…answer is proving more difficult to research than I'd anticipated.
It’s a pretty hard quiz. (I wouldn’t have known the answer if I hadn’t listened to a recent AZN webcast.)
To answer the question, it’s useful to employ the automobile (gas-pedal/brake) analogy for immuno-oncology drugs. I.e., PD-(L)1 drugs rev the gas, while CTLA-4 releases the brake. Once you release the brake, it stays released, but the gas pedal must be continuously depressed or motion stops. (Cruise control is not part of this analogy, LOL.)
So, AZN thinks that four cycles of Tremelimumab are sufficient to permanently release the brake and thereby allow Durvalumab to attack the cancer (prime the gas) effectively.
“The efficient-market hypothesis may be the foremost piece of B.S. ever promulgated in any area of human knowledge!”