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In this double-blind, Phase 2 study, 220 patients with relapsed/refractory multiple myeloma were randomly assigned 1:1:1 to receive placebo ( N = 72), tabalumab 100 mg ( N = 74), or tabalumab 300 mg ( N = 74), each in combination with dexamethasone 20 mg and subcutaneous bortezomib 1·3 mg/m2 on a 21-day cycle. No significant intergroup differences were observed among primary (median progression-free survival [ mPFS]) or secondary efficacy outcomes. The mPFS was 6·6, 7·5 and 7·6 months for the tabalumab 100, 300 mg and placebo groups, respectively (tabalumab 100 mg vs. placebo Hazard ratio ( HR) [95% confidence interval ( CI)] = 1·13 [0·80-1·59], P = 0·480; tabalumab 300 mg vs. placebo HR [95% CI] = 1·03 [0·72-1·45], P = 0·884). The most commonly-reported treatment-emergent adverse events were thrombocytopenia (37%), fatigue (37%), diarrhoea (35%) and constipation (32%). Across treatments, patients with low baseline BAFF (also termed TNFSF13B) expression ( n = 162) had significantly longer mPFS than those with high BAFF expression ( n = 55), using the 75th percentile cut-off point ( mPFS [95% CI] = 8·3 [7·0-9·3] months vs. 5·8 [3·7-6·6] months; HR [95% CI] = 1·59 [1·11-2·29], P = 0·015). Although generally well tolerated, PFS was not improved during treatment with tabalumab compared to placebo. A higher dose of 300 mg tabalumab did not improve efficacy compared to the 100 mg dose. Nonetheless, BAFF appears to have some prognostic value in patients with multiple myeloma. [ABSTRACT FROM AUTHOR]