Aug 17 (Reuters) - Belgian-Dutch immunology company argenx said on Monday its autoimmune drug Vyvgart met the primary endpoint in a late-stage study in autoimmune myositis, ⁠potentially opening a major new market for the medicine beyond its core neurology franchise.

Shares in argenx rose 11% as investors reacted to what could ⁠become Vyvgart's first entry into rheumatology, a key growth area as the company seeks to expand the drug across multiple ⁠autoimmune diseases.

The Phase 3 study showed rapid ‌and sustained benefit over placebo in adults with autoimmune myositis, with patients receiving Vyvgart posting a 15.4-point greater improvement in mean total improvement score after 52 weeks.

The results were driven by strong efficacy in immune-mediated necrotising myopathy (IMNM), a severe muscle-wasting disease with no approved treatments and one of the most underserved forms of autoimmune myositis.

Patients ‌with the related dermatomyositis subtype showed similar clinical improvements, although the results did not ​reach statistical significance ‌because of the smaller patient population enrolled in ‌the study, the company said.

"This is a great day for patients," Chief Executive Karen Massey told Reuters, noting that around 20,000 people ⁠in the United States suffer from IMNM, a rapidly progressing ‌condition in which the immune ⁠system attacks muscle tissue.

Vyvgart has become one ​of Europe's most successful biotech launches through its ‌use in neurological disorders, and argenx sees rheumatology as a potentially large new growth opportunity.

"If approved, we will be the first FcRn to launch into rheumatology," Massey said.

Argenx is also studying Vyvgart in Sjogren's disease ​and systemic sclerosis, part of a broader effort to extend the medicine ‌beyond ‌neurology and establish it as a multi-indication autoimmune franchise.

Massey said the company plans to move quickly with the ‌U.S. Food and Drug Administration ​and other regulators, noting that the IMNM programme has received Breakthrough Therapy designation from the FDA.

Vyvgart generated $1.52 billion in second-quarter sales, up 60% from a year earlier, making it one of the ⁠fastest-growing autoimmune medicines on the market.

Investors and analysts have increasingly focused on whether ‌argenx can replicate that success outside neurology. The myositis data suggest the company may be a step ​closer.

(Reporting by Lucie Barbier; Editing by Matt Scuffham)