** Shares of drugmaker Regeneron REGN.O fall 13.28% to $525 premarket

** Company and partner Sanofi SASY.PA say their experimental drug, itepekimab, failed in one late-stage trial testing it for a lung disease commonly called "smoker's lung", but met the goal of another trial

** Sanofi's U.S.-listed shares down ~6.5% before the bell

** Itepekimab was being tested in former smokers in two studies for chronic obstructive pulmonary disease, a condition that mostly affects smokers but can also be caused by pollutants

** Both trials were testing if the drug was statistically significant in reducing moderate or severe flare-ups of the condition in former smokers

** Companies are assessing the data and will discuss next steps with regulatory authorities

** REGN shares down ~15% YTD

(Reporting by Manas Mishra in Bengaluru)

((Manas.Mishra@thomsonreuters.com; www.twitter.com/Manaswrites15;))