• Change from Baseline to Weeks 1, 2, 4, 6, 8, and 12 in the mBPI-sf score for Pain. [ Time Frame: 12 weeks ] [ Designated as safety issue: No ]
• Response as defined by a ≥30% and a ≥50% reduction from Baseline in the LBPI NRS score derived from the patient daily diary at Weeks 1, 2, 4, 6, 8, and 12. [ Time Frame: 12 weeks ] [ Designated as safety issue: No ]
• Time to a ≥30% and a ≥50% reduction from Baseline in the daily average LBPI NRS score (sustained for a minimum of 4 consecutive days). [ Time Frame: 12 weeks ] [ Designated as safety issue: No ]
• Change from Baseline to Weeks 1, 2, 4, 6, 8 and 12 in the Modified Brief Pain Inventory-short form (mBPI-sf) scores for Worst Pain and Average Pain obtained at study visits. [ Time Frame: 12 weeks ] [ Designated as safety issue: No ]
• Response as defined by a score of ≤2 in the mean daily average LBPI NRS score during Weeks 1, 2, 4, 6, 8, and 12 as derived from the daily patient e-diary. [ Time Frame: 12 weeks ] [ Designated as safety issue: No ]
• Percent change from Baseline in the LBPI NRS score through Week 6. [ Time Frame: 6 weeks ] [ Designated as safety issue: No ]
• Change from Baseline to Weeks 1, 2, 4, 6, 8, and 12 in Roland-Morris Disability Questionnaire total score. [ Time Frame: 12 weeks ] [ Designated as safety issue: No ]
• Patient's Global Evaluation of Study Medication (treatment response) score at Weeks 1, 2, 4, 6, 8 and 12. [ Time Frame: 12 weeks ] [ Designated as safety issue: No ]
• Response as defined by a ≥30% and a ≥50% reduction from Baseline in the daily average LBPI NRS score (sustained for a minimum of 4 consecutive days). [ Time Frame: 12 weeks ] [ Designated as safety issue: No ]
• Change from Baseline to Weeks 1, 2, 4, 6, 8, and 12 in Patient's Global Assessment of Low Back Pain (disease activity) score. [ Time Frame: 12 weeks ] [ Designated as safety issue: No ]
• Total duration of response as defined by days with a ≥30% and a ≥50% reduction from Baseline in the daily average LBPI NRS score. [ Time Frame: 12 weeks ] [ Designated as safety issue: No ]
• Change from Baseline to Weeks 1, 2, 4, 8 and 12 in the LBPI NRS score derived from the patient daily diary. [ Time Frame: 12 weeks ] [ Designated as safety issue: No ]
• Change from Baseline in average pain over Weeks 1 to 4, 1 to 8, 1 to 12, 5 to 8, and 5 to 12 in the LBPI NRS score derived from the patient daily diary. [ Time Frame: 12 weeks ] [ Designated as safety issue: No ]

• Change from Baseline to Weeks 1, 2, 4, 8 and 12 in the LBPI NRS score derived from the patient daily diary.
• Change from Baseline in average pain over Weeks 1 to 4, 1 to 8, 1 to 12, 5 to 8, and 5 to 12 in the LBPI NRS score derived from the patient daily diary.
• Change from Baseline to Weeks 1, 2, 4, 6, 8 and 12 in the Modified Brief Pain Inventory-short form (mBPI-sf) scores for Worst Pain and Average Pain obtained at study visits.
• Response as defined by a score of ≤2 in the mean daily average LBPI NRS score during Weeks 1, 2, 4, 6, 8, and 12 as derived from the daily patient e-diary.
• Percent change from Baseline in the LBPI NRS score throuh Week 6.
• Response as defined by a ≥30% and a ≥50% reduction from Baseline in the daily average LBPI NRS score (sustained for a minimum of 4 consecutive days).
• Response as defined by a ≥30% and a ≥50% reduction from Baseline in the LBPI NRS score derived from the patient daily diary at Weeks 1, 2, 4, 6, 8, and 12.
• Time to a ≥30% and a ≥50% reduction from Baseline in the daily average LBPI NRS score (sustained for a minimum of 4 consecutive days).
• Total duration of response as defined by days with a ≥30% and a ≥50% reduction from Baseline in the daily average LBPI NRS score.
• Change from Baseline to Weeks 1, 2, 4, 6, 8, and 12 in Roland-Morris Disability Questionnaire total score.
• Change from Baseline to Weeks 1, 2, 4, 6, 8, and 12 in the mBPI-sf score for Pain.
• Change from Baseline to Weeks 1, 2, 4, 6, 8, and 12 in Patient's Global Assessment of Low Back Pain (disease activity) score.
• Patient's Global Evaluation of Study Medication (treatment response) score at Weeks 1, 2, 4, 6, 8 and 12.

The primary objective of this study is to evaluate the analgesic efficacy of RN624 compared with placebo and compared with naproxen in the treatment of adult patients with chronic low back pain.

• Drug: Naproxen
• Drug: Placebo
• Drug: PF-04383119 (RN624)

Inclusion Criteria:

• Male or female of any race, >18 years of age and have BMI ≤39 kg/m2
• Present with duration of chronic low back pain of ≥3 months requiring regular use of analgesic medication (>4 days per week for the past month)
• Primary location of low back pain is between the 12th thoracic vertebra and the lower gluteal folds, with or without radiation into the posterior thigh, classified as Category 1 or 2 according to the classification of the Quebec Task Force in Spinal Disorders
• Must have a score of ≥4 for Low Back Pain Intensity (NRS) while on current treatment at Screening, and completes at least 4 daily pain diaries during the 5 days prior to Randomization, with an average Low Back Pain Intensity (NRS) score of ≥4

Exclusion Criteria:

• Back pain due to visceral disorder (i.e. endometriosis) or Back pain due to recent major trauma (i.e. vertebral fracture, post-traumatic spondylolisthesis)
• History of lumbosacral radiculopathy, spinal stenosis associated with neurological impairment, or neurogenic claudication
• Osteoporotic compression fracture within the last 6 months
• Known history of: Rheumatoid arthritis; Seronegative spondyloarthropathy (i.e., ankylosing spondylitis, psoriatic arthritis, reactive arthritis, inflammatory bowel disease-related arthropathy); Paget's disease of spine, pelvis or femur; Fibromyalgia; Tumors or infections of the spinal cord
• Patients receiving acetaminophen only to manage their chronic low back pain
• Any uncontrolled or untreated chronic disease