In a clinical trial for an experimental breast cancer drug made by Pfizer, the number of patients who lived without the disease worsening doubled, but overall survival was not quite as significantly improved.

According to Reuters, women who took letrozole, a hormone drug, alone lived for 10.2 months before their breast cancer progressed in the Phase 2 study. Women who took Pfizer's palbociclib along with letrozole lived for about 20.2 months before the disease progressed.

The U.S. Food and Drug Administration (FDA) felt the results were impressive enough to warrant a "breakthrough" label. However, the researchers noted that the drug has not yet proven it can consistently help patients survive breast cancer.

"The curves are starting to separate... It hasn't reached statistical significance, but patients are still being followed," study lead author Dr. Richard Finn, a UCLA associate professor of medicine, told Reuters.

The drug is part of a new class that specifically tries to block the proteins of tumors. The drug may also eventually be adapted to treat other forms of cancer. Pfizer's new experiment is designed to treat cancers that are estrogen receptor positive, the most common form of breast cancer.

"This hits a hijacked molecule that could be in many different cancers. There's data in prostate cancer, lung cancer, sarcoma that this drug has benefits. So [it has] potential for really making an impact on cancer," CBS News' medical contributor, who was not involved in the study, Dr. David Agus, director of the Westside Cancer Center at the University of Southern California, said on "CBS This Morning."

Breast cancer is the most common form of cancer among women, killing about 40,000 per year, according to the most recent data from the Centers for Disease Control and Prevention.

"We took a new drug and asked how it works in a specific set of breast cancers based on science," Finn told Bloomberg News. "This is really the type of benefit we're aiming for, and it's the type of benefit that results from a rational development program."