Regeneron slumps to four-year low after closely watched smoker's lung drug fails trial

Shares of drugmaker Regeneron REGN.O fall as much as 18% to a four-year low $493.15, stock set for worst day since 2011

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 ~5.2%

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 30.3% YTD