CDK 4/6 Inhibitors - Part 2

According to a clinical trial, the combination therapy of ribociclib and letrozole increased the likelihood of progression free survival to 63% in the first 18 months in comparison to 42% with letrozole alone (3). Palbociclib and letrozole in combination slowed progression of the cancer by 26.1 months compared to 7.5 months with letrozole alone. Letrozole is ineffective in premenopausal women since the main source of estrogen is from the ovaries and not the peripheral tissues (4). The logic behind these combination therapies is that if more growth and survival pathways are blocked in tumors, they will have more difficulty with proliferation. Clinically, side effects of these CDK 4/6 inhibitors seem to be less severe than those of chemotherapies (3). Diarrhea is more common in abemaciclib than ribo- and palbociclib, while neutropenia is most associated with palbociclib. Fatigue is more common in abema- and palbociclib than ribociclib. However, patients are able to switch which CDK 4/6 inhibitor they are taking based on the side effects they experience.