Bifunctional ligands that target cells displaying the αvβ3 integrin

Strategies to eliminate tumor cells have long been sought. We envisioned that a small molecule could be used to decorate the offending cells with immunogenic carbohydrates and evoke an immune response. To this end, we describe the synthesis of bifunctional ligands possessing two functional motifs: one binds a cell-surface protein and the other binds a naturally occurring human antibody. Our conjugates combine an RGD-based peptidomimetic, to target cells displaying the alpha(v)beta(3) integrin, with the carbohydrate antigen galactosyl-alpha(1-3)galactose [Gal alpha(1-3)Gal or alpha-Gal]. To generate such bifunctional ligands, we designed and synthesized RGD mimetics 1 b and 2 c, which possess a free amino group for modification. These compounds were used to generate bifunctional derivatives 1 c and 2 d, with dimethyl squarate serving as the linchpin; thus, our synthetic approach is modular. To evaluate the binding of our peptidomimetics to the target alpha(v)beta(3)-displaying cells, we implemented a cell-adhesion assay. Results from this assay indicate that the designed small-molecule ligands inhibit alpha(v)beta(3)-dependent cell adhesion. Additionally, our most effective bifunctional ligand exhibits a high degree of selectivity (4000-fold) for alpha(v)beta(3) over the related alpha(v)beta(5) integrin, a result that augurs its utility in specific cell targeting. Finally, we demonstrate that the bifunctional ligands can bind to alpha(v)beta(3)-positive cells and recruit human anti-Gal antibodies. These results indicate that both the integrin-binding and the anti-Gal-binding moieties can act simultaneously. Bifunctional conjugates of this type con facilitate the development of new methods for targeting cancer cells by exploiting endogenous antibodies. We anticipate that our modifiable alpha(v)beta(3)-binding ligands will be valuable in a variety of applications, including drug delivery and tumor targeting.