Targeting drug delivery in the vascular system: Focus on endothelium

Abstract

The bloodstream is the main transporting pathway for drug delivery systems (DDS) from the site of administration to the intended site of action. In many cases, components of the vascular system represent therapeutic targets. Endothelial cells, which line the luminal surface of the vasculature, play a tripartite role of the key target, barrier, or victim of nanomedicines in the bloodstream. Circulating DDS may accumulate in the vascular areas of interest and in off-target areas via mechanisms bypassing specific molecular recognition, but using ligands of specific vascular determinant molecules enables a degree of precision, efficacy, and specificity of delivery unattainable by non-affinity DDS. Three decades of research efforts have focused on specific vascular targeting, which have yielded a multitude of DDS, many of which are currently undergoing a translational phase of development for biomedical applications, including interventions in the cardiovascular, pulmonary, and central nervous systems, regulation of endothelial functions, host defense, and permeation of vascular barriers. We discuss the design of endothelial-targeted nanocarriers, factors underlying their interactions with cells and tissues, and describe examples of their investigational use in models of acute vascular inflammation with an eye on translational challenges.

Keywords: Endothelium; Inflammation; Nanocarriers; Vascular system.

Graphical abstract

Fig. 1

The endothelium provides an extremely large surface area for drug delivery.

Fig. 2

Inflammation affects surface expression and cellular localization of endothelial targets. Upper panel: Receptor expression on quiescent endothelium. Certain proteins are localized to specific cell surface microdomains, such as tetraspanin-rich (ICAM-1), cell-cell junctions (PECAM-1, VE-Cadherin), and caveolae (APP2, PLVAP), while others may be found throughout the cell surface (ACE, thrombomodulin, TfR). Lower panel: Receptor expression, localization, and accessibility change upon endothelial activation. ICAM-1 and VCAM-1 expression in the tetraspanin domains is increased, largely via de novo synthesis. Accessibility to endothelial junction proteins PECAM-1 and VE-Cadherin may change due to reduced cell-cell interactions. P-selectin is upregulated on the membrane through mobilization of intracellular stores found in Weibel-Palade Bodies. Other proteins such as ACE and thrombomodulin are lost from the cell surface through a shedding mechanism.

Fig. 3

Targeting to VCAM-1 enables selective delivery to the inflamed cerebral vasculature. Left panel: Absolute uptake of anti-VCAM-1 mAb in the brain of mice injured via an intrastriatal injection of TNF-α exceeds that of the ‘gold standard’ for brain delivery, Transferrin Receptor (TfR), by an order of magnitude. Middle panel: Flow cytometry on brain homogenates reveals that over 50% of endothelial cells are positive for VCAM-targeted agents (either mAb or liposome) following IV injection. Following IV injection of fluorescently labeled mAbs and liposomes under the same conditions as in the left panel, brains were disaggregated and stained to determine mAb/liposome association with leukocytes and endothelial cells. Inset: Typical dot plot showing cell types identified via this approach. Right panel: SPECT imaging of VCAM-targeted liposomes labeled with ¹¹¹In demonstrates selective uptake in the injured hemisphere of the brain. Units in the scale bar are presented as arbitrary intensity units. Figures adapted from [87]. Colors are consistent across panels. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 4

Carrier flexibility modulates engagement of targeted carriers with caveolar plasmalemma vesicle associated protein (PLVAP) in lung endothelium. Carrier mechanical properties can affect how well carriers access targets (e.g. by soft particles “squeezing” into small spaces, as depicted in (a)). PLVAP is located in a sterically concealed position, the caveolar neck, in lung endothelial cells. Soft dextran particles (a) were able to target PLVAP more effectively than crosslinked dextran particles (b) or rigid polystyrene particles (c) of similar size. As evaluated in radioisotope tracing (d) and fluorescence (e) studies, PLVAP targeting efficacy in the lungs decreased as nanoparticle Young's modulus (as determined by AFM) increased (d). Adapted from [131,228].

Fig. 5

Utility of the first-pass effect in tissue targeting. A: Intravenous (IV) infusion of ICAM-targeted nanocarriers (NC) results in a large portion of the dose being deposited in the lungs. On the other hand, infusion into the internal carotid artery bypasses the lung first pass and permits delivery to the brain. B: Ex vivo adsorption of untargeted NC onto RBC, termed RBC hitchhiking (RBC–H), allows transfer of NC to endothelium in the first capillary downstream of the injection site. This technology has been shown to increase delivery to several tissues by varying the injection site. Additional abbreviations used in figure: ICA (internal carotid artery), CCA (common carotid artery). Figures adapted from [231,232].

Fig. 6

Endothelial receptors for conjugate targeting in acute vascular pathology. Shown is a small vessel or capillary in cross-section, lined by endothelial cells (blue). Receptors (orange) are bound by drug conjugate (green circle) plus targeting moiety/ nanocarrier (green rectangle). Many receptors including PECAM and ICAM enable both intravascular and extravascular drug targeting. Factors that promote intracellular uptake (with retention of drug conjugate and recycling of receptor) include multivalent targeting moieties and low-flow state. Factors that modify uptake include ligand density, carrier geometry, and size. Ischemia/reperfusion injury (A) and acute vascular inflammation (B) both result in increased leukocyte recruitment and leaky tight junctions between endothelial cells. Treatment requires drug to be delivered intracellularly to the endothelium and to the interstitial space. Drug targeting to the specific receptors shown has resulted in treatment effect in animal models of these pathologies. (C) Shown is a blood clot within the lumen. Thrombosis and embolism both require the drug to remain in the vessel lumen to achieve direct therapeutic effect. Drug targeting to the receptors shown has resulted in treatment effect in animal models of thrombosis. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 7

Role of ROS in cytokine- and TLR-induced signaling and potential use of intracellular antioxidant delivery for anti-inflammatory protection. Cytokine or PAMP binds to counterpart receptor, causing NOX activation and the complex internalization forming redox-active endosome. Generated ROS transfers into cytosol via CLC3 channel and stimulates proinflammatory NFκB cascade. Delivery of antioxidant to the endosome or overexpression of cytosolic SOD can reduce the inflammatory signaling and attenuate cellular injury [307,308]. Inset shows SOD-loaded nanoparticles (green) enter endosomes (red), SOD-containing endosomes seen as yellow. Cell nuclei are shown blue. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)