Antibody-coupled photosensitive molecules can achieve an ideal tumor-specific photodynamic therapy (PDT) and show strong clinical application potential. However, some inherent disadvantages, such as long circulation half-life, poor permeation into solid tumors, and difficulty in obtaining uniform coupling products, present potential problems to clinical applications. In this study, we propose a novel design of targeting photosensitizers, based on a very small targeting protein (an affibody molecule) coupled with photosensitive compounds, to address these problems. In the synthesis, photosensitive pyropheophorbide-a (Pyro) is modified with a PEG linker (molecular weight of 727 Da) and then site specifically coupled to the anti-HER2 ZHER2:2891 affibody protein to provide a homogeneous protein-coupled photosensitizer via a convenient process. In vitro and in vivo experiments show that this molecule has an ideal selectivity for binding and photocytotoxicity against HER2-positive cells (more than 50-fold selectivity between HER2-high expression and HER2-low expression cells) and highly specific tumor accumulation; at a relatively low dose, it effectively eliminated HER2-high expression NCI-N87 tumors in a mouse model. It is worth noting that Pyro only has a moderate photodynamic activity; however, the affibody-coupled Pyro molecule (Pyro-Linker-ZHER2) still shows excellent tumor therapeutic function. The more ideal tumor permeability of small ligands may be helpful to enhance the drug concentration in the tumor site and the ability to penetrate deeply inside the tumor. Coupling photosensitive compounds with affibody proteins may provide a new way for targeting PDT of tumors.
Keywords: affibody; conjugate; human epidermal growth factor receptor 2 (HER2); photodynamic therapy; tumor targeting.