(53f) Implication of Particle-Cell Dynamics In Physiological Blood Flow On the Efficacy of Vascular-Targeted Drug Carriers

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Introduction: Nanospheres are commonly used as drug carriers for intravenous delivery due to their ability to pass through microcirculation without occlusion and their ability to avoid eliciting immune response. In addition to these abilities, ideal targeted drug carriers must also effectively adhere at the targeted location to release the therapeutic agents. Recent studies focused on elucidating the vascular wall binding efficiency of targeted spheres with sizes ranging from nano- to micrometer in reconstituted human blood (platelet and leukocyte depleted blood) demonstrated that particle binding increases with increasing particle size, i.e. nanospheres exhibit minimal adhesion to inflamed endothelium from blood flow relative to microspheres.  Since many vascular wall target molecules (e.g. selectin and ICAM) are also expressed on other cells of the blood, it becomes important to understand the possible physical/molecular interactions that exist between blood cells and vascular-targeted drug carriers. Thus, this study aims to elucidate the influence of leukocytes (WBCs) and platelets on the margination and adhesive dynamics of vascular-targeted drug carriers under physiological blood flows as a function of particle size and targeting moiety.

Materials and Methods: Dynamics of 0.2, 0.5, 2 and 5 mm spherical particles in blood were assessed in a parallel plate flow chamber equipped with a straight or step channel. Human Umbilical Vein Endothelial Cells (HUVECs) activated by IL1-b were employed as inflamed endothelial cells. These cells were attached at the bottom of the chamber and used in flow adhesion assays. Spheres were either targeted to endothelial selectins via surface-linked sLe^A or ICAM-1 via anti-ICAM-1 antibody (surface density for each targeting ligand was fixed for all particle sizes). Particle adhesion was observed in human reconstituted blood (plasma and RBCs, at ~ 40% Hct), whole blood, platelet-depleted blood and leukocyte-depleted blood flow with steady, pulsing or recirculating profile.

Results and Discussion: The level of particle binding in whole blood flow is a function of particle size and type of blood flow. Specifically, in laminar and pulsatile whole blood flow, 2 mm spheres showed the highest binding over other sizes whereas in steady recirculating whole blood flow, 5 mm had maximum binding. Overall, adhesion of nanospheres (0.2 and 0.5 mm) was minimal in all flow conditions compared to microspheres. We find that the higher adhesion observed for 2 mm spheres over 5 mm ones in laminar and pulsatile whole blood flow was due to their physical and molecular interactions with blood cells in whole blood that served to improve their recruitment to the wall. However, the presence of leukocytes in blood appears to negatively affected the adhesion of 5 mm spheres in laminar and pulsatile flow via: (1) competition for the available binding space and (2) physical interaction with the bound particles that removes them from the adhesive state. We propose that the higher adhesion of 5 mm spheres in recirculating flow is due to the rotational motion induced by the step channel that cause the depletion of white blood cells in the vortex area and thus eliminate the competition for binding between particles and white blood cells.

Conclusions: The binding efficiency of drug carriers in blood flow depends on the interplay between particle size and other factors such as the blood hematocrit, interaction with non-erythrocytes blood cells and type of flows.  In addition to elucidating the physical and molecular dynamics of spherical particles in blood flow, our results can serve as a road map for designing efficient drug carrier systems for targeted delivery applications in many human diseases.