US regulators have agreed to undertake a speedy review of Pfizer’s breast cancer drug palbociclib, signalling their belief that it could offer a major treatment advance for patients.

The US Food and Drug Administration has assigned a Priority Review status to the application to market the drug, which means its evaluation process should take just six months rather than the standard 10 months.

Pfizer is seeking approval for palbociclib, in combination with the aromatase inhibitor letrozole, to treat postmenopausal women with oestrogen receptor positive (ER+), human epidermal growth factor receptor 2 negative (HER2-) advanced breast cancer, but only in those who have not received previous systemic treatment for it.

The submission is based on data from the Phase II PALOMA-1 trial, which showed that first-line treatment with the palbociclib/letrozole combination boosted progression-free survival (PFS) by around 50% compared to letrozole alone (PFS of 20.2 months versus 10.2 months, respectively).

Palbociclib, a first-in-class cyclin-dependent kinase 4/6 inhibitor that inhibits cell proliferation, was also awarded breakthrough therapy designation in the US in April 2013, after interim data showed also showed a statistically significant improvement in median PFS compared to letrozole alone (26.1 months vs 7.5 months).

Pfizer is expecting a final decision from the FDA by April 13, 3015.