First thing to remember: cancer drugs attack human cells. Because of this they can very unexpected and traumatic side-effects.

Because of this initial trials consume lots of medical staff to deal with the potential side effects. Normal side effects for cancer treatments include:

  * Your gut lining dissolves, your shit leaks into your body cavity, and you get sepsis.

  * Your heart stops during the infusion.

  * Cumulative poisoning that nobody understands. (E.g. some agents have lifetime limits, and if you go beyond that, then you die. Guess how we found out.)

  * Your immune system, and you get things like disseminated fungal infections.

The danger of side-effects like this requires a medical team largely dedicated to the experimental patients.

This puts a limit on how many patients you can put into a trial. I'm under the impression that cancer trials are pretty much always full.