Queen B.

Forget oligo-archy, this is a monarch-3. And breast cancer is queen. With this week’s publication of MONARCH-3, we are now only one short of a royal flush of cyclin-dependent kinase (CDK) 4 and 6 inhibitors proven effective for breast cancer. Together with PALOMA-2 (palbociclib) and MONALEESA-2 (ribociclib), results of MONARCH-3 (abemaciclib) represent a paradigm shift in the first-line management of advanced estrogen receptor (ER)-positive breast cancer. So with all of these options, how are we supposed to choose? Both palbociclib and ribociclib seem to cause higher rates of myelosuppression, particularly neutropenia. Abemaciclib, on the other hand, appears to cause a lot more diarrhea. Another important factor to keep in mind is cost: Currently there are no known biomarkers to select which patients are set to benefit (i.e., expand the cost-benefit ratio) most with the addition of CDK4/6 inhibition. But MONARCH-3’s exploratory analyses indicate that patients with long disease-free intervals and/or bone-only mets do just as well with endocrine monotherapy. So those with more aggressive recurrences are likely to give you the most disease-free bang for your CDK4/6 buck...but that's a problem for the peasants (actually the insurers).