Shares of Eli Lilly & Co. /zigman2/quotes/200106384/composite LLY +3.01% tumbled 3.1% in trading on Tuesday after the drug maker confirmed a data safety monitoring board had paused enrollment in a clinical study testing its experimental COVID-19 monoclonal antibody treatment in combination with Gilead Sciences Inc.'s /zigman2/quotes/210293917/composite GILD +1.82% remdesivir. "Safety is of the utmost importance," a company spokesperson said in an email. The New York Times had first reported Tuesday afternoon that the trial had been paused due to safety concerns. The trial, which is sponsored by the National Institutes of Health, is testing Lilly's monoclonal antibody treatment, LY-CoV555, in combination with remdesivir, an antiviral drug that has demonstrated that it can reduce recovery times in COVID-19 patients. Lilly said last week that it had submitted a request for an emergency use authorization to the Food and Drug Administration for LY-CoV555 as a standalone treatment based on findings from a separate trial evaluating the investigational therapy in patients with mild to moderate symptoms. The pause is notable for two reasons. Lilly's investigational antibody drug is similar to the Regeneron Pharmaceuticals Inc.'s experimental antibody treatment that was prescribed to President Donald Trump. It is also the third major clinical trial in this pandemic to be paused for safety reasons, which experts say is a common occurrence but likely one being scrutinized given the lack of treatment or prevention options against the coronavirus. Stat News reported Monday that Johnson & Johnson /zigman2/quotes/201724570/composite JNJ +1.49% had paused its COVID-19 vaccine trial over an "unexplained illness," and AstraZeneca had halted a trial for its vaccine candidate in September for the same reason. Lilly's stock has gained 13.9% so far this year, while the S&P 500 /zigman2/quotes/210599714/realtime SPX +0.36% is up 9.4%.