PURPOSE: Fulranumab is an investigational, fully human recombinant monoclonal antibody (IgG2) that neutralizes the biological actions of human nerve growth factor. Low back pain is a common cause of noncancer chronic pain and represents one of the most significant socioeconomic health-related problems in developed countries. This randomized, double-blind, placebo-controlled study was conducted to evaluate the analgesic effect of fulranumab in patients with moderate-to-severe chronic low back pain. METHODS: Patients (aged 18-80 years) were randomized to receive subcutaneous injections every 4 weeks in 1 of 5 parallel treatment groups: placebo or fulranumab 1mg (1mgQ4wk), 3mg (3mgQ4wk), 3mg after a 6mg loading dose (6mgLD+3mgQ4wk), or 10mg (10mgQ4wk) every 4 weeks. FINDINGS: A total of 385 patients (median age, 53 years; women, 54%) received at least 1 injection of study medication. No statistically significant differences were observed in improvement of pain intensity scores between the fulranumab treatment regimens and the placebo group at week 12 (primary end point; mean [SD], placebo: -2.0 [2.17], 1mgQ4wk: -1.9 [2.14], 3mgQ4wk: -2.2 [1.89], 6mgLD+3mgQ4wk: -2.0 [1.72] and 10mgQ4wk: -2.1 [2.18]). Results for secondary efficacy parameters (change in the Oswestry Disability Index, Brief Pain Inventory-Short Form, and Patient Global Assessment scales) were consistent with the primary outcome. A placebo effect was observed; the overall percentage of patients with treatment-emergent adverse events (TEAEs) was similar between the placebo (76%) and fulranumab treatment groups (77%-90%). Across all phases, the most common TEAEs in at least 10% of patients (combined fulranumab group vs placebo) were arthralgia (15% vs 12%), back pain (15% vs 18%), upper respiratory tract infection (15% vs 8%), paresthesia (14% vs 8%), diarrhea (12% vs 4%), headache (12% vs 8%), hypoesthesia (11% vs 5%), pain in extremity (11% vs 8%), sinusitis (10% vs 5%), and nasopharyngitis (10% vs 9%). Across all phases, neurologic TEAEs were less frequent in the placebo group (14%) versus the fulranumab treatment groups (25%). In the posttreatment phase, 8 patients had joint replacement operations, which were considered a result of normal progression of osteoarthritis. One case in the 10-mg group was determined to be rapid progession of osteoarthritis and was considered to be possibly related to study drug. IMPLICATIONS: Fulranumab did not demonstrate efficacy compared with placebo in patients with chronic low back pain but was generally well-tolerated. ClinicalTrials.gov identifier: NCT00973024.