In the phase II PALOMA-1/TRIO-18 study, patients with estrogen receptor (ER)—positive postmenopausal breast cancer were randomized to receive first-line letrozole plus palbociclib, a cyclin-dependent kinase 4/6 (CDK4/6) inhibitor, or letrozole alone, says Richard Finn, MD. Progression-free survival (PFS) was 20 months versus 10.2 months, respectively, for the combination versus the single-agent. Based on these data, the FDA approved palbociclib plus letrozole in February 2015.

Several phase III studies are continuing to look at frontline use of palbociclib. The phase III PALOMA-3 trial examining the palbociclib regimen in ER-positive breast cancer was halted after an independent panel determined it met the primary endpoint of improving PFS. Full results from the study will be presented at the 2015 ASCO Annual Meeting.

The main side effect of palbociclib is neutropenia, although many patients were able to remain on the medication for a long period of time without having neutropenic fever or requiring growth factor, notes Finn. Lee Schwartzberg, MD, agrees that the toxicities he has seen in these patients have been manageable.

In addition to palbociclib, Kim Blackwell, MD, adds that there are at least 2 other CDK inhibitors currently in similar trials as the PALOMA study. These agents have also shown promising results, suggesting there could be a wealth of treatment options in the future.