A new promising drug to treat metastatic breast cancer was reported by a research team of the University of California Los Angeles (UCLA). The study entitled “The cyclin-dependent kinase 4/6 inhibitor palbociclib in combination with letrozole versus letrozole alone as first-line treatment of oestrogen receptor-positive, HER2-negative, advanced breast cancer (PALOMA-1/TRIO-18): a randomized phase 2 study” was published in The Lancet Oncology. Many risk factors can contribute to the development of breast cancer, which can be based on biological markers such as the estrogen receptor-positive (ER) and the human protein growth factor receptor 2 (HER2). The research team led by Professor Dennis Slamon, which established the successful breast cancer drug Herceptin, tested the safety and efficacy of a CDK4/6 inhibitor Ibrance (palbociclib - Pfizer) in combination with letrozole, an aromatase inhibitor that lowers estrogen levels, as first-line treatment of post-menopausal women with advanced, estrogen receptor-positive, HER2-negative breast cancer. Ibrance targets cyclin D kinase 4 (CDK 4) and cyclin D kinase 6 (CDK 6) present in cancer cells and inhibits their proliferation capacity.