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Trial Examines Treatment Combination for Metastatic Breast Cancer

Study to compare letrozole plus new medicine versus letrozole alone

Trial Starts: February 2013 Trial Ends: March 2015

Written By Nicole Katze, MA, Editor and Manager, Content Development
Reviewed By Virginia Kaklamani, MD, DSc

This trial is currently seeking participants

A phase III trial is comparing the effects of combining the aromatase inhibitor letrozole (Femara) and a new kinase inhibitor, PD-0332991 (PD-991), with the effects of letrozole alone in postmenopausal women with estrogen receptor-positive (ER positive), human epidermal growth factor receptor 2- negative (HER2 negative) metastatic breast cancer.

Aromatase inhibitors are medicines that block the enzyme aromatase to stop the body from making the estrogen that fuels tumor growth in ER positive cancer. Kinase inhibitors are medicines that target a specific kind of enzyme, kinase, that plays a part in cell division and growth.

Study Background

PD-991 works against cyclin-dependent kinase (CDK) 4 and 6, two kinases that are known to aid tumor cell growth. Positive results in early trials led researchers to test it in women with estrogen receptor-positive, HER2 negative metastatic breast cancer. 

Letrozole is a common treatment for estrogen receptor-positive breast cancer. Researchers now want to see if combining letrozole with PD-991 will better treat ER positive metastatic breast cancer.

Pfizer, the maker of PD-991, is sponsoring the trial.

Study Goals

The primary goal of this trial is to assess progression-free survival, how long the cancer stays stable. Secondary goals include assessing the combination’s safety and its impact on overall survival, how long participants live after starting treatment.

Study Design

Participants will be randomly and blindly assigned to one of two treatment arms (groups):

  • Arm I will take 125 mg of PD-991 once daily for 21 days of each 28-day cycle, along with 2.5 mg of letrozole once daily. Both PD-991 and letrozole are taken in pill form by mouth.
  • Arm II will take a placebo once daily for 21 days of each 28 day cycle, along with 2.5 mg of letrozole once daily. Both the placebo and letrozole are taken in pill form by mouth. A placebo is an inactive substance.

Progression-free survival will be assessed up to 2.5 years; overall survival will be assessed up to 6 years. This study is double-blind, which means that neither the participant nor the doctor knows which treatment is being given.


You may be eligible for this clinical trial if you:

  • Are a woman 18 years of age or older
  • Have been diagnosed with ER positive-, HER2 negative metastatic breast cancer
  • Are postmenopausal
  • Have not been treated with systemic therapy for metastatic disease
  • Have not taken letrozole or anastrozole for early-stage cancer within the last year

Study Locations

This study is being held in several locations throughout the United States, as well as in one location in Belgium. For full location details and contact information, see trial NCT01740427 on