A novel porphyrin-based theranostic agent activated by cysteine over-expressed in cancer cells shows promise for tumour-targeted monitoring and phototherapy

l-Cysteine (Cys) is a tumor-related biomarker. The photo- and Cys-activatable theranostic agent 4 was designed and synthesized based on 5,10,15,20-tetrakis(4-hydroxyphenyl)porphyrin (3). Compound 4 exhibited negligible fluorescence and photosensitizing activity in the absence of Cys. However, when activated by Cys, 3 was released, resulting in fluorescence and photosensitization. Moreover, upon irradiation with 660 nm light, 4 displayed selective and effective fluorescence and photo-cytotoxicity exclusively against cancer cells, such as HeLa and A549 cells, which express high levels of Cys. This wavelength of light falls within the phototherapeutic window.