Bifunctional Ligands that Target Cells Displaying the αvβ3 Integrin

• Published in: Chembiochem : a European journal of chemical biology 2007-01, Vol.8 (1), p.68-82
• Main Authors: Owen, Robert M., Carlson, Coby B., Xu, Jinwang, Mowery, Patricia, Fasella, Elisabetta, Kiessling, Laura L.
• Format: Article
• Language: English
• Subjects: cancer; carbohydrates; integrins; ligand design; peptidomimetics
• ISSN: 1439-4227; 1439-7633
• DOI: 10.1002/cbic.200600339

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 αvβ3 integrin, with the carbohydrate antigen galactosyl‐α(1–3)galactose [Galα(1–3)Gal or α‐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 αvβ3‐displaying cells, we implemented a cell‐adhesion assay. Results from this assay indicate that the designed, small‐molecule ligands inhibit αvβ3‐dependent cell adhesion. Additionally, our most effective bifunctional ligand exhibits a high degree of selectivity (4000‐fold) for αvβ3 over the related αvβ5 integrin, a result that augurs its utility in specific cell targeting. Finally, we demonstrate that the bifunctional ligands can bind to αvβ3‐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 can facilitate the development of new methods for targeting cancer cells by exploiting endogenous antibodies. We anticipate that our modifiable αvβ3‐binding ligands will be valuable in a variety of applications, including drug delivery and tumor targeting. Targeting cancer: We have synthesized an RDG‐based peptidomimetic that binds selectively to the integrin αvβ3, which is displayed on cancer cells. The integrin ligand can be elaborated to generate conjugates for imaging, delivering chemotherapeutics, or recruiting an immune response. When the ligand is attached to an antigenic carbohydrate, (α‐Gal), the resulting small molecule selectively recruits endogenous anti‐Gal antibodies to the target cells.