A study presented at the 2016 American Society of Clinical Oncology annual meeting confirmed the practice-changing findings that led to the U.S. Food and Drug Administration approval of palbociclib in combination with letrozole for first-line treatment in women with estrogen receptor -positive, HER2-negative advanced breast cancer.

Researchers in the new, phase III trial studied the treatment in 666 women and found a greater than 10-month improvement in progression-free survival compared to letrozole alone (24 months versus 14.5 months). This is equivalent to a 42% reduction in risk. The previous trial also found a 10-month improvement, demonstrating a consistent clinical benefit.

The most common adverse events were neutropenia (1.8%) in the combination treatment arm and fatigue (0.9%) in the letrozole-alone arm. Serious adverse events occurred in less than 1% of patients in either arm, except for febrile neutropenia, which occurred in 1.6% of the palbociclib arm, and pulmonary embolism, which occurred in 0.9% of the treatment arm and 1.4% of the control arm.