Topical delivery of gabapentin (Gaba Gel™) for neuropathic pain: A 'proof of concept' study

Focal points • Some patients cannot tolerate oral gabapentin for neuropathic pain due to significant central side effects • Topical gabapentin, an NHS Pharmaceutical unlicensed 'special', has been used as a treatment alternative • Significant reduction in pain scores after treatment were demonstrated (n = 23) with no reported side effects • Topical gabapentin is an alternative treatment option for refractory, focal, peripheral neuropathic pain Introduction Some patients cannot tolerate oral gabapentin for neuropathic pain due to significant side effects such as nausea or dizziness. Topical gabapentin, a pharmaceutical 'special', is used as a treatment alternative in chronic pain clinics1. Professional opinion, locally and nationally, indicate that this product meets immediate clinical need however there is lack of robust evidence to support safety, efficacy and quality. We performed a local retrospective observational study to assess efficacy and safety of 6% w/w gabapentin gel, manufactured by a local NHS pharmacy manufacturing unit, for the treatment of peripheral neuropathic pain conditions. The wider collaborative research group also explored mechanisms of action using human tissue and animal studies and quality aspects of formulation development. We present here a 'proof of concept' study to investigate safety and efficacy of topical gabapentin for a variety of peripheral neuropathic pain conditions. Methods We present a single centre, retrospective, observational study (n = 23) where pain scores pre and post treatment were assessed using the British Pain Inventory numeric rating scale (BPI NRS) where 0 = no pain and 10 = maximal pain. Patients attended monthly assessment clinics where pain scores were routinely recorded in medical notes. Ethics and governance approvals were obtained to collect data from the notes of consecutive consenting patients (n = 25) being treated with topical gabapentin for refractory, focal peripheral neuropathic pain. A clinically significant reduction in pain score was deemed to be 22. The case mix included post herpetic neuralgia (PHN), painful diabetic peripheral neuropathy (PDPN), chronic post surgical pain (CPSP), complex regional pain syndrome (CRPS) and vulvodynia. Patient treatment regimens followed local neuropathic pain guidelines. Patients were only treated with topical gabapentin if proven refractory or suffering intolerable side effects to standard regimens. Results Data analysis demonstrates statistically and clinically significant reduction in pain scores after one month treatment (n = 20) with remaining patients (n = 3) reporting unaltered pain scores. Pain scores at baseline and one month after treatment changed from 8.2 (SD 1.5) to 5.6 (SD 1.6), p <0.001. Post process analysis of results with Wilcoxon signed rank test also indicated a strong tendency for the pain score to decrease after 1 month (-0.988). Individual patients reported between 30-80% pain relief (BPI NRS) after treatment. All patients with vulvodynia (4/4), PDPN (1/1) and 2/3 patients with PHN responded positively to topical gabapentin. Patients with pain reduction at one month showed similar improvements at 6 months. Individual analysis of symptoms from each case indicated the gel was effective for burning pain, autonomic hyperactivity and post surgical neuropathic pain and was ineffective for dysaesthesias. However this study is limited by small sample size and absence of specific neuropathic pain measures. No side effects were reported. Discussion Our study suggests that topical delivery of gabapentin can elicit a therapeutic response. A reduction in any experienced central side effects and a lower delivery dose were observed when compared to oral delivery. Topical gabapentin resulted in rapid improvement of symptoms (