In patients with measurable disease, the objective response rate was 45 percent for those women who received PD-991 plus letrozole versus 31 percent 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 percent versus 44 percent, respectively. The differences observed in the objective response rate and clinical benefit rate were statistically significant. The most frequently reported treatment-related Grade 3/4 adverse events (AEs) in patients who received the combination therapy were neutropenia, leucopenia, anemia and fatigue.Both Part 1 and Part 2 of this Phase 2 evaluation are ongoing but no longer enrolling new patients. Final efficacy and safety data are expected to be presented at a future medical congress. “In demonstrating very strong efficacy and a manageable tolerability profile, these new data represent a potential major advancement in breast cancer clinical research and our continued efforts to identify new medicines that target patients most likely to have an optimal response,” said Dr. Richard S. Finn, Associate Professor of Medicine, Revlon/UCLA Women’s Cancer Research Program at Jonsson Comprehensive Cancer Center, UCLA, and lead investigator of the Phase 2 trial. “The oncology community is looking forward to the further evaluation of PD-991 in the planned Phase 3 trial and very interested in the potential for this novel CDK 4 and 6 inhibitor to improve the treatment landscape for patients with advanced breast cancer.” About PD-991 PD-991 is an investigational, oral and selective inhibitor of the CDK 4 and 6 kinases. CDK 4 and 6 are two closely related kinases that enable tumor cell progression during phase G1 to phase S in the cell cycle. This progression is necessary for DNA replication and cell division. Inhibition of CDK 4 and 6 has been shown to prevent the deactivation of retinoblastoma, a tumor suppressor protein, and interfere with tumor cell progression. In pre-clinical studies, PD-991 was shown to be an inhibitor of cell growth and a suppressor of DNA replication by preventing cells from entering S phase.