Why argenx Stock Has Surged on Monday

Shares of argenx SE (NASDAQ: ARGX) rocketed higher on Monday, closing up roughly 16% at $990.80 after touching an intraday high near $998.82, as investors cheered positive Phase 3 trial results for the company’s flagship drug efgartigimod.

The Amsterdam-based immunology company announced before markets opened that its ALKIVIA Phase 3 study evaluating VYVGART Hytrulo in adults with autoimmune myositis met its primary endpoint with strong statistical significance (p=0.0011).

Patients treated with efgartigimod showed a 15.4-point greater improvement in Total Improvement Score at Week 52 versus placebo, with benefits emerging as early as Week 4 and sustained through a full year of treatment, even as steroids were tapered.

Notably, the drug also hit statistical significance in immune-mediated necrotizing myopathy (IMNM), a myositis subtype that currently has no approved targeted therapy — a milestone that analysts said meaningfully expands the drug’s addressable market.

Roughly 100,000 people in the U.S. live with autoimmune myositis, including about 20,000 with IMNM and 40,000 with dermatomyositis, most of whom currently rely only on corticosteroids and broad immunosuppressants.

Wall Street reacted swiftly. Several analysts framed the data as opening a new, high-value indication for VYVGART, which is already approved for generalized myasthenia gravis and CIDP. Coverage from GuruFocus noted that even after the rally, argenx still appeared meaningfully undervalued relative to its GF Value estimate.

Management hosted an investor call Monday afternoon to detail the results further, with detailed data expected to be presented at a future medical conference. The stock’s move ranked among the largest single-day gains for any large-cap biotech this year.