A new catechol-functionalized polyamidoamine as an effective SPION stabilizer

Abstract

A synthetic strategy was established for decorating and stabilizing superparamagnetic iron oxide nanoparticles (SPIONs) with a zwitterionic linear polyamidoamine (PAA). The strategy was successfully tested with a PAA coded ISA23 previously found endowed with interesting biological properties, such as biocompatibility, degradability in aqueous media and stealth-like properties when injected in test animals. A post-synthetic functionalization with catechol-bearing moieties of a preformed PAA was successfully carried out. ISA23 was obtained by polyaddition reactions of methyl-piperazine and 2,2-bis(acrylamidoacetic) acid. It was functionalized using nitrodopamine and 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide as coupling agent, to randomly form amide bonds with 17% of ISA23 carboxylic groups (ISA23-ND). SPIONs were prepared by a thermal decomposition synthesis in 1-octadecene with oleic acid, and then transferred in water by two distinct ligand exchange procedures: i) the direct displacement of oleate molecules from SPION surface by ISA23 in a biphasic (n-hexane/water) environment; ii) the two-step method involving an intermediate small molecule, tetramethylammonium hydroxide, used as a transient transfer agent, which was in turn exchanged with ISA23-ND in a second exchange step occurring in water. The two-step procedure provided a SPION@PAA nanocomposite more stable than that obtained by the one-step procedure in the presence of an applied external magnetic field. ATR-FTIR spectroscopy, ζ-potential and thermogravimetric analysis (TGA) showed the presence of the ISA23 on the SPION surface. In particular, TGA showed that the ISA23-ND amount on the NPs accounted for 26% of the overall nanocomposite mass. The nanocomposite size was determined by both TEM (21.1±2.9 nm) and DLS measurements (hydrodynamic size 100±28 nm). SPION@ISA23-ND were re-suspended after lyophilization reverting to their pristine dimensions. The SPION@ISA23-ND adsorption of BSA in water, considered as the first stage of phagocytosis, was very low, suggesting that ISA23 could impart stealthiness to SPION@ISA23-ND. ¹H-NMR relaxivity measurements showed an r₂ value of 158 s^-1 mmol^-1 L (vs 100 s^-1 mmol^-1L for Endorem^®) at relevant clinical fields for magnetic resonance imaging (from 0.2 to 1.5 T). SPION@ISA23-ND was tested on HeLa cells and their internalization was visualized by reflectance microscopy. Finally, with the aim of prepare a new dual magneto-optical system, a synthetic procedure to decorate SPION@ISA23-ND with a fluorescent dye was devised, even though the emission intensity of the resultant conjugate was lower than expected, possibly due to luminescence quenching caused by the closeness of emitting moieties to the SPION surface.

Keywords: Ligand exchange; Polyamidoamine; Relaxivity; SPION.