Multifactor analysis still requires an underlying commonality. People taking multiple drugs are all still taking the drug being trialed. You’re removing the confounding factors. If every treatment is a unique cancer protein there is no common factor. The treatment is the confounding factor.

To put it another way. A safety trial has to prove that any protein administered is safe.

Edit: just realised you’re probably talking about efficacy trials, whereas I’m more concerned with safety.