The single-blind, randomized, controlled, non-inferiority (NI) trial was carried out in adult patients aged between 18 and 75 years and diagnosed with lower back pain for at least one month.

The neuropathic component of lower back pain was screened by the Thai version of DN4 (DouleurNeuropathiqueen 4 Questions). Patients had to have at least two positive responses to the DN4 questions. Pain had to be located in the lower back, buttock and lower extremity. Patients had to have moderate pain intensity assessed by visual analogue scale (VAS) that scored at least 40 mm.

Patients were randomized into two treatment groups. The first group received generic gabapentin Sandoz and the second group received originator gabapentin (Neurontin). The amount of medication was increased to maintain VAS less than 40 mm and tapered off in case of adverse event.

Forty-one patients were randomized into the generic gabapentin group (n = 21) and the originator gabapentin group (n = 20).

The causes of lower back pain were herniated nucleus pulpous (n = 12, 29.27%), spondylolisthesis (n = 11, 26.83%), spinal stenosis (n = 10, 24.39%), spondylosis (n = 7, 17.07%) and other (n = 1, 2.47%). The number of patients having true neuropathic pain determined by DN4 score greater than 4 were 14 and 13 patients in the generic gabapentin and originator gabapentin groups, respectively.

At week 8, the VAS and ODI scores significantly decreased in both groups. The difference in VAS score between the two groups was 2.9 mm with a 95% confidence interval (CI) of ‑17.7 and 11.8 mm. The effective gabapentin dosage was between 600 and 1,200 mg in both groups. The average effective dose was 700 mg in the generic gabapentin group and 778.6 mg in the originator gabapentin group (p-value = 0.46). Both groups revealed that 83% of patients used gabapentin at a dose of at least 600 mg. The average duration until reaching the effective dose was 3.6 weeks in the generic gabapentin group and four weeks in the originator gabapentin group (p-value = 0.58).

The ODI score was significantly lower in both groups after treatment. The difference in the ODI score between both groups was 3.5%, with a 95% CI of ‑12.3% and 5.3%. The lumbar range of motion was significantly improved only for flexion in both groups.

Regarding other medications used for pain relief such as rescue medications, acetaminophen and combination of acetaminophen and orphenadrine citrate were the most commonly used in both groups. Nonsteroidal Anti-Inflammatory Drugs (NSAIDs) group was the second most commonly used, whereas opioid was rarely used in both groups. The frequencies of rescue medication usage in the generic gabapentin and originator gabapentin groups were 184 and 127 times, respectively. After completing the protocol, the less frequent usage of rescue medication was significantly revealed in both groups.

The generic gabapentin group had pain reduction within the 20% pre-specified margin; thus, generic gabapentin provided non-inferior effectiveness for pain relief in comparison with the originator form. Furthermore, the effective dose and duration reaching the effective dose remained similar in both generic and originator forms. Interestingly, some patients in both groups still used acetaminophen based and NSAIDs. These findings indicated that back pain for these patients was originated partly from a nociceptive mechanism.

As for functional improvement, both treatment groups have significantly gained in functional improvement, determined by improved lumbar spine’s flexion, and reduced ODI score. Improvement of back flexion could be possibly explained by the most common cause of back pain, i.e. herniated nucleus pulposus.

The non-inferior effectiveness for pain reduction and back function improvement in the generic group compared to the originator group led the authors to recommend use of generic gabapentin in Thailand.

Conflict of interest
The authors of the research paper [1] declared that their study was funded by Novartis (Thailand) Limited; however, the research process was not influenced by any conflict of interest.

Abstracted by Dr Natthiya Tantisiriwa, Department of Rehabilitation Medicine, King Chulalongkorn Memorial Hospital, Thailand.

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Reference
1.  Kantito S, Kantito S, Tantisiriwat N, Piravej K. Comparison of the effectiveness between generic and original form of gabapentin for pain relief in suspected neuropathic component of low back pain. J Med Assoc Thai. 2014;97(7):767-75.

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