Shares of Regeneron Pharmaceuticals Inc. /zigman2/quotes/203149337/composite REGN +0.66% were down 2.4% and Sanofi's /zigman2/quotes/201967021/composite SNY -0.02% stock declined 1.7% in after-hours trading on Friday after the companies halted a Phase 3 clinical trial testing their rheumatoid arthritis drug Kevzara as a COVID-19 treatment. The drug, which was being tested in 194 hospitalized COVID-19 patients receiving mechanical ventilation, did not meet the primary or key secondary endpoints in the study. The findings will be published in a peer-reviewed medical journal later this year. Sanofi is continuing to evaluate Kevzara using a different dose in a trial of COVID-19 patients taking place outside of the U.S. Since the start of the year, Regeneron's stock is up 65.7%, shares of Sanofi are up 2.6%, and the S&P 500 /zigman2/quotes/210599714/realtime SPX -0.22% is down 3.5%.