A passive diffusion model of fluorescein derivatives in an in vitro human brain microvascular endothelial cell (HBMEC) monolayer

Eukaryotic cells have a protective plasma membrane, which restricts the free movement of molecules from the external environment to the internal environment. This study aims to computationally model the transport of fluorescein derivatives across the monolayer of human brain microvascular endothelial cells (HBMEC). The determination of plausible effective diffusion constants (𝐷eff) will allow models to be built that could be useful beyond in vitro experimentation. Fluorescein-5-isothiocyanate (FITC) modeling produced a 𝐷effrange of 1E-20 to 5E-20 cm²/s at a 1 μm cell monolayer thickness and a 𝐷eff constant near 5E-29 cm²/s at a 5 μm cell monolayer thickness. Both fluorescein and sodium fluorescein (NaFl) modeling at the 1 and 5 μm thicknesses did not produce simulations that closely resembled the HBMEC in vitro model. Overall, it is possible that the fluorescent intensity noted with fluorescein and NaFl may be better explained by a mechanism other than passive diffusion. Simulations of FITC diffusion produced a narrow range of 𝐷eff constants that closely matched the in vitro HBMEC fluorescent intensity.