pH-Responsive Nanostructured Calcium Phosphate Microrods as Pulmonary Delivery Platform: Fabrication, Characterization, and Comparative Assessment of Cytotoxic and Transcriptomic Responses in Alveolar Macrophages

Background: Nanostructured, rod-shaped microparticles represent a promising drug delivery platform for the pulmonary delivery and targeting of alveolar macrophages by exploiting the aerodynamic advantages of fiber-like geometries. These microrods feature a hierarchical architecture, designed for potential macromolecular payloads, and silica (SiO2)-based systems have previously been shown to successfully deliver oligonucleotides in vitro. However, current microrod systems mainly rely on nanoparticulate SiO2-based frameworks with limited biodegradability and lack a specific escape mechanism to the cytosol. Therefore, a nanostructured calcium phosphate (CaP) framework is proposed as a biodegradable and resorbable alternative, featuring pH-responsive dissolution under endolysosomal conditions. Methods and Results: This study presents the fabrication of nanostructured, rod-shaped calcium phosphate microparticles and discusses their suitability as a potential pulmonary drug delivery platform. The particles feature dissolution-driven disintegration in acidic and ion-rich environments relevant to phagolysosomes. In addition, the particles exhibited a favorable acute cytotoxicity profile in the murine alveolar macrophage cell line MH-S compared with their SiO2-based counterparts. Comparative RNA-seq analysis of MH-S exposed to the particles indicates a mild transcriptomic response, while canonical signatures of classical or alternative macrophage activation programs were not observed, supporting a generally well-tolerated exposure profile of the carrier. Conclusions: Together, these findings establish key prerequisites for employing calcium phosphate microrods as a biodegradable pulmonary carrier platform in subsequent studies incorporating therapeutic cargos.