Preclinical Evaluation of an Integrin α_vβ₆-Targeted Photodynamic Therapy

Photodynamic therapy (PDT) is a minimally invasive treatment in which an external light source activates an injected photosensitizer (PS) to generate reactive oxygen species, causing localized cell death. Although the first PDT received FDA approval in 1995, clinical adoption has been limited, in part due to the limited tumor selectivity of PSs. The goal of this study was to develop a PS with improved tumor selectivity by incorporating a targeting peptide that selectively binds to the integrin α_vβ₆. The integrin α_vβ₆ is an epithelial-specific cell-surface receptor that is overexpressed in several cancer types, with expression level often linked to poor overall survival. The integrin α_vβ₆ targeting peptide (ABM-5G) was conjugated onto a water-soluble PS (IRDye700DX, IR700) in solution phase, and the resulting PS-α_vβ₆-targeted-peptide conjugate (IR700-ABM-5G, 1) demonstrated excellent photochemical and photophysical properties, including high extinction coefficient and singlet oxygen productivity similar to the nontargeted PS (free IR700). In vitro, 1 showed α_vβ₆-selective binding to and internalization into DX3puroβ6 (α_vβ₆+) cells vs DX3puro (α_vβ₆-) cells, and α_vβ₆-selective phototoxicity with EC₅₀s of 1.6 nM for DX3puroβ6 cells and ≥ 250 nM for DX3puro cells. In mice bearing paired DX3puroβ6 (α_vβ₆+) and DX3puro (α_vβ₆-) tumor xenografts, the fluorescence intensity of 1 in DX3puroβ6 (α_vβ₆+) tumors was 2.5- to 7-fold higher than that of the other tissues (including DX3puro (α_vβ₆-) tumors, p < 0.0001), except for the kidneys and stomach. A single treatment of 1 (1.4 nmol per mouse) combined with near-infrared light exposure significantly suppressed the growth of DX3puroβ6 (α_vβ₆+) tumors (198 ± 112 mm³ vs 714 ± 251 mm³ for saline control, p < 0.0001, on day 37 post treatment). In summary, PDT treatment with 1 demonstrated α_vβ₆-selective therapeutic efficacy both in vitro and in vivo and is a promising targeted therapy for the treatment of a range of α_vβ₆-expressing cancers.