Delivery of nanoparticle: complexed drugs across the vascular endothelial barrier via caveolae

Abstract

The endothelial cell monolayer lining the vessel wall forms a size-selective, semi-permeable barrier between the blood and tissue that must be crossed by blood borne therapeutic agents to reach diseased extravascular tissue. Nanoparticles engineered to carry drugs present an opportunity to enhance the specificity and efficacy of drug delivery. Therefore, an understanding of how these engineered nanoparticles are transported across the vessel wall will help us to more fully exploit this powerful therapeutic technology. Vascular endothelial cells are rich in caveolae, cell surface invaginations 50-100 nm in diameter that mediate endocytosis of lipids, proteins, and viruses. Caveolar invaginations pinch off to form intracellular vesicles that can transport cargo across the cell and release the cargo into the extravascular space via exocytosis. Here, we will review the current concepts and state of development for delivering engineered nanoparticles across the endothelium via the caveolae-mediated pathway.

Figure 1

Two populations of caveolae in endothelial cells detected by a nanoruler using dual-color nanoparticle pairs. (A) The principle of a nanoruler using dual-color nanoparticle pairs. Left: Two nanoparticles (green dots) with similar fluorescence spectrum are separated at a distance “D” of less than half the emission wavelength. The use of separate detectors will measure the same fluorescent profile at the optical diffraction limit (large green spot in the image plane). Right: Two nanoparticles having different fluorescence spectrums (red and green). Red and green nanoparticles detected using appropriate filters will appear as two circular diffraction limited spots in the image (as indicated by red and green spots) and measuring the distance between the two centers gives the linear distance between particles. (B) Merged images of dual-color pairs of 40 nm BSA-coated nanoparticles in endothelial cells (green particle emission at 515 nm and red particle emission at 605 nm). The image of the nanoparticles was acquired at 488 nm for green and 543 nm for red. Left: Showing co-localization of red and green nanoparticles (red line on the image shows the cross-section of the image representing the image size profile on the right); Right: image size profile of red and green nanoparticles showing the separation between two nanoparticles obtained by measuring the distance between centers of diffraction images of red and green nanoparticles; for example, in this case, the separation is 80 nm, i.e., less than the diffraction limit. (C) Diagram shows caveolae and aggregates of caveolae sharing dual-color albumin-conjugated nanoparticles. Using different sizes of albumin-conjugated nanoparticles allows the measurement of the size of caveolae and of caveolae aggregates.

Figure 2

Transcytosis of gold-labeled albumin in perfused lung microvessels. Vesicles 1, 2 and 3 (V1, V2 and V3) represent caveolae-mediated internalization of gold nanoparticles. V3 additionally shows gp60 receptor-bound gold nanoparticles from binding at the cell surface. The release of gold nanoparticles from vesicles by exocytosis at the abluminal surface is indicated by V4 and V5. (Figure from reference #1).

Figure 3

Caveolae-mediated pathways of nanoparticle transport. The red arrows represent caveolae-mediated transport of nanoparticles across endothelial cells (transcytosis), and the blue arrows respresent the interactions between nanoparticle-loaded caveolae and intracellular compartments, such as the endosomal and lysosomal compartments.

Figure 4

Multifunctional nanoparticles for targeting using caveolar pathway. The multifunctional engineered nanoparticle has the capability of simultaneously carrying therapeutic agents, targeting molecules and imaging agents. Therapeutic agents include siRNA, proteins and small drug molecules, and targeting molecules (specific antibodies and recognition peptides) while imaging agents include fluorescent probes and magnetic contrast agents.