Challenges in design and characterization of ligand-targeted drug delivery systems
Graphical abstract
Numerous factors contribute to the relative efficacy and potential for clinical translation of drug targeting approaches, including complex physiological factors and design parameters relative to the target itself, the targeting moiety or ligand, and the final drug–ligand formulation.
Introduction
A great variety of complex and intertwined factors governs the efficacy of drugs aimed to provide prophylaxis or, most commonly, therapeutic alleviation of clinical conditions. This includes intrinsic drug features, such as the specificity of action of the drug itself and properties such as its molecular size and chemical composition, determining its overall solubility in body fluids, penetration within tissues, and uptake by cells (reviewed by [1], [2]). In addition to this, the therapeutic outcome largely depends on various processes related to the handling of the drug by the body, which determine drug circulation half-life, as well as drug clearance by certain organs, potential metabolic transformation, degradation, and disposal [1], [2]. New and refined means of drug discovery and development continue to render agents with novel and more powerful therapeutic properties, In addition, the design of systems capable of more efficiently and safely carrying and delivering drugs to areas of the body requiring intervention offers the opportunity to improve not only the overall therapeutic outcome of the new drugs being discovered, but also that of agents that, despite been clinically approved, display otherwise sub-optimal effects. Targeting of drugs with desirable properties to precise sites of the body affected by disease holds, therefore, great promise with regard to achieving enhanced therapeutic results while reducing side effects. This provides a means to lower the administered dose and, in turn, the cost of clinical treatments. As such, drug targeting is a very active area of research and development.
The concept of drug targeting was first proposed early in the twentieth century by Paul Ehrlich, who coined the term “magic bullet”, referring to a theoretical entity composed of a therapeutic agent linked to a component capable of recognizing a disease target, hence, providing precise transport of the drug (reviewed by [3]) . This notion became a plausible option after achieving production of monoclonal antibodies in the early 1970's [4], supporting high affinity and specificity of binding toward particular antigens of interest. Since then, most prominently during the last two decades, the seminal idea of drug targeting has greatly evolved to encompass a great variety of strategies that range from relatively simple concepts, similar to those devised by Ehrlich, to complex approaches involving design of new materials with highly controlled properties (reviewed by [5], [6]). These strategies are often classified within four major categories, including passive, inverse, active, and combined targeting (Fig. 1).
Passive targeting refers to the accumulation of drugs into particular regions of the body, due to the natural features and physiological role of said tissues. This is the case for drug accumulation in organs of the reticulo-endothelial system (RES), mainly the liver and spleen, which “capture” foreign substances and objects that reach the systemic circulation, as well as the monocyte/macrophage system at the cellular level, given the specialization of these cells in taking into their bodies said foreign materials for disposal (reviewed by [7], [8]). Drugs aimed to cope with pathological conditions in these tissues accumulate at these sites passively. Another common example of passive targeting is illustrated by the enhanced permeability and retention (EPR) effect associated to the tumors' vasculature (first described by [9]), where the loose junctions between endothelial cells that line the tumor blood vessels allow passive leakage of circulating drugs into the tumor parenchyma. Since no targeting properties are “externally” imposed on the drug itself, passive targeting is often referred to as no targeting.
Inverse targeting is related to the previous strategy in that the goal of this approach is to block sites in the body associated to passive targeting, to allow a drug to better accumulate in other areas. This has been attained by administering certain sugar polymers or lipid microemulsions that saturate the RES prior to administering a therapeutic compound of interest, shifting its biodistribution pattern [10].
The most recognized form of targeting is that where targeting properties are imposed on the drug, which is known as active targeting. This approach comprises strategies aimed to design drugs that exert very specific activities toward precise molecules in the body, whose structure/function needs to be therapeutically modified. Hence, this targeting is an intrinsic signature of the drug itself. Most other active targeting approaches, though, refer to strategies where the targeting property is extrinsically imposed on the drug, e.g., by coupling to other component possessing targeting features (reviewed by [6]). In this case, a drug can be coupled to a component that does not display affinity and binding toward a particular target, but allows for programmed release of the drug under particular environmental signatures of the disease site, such as temperature, pH, etc., which is known as physical targeting (reviewed by [5], [11], [12]. In other cases, active targeting is achieved by coupling the drug to a component (called ligand) that displays affinity and therefore binds to a particular element present at the disease site (reviewed by [13]). Furthermore, ligand-based targeting can be achieved by either coupling the drug to the targeting ligand directly, e.g., biologically or by chemical conjugation, or indirectly through a carrier that encapsulates the drug and displays the targeting ligand (reviewed by [14], [15], [16], [17]). Such strategies of ligand-based targeting are the most commonly referred forms of active targeting, and many authors use the term “active targeting” to refer to this type of approach.
Furthermore, numerous approaches combine several elements of the drug targeting categories described above, such as the case of targeted delivery of cancer therapeutics, where relatively insoluble and toxic drugs can be encapsulated into pH-sensitive polymeric materials coupled to ligands that recognize cancer markers (reviewed by [6]). Such composites can favor the solubility and diminish the toxicity of chemotherapeutics, accumulate at tumor sites due to the EPR effect, bind to and enter cancer cells driven by the ligand, where the drug can be released due to the low pH of the cancer-cell environment or within intracellular lysosomal compartments. This illustrates the degree of flexibility and potential for fine-tune design of targeted drug delivery systems.
In addition, all of these strategies of drug targeting need to be considered under the light of the different levels of complexity and organization encountered in biological systems; that is, from the macro-scale view of organs and tissues, to the molecular and even atomic level of drug interaction with its precise targets (Fig. 2). A site affected by disease is typically confined to a particular organ or tissue in the body (e.g., the brain versus the lung or the liver), likely associated to particular cell type within that organ or tissue (e.g., dopaminergic neurons versus other neurons or glial cells in the brain) and, moreover, located in certain sub-cellular compartment (e.g., the nucleus versus cytosol, the mitochondria versus the Golgi apparatus, etc.). While the drug must attain specific and effective action against the molecular determinant involved in a disease, drug targeting strategies must overcome challenges posed by a variety of barriers in the body in order provide safe and efficient transport of the therapeutic agent from the organ, to cell, to sub-cellular level.
Importantly, no drug targeting strategy serves as a universal platform (reviewed by [18], [19]). Different strategies are designed to overcome different barriers, and often the advantages offered by a particular strategy regarding a certain barrier represent disadvantages from the perspective of overcoming other obstacles. These problems have largely impacted the translational success of drug targeting systems, (reviewed by [6], [19]). Only a few protein-, liposomal-, and polymer-based examples have successfully reached the clinics, due to: (i) a major effort investment in design but lack of concomitant systematic studies, (ii) the inherent complexity of these systems, (iii) partial understanding of their properties in vivo, and (iv) still incomplete fundamental knowledge on key regulatory parameters of the physiological environment. For instance, by taking advantage of natural physiological features of the body, passive targeting requires fewer modifications of the drug formulation than in the case of active targeting. This simplifies design and production, therefore reducing cost and facilitating the translational process (reviewed by [6]). However, the potential efficiency of passive targeting is rather restricted, such as the case of the EPR effect for reaching tumors, narrowing down the field of applicability of these systems as compared to the broad Pharma interest. Even in the case of cancer therapeutics, taking advantage of the passive EPR effect requires relatively prolonged circulation of the therapeutic agent or its carrier, and this poses additional design complexity and production obstacles [19]. However, despite design of long circulating formulations, passive transport into the tumor area effect is often counteracted by the high hydrostatic tumor pressure and lymphatic drainage within the tumor parenchyma, resulting in sub-optimal drug accumulation at theis site [19]. Similar requirements and complications apply to the design of strategies taking advantage of other passive accumulation mechanisms, e.g., for drug delivery to RES organs or inflammatory conditions characterized by increased vascular permeability.
In turn, active drug targeting strategies, e.g., using ligand-targeted carriers, may provide more controllable pharmacokinetics and bioavailability features. However, these strategies are also pronged with numerous obstacles. For example, they require complex design and multiple components, making production difficult, not fully controllable, and rather costly. Conjugation of drugs to targeting moieties and, mainly, their loading in targeted carriers increases the size of these formulations as compared to free non-coupled therapeutics. As a consequence, these strategies often suffer from lower diffusion and penetration through tissues (e.g., the tumor parenchyma or that of other therapeutic sites), with formulations remaining entrapped in the proximity of blood vessels, unable to reach the target cells (reviewed by [18], [19]). Further, other obstacles arise from the inherent tissue heterogeneity, e.g., regarding composition, structure, and function, variability in the expression of the target, and numerous intracellular barriers (described in more detail in the following sections) [18], [19].
Altogether, these difficulties hinder practical translation of drug targeting strategies, requiring long and costly development, with only a minimal fraction of experimental technologies making their way to the marketplace. Understanding the advantages and disadvantages of these different drug targeting strategies and how they relate to the physiological system, is crucial to more logically design these therapeutic approaches while holding realistic expectations regarding their performance. This review will discuss some of the challenges affecting the design and characterization of drug targeting, particularly focusing on ligand-targeted drug delivery systems.
Section snippets
The target
As described above, ligand-targeted drug delivery requires coupling (either directly or through a carrier) of the drug of interest to an affinity moiety that recognizes a feature present in areas of the body affected by disease and (preferably) missing from healthy tissues, favoring drug delivery where needed (reviewed by [20]). Most typically, this is implemented by targeting certain molecular signatures associated to the diseased cells, such as a specific protein. Table 1 below shows only
The targeting moiety
Achieving binding to the selected target is accomplished by using elements that display affinity toward that target. A variety of molecules can be used for this purpose. These include proteins and peptides, which represent by far the most common affinity elements employed for ligand-based drug targeting. Antibodies are often used given their high affinity and specificity, particularly to establish proof-of-principle but also from a translational perspective, since they can be easily modified
The coupling strategy
Once the molecular target and amenable targeting moiety have been selected, coupling of the latter to the drug of interest can be achieved by a variety of strategies (Fig. 8), including direct coupling approaches such as co-synthesis (applicable to the case of therapeutic proteins that can be synthesized as fusion proteins) or conjugation of both elements after their independent synthesis or production (reviewed by [6], [87], [88]). Conjugation strategies encompass covalent conjugation via
Conclusion
Drug targeting holds great promise as a means to enhance the therapeutic outcome of clinical treatments by improving site-specificity and safety of drugs. Yet, after several decades of a considerable experimental effort and apart from a few success stories, the translational output of drug targeting systems does not seem to have met the expectations. Despite undeniable advances, the high number of variables impacting the design of drug delivery systems, lack of homogeneity and level of
Acknowledgments
The author thanks Dr. Muzykantov (University of Pennsylvania, Philadelphia, PA) and Dr. Garnacho (University of Seville, Seville, Spain) for their contribution to the research on anti-ICAM carriers reviewed in this article, Daniel Serrano (University of Maryland, College Park, MD) for careful reading of the manuscript, and funding from the National Institutes of Health (Grant R01-HL98416) and the American Heart Association (Grant 09BGIA2450014).
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