Development of a Cysteine-Conjugatable Disulfide FRET Probe: Influence of Charge on Linker Cleavage and Payload Trafficking for an Anti-HER2 Antibody Conjugate

Disulfide-linked bioconjugates allow the delivery of pharmacologically active or other cargo to specific tissues in a redox-sensitive fashion. However, an understanding of the kinetics, subcellular distribution, and mechanism of disulfide cleavage in such bioconjugates is generally lacking. Here, we report a modular disulfide-linked TAMRA-BODIPY based FRET probe that can be readily synthesized, modified, and conjugated to a cysteine-containing biomolecule to enable real-time monitoring of disulfide cleavage during receptor-mediated endocytosis in cells. We demonstrate the utility of this probe to study disulfide reduction during HER2 receptor-mediated uptake of a Cys-engineered anti-HER2 THIOMAB antibody. We found that introduction of positive, but not negative, charges in the probe improved retention of the BODIPY catabolite. This permitted the observation of significant disulfide cleavage in endosomes or lysosomes on par with proteolytic cleavage of a similarly charged valine–citrulline peptide-based probe. In general, the FRET probe we describe should enable real-time cellular monitoring of disulfide cleavage in other targeted delivery systems for mechanistic or diagnostic applications. Furthermore, modifications to the released BODIPY moiety permit evaluation of physicochemical properties that govern lysosomal egress or retention, which may have implications for the development of next-generation antibody–drug conjugates.