Studies of two other drugs in the same class, which also stop progression of breast cancer, have not found a survival benefit. Research on them is still underway.
Dr. Tripathy said it was not clear whether the survival benefit from ribociclib was unique or would eventually show up with the other drugs, too. He added that it was also not known whether ribociclib might lengthen survival in postmenopausal patients as well as younger ones.
The study included 672 women aged 18 to 59, with 72 percent aged 40 or older. All had advanced breast cancer, meaning it had recurred after treatment or had begun to spread, and was no longer considered curable. All had estrogen-sensitive tumors.
In the past, the standard treatment for women in that situation involved hormonal treatments such as tamoxifen, to block the effects of estrogen, or drugs called aromatase inhibitors, to help stop estrogen production. Premenopausal women taking aromatase inhibitors need another drug as well, to shut down their ovaries.
But in women with advanced cancer, the hormonal therapy often stops working after a year or so.
All the women in the study got some form of hormonal treatment, and half were picked at random to also take ribociclib.
Sheila Hidalgo, 50, of Orange, Tex., was a participant. Breast cancer, first diagnosed in 2009 when she was 40, had spread to her liver, lungs and bones.
She took the Novartis drug from January 2015 until September 2017. During that time, a tumor in her lung and several in her liver disappeared, and a large one in her liver shrank from more than 6 centimeters down to half a centimeter, she said.