I learned this in my own treatment of PH+ ALL. In reality they almost started on a chemo regimen intended for AML but waited for results of one test, it showed I didn't have AML. My treatment was dramatically changed, the chemo began nearly a day later.
Even today, approaching 6 years post stem cells I'm still on a form of pill chemo that was intended for AML and I don't believe has been in clinical trials for my form of ALL, but the Doctors know it works, and the insurance companies don't balk. My Dr. who heads hematology at City of Hope has been involved with the development of many drugs. He's made it clear to me that much that's commonly done is off label. In just a few of the meds he's prescribed over the years he needed to fight a bit for insurance clearance, normally it didn't take more than a day or two.
I cannot say if our AML drug will prove effective in other forms of leukemia, but I'd be willing to bet that if other drugs fail to help a patient, it will be tried. Many of the patients I've met at City of Hope had been given no hope where they had been treated. Several of them were people I just happened to meet on the one day a year they have an appointment there, it's more of a reunion, but they do run tests to assure that all's well even after decades. I believe that for those traveling substantial distances the appointments are often schedule within days of their BMT reunions so they can attend both with a single trip. The reunion brings in several thousands, last year it was done virtually, I hope it won't be this year.
My quarterly appointments at COH have become phone conferences after coming in for labs. Hopefully it won't be that much longer. I'd rather be there live in asking my Dr. about their trial of our drug, it's being done by someone on his staff who did see me a few times in my Drs. absents.