Efficient Tumor-Targeted Photosensitizer Based on Nanobody-Coupled Pyropheophorbide-a for Precise Photodynamic Therapy of Tumors

The use of antibody-coupled photosensitizers is a promising strategy for tumor-targeted photodynamic therapy (PDT). However, some inherent disadvantages, including poor permeability into solid tumors, a long circulation half-life and random coupling of antibodies and photosensitizers, pose problems for their clinical application. In this study, we proposed an improved design for antibody-coupled photosensitizers based on microbial transglutaminase (mTGase)-catalyzed site-specific coupling of a small-sized nanobody with a stable and easily available photosensitive moiety, pyropheophorbide-a (Pyro), to obtain the N_HER2-PEG-Pyro conjugate, in which the high hydrophobicity of Pyro was alleviated by introducing a hydrophilic polyethylene glycol (PEG) chain. In vitro and in vivo experiments confirmed that N_HER2-PEG-Pyro had excellent binding selectivity and photodynamic activity toward highly HER2-expressing tumor cells, strongly accumulated in highly HER2-expressing tumor tissues, and eliminated highly HER2-expressing NCI-N87 tumors at a relatively low dose (20 nmol/mouse) as a single therapy. This work demonstrated the excellent therapeutic ability of this nanobody-coupled photosensitizer and highlighted the potential of Pyro as an alternative to IRDye 700DX in the development of tumor-targeted photosensitizers. The advantages of the nanobody and Pyro, along with their site-specific conjugation, confer the conjugate with good potential for clinical application.