NPC Archive Item: High-concentration capsaicin patch for the treatment of postherpetic neuralgia – more data needed

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In patients who had had postherpetic neuralgia for at least 6 months, a single 60-minute application of an 8% capsaicin patch proved more effective at providing pain relief than a 0.04% capsaicin patch (control) in this randomised, double-blind 12 week study. However, greater use of other medication by patients in this study using the 8% capsaicin patch means that the clinical significance of this data is unclear.

NGX-4010 does not currently have a marketing authorisation and should it reach the market it may be expensive. Treatment of PHN is difficult and there might be interest in the product due to the possible lack of systemic adverse effects. However, until there are high quality data over a longer time period existing treatment pathways should be followed.

What is the background to this?
Postherpetic neuralgia (PHN) is a common complication of shingles in adults. Clinical Knowledge Summaries (CKS) details a treatment pathway where paracetamol, either alone or in combination with codeine is recommended is as first line. If this should prove inappropriate then a tricyclic antidepressant or gabapentin may be considered. Capsaicin 0.075% cream can be tried if other treatments are not appropriate or the person prefers a topical treatment. Lidocaine 5% patches can be considered for patients in whom oral or other topical treatments are poorly tolerated or unsuitable.

What does this study claim?
One 60-minute application of an 8% capsaicin patch (NGX–4010) was compared with an 0.04% control patch in patients with postherpetic neuralgia. The primary endpoint was percentage change in Numeric Pain Rating Scale (NPRS) from baseline to the mean score during weeks two to eight. NPRS is an 11-point scale that ranges from 0 to 10 points, where 0 points is no pain and 10 points is the worse pain possible. The mean changes in NPRS score were −29.6% in the higher-dose group versus −19.9% in the control group (difference 9.7%, 95% confidence interval (CI) 15.47 to 3.95; P=0.001).

How does this relate to other studies?
This is one of a number of trials for the use of this product for PHN or other painful conditions. There are no head to head studies comparing NGX–4010 with capsaicin 0.075% cream, lidocaine 5% patch or recommended oral treatments for PHN.  NICE is due to produce a clinical guideline on the management of neuropathic pain in 2010.

So what?
A reduction of approximately three points or 30% on acute and chronic pain scales is accepted as representing a clinically important difference. Adding in the higher dose capsaicin patch achieved this mean improvement whereas the lower dose patch did not. However, more patients in the NGX–4010 took concomitant pain medication than in the control group. In addition the study was of short duration, and we therefore have no data indicating how long any additional pain relief might persist or what long-term adverse events might occur.

Study details

Backonja M et al for the NGX-4010 C116 Study Group. NGX–4010, a high-concentration capsaicin patch, for the treatment of postherpetic neuralgia: a randomised, double-blind study. Lancet Neurol 2008;7:1106–12 DOI:10.1016/S1474-4422(08)70228-X

Design: phase III, double-blind, randomised trial.

Patients: patients aged 18–90 years (n=402) with PHN for at least 6 months and an average baseline NPRS score of 3 to 9. Mean score at baseline for both study groups was 6.0.  Patients were allowed to stay on a stable dose of their long-term oral pain medication during the study period.

Intervention: single capsaicin 8% patch applied for 60 minutes (n=206).

Comparison: single capsaicin 0·04% patch applied for 60 minutes (n=196). The low concentration capsaicin patch was used in place of placebo to ensure effective blinding, because topical capsaicin produces erythema and a local burning sensation. Patients were pre-treated with a local anaesthetic cream and could use rescue medication (5mg hydrocodone bitartrate and 500mg paracetamol) for up to 5 days after application of either patch.

Outcomes: primary endpoint was the percentage change in NPRS score from baseline to mean of scores between weeks two to eight (intention-to-treat analysis).

Results: mean changes in NPRS score were -29.6% (95%CI -33.63 to -25.59) in the higher-dose group versus -19.9% (95%CI -24.02 to -35.78) in the control group (difference 9.7%, 95% CI 15.47 to 3.95; P=0.001).  A total of 87 (42%) patients who received NGX-4010 and 63 (32%) of those administered the control had a 30% or greater reduction in mean NPRS score (odds ratio [OR] 1.56, 95% CI 1.03 to 2.37; P=0.03), a secondary end point.  More patients in the NGX–4010 group took concomitant pain medication at baseline than in the control group, 50% vs. 38%, respectively; P=0.021.

Adverse Events: erythema was reported in 94% of the treatment group (n=205) versus 65% of the control group (n=197). Pain at the site of application was reported in 56% versus 22% of patients, respectively. Most application site reactions were transient, and mild to moderate in severity.  The incidence of serious adverse events was 5% in the NGX-4010 group and 3% in the control group.  Hypertension was reported by 3% of the treatment group versus 2% of the control group.

Sponsorship: NeurogesX.

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