Sept 9 (Reuters) - An experimental Eli Lilly and Co drug taken with standard therapy reduced the risk of disease progression by 46 percent versus standard therapy alone in women not previously treated for advanced breast cancer, according to interim data from a late stage study presented at a medical meeting.
Adding the drug, abemaciclib, led to significant tumor shrinkage in 59 percent of patients compared with 44 percent for the control group in the ongoing study dubbed Monarch-3, researchers reported.
For the study's main goal of progression-free survival (PFS), women who received standard endocrine therapy of letrozole or anastrazole on average went 14.7 months before disease worsening. Median PFS had not yet been reached for the abemaciclib group after 18 months of treatment.
The overall survival rate also was not yet determined for the Lilly drug.
"These data early on are very promising," said Dr. Matthew Goetz, the study's co-primary investigator from Mayo Clinic.
Abemaciclib belongs to a new class of oral medicines called CDK 4/6 inhibitors that block cancer cells' ability to divide and proliferate. If approved, as expected, it will compete with Ibrance from Pfizer and Novartis' Kisqali.
It is forecast to reach annual sales of $1.8 billion by 2023, according to Thomson Reuters data.
Monarch-3 enrolled 493 postmenopausal women who had not yet received systemic treatment for advanced breast cancer that had spread.
Five patients, or about 2 percent, who received abemaciclib experienced complete responses, meaning no detectable cancer, a number that could rise, researchers said.
"The longer you're on (these drugs) the greater the chance that you can have complete responses," Goetz said.
The data was being presented on Sunday at ESMO, the European cancer meeting in Madrid.
Researchers reported substantial benefit from abemaciclib in patients with particularly challenging metastases, such as tumors in the liver or lungs. However, they said patients with bone metastases had a good prognosis with endocrine therapy alone.
Abemaciclib is given continuously rather than the on-and-off cycles used for its approved rivals as it does not appear to cause as high rates of neutropenia, a suppression of white blood cells that can lead to infection, Goetz explained.
The most prevalent abemaciclib adverse side effect was diarrhea, which researchers said was easily controlled by medicine or dose reduction.
"This trial gives us full confirmation that CDK 4/6 inhibitors are here to stay. They're a new standard of care for breast cancer," Goetz said. (Reporting by Bill Berkrot; Editing by James Dalgleish)