Preparation of poly(N-isopropylacrylamide)-block-(acrylic acid)-encapsulated proteinaceous microbubbles for delivery of doxorubicin.

Inspired by theranostic technologies, we electrostatically loaded proteinaceous microbubbles (MBs) with a model drug, doxorubicin (Dox) to couple their utilizations in diagnostic imaging with drug loading. A temperature-sensitive polymer, poly(N-isopropylacrylamide-block-acrylic acid) (poly(NIPAM-b-AAc)) was used to encapsulate the Dox-loaded MBs to prevent premature release and to control the Dox release thermally. An LCST of 39°C, slightly higher than normal body temperatures, enables the release of Dox through a conformational change of the polymer shell upon moderate heating. The successive loadings of Dox and poly(NIPAM-b-AAc) were confirmed by fluorescent confocal laser scanning microscope (CLSM) imaging, zeta potential measurement, Fourier transform infrared spectroscopy (FTIR), and quartz crystal microbalance with dissipation (QCM-D). Without a polymer shell, Dox-loaded MBs showed a poor in vitro retention of Dox at room temperature, releasing ∼75% within 8h, whereas the polymer-shelled, Dox-loaded MBs did not show any premature release of Dox. From 37°C to 39°C, the cumulative release of Dox from the polymer-encapsulated MBs was increased from ∼20 to ∼90% over a period of 18h based on in vitro release testing (IVRT). However, the release profiles of Dox from the shell-free, Dox-loaded MBs did not exhibit any similar temperature-controlled behavior, releasing ∼90% of Dox within 5h at both 37°C and 39°C.