Using the singlet oxygen scavenging property of carotenoid in photodynamic molecular beacons to minimize photodamage to non-targeted cells

We recently introduced the concept of photodynamic molecular beacons (PMB) for selective control of photodynamic therapy (PDT). The PMB consists of a peptide linker that is sequence specific to a cancer-associated protease. A photosensitizer (PS) and a singlet oxygen (¹O₂) quencher are conjugated to the opposite ends of this linker. Proximity of the PS and quencher can efficiently inhibit ¹O₂ generation. In the presence of a targeted protease, the substrate sequence is cleaved and the PS and quencher will separate so that the PS can be photo-activated. There are two ways to optimize the PMB selectivity to cancer cells. The first is to increase the protease specificity to targeted cells and the second is to minimize the phototoxicity of intact (uncleaved) PMBs in non-targeted (normal) cells. Carotenoids (CARs) are well known in nature for their role in quenching excited states of PS and in directly scavenging ¹O₂. The purpose of this study is to evaluate whether the CAR with dual quenching modes (PS excited states deactivation and ¹O₂ scavenging) can be used to minimize the photodamage of intact PMBs to non-targeted cells. Thus, we synthesized a beacon (PPC) with a caspase-3 cleavable peptide linking a PS and a CAR quencher. It was confirmed that CAR deactivates the PS excited states and also directly scavenges ¹O₂. Moreover, the in vitro PDT response showed that CAR completely shuts off the photodynamic effect in non-targeted HepG₂ cells, while PS without CAR (control) remains highly potent even at a much lower (30-fold) dose. © The Royal Society of Chemistry and Owner Societies.