José Baselga, MD, PhD, Physician-in-Chief, Memorial Sloan-Kettering Cancer Center, compares results from two trials analyzing the mTOR inhibitors temsirolimus and everolimus in breast cancer.

A phase II trial looked at a 25 mg/week dose of temsirolimus in patients with pretreated breast cancer who tested positive for ER, PR or HER2. The study found that temsirolimus as a single agent has minimal activity. However, the BOLERO-2 study that analyzed everolimus at 10 mg/day and exemestane at 25 mg/day for postmenopausal patients achieved its primary endpoint of progression-free survival.

Baselga says that the oncology community is trying to determine why temsirolimus and everolimus, both mTOR inhibitors, produced vastly different results in similar studies.

Baselga suggests that everolimus could only be working in patients with refractory disease. A more likely reason of the variance in results, according to Baselga, is dosing differences, which could explain the lower rate of mucositis in the temsirolimus study.