Highly selective organ distribution and cellular uptake of inorganic-organic hybrid nanoparticles customized for the targeted delivery of glucocorticoids

We previously reported that inorganic-organic hybrid nanoparticles (IOH-NPs) containing the synthetic glucocorticoid (GC) betamethasone show efficient anti-inflammatory activity in mice. Here, we employed IOH-NPs with the chemical composition Gd-2(3+) [AMP](3)(2-) (AMP: adenosine monophosphate) to determine their in vivo distribution by magnetic resonance imaging after intraperitoneal injection. We show that IOH-NPs distribute throughout the peritoneal cavity from where they get rapidly cleared and then localize to abdominal organs. Our findings were confirmed by analyzing individual mouse organs ex vivo following injection of IOH-NPs with the chemical composition [ZrO](2+)[(BMP)(0.9)(FMN)(0.1)](2-) (BMP: betamethasone phosphate, FMN: flavin mononucleotide) or [ZrO](2+)[(HPO4)(0.9)(FMN)(0.1)](2-) using inductively coupled plasma mass spectrometry and flow cytometry. To characterize the mechanism of cellular uptake in vitro, we tested different cell lines for their ability to engulf IOH-NPs by flow cytometric analysis taking advantage of the incorporated fluorescent dye FMN. We found that IOH-NPs were efficiently taken up by macrophages, to a lesser extent by fibroblasts, epithelial cells, and myoblasts, and hardly at all by both T and B lymphocytes. Characterization of the endocytic pathway further suggested that IOH-NPs were internalized by macropinocytosis, and imaging flow cytometry revealed a strong colocalization of the engulfed IOH-NPs with the lysosomal compartment. Intracellular release of the functional anions from IOH-NPs was confirmed by the ability of the GC betamethasone to downregulate the expression of surface receptors on bone marrow-derived macrophages. Taken together, our findings unveil the mechanistic basis of an anti-inflammatory GC therapy with IOH-NPs, which may entail translational approaches in the future.