Objective: Suplatast tosilate, a Th2 cytokine inhibitor, was predicted to relieve interstitial cystitis symptoms. Four studies with suplatast tosilate in Japanese interstitial cystitis patients have been conducted: a single-arm clinical study, a phase II dose-ranging trial, a phase III trial with placebo, and a second phase PIII trial with placebo. Treatment efficacy was observed in the first two studies; however, in the phase PIII trials, no significant difference in interstitial cystitis symptom score changes was observed between suplatast tosilate and placebo. We summarized these four studies to investigate factors causing the difference in observed efficacy.
Methods: Placebo effects in the first two studies and differences regarding study design between the four studies were considered to be possible factors. Therefore, placebo effects were investigated by comparing interstitial cystitis symptom score changes, and the study designs were compared to investigate the effects on observed efficacy.
Results: Interstitial cystitis symptom score changes in the phase PII treatment groups increased in a dose-dependent manner and showed an almost linear relationship with interstitial cystitis symptom score changes observed in placebo groups of 2 phase PIII studies. A major difference regarding the phase PIII study design was the use of diagnostic hydrodistention. Diagnostic hydrodistention and its washout period were applied only in the phase PIII trials.
Conclusions: Comparison of interstitial cystitis symptom score changes suggested that the placebo effect was very small. Use of diagnostic hydrodistention was considered to be a major difference in the population characteristics of the studies and may have resulted in different observed efficacies. Diagnostic hydrodistention, which potentially influences the treatment effect, is probably not essential for trials of suplatast in interstitial cystitis patients.
Keywords: diagnostic hydrodistention; interstitial cystitis; placebo effect; suplatast tosilate.
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