Structural insights into pH-responsive drug release of self-assembling human serum albumin-silk fibroin nanocapsules

Abstract

Inflammation processes are assocd. with significant decreases in tissue or lysosomal pH from 7.4 to 4, a fact that argues for the application of pH-responsive drug delivery systems. However, for their design and optimization a full understanding of the release mechanism is crucial. In this study we investigated the pH-depending drug release mechanism and the influence of silk fibroin (SF) concn. and SF degrdn. degree of human serum albumin (HSA)-SF nanocapsules. Sonochem. produced nanocapsules were investigated regarding particle size, colloidal stability, protein encapsulation, thermal stability and drug loading properties. Particles of the monodisperse phase showed av. hydrodynamic radii between 438 and 888 nm as measured by DLS and AFM and a zeta potential of -11.12 ± 3.27 mV. Together with DSC results this indicated the successful prodn. of stable nanocapsules. ATR-FTIR anal. demonstrated that SF had a pos. effect on particle formation and stability due to induced beta-sheet formation and enhanced crosslinking. The pH-responsive release was found to depend on the SF concn. In in-vitro release studies, HSA-SF nanocapsules composed of 50% SF showed an increased pH-responsive release for all tested model substances (Rhodamine B, Crystal Violet and Evans Blue) and methotrexate at the lowered pH of 4.5 to pH 5.4, while HSA capsules without SF did not show any pH-responsive drug release. Mechanistic studies using confocal laser scanning microscopy (CLSM) and small angle X-ray scattering (SAXS) analyses showed that increases in particle porosity and decreases in particle densities are directly linked to pH-responsive release properties. Therefore, the pH-responsive release mechanism was identified as diffusion controlled in a novel and unique approach by linking scattering results with in-vitro studies. Finally, cytotoxicity studies using the human monocytic THP-1 cell line indicated non-toxic behavior of the drug loaded nanocapsules when applied in a concn. of 62.5 mg mL-1. Based on the obtained release properties of HSA-SF nanocapsules formulations and the results of in-vitro MTT assays, formulations contg. 50% SF showed the highest requirements arguing for future in vivo expts. and application in the treatment of inflammatory diseases.