Application of Strontium Chloride Hexahydrate to Synthesize Strontium-Substituted Carbonate Apatite as a pH-Sensitive, Biologically Safe, and Highly Efficient siRNA Nanocarrier

Naked siRNAs are sensitive to enzymatic degradation, phagocytic entrapment, quick renal excretion, membrane impermeability, endosomal escape, and off-target effects. Designing a safe and efficient nanocarrier for siRNA delivery to the target site without toxicity remains a significant hurdle in gene therapy. CA is a unique derivative of hydroxyapatite and a highly pH-sensitive nanocarrier with strong particle aggregation and a high polydispersity index. Strontium (Sr²⁺), a group two divalent metal in the periodic table, has been reported for substituting calcium (Ca²⁺) ions from the apatite lattice and limiting particle growth/aggregation. This study used strontium chloride hexahydrate (SrCl₂·6H₂O) salt to develop a Sr-substituted CA (Sr-CA) nanocarrier with ∼30 nm size, spherical shape, less aggregation, homodispersity, and a fair anionic charge. Sr-CA demonstrated a large surface area-to-volume ratio, an improved cargo loading efficiency, and enhanced cellular uptake in HEK-293 cells. Moreover, Sr-CA is a pH-responsive nanocarrier responsible for its long physiological stability, efficient endosomal escape, and optimal cargo delivery within cells. These NPs have differential effects on MAPK1, MAP2K4, PIK3Ca, CAMK4, and p53 gene expression in HEK-293 cells without showing any significant cytotoxicity in cell growth properties. Gene silencing by Sr-CA-mediated siRNA delivery against MAPK1, MAP2K4, PIK3Ca, and CAMK4 genes significantly decreased the level of target gene expression and cell survival, demonstrating successful intracellular siRNA delivery in HEK-293 cells. Additionally, biocompatibility testing confirmed the biological safety of the Sr-CA nanocarrier in mice. These findings suggest that Sr-CA nanocarriers are a promising siRNA delivery system, combining high efficiency with pH-sensitive release and excellent biocompatibility, making them a viable option for future therapeutic applications.