Pfizer has presented detailed mid-stage data on its closely-watched breast cancer drug palbociclib showing that it significantly prolonged progression-free survival when combined with the aromatase inhibitor letrozole.

For women treated with the combo in the Paloma-1 Phase II study, median PFS was 20.2 months compared to 10.2 months for those who received letrozole alone. An initial assessment of overall survival, a secondary endpoint, was also performed, showing a median of 37.5 months versus 33.3 months, which was not statistically significant.

The OS data is not a major issue at present for some observers, given the relatively low number of study participants who have died. A follow-up OS analysis will be conducted to determine whether statistical significance is reached later on.

Commenting on the data, which was presented at the American Association for Cancer Research meeting in San Diego, Mace Rothenberg, chief medical officer for Pfizer's oncology unit, said they demonstrate the potential of palbociclib "to be a major advance in the treatment of women with this type of advanced breast cancer". The drug belongs to a promising new class of investigational anticancer agents that selectively inhibit cyclin-dependent kinases (CDKs) 4 and 6 and it is in two Phase III studies in metastatic breast cancer.

Dennis Slamon, a professor of medicine at UCLA, was very enthusiastic, saying that "the potential impact of this study could be huge". He noted that "we are doing further Phase III work with the drug, but the current data are as exciting as the initial studies we were involved in when testing Herceptin (trastuzumab).