Mock et al. generated a panel of cytokine–antibody fusion proteins comprising different formats of murine 4-1BB ligand and the antibody F8, which binds to the human (and mouse) tumor-associated, alternatively spliced EDA domain of fibronectin. The top candidate, F8–4-1BBL was inactive in solution, but regained agonist 4-1BB activity upon binding EDA. In vivo biodistribution studies showed that F8-4-1BBL selectively localized to EDA+ tumor vasculature with no binding to normal tissues. In tumor models, the fusion protein exhibited potent antitumor efficacy alone and in combination with PD-1 blockade, without apparent toxicity.
Contributed by Katherine Turner
ABSTRACT: Engineered cytokines are gaining importance in cancer therapy, but these products are often limited by toxicity, especially at early time points after intravenous administration. 4-1BB is a member of the tumor necrosis factor receptor superfamily, which has been considered as a target for therapeutic strategies with agonistic antibodies or using its cognate cytokine ligand, 4-1BBL. Here we describe the engineering of an antibody fusion protein, termed F8-4-1BBL, that does not exhibit cytokine activity in solution but regains biological activity on antigen binding. F8-4-1BBL bound specifically to its cognate antigen, the alternatively spliced EDA domain of fibronectin, and selectively localized to tumors in vivo, as evidenced by quantitative biodistribution experiments. The product promoted a potent antitumor activity in various mouse models of cancer without apparent toxicity at the doses used. F8-4-1BBL represents a prototype for antibody-cytokine fusion proteins, which conditionally display "activity on demand" properties at the site of disease on antigen binding and reduce toxicity to normal tissues.