Transforming a Sword into a Knife: Persistent Phototoxicity Inhibition and Alternative Therapeutical Activation of Highly-Photosensitive Phytochlorin

The phototoxicity of photosensitizers (PSs) is a double-edged sword with one edge beneficial for destroying tumors while the other is detrimental to normal tissues, and the conventional "OFF-ON" strategy provides temporary inhibition so that phototoxicity would come sooner or later due to the inevitable retention and transformation of PSs in vivo. We herein put forward a strategy to convert "double-edged sword" PSs into "single-edged knife" ones with simultaneously persistent phototoxicity inhibition and alternative multiple therapeutical activation. The Chlorin e6 (Ce6) as the PS model directly assembles with Cu2+ ions into nanoscale frameworks (nFs) whose Cu2+-coordination includes both carboxyl groups and a porphyrin ring of Ce6 instead of Fe3+/Mn2+-coordination with only carboxyl groups. Compared to the high phototoxicity of Ce6, the nFs exhibit efficient energy transfer due to the dual-coordination of paramagnetic Cu2+ ions and the aggregation, achieving the persistent and high phototoxicity inhibition rate of >92%. Alternatively, the nFs not only activate a high photoacoustic contrast and near-infrared (NIR)-driven photothermal efficacy (3.5-fold that of free Ce6) due to the aggregation-enhanced nonradiative transition but also initiate tumor microenvironment modulation, structure disassembly, and chemodynamic effect by Cu2+ ions. Given these merits, the nFs achieve long-term biosecurity, no retina injury under sunlight, and a higher therapeutical output than the photodynamic effect of Ce6. This work presents a possibility of converting numerous highly phototoxic porphyrins into safe and efficient ones.