May 30 (Reuters) - Regeneron REGN.O shares fell 10% before the bell on Friday after its experimental drug for patients with a type of lung condition commonly called "smoker's lung" failed a late-stage trial, although it succeeded in another.

Regeneron and partner Sanofi SASY.PA were studying the drug, which some analysts expect could bring in peak sales of as much as $5 billion, for treating chronic obstructive pulmonary disease (COPD).

The disease has limited treatment options and the drug, itepekimab, was expected to target a broader patient population compared to Regeneron and Sanofi's blockbuster Dupixent, which is also approved for the condition.

But after Friday's data, Barclays analyst Emily Field said Dupixent is likely to remain doctors' first choice.

Regeneron's itepekimab showed a significant reduction in exacerbations or flare-ups in the condition by 27% compared to placebo at 52 weeks in a 1,127-patient study.

But the second study - which had enrolled fewer former smokers compared to the first - did not meet its goal, although a benefit was seen earlier in the trial.

"Given the mixed data, approval on the basis of these trials seems challenging to us," said J.P. Morgan analyst Richard Vosser.

At least three analysts said the companies may need to conduct additional studies for a potential approval.

U.S.-listed shares of French drugmaker Sanofi fell 4.83% to $49.82 in premarket trading.

The experimental drug binds to and inhibits interleukin-33, a type of protein that causes inflammation in COPD.

The common lung disease causes restricted airflow and breathing problems. It typically affects smokers, but can also be caused by pollutants.

Regeneron's price-to-earnings ratio, a common benchmark for valuing stocks, was 16.15, compared with 13.62 for Gilead GILD.O and 7.29 for Bristol Myers Squibb BMY.N.

Shares of the company, which have already fallen 15% this year, were down at $549.69. The company has a market capitalization of $65.36 billion.