A Fluorescent Probe for Investigating the Activation of Anticancer Platinum(IV) Prodrugs Based on the Cisplatin Scaffold

Following the discovery of its potent antitumoral activity in 1965, the inorganic drug cisplatin has become one of the most important anticancer agents in clinical use.1 It exhibits exceptional activity against germ-cell testicular cancer, for which it is the first-line therapeutic option.2 Its success led to the development of second-generation drugs, namely carboplatin and oxaliplatin, both of which are also based on the cis-diam(m)ineplatinum(II) pharmacophore. Together, these FDA-approved PtII drugs constitute some of the most widely used chemotherapeutic agents (Figure 1).3 Yet, despite their clinical success, the use of PtII drugs is limited by their high toxicity, severe side-effects, and incidences of drug resistance.4 To address some of these limitations, researchers have developed stable PtIV carboxylate complexes as anticancer prodrugs that can be activated by intracellular reduction to release their latent cytotoxic activity.5 This versatile strategy has been used to develop PtIV prodrug complexes with highly tuned properties,6 which are capable of targeting,7 as well as delivering novel modes of action.8 The most prominent example is satraplatin, which is currently undergoing Phase III clinical trials against hormone-refractory prostate cancer (Figure 1).9 Although there is much interest in understanding and exploiting this prodrug strategy as a springboard towards the next generation of Pt drugs, tools capable of directly visualizing the uptake and accumulation of these clinically important compounds are lacking. Herein, we describe a fluorescent probe that was custom-built for the detection of PtII drugs such as cisplatin. Using confocal microscopy, we could determine the localization of the PtIV prodrugs in cancer cells in vitro after cell entry and intracellular reduction