Breast cancer patients with particularly aggressive tumor profiles were treated to some good news on Thursday. Clinical trial results from Merck (NYSE:MRK) showed that adding Keytruda to standard chemotherapy reduced those patients' risk of disease progression in the first-line treatment setting.

A win for triple-negative breast cancer patients

Targeted cancer therapies work well for breast cancer patients with tumors that test positive for estrogen receptor, progesterone receptor, or human epidermal growth factor receptor 2 (HER2). Unfortunately, around 15% to 20% of all breast cancer patients have tumors that don't express any of those targets -- thus their designation as "triple-negative" breast cancers. Merck's Keynote-355 trial tested Keytruda as a treatment for triple-negative breast cancer (TNBC) patients who have significant amounts of programmed death-ligand 1 (PD-L1) on their tumors, and the results of the study are compelling.

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Roughly two-fifths of all TNBC patients have PD-L1 positive tumors, and it looks like Keytruda makes it harder for those tumors to shut down the immune system when it attacks them. Patients with PD-L1 combined positive scores above 10 who were treated with a combination of Keytruda and chemotherapy were 35% less likely to die or show signs of disease progression than patients treated with standard chemotherapy on its own.

Treatment duration

Keytruda is already on pace to generate sales north of $15 billion in 2020; an approval to treat TNBC patients in the first-line setting could lead to several billion dollars in additional annual revenue. Patients generally respond longest to their first line of treatment, and TNBC patients diagnosed early could wind up taking Keytruda plus chemo for a long time.

During this interim assessment of the Keynote-355 trial, a majority of patients in the Keytruda plus chemo group had been on therapy for 25.9 months; the trial still hasn't run long enough to produce overall survival data.