Discovery of targeting ligands for breast cancer cells using the one-bead one-compound combinatorial method.

Four "one-bead one-compound" (OBOC) combinatorial libraries were designed, synthesized, and screened against MDA-MB-231 breast cancer cells. A novel cyclic peptide 1 (LXY1) with high binding specificity to alpha3 integrin was identified. Molecular interactions between alpha3 integrin and 1 were characterized by using a series of K562 cells transfected with various mutant alpha3 integrins. Using analytic flow cytometry, the binding affinity (K(d)) of 1 to alpha3 integrin on MDA-MB-231 breast cancer cells was determined to be approximately 0.4 microM. Based on the established structure-activity relationship (SAR) study, two highly focused cyclic peptide libraries were further designed, synthesized, and screened against MDA-MB-231 breast cancer cells under stringent conditions. A novel cyclic peptide 2 (LXY3) with a high binding affinity (IC(50) = 57 nM) was identified. Moreover, the targeting efficiency and specificity of 2 to the breast adenocarcinoma tumors in mouse xenografts were further confirmed by in vivo and ex vivo near-infrared fluorescence optical imaging.