Why Is European Drugmaker Argenx Stock Sinking Today?

Argenx SE ARGX shares plunged after the company released topline results from the ADDRESS Phase 3 study evaluating efgartigimod subcutaneous (SC) (efgartigimod alfa and hyaluronidase-qvfc) in adults with pemphigus vulgaris (PV) and pemphigus foliaceus (PF). 

Pemphigus is an autoimmune disease in which the immune system attacks cells in the top layer of the skin (epidermis) and the mucous membranes. 

The results show that the proportion of PV patients achieving the primary endpoint of complete remission on a minimal dose of steroids (CRmin) was not significantly different between efgartigimod SC and placebo.

Argenx will not pursue additional development in pemphigus and plans to prioritize clinical development of efgartigimod in its ongoing severe autoimmune indications.

Efgartigimod SC led to total immunoglobulin G (IgG) and desmoglein autoantibody (DSG-1 and DSG-3) reductions of up to 75%. The observed PD effect was consistent with previous clinical trials of efgartigimod.

There was a higher-than-expected response to background treatment with corticosteroids, which showed reduced DSG-1 and DSG-3 levels of up to 70% in the placebo arm and correlated to sustained clinical benefit. 

The level of autoantibody reduction driven by corticosteroids in both treatment arms was sufficient for patients to achieve CRmin. The significant PD effect of corticosteroids was specific to DSG-1 and DSG-3 autoantibodies, while the observed effect on total IgG reduction was in line with the literature (up to 10%).

Treatment with efgartigimod SC led to CRmin in 35.5% of patients compared to 30.3% with placebo (p=0.5956). Secondary endpoints were also not met, including CRmin in the overall pemphigus population, cumulative dose of corticosteroids, and time to disease control or complete remission. Efgartigimod SC was well-tolerated.

In November, Argenx released disppointing topline results from the ADVANCE-SC study of Vyvgart Hytrulo (efgartigimod alfa and hyaluronidase-qvfc) in adults with primary immune thrombocytopenia (ITP), an autoimmune disorder defined by a significantly reduced number of platelets in blood circulation.

Price Action: ARGX shares are down 23.90% at $344.27 on the last check Wednesday.