Targeting therapeutics to endothelium: are we there yet?

Abstract

Vascular endothelial cells represent an important therapeutic target in many pathologies, including inflammation, oxidative stress, and thrombosis; however, delivery of drugs to this site is often limited by the lack of specific affinity of therapeutics for these cells. Selective delivery of both small molecule drugs and therapeutic proteins to the endothelium has been achieved through the use of targeting ligands, such as monoclonal antibodies, directed against endothelial cell surface markers, particularly cell adhesion molecules (CAMs). Careful selection of target molecules and targeting agents allows for precise delivery to sites of inflammation, thereby maximizing therapeutic drug concentrations at the site of injury. A good understanding of the physiological and pathological determinants of drug and drug carrier pharmacokinetics and biodistribution may allow for a priori identification of optimal properties of drug carrier and targeting agent. Targeted delivery of therapeutics such as antioxidants and antithrombotic agents to the injured endothelium has shown efficacy in preclinical models, suggesting the potential for translation into clinical practice. As with all therapeutics, demonstration of both efficacy and safety are required for successful clinical implementation, which must be considered not only for the individual components (drug, targeting agent, etc.) but also for the sum of the parts (e.g., the drug delivery system), as unexpected toxicities may arise with complex delivery systems. While the use of endothelial targeting has not been translated into the clinic to date, the preclinical results summarized here suggest that there is hope for successful implementation of these agents in the years to come.

Keywords: Antioxidants; Antithrombotic drugs; Drug delivery; Endothelial targeting; Vascular immunotargeting.

Figure 1

Endothelial determinants for targeted drug delivery. (A) Pan-endothelial molecules clustered in apical plasmalemma, (B) located in the caveolae; (C) Cell adhesion molecules (CAMs); (D) Molecules expressed during angiogenesis or tumor-related; (E) Molecules concentrated in cellular junctions.

Figure 2

Mathematical modeling of the impact of local injury on biotherapeutic pharmacokinetics. (A) General structure of a semi-physiologic model for nanocarrier disposition, (B) Lung tissue model for nanocarrier disposition, separating the tissue into injured and healthy regions, and (C) pathological processes involved in altering nanocarrier pharmacokinetics and biodistribution.

Figure 3

Reactive oxygen species (ROS) in vascular cell signaling pathways and oxidative stress. Superoxide is produced by several cellular enzyme systems including respiratory chain, NADPH-oxidases, xanthine oxidase (XO), cyclooxygenase (COX), etc. It can react with NO decreasing functional NO pool and producing highly reactive peroxynitrate anion ONOO− and. Superoxide spontaneously or by action of superoxide dismutase (SOD) may be reduced into hydrogen peroxide H2O2. Hydrogen peroxide can produce highly reactive hydrogen radical •OH in the presence of transition metals or hypochlorous acid by myeloperoxidase. Catalase and glutathione peroxidases protect cells against hydrogen peroxide. ARDS, acute respiratory distress syndrome; GSHPx, glutathione peroxidases; MPO, myeloperoxidase.

Figure 4

Endothelial targeting to boost endogenous antithrombotic and anti-inflammatory pathways. The endothelial protein C (PC) pathway, which centers on the membrane glycoproteins, thrombomodulin (TM) and endothelial protein C receptor (EPCR), is a critical regulator of inflammation, thrombosis, and vascular permeability. Infusion of soluble biotherapeutics, like soluble human TM (shTM) and recombinant human APC (rhAPC), suffers from poor PK and lack of interaction with key components of the endogenous system. Anchoring recombinant TM and/or EPCR to endothelial cell adhesion molecules, like PECAM-1 and ICAM-1, localizes therapeutic action to the surface membrane and may exert superior effects through restoration of natural anticoagulant effects and signaling through protease-activated receptors (PARs). For these purposes, scFv fusion protein biotherapeutics may be preferable to large monoclonal antibody (mAb)-protein complexes, which can crosslink CAMs and induce endosomal uptake and endothelial disruption.

Figure 5

Proposed model of CEPAL to PECAM-1. Proposed hypothetical model of CEPAL to PECAM-1. (A and B) Targeting of drug conjugates with antibody mAb1 (a) or mAb2 (B). Collaborative enhancement of antibody targeting observed in CEPAL. (D) Examples of in vivo endothelial targeting: Lung to blood ratios in biodistribution of anti-muPECAM-1 [125I]-mAbs 390 and MEC13.3 [1] (left), coadministration of Mec13 scFv/EPCR with 390 scFv/TM increases pulmonary targeting of 390 scFv/TM [12] (right).