Molecular Dynamics Simulations of cRGD-Conjugated PEGylated TiO2 Nanoparticles for Targeted Photodynamic Therapy

Active targeting strategies, exploiting the biological interaction between ligands on the surface of nanoparticles and the cell targets, are known to increase the therapeutic efficacy of cancer treatments with respect to passive targeting strategies that are mainly based on the enhanced permeability and retention effect of tumor tissues. In this respect, the conjugation of nanoparticles with integrin αVβ3-affine cyclic RGD peptides (cRGD) is a promising approach in nanomedicine to efficiently reduce off-targeting effects and enhance the cellular uptake by integrin-overexpressing cancer cells. We used atomistic molecular dynamics simulations to evaluate key structural-functional parameters of cRGD ligands conjugated to PEGylated TiO2 nanoparticles for an effective binding activity towards αVβ3 integrins. An increasing number of ligands has been conjugated to PEG chains, grafted to highly curved TiO2 nanoparticles, to unveil the impact of cRGD ligand density on its presentation, diffusion, and conformation in an explicit aqueous environment. We find that a low density leads to an optimal spatial presentation of cRGD ligands out of the "stealth" PEGylated layer around the nanosystem, favoring a straight upward orientation and a spaced distribution of the targeting ligands in the bulk-water phase. On the contrary, a high density favors the clustering of cRGD ligands, driven by a concerted mechanism of enhanced ligand-ligand interactions and reduced water accessibility to the ligand's molecular surface. These findings strongly suggest that the ligand density modulation is a key factor in the design of cRGD-conjugated nanodevices to maximize their binding efficiency to over-expressed αVβ3 integrin receptors.