Palbociclib for previously treated hormone-dependent breast cancer

The authors aimed to determine whether palbociclib (Ibrance) could help treat women hormone-receptor positive (HR+) and human epidermal growth factor 2 negative (HER2-) metastatic (cancer has spread) breast cancer.

The addition of palbociclib showed consistent success against breast cancer that had become resistant to hormone therapy.

HR+ breast cancers are those dependent on the hormones estrogen or progesterone for growth. A main treatment option for patients with HR+ breast cancer is hormone therapy. In this type of therapy, the activity or production of these hormones are blocked, decreasing cancer growth. Some patients will become resistant to this type of treatment, however.

Palbociclib is a therapy designed to inhibit (block) certain proteins involved in cell growth in HR+, HER2- (not dependent on HER2 for growth) breast cancer. The combination of palbociclib and fulvestrant (Faslodex, another hormone therapy) has already been shown to be effective. It is not fully understood, however, if previous treatments used by patients (such as hormone therapy) affects the effectiveness of palbociclib.

The aim of this study was to determine the effects of palbociclib on women with HR+, HER2- breast cancer who had been previously treated with hormone therapy.

A total of 521 women with HR+, HER2- breast cancer were evaluated for this study. 347 women were randomly assigned to receive fulvestrant and palbociclib. The remaining 147 received fulvestrant with a placebo (substance with no effect on the body). 40% of the palbociclib group and 35% of the placebo received two previous lines of endocrine therapy. 67% had cancer that had spread to more than 2 other sites.

Average progression-free survival (PFS; time from treatment until disease progression) was longer in the group who received palbociclib (9.5 months) compared to those who received the placebo (4.6 months). Patients treated with palbociclib were 54% more likely to have a longer PFS than those treated with placebo. There was no significant difference in PFS due to previous hormone therapy. There was also no difference in PFS in women with a certain genetic mutation (permanent change in the PIK3CA gene) that could affect hormone therapy sensitivity.

Serious side effects were experienced by 73% of patients treated with palbociclib and 22% treated with placebo. The most common severe side effect was low levels of white blood cells. This occurred in 65% of those treated with palbociclib and 1% of those treated with placebo.

The authors confirmed that pablociclib used with fulvestrant significantly improved PFS for women with HR+, HER2- breast cancer previously treated with hormone therapy.

This study was funded by Pfizer, the manufacturer of palbociclib.