Clinical Data Topline ResultsThirty-eight patients on standard of care therapy with platelet count ≤ 30 x109/L at screening were enrolled in the double-blind, randomized, placebo-controlled Phase 2 trial to receive either 5mg/kg or 10mg/kg of intravenous (IV) efgartigimod, or placebo. argenx initiated an open-label extension study approximately halfway through the Phase 2 trial enabling 12 patients across the three cohorts to enroll and receive 10mg/kg of efgartigimod. This included four patients from the placebo cohort who received efgartigimod treatment for the first time.The primary endpoint of the Phase 2 trial was safety and tolerability. Efgartigimod was reported to be well-tolerated in all patients, with most adverse events (AEs) characterized as mild and deemed unrelated to the study drug. One serious adverse event was reported in the primary study and was deemed unrelated to the study drug.Post-hoc analyses around secondary endpoint measures relating to magnitude of effect, onset of action and durability showed efgartigimod treatment was associated with clinically meaningful improvements in platelet count to ≥ 50x109/L.Magnitude of effect:

• 46% of patients improved platelet count to ≥ 50x109/L during two or more visits in each of the 5mg/kg and 10 mg/kg dosing cohorts compared to 25% in the placebo cohort.
• 58% of patients (n=12) improved platelet count to ≥ 50x109/L during two or more visits following the first dosing cycle of the open-label extension study.

Onset of action:

• Onset of platelet count reaching 50x109/L for the first time ranged from week 1 to week 10, consistent with disease heterogeneity.
• All efgartigimod-treated patients showed a rapid and deep reduction of total IgG levels, consistent with the pharmacodynamic effects observed in previous clinical trials.

Durability:

• For efgartigimod-treated patients with clinically meaningful platelet responses (≥ 50x109/L during two or more visits), the mean duration of platelet response was 40 days versus 16 days for placebo treated patients, with responses lasting the study duration.
• 38% of efgartigimod-treated patients showed durable platelet count improvements to clinically meaningful and statistically significant levels of ≥ 50x109/L for at least 10 cumulative days, compared to 0% of placebo patients (p=0.03).

argenx plans to present the full data from the Phase 2 proof-of-concept trial of efgartigimod in ITP during a workshop around the American Society of Hematology Annual Meeting (San Diego, December 1-4, 2018).Based on these data, argenx plans to advance efgartigimod (IV) to Phase 3 development in ITP. The Company also expects to initiate a Phase 2 trial in ITP using a subcutaneous formulation of efgartigimod. Efgartigimod is currently being evaluated in a global Phase 3 registration trial in gMG and a Phase 2 proof-of-concept trial in pemphigus vulgaris (PV).