferences, the PBCS Working Group considered several high-level questions that were raised to frame the classification system. In addition, the PBCS Working Group identified a number of knowledge gaps that need to be addressed to develop a rigorous PBCS. RESULTS It was determined that for a PBCS to be truly meaningful, it needs to be broken down into several different age groups that account for developmental changes in intestinal permeability, luminal contents, and gastrointestinal (GI) transit. Several critical knowledge gaps were identified, including (1) a lack of fully understanding the ontogeny of drug metabolizing enzymes and transporters along the GI tract, in the liver, and in the kidney; (2) an incomplete understanding of age-based changes in the GI, liver, and kidney physiology; (3) a clear need to better understand age-based intestinal permeability and fraction absorbed required to develop the PBCS; (4) a clear need for the development and organization of pediatric tissue biobanks to serve as a source for ontogenic research; and (5) a lack of literature published in age-based pediatric pharmacokinetics to build physiologically- and population-based pharmacokinetic (PBPK) databases. CONCLUSIONS To begin the process of establishing a PBPK model, 10 pediatric therapeutic agents were selected (based on their adult BCS classifications). These agents should be targeted for additional research in the future. The PBCS Working Group also identified several areas where greater emphasis on research was needed to enable the development of a PBCS.

Effects of di-(2-ethylhexyl)-phthalate and its metabolites on the lipid profiling in rat HRP-1 trophoblast cells

The highly directional maternal-to-fetal transfer of essential fatty acids (EFAs) across the placenta plays a critical role in guiding proper fetal development. Exposure to xenobiotics that may alter the fetal supply of EFAs/lipids could lead to fetal toxicity. Since the placenta is the first fetal arising organ that regulates fetal fatty acid homeostasis, the fatty acid/lipid composition in the placenta may serve as an indicator of fetal composition. In this study, we investigated the effects of the peroxisome proliferator chemical di-(2-ethylhexyl)-phthalate (DEHP), a widely used plasticizer and ubiquitous environmental contaminant, and its selective metabolites, mono-(2-ethylhexyl)-phthalate (MEHP) and 2-ethylhexanoic acid (EHA) on the lipid metabolome in a rat HRP-1 trophoblast model. The concentrations of ten lipid classes (cholesterol esters, diacylglycerol, triacylglycerides, phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, lysophosphatidylcholine, cardiolipin, and sphingomyelin) were determined, as well as the individual fatty acid compositions, especially the ω-3 and ω-6 family of EFAs. The level of each lipid class was significantly increased upon exposure to the agents, with MEHP and EHA generally showing higher increases than DEHP. The same trends were observed in comparing the fatty acid compositions. For example, the ω-3/ω-6 fatty acids ratio did not change, although the levels of ω-3 and ω-6 fatty acids were significantly elevated upon exposure. These results suggest that DEHP and its metabolites can alter lipid metabolome in a rat placental cell line, implying that these compounds may contribute to aberrant placental EFA/lipid homeostasis caused by peroxisome proliferation, and potentially result in abnormal fetal development.

Cys34-PEGylated Human Serum Albumin for Drug Binding and Delivery

Polyethylene glycol (PEG) derivatives were conjugated onto the Cys-34 residue of human serum albumin (HSA) to determine their effects on the solubilization, permeation, and cytotoxic activity of hydrophobic drugs such as paclitaxel (PTX). PEG(C34)HSA conjugates were prepared on a multigram scale by treating native HSA (n-HSA) with 5- or 20-kDa mPEG-maleimide, resulting in up to 77% conversion of the mono-PEGylated adduct. Nanoparticle tracking analysis of PEG(C34)HSA formulations in phosphate buffer revealed an increase in the number of nanosized aggregates relative to n-HSA, both in the absence and presence of PTX. Cell viability studies conducted with MCF-7 breast cancer cells indicated that PTX cytotoxicity was enhanced by PEG(C34)HSA when mixed at 10:1 mol ratios, up to a 2-fold increase in potency relative to n-HSA. The PEG(C34)HSA conjugates were also evaluated as PTX carriers across monolayers of HUVEC and hCMEC/D3 cells, and found to have permeation profiles nearly identical to those of n-HSA.

Pharmacogenomics of acetaminophen in pediatric populations: a moving target

Acetaminophen (APAP) is widely used as an over-the-counter fever reducer and pain reliever. However, the current therapeutic use of APAP is not optimal. The inter-patient variability in both efficacy and toxicity limits the use of this drug. This is particularly an issue in pediatric populations, where tools for predicting drug efficacy and developmental toxicity are not well established. Variability in toxicity between age groups may be accounted for by differences in metabolism, transport, and the genetics behind those differences. While pharmacogenomics has been revolutionizing the paradigm of pharmacotherapy for many drugs, its application in pediatric populations faces significant challenges given the dynamic ontogenic changes in cellular and systems physiology. In this review we focused on the ontogenesis of the regulatory pathways involved in the disposition of APAP and on the variability between pediatric, adolescent, and adult patients. We also summarize important polymorphisms of the pharmacogenes associated with APAP metabolism. Pharmacogenetic studies in pediatric APAP treatment are also reviewed. We conclude that while a consensus in pharmacogenetic management of APAP in pediatric populations has not been achieved, a systems biology based strategy for comprehensively understanding the ontogenic regulatory pathway as well as the interaction between age and genetic variations are particularly necessary in order to address this question.

Persistent pharmacokinetic challenges to pediatric drug development

The development of new therapeutic agents for the mitigation of pediatric disorders is largely hindered by the inability for investigators to assess pediatric pharmacokinetics (PK) in healthy patients due to substantial safety concerns. Pediatric patients are a clinical moving target for drug delivery due to changes in absorption, distribution, metabolism and excretion (ADME) and the potential for PK related toxicological (T) events to occur throughout development. These changes in ADMET can have profound effects on drug delivery, and may lead to toxic or sub-therapeutic outcomes. Ethical, economical, logistical, and technical barriers have resulted in insufficient investigation of these changes by industrial, regulatory, and academic bodies, leading to the classification of pediatric patients as therapeutic orphans. In response to these concerns, regulatory agencies have incentivized investigation into these ontogenic changes and their effects on drug delivery in pediatric populations. The intent of this review is to briefly present a synopsis of the development changes that occur in pediatric patients, discuss the effects of these changes on ADME and drug delivery strategies, highlight the hurdles that are still being faced, and present some opportunities to overcome these challenges.