Synthesis of a macrocyclic rhodamine 110 enzyme substrate as an intracellular probe for caspase 3 activity

The synthesis of the macrocyclic rhodamine 110 caspase 3 substrate 8 is described. The key step is a high dilution intramolecular cyclization reaction of an in situ generated primary amine with a 4-nitrophenyl ester. Substrate 8 reacts with recombinant caspase 3 to yield a fluorescent signal but virtually no signal is detected in the absence of caspase 3 or in the presence of the caspase 3 inhibitor Ac-DEVD-CHO. Notably, 8 selectively stains live cells that have been induced to undergo apoptosis with etoposide.