@snowonn1
I'm sure each of our breast cancers have their own nuances that may make an oncologist choose one treatment plan over another. My oncologist did say if Kisqali stopped working, he may try Verzenio next. That's curious since they seem so similar, but there are some differences.

Here is an article I found today comparing data for Verzenio (Abemaciclib), Ibrance (Palbociclib) and Kisqali (Ribociclib). Some seem to perform better in one area and others in another area. I'll let you draw your own conclusions. This may help with questions for your oncologist. It would be nice to have a single study that included all three drugs so the study criteria, researchers, etc. would be identical for all three.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10217927/

@snowonn1 in response to your question about my cancer.

2013: Stage 1 BC, BRCA2+, double mastectomy, Ki-67 17% (moderately aggressive) ER+ PR+ HER2-. Told 3% chance of recurrence.

2020: BC appeared in chest wall/pectoral muscle. Ki-67 50% this time which means 50% of the cells in the tumor are dividing. I was told it's stage IV. I was told due to its highly aggressive nature this time, cancer cells likely traveled by the time it was discovered. He said if we did nothing the cancer would be everywhere in a matter of months. Had surgery (positive margin left behind), 37 radiation treatments, not a candidate for chemo due to BC personality and pre-existing neuropathy, now on Kisqali and Letrozole for 2.5 years so far.

I had a neighbor (she moved) with stage IV breast cancer who was taking Ibrance. A friend of a friend with stage IV breast cancer took Verzenio and did have bad diarrhea from it. So, I'm not sure why one CDK 4/6 inhibitor is selected over the others.