Inclusion Criteria:

• Male or female 18 to 75 years of age (inclusive) at the time of signing initial consent, with all of the following at Screening:
  • Diagnosis of SjS based on AECG classification;
  • Active SjS, with ESSDAI ≥5;
  • Seropositivity for anti-SSA or anti-SSB, per central laboratory.
• A negative serum pregnancy test at Screening and a negative urine pregnancy test at Day 1 are required for female subjects of child-bearing potential (as defined in Appendix 5).
• Starting at the time of written consent, through the study, and for 90 days (male) and 36 days (female) following their last dose of study drug:
  • Male and female subjects of childbearing potential who engage in heterosexual intercourse must agree to use protocol specified method(s) of contraception as described in Appendix 5;
  • Male subjects must agree to refrain from sperm donation;
  • Female subjects must agree not to breastfeed or to donate/harvest eggs for the purpose of fertilization.
• Subjects with prior exposure to a B-cell depleting bDMARD (at any time) must have a documented return of CD19+ B cells at Screening.
• Stable dose (defined as no change in prescription for at least 4 weeks prior to Day 1) of NSAIDs, HCQ, MTX and/or oral corticosteroids is permitted during the study, but not required (see protocol Section 5.8 for definitions, full list of allowed prior and concomitant medications and permitted doses).
• Meet one of the following (a or b) tuberculosis (TB) Screening criteria:
  • No evidence of active or latent TB:
    • Negative history of TB infection and
    • Negative QuantiFERON® TB-Gold In-Tube test and
    • Note: QuantiFERON® tests with inconclusive results may be repeated one time.  If the repeat result is also inconclusive, the subject is excluded from the study.
    • Negative chest X-ray results (radiographs taken at Screening or within 90 days prior to Screening with films or report available for investigator review).
  • Subjects with prior latent TB who have been treated with a full course of prophylaxis as per local guidelines.
    • Note: Appropriate documentation of previous treatment is required.  In these cases, a QuantiFERON® TB-Gold In-Tube test is not needed, but a chest radiograph must be obtained within 3 months prior to Screening (report or films must be available).  In addition, these cases must be approved by the Medical Monitor prior to enrollment.  Subjects with a new diagnosis of latent TB or prior untreated/partially treated latent TB are NOT allowed (ie, subjects who require prophylactic therapy for TB during the study). Subjects with any prior active TB are excluded regardless of treatment.
• Are willing and able to sign the informed consent form and comply with study requirements, procedures, and scheduled visits.
  • Note: Subjects who cannot read or understand the ICF may not be enrolled by a guardian, representative, or any other individual

Exclusion Criteria

• Concurrent treatment at Screening with any bDMARD (prior bDMARD treatment allowed with appropriate washout as defined in Section 5.8).
• Any prior use of cyclophosphamide.
• Use of cyclosporine within 4 weeks prior to the first dose of study drug (Day 1) or anticipated chronic use while on study (other than ophthalmic cyclosporine as described as per Section 5.8).
• Clinically significant abnormalities on 12-lead ECG as judged by the investigator.
• Treatment with moderate or strong CYP3A inducers or inhibitors within 2 weeks, or strong P-gp inducers within 3 weeks prior to the first dose of study drug (Day 1) or anticipated chronic use while on study (see Section 5.8).
• Participation in any clinical study of an investigational drug within 4 weeks or 5 drug half-lives prior to Screening, whichever is longer. Washout duration for exposure to investigational biologics should be discussed with the sponsor.
• Treatment with any commercially available or investigational drug listed below within 3 months of Screening:
  • BTK inhibitor, such as ibrutinib; or
  • SYK inhibitor, such as fostamatinib; or
  • JAK inhibitor, such as tofacitinib or baricitinib.
• History of opportunistic infection or immunodeficiency syndrome which would put the subject at risk, as per investigator judgment.
• History of symptomatic Herpes zoster or Herpes simplex infection within 12 weeks prior to Screening or history of disseminated/complicated Herpes zoster infection (multi-dermatomal involvement, ophthalmic zoster, CNS involvement or postherpetic neuralgia) at any time.
• Known hypersensitivity to the study drug, its metabolites, or any of the excipients, or previous clinically significant allergic reaction to any drug, per judgment of the investigator.
• Another highly active inflammatory/autoimmune/rheumatic disease (such as highly active RA, highly active SLE, highly active nephritis or CNS inflammation) which would compromise subject safety or interfere with the conduct of the study.
  • Note: Subjects with stable autoimmune disease, (eg, RA, SLE, or thyroiditis) not requiring prohibited medications are permitted.
• History of head/neck irradiation, sarcoidosis, graft v host disease, or IgG4 related disease {Stone 2012}, {Umehara 2012}.
• Female subjects who are pregnant, breastfeeding, or planning to become pregnant or breastfeed during the study up to 36 days after the last dose of study drug or males who are planning to father a child during the study up to 90 days after the last dose of study drug (as per Appendix 5).
• Major surgery (requiring regional block or general anesthesia) within 30 days prior to Screening, or planned during the study.
• History of live or attenuated vaccines within 30 days of Day 1, or planned during the study period or for 12 weeks after the subject's last dose of study drug. Refer to Section 5.9.
• History of malignancy within the past 5 years prior to Screening (except for adequately treated basal cell carcinoma or non-metastatic squamous cell carcinoma of the skin or cervical carcinoma in situ, with no evidence of recurrence).
• Positive serology for human immunodeficiency virus (HIV) 1 or 2 at Screening.
• Hepatitis B virus (HBV) surface antigen (HBsAg) or positive HBV core antibodies at Screening.
• Hepatitis C virus (HCV) antibodies and positive HCV RNA viral load (VL).
  • Note: Subjects with positive HCV Ab, but negative HCV RNA VL are eligible per investigator judgment, but require monitoring as outlined in Appendix 2.  Subject with active HCV during the study as evidenced by RNA positivity will be discontinued from study drug as outlined in Section 3.4.
• Serious active infection of any kind at Screening or Day 1.
• Blood loss (> 500 mL) or blood product transfusion within 12 weeks of Day 1.
• Known bleeding disorder or hypercoagulable state; antiphospholipid antibody syndrome with prior clinically significant event (per judgment of investigator); or on chronic anticoagulation or anti-platelet therapy.
  • Note: Aspirin therapy ≤325 mg/day for cardiovascular prophylaxis is permitted.
•  Current drug or alcohol abuse, or heavy tobacco use, as per judgment of investigator.
•  Clinically significant laboratory values at Screening, including the following:
  • Hemoglobin <9 g/dL (International System of Units [SI]: <90 g/L);
  • Neutrophil count <1.5 x 109/L (SI: <1.5 x 109 cells/L);
  • Platelets <100 x 103 cells/mm3 (SI: <100 x 109 cells/L);
  • Alanine aminotransferase (ALT) or aspartate aminotransferase (AST) ≥1.5x upper limit of normal (ULN);
  • Total bilirubin level ≥2x ULN unless the subject has been diagnosed with Gilbert’s disease and this is clearly documented;
  • Estimated creatinine clearance <60 mL/min per Cockcroft-Gault equation.
    • Note: Abnormal values may be rechecked one time, at discretion of the investigator if an error is suspected.