Merck & Co.'s (NYSE:MRK) cancer-fighting drug Keytruda has been on a roll lately, delivering top-notch efficacy and winning FDA approval for use across a host of cancer indications. The company's winning streak, however, could be in jeopardy. On Monday, management halted enrollment in two studies evaluating Keytruda's use in multiple myeloma after a monitoring committee raised questions regarding "more reports of death" in patients receiving Keytruda.

Before we discuss these two studies, let's examine the drug itself.

What is Keytruda?

Keytruda is a checkpoint inhibitor that halts the activity of PD-1, a protein used by the immune system's T-cells to decide whether or not a cell should be destroyed or left alone.

A businessman holds up his hand to signal a stop.

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Because cancer cells can trick the immune system into thinking they're healthy cells by activating PD-1, Keytruda shuts off the protein, allowing more cancer cells to be found, and thus, destroyed.

Does it work?

So far, Keytruda has been very effective in cancer patients across a variety of indications. It's already won FDA approval for use in melanoma, non-small cell lung cancer, head and neck cancer, Hodgkin lymphoma, and bladder cancer.

Initially, Keytruda's approvals have been for its use after cancer has returned following existing standard of care. However, Keytruda's use earlier in patient treatment is growing. For instance, Keytruda was shown to be effective as a first-line lung cancer treatment last year, and the FDA approved its use in that indication last October.

More recently, the FDA approved its use in patients with a specific biomarker, regardless of tumor location. That approval marked the first time regulators have approved a cancer drug based on a biomarker rather than a cancer's origin.

Keytruda's broad efficacy has led to the company conducting dozens of studies that could further expand its use, including combination studies wherein Keytruda is being matched up with cancer drugs made by other companies. For instance, the KEYNOTE-183 and KEYNOTE-185 multiple myeloma trials are pairing Keytruda up with Pomalyst and dexamethasone (a chemotherapy), and Revlimid and dexamethasone, respectively.

Hitting a speed bump

Multiple myeloma is a multibillion-dollar indication, so success in KEYNOTE-183 and KEYNOTE-185 could significantly move the needle for Merck. Unfortunately, the future of these two trials is in question following news that Merck & Co. is pausing enrollment based on a recommendation from the trials' external data monitoring committee.

KEYNOTE-183 is evaluating Keytruda, Pomalyst, and dexamethasone in patients whose disease has returned or failed to improve following two prior treatments, while KEYNOTE-185 is evaluating Keytruda, Revlimid, and dexamethasone in newly diagnosed patients who aren't candidates for autologous stem cell transplant, a common treatment approach.

The committee recommended halting enrollment so that the company could collect additional data related to patient deaths in the Keytruda arms of these studies. Merck & Co. didn't offer up much insight into the deaths in its press release, but we can assume that the number of deaths in the Keytruda arm were unexpected based on what was happening in the control arm of the studies.

An eye on the future

We won't know the extent of Keytruda's blame until the company fully explores the data, so it's too early to know for sure if Keytruda's hopes in multiple myeloma are dashed. It's certainly possible the deaths were due to a trial population that's gravely ill, but it's also possible that Keytruda causes off-target activity in multiple myeloma patients that makes it unsafe in this indication.

If Keytruda is to blame, then it will be important to understand whether the risks are class-specific across all PD-1 drugs. If they are, that could derail hopes for competitors, too, including Bristol-Myers Squibb's Opdivo, another PD-1 inhibitor.

In any event, Keytruda's stumble is a good reminder that cancer drug development isn't easy, and that success in one indication never guarantees success in another. That being said, investors will want to keep this news in perspective. After all, Keytruda's significant wins in other cancers are fueling big sales and profit growth at Merck & Co., and that's unlikely to dramatically change unless safety risks emerge in these other indications, too.

Todd Campbell has no position in any stocks mentioned. His clients may have positions in the companies mentioned. The Motley Fool has no position in any of the stocks mentioned. The Motley Fool has a disclosure policy.