 The purpose of this trial is to evaluate the safety and efficacy of once daily orally administered GRT6005 in a total of 3 fixed doses compared to placebo in subjects with moderate to severe chronic Low Back Pain LBP. The study includes a maximum of 21 days screening period followed by a 2-week titration period and 12-week maintenance double-blind treatment period and a 10-14 day safety follow up period. Patients who are eligible for the double-blind treatment period will be randomized to one of the following treatment groups: GRT6005 low-dose, GRT6005 medium dose, GRT6005 high-dose, Tapentadol or placebo. 