(119e) Effect of Material Type and Particle Density On the Margination of Vascular-Targeted Carriers

Effect of Material Type and Particle Density on the Margination of Vascular-Targeted Carriers

Alex Thompson¹
and Omolola Eniola-Adefeso¹

(1)
Chemical Engineering, University of Michigan, Ann Arbor, MI

Introduction:  
The development
of vascular-targeted carriers (VTC) for the localized delivery of therapeutics
could greatly improve the treatment of many human diseases.  The ability to deliver a high fraction of
an injected dose to a targeted diseased area is the paramount goal of vascular
targeting.  In order for VTCs to be
effective, they must be able to marginate (localize
and adhere) to diseased endothelium from blood flow. Recent research has shown
that particle size and shape affect particle margination
in the presence of red blood cells.^1,2,3 Neutrally
buoyant microparticles (relative to blood) with
equivalent spherical diameter (ESD) 
³ 2 µm undergo a forced localization to the red blood cell free layer
(RBC-FL); analogous to the mechanism by which red blood cells force leukocytes
and platelets to the RBC-FL, allowing them to effectively interact with the endothelium.  Smaller, neutrally buoyant spherical
particles (100-1000 nm ESD) do not benefit from this effect, therefore making
them suboptimal for use as VTCs, particularly in treating diseases that occur
in M/LBV, such as atherosclerosis. VTCs can be made from a variety of materials
which have densities different from that of blood. The purpose of this study is
to examine how the size and density of potential VTCs prescribe their ability
to localize the RBC-FL in vitro in human blood flow under physiological
conditions.

Materials and
Methods:  Fluorescent spherical particles of
different material with different densities were conjugated with targeting
molecules (sialyl Lewis a, sLe^a)
which bind to selectins at a fixed surface
density.  Targeted particles were
tested for their margination efficiency from blood
flow in vitro using parallel plate
flow chamber (PPFC) assays.  The particles explored ranged in
diameter from 100-1000 nm.

vascular-targeted spheres at a fixed
concentration were mixed with blood from human donors and allowed to flow over
a layer of IL-1β activated human umbilical vein endothelial cells (HUVEC) lining
one wall of a parallel plate flow chamber. Adhesion of VTCs from laminar,
steady blood flow from a range of wall shear rates (200-1000 s^-1)
was investigated.  Channel
orientation was varied in four different configurations in relation to gravity
to examine how particles of different densities marginate
from blood flow.  Particle adhesion in
recirculating blood flow, in which a sudden expansion in the PPFC causes a
region of recirculating flow to occur in the channel, was also examined to see
if a disturbed flow profile helps particles of different densities from blood
separate from the RBC core more efficiently.  The number of adherent particles under
various flow conditions was imaged/quantified using brightfield
and fluorescence microscopy. Also, the localization to the RBC-FL during blood
flow was visualized using confocal microscopy.

Results and
Discussion:   Results from in
vitro flow assays show that, as expected, the adhesion of neutrally buoyant
polystyrene (PS) microspheres was not affected by chamber orientation.  Silica (Si) microspheres with 1 µm
diameter display roughly double the adhesion of equivalent polystyrene (PS)
spheres when the chamber was oriented with the HUVEC monolayer at the bottom;
however, the same Si microspheres adhere at roughly the same level as PS
microspheres when the chamber is inverted so that the monolayer is at the top
of the chamber. When the target site is in the downward direction, gravity
favors the more dense Si particle in terms of adhesion; however when the target
site is upward, perhaps the presence of the RBC core below allows similar
levels of Si microparticles to remain in the RBC-FL
to interact with the endothelium. In recirculation flow, our results show that
at lower shear rates (200 s^-1) Si microparticles
adhere at higher levels than PS microparticles
whether the chamber is oriented with the monolayer at the top or the
bottom.  It appears that the flow
normal to the monolayer caused by the recirculation region helps Si
redistribute to the RBC-FL in this disturbed flow, resulting in an improved
adhesion in this area.  This finding
could have implication in targeting atherosclerosis, which is known to occur in
areas of the vasculature where this type of disturbed flow is found.  Ongoing work is investigating the margination smaller (20-200 nm) and more dense (FeO, Au, etc) particles with the
goal of identifying particle characteristics which would make them suitable for
targeting specific areas of the vasculature. 

Conclusions:   Our study shows that particle density can affect the margination of vascular-targeted carriers, which can be
particularly useful in designing carriers which target diseases, such as
atherosclerosis, which occur in areas of the vasculature that exhibit unique
hemodynamics.