Human Interleukin-6 Receptor Super-Antagonists With High Potency and Wide Spectrum on Multiple Myeloma Cells

Interleukin-6 (IL-6) is the major growth factor for myeloma cells and is believed to participate in the pathogenesis of chronic autoimmune diseases and postmenopausal osteoporosis. IL-6 has been recently shown to possess three topo-logically distinct receptor binding sites: site 1 for binding to the subunit specific chain IL-6Rα and sites 2 and 3 for the interaction with two subunits of the signaling chain gp130. We have generated a set of IL-6 variants that behave as potent cytokine receptor super-antagonists carrying substitutions that abolish interaction with gp130 at either site 2 alone (site 2 antagonist) or at both sites 2 and 3 (site 2 + 3 antagonist). In addition, substitutions have been introduced in site 1 that lead to variable increases in binding for IL-6Rα up to 70-fold. IL-6 super-antagonists inhibit wild-type cytokine activity with efficacy proportional to the increase in receptor binding on a variety of human cell lines of different origin, and the most potent molecules display full antagonism at low molar excess to wild-type IL-6. When tested on a representative set of IL-6–dependent human myeloma cell lines, although site 2 super-antagonists were in general quite effective, only the site 2 + 3 antagonist Sant7 showed antagonism on the full spectrum of cells tested. In conclusion, IL-6 super-antagonists are a useful tool for the study of myeloma in vitro and might constitute, in particular Sant7, effective IL-6 blocking agents in vivo.