The potential cytotoxic effect of nHAP/IO and nHAP/IO@miR-21/124 was tested based on ultrastructural features of cells, and mitochondrial metabolism determined using Alamar Blue assay. The metabolic activity of cells measured with Alamar Blue assay was evaluated after 4 days of the experiment, using the protocol established previously and according to the manufacturer's instructions.28 (link) For confocal imaging, cultures were fixed with 4% paraformaldehyde (PFA) for 15 min at room temperature. The mitochondria network structure was determined after its staining with MitoRed dye (Sigma-Aldrich, Munich, Germany). Actin cytoskeleton was visualised using a solution of phalloidin-Atto 488 (Sigma-Aldrich, Munich, Germany) prepared at a concentration of 1:800 in phosphate-buffered saline pH 7.4. Mitochondria and cytoskeleton staining were described in detail elsewhere.29 (link) The specimens were preserved using Mounting Medium with DAPI ([4ʹ,6-diamidino-2-phenylindole], Thermo Fisher Scientific, Waltham, MA, USA) for nuclei counterstain. The specimens imaging was done with confocal fluorescence microscopy (Leica TCS SPE, Leica Microsystems, Wetzlar, Germany) under 630× magnification. The analysis was also supported by measuring mRNA-coding anti- and pro-apoptotic molecules, Bcl-2 and Bax, respectively. The description of transcripts determination is described below.