Effect of duration of postherpetic neuralgia on efficacy analyses in a multicenter, randomized, controlled study of NGX-4010, an 8% capsaicin patch evaluated for the treatment of postherpetic neuralgia

Abstract

Background: Postherpetic neuralgia (PHN) is a painful and difficult to treat complication of acute herpes zoster. Current treatment options provide only partial relief and are often limited by poor tolerability. We evaluated the safety and efficacy of a single 60-minute application of NGX-4010, an 8% capsaicin patch, in patients with PHN.

Methods: This multicenter, double-blind, controlled study randomized 155 patients 2:1 to receive either NGX-4010 or a 0.04% capsaicin control patch. Patients were at least 18 years old with PHN for at least 3 months, and an average Numeric Pain Rating Scale (NPRS) score of 3 to 9. The primary efficacy endpoint was the percentage change in NPRS score from baseline to weeks 2-8.

Results: The mean percent reduction in "average pain for the past 24 hours" NPRS scores from baseline to weeks 2-8 was greater in the NGX-4010 group (36.5%) compared with control (29.9%) although the difference was not significant (p = 0.296). PGIC analysis demonstrated that more NGX-4010 recipients considered themselves improved (much, or very much) compared with control at weeks 8 and 12, but the differences did not reach statistical significance. Post hoc analyses of patients with PHN for at least 6 months showed significantly greater reductions in "average pain for the past 24 hours" NPRS scores from baseline to weeks 2-8 in NGX-4010 patients compared to controls (37.6% versus 23.4%; p = 0.0291). PGIC analysis in this subgroup demonstrated that significantly more NGX-4010 recipients considered themselves much or very much improved compared with control at week 12 (40% versus 20%; p = 0.0403;).

Conclusions: Although treatment appeared to be safe and well tolerated, a single 60-minute application of NGX-4010 failed to show efficacy in this study which included patients with PHN for less than 6 months. Large reductions in pain observed among control patients with pain for less than 6 months may have been due to spontaneous resolution of PHN, may have confounded the results of the prespecified analyses, and should be taken into account when designing PHN studies.

Trial registration: NCT00068081.

Figure 1

Study randomization and disposition.

Figure 2

Percent change from baseline in "average pain for the past 24 Hours" NPRS scores by week. Values shown are mean and standard error of the mean (SEM). NGX-4010 showed greater improvement in pain scores at weeks 1 through 8 but the differences between the two groups diminished over time largely due to continued improvement in the control group.

Figure 3

Change in "Pain Now" NPRS scores from pre-anesthetic time point during the treatment procedure. Values shown are mean and SEM. "Pain Now" NPRS scores decreased following topical anesthetic and increased following patch application in both treatment groups. Patients receiving NGX-4010 reported a mean maximum increase of 1.9 from the pre-anesthetic time point. NPRS scores did not increase above pre-procedure levels in those receiving control. NPRS scores in the NGX-4010 group decreased to below pre-procedure levels at 85 minutes after patch removal.