Randomized, Double-Blind, Placebo-Controlled, Multicenter Study of Siltuximab in High-Risk Smoldering Multiple Myeloma

Purpose: IL6 is important for the growth and survival of myeloma cells. This study evaluated blocking IL6 with siltuximab to delay the transition from high-risk smoldering multiple myeloma (SMM) to multiple myeloma. Patients and Methods: In a randomized, double-lind, placebo-controlled, multicenter study, 85 patients with high-risk SMM were randomized to 15 mg/kg siltuximab (43 patients) or placebo (42 patients). The primary endpoint was 1-year progression-free survival (PFS) rate, based on IMWG CRAB criteria. Secondary endpoints included progressive disease indicator rate, PFS, and safety. Results: Median age was 62 years (range: 21-84); 57% were male and 87% had a baseline Eastern Cooperative Oncology Group score of 0. The 1-year PFS rate was 84.5% (siltuximab) and 74.4% (placebo). After a median followup of 29.2 months, 32.6% of PFS events occurred with siltuximab and 42.9% with placebo. Median PFS was not reached with siltuximab but was 23.5 months with placebo [HR 0.50 (95% confidence interval, 0.24-.04); P = 0.0597]. The safety profile of siltuximab was comparable with placebo. Most adverse events in the siltuximab group were grade 2/3; the most common serious adverse events were infections/infestations, and renal/urinary disorders. Mortality was low in both groups (3 deaths in the siltuximab group and 4 in the placebo group). Conclusions: Although this study did not meet the prespecified protocol hypothesis criteria, data suggest that siltuximab may delay the progression of high-risk SMM.