US regulators have granted accelerated approval to Pfizer’s first-in-class cyclin-dependent kinase 4/6 inhibitor Ibrance (palbociclib), offering a novel approach in the treatment of postmenopausal women with certain forms of advanced breast cancer.

On the back of impressive mid-stage progression-free survival data, the drug has been approved for use in alongside letrozole to treat postmenopausal women with oestrogen receptor-positive, human epidermal growth factor receptor 2-negative (ER+/HER2-) advanced breast cancer as initial endocrine-based therapy for their metastatic disease.

The US Food and Drug Administration said it would undertake a priority review of Pfizer’s application for Ibrance back in October, after Phase II data from the PALOMA-1 trial showed first-line treatment with the palbociclib/letrozole combination boosted PFS by around 50% compared to letrozole alone (20.2 months versus 10.2 months, respectively).

The most common side effects were a decrease in infection-fighting white blood cells called neutrophils, low levels of white blood cells, fatigue, low red blood cell counts, upper respiratory infection, nausea, inflammation of the lining of the mouth, hair loss, diarrhoea, low blood platelet counts, decreased appetite, vomiting, lack of energy and strength, damage to the peripheral nerves and nosebleed. Healthcare professionals should inform patients of these risks, the regulator said.

Breast cancer in women is the second most common type of cancer in the US. The National Cancer Institute estimates that 232,670 American women were diagnosed with breast cancer and 40,000 died from the disease in 2014.