The delivered dose: Applying particokinetics to in vitro investigations of nanoparticle internalization by macrophages.

PURPOSE: The aim of this study was to investigate the impact of nanoparticle dosimetry on the interpretation of results from in vitro experiments involving particle-cell interactions. Three different dose metrics were evaluated: 1) The administered dose (particle mass, number or surface area administered per volume media at the onset of an experiment), 2) the delivered dose (particle mass, number or surface area to reach the cell monolayer via diffusion and sedimentation over the duration of an experiment) and 3) the cellular dose (particle mass, number or surface area internalized by the cells during the experiment). The In Vitro Sedimentation and Diffusion and Dosimetry model (ISDD) was used to calculate particle sedimentation and diffusion in cell culture media to predict delivered dose values. These were compared with administered doses and experimentally determined cellular dose values.
METHODS: Dosing conditions and predicted delivered dose values were computed in silico using ISDD. In vitro cell association experiments were performed by exposing fluorescently labelled polystyrene beads of 50, 100, 200, 700 and 1000nm diameter to J774A.1 macrophage-like cells and determining the internalized particle content (cellular dose) via fluorescence spectroscopy. Experiments were repeated using lipopolysachharide (LPS) to activate and cytochalasin D to inhibit phagocytosis.
RESULTS: Only a small fraction (0.03-0.33%) of the administered dose was able to interact with the cells for all particle sizes tested. Measured cellular doses in non-activated J774A.1 cells corresponded well with computed delivered dose values for all particle sizes tested under six different exposure conditions. When cellular doses were averaged and normalized to their corresponding delivered doses, the percentage values of cell-associated particles were: 36 ± 10%(50 nm), 15 ± 3%(100 nm), 22 ± 6%(200 nm), 18 ± 4%(700 nm), and 42 ± 19%(1000 nm). Activation of J774A.1 cells with LPS significantly increased the cellular dose (normalized to the delivered dose) in all particle sizes except 50 nm, while cytochalasin D treatment significantly reduced the cellular dose of 100, 200 and 1000 nm particles.
CONCLUSIONS: This study demonstrates that dose correction using the ISDD model (i.e. normalization of cellular dose values to the delivered dose) is essential for accurate interpretation of results derived from in vitro particle-cell interaction studies (e.g. particle uptake, cytotoxicity, mechanisms of action, pharmacodynamic studies, etc.). It is of particular relevance to the field of particulate drug delivery systems, because the low density nature of most biomaterials used as drug carriers will result in very low fractions of the administered particle dose reaching the cell monolayer under most commonly used experimental conditions.