argenx's Vyvgart Hytrulo Hits Phase 3 Primary Endpoint in Autoimmune Myositis

argenx (NASDAQ: ARGX) reported positive topline results from ALKIVIA, the Phase 3 trial of its subcutaneous FcRn blocker VYVGART Hytrulo (efgartigimod alfa and hyaluronidase-qvfc) in adults with autoimmune myositis. The stock closed at $1,038.12 on August 20, up about 22% from $851.29 on August 14, the session before the August 17 readout.

The trial met its primary endpoint, with patients on efgartigimod showing a 15.4-point greater improvement in mean Total Improvement Score (TIS) at Week 52 versus placebo (47.95 vs 32.56, p=0.0011). Treatment benefit appeared by Week 4 and was sustained through the full year, even as patients tapered corticosteroids.

The win held in the immune-mediated necrotizing myopathy (IMNM) subtype, where efgartigimod delivered a 14.8-point improvement over placebo (45.05 vs 30.24, p=0.0048). In dermatomyositis (DM), the 14.5-point improvement (51.51 vs 36.96) was clinically meaningful but did not reach statistical significance in the smaller cohort (p=0.1093). Efgartigimod was well tolerated, with a safety profile consistent with prior studies.

Why It Matters

Autoimmune myositis affects roughly 100,000 people in the U.S., about 20,000 of them with IMNM, a subtype with no approved therapy. The standard of care has been corticosteroids and broad immunosuppressants, which carry serious cumulative toxicity. ALKIVIA is the first Phase 3 study to show a statistically significant and clinically meaningful improvement in IMNM disease activity.

For argenx, the readout extends the VYVGART franchise beyond its two approved indications, generalized myasthenia gravis and CIDP, into a third autoimmune disease. The same FcRn mechanism that drives VYVGART also underpins argenx’s pipeline in Sjögren’s disease and systemic sclerosis. Our primer on the FcRn inhibitor mechanism explains why this class keeps producing readouts across indications.

The result lands a month after argenx agreed to acquire Forte Biosciences for about $2.2 billion, its push into vitiligo. The myositis win reinforces the immunology platform thesis that runs through both moves.

What Happens Next

argenx said detailed results will be presented at an upcoming medical meeting and that it plans to discuss the data with regulators. The ALKIVIA study enrolled 264 patients in a combined Phase 2/3 design, with 175 patients in the Phase 3 portion. The next catalysts are the full data presentation and a potential regulatory submission in myositis.