Argenx's Vyvgart Shows Positive Phase 3 Results in Seronegative Myasthenia Gravis on August 27, 2025 AChR-Ab adaptation Argenx expansion Labels (device) LRP4 gene Musk Myasthenia Gravis Phase 3 SERON Seronegative triple seronegative United States Food and Drug Administration Vyvgart +