Shares of Vir Biotechnology Inc. /zigman2/quotes/214486077/composite VIR -1.12% tumbled 5.4% in premarket trading on Thursday, the day after the company said the experimental COVID-19 treatment that it's developing with GlaxoSmithKline /zigman2/quotes/209463850/composite GSK +4.32% may not benefit patients. U.S.-listed shares of GSK /zigman2/quotes/209463850/composite GSK +4.32% were up 0.6% in trading before the market opened on Thursday. The investigational monoclonal antibody therapy was being tested in hospitalized COVID-19 patients in a Phase 3 clinical trial conducted by the National Institutes of Health until an independent safety board recommended that the trial investigators close the study to enrollment at this time, Vir said. "While we are disappointed with the recommendation ... we are encouraged by the safety profile of VIR-7831 and by the possibility of a benefit on top of remdesivir and corticosteroids in this advanced cohort of patients," Vir CEO George Scangos said in a news release. (Remdesivir, which is also called Veklury, was developed by Gilead Sciences Inc. /zigman2/quotes/210293917/composite GILD +0.72% and is the only COVID-19 treatment to have a full Food and Drug Administration approval.) Vir's stock is down 6.9% over the past 12 months, while the broader S&P 500 /zigman2/quotes/210599714/realtime SPX +1.11% has gained 27.2%.