Safety/Efficacy Study of Retigabine vs. Placebo in Post-Herpetic Neuralgia (PHN)

Participants rated their pain during the previous 24 hours at all clinic visits using an 11-point Numerical Rating Scale (NRS): 0, no pain; 1 to 3, mild; 4 to 6, moderate; 7 to 10, severe pain (10=worst possible pain). Change in In-clinic Pain Assessment was calculated by subtracting the average score on the NRS at Week 2 and Week 4 (values for each week were observed cases) of the MP from the average score on the NRS at Baseline (the last non-missing measurement prior to taking study drug).

Participants recorded the number of acetaminophen tablets taken during the previous 24 hours in a participant diary. Rescue medication was summarized as the mean number of doses taken per day during each week of the Maintenance Phase (MP) and the mean number of doses taken during all MP weeks.

Least square mean (LSM) of pain intensity was calculated from the NRS score entered by the participants in their diaries at each week during the MP. Participants rated their pain during the previous 24 hours at all clinic visits using an NRS: 0, no pain; 1-3, mild; 4-6, moderate; 7-10 (worst possible pain), severe pain. Change from Baseline was calculated by subtracting the value of the average of the LSM of the NRS score at each week during the MP (the last 7 available diary entries in the MP were used, provided at least 3 existed) from the average Baseline value of the LSM of the NRS score.

Responders were defined as participants achieving a mean >=50% or >=30% pain reduction based on the NRS score from Baseline to the last 7 days of the MP. Those participants who did not have at least 3 diary entries in the MP, those who withdrew during the Titration Phase (TP), and non-completers (NC: who did not complete the study) were classified as non-responders. The number of responders and responders including non-completers from Baseline to the last 7 days of the MP were reported.

Change from BL (average value of MOS Sleep Scale score including Overall Sleep Problem [OSP] Index at end of MP minus average BL value) to the end of the MP was summarized for the OSP Index, in addition to the following subscales of the MOS Sleep Scale: Sleep Disturbance; Sleep Adequacy; Snoring; Awakening with Shortness of Breath or with a Headache; Somnolence; and Optimal Sleep. Each item was transformed to a scale with a range of 0-100. For each subscale, except Optimal Sleep, higher scores indicate a greater level of what was being measured (i.e., more snoring, more sleep adequacy, etc.).

Change from Baseline in sleep quantity was calculated by subtracting the average value of sleep quantity, calculated in hours, at the end of the MP from the average Baseline value.

Optimal Sleep was based on the Sleep Quantity domain of the MOS Sleep Scale and included the options of "Yes" if sleep quantity was 7-8 hours, and "No" otherwise. "Improved" indicated a change in response of no at baseline to yes at the end of the MP, "Same" indicated no change in response, and "Worse" indicated a change in response from yes at baseline to no at the end of the MP.

For the PGIC assessment, participants were asked to assess their overall status since they initiated the study drug to the end of the Maintenance Phase using a 7-point categorical scale: 1=very much improved, 2=much improved, 3=minimally improved, 4=no change, 5=minimally worse, 6=much worse, and 7=very much worse. All participants who completed Week 4 of the Maintenance Phase and participants who terminated early during the Maintenance Phase were assessed.

The TSQM assessed the participant's overall satisfaction with treatment, including subscales to assess effectiveness, side effects, convenience, and global satisfaction. Raw scores from the scale were transformed into a numeric scale ranging from 0 to 100, where higher scores indicated greater satisfaction with treatment. TSQM was reported at the end of the MP. Participants who completed Week 4 of the MP and who terminated early during the MP were assessed.

The BPI-SF assessed pain intensity, pain relief from medication, and pain interference with function over the previous 24 hours. Pain intensity was assessed by the mean of 4 intensity items rated on a 0-10 categorical scale: 0=no pain, 10=pain as bad as you can imagine. Pain interference was assessed by determining the mean of the 7 interference items on a 0-10 categorical scale ranging from 0=does not interfere to 10=completely interferes. The level of pain relief provided by treatment was assessed on an 11-point categorical scale ranging from 0% to 100%

Least square (LS) mean calculated based on participant's assessment of SF-36,a quality of life questionnaire consisting of 36 items grouped into 8 domains. These 8 domains further grouped into 2 overall summary measures,physical health and mental health. Higher scores on SF-36 represented better state of health. Physical/mental components summarized the data of all the physical/mental domains of SF-36 and higher scores represented better state of health.

The least square (LS) mean score for reported health transition was calculated based on the scores (ranging from 1 to 5) given by the participant in answer to the following question: "Compared to 1 year ago, how would you rate your health in general now?". Lower numbers represent a better state of health.

At Baseline, the investigator conducted the Neuropathic Pain Physical Examination (NPPE) to examine hyperalgesia and allodynia (tactile and cold). Tactile allodynia was assessed with a foam brush, based on the question:"How painful was the affected side compared to the opposite side?".

At Baseline the investigator conducted the NPPE to examine hyperalgesia and allodynia (tactile and cold). Cold threshold and allodynia was assessed to determine if a metal bar felt cool and if it felt painful, based on the questions:"Is it cool?" and "Is it painful?"

At Baseline the investigator conducted the NPPE to examine hyperalgesia and allodynia (tactile and cold). Hyperalgesia is defined as increased sensitivity to pain, which may be caused by damage to peripheral nerves. It was assessed with a pinprick brush, based on the questions: "How sharp was the affected side compared to the opposite side?" and "How painfule was the affected side compared to the opposite side?"

At the end of the MP, the investigator conducted the NPPE (an examination of the effect of retigabine on sensory abnormalities) to examine hyperalgesia and allodynia (tactile and cold). Tactile allodynia was assessed with a foam brush, based on the question:"How painful was the affected side compared to the opposite side?" End of Maintenance Phase includes participants who completed Week 4 of the MP and who terminated early during the MP.

At the end of the MP, the investigator conducted the NPPE (an examination of the effect of retigabine on sensory abnormalities) to examine hyperalgesia and allodynia (tactile and cold). Cold threshold and allodynia was assessed to determine if a metal bar felt cool and if it felt painful, based on the questions:"Is it cool?" and "Is it painful?"

At the end of the MP, the investigator conducted the NPPE (an examination of the effect of retigabine on sensory abnormalities) to examine hyperalgesia and allodynia (tactile and cold). Hyperalgesia is defined as increased sensitivity to pain, which may be caused by damage to peripheral nerves. It was assessed with a pinprick brush, based on the questions:"How sharp was the affected side compared to the opposite side?" and How painful was the affected side compared to the opposite side?"

Participants were stratified into four different PHN subtypes based on the NPPE. Participants with pain, abnormal sensitization of the specific receptor (irritable nociceptors), and with minimal sensory loss were stratified in the"irritable nociceptors" subtype. Based on their pain experienced during the previous 24 hours, participants assessed their pain every evening at bedtime in an electronic diary by choosing the appropriate number on an 11-point NRS: 0, no pain; 1 to 3, mild; 4 to 6, moderate; 7 to 10, severe pain.

Participants stratified into 4 different PHN subtypes based on NPPE. Participants with marked sensory loss associated with severe burning pain upon slight mechanical stimuli (allodynia) were stratified in the "deafferentation type 1" subtype. Based on their pain experienced during the previous 24 hours, participants assessed their pain every evening at bedtime in an electronic diary by choosing the appropriate number on an 11-point NRS: 0, no pain; 1 to 3, mild; 4 to 6, moderate; 7 to 10, severe pain.

Participants stratified into 4 different PHN subtypes based on NPPE. Participants with marked sensory loss and severe spontaneous burning pain without allodynia associated with reorganization of central nerve fibers were stratified in the "deafferentation type 2" subtype. Based on their pain experienced during the previous 24 hours, participants assessed their pain every evening at bedtime in an electronic diary by choosing the appropriate number on an 11-point NRS: 0, no pain; 1 to 3, mild; 4 to 6, moderate; 7 to 10, severe pain.

Participants stratified into 4 different PHN subtypes based on NPPE. Participants who could not be specifically differentiated in any of the other three subtypes were stratified as "unclassifiable" meaning that the participants could not be classified into any of the predefined PHN subtypes. Based on their pain experienced during the previous 24 hours, participants assessed their pain every evening at bedtime in an electronic diary by choosing the appropriate number on an 11-point NRS: 0, no pain; 1 to 3, mild; 4 to 6, moderate; 7 to 10, severe pain.