Chalk up another win for Keytruda, Merck's (NYSE:MRK) cancer drug that seems to be able to be useful in treating almost every type of cancer it's tested against. The most recent results come from a late-stage clinical trial in patients with metastatic triple-negative breast cancer with tumors that express the protein that Keytruda blocks.

Merck said results from the Keynote-355 study showed that Keytruda plus chemotherapy delayed cancer progression and kept patients alive longer than chemotherapy alone. The company did not disclose how much better the combination performed, however; it's saving the details for a presentation at a future medical meeting.

This was only an interim peek at the data, and the clinical trial will continue so it can measure overall survival, which is arguably a more important endpoint for patients and doctors. In the meantime, the pharmaceutical company plans to discuss the results with the FDA and other regulators to potentially get the drug approved to treat this type of cancer.

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Triple-negative breast cancer gets its name because the cancer cells lack receptors for both estrogen and progesterone, and do not produce excessive amounts of human epidermal growth factor receptor 2 (HER2). A large share of other cancer drugs disrupt the effects of estrogen, progesterone, or HER2 on breast cancer cells, and are therefore ineffective against the triple-negative variety. This limits the treatment options, which means that triple-negative breast cancer tends to be particularly aggressive and has a high recurrence rate.

Keytruda blocks the interaction of a protein on immune cells called programmed cell death 1 (PD-1) with a protein expressed on cancer cells (PD-L1) that tells immune cells not to attack them. By blocking the interaction, Keytruda induces the immune cells to attack the tumor. Since the mechanism is a fairly universal way that tumors avoid being targeted by the immune system, Merck has been able to show that the drug improves outcomes in more than a dozen different types of cancer.

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