Based on the lack of observed efficacy in the primary study ARGX-113-1904, the sponsor decided to discontinue the open-label extension study.

This was a prospective, multicenter, open label extension (OLE) trial on the efficacy, safety, patient outcome measures, tolerability, immunogenicity, PK and PD of efgartigimod PH20 SC in adult PV or PF participants, who participated in antecedent trial ARGX-113-1904. This trial provided extension of efgartigimod PH20 SC treatment and retreatment options for participants who had been randomized to efgartigimod PH20 SC treatment arm in the trial ARGX-113-1904, and first treatment of efgartigimod PH20 SC and retreatment options for participants who had been randomized to the placebo arm in trial ARGX-113-1904. The participants could also receive concomitant prednisone therapy. Investigators could increase or decrease the prednisone dose based on protocol-specified criteria.

Trial ARGX-113-1905 evaluated the ability to (further) taper prednisone therapy and achieve Clinical Remission (CR) off therapy (CRoff), the ability to achieve CR and CR on minimal therapy (CRmin) for participants who had not yet achieved CR or CRmin, and the ability to treat flare; it also assessed patient outcome measures and the safety, PD, PK and immunogenicity of efgartigimod PH20 SC over the duration of trial.

Study duration: Up to 60 weeks for participants who receive IMP administration up to 52 weeks and with a follow-up period of 8 weeks after the last IMP administration

Inclusion Criteria:

1. Ability to understand the requirements of the trial, to provide written informed consent (including consent for the use and disclosure of research-related health information), willingness and ability to comply with the trial protocol procedures (including required trial visits).

2. The participant participated in trial ARGX-113-1904 and completed the study or has the defined criteria for rollover.

3. Contraceptive use by men and women should be consistent with local regulations regarding the methods for contraception for those participating in clinical trials and:

   1. Male participants:

      Male participants must agree to use an acceptable method of contraception as described in the protocol, from signing the ICF until the last dose of the study drug.

   2. Female participants

Women of childbearing potential (WOCBP) must:

• have a negative urine pregnancy test at baseline before the IMP can be administered,
• agree to use a highly effective or acceptable contraception method (as described in the protocol), which should be maintained at minimum until after the last dose of IMP

Exclusion Criteria:

1. Pregnant and lactating women and those intending to become pregnant during the trial.
2. Participants with clinical evidence of other significant serious disease or participants who recently underwent or have planned a major surgery during the period of the trial, or any other condition in the opinion of the investigator, that could confound the results of the trial or put the participant at undue risk.
3. Known hypersensitivity to any of the components of the administered treatments.