Differences in binding of ^99mTc-disintegrins to integrin αvβ3 on tumor and vascular cells

Abstract 

Disintegrins, which contain an Arg-Gly-Asp sequence in their binding domains are antagonists of integrins such as αvβ3. The purpose of this study was to compare a range of disintegrins with different integrin selectivities for their binding behavior in vitro to vascular endothelial cells bearing αvβ3 and to cultured tumor cells which express αvβ3.

Methods

Five disintegrins (bitistatin, kistrin, flavoridin, VLO4 and echistatin) and a cyclic pentapeptide, c[RGDyK], were radiolabeled with ^99mTc and tested for binding to cells in vitro.

Results

^99mTc-Kistrin, flavoridin and VLO4 had the highest binding, ^99mTc-echistatin had moderate binding, and ^99mTc-bitistatin and ^99mTc-c[RGDyK] had low binding to cells. The observed binding was attributed to αvβ3 to various extents: echistatin, bitistatin>kistrin>flavoridin>VLO4. Cancer cells internalized bound disintegrins after binding, but endothelial cells did not. After binding to endothelial cells, ^99mTc-kistrin was not displaced by competing peptide or plasma proteins.

Conclusions

These data suggest that radiolabeled kistrin, flavoridin and VLO4 may have advantages over labeled bitistatin and small cyclic peptides for targeting αvβ3 in vivo.

Since receptor-bound radioligand is not internalized by endothelial cells, disintegrins may provide an advantage for targeting αvβ3 on vasculature because they bind strongly to surface receptors and are not readily displaced.

Keywords: Disintegrins, Integrin αvβ3, Angiogenesis, Tumors, Tc-99m, c[RGDyK]