Study of BOS161721 in Systemic Lupus Erythematosus (SLE) Patients on a Background of Limited Standard of Care

Inclusion Criteria:

- Men and women, ages 18 to 70 years, inclusive

- Participants must be mentally capable of giving consent and there must be evidence of
a personally signed and dated informed consent document indicating that the
participant has been informed of all pertinent aspects of the study

- Participants must have Systemic Lupus Erythematosus (SLE) as defined by meeting 4 of
the Systemic Lupus International Collaborating Clinics classification criteria for SLE
(with at least 1 clinical and 1 immunologic criterion OR Lupus nephritis as the sole
clinical criterion in the presence of anti-nuclear antibodies (ANA) or anti-double
stranded DNA (dsDNA) antibodies), either sequentially or simultaneously

- At screening, participants must have at least 1 of the following:

1. Elevated ANA ≥ 1:80 via immunofluorescent assay at the central laboratory

2. Positive anti-dsDNA or anti-Smith (anti-Sm) above the normal level as determined
by the central laboratory

3. C3 or C4 levels below normal as determined by the central laboratory

- At screening, the Systemic Lupus Erythematosus Disease Activity Index 2000 (SLEDAI-2K)
must be ≥ 6, including points from at least 1 of the following clinical components:
Arthritis, rash, myositis, mucosal ulcers, pleurisy, pericarditis, and vasculitis

1. Excluding parameters which require central laboratory results: (hematuria,
pyuria, urinary casts, proteinuria, positive anti-dsDNA, decreased complement,
thrombocytopenia, and leukopenia)

2. Points from lupus headache and organic brain syndrome will also be excluded.

- On Day 0, the SLEDAI-2K must be ≥ 6, including points from at least 1 of the following
clinical components: Arthritis, rash, myositis, mucosal ulcers, pleurisy,
pericarditis, and vasculitis

1. Excluding parameters which require central laboratory results: hematuria, pyuria,
urinary casts, proteinuria, positive anti-dsDNA, decreased complement,
thrombocytopenia and leukopenia

2. Points from lupus headache and organic brain syndrome will also be excluded.

- Participants must have at least 1A or 2Bs from the following manifestations of SLE, as
defined by the British Isles Lupus Assessment Group (BILAG) criteria as modified for
use in this study, which must be confirmed by the central data reviewer:

1. BILAG A or B score in the mucocutaneous body system

2. BILAG A or B score in the musculoskeletal body system due to active polyarthritis

If only one "B" and no "A" score is present in the mucocutaneous body system or in the
musculoskeletal body system due to arthritis, then at least 1 "B" must be present in the
other body systems for a total of 2 "B" BILAG body system scores

- Participants must be currently receiving at least 1 of the following:

1. Administration for a minimum of 12 weeks, and a stable dose for at least 56 days
(8 weeks prior to signing consent) of the following permitted steroid sparing
agents: Azathioprine, mycophenolate mofetil or mycophenolic acid, chloroquine,
hydroxychloroquine, or methotrexate

2. Prednisone (or prednisone-equivalent) cannot exceed 30 milligrams per day
(mg/day) at screening for a participant to be eligible and must be stable at a
maximum of 10 mg/day for at least 5 days prior to Day 0 (randomization)

- Women of childbearing potential (WOCBP):

1. Must have a negative serum pregnancy test at screening. Urine pregnancy test must
be negative prior to first dose

2. Must not be breastfeeding

3. Must agree to follow instructions for method(s) of contraception for the duration
of treatment with study drug plus 52 weeks

- Men who are sexually active with WOCBP must agree to follow instructions for method(s)
of contraception for the duration of treatment with study drug plus 52 weeks

- Participants must demonstrate willingness and ability to comply with the scheduled
study visits, treatment plans, laboratory tests, and other procedures

Exclusion Criteria:

Participants presenting with any of the following will not be included in this study:

- Drug-induced SLE, rather than "idiopathic" SLE

- Other systemic autoimmune disease (eg, erosive arthritis, rheumatoid arthritis [RA],
multiple sclerosis [MS], systemic sclerosis, or vasculitis not related to SLE).
RA-Lupus overlap (Rupus), and secondary Sjogren syndrome are allowed.

- Any major surgery within 6 weeks of study drug administration (Day 0) or any elective
surgery planned during the course of the study

- Any history or risk for tuberculosis (TB), specifically those with:

1. Current clinical, radiographic, or laboratory evidence of active TB

2. History of active TB

3. Latent TB defined as positive QuantiFERON-TB Gold In Tube or other diagnostic
test in the absence of clinical manifestations. Latent TB is not excluded if the
participant has documented completion of adequate course of prophylactic
treatment with regimen recommended by local health authority guideline, or the
participant has started treatment with isoniazid, or other regimen recommended by
local health authority guidelines for at least 1 month before Day 0 and continues
to receive the prophylactic treatment during study until the treatment course is
completed

- Active or unstable lupus neuropsychiatric manifestations, including but not limited to
any condition defined by BILAG A criteria, with the exception of mononeuritis
multiplex and polyneuropathy, which are allowed

- Severe proliferative lupus nephritis (World Health Organization Class III, IV), which
requires or may require induction treatment with cytotoxic agents or high dose
corticosteroids

- Concomitant illness that, in the opinion of the investigator, is likely to require
additional systemic glucocorticosteroid therapy during the study, (eg, asthma), is
exclusionary. However, treatment for asthma with inhalational corticosteroids therapy
is allowed.

- Use or planned use of concomitant medication outside of standard of baseline treatment
for SLE from Day -1 or for any time during the study

- Active and clinically significant infection (bacterial, fungal, viral, or other)
within 60 days prior to first dose of study drug. Clinically significant is defined as
requiring systemic parenteral antibiotics or hospitalization

- A history of opportunistic infection, or a history of recurrent or severe disseminated
herpes zoster or disseminated herpes simplex within the last 3 years

- Chronic viral hepatitis B (HBV) and hepatitis C (HCV), unless participant received
curative treatment for HCV and has a documented negative viral load, known human
immunodeficiency virus (HIV), or acquired immunodeficiency syndrome (AIDS)-related
illness

- Cryptosporidium in the stool sample at screening

- White blood cells < 1,200/millimeters cubed (mm^3) (1.2 × 10^9/Liter [L]) at screening

- Absolute neutrophil count < 500/mm^3 at screening

- CD4+ count < 350/microliter (µL) at screening

- Platelets < 50,000/mm^3 (50 × 10^9/L) or < 35,000/mm^3 (35 × 10^9/L) if related to
SLE, at screening

- Hemoglobin < 8 grams per deciliter (g/dL) or < 7 g/dL at screening if related to SLE

- Proteinuria > 3.0 g/day (3000 milligrams per day [mg/day]) at screening or equivalent
level of proteinuria as assessed by protein/creatinine ratio (3 mg/mg or 339
milligrams per millimole [mg/mmol])

- Serum creatinine > 2.0 mg/dL at screening or creatinine clearance < 40 milliliters per
minute (mL/minute) based on Cockcroft-Gault calculation

- Serum alanine aminotransferase and/or serum aspartate aminotransferase > 2 × the upper
limit of normal (ULN) at screening, unless explicitly related to lupus based on the
investigator's judgment

- Creatinine kinase > 3.0 × ULN at screening unless related to lupus myositis

- Direct bilirubin > 1.5 × ULN at screening (unless related to Gilbert's syndrome)

- Any other laboratory test results that, in the opinion of the Investigator, might
place a participant at unacceptable risk for participating in this study

- History of allergic or anaphylactic reaction to any therapeutic or diagnostic
monoclonal antibodies (mAb) (eg, IgG protein) or molecules made of components of mAbs

- History substance and/or alcohol abuse, or dependence within the past 1 year, at the
investigator's judgment

- History of cancer within the last 5 years (except for cutaneous basal cell or squamous
cell cancer, or cervical cancer in situ resolved by excision)

- Any other severe acute or chronic medical or psychiatric condition, including recent
(within the past year) medical conditions (eg, cardiovascular conditions, respiratory
illnesses) that may increase the risk associated with study participation or
investigational product administration or may interfere with the interpretation of
study results and, in the judgment of the investigator, would make the participant
inappropriate for entry into this study

- Investigational site staff members directly involved in the conduct of the trial and
their family members, site staff members otherwise supervised by the investigator, or
participants who are employees of the sponsor or directly involved in the conduct of
the trial

- Currently participating in, or who have participated in other interventional (drug or
device) clinical study within 30 days or 5 half-lives of baseline, whichever is longer

- Recent (within the past 12 months) or active suicidal ideation or behavior based on
participant responding "yes" to question 3, 4, or 5 on the Columbia Suicide severity
Rating Scale

- Current or pending incarceration

- Current or pending compulsory detainment for treatment of either a psychiatric or
physical (eg, infectious disease) illness