Shares in Pfizer have climbed after regulators in the USA agreed to fast-track its new breast cancer drug palbociclib.

Specifically, palbociclib, an oral and selective inhibitor of cyclin dependent kinases (CDK) 4 and 6, has received 'breakthrough therapy designation' by the US Food and Drug Administration for the potential treatment of patients with breast cancer. The latter, a recent innovation by the agency, is granted to expedite the development and review of a medicine if it is “intended, alone or in combination with one or more other drugs, to treat a serious or life-threatening disease", and if preliminary clinical evidence "indicates that the drug may demonstrate substantial improvement over existing therapies".

Breakthrough therapy designation is distinct from the FDA’s other mechanisms such as accelerated approval and priority review, although these can also be granted if relevant criteria are met. It has been granted to palbociclib on the basis of preliminary Phase II data presented at the San Antonio Breast Cancer Symposium in December.

That interim data showed that women treated with the combination of palbociclib plus letrozole achieved a statistically significant improvement in median progression free survival compared to those on letrozole alone (26.1 months versus 7.5 months). Pfizer has initiated a Phase III study evaluating the combo versus letrozole alone as a first-line treatment for post-menopausal patients with ER+, HER2- locally-advanced breast cancer, which represents about 60% of cases.

The news has gone down well with investors (Pfizer ended the day up 2.8% at $29.92) and analysts. Seamus Fernandez at Leerink Swann issued a research note saying that "given median PFS of eight-ten months with letrozole in the first-line setting, we assume data at 18 months (possible by late 2014/early 2015) would be sufficient for approval".

He believes palbociclib could be a $5 billion drug, with potential for sales of $3 billion in first-line metastatic breast cancer alone.