The clinical studies used for the pooled analysis were of similar design. All were rigorous, multicenter, randomized, double-blind, parallel-design, placebo-controlled trials, and all of them used the same measurements to demonstrate the efficacy of gabapentin in the treatment of PHN. Patients were randomly assigned to receive either or placebo. They then entered a period of titration to the maximum dose and were monitored for seven to eight weeks.
In one of the two pivotal studies, 229 patients were evaluated for eight weeks. The initial gabapentin dose of 300 mg/day was increased to a maximum of 3,600 mg/day, administered as 1,200 mg three times a day. Only patients who were able to tolerate doses of 1,200 mg/day or greater were included in the analysis of this study.
The second key study evaluated 334 patients for seven weeks. The dosages were titrated to a fixed dose of either 1,800 mg/day or 2,400 mg/day and were eliminated if patients could not tolerate these fixed doses. Additional patient-reported outcomes data were obtained from 40 PHN patients in a study of various neuropathic pain syndromes; in that study, patients received gabapentin canadian doses titrated from 900 mg/day to a maximum of 2,400 mg/day.
In all of the studies, the patients were at least 18 years of age, had had pain for three or more months after healing of the zoster skin rash, and had at least moderate pain, as indicated by a score of 4 or more on a pain scale of 0 to 10 points (the Likert scale); 0 represented “no pain,” and 10 indicated the “worst possible pain.” The pooled analysis included only PHN patients who received at least 1,800 mg/day of gabapentin.
The primary efficacy measure was the weekly mean pain score on the Likert scale, as determined by averaging daily pain scores. The secondary endpoint measurements included the Short-Form McGill Pain Questionnaire (SF-MPQ). The SF-McGill Questionnaire included a 100-mm Visual Analogue Scale (VAS), in which patients marked their pain intensity along a 100-mm line; 0 mm indicated “no pain,” and 100 mm indicated the “worst possible pain.” Secondary measures also included the SF-36 Quality-of-Life Questionnaire, the patient-rated Patients’ Global Impression of Change (PGI-C) Questionnaire, and the investigator-rated Clinicians’ Global Impression of Change (CGI-C) Questionnaire.
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For the two pivotal efficacy trials, an analysis of covariance (ANCOVA) was used to analyze the intent-to-treat (ITT) population, and the Last Observation Carried Forward (LOCF) imputation rule was used to impute missing observations. “Baseline” was defined as the last seven days before administration of the study medication. The endpoint measure was the mean of the last seven days of available pain diary data. The endpoint change from baseline was the endpoint measure minus the baseline measure. If either baseline or endpoint was missing, the endpoint change from baseline was set to “missing.” For the pooled analysis, all PHN patients from the three studies were combined, and the ANCOVA and LOCF imputation rule were used to assess the combined ITT population.
The SF-36 is a validated tool for measuring patients’ quality of life. Domains were grouped and analyzed by ANCOVA using baseline values as covariate and treatment as main effect. The CGI-C and PGI-C scores were assessed on a seven-point scale by which the clinician or patient, respectively, assessed changes in the patient’s overall status. The CGI-C and PGI-C scores were combined for each category: “very much improved,” “much improved,” “minimally improved,” “no change,” or “worsened.”