It's not a great Tuesday for MacroGenics Inc. (MGNX) shareholders. The biotech company's stock is taking a significant hit in premarket trading after announcing that the U.S. Food and Drug Administration has slapped a partial clinical hold on one of its key cancer studies.

Think of a clinical hold as the FDA hitting the pause button. In this case, the regulator is concerned about safety events in the Phase 2 LINNET trial, which is testing an experimental drug called lorigerlimab in patients with gynecologic cancers. The company is now in the familiar, though unpleasant, position of having to work closely with the FDA to address these concerns before it can move forward.

So, what exactly prompted the FDA to step in? The hold was initiated due to recent safety events affecting four patients. The most serious of these was a patient death, classified as a Grade 5 event. The other events were severe medical complications: two patients experienced Grade 4 thrombocytopenia (a dangerous platelet deficiency), one had Grade 4 myocarditis (inflamed heart muscles), and another suffered from Grade 4 neutropenia (critically low white blood cells) along with septic shock, which ultimately led to the fatal outcome.

As a result, MacroGenics has paused enrollment of new participants in the trial. To date, 41 people have already been dosed in the study. The trial's main goal is to measure the objective response rate—essentially, how many patients see their tumors shrink. It was designed so that enrollment could potentially be expanded if the initial results looked promising, but that plan is now on ice.

"At MacroGenics, our top priority is patient safety," said Eric Risser, the company's President and CEO. "MacroGenics is fully committed to working closely with the FDA to resolve the partial clinical hold and we intend to resume study enrollment as soon as possible."

This isn't the first time MacroGenics has faced serious safety questions in a clinical trial recently. It feels a bit like déjà vu. Back in May 2024, the company released interim data from a different Phase 2 study, called TAMARACK, for a prostate cancer drug. That update reported five patient deaths and a troubling rate of severe adverse events—over 50% in both groups of patients receiving the drug.

For investors, the timing adds another layer of uncertainty. The company's next scheduled financial update isn't until March 19, 2026. Analysts are currently expecting the company to report a loss of 18 cents per share, which is an improvement from an expected loss of 25 cents. Revenue is also projected to be higher, at $26.32 million compared to a prior estimate of $19.35 million.

In the meantime, the market is delivering its own verdict. According to market data, MacroGenics shares were down 18.29% at $1.43 during premarket trading on Tuesday.