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WASHINGTON -- The FDA has approved the cyclin dependent kinase (CDK)-4/6 inhibitor palbociclib (Ibrance) for treatment of advanced, hormone receptor (HR)-positive postmenopausal breast cancer.

It's the first CDK-4/6 inhibitor to be OK'd in the U.S.

The approval stipulates use of palbociclib in combination with letrozole (Femara) in patients with metastatic breast cancer who have received no prior endocrine therapy. The FDA previously designated palbociclib for breakthrough therapy status, which put the drug on the fast track to approval.

"The addition of palbociclib to letrozole provides a novel treatment option to patients diagnosed with metastatic breast cancer," , director of the FDA's Office of Hematologic and Oncology Products, said in a statement.

As previously reported, a randomized clinical trial showed that the combination of palbociclib and letrozole led to a doubling of median progression-free survival (PFS) in advanced HR-positive breast cancer from 10.2 months with letrozole alone to 20.2 months. Overall survival data remain immature, according to the FDA announcement. Initial data on median survival showed a 4-month improvement with the combination of palbociclib and letrozole, but the difference did not achieve statistical significance.

In the normal state, CDK-4/6 facilitates progression from the growth phase (G1) of the cell cycle into the S-phase, which is associated with DNA synthesis. In breast cancer and many other types of cancer, activation of CDK-4/6 leads to loss of control over the cell cycle, contributing to the cascade of epigenetic and molecular events that results in the uncontrolled cell growth that typifies cancer. Preclinical studies showed that inhibition of CDK-4/6 by palbociclib restored the normal cell-cycle process.

The most common adverse events associated with palbociclib treatment have been neutropenia, leukopenia, fatigue, and anemia.

Palbociclib is marketed by Pfizer.

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