(Adds analyst comment, updates shares)

By Bill Berkrot

March 28 (Reuters) - The U.S. Food and Drug Administration on Tuesday approved Regeneron Pharmaceuticals Inc and Sanofi SA's drug for moderate-to-severe atopic dermatitis, a product widely seen as the most important growth driver for the two companies.

Sanofi and Regeneron said the drug, Dupixent, will have a list price of $37,000 a year. But while the price before discounts and rebates to insurers is far more expensive than topical medicines and steroids currently used to treat eczema, it is less pricey than other injectable antibody drugs for serious skin conditions, such as psoriasis, that list for about $50,000 a year.

"With this price, we don't anticipate payer restrictions being too onerous ... which will allow more moderate patients to receive the drug," Bernstein analyst Ronny Gal said in a research note.

Sanofi said is has been encouraged by conversations with health plans and pharmacy benefit managers about patient coverage for Dupixent.

Atopic dermatitis is a chronic type of skin inflammation also known as eczema, which in severe cases causes constant, often unbearable, itching.

Wall Street analysts forecast annual sales exceeding $4 billion by 2022 for the biotech drug known chemically as dupilumab, according to Thomson Reuters data.

Dupilumab had earned the FDA's breakthrough therapy designation given to expedite development and review of medicines for serious or life-threatening diseases lacking effective treatments.

"This drug is really a game changer," said Dr. Emma Guttman-Yassky, Vice Chair of the department of dermatology at Mount Sinai Hospital in New York who conducted clinical trials of dupilumab.

"This condition is terrible for patients. They don't sleep at night. It's like having poison ivy all the time all over the body," she said, adding that patients lose work and can become so desperate they contemplate suicide.

Dupixent is also being developed for severe asthma, where it will compete with a wave of other new biotech medicines, such as GlaxoSmithKline's Nucala, as well as for nasal polyps.

In a pivotal late stage study, after 16 weeks of treatment with 300 milligrams of Dupixent either weekly or every two weeks along with topical corticosteroids, 39 percent had achieved clear or nearly clear skin compared with 12 percent of patients who received the topical treatment alone.

Eczema affects an estimated 2 percent of U.S. adults, and as many as 10 percent to 20 percent of children. About a third of adults have moderate to severe disease.

Regeneron shares after rising slightly fell nearly 1 percent to $380.24 following the widely expected approval.

(Additional reporting By Deena Beasley in Los Angeles and Ben Hirschler in London; Editing by Bernard Orr and Cynthia Osterman)