Sample size calculation

Hi everyone!

Can anyone helps with calculation sample size for trial?

We are planned 2/3 phase with group-sequential design (two stage) with re-estimation procedure for some rare disease (to prove superiority), expected succes rate in treatment group is around 85%, and 75% for control group (based on observation result). If use standard method for calsulation size - the total samle size is around 500 subjects at final stage, and around 250 subjects at interim stage (~50%). It too large number of patients for this disease.

How may to reduce samle size? Or is there method that allow to correct/ adjust sample size by population number? or it is necesserary to imply other approach

Thank in advance!
Totally agree!
Phase 2/3 is type of seamless design in what we conduct interim analysis (in my example when 50% of total subjects are finished study), assess the primary endpoint and corresponding p-value, and according to fulitlity and efficacy boundaries, we may conclude to continued trial or stoped it, or we also re-estimate sample size