Immunotherapy is indeed the most promising approach to cancer therapy so I'm optimistic, but the odds of success here are 1-in-20 if this is an average study, or perhaps 1-in-5 at best.
You certainly have chosen the right user name, but it is a logical error to use the success rate of all cancer trials to calculate the chance of success of this immunotherapy trial.
Immunotherapy is indeed the most promising approach to cancer therapy so I'm optimistic, but the odds of success here are 1-in-20 if this is an average study, or perhaps 1-in-5 at best.