Selective ultrasound contrast enhancement in the tumor by nanocapsules with perfluorooctylbromide: effect of PLGA–PEG proportion

We used PLGA–COOH and PLGA–PEG–COOH blended polymer material to encapsulate perfluorooctyl bromide to prepare nanocapsules (NCs) as nano-ultrasound contrast agents. The aim of this study was to assess the effect of PLGA–PEG proportion on the physical, biological and acoustic characteristics of the nanocapsules, and to develop optimal nanocapsules for selective ultrasound contrast enhancement in tumors. The weight ratio of PLGA–PEG in the formulation was 0, 25%, 50%, 75%, and 100%, and the corresponding nanocapsules were designated NCs(PLGA), NCs(25% PLGA–PEG,) NCs(50% PLGA–PEG,) NCs(75% PLGA–PEG) and NCs(100% PLGA–PEG). As the PLGA–PEG proportion increased, the diameter and bulk modulus of the NCs gradually decreased, and the originally smooth surface of NCs was roughened. NCs(PLGA), NCs(25% PLGA–PEG) and NCs(50% PLGA–PEG) had regular spherical shape and relatively distinct boundaries compared with NCs(75% PLGA–PEG) and NCs(100% PLGA–PEG), which showed heavy agglomeration. The proportion of PLGA–PEG in the formula could also change the uptake rate of NCs by RAW 264.7 cells. NCs(50% PLGA–PEG) and NCs(75% PLGA–PEG) had the lowest uptake by RAW 264.7 cells. In vitro, the ultrasonic gray values of NCs(50% PLGA–PEG), NCs(75% PLGA–PEG) and NCs(100% PLGA–PEG) were obviously higher than those of NCs(PLGA) and NCs(100% PLGA–PEG). NCs(PLGA), NCs(50% PLGA–PEG) and NCs(100% PLGA–PEG) were injected into mice via the tail vein, but only NCs(50% PLGA–PEG) could produce persistent gray contrast enhancement in tumors after 24 h. Histological fluorescence of the tumor tissue confirmed that NCs(50% PLGA–PEG) and NCs(100% PLGA–PEG) gathered in tumor tissues. Our results indicate that the PLGA–PEG proportion in the formula is an important factor in constructing optimal nano-ultrasound contrast agents with a liquid core, and could change the nanocapsule size, surface morphology, elastic modulus, macrophage cellular uptake, and ultrasonic reflection. An appropriate PLGA–PEG proportion could help nanoparticles to achieve selective gray contrast enhancement in tumors.