Development of a fluorescence polarization binding assay for folate receptor

Folate receptor (FR) has been actively investigated for targeted delivery of therapeutics into cancer cells because this receptor is selectively and highly expressed in carcinomas. Because FR rapidly cycles between the cell surface and cytoplasm, folic acid conjugated to a therapeutic agent can drive targeted therapeutic delivery to cancer cells. We prepared a novel fluorescent ligand Cy5–folate and used it to develop a fluorescence polarization (FP) FR binding assay to determine the binding affinities of FR-targeted molecules. The assay was performed in 96-well microplates using membrane preparations from human KB cells as a source of FR and Cy5 fluorophore-labeled folic acid as a tracer. This high-throughput homogeneous assay demonstrates advantages over existing multistep methods in that it minimizes both time and resources spent determining binding affinities. At the optimized conditions, a Z′ of 0.64 was achieved in a 96-well format.