May 31 (Reuters) - Sanofi and partner Regeneron Pharmaceuticals have secured the European regulator's backing for use of Dupixent in patients with a chronic lung disease even as the U.S. Food and Drug Administration (FDA) extended its review.

French drugmaker Sanofi and its U.S. partner have been counting on expanding the use of the drug as an add-on maintenance treatment for chronic obstructive pulmonary disease (COPD) - also called 'smoker's lungs' as the lifelong condition commonly affects cigarette smokers.

The disease, which causes restricted airflow and breathing problems, affects nearly 16 million U.S. adults and over 35 million people in Europe, according to government data.

French drugmaker Sanofi said in April it expects the target patient population eligible for the drug to be roughly 300,000 patients in the U.S. alone. It anticipated steeper growth in Dupixent sales in 2025 after its launch for COPD in the country.

But the FDA pushed back its deadline for the review by three months to Sept. 27 as it sought additional data from the drugmakers on the efficacy of Dupixent as an add-on COPD treatment, the drugmakers said on Friday.

The drugmakers said they were still "confident" the U.S. approval would come "as quickly as possible".

On the other hand, the European Medicines Agency's advisory panel recommended approval of Dupixent in Europe for patients with COPD for whom systemic corticosteroids or surgery did not provide adequate disease control, the agency said on Friday.

In Europe, the final decision rests with the European Commission, which usually follows the guidance from the EMA's Committee for Medicinal Products for Human Use.

The drug is already approved in many countries for various immune system-related conditions like asthma and eczema. In 2023, Dupixent generated 10.72 billion euros ($11.63 billion) in revenue, according to Sanofi's data. ($1 = 0.9215 euros) (Reporting by Leroy Leo in Bengaluru and Diana Mandiá, editing by Tassilo Hummel and Devika Syamnath)