Abstract
Targeted drug delivery to sites of inflammation will provide effective, precise, and safe therapeutic interventions for treatment of diverse disease conditions, by limiting toxic side effects and/or increasing drug action. Disease-site targeting is believed to play a major role in the enhanced efficacy observed for a variety of drugs when formulated inside lipid vesicles. This article will focus on the factors and mechanisms involved in drug targeting to sites of inflammation and the importance of cell adhesion molecules, in particular intercellular adhesion molecule-1, in this process.
INTRODUCTION
Targeted delivery of therapeutics to sites of inflammation is an important goal. The endothelium represents a key target for pharmacological interventions in many disease conditions, including rheumatological, cardiovascular, hematological, pulmonary, and oncological (; ; ; ). The goal of endothelial targeting is to achieve specific and safe delivery of a drug to, into, or across endothelial cells, in order to localize effects in the lumen, desired intracellular endothelial cell compartments, or extravascular space, thereby improving pharmacological interventions. However, due to their lack of affinity to the endothelium, only a small fraction of injected therapeutics binds to endothelial cells (). Progress in understanding disease mechanisms provides better selection of drugs for endothelial interventions and a deeper insight into designing drug delivery carriers to target inflammatory specific destinations.
DRUG TARGETING TO SITES OF INFLAMMATION
In inflamed tissues, the permeability of the vasculature is often increased to the extent that particulate carriers, which are normally excluded from these tissues, can extravasate and localize in the tissue interstitial space. Endothelial cells also start to express several types of adhesion molecules: the selectins, the integrins, and the immunoglobulins, which mediate recruitment of leukocytes into the inflamed tissue (; ; ). Furthermore, the process of angiogenesis (formation of new blood vessels from pre-existing vasculature) may occur in several chronic inflammatory disorders, such as rheumatoid arthritis, psoriasis, and inflammatory bowel disease (; ). In this complex cascade of events, numerous cell-surface receptors, adhesion molecules, and growth factors are involved, which may serve as potential targets for therapeutic intervention.
Targeted intervention in inflammatory disease at the vascular endothelial cell (VEC) level has great potential. Rational design of such drug delivery systems includes: (1) selection of proper target determinants on endothelial surfaces, such as cell adhesion molecules (CAMs); (2) production of affinity ligands useful for targeting, such as affinity peptides, antibodies, or their fragments; (3) selection and adopting of suitable delivery vehicles, such as liposomes; and (4) formulation of drug delivery system with optimal targeting and therapeutic features (). Specific drug delivery should concentrate the drug at the targeted site, increasing efficacy, and also decreasing side effects in other tissues (; ). This concept is particularly attractive in cancer therapy, where the dose a patient can tolerate is limited due to high toxicity to non-target cells. Targeted delivery to tumor tissue may allow the use of lower drug-concentrations. Moreover, targeting therapeutic agents to the vasculature of tumors offer additional advantages; in particular blood vessels are more readily accessible to intravenously administered therapy than tumor cells (). In the same respects, targeted drug delivery of opioid analgesics to peripheral opioid receptors upregulated at sites of inflammation will significantly alleviate nociception without the central opioid mediated side effects ().
TARGETING ADHESION MOLECULES
Adhesion molecules are glycoproteins expressed on cell surfaces, where they mediate the contact between two cells (both homotypic and heterotypic interactions) or between cells and the extracellular matrix. They are essential for the regulation of immune cell responses and migration of inflammatory cells from the blood vessels into inflamed tissues (; ). In fact, the expression of particular CAMs [e.g., intercellular adhesion molecule-1 (ICAM-1), E-selectin, P-selectin, vascular cell adhesion molecule-1 (VCAM-1)] are locally induced or enhanced at areas of inflammation (; ; ; ; ; ; ). Upregulated and/or overexpressed CAMs can be found in a multitude of clinical diseases where inflammation and immune cells are involved (e.g., ischemia-reperfusion injury, transplant rejection, and inflammatory diseases of the cardiovascular system, skin, kidneys, gastrointestinal tract, brain, and liver; ; ; ; ; ). Additionally, tumor cells use adhesion molecules to grow and spread throughout the body ().
Of particular note is the implication of CAMs in the pathogenesis of several rheumatic diseases. For example, rheumatoid arthritis is a chronic inflammatory disease in which adhesion molecules play an important role in the invasion of leukocytes into synovial tissues, leading to tissue damage (; ). Not only have increased expression of E-selectin, VCAM-1, and ICAM-1 been found on the vascular endothelium of synovial tissues, but immunohistochemical studies have shown elevated levels of adhesion molecule expression in ongoing inflammatory lesions (; ). Although reduction or blockade of the expression or function of a specific CAM is a possible therapeutic way to diminish infiltration and/or activation of inflammatory immune cells in order to reduce inflammation, this approach is complicated by the fact that most types of adhesion molecules are expressed on more than one cell type, that most cells express more than one adhesion molecule on their surface, and that several molecules can function as a ligand for a single adhesion molecule (; ). Importantly, blockade of CAMs can interfere with functions of immune cells essential for host defense (; ). Induction and/or increased expression of certain CAMs at inflammatory loci associated with various diseases offers opportunities for the development of new therapeutic strategies aimed toward selective drug-targeting. Adhesion molecules represent an easily accessible target molecule for therapeutics circulating in the blood compartment (; ; ; ; ).
INTERCELLULAR ADHESION MOLECULE-1
Intercellular adhesion molecules (ICAMs) are structurally related transmembrane glycoproteins of the immunoglobulin supergene family and are ligands for the β2 integrin molecules present on leukocytes (; ). Of the five ICAMs identified, ICAM-1 is the most extensively studied (; ). ICAM-1 specifically participates in trafficking of inflammatory cells, in leukocyte effector functions, in adhesion of antigen-presenting cells to T lymphocytes, in microbial pathogenesis, and in signal transduction pathways through outside-in signaling events (; ; ). This adhesion molecule is localized to both the apical and basolateral surface of endothelial cells, making it ideally positioned to facilitate transendothelial migration of leukocytes (). In fact, ICAM-1 (along with VCAM-1) is considered to represent the most important adhesion molecule for leukocyte recruitment to inflamed sites (). Additionally, ICAM-1 has been shown to exist in a soluble form in circulation, which results from proteolytic cleavage mediated by neutrophil proteases (leukocyte elastase and cathepsin G) in a process independent of ICAM-1 surface density ().
Intercellular adhesion molecule-1 is widely distributed and expressed constitutively at low levels on leukocytes, VECs, fibroblasts, and epithelial cells. Although ICAM-1 is present in several cell types, the level of expression is orders of magnitude lower than that of VECs (; ; ; ; ; , ; ). Stimulation of a variety of cells with inflammatory cytokines such as interleukin-1 (IL-1), tumor necrosis factor-α (TNF-α), and interferon-γ (IFN-γ) has been documented to increase ICAM-1 expression on multiple cell types (; ; ; , ; ). Strong upregulation of ICAM-1 is observed under inflammatory conditions within 24 h (). In contrast to the selectins, which are rapidly down regulated after induction, ICAM-1 and VCAM-1, once upregulated, remain on the cell surface for more than 48 h (). Information on the internalization capacity of ICAM-1 is, however, rather contradictory (). Some authors report the total absence of internalization on TNF-activated human umbilical vein endothelial cells (HUVEC) or a rather slow process of internalization by HUVECs, whereas others report on (rapid) internalization of ICAM-1-binding peptides by lymphocytes, antibody-targeted liposomes or poly lactic-co-glycolic acid (PLGA) nanoparticles by epithelial cells (; ). In fact, it has been reported that ICAM-1 internalization levels are practically indistinguishable from background (<10% of surface expressed ICAM-1; ). These contradictory findings may not only be attributed to the difference in cell types and targeting ligands used, but also to the timeframe of the study (). It is therefore of great importance to test the internalization capacity of the developed drug delivery system. Targeting to non- or slow-internalizing epitopes may be of specific interest for drugs that work at the luminal site of the VECs, whereas fast-internalizing epitopes are interesting for drugs with an intracellular address (; ).
Several adhesion molecules involved in the leukocyte adhesion cascade, in principle, comply with the requirements for achieving targeted delivery of drugs into VECs. However of these, ICAM-1 represents an attractive target since it is a high-density determinant stably exposed from the endothelial surface, which is upregulated and functionally involved in inflammation (; ; ; ; ; , ; ). In particular, ICAM-1 seems to be well suited for drug targeting to the luminal surface, due to ineffective internalization of either monomolecular or large anti-ICAM conjugates (). Potentially this will allow extravasation of the delivery carrier across the endothelial cells and release of the therapeutic drug specifically into the inflammatory site of action (; ). This feature distinguishes ICAM from other similarly prevalent endothelial determinants all of which are rapidly internalized, therefore leading to early release of the drug within the endothelial cells themselves (; ).
Antibodies to CAMs are being explored as therapeutics and delivery carriers in cell cultures, animal models, and early clinical studies (). A small number of studies have demonstrated the validity of such an approach, in particular showing specific binding and drug delivery to VECs in vitro and in vivo (; ; ; ; ; ; ; ; ; ; , ; ; ; ). Presumably, the specific and strong upregulation of these CAMs at sites of inflammation still allows specific targeting to be observed. Therefore, ICAM-1 targeting seems attractive, as this CAM shows basal levels of expression on VECs in general, but is strongly upregulated on VECs at inflamed sites (; ; ; ; ; , ; ). These developments in drug targeting to VECs will result in increasing knowledge on the role of the endothelium in inflammatory disorders and will further improve clinical therapy.
SELECTIVE INTERACTION WITH ICAM-1 AND UPTAKE BY TARGET CELLS
There are a number of potential modes of delivery of encapsulated therapeutics from ICAM-1 targeted carriers, which will affect its therapeutic availability and action. Contradicting results have been reported of the extent of internalization of ICAM-1-directed carriers by endothelial cells (). The capacity of endothelial cells to uptake anti-CAM multimeric conjugates may depend on the size of the particles, with conjugates having diameters from 100 to 300 nm readily entering endothelial cells, whereas conjugates of larger size (500 nm to 1 μm) remained attached to the cell surface at 37°C (; ; ). The notion that small multimeric ligands can undergo internalization within endothelial cells by CAM-mediated endocytosis is of pharmacological and physiological relevance (; ; ). The signaling and cytoskeletal events involved in endothelial internalization of anti-CAM conjugates are similar to those triggered by CAM-clustering in course of leukocyte adhesion and transmigration (). This parallelism supports the notion that intracellular drug delivery mediated by anti-CAM conjugates may be further enhanced in inflammation and pathological conditions that activate such transduction pathways in endothelial cells ().
In addition to delivering therapeutic cargoes intracellularly or to the luminal surface to have an anti-inflammatory effect on the endothelial cells involved in inflammation (; ), it is plausible for liposomes under pathological conditions to extravasate through the endothelial barrier directed by ICAM-1 on the surface of endothelial cells at sites of inflammation to release drugs within the extravascular tissue space (; ; ; ; ; ).
FACTORS INFLUENCING TARGET ACCUMULATION IN INFLAMMATION
Drug targeting using liposomes as carriers holds much promise, especially in reducing toxicity and targeting delivery to pathological sites of inflammation (e.g., musculoskeletal conditions, infection, burns, tumors) that are characterized by increased vascular permeability (; ; ; ; ; ; ; ; ; ; ; ). Long-circulating liposomes are currently used in targeted drug delivery to tumors and inflammatory regions, and have shown impressive improvement of the therapeutic index of encapsulated drugs (; , ; ; ; ; ; ). For example, rats and mice with arthritis treated with a single intravenous (IV) injection of sterically stabilized liposomes (SL) containing prednisolone phosphate resulted in complete remission of paw inflammation for 1 week in comparison to free drug (). Mechanistic studies showed that the increased therapeutic benefit was a result of selective joint targeting ().
Within inflamed tissues the permeability of the vasculature is often increased to the extent that particulate carriers, which are normally excluded from these tissues, can extravasate and localize in the tissue interstitial space (; ). This selective accumulation and increase in drug concentration at inflamed target sites is due to the so-called enhanced permeability and retention (EPR) effect (; ). Inflammation results in a dramatic change in blood vessel permeability as the capillary vasculature undergoes structural remodeling to allow leukocyte diapedesis into the peripheral tissue (). The width of the tight junctional regions between endothelial cells in vivo has been reported to be from 12 to 20 nm (), however exposure of endothelial cells to inflammatory mediators increases permeability of the microvasculature, with the formation of gaps of up to 1 μm (). In fact, pore sizes ranging from 0.2 to 1.2 μm have been observed, though the size and number of pores are dependent upon the microenvironment of the pathological site (; ). Observations using fluorescence and electron microscopy have shown that SL can indeed extravasate beyond the endothelial barrier, mainly in postcapillary venules, with SL ranging from 100 to 200 nm in diameter having a higher probability of encountering the leaky vessels of the inflamed tissue (; ; ). Leukocytes are able to open intercellular junctions of the endothelium monolayer by stimulating contraction of the endothelial cells or by causing a gap by passing between the cells (). It is therefore plausible that liposomal carriers may cross the monolayer in association with leukocytes or migrate independently across gaps formed in the monolayer by leukocyte migration (; ; ; ).
Currently, systemic liposome targeting strategies investigated are able to deliver no more than a few percent of the administered dose to their desired sites in vivo (). Although these formulations represent significant improvements over corresponding conventional drug therapies, much of the administered dose is still delivered to non-targeted tissues (). For example, a biodisposition study of polyethylene glycol (PEG)-coated lipid microspheres of indomethacin in arthritic rats reported an overall drug targeting efficiency of 7.5-fold higher than the conventional lipid microspheres (). The enhanced accumulation of the drug in the inflammatory tissue may be attributed to extravasation through the leaky vasculature and their possible uptake by circulating monocytes, which would subsequently be concentrated in the rheumatic joints (). Importantly, formulation as lipid microspheres drastically reduced the concentration of the drug in the brain (Cmax) from 1.73 to 0.69 μg/g of the tissue, thereby reducing central nervous system (CNS) adverse effects (). PEG-coated lipid microspheres further reduced the concentration to 0.58 μg/g (). The lower accumulation in sensitive non-target tissues (e.g., brain, kidneys) may be due to the reduced availability of the free drug in the blood (). It should be noted that the blood–brain barrier is often the rate-limiting factor in determining permeation of therapeutic drugs into the brain due to both physical (tight junctions) and metabolic (enzymes) barriers (; ). Thus liposomal carriers are only able to localize in the brain more efficiently when this barrier has been altered (; ; ).
Attachment of target-specific ligands to the liposome surface (active targeting) has been shown to further enhance targeting to specific cells or tissues (; , ; ; ; ; ; ). Targeting endothelial cells by exploiting cell-specific surface markers has been widely investigated in vitro (; ; ; ; ). Liposomes have been modified with ligands that can selectively interact with E-selectin (; ; ), ICAM-1 (; ; ; ; ,; ; ) and VCAM-1 () molecules that are upregulated on the surface of endothelial cells following activation by inflammatory signals. For example, P0-peptide-1 linked to liposome surfaces is capable of mediating the specific binding of liposomes to melanoma cells expressing high levels of ICAM-1, thus making it possible to target cancerous melanocytes in lymph nodes and skin melanomas (). Most of the work with liposomes in inflammatory disorders are based on imaging agents, with only few in vivo studies having been conducted using ligand-targeted liposomes incorporating therapeutic agents (). For example, the biodistribution and target localization of E-selectin-targeted dexamethasone-containing liposomes was examined in a murine delayed-type hypersensitivity model, which reported enhanced uptake by activated endothelium at inflamed sites as compared with control tissue (). Similarly, selective interaction with target cells following extravasation of targeted liposomes into the inflamed tissue has hardly been addressed in vivo (). reported that the surface receptor CD134, specifically expressed by auto-aggressive T cells at sites of inflammation, could efficiently be targeted by liposomes modified with anti-CD134 antibody. It was observed that encapsulation of 5′-fluorodeoxyuridine dipalmitate in these liposomes could lead to inactivation of auto-aggressive T cells and amelioration of experimental arthritis (). In addition, loperamide-encapsulated ICAM-1 targeted immunoliposomes have been shown to induce significant peripheral antinociceptive and anti-inflammatory activity in rats with complete Freund’s adjuvant-induced inflammation of the paw via an opioid receptor dependent mechanism ().
This phenomenon of disease-site targeting is believed to play a major role in the enhanced efficacy observed for a variety of drugs when formulated inside lipid vesicles (; ; ; ; ; ; ). Formulation of ICAM-1-directed sterically stabilized immunoliposomes (SIL) will not only allow prolonged circulation but also active targeting to sites of inflammation (; ; ; ; ). Such drug carriers may escape from the gaps between adjacent endothelial cells and openings at the vessel termini during inflammation by passive convective transport and/or ligand-directed targeting (; ). It is also plausible that some liposomes can attach onto activated leukocytes undergoing diapedesis into inflammatory sites (), as CAMs such as ICAM-1 are expressed not only on the surface of vascular endothelium and neurones, but also by activated T lymphocytes (; ; ). This field of research of targeting therapeutics to sites of inflammation specific to pathological disease states will improve the efficacy of therapeutic agents and reduce the toxicity to other parts of the body.
Statements
Acknowledgments
This work was supported by The Pharmacy Research Trust of New South Wales.
Conflict of interest
The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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Summary
Keywords
inflammation, targeted drug delivery, ICAM-1, cell adhesion molecules, liposomes
Citation
Hua S (2013) Targeting sites of inflammation: intercellular adhesion molecule-1 as a target for novel inflammatory therapies. Front. Pharmacol. 4:127. doi: 10.3389/fphar.2013.00127
Received
04 August 2013
Accepted
14 September 2013
Published
04 October 2013
Volume
4 - 2013
Edited by
Peter John Cabot, The University of Queensland, Australia
Reviewed by
Muzamil Ahmad, Indian Institute of Integrative Medicine, India; Heather Hain, Psychogenics, Inc., USA
Copyright
© Hua.
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Susan Hua, The School of Biomedical Sciences and Pharmacy, The University of Newcastle, Callaghan, New South Wales 2038, Australia e-mail: susan.hua@newcastle.edu.au
This article was submitted to Neuropharmacology, a section of the journal Frontiers in Pharmacology.
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