PARIS, Oct 20 (Reuters) - French drugmaker Sanofi and its U.S. partner Regeneron said on Monday they had started a Phase III clinical study of their experimental drug dupilumab in patients with atopic dermatitis.

In a mid-stage trial, the injectable drug - which is also being tested against asthma and chronic sinusitis - had significantly reduced skin lesions and itching in adult patients suffering from atopic dermatitis, a chronic form of eczema that is poorly controlled by standard drugs.

The first phase III trial will enroll around 700 patients with the primary objective of demonstrating the efficacy of dupilumab in adults with moderate to severe disease in combination with topical corticosteroids through 16 weeks, Sanofi said in a statement. Secondary objectives of the study will evaluate the drug's long-term safety and efficacy over up to 52 weeks.

(Reporting by Natalie Huet)