A first interim analysis, conducted in the unselected patient population and previously reported at the IMPAKT Breast Cancer Conference, showed that progression-free survival, the trial’s primary endpoint, was significantly better with the combination than with letrozole alone (hazard ratio, 0.35; P = .006).
A second interim analysis, conducted in the entire trial population and the one being reported in San Antonio, showed that median progression-free survival was 26.1 months with the combination versus 7.5 months with letrozole alone (hazard ratio, 0.37; P less than .001), according to Dr. Finn.
The combination was also associated with a better overall response rate both in the entire trial population (34% vs. 26%) and in the subset having measurable disease (45% vs. 31%).
Dose modifications were more common with addition of PD 0332991. The rate of treatment discontinuation because of adverse events was 10% with the combination vs. 1% with letrozole monotherapy.
Compared with monotherapy, the combination was associated with higher rates of grade 3/4 neutropenia (51% vs. 1%), leukopenia (14% vs. 0%), and anemia (5% vs. 0%).
However, "I need to stress that this was not clinically significant neutropenia," Dr. Finn commented. "There was no evidence of neutropenic fever, there was no use of growth factors in this study either."
Alopecia was more common with the combination as well, but all cases were of grade 1 severity.
Dr. Finn disclosed that he receives research from funding from Novartis Pharmaceuticals. The trial was sponsored by Pfizer.