Ranpirnase (Frog RNase) Targeted with a Humanized, Internalizing, Anti–Trop-2 Antibody Has Potent Cytotoxicity against Diverse Epithelial Cancer Cells

Ranpirnase (Rap), an amphibian RNase, has been extensively studied both preclinically and clinically as an antitumor agent. Rap can be administered repeatedly to patients without any untoward immune response, with reversible renal toxicity reported to be dose limiting. To enhance its potency and targeted tumor therapy, we describe the generation of a novel IgG-based immunotoxin, designated 2L-Rap(Q)-hRS7, comprising Rap(Q), a mutant Rap with the putative N-glycosylation site removed, and hRS7, an internalizing, humanized antibody against Trop-2, a cell surface glycoprotein overexpressed in variety of epithelial cancers. The immunotoxin was generated recombinantly by fusing Rap(Q) to each of the two hRS7 light (L) chains at the NH(2) terminus, produced in stably transfected myeloma cells, purified by Protein A, and evaluated by a panel of in vitro studies. The results, including size-exclusion high-performance liquid chromatography, SDS-PAGE, flow cytometry, RNase activity, internalization, cell viability, and colony formation, showed its purity, molecular integrity, comparable affinity to hRS7 for binding to several Trop-2-expressing cell lines of different cancer types, and potency to inhibit growth of these cell lines at nanomolar concentrations. In addition, 2L-Rap(Q)-hRS7 suppressed tumor growth in a prophylactic model of nude mice bearing Calu-3 human non-small cell lung cancer xenografts, with an increase in the median survival time from 55 to 96 days (P < 0.01). These results warrant further development of 2L-Rap(Q)-hRS7 as a potential therapeutic for various Trop-2-expressing cancers, such as cervical, breast, colon, pancreatic, ovarian, and prostate cancers.