Shares of Argenx climbed more than 13% on Monday morning after the Dutch biopharmaceutical company reported positive data from a Phase 3 trial evaluating its blockbuster drug Vyvgart in a rare autoimmune condition, potentially broadening the medicine's approved uses.

In a press release issued Monday, Argenx said that an under-the-skin injection formulation of Vyvgart, known as Vyvgart Hytrulo, met its primary endpoint in a late-stage combined study involving patients with myositis—a rare autoimmune disease that attacks muscles and currently has limited treatment options. Compared with placebo, Vyvgart demonstrated improvements across two disease subtypes, as measured by the Total Improvement Score (TIS), a scale that evaluates muscle strength and physical health.

“For people living with myositis, the goal is straightforward: regain strength and function, and get off long-term steroids. Until now we have had limited targeted therapies to offer patients,” said Rohit Aggarwal, co-director of the Myositis Center at the University of Pittsburgh and an ALKIVIA investigator, in a statement.

The study enrolled individuals with two forms of myositis: dermatomyositis (DM), which causes skin rash, and immune-mediated necrotising myopathy (IMNM), a more aggressive subtype that breaks down muscle cells and has no approved treatment options. The combined patient population for these conditions is estimated at approximately 20,000.

When compared with placebo, Vyvgart showed “rapid and sustained benefit” beginning as early as week four in adults with the condition, with effects lasting up to one year alongside steroid tapering. According to Argenx, the drug also produced a 15.4-point greater improvement in mean TIS after 52 weeks. The safety profile was consistent with prior studies.

The positive overall results were largely driven by robust efficacy in the IMNM subgroup. While Vyvgart also showed meaningful improvement in patients with DM, the difference did not reach statistical significance.

Despite the mixed subset data, investor sentiment remained upbeat. Leerink Partners analyst Thomas Smith told clients in a Monday note that the results represent “a positive outcome representing a meaningful expansion opportunity for the Vyvgart franchise, particularly in the IMNM subset where there are no approved therapies and profound unmet need.”

Vyvgart has helped make Argenx one of biotech’s most valuable companies, with a market capitalization now exceeding $60 billion. The drug, which reduces immunoglobulin G antibodies, first received approval in 2021 for the treatment of generalized myasthenia gravis (gMG), as noted in an FDA announcement. Since then, a subcutaneous formulation was developed and received label expansion for another autoimmune condition, generating billions in revenue. Between January and June of this year, sales have climbed to nearly $2.9 billion.

Argenx has been actively seeking to expand Vyvgart’s indications, but has previously encountered setbacks in other conditions.

The latest results offer another potential expansion avenue for Vyvgart. RBC Capital Markets analyst Luca Issa called the trial a “big win” for the company. In a Monday note, Issa said that while the drug missed its primary goal in DM, he believes the miss was due to a small sample size, and that Vyvgart will ultimately gain approval “either on this trial or a new larger trial replicating this evidence should the FDA decide to play hard ball.”

Argenx said detailed results will be presented at an upcoming medical meeting.