A fluorogenic cyclic peptide for imaging and quantification of drug-induced apoptosis

Programmed cell death or apoptosis is a central biological process that is dysregulated in many diseases, including inflammatory conditions and cancer. The detection and quantification of apoptotic cells in vivo is hampered by the need for fixatives or washing steps for non-fluorogenic reagents, and by the low levels of free calcium in diseased tissues that restrict the use of annexins. In this manuscript, we report the rational design of a highly stable fluorogenic peptide (termed Apo-15) that selectively stains apoptotic cells in vitro and in vivo in a calcium-independent manner and under wash-free conditions. Furthermore, using a combination of chemical and biophysical methods, we identify phosphatidylserine as a molecular target of Apo-15. We demonstrate that Apo-15 can be used for the quantification and imaging of drug-induced apoptosis in preclinical mouse models, thus creating opportunities for assessing the in vivo efficacy of anti-inflammatory and anti-cancer therapeutics.N.D.B. acknowledges funding from OPTIMA (EP/L016559/1). R.S.F. acknowledges an MSCA Individual Fellowship (659046). L.M.T. acknowledges the support of Fundacion Antonio Martin Escudero (FAME) in the form of a post-doctoral fellowship. S.T.H. is an Australian Research Council Future Fellow (FT150100398). R.L. acknowledges funding from MINECO (CTQ2015-67870-P and ERA NET PCIN-2015-224). F.M.G. acknowledges financial help from the Spanish Ministry of Economy (FEDER MINECO PGC2018-099857-B-I00) and the Basque Government (IT264-19). M.E. acknowledges funding from Cold Spring Harbor Laboratory Cancer Center (P30-CA045508), Northwell Health, and the Thompson Family Foundation. M.V. acknowledges funding from an ERC Consolidator Grant (771443), the Biotechnology and Biological Sciences Research Council (BB/M025160/1), and The Royal Society (RG160289).Peer reviewe