Evaluation of phosphatidylserine-binding peptides targeting apoptotic cells

The inhibition or dysregulation of apoptosis plays an intimate role in the initiation and progression of cancer by confounding normal tissue homeostasis. We currently do not have a clinical method to assess apoptosis induced by cancer therapies. Phosphatidylserine (PS) is an attractive target for imaging apoptosis because it is on the exterior of the apoptotic cells and PS externalization is an early marker of apoptosis. PS-binding peptides are an attractive option for developing an imaging probe to detect apoptosis using positron emission tomography. In this study, four peptides were evaluated for PS-binding characteristics using a plate-based assay system, a liposome mimic of cell membrane PS presentation, and a cell assay of apoptosis. This work also describes two screening techniques to enable researchers to identify and optimize compounds that bind to PS. The results of our study indicate that all four peptides bind to PS and are specific to apoptotic cells. Two of the peptides in particular that have an additional cysteine residue are good potential candidates for development into imaging probes because they bind to PS with high affinity and specificity and they can be easily radiolabelled with (18)F.