In a clinical trial, patients with advanced breast cancer that was estrogen-receptor positive (ER+) and HER2-negative (HER2-), and who were given palbociclib (PD 0332991, Pfizer Inc.) in addition to the standard anti-estrogen treatment of letrozole had significantly higher progression-free survival — the length of time a patient is on treatment without tumor growth — than patients taking letrozole alone.
Enacted as part of the 2012 FDA Safety and Innovation Act, the breakthrough therapy designation was created by the agency to expedite the development and review of a potential new medicine if it is "intended, alone or in combination with one or more other drugs, to treat a serious of life-threatening disease or condition and preliminary clinical evidence indicates that the drug may demonstrate substantial improvement over existing therapies on one or more clinically significant endpoints."
Dr. Richard S. Finn, associate professor of medicine at the Jonsson Cancer Center, initially reported the phase 2 clinical data supporting the designation in December 2012 at the CTRC-AACR San Antonio Breast Cancer Symposium. The clinical study was built on laboratory work from the Translational Oncology Research Laboratory directed by Dr. Dennis Slamon, professor of medicine at UCLA and director of the Jonsson Cancer Center's Revlon/UCLA Women's Cancer Research Program.
In preclinical work, palbociclib was tested in a panel of human breast cancer cells growing in culture dishes and showed very encouraging activity, specifically against ER+ cancer cells. These preclinical observations were then moved into phase 1 clinical studies. Led by Finn and Slamon at UCLA, the studies were designed to determine the doses and safety of a combination with letrozole, a commonly used drug for ER+ breast cancer.
Once the phase 1 studies were completed, the phase 2 studies were performed in 165 patients with breast cancer with ER+ disease. The drug was designated as a breakthrough therapy by the FDA based on the preliminary analysis of the phase 2 data showing that the median progression-free survival of patients given the palbociclib-letrozole combination was 26.1 months, compared with 7.5 months for those given letrozole alone. Among patients with measurable disease, 45 percent receiving the combination had confirmed responses, compared with 31 percent for letrozole alone, and the clinical benefit rates (tumor shrinkage and/or stable disease for a minimum of six months) were 70 percent for those receiving the combination therapy, versus 44 percent for letrozole only.
"This drug combination demonstrated a dramatic and clinically meaningful effect on progression-free survival in women with ER+ breast cancer," Finn said. "These results confirm the preclinical work we began at the Translational Lab."
Finn and colleagues have initiated a randomized, multicenter, double-blind phase 3 study to evaluate palbociclib combined with letrozole, compared with letrozole alone, as a first-line treatment for post-menopausal patients with ER+, HER2-, locally advanced or metastatic breast cancer. The researchers will continue to work closely with Pfizer and the FDA to better understand the implications of the breakthrough therapy designation with the hope that further study will support potential regulatory submission.
Slamon said the phase 2 study results validate the Translational Laboratory's approach.
"By identifying the correct targets for treatment in the right patient population, we move forward with personalized oncology that we hope will greatly improve the outcomes for this group of breast cancer patients," he said. "These results are as exciting as the initial results we saw for trastuzumab (Herceptin) in HER2+ breast cancers but represent a new approach for a different and larger subset of breast cancers, namely those that are ER+."