IL6 and plasma cell leukemia: Klein et al. showed biological activity of blocking IL-6 with an anti-IL-6 monoclonal antibody in 10 patients with relapsed/refractory MM and plasma cell leukemia, with reduction in proliferative indices, though no responses were seen.14 Pre-clinical studies demonstrated that the combination of siltuximab and bortezomib had a potentially synergistic effect in inducing apoptosis in both IL-6-dependent and IL-6-independent MM cell lines.