argenx SE’s U.S.-listed shares traded lower on Tuesday, April 21, 2026, with the move fitting a classic pre-catalyst “de-risking” pattern rather than a single headline shock. The company is in the middle of a busy news window around the American Academy of Neurology (AAN) meeting in Chicago (April 18–22, 2026), and investors are also positioning ahead of the next major regulatory event for its blockbuster franchise VYVGART.

The market’s key date is May 10, 2026, the FDA target action date (PDUFA) for expanding VYVGART into anti–acetylcholine receptor antibody–negative (seronegative) generalized myasthenia gravis. That decision is meaningful because it could broaden the addressable MG population and reinforce VYVGART’s leadership in FcRn inhibition, but it also concentrates near-term risk into a single binary event. The stock’s slide looks consistent with investors trimming exposure as that decision approaches and as additional AAN readouts circulate.

argenx has been priced as a premium-growth biotech with a now-profitable commercial engine, and that valuation can amplify routine pullbacks on quiet days. With the stock elevated and headline risk rising into an FDA decision and a high-visibility medical meeting, traders frequently rotate into profits, tighten stops, or add hedges—producing a down session even when the underlying thesis hasn’t changed.

Investors will be tracking any incremental color from AAN 2026 presentations on VYVGART across MG and CIDP and how clinicians interpret the newest datasets. The next high-impact inflection remains the May 10, 2026 FDA decision for seronegative gMG; a positive outcome would likely refocus attention on label breadth and launch execution, while any delay or negative outcome would likely drive near-term multiple compression.