Proton Oriented-"Smart Depot" for Responsive Release of Ca to Inhibit Peptide Acylation in PLGA Microspheres.

Purpose: The purpose of this study was to characterize and detail the mechanism of a smart Ca release depot (Ca(PO)) about its ability for sustainable inhibition on peptide acylation within PLGA microspheres.

Methods: The octreotide acetate release and acylation kinetics were analyzed by RP-HPLC. Changes of Ca concentration and adsorption behavior were determined by a Calcium Colorimetric Assay Kit. The inner pH changes were delineated by a classic pH sensitive probe, Lysosensor yellow/ blue® dextran. Morphological changes of microspheres, adsorption between polymer and additive, transformation of Ca(PO) were characterized using SEM, FTIR and SSNMR separately.

Results: Before and after microspheres formulation, the property and effectiveness of Ca(PO) were investigated. Compared with a commonly used calcium salt (CaCl), high encapsulation efficiency (96.56%) of Ca(PO) guarantees lasting effectiveness. In an increasingly acidic environment that simulated polymer degradation, the poorly water-soluble Ca(PO) could absorb protons and transform into the more and more soluble CaHPO and Ca(HPO) to produce sufficient Ca according to severity of acylation. The corresponding Ca produce capacity fully met the optimum inhibition requirement since the real-time adsorption sites (water-soluble carboxylic acids) inside the degrading microspheres were rare. A sustained retention of three switchable calcium salts and slow release of Ca were observed during the microsphere incubation. FTIR results confirmed the long-term inhibition effect induced by Ca(PO) on the adsorption between drug and polymer.

Conclusions: With the presence of the smart Ca depot (Ca(PO)) in the microspheres, a sustainable and long-term inhibition of peptide acylation was achieved.