The addition of the CDK4/6 inhibitor ribociclib to letrozole therapy significantly improves progression-free survival in postmenopausal women with hormone receptor-positive advanced breast cancer, researchers reported today at the European Society for Mediical Oncology (ESMO) 2016 Congress in Copenhagen. The first interim analysis of data from the randomized, double-blind MONALEESA2 study showed a 44% improvement in progression-free survival with ribociclib plus letrozole as a first-line treatment combination. "This was THE definitive study to demonstrate the superiority of the combination of ribociclib and letrozole over letrozole alone," said principle investigator, Professor Gabriel Hortobagyi, from the University of Texas MD Anderson Cancer Center in Houston, Texas, US. Researchers randomized 668 postmenopausal women with hormone receptor-positive, HER2-negative advanced breast cancer, who had not undergone any prior systemic treatment, to ribociclib (600 mg/day, 3 weeks on/1 week off) and letrozole (2.5 mg/day, continuous), or letrozole plus placebo.In the ribociclib arm, there was a 44% improvement in the primary objective of progression-free survival compared to the placebo arm (HR: 0.556, p = 0.00000329). Median progression-free survival was 14.7 months in the placebo arm, but was not reached in the ribociclib arm at data cut-off. "The results of this trial represent a compelling proof of principle, and suggest a paradigm shift in metastatic, HR+ breast cancer. They also suggest that testing combinations of ribociclib with other inhibitors of various signaling pathways might lead to additional progress in the management of several subtypes of breast cancer," Dr. Hortobagyi said.
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