The use of pH-sensitive positively charged polymeric micelles for protein delivery.

In this investigation, a nano-sized protein-encapsulated polymeric micelle was prepared by self-assembling human serum albumin (HSA) as a model protein and degradable block copolymer methoxy poly(ethylene glycol)-poly(β-amino ester) (PEG-PAE) with piperidine and imidazole rings. From the zeta potential measurement, the protein-encapsulated polymeric micelle showed a pH-tuning charge conversion from neutral to positive when pH decreases from 7.8 to 6.2. It was envisioned that the pH-tunable positively charged polymeric micelle could enhance the protein delivery efficiency and, simultaneously, target to the pH-stimuli tissue, such as cancerous tissue or ischemia. The pH-dependent particle size and scattering intensity were also measured and showed 50-70 nm particle size. Consequently, the circular dichroism (CD) spectroscopy confirmed that the secondary structure of albumin was unaffected during the pH changing process. The in vitro cytotoxicity for the polymeric micelle was evaluated on MDA-MB-435 cell lines and no obvious toxicity could be observed when the polymer concentration was below 200 μg/mL. To assess the ability of this pH-tunable positively charged polymeric micelle as a vehicle for protein delivery to in vivo acidic tissues, we utilized a disease rat model of cerebral ischemia that produced an acidic tissue due to its pathologic condition. The rat was intravenously injected with the Cy5.5-labled albumin-encapsulated polymeric micelle. We found a gradual increase in fluorescence signals of the brain ischemic area, indicating that the pH-tuning positively charged protein-encapsulated polymeric micelle could be effective for targeting the acidic environment and diagnostic imaging.