Challenging RBC Hitchhiking as a Generic Concept for Targeted Delivery:—Towards an Understanding of the Bionano-Interface

Non-covalent surface binding of nanocarriers on red blood cells (RBCs), commonly referred to as RBC hitchhiking, has shown great potential in improving targeting and reducing rapid clearance of nanoparticles (NPs). RBC hitchhiking is particularly of interest for improved targeting towards the lungs, aided by a high capillary shear stress and a high local blood flux. In the case of pulmonary inflammatory diseases, local acidosis catalyzes the breakdown of, especially polymeric, NPs in the region of interest.<br/>However, given the non-covalence of NP attachment, variations in RBC characteristics, particle characteristics and the conditions of the immediate surroundings of the complex, might influence the nature and magnitude of interactions between the NPs and the RBCs. Gaining insights into the parameters in play, and their respective effects, is crucial for demarcating the versatility of RBCs as a delivery platform and assuring possible clinical translation of RBC-nanoformulations.<br/>Therefore, we conducted extensive research on the effects of different polymeric NP and RBC surface parameters, including RBC interspecies differences, NP surface hydrophobicity, NP surface charge, NP surfactant and drug encapsulation, on the success and biocompatibility of NP adsorption. Techniques used were, among others, nanoprecipitation, dynamic light scattering, nanoparticle tracking analysis, HPLC, scanning electron microscopy, agglutination, fluorescence analysis and image-based flow cytometry.<br/>Our results showed that major differences in RBC characteristics between different species have considerable effects on NP adsorption. Variations in NP hydrophobicity, zetapotential and surfactant were shown to impact the overall interaction balance (consisting of hydrophobic interactions, electrostatic forces, hydrogen bonding and Van der Waals forces) in such a way that some NP types, for example, strongly positive NPs, lead to biocompatibility issues, which manifests differently depending on RBC origin. Additionally, despite the successful adsorption of different drug encapsulated NPs (e.g. paclitaxel), the substantial interference of drug molecules in adsorption outcomes entails the need for meticulous characterization of each drug-NP-RBC complex.<br/>Our research provides a more thorough insight into the parameters in play for NP adsorption and will aid researchers in experimental design choices for the use of RBC hitchhiking as a delivery strategy for nanotherapeutics. Additionally, the limits of RBC hitchhiking, with the current understanding, are highlighted, calling for more fundamental research on the nanoparticle-RBC interface to fully unlock the potential of RBCs as delivery vehicle of nanocarriers.