While screening for novel IL-6 inhibitors, we synthesized 20S, 21-epoxy-resibufogenin-3-acetate (ERBA). ERBA dose-dependently suppressed IL-6-induced cell growth with an IC50 value of 5.3 muM and caused a parallel rightward shift of dose-response curves to IL-6. Analysis of data yields a pA2 of 5.83 and a slope of 0.99. ERBA did not affect IL-2-, IL-3-, and GCSF-dependent cell growth, or tumor necrosis factor alpha-induced growth suppression, nor did ERBA affect osteoclast formation induced by IL-11, leukemia inhibitory factor, and 1alpha,25-dihydroxyvitamin D-3. Receptor assay showed that ERBA dose-dependently suppressed IL-6 binding to IL-6 receptor (IL-6R). Furthermore, no band existing at the position of IL-6R in Western blots of ERBA-treated cells when stimulated with IL-6:ERBA suppresses IL-6 activity by blocking the binding of IL-6 to IL-6R. In an experimental model of colon 26-induced cancer cachexia, ERBA markedly inhibited body weight loss. ERBA is a specific small molecule with IL-6R-antagonist activity. (C) 2004 Published by Elsevier Inc.