Specific E-selectin targeting with a superparamagnetic MRI contrast agent

Targeting of the endothelial inflammatory adhesion molecule E-selectin by magnetic resonance imaging (MRI) was performed with a superparamagnetic contrast agent in the context of in vitro and in vivo models of inflammation. The specific contrast agent was obtained by grafting a synthetic mimetic of sialyl Lewis(x) (sLe(x)), a natural ligand of E-selectin expressed on leukocytes, on the dextran coating of ultrasmall particles of iron oxide (USPIO). This new contrast agent, called USPIO-g-sLe(x), was tested, in vitro, on cultured human umbilical vein endothelial cells (HUVECs) stimulated to express inflammatory adhesion molecules, and in vivo, on a mouse model of hepatitis. In vitro, HUVECs were stimulated with the pro-inflammatory cytokine tumor necrosis factor alpha (TNF-alpha) and were then incubated with USPIO-g-sLe(x) or ungrafted USPIO. In vivo, hepatitis was induced on NMRI mice by injection of concanavalin A (Con A). USPIO-g-sLe(x) and ungrafted USPIO were injected intravenously. In vitro results showed an extensive retention of USPIO-g-sLe(x) on TNF-alpha stimulated HUVECs. Image intensity and R(2) measurements performed on T(2)-weighted MR images demonstrated a significantly higher binding of USPIO-g-sLe(x) on stimulated HUVECs. In vivo, USPIO are known to pass through the fenestrae of the liver and to be captured by Kupffer cells, inducing a loss of signal intensity on T(2)-weighted MR images. Unexpectedly, when injected to Con A-treated mice, USPIO-g-sLe(x) induced a significantly lower attenuation of liver signal intensity than USPIO or USPIO-g-sLe(x) injected to healthy mice, or USPIO injected to Con A-treated mice, suggesting that the specific contrast media is retained extracellularly by an interaction with E-selectin overexpressed on the vascular endothelium. Both in vitro and in vivo results therefore indicate that USPIO-g-sLe(x) is recognizing endothelial E-selectin. USPIO-g-sLe(x) is thus well suited for the MRI diagnosis of inflammation and for the in vitro evaluation of endothelial cells activation.