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Inappropriate expression of the receptor tyrosine kinase Met and its ligand is usually associated with an aggressive solid-tumor phenotype (angiogenesis, invasiveness, and metastasis) and poor clinical prognosis. We are developing antibody-based agents that target the Met-ligand complex for clinical diagnostic and therapeutic applications. In this study, we constructed a human naïve Fab phage-display library with a diversity of 2.0 × 109 and screened out a Fab fragment (designated hFab-Met-1) from this library that specifically reacts with Met by using biopanning with Met-positive cell line S114. The specificity of hFab-Met-1 was characterized by immunoprecipitation, Western blotting, and flow cytometry. The results demonstrate that hFab-Met-1 reacts with the extracellular domain of Met in its native conformation. To confirm that hFab-Met-1 interacts with Met-expressing tumors in vivo, I-125-labeled hFab-Met-1 was injected intravenously into nude mice bearing subcutaneous xenografts of the Met- and HGF/SF-expressing human leiomyosarcoma line SK-LMS-1/HGF, and total body scintigrams were obtained between 1 hour (1h) and 48h postinjection (PI). Tumor-associated activity was imaged as early as 1h PI, and remained visible in some animals as late as 24 hours PI. As expected, activity was highest in the kidneys in early images, while thyroid activity became predominant in later images. In summary, hFab-Met-1 interacts with Met both in vitro and in vivo. Thus, a naïve library can be used to obtain high-affinity, functional human antibody fragments to a wide variety of targets, and to develop potential agents for clinical applications to cancers.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]