NTR rapidly activated photosensitizers for high-signal-to-noise imaging and type I photodynamic therapy

Although nitroreductase (NTR) response-based photosensitive dyes have been widely developed for active photodynamic therapy, their small Stokes shift, uncontrolled enzymatic reaction rates, and poor photodynamic effect have resulted in unsatisfactory biological imaging and therapeutic applications of dyes. In particular, the sensitivity of the probe is more required to reveal the relationship between hypoxia-inducible factors-1α (HIF-1α) and NTR. Traditional hemicyanine dyes by the introduction of benzyl nitrobenzene were used for bioimaging and photodynamic therapy. Due to the relatively weak electron-withdrawing contribution of indole salt, the dye has longer absorption wavelength and slow response to NTR with a small Stokes shift. Thus, we choose pyridine salt with strong electron-withdrawing ability to replace the indole part. The increased ability to electron-withdrawing leads to an uneven charge distribution, which shortens the conjugation length resulting in the absorption blue shift and the Stokes shift increasing. Further, the nitro group directly was connected on the other side of the fluorophore to improve the reduction potential of the dye. In this way, not only NTR can respond nitro quickly, but also photosensitive dye is prone to electron transfer, which can realize type I photodynamic therapy. Interestingly, we found that the HIF-1α can regulate NTR level through bioimaging with such a well-performing probe, and that inhibitors of HIF-1α can inhibit the expression of NTR, while inhibitors of NTR are ineffective in inhibiting HIF-1α.