Pfizer announced randomized Phase 2 data that showed PD-0332991 (PD-991) in combination with letrozole (Femara; Novartis) significantly extended progression free survival (PFS) compared with letrozole alone in post-menopausal patients with estrogen receptor positive (ER+), human epidermal growth factor receptor 2 negative (HER2-) locally advanced or metastatic breast cancer. PD-991 is an investigational, oral and selective inhibitor of the CDK 4 and 6 kinases.

A total of 66 patients were enrolled in Part 1 of the study. Preliminary results of Part 1, announced in May 2012, showed statistically significant improvement in median PFS in the PD-991 plus letrozole arm vs. the letrozole arm. Part 2 enrolled 99 additional patients whose tumors were selected for presence of biomarkers: cyclin D1 amplification and/or p16 loss. The results announced today reflect the combined interim data analysis of Parts 1 and 2.

For patients treated with the combination of PD-991 plus letrozole, median PFS was 26.1 months, a statistically significant improvement compared to the median PFS in women who received letrozole alone, which was 7.5 months (HR=0.37 [95% CI: 0.21, 0.63]; P<0.001). In patients with measurable disease, the objective response rate was 45% for those women who received PD-991 plus letrozole vs. 31% for those who received letrozole alone. The clinical benefit rate (defined as complete response plus partial response plus stable disease for ≥24 weeks) was 70% vs. 44%, respectively. The differences observed in the objective response rate and clinical benefit rate were statistically significant.

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This article originally appeared on MPR