Utility of Göttingen minipigs for Prediction of Human Pharmacokinetic Profiles After Dermal Drug Application

Pharmacokinetics and pharmacodynamics of standard nerve agent medical countermeasures in Göttingen Minipigs

Animal research continues to serve a critical role in the testing and development of medical countermeasures. The Göttingen minipig, developed for laboratory research, may provide many benefits for addressing research questions within chemical defense. Targeted development of the Göttingen minipig model could reduce reliance upon non-human primates, and improve study design, statistical power, and throughput to advance medical countermeasures for regulatory approval and fielding. In this vein, we completed foundational pharmacokinetics and physiological safety studies of intramuscularly administered atropine sulfate, pralidoxime chloride (2-PAM), and diazepam across a broad range of doses (1-6 autoinjector equivalent) using adult male Göttingen minipigs (n=11; n=4-8/study) surgically implanted with vascular access ports and telemetric devices to monitor cardiovascular, respiratory, arterial pressure, and temperature signals. Pharmacokinetic data were orderly and the concentration maximum mirrored available human data at comparably scaled doses clearly for atropine, moderately for 2-PAM, and poorly for diazepam. Time to peak concentration approximated 2, 7, and 20 min for atropine, 2-PAM, and diazepam, respectively, and the elimination half-life of these drugs approximated 2 hr (atropine), 3 hr (2-PAM), and 8 hr (diazepam). Atropine sulfate dose-dependently increased the magnitude and duration of tachycardia and decreased the PR and ST intervals (consistent with findings obtained from other species). Mild hypothermia was observed at the highest diazepam dose. Göttingen minipigs appear to provide a ready and appropriate large animal alternative to non-human primates, and further development and evaluation of novel nerve agent medical countermeasures and treatment strategies in this model are justified.

Utility of Göttingen minipigs for the prediction of human pharmacokinetic profiles after intravenous drug administration

Göttingen minipigs are increasingly used to evaluate the pharmacokinetic (PK) profiles of drug candidates. However, their accuracy in predicting human PK parameters is unclear. In this study, we investigated the utility of Göttingen minipigs for predicting human PK profiles. We evaluated the PK parameters of 30 compounds with diverse metabolic pathways after intravenous administration in minipigs. Human total clearance (CLtotal) was corrected using the blood to plasma ratio, and the volume of distribution at steady state (Vd(ss)) was corrected with plasma unbound fraction (fup). CLtotal and Vd(ss) were predicted using single-species allometric scaling using data from minipigs and other reported animal models (monkeys, human liver chimeric mice, and rats). The predicted values were compared with actual values reported in humans. Göttingen minipig were superior to rats because of their better predictability of Vd(ss) and CLtotal, as represented by lower absolute average fold error values. However, their predictability for Vd(ss) was inferior to monkey and human liver chimeric mice. Prediction of CLtotal from blood-based minipig data showed excellent correlation with human data, and comparable predictability with monkey and human liver chimeric mice. Thus, Göttingen minipigs can be used as an optional model for preclinical pharmaceutical research for predicting human CLtotal.

Design and development of topical liposomal formulations in a regulatory perspective

The skin is the absorption site for drug substances intended to treat loco-regional diseases, although its barrier properties limit the permeation of drug molecules. The growing knowledge of the skin structure and its physiology have supported the design of innovative nanosystems (e.g. liposomal systems) to improve the absorption of poorly skin-permeable drugs. However, despite the dozens of clinical trials started, few topically applied liposomal systems have been authorized both in the EU and the USA. Indeed, the intrinsic complexity of the topically applied liposomal systems, the higher production costs, the lack of standardized methods and the more stringent guidelines for assessing their benefit/risk balance can be seen as causes of such inefficient translation. The present work aimed to provide an overview of the physicochemical and biopharmaceutical characterization methods that can be applied to topical liposomal systems intended to be marketed as medicinal products, and the current regulatory provisions. The discussion highlights how such methodologies can be relevant for defining the critical quality attributes of the final product, and they can be usefully applied based on the phase of the life cycle of a liposomal product: to guide the formulation studies in the early stages of development, to rationally design preclinical and clinical trials, to support the pharmaceutical quality control system and to sustain post-marketing variations. The provided information can help define harmonized quality standards able to overcome the case-by-case approach currently applied by regulatory agencies in assessing the benefit/risk of the topically applied liposomal systems.

In vivo characterization of minipig skin as a model for dermatological research using multiphoton microscopy

Minipig skin is one of the most widely used non-rodent animal skin models for dermatological research. A thorough characterization of minipig skin is essential for gaining deeper understanding of its structural and functional similarities with human skin. In this study, three-dimensional (3-D) in vivo images of minipig skin was obtained non-invasively using a multimodal optical imaging system capable of acquiring two-photon excited fluorescence (TPEF) and fluorescence lifetime imaging microscopy (FLIM) images simultaneously. The images of the structural features of different layers of the minipig skin were qualitatively and quantitatively compared with those of human skin. Label-free imaging of skin was possible due to the endogenous fluorescence and optical properties of various components in the skin such as keratin, nicotinamide adenine dinucleotide phosphate (NAD(P)H), melanin, elastin, and collagen. This study demonstrates the capability of optical biopsy techniques, such as TPEF and FLIM, for in vivo non-invasive characterization of cellular and functional features of minipig skin, and the optical image-based similarities of this commonly utilized model of human skin. These optical imaging techniques have the potential to become promising tools in dermatological research for developing a better understanding of animal skin models, and for aiding in translational pre-clinical to clinical studies.

Large Animal Models in Regenerative Medicine and Tissue Engineering: To Do or Not to Do

Rapid developments in Regenerative Medicine and Tissue Engineering has witnessed an increasing drive toward clinical translation of breakthrough technologies. However, the progression of promising preclinical data to achieve successful clinical market authorisation remains a bottleneck. One hurdle for progress to the clinic is the transition from small animal research to advanced preclinical studies in large animals to test safety and efficacy of products. Notwithstanding this, to draw meaningful and reliable conclusions from animal experiments it is critical that the species and disease model of choice is relevant to answer the research question as well as the clinical problem. Selecting the most appropriate animal model requires in-depth knowledge of specific species and breeds to ascertain the adequacy of the model and outcome measures that closely mirror the clinical situation. Traditional reductionist approaches in animal experiments, which often do not sufficiently reflect the studied disease, are still the norm and can result in a disconnect in outcomes observed between animal studies and clinical trials. To address these concerns a reconsideration in approach will be required. This should include a stepwise approach using in vitro and ex vivo experiments as well as in silico modeling to minimize the need for in vivo studies for screening and early development studies, followed by large animal models which more closely resemble human disease. Naturally occurring, or spontaneous diseases in large animals remain a largely untapped resource, and given the similarities in pathophysiology to humans they not only allow for studying new treatment strategies but also disease etiology and prevention. Naturally occurring disease models, particularly for longer lived large animal species, allow for studying disorders at an age when the disease is most prevalent. As these diseases are usually also a concern in the chosen veterinary species they would be beneficiaries of newly developed therapies. Improved awareness of the progress in animal models is mutually beneficial for animals, researchers, human and veterinary patients. In this overview we describe advantages and disadvantages of various animal models including domesticated and companion animals used in regenerative medicine and tissue engineering to provide an informed choice of disease-relevant animal models.

Utility of hairless rats as a model for predicting transdermal pharmacokinetics in humans

This study investigated the use of HWY hairless rats to predict human plasma concentrations of drugs following dermal application.Utilizing a deconvolution method, pharmacokinetic parameters (e.g. in vivo absorption rates) were determined for six transdermal drugs in hairless rats. Obtained data were used to simulate the human plasma concentration-time profiles of transdermal drugs, which were then compared with clinical data in humans. Because hairless rats have lower hair follicle density than do humans, the impact of hair follicle density on skin permeability to hydrophilic compounds was also evaluated.Pharmacokinetic parameters showed low intra-individual variability in hairless rats. Simulated concentration profiles for compounds with logarithm of the octanol-water partition coefficient exceeding two were comparable to clinical data, but simulated concentration profiles for hydrophilic compounds (i.e. bisoprolol and nicotine) at maximum concentration differed from clinical data by more than two-fold. Finally, in vitro permeability to bisoprolol and nicotine was higher in human skin than in hairless rat skin, but hair follicle plugging reduced human skin permeability.In vivo skin absorption data from HWY hairless rats help to predict human concentration profiles for lipophilic compounds. However, the data underestimate human absorption of hydrophilic compounds.

Application of the Tissue Composition-Based Model to Minipig for Predicting the Volume of Distribution at Steady State and Dermis-to-Plasma Partition Coefficients of Drugs Used in the Physiologically Based Pharmacokinetics Model in Dermatology

The minipig continues to build a reputation as a viable alternative large animal model to predict humans in dermatology and toxicology studies. Therefore, it is essential to describe and predict the pharmacokinetics in that species to speed up the clinical candidate selection. Essential input parameters in whole-body physiologically based pharmacokinetic models are the tissue-to-plasma partition coefficients and the resulting volume of distribution at steady-state (V_ss). Mechanistic in vitro- and in silico-based models used for predicting these parameters of tissue distribution of drugs refer to the tissue composition-based model (TCM). Robust TCMs were initially developed for some preclinical species (e.g., rat and dog) and human; however, there is currently no model available for the minipig. Therefore, the objective of this present study was to develop a TCM for the minipig and to estimate the corresponding tissue composition data. Drug partitioning into the tissues was predominantly governed by lipid and protein binding effects in addition to drug solubilization and pH gradient effects in the aqueous phase on both sides of the biological membranes; however, some more complex tissue distribution processes such as drug binding to the collagen-laminin material in dermis and a restricted drug partitioning into membranes of tissues for compounds that are amphiphilic and contain sulfur atom(s) were also challenged. The model was validated by predicting V_ss and the dermis-to-plasma partition coefficients (K_p-dermis) of 68 drugs. The prediction of K_p-dermis was extended to humans for comparison with the minipig. The results indicate that the extended TCM provided generally good agreements with observations in the minipig showing that it is also applicable to this preclinical species. In general, up to 86% and 100% of the predicted V_ss values are respectively within 2-fold and 3-fold errors compared to the experimentally determined values, whereas these numbers are 78% and 94% for K_p-dermis when the anticipated outlier compounds are not included. Binding data to dermis are comparable between minipigs and humans. Overall, this study is a first step toward developing a mechanistic TCM for the minipig, with the aim of increasing the use of physiologically based pharmacokinetic models of drugs for that species in addition to rats, dogs, and humans because such models are used in preclinical and clinical transdermal studies.

Porcine Prediction of Pharmacokinetic Parameters in People: A Pig in a Poke?

The minipig has become an animal of considerable interest in preclinical drug development. It has been used in toxicology research and in examining/establishing regulatory guidelines as a nonrodent animal model. We have reviewed some basic issues that one would want to consider in the development and testing of any animal model for humans. The pig is a reasonable alternative to the dog, but there are some clear limitations and unexplained disparities in the literature, which require further study; primary among these is the need for standardization in choice of breed and sex and routine protocols. The minipig offers numerous advantages over other established animal models, and it has similarities to the human with regard to anatomy, physiology, and biochemistry. The gastrointestinal tract is structurally and functionally similar to humans. This appears to be true for enzymes and transporters in the gut as well, but more study is needed. One major concern is assessment of oral drug absorption, especially with regard to potential food effects due to gastric emptying differences, yet this does not appear to be a consistent observation. Hepatic metabolism seems to reflect enzymatic patterns in humans, with some differences. Kidney function seems similar to humans but requires further study. We have analyzed literature data that suggest the pig would offer a reasonable model for human oral bioavailability and for allometric predictions of clearance. The minipig appears to be the model for dermal absorption in humans, and we discuss this in terms of literature data and our own in-house experience.