TISSUE BARRIERS 2016, VOL. 4, NO. 2, e1187981 (5 pages) http://dx.doi.org/10.1080/21688370.2016.1187981
PREFACE
Introduction for the special issue on recent advances in drug delivery across tissue barriers Randall J. Mrsnya and David J. Braydenb a Department of Pharmacy and Pharmacology, University of Bath Claverton Down, Bath, UK; bUCD School of Veterinary Medicine and Conway Institute, University College Dublin, Belfield, Dublin 4, Ireland
ABSTRACT
This special issue of Tissue Barriers contains a series of reviews with the common theme of how biological barriers established at epithelial tissues limit the uptake of macromolecular therapeutics. By improving our functional understanding of these barriers, the majority of the authors have highlighted potential strategies that might be applied to the non-invasive delivery of biopharmaceuticals that would otherwise require an injection format for administration. Half of the articles focus on the potential of particular technologies to assist oral delivery of peptides, proteins and other macromolecules. These include use of prodrug chemistry to improve molecule stability and permeability, and the related potential for oral delivery of poorly permeable agents by cellpenetrating peptides and dendrimers. Safety aspects of intestinal permeation enhancers are discussed, along with the more recent foray into drug-device combinations as represented by intestinal microneedles and externally-applied ultrasound. Other articles highlight the crossover between food research and oral delivery based on nanoparticle technology, while the final one provides a fascinating interpretation of the physiological problems associated with subcutaneous insulin delivery and how inefficient it is at targeting the liver.
The vast majority of mucosal surfaces that separate internal structures from the external world are composed of a single layer of polarized epithelial cells, e.g. the gastrointestinal tract and the respiratory tract. Despite their delicate nature, these epithelial sheets provide a formidable barrier to the casual and un-regulated uptake of most materials that the body is exposed to through ingestion and inhalation. While protecting the body from the many toxins and pathogens that the body is exposed to on a daily basis, this barrier also limits the efficient uptake of biopharmaceuticals including peptides, proteins, and DNA- and RNA-based drugs. Since selected macromolecules as well as certain toxins and pathogens traverse these simple epithelia barriers without inducing overt physical damage, one must consider that such pathways and mechanisms might be harnessed for the non-invasive of biopharmaceuticals. The identification of possible mechanisms to transport macromolecular structures across simple epithelia is a current need for the pharmaceutical industry, particularly in the context of increasing numbers of novel biopharmaceuticals being discovered and recent CONTACT Randall J. Mrsny © 2016 Taylor & Francis
[email protected]; David J. Brayden
ARTICLE HISTORY
Received 4 May 2016 Accepted 6 May 2016 KEYWORDS
cell penetrating peptides; food-derived nanoparticles; microneedles; oral delivery; oral peptide prodrugs; permeation enhancer toxicology; sonophoresis
advances in nanotechnologies, which can improve pharmacokinetics, localize molecules to target tissue, and potentially reduce toxicity. One of the most powerful reasons for this current need is greater appreciation by pharmaceutical industry leadership of the potential benefits of trans-epithelial delivery to not only improve the benefits of current drugs but to open up entirely novel biological paradigms for therapy. Previously, the focus behind finding trans-epithelial delivery strategies was to improve patient convenience and compliance in respect of already marketed drugs,1 an important although somewhat limited research goal. While these are clearly important in gaining maximum benefit to patients for drugs that currently must be administered by intravenous or subcutaneous injection or by implants, greater scientific benefits are now being appreciated in terms of the physiological and toxicological benefits of not flooding all tissues with the molecule, in contrast to localizing concentrations to the intended target. One prominent area of potential benefit involves the uptake of glucose-regulating hormones, such as insulin and glucagon-like peptide-1 (GLP-1), across
[email protected]
e1187981-2
R. J. MRSNY AND D. J. BRAYDEN
the intestinal epithelium. While these agents already have dramatically changed the lives of diabetic patients when administered by injection, one can envisage multiple added benefits to their therapeutic actions if they were to enter the body via the hepatic portal vein following uptake across the intestinal mucosa. This uptake route would emulate how the liver normally experiences such agents that are released from the basal surface of intestinal epithelial cells or pancreatic islet cells. Oral delivery of glucoseregulating hormones would, therefore, provide not only a more convenient but also a more physiologically-relevant outcome for these agents.2 In this regard, the corporate leadership of Novo-Nordisk has gone on record that they will invest heavily in research and manufacturing capacity with the goal of leveraging their diabetes franchise to also encompass oral delivery formats.3 A second focus of identifying methods to deliver poorly absorbed drugs across simple epithelial barriers relates to opportunities for local delivery to the submucosa. Many of the biopharmaceutical approaches have the ability to selectively target discrete cell populations within the body and thus perform very well following systemic injection. Others, however, are not as selective and thus patients can suffer from systemic side effects that limit the efficacy of these agents. Importantly, some of these agents target cell populations present in the submucosa of the gut and/or lung. Here, the identification of methods for efficient uptake across the simple epithelia could locally administer these molecules preferentially to the site where their target cell population resides, shifting the safety to efficacy profile dramatically. In particular, these submucosal spaces contain cells that represent the largest immune organs of the body and direct delivery of immune-modulating drugs to these sites could open entirely new therapies for a wide array of immune-related conditions including asthma, colitis, lupus, and multiple sclerosis4. Thus, the future of trans-epithelial delivery approaches may be used for inducing both systemic actions and local events, changing the therapeutic profile of many biopharmaceutical medicines. Efficient delivery of biopharmaceuticals across simple epithelia requires overcoming potent physical, chemical, biological, and physiological barriers. The first challenge is to reach these epithelial surfaces and to overcome proteolytic barriers in the gut, as well as mucus-driven removal and cell-based clearance
processes in the lung. A variety of technologies related to particle production, protection, and delivery have now been identified that can overcome many of these issues to allow for the placement of biologically active biopharmaceuticals at the luminal surface of simple epithelia, the limiting step to effective biopharmaceutical uptake. The physical barrier properties of simple epithelia provide a thermodynamic barrier to the movement of large molecular weight, typically charged and hydrophilic pharmaceutical agents.5 Such is the challenge that those molecules that are taken up by the apical membrane of these epithelial cells are commonly directed to intracellular cytosolic lysosomes where they are destroyed. There are two routes that might be explored to deliver biopharmaceuticals across these cellular barriers: between or across the epithelium. Movement between adjacent epithelial cells is defined as the paracellular route, whereas transcellular movement across cells involves migration across membrane bilayers and into the cell cytoplasm; transcytosis involves access to intracellular vesicles that might traffic across the cell to release their contents on the basal surface. As one might expect, successful strategies to utilize these routes of delivery must overcome barriers that are designed to thwart un-restricted movement through routes. Tight junction (TJ) complexes at the apical neck of adjacent epithelial cells limit paracellular flux of macromolecules; strategies to transiently modulate the barrier properties of TJ structures have been described, one of which resulted in a New Drug Application (NDA) for the peptide, octreotide, given in oral capsules.6 Transcellular intestinal permeability (aside from that mediated by transporters and carriers) has also been suggested as a contributing additional mechanism of permeation for some macromolecules including peptides when presented with selected permeation enhancers7, and part of the mechanism is likely to involve interaction with small intestinal bile salts to form micelles and mixed micelles.8 Finally, transcytosis pathways used by pathogen-derived materials to enter the body are also being explored to facilitate the uptake of biopharmaceuticals.9 Validation of these transport-enhancing approaches for oral delivery typically involves initial screening using in vitro models of polarized monolayers of epithelial cells followed by in vivo rodent models. While several transport-enhancing strategies have looked
TISSUE BARRIERS
promising using these in vitro and in vivo screens, translation for man have for the most part resulted in disappointing outcomes. Two important issues are likely the main challenges to this lack of successful translation. The first is that these models fail to adequately include scaled down elements of the actual solid dosage form required for dosing humans. This issue leads into the second, and likely more important, reason for these poor outcomes: most of these strategies are identified through empirical screens and lack a clearly defined mechanisms of action (MoA) for how the delivery system interacts with the payload at a molecular level, how the physicochemical properties of the combined system change in the human GI tract, and how the components of the system interact with the epithelium. The issue here is that, in the absence of a MoA, subtle differences in the physical, chemical, biological, and physiological barriers between these models and patients might dramatically change the safety and efficacy outcomes for an oral delivery strategy; a lack of MoA limits the potential for hypothesis driven studies that might be performed to correct these species-related differences.10 In this special issue of Tissue Barriers, a several articles are presented that describe novel methods to enhance the transport of biopharmaceuticals across simple epithelial barriers. Each group uses MoA strategies to identify potential strategies that might be translated to achieve improved uptake of proteins, peptides, and RNA-based drugs. In Buckley et al11, a team from Novo-Nordisk explores how peptides can be made more stable and permeable to improve their suitability for oral delivery. They describe how conjugation to cell penetrating peptides (CPP) and to ligands that target epithelial cell transporters and receptors in the intestine can potentially have advantageous benefits. The use of PEG and other polymers as conjugates to improve physicochemical properties of a biopharmaceutical is also discussed, along with an alternative approach of lipidization. While few of these approaches have yet been moved to clinical trials, possibly because they involve use of more risky new chemical entities, the authors provide a very informative table on the practicalities required to construct a viable dosage form based around achieving a target bioavailability value for potent and notso-potent peptides in man. Kristensen and Nielsen12 from University of Copenhagen then address the
e1187981-3
topic of CPP enhancing the oral delivery of peptides in greater detail. They compared preclinical data on conjugates and mixtures of CPP analogues based on the penetratin motif for insulin delivery and focus on the MoA as the driver for progress. Advanced techniques including single angle x-ray scattering, two photon microscopy, isothermal calorimetry and surface plasma resonance are providing detailed information on the nature of the interactions between CPP and peptides which, together with advanced cellular and in vivo imaging, will guide optimization of constructs based on CPP molecular structure and performance of the CPP-peptide mixture/conjugate in the conditions of the human small intestine in vivo. Yellepeddi and Ghandehari13 from Utah University continue the theme of improving intestinal uptake of poorly permeable molecules by updating readers on the preclinical progress with poly (amidoamine) (PAMAM) dendrimers. Much of the recent work has focused on the MoA of how dendrimers move across intestinal epithelial cells and how they can alter TJ permeability, with the authors predicting potentially very high oral bioavailability for dendrimers per se. The current drive appears to be in respect of how PAMAM dendrimers can be conjugated or complexed with some of the newer anti-cancer molecules in order to improve bioavailability and reduce toxicity by the oral route. McCartney et al14 from University College Dublin provide a comprehensive analysis of the safety of oral permeation enhancers, with a focus on those strategies in clinical trials. The authors again point out the discrepancies of trying to predict epithelial damage from in vitro models compared to the human intestine in vivo, which can repair mild perturbations induced by food and other challenges within minutes. Without dismissing the importance of studying the toxicology of repeat-dose administration on the epithelial damage-repair cycle in human trials, these authors point out that the current trial data on medium chain fatty acids, acyl carnitines and various excipients, as well as other agents with generallyregarded-as-safe status have not flagged intestinal toxicity as a problem (to date), in contrast to low efficacy and wide variability in bioavailability seen between human subjects. They also suggest that use
e1187981-4
R. J. MRSNY AND D. J. BRAYDEN
of such enhancers should be avoided in patients with inflammatory bowel disease (IBD) and coeliac disease, since they may already have permeability issues and dysregulated tight junctions. Schoellhammer et al15 from MIT argue that the current approaches for oral delivery as discussed above will only provide incremental benefits and that radically different thinking is required to shift oral bioavailability of biopharmaceuticals to achieve much higher values. Taking learnings from the transdermal microneedle field where constructs can deliver payloads from patches to just beneath the stratum corneum, the premise of the systems advocated by these authors is that intestinal tissue will similarly not convey pain. The authors reviewed patented systems ranging from drug-loaded capsules with projections made from stainless steel or dissolvable sugars that penetrate the small intestinal epithelium to potentially deliver insulin and antibodies. Studies in pigs showed that such systems can be monitored in pigs and revealed normal tissue histology when exposed to stainless steel needles. It is clear that orally-delivered, bioadhesive intestinal capsules and patches with associated projections that can deliver biopharmaceuticals has become an important research area and these efforts highlight the requirement for large animal models in certain approaches that cannot be readily assessed using rodents. The fact that Rani Corporation (CA, USA) has publicized new commercial relationships with Astra-Zeneca/Medimmune (USA) and Novartis (Switzerland) based on capsules with sugar microneedles to deliver impermeable payloads16 may spur interest in developing similar technologies. A trend in recent literature is how previously unrelated fields are intersecting to provide complementary knowledge. Zhang et al17 provide a perfect example of this by connecting exosome research in cancer to nanomedicines for drug delivery, and then linking both to food as a source of edible nanoparticles with exosome features. Starting with a hypothesis that, like mammalian cells, the plant kingdom also uses exosomes to convey information between cells, the authors reviewed how edible nanoparticles with potentially interesting pharmacology could be made from ginger, grapefruit, carrots and grapes. Orallydelivered grape nanoparticles with exosome features seemed to have stability and epithelial interaction advantages over many current prototypes made by pharmaceutical scientists from synthetic polymers and
lipids. Moreover, plant-derived lipid nanoparticles are being researched in animal models of disease including IBD and cancer. There will likely be more to come on this topic as nanoparticle science in food and pharmaceuticals converge. Finally, Shau et al18 from the University of Southern California remind us that although sub-cutaneous (s.c.) insulin has been a life-saving drug for decades, there is still much that is poorly understood in relation to the interaction between peripheral insulin and its effects in the liver. The premise is that s.c-delivered insulin is actually quite poor at accessing the liver and that downside outcomes of peripherally-dosed insulin are an increased potential for hypoglycaemia excursion and weight gain. The authors provide a mechanistic rationale for improving access of injectable insulin to the liver based on understanding insulin gradients and tissue barriers between the two. This data provides the basis of their argument for liver-targeted insulin injections by passive and active targeting of liver receptors using PEG- and receptor-targeted conjugates. We thank all the co-authors who contributed to this highly provocative set of seven related articles and to the Editorial staff of Tissue Barriers for assembling them.
Abbreviations CPP cell-penetrating peptides NP nanoparticle F bioavailability
Disclosure of potential conflicts of interest DB has consulted for some of the companies mentioned above.
References [1] Lewis AL, Richard J. Challenges in the delivery of peptide drugs: an industry perspective. Therapeutic Delivery 2014; 6:149-163. [2] Lakkireddy HR, Urmann M, Besenius M, Werner U, Haack T, Brun P, et al. Oral delivery of diabetes peptides Comparing standard formulations incorporating functional excipients and nanotechnologies in the translational context. Adv Drug Deliv Rev 2016 Mar 8. pii: S0169-409X(16)30076-X. doi: 10.1016/j.addr.2016.02.011 [3] http://www.in-pharmatechnologist.com/Drug-Delivery/ Novo-Nordisk-to-pump-2bn-into-network-on-back-oforal-GLP-1-milestone
TISSUE BARRIERS
[4] Winer DA, Luck H, Tsai S, Winer S. The intestinal immune system in obesity and insulin resistance. Cell Metabolism 2016; 23:413-426. [5] Turner JR, Buschmann MM, Romero-Calvo I, Sailer A, Shen L. The role of molecular remodeling in differential regulation of tight junction permeability. Semin Cell Dev Biol. 2014; 36:204-212. [6] Melmed S, Popovic V, Bidlingmaier M, Mercado M, van der Lely AJ, Biermasz N, Bolanowski M, et al. Safety and efficacy of oral octreotide in acromegaly: results of a multicenter Phase III trial. J. Clin. Endocrinol. Metab. 2015; 18;100:1699-1708. [7] Brayden DJ, Gleeson J, Walsh E. A head-to-head multiparametric high content analysis of a series of medium chain fatty acid intestinal permeation enhancers in Caco2 cells. Eur. J. Pharm. Biopharm. 2014; 88: 830-839. [8] Gradauer K, Nishiumi A, Unrinin K, Higashino H, Kataoka M, Pedersen BL, Buckley ST, Yamashita S. Interaction with mixed micelles in the intestine attenuates the permeation enhancing potential of alkyl-maltosides. Mol. Pharm. 2015; 12:2245-253. [9] Mrsny RJ, Daugherty AL, McKee ML, FitzGerald DJ. Bacterial toxins as tools for mucosal vaccination. Drug Discovery Today 2002; 7:247-258. [10] Yun YH, Lee BK, Park K. Controlled Drug Delivery: Historical perspective for the next generation. J. Control Release 2015; 219:2-7.
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[11] Buckley ST, Hubalek F, and Rahbek, UL. Chemically modified peptides and proteins - critical considerations for oral delivery. Tissue Barriers 2016; 4(2): e1156805 [10p.]. [12] Kristensen M, Nielsen HM. Cell-penetrating peptides as tools to enhance non-injectable delivery of biopharmaceuticals. Tissue Barriers 2016; 4(2): e1178369 [15p.]. [13] Yellepeddi VK, Ghandehari H. Poly(amidoamine) dendrimers in oral delivery. Tissue Barriers 2016; 4(2): e1173773 [12p]. [14] McCartney F, Gleeson JP, Brayden DJ. Safety concerns over the use of intestinal permeation enhancers: A minireview. Tissue Barriers 2016; 4(2): e1176822 [13p.]. [15] Schoellhammer CM, Langer R, Traverso G. Of microneedles and ultrasound: Physical modes of gastrointestinal macromolecule delivery. Tissue Barriers 2016; 4(2): e1150235 [5p]. [16] http://www.prnewswire.com/news-releases/rani-therapeu tics-announces-collaboration-with-medimmune-in-themetabolic-disease-field-300200271.html [17] Zhang M, Viennoisa E, Xua C, Merlin D. Plant derived edible nanoparticles as a new therapeutic approach against diseases. Tissue Barriers 2016, 4(2):e1134415 [9p.]. [18] Shao J, Zaro JL, Shen, W-C. Tissue barriers and novel approaches to achieve hepatoselectivity of subcutaneously-injected insulin therapeutics. Tissue Barriers 2016; 4(2): e1156804 [6p.].
PREFACE
Introduction for the special issue on recent advances in drug delivery across tissue barriers Randall J. Mrsnya and David J. Braydenb a Department of Pharmacy and Pharmacology, University of Bath Claverton Down, Bath, UK; bUCD School of Veterinary Medicine and Conway Institute, University College Dublin, Belfield, Dublin 4, Ireland
ABSTRACT
This special issue of Tissue Barriers contains a series of reviews with the common theme of how biological barriers established at epithelial tissues limit the uptake of macromolecular therapeutics. By improving our functional understanding of these barriers, the majority of the authors have highlighted potential strategies that might be applied to the non-invasive delivery of biopharmaceuticals that would otherwise require an injection format for administration. Half of the articles focus on the potential of particular technologies to assist oral delivery of peptides, proteins and other macromolecules. These include use of prodrug chemistry to improve molecule stability and permeability, and the related potential for oral delivery of poorly permeable agents by cellpenetrating peptides and dendrimers. Safety aspects of intestinal permeation enhancers are discussed, along with the more recent foray into drug-device combinations as represented by intestinal microneedles and externally-applied ultrasound. Other articles highlight the crossover between food research and oral delivery based on nanoparticle technology, while the final one provides a fascinating interpretation of the physiological problems associated with subcutaneous insulin delivery and how inefficient it is at targeting the liver.
The vast majority of mucosal surfaces that separate internal structures from the external world are composed of a single layer of polarized epithelial cells, e.g. the gastrointestinal tract and the respiratory tract. Despite their delicate nature, these epithelial sheets provide a formidable barrier to the casual and un-regulated uptake of most materials that the body is exposed to through ingestion and inhalation. While protecting the body from the many toxins and pathogens that the body is exposed to on a daily basis, this barrier also limits the efficient uptake of biopharmaceuticals including peptides, proteins, and DNA- and RNA-based drugs. Since selected macromolecules as well as certain toxins and pathogens traverse these simple epithelia barriers without inducing overt physical damage, one must consider that such pathways and mechanisms might be harnessed for the non-invasive of biopharmaceuticals. The identification of possible mechanisms to transport macromolecular structures across simple epithelia is a current need for the pharmaceutical industry, particularly in the context of increasing numbers of novel biopharmaceuticals being discovered and recent CONTACT Randall J. Mrsny © 2016 Taylor & Francis
[email protected]; David J. Brayden
ARTICLE HISTORY
Received 4 May 2016 Accepted 6 May 2016 KEYWORDS
cell penetrating peptides; food-derived nanoparticles; microneedles; oral delivery; oral peptide prodrugs; permeation enhancer toxicology; sonophoresis
advances in nanotechnologies, which can improve pharmacokinetics, localize molecules to target tissue, and potentially reduce toxicity. One of the most powerful reasons for this current need is greater appreciation by pharmaceutical industry leadership of the potential benefits of trans-epithelial delivery to not only improve the benefits of current drugs but to open up entirely novel biological paradigms for therapy. Previously, the focus behind finding trans-epithelial delivery strategies was to improve patient convenience and compliance in respect of already marketed drugs,1 an important although somewhat limited research goal. While these are clearly important in gaining maximum benefit to patients for drugs that currently must be administered by intravenous or subcutaneous injection or by implants, greater scientific benefits are now being appreciated in terms of the physiological and toxicological benefits of not flooding all tissues with the molecule, in contrast to localizing concentrations to the intended target. One prominent area of potential benefit involves the uptake of glucose-regulating hormones, such as insulin and glucagon-like peptide-1 (GLP-1), across
[email protected]
e1187981-2
R. J. MRSNY AND D. J. BRAYDEN
the intestinal epithelium. While these agents already have dramatically changed the lives of diabetic patients when administered by injection, one can envisage multiple added benefits to their therapeutic actions if they were to enter the body via the hepatic portal vein following uptake across the intestinal mucosa. This uptake route would emulate how the liver normally experiences such agents that are released from the basal surface of intestinal epithelial cells or pancreatic islet cells. Oral delivery of glucoseregulating hormones would, therefore, provide not only a more convenient but also a more physiologically-relevant outcome for these agents.2 In this regard, the corporate leadership of Novo-Nordisk has gone on record that they will invest heavily in research and manufacturing capacity with the goal of leveraging their diabetes franchise to also encompass oral delivery formats.3 A second focus of identifying methods to deliver poorly absorbed drugs across simple epithelial barriers relates to opportunities for local delivery to the submucosa. Many of the biopharmaceutical approaches have the ability to selectively target discrete cell populations within the body and thus perform very well following systemic injection. Others, however, are not as selective and thus patients can suffer from systemic side effects that limit the efficacy of these agents. Importantly, some of these agents target cell populations present in the submucosa of the gut and/or lung. Here, the identification of methods for efficient uptake across the simple epithelia could locally administer these molecules preferentially to the site where their target cell population resides, shifting the safety to efficacy profile dramatically. In particular, these submucosal spaces contain cells that represent the largest immune organs of the body and direct delivery of immune-modulating drugs to these sites could open entirely new therapies for a wide array of immune-related conditions including asthma, colitis, lupus, and multiple sclerosis4. Thus, the future of trans-epithelial delivery approaches may be used for inducing both systemic actions and local events, changing the therapeutic profile of many biopharmaceutical medicines. Efficient delivery of biopharmaceuticals across simple epithelia requires overcoming potent physical, chemical, biological, and physiological barriers. The first challenge is to reach these epithelial surfaces and to overcome proteolytic barriers in the gut, as well as mucus-driven removal and cell-based clearance
processes in the lung. A variety of technologies related to particle production, protection, and delivery have now been identified that can overcome many of these issues to allow for the placement of biologically active biopharmaceuticals at the luminal surface of simple epithelia, the limiting step to effective biopharmaceutical uptake. The physical barrier properties of simple epithelia provide a thermodynamic barrier to the movement of large molecular weight, typically charged and hydrophilic pharmaceutical agents.5 Such is the challenge that those molecules that are taken up by the apical membrane of these epithelial cells are commonly directed to intracellular cytosolic lysosomes where they are destroyed. There are two routes that might be explored to deliver biopharmaceuticals across these cellular barriers: between or across the epithelium. Movement between adjacent epithelial cells is defined as the paracellular route, whereas transcellular movement across cells involves migration across membrane bilayers and into the cell cytoplasm; transcytosis involves access to intracellular vesicles that might traffic across the cell to release their contents on the basal surface. As one might expect, successful strategies to utilize these routes of delivery must overcome barriers that are designed to thwart un-restricted movement through routes. Tight junction (TJ) complexes at the apical neck of adjacent epithelial cells limit paracellular flux of macromolecules; strategies to transiently modulate the barrier properties of TJ structures have been described, one of which resulted in a New Drug Application (NDA) for the peptide, octreotide, given in oral capsules.6 Transcellular intestinal permeability (aside from that mediated by transporters and carriers) has also been suggested as a contributing additional mechanism of permeation for some macromolecules including peptides when presented with selected permeation enhancers7, and part of the mechanism is likely to involve interaction with small intestinal bile salts to form micelles and mixed micelles.8 Finally, transcytosis pathways used by pathogen-derived materials to enter the body are also being explored to facilitate the uptake of biopharmaceuticals.9 Validation of these transport-enhancing approaches for oral delivery typically involves initial screening using in vitro models of polarized monolayers of epithelial cells followed by in vivo rodent models. While several transport-enhancing strategies have looked
TISSUE BARRIERS
promising using these in vitro and in vivo screens, translation for man have for the most part resulted in disappointing outcomes. Two important issues are likely the main challenges to this lack of successful translation. The first is that these models fail to adequately include scaled down elements of the actual solid dosage form required for dosing humans. This issue leads into the second, and likely more important, reason for these poor outcomes: most of these strategies are identified through empirical screens and lack a clearly defined mechanisms of action (MoA) for how the delivery system interacts with the payload at a molecular level, how the physicochemical properties of the combined system change in the human GI tract, and how the components of the system interact with the epithelium. The issue here is that, in the absence of a MoA, subtle differences in the physical, chemical, biological, and physiological barriers between these models and patients might dramatically change the safety and efficacy outcomes for an oral delivery strategy; a lack of MoA limits the potential for hypothesis driven studies that might be performed to correct these species-related differences.10 In this special issue of Tissue Barriers, a several articles are presented that describe novel methods to enhance the transport of biopharmaceuticals across simple epithelial barriers. Each group uses MoA strategies to identify potential strategies that might be translated to achieve improved uptake of proteins, peptides, and RNA-based drugs. In Buckley et al11, a team from Novo-Nordisk explores how peptides can be made more stable and permeable to improve their suitability for oral delivery. They describe how conjugation to cell penetrating peptides (CPP) and to ligands that target epithelial cell transporters and receptors in the intestine can potentially have advantageous benefits. The use of PEG and other polymers as conjugates to improve physicochemical properties of a biopharmaceutical is also discussed, along with an alternative approach of lipidization. While few of these approaches have yet been moved to clinical trials, possibly because they involve use of more risky new chemical entities, the authors provide a very informative table on the practicalities required to construct a viable dosage form based around achieving a target bioavailability value for potent and notso-potent peptides in man. Kristensen and Nielsen12 from University of Copenhagen then address the
e1187981-3
topic of CPP enhancing the oral delivery of peptides in greater detail. They compared preclinical data on conjugates and mixtures of CPP analogues based on the penetratin motif for insulin delivery and focus on the MoA as the driver for progress. Advanced techniques including single angle x-ray scattering, two photon microscopy, isothermal calorimetry and surface plasma resonance are providing detailed information on the nature of the interactions between CPP and peptides which, together with advanced cellular and in vivo imaging, will guide optimization of constructs based on CPP molecular structure and performance of the CPP-peptide mixture/conjugate in the conditions of the human small intestine in vivo. Yellepeddi and Ghandehari13 from Utah University continue the theme of improving intestinal uptake of poorly permeable molecules by updating readers on the preclinical progress with poly (amidoamine) (PAMAM) dendrimers. Much of the recent work has focused on the MoA of how dendrimers move across intestinal epithelial cells and how they can alter TJ permeability, with the authors predicting potentially very high oral bioavailability for dendrimers per se. The current drive appears to be in respect of how PAMAM dendrimers can be conjugated or complexed with some of the newer anti-cancer molecules in order to improve bioavailability and reduce toxicity by the oral route. McCartney et al14 from University College Dublin provide a comprehensive analysis of the safety of oral permeation enhancers, with a focus on those strategies in clinical trials. The authors again point out the discrepancies of trying to predict epithelial damage from in vitro models compared to the human intestine in vivo, which can repair mild perturbations induced by food and other challenges within minutes. Without dismissing the importance of studying the toxicology of repeat-dose administration on the epithelial damage-repair cycle in human trials, these authors point out that the current trial data on medium chain fatty acids, acyl carnitines and various excipients, as well as other agents with generallyregarded-as-safe status have not flagged intestinal toxicity as a problem (to date), in contrast to low efficacy and wide variability in bioavailability seen between human subjects. They also suggest that use
e1187981-4
R. J. MRSNY AND D. J. BRAYDEN
of such enhancers should be avoided in patients with inflammatory bowel disease (IBD) and coeliac disease, since they may already have permeability issues and dysregulated tight junctions. Schoellhammer et al15 from MIT argue that the current approaches for oral delivery as discussed above will only provide incremental benefits and that radically different thinking is required to shift oral bioavailability of biopharmaceuticals to achieve much higher values. Taking learnings from the transdermal microneedle field where constructs can deliver payloads from patches to just beneath the stratum corneum, the premise of the systems advocated by these authors is that intestinal tissue will similarly not convey pain. The authors reviewed patented systems ranging from drug-loaded capsules with projections made from stainless steel or dissolvable sugars that penetrate the small intestinal epithelium to potentially deliver insulin and antibodies. Studies in pigs showed that such systems can be monitored in pigs and revealed normal tissue histology when exposed to stainless steel needles. It is clear that orally-delivered, bioadhesive intestinal capsules and patches with associated projections that can deliver biopharmaceuticals has become an important research area and these efforts highlight the requirement for large animal models in certain approaches that cannot be readily assessed using rodents. The fact that Rani Corporation (CA, USA) has publicized new commercial relationships with Astra-Zeneca/Medimmune (USA) and Novartis (Switzerland) based on capsules with sugar microneedles to deliver impermeable payloads16 may spur interest in developing similar technologies. A trend in recent literature is how previously unrelated fields are intersecting to provide complementary knowledge. Zhang et al17 provide a perfect example of this by connecting exosome research in cancer to nanomedicines for drug delivery, and then linking both to food as a source of edible nanoparticles with exosome features. Starting with a hypothesis that, like mammalian cells, the plant kingdom also uses exosomes to convey information between cells, the authors reviewed how edible nanoparticles with potentially interesting pharmacology could be made from ginger, grapefruit, carrots and grapes. Orallydelivered grape nanoparticles with exosome features seemed to have stability and epithelial interaction advantages over many current prototypes made by pharmaceutical scientists from synthetic polymers and
lipids. Moreover, plant-derived lipid nanoparticles are being researched in animal models of disease including IBD and cancer. There will likely be more to come on this topic as nanoparticle science in food and pharmaceuticals converge. Finally, Shau et al18 from the University of Southern California remind us that although sub-cutaneous (s.c.) insulin has been a life-saving drug for decades, there is still much that is poorly understood in relation to the interaction between peripheral insulin and its effects in the liver. The premise is that s.c-delivered insulin is actually quite poor at accessing the liver and that downside outcomes of peripherally-dosed insulin are an increased potential for hypoglycaemia excursion and weight gain. The authors provide a mechanistic rationale for improving access of injectable insulin to the liver based on understanding insulin gradients and tissue barriers between the two. This data provides the basis of their argument for liver-targeted insulin injections by passive and active targeting of liver receptors using PEG- and receptor-targeted conjugates. We thank all the co-authors who contributed to this highly provocative set of seven related articles and to the Editorial staff of Tissue Barriers for assembling them.
Abbreviations CPP cell-penetrating peptides NP nanoparticle F bioavailability
Disclosure of potential conflicts of interest DB has consulted for some of the companies mentioned above.
References [1] Lewis AL, Richard J. Challenges in the delivery of peptide drugs: an industry perspective. Therapeutic Delivery 2014; 6:149-163. [2] Lakkireddy HR, Urmann M, Besenius M, Werner U, Haack T, Brun P, et al. Oral delivery of diabetes peptides Comparing standard formulations incorporating functional excipients and nanotechnologies in the translational context. Adv Drug Deliv Rev 2016 Mar 8. pii: S0169-409X(16)30076-X. doi: 10.1016/j.addr.2016.02.011 [3] http://www.in-pharmatechnologist.com/Drug-Delivery/ Novo-Nordisk-to-pump-2bn-into-network-on-back-oforal-GLP-1-milestone
TISSUE BARRIERS
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