Methodological Issues in Conducting Pilot Trials in Chronic Pain as Randomized, Double-blind, Placebo-controlled Studies

Background: The efficacy of tapentadol extended release (ER) for managing chronic pain has been demonstrated in large-scale, randomized, controlled, phase 3 studies (N = 318-1,030) in patients with chronic osteoarthritis (OA) pain, low back pain (LBP), and pain related to diabetic peripheral neuropathy (DPN), which led to registration in many regions, including the United States and Europe. 2 pilot 12-week, randomized, double-blind, placebo-controlled phase 2 studies of tapentadol ER for chronic pain (OA knee pain or LBP, n = 91; DPN or peripheral herpetic neuralgia [PHN] pain; n = 91) were conducted in Japan. These small exploratory studies were substantially underpowered compared with the registration trials. Methods: Patients in both studies were randomized (2: 1) to tapentadol ER (25-250 mg) or placebo for 12 weeks (<= 6-week titration plus maintenance periods). Results: For the primary efficacy endpoint (change in pain intensity from baseline to last week of treatment; last observation carried forward), both studies failed to differentiate between tapentadol ER and placebo; least-squares mean differences (95 % confidence intervals) for tapentadol ER vs. placebo were -0.1 (-1.04, 0.80) in the OA/LBP study and -0.1 (-1.10, 0.99) in the DPN/PHN study. More than 80 % of patients took concomitant analgesics during these studies. Tapentadol was well tolerated. Conclusions: Both studies were associated with methodological issues, including populations with different disease entities, small sample sizes, use of concomitant analgesics, and possible placebo effect that may have led to the failure to differentiate between tapentadol ER and placebo.