A 2-Part Study to Learn Whether Litifilimab (BIIB059) Injections Can Improve Symptoms of Adult Participants Who Have Active Cutaneous Lupus Erythematosus (AMETHYST)

In this study, researchers will learn more about a study drug called litifilimab (BIIB059) in participants with cutaneous lupus erythematosus (CLE). The study will focus on participants who have either active subacute CLE or chronic CLE, or both. They may also have systemic lupus erythematosus (SLE). The participants did not respond to antimalarial therapy or had problems with the treatment that made it hard to continue.

The main objective of the study is to learn about the effect litifilimab has on lowering the activity of the skin disease. Researchers will measure symptoms of CLE over time using a variety of scoring tools. These include the Cutaneous Lupus Erythematosus Disease Area and Severity Index (CLASI), the Cutaneous Lupus Activity of Investigator's Global Assessment-Revised (CLA-IGA-R), and the SELENA-SLEDAI Flare Index (SFI).

The main questions researchers want to answer are:

* How many participants have a score of 0 or 1 on the CLA-IGA-R looking at skin redness after treatment?
* How many participants have their skin disease activity go down by at least 70%?

Researchers will also learn more about the safety of litifilimab. They will study how participants' immune systems respond to litifilimab. Additionally, they will measure the effect litifilimab and CLE have on the quality of life of participants using a group of questionnaires.

The study will be split into 2 parts - Part A and Part B. Both parts will be done as follows:

* After screening, participants will be randomized to receive either litifilimab or placebo for the 1st treatment period. A placebo looks like the study drug but contains no real medicine.
* Participants will receive either litifilimab or placebo as injections under the skin once every 4 weeks.
* The 1st treatment period will be double blinded which means neither the researchers nor the participants will know if the participants are receiving litifilimab or placebo.
* This double blinded treatment period will last 24 weeks, after which the 2nd treatment period will begin.
* During the 2nd treatment period, all participants will receive litifilimab for 28 weeks.
* After completing treatment in this study, participants that qualify will be given the choice to join the Long-Term Extension study, 230LE305. If they do not, they will move into a follow-up safety period that will last up to 24 weeks.
* The total study duration for participants will be up to 80 weeks

Litifilimab is a humanized immunoglobulin G1 (IgG1) monoclonal antibody targeting blood dendritic cell antigen 2 and is being investigated for the potential treatment of systemic lupus erythematosus and cutaneous lupus erythematosus. The primary objectives of the study are to evaluate the efficacy of litifilimab compared with placebo in reducing skin disease activity measured by the CLA-IGA-R score \[Parts A and B (US)\] and the Cutaneous Lupus Erythematosus Disease Area and Severity Index Activity (CLASI-A) score \[Part B (ROW)\] in participants with active SCLE and/or CCLE with or without systemic manifestations and refractory and/or intolerant to antimalarials. The secondary objectives of the study are to evaluate the efficacy of litifilimab in reducing SCLE and/or CCLE disease activity by CLA-IGA-R, CLASI-A; to evaluate additional efficacy parameters of litifilimab in reducing SCLE and/or CCLE disease activity; safety; tolerability; and immunogenicity of litifilimab \[Parts A and B\].

Eligibility Criteria:
Key Inclusion Criteria:

1. Histologically confirmed (in the past or during the Screening period) diagnosis of CLE with or without systemic manifestations.
2. Must have active cutaneous manifestations that meet study criteria.
3. Must have a CLASI-A score ≥10.
4. Must have an active CLE lesion despite an adequate trial of antimalarial treatment.

Key Exclusion Criteria:

1. Any active skin conditions other than CLE that may interfere with the study assessments of CLE.
2. Diagnosis of mixed connective tissue disease \[(within 1 year of signing the informed consent form (ICF)\] or any history of overlap syndromes of SLE including concomitant presence with rheumatoid arthritis, dermatomyositis and/or polymyositis, systemic sclerosis, psoriatic arthritis, or any other autoimmune disease that may confound the evaluation of the disease activity or the effect of the investigational product. Exceptions for overlap syndrome of SLE include participants with overlap syndrome of SLE with myositis and secondary Sjögren's syndrome at screening is permitted provided the participant also meets the criteria for classification as SLE. A past history of mixed connective tissue disease that over time has developed into a diagnosis of SLE is permitted, provided diagnosis of SLE has been present for at least 1 year.
3. Active severe lupus nephritis.
4. Active neuropsychiatric SLE.
5. Use of intralesional corticosteroids within 1 week prior to Screening and during the study.
6. Use of immunosuppressive or disease-modifying treatments for SLE or CLE \[via an oral, intravenous (IV), or SC route\] that were initiated less than 12 weeks prior to randomization, have not been at a stable and allowable dose.

NOTE: Other protocol-defined Inclusion/Exclusion criteria may apply