In situ reaction of 20 with h2P2 ( ) restored cytotoxicity almost completely, resulting in an IC50 of 3.3 nM. Notably, a Tz concentration of only 1 µM was required to achieve this 300-fold 'turn-on' in toxicity with h2P2 (5), in contrast to 5 µM for DMT (1) (Fig. ). Furthermore, Tz 5 showed no effects on cell proliferation and viability at the tested concentrations.