Collaborative Enhancement of Endothelial Targeting of Nanocarriers by Modulating Platelet-Endothelial Cell Adhesion Molecule-1/CD31 Epitope Engagement

Nanocarriers (NCs) coated with antibodies (Abs) to extracellular epitopes of the transmembrane glycoprotein PECAM (platelet endothelial cell adhesion molecule-1/CD31) enable targeted drug delivery to vascular endothelial cells. Recent studies revealed that paired Abs directed to adjacent, yet distinct epitopes of PECAM stimulate each other's binding to endothelial cells in vitro and in vivo ("collaborative enhancement"). This phenomenon improves targeting of therapeutic fusion proteins, yet its potential role in targeting multivalent NCs has not been addressed. Herein, we studied the effects of Ab-mediated collaborative enhancement on multivalent NC spheres coated with PECAM Abs (Ab/NC, ∼180 nm diameter). We found that PECAM Abs do mutually enhance endothelial cell binding of Ab/NC coated by paired, but not "self" Ab. In vitro, collaborative enhancement of endothelial binding of Ab/NC by paired Abs is modulated by Ab/NC avidity, epitope selection, and flow. Cell fixation, but not blocking of endocytosis, obliterated collaborative enhancement of Ab/NC binding, indicating that the effect is mediated by molecular reorganization of PECAM molecules in the endothelial plasmalemma. The collaborative enhancement of Ab/NC binding was affirmed in vivo. Intravascular injection of paired Abs enhanced targeting of Ab/NC to pulmonary vasculature in mice by an order of magnitude. This stimulatory effect greatly exceeded enhancement of Ab targeting by paired Abs, indicating that '"collaborative enhancement"' effect is even more pronounced for relatively large multivalent carriers versus free Abs, likely due to more profound consequences of positive alteration of epitope accessibility. This phenomenon provides a potential paradigm for optimizing the endothelial-targeted nanocarrier delivery of therapeutic agents.

Vascular targeting of nanocarriers: perplexing aspects of the seemingly straightforward paradigm

Targeted nanomedicine holds promise to find clinical use in many medical areas. Endothelial cells that line the luminal surface of blood vessels represent a key target for treatment of inflammation, ischemia, thrombosis, stroke, and other neurological, cardiovascular, pulmonary, and oncological conditions. In other cases, the endothelium is a barrier for tissue penetration or a victim of adverse effects. Several endothelial surface markers including peptidases (e.g., ACE, APP, and APN) and adhesion molecules (e.g., ICAM-1 and PECAM) have been identified as key targets. Binding of nanocarriers to these molecules enables drug targeting and subsequent penetration into or across the endothelium, offering therapeutic effects that are unattainable by their nontargeted counterparts. We analyze diverse aspects of endothelial nanomedicine including (i) circulation and targeting of carriers with diverse geometries, (ii) multivalent interactions of carrier with endothelium, (iii) anchoring to multiple determinants, (iv) accessibility of binding sites and cellular response to their engagement, (v) role of cell phenotype and microenvironment in targeting, (vi) optimization of targeting by lowering carrier avidity, (vii) endocytosis of multivalent carriers via molecules not implicated in internalization of their ligands, and (viii) modulation of cellular uptake and trafficking by selection of specific epitopes on the target determinant, carrier geometry, and hydrodynamic factors. Refinement of these aspects and improving our understanding of vascular biology and pathology is likely to enable the clinical translation of vascular endothelial targeting of nanocarriers.

Delivering nanoparticles to lungs while avoiding liver and spleen through adsorption on red blood cells

Nanoparticulate drug delivery systems are one of the most widely investigated approaches for developing novel therapies for a variety of diseases. However, rapid clearance and poor targeting limit their clinical utility. Here, we describe an approach to harness the flexibility, circulation, and vascular mobility of red blood cells (RBCs) to simultaneously overcome these limitations (cellular hitchhiking). A noncovalent attachment of nanoparticles to RBCs simultaneously increases their level in blood over a 24 h period and allows transient accumulation in the lungs, while reducing their uptake by liver and spleen. RBC-adsorbed nanoparticles exhibited ∼3-fold increase in blood persistence and ∼7-fold higher accumulation in lungs. RBC-adsorbed nanoparticles improved lung/liver and lung/spleen nanoparticle accumulation by over 15-fold and 10-fold, respectively. Accumulation in lungs is attributed to mechanical transfer of particles from the RBC surface to lung endothelium. Independent tracing of both nanoparticles and RBCs in vivo confirmed that RBCs themselves do not accumulate in lungs. Attachment of anti-ICAM-1 antibody to the exposed surface of NPs that were attached to RBCs led to further increase in lung targeting and retention over 24 h. Cellular hitchhiking onto RBCs provides a new platform for improving the blood pharmacokinetics and vascular delivery of nanoparticles while simultaneously avoiding uptake by liver and spleen, thus opening the door for new applications.

NOX2 in lung inflammation: quantum dot based in situ imaging of NOX2-mediated expression of vascular cell adhesion molecule-1

Quantum dot (QD) imaging is a powerful tool for studying signaling pathways as they occur. Here we employ this tool to study adhesion molecule expression with lung inflammation in vivo. A key event in pulmonary inflammation is the regulation of vascular endothelial cell adhesion molecule-1 (VCAM), which drives activated immune cell adherence. The induction of VCAM expression is known to be associated with reactive oxygen species (ROS) production, but the exact mechanism or the cellular source of ROS that regulates VCAM in inflamed lungs is not known. NADPH oxidase 2 (NOX2) has been reported to be a major source of ROS with pulmonary inflammation. NOX2 is expressed by both endothelial and immune cells. Here we use VCAM-targeted QDs in a mouse model to show that NOX2, specifically endothelial NOX2, induces VCAM expression with lung inflammation in vivo.

Vascular immunotargeting to endothelial determinant ICAM-1 enables optimal partnering of recombinant scFv-thrombomodulin fusion with endogenous cofactor

The use of targeted therapeutics to replenish pathologically deficient proteins on the luminal endothelial membrane has the potential to revolutionize emergency and cardiovascular medicine. Untargeted recombinant proteins, like activated protein C (APC) and thrombomodulin (TM), have demonstrated beneficial effects in acute vascular disorders, but have failed to have a major impact on clinical care. We recently reported that TM fused with an scFv antibody fragment to platelet endothelial cell adhesion molecule-1 (PECAM-1) exerts therapeutic effects superior to untargeted TM. PECAM-1 is localized to cell-cell junctions, however, whereas the endothelial protein C receptor (EPCR), the key co-factor of TM/APC, is exposed in the apical membrane. Here we tested whether anchoring TM to the intercellular adhesion molecule (ICAM-1) favors scFv/TM collaboration with EPCR. Indeed: i) endothelial targeting scFv/TM to ICAM-1 provides ~15-fold greater activation of protein C than its PECAM-targeted counterpart; ii) blocking EPCR reduces protein C activation by scFv/TM anchored to endothelial ICAM-1, but not PECAM-1; and iii) anti-ICAM scFv/TM fusion provides more profound anti-inflammatory effects than anti-PECAM scFv/TM in a mouse model of acute lung injury. These findings, obtained using new translational constructs, emphasize the importance of targeting protein therapeutics to the proper surface determinant, in order to optimize their microenvironment and beneficial effects.

Reduction of nanoparticle avidity enhances the selectivity of vascular targeting and PET detection of pulmonary inflammation

Targeting nanoparticles (NPs) loaded with drugs and probes to precise locations in the body may improve the treatment and detection of many diseases. Generally, to achieve targeting, affinity ligands are introduced on the surface of NPs that can bind to molecules present on the cell of interest. Optimization of ligand density is a critical parameter in controlling NP binding to target cells, and a higher ligand density is not always the most effective. In this study, we investigated how NP avidity affects targeting to the pulmonary vasculature, using NPs targeted to ICAM-1. This cell adhesion molecule is expressed by quiescent endothelium at modest levels and is upregulated in a variety of pathological settings. NP avidity was controlled by ligand density, with the expected result that higher avidity NPs demonstrated greater pulmonary uptake than lower avidity NPs in both naive and pathological mice. However, in comparison with high-avidity NPs, low-avidity NPs exhibited several-fold higher selectivity of targeting to pathological endothelium. This finding was translated into a PET imaging platform that was more effective in detecting pulmonary vascular inflammation using low-avidity NPs. Furthermore, computational modeling revealed that elevated expression of ICAM-1 on the endothelium is critical for multivalent anchoring of NPs with low avidity, while high-avidity NPs anchor effectively to both quiescent and activated endothelium. These results provide a paradigm that can be used to optimize NP targeting by manipulating ligand density and may find biomedical utility for increasing detection of pathological vasculature.

Acute and chronic shear stress differently regulate endothelial internalization of nanocarriers targeted to platelet-endothelial cell adhesion molecule-1

Intracellular delivery of nanocarriers (NC) is controlled by their design and target cell phenotype, microenvironment, and functional status. Endothelial cells (EC) lining the vascular lumen represent an important target for drug delivery. Endothelium in vivo is constantly or intermittently (as, for example, during ischemia-reperfusion) exposed to blood flow, which influences NC-EC interactions by changing NC transport properties, and by direct mechanical effects upon EC mechanisms involved in NC binding and uptake. EC do not internalize antibodies to marker glycoprotein PECAM(CD31), yet internalize multivalent NC coated with PECAM antibodies (anti-PECAM/NC) via a noncanonical endocytic pathway distantly related to macropinocytosis. Here we studied the effects of flow on EC uptake of anti-PECAM/NC spheres (~180 nm diameter). EC adaptation to chronic flow, manifested by cellular alignment with flow direction and formation of actin stress fibers, inhibited anti-PECAM/NC endocytosis consistent with lower rates of anti-PECAM/NC endocytosis in vivo in arterial compared to capillary vessels. Acute induction of actin stress fibers by thrombin also inhibited anti-PECAM/NC endocytosis, demonstrating that formation of actin stress fibers impedes EC endocytic machinery. In contrast, acute flow without stress fiber formation, stimulated anti-PECAM/NC endocytosis. Anti-PECAM/NC endocytosis did not correlate with the number of cell-bound particles under flow or static conditions. PECAM cytosolic tail deletion and disruption of cholesterol-rich plasmalemma domains abrogated anti-PECAM/NC endocytosis stimulation by acute flow, suggesting complex regulation of a flow-sensitive endocytic pathway in EC. The studies demonstrate the importance of the local flow microenvironment for NC uptake by the endothelium and suggest that cell culture models of nanoparticle uptake should reflect the microenvironment and phenotype of the target cells.

Targeted Nanocarriers for Imaging and Therapy of Vascular Inflammation

Vascular inflammation is a common, complex mechanism involved in pathogenesis of a plethora of disease conditions including ischemia-reperfusion, atherosclerosis, restenosis and stroke. Specific targeting of imaging probes and drugs to endothelial cells in inflammation sites holds promise to improve management of these conditions. Nanocarriers of diverse compositions and geometries, targeted with ligands to endothelial adhesion molecules exposed in inflammation foci are devised for this goal. Imaging modalities that employ these nanoparticle probes include radioisotope imaging, MRI and ultrasound that are translatable from animal to human studies, as well as optical imaging modalities that at the present time are more confined to animal studies. Therapeutic cargoes for these drug delivery systems include diverse anti-inflammatory agents, anti-proliferative drugs for prevention of restenosis, and antioxidants. This article reviews recent advances in the area of image-guided translation of targeted nanocarrier diagnostics and therapeutics in nanomedicine.

A Biocompatible Arginine-based Polycation

Self assembly between cations and anions is ubiquitous throughout nature. Important biological structures such as chromatin often use polyvalent assembly between a polycation and a polyaninon. Biomedical importance of synthetic polycations arises from their affinity to polyanions such as nucleic acid and heparan sulfate. However, the limited biocompatibility of synthetic polycations hampers the realization of their immense potential. By examining biocompatible cationic peptides, we hypothesize that a biocompatible polycation should be biodegradable and made from endogenous cations. We designed an arginine-based biodegradable polycation and demonstrated that it was orders of magnitude more compatible than conventional polycations in vitro and in vivo. This biocompatibility diminishes when L-arginine is substituted with D-arginine or when the biodegradable ester linker changes to a biostable ether linker. We believe this design can lead to many biocompatible polycations that can significantly advance a wide range of applications including controlled release, tissue engineering, biosensing, and medical devices.

Control growth factor release using a self-assembled [polycation:heparin] complex

The importance of growth factors has been recognized for over five decades; however their utilization in medicine has yet to be fully realized. This is because free growth factors have short half-lives in plasma, making direct injection inefficient. Many growth factors are anchored and protected by sulfated glycosaminoglycans in the body. We set out to explore the use of heparin, a well-characterized sulfated glycosaminoglycan, for the controlled release of fibroblast growth factor-2 (FGF-2). Heparin binds a multitude of growth factors and maintains their bioactivity for an extended period of time. We used a biocompatible polycation to precipitate out the [heparin∶FGF-2] complex from neutral buffer to form a release matrix. We can control the release rate of FGF-2 from the resultant matrix by altering the molecular weight of the polycation. The FGF-2 released from the delivery complex maintained its bioactivity and initiated cellular responses that were at least as potent as fresh bolus FGF-2 and fresh heparin stabilized FGF-2. This new delivery platform is not limited to FGF-2 but applicable to the large family of heparin-binding growth factors.