An ultra-small bispecific protein augments tumor penetration and treatment for pancreatic cancer.

Purpose: The once highly anticipated antibody-based pathway-targeted therapies have not achieved promising outcomes for deadly pancreatic ductal adenocarcinoma (PDAC), mainly due to drugs' low intrinsic anticancer activity and poor penetration across the dense physiological barrier. This study aims to develop an ultra-small-sized, EGFR/VEGF bispecific therapeutic protein to largely penetrate deep tumor tissue and effectively inhibit PDAC tumor growth in vivo.

Methods: The bispecific protein, Bi-fp50, was constructed by a typical synthetic biology method and labeled with fluorescent dyes for in vitro and in vivo imaging. Physicochemical properties, protein dual-binding affinity, and specificity of the Bi-fp50 were evaluated in several PDAC cell lines. In vitro quantitatively and qualitatively anticancer activity of Bi-fp50 was assessed by live/dead staining, MTT assay, and flow cytometry. In vivo pharmacokinetic and biodistribution were evaluated using blood biopsy samples and near-infrared fluorescence imaging. In vivo real-time tracking of Bi-fp50 in the local tumor was conducted by fibered confocal fluorescence microscopy. The subcutaneous PDAC tumor model was used to assess the in vivo antitumor effect of Bi-fp50.

Results: Bi-fp50 with an ultra-small size of 50 kDa (5 ~ 6 nm) showed an excellent binding ability to VEGF and EGFR simultaneously and had enhanced, accumulated binding capability for Bxpc3 PDAC cells compared with anti-VEGF scFv and anti-EGFR scFv alone. Additionally, bi-fp50 significantly inhibited the proliferation and growth of Bxpc3 and Aspc1 PDAC cells even under a relatively low concentration (0.3 µM). It showed synergistically enhanced therapeutic effects relative to two individual scFv and Bi-fp50x control in vitro. The half-life of blood clearance of Bi-fp50 was 4.33 ± 0.23 h. After intravenous injection, Bi-fp50 gradually penetrated the deep tumor, widely distributed throughout the whole tissue, and primarily enriched in the tumor with nearly twice the accumulation than scFv2 in the orthotopic PDAC tumor model. Furthermore, the Bi-fp50 protein could induce broad apoptosis in the whole tumor and significantly inhibited tumor growth 3 weeks after injection in vivo without other noticeable side effects.

Conclusion: The proof-of-concept study demonstrated that the ultra-small-sized, bispecific protein Bi-fp50 could be a potential tumor suppressor and an efficient, safe theranostic tool for treating PDAC tumors.