This is a first-in-human, placebo-controlled, single dose, dose-escalation phase 1 study to evaluate the safety, pharmacokinetics and antiviral activity of a highly potent neutralizing anti-HBV monoclonal antibody (mAb), HepB mAb19, which targets the S-protein in individuals with chronic hepatitis B (CHB) on nucleos(t)ide analog therapy (NRTI).

The study has a dose escalation design. In Groups 1-4, eligible participants will be randomized at a 3:1 ratio to receive a single intravenous infusion of HepB mAb19 or placebo (normal saline) at one of four increasing dose levels (1 mg/kg, 3 mg/kg, 10 mg/kg and 30 mg/kg). In Group 5 participants will receive HepB mAb19 at the maximum tolerated dose (MTD). Participants will be followed for 48 weeks after HepB mAb19 or placebo infusion.

Inclusion Criteria:

• Age 18 to 70;
• HBV infection confirmed by positive HBsAg for >/= 6 months;
• On HBV-active nucleos(t)ide therapy for >/= 6 months without change in NRTI in the previous 3 months;
• The following laboratory values within 49 days from study entry (day 0):
• HBV DNA below lower limit of quantification;
• HBsAg > 10 IU/mL;
• HBs antibody negative;
• Ability and willingness to provide informed consent;
• For participants who can become pregnant (i.e., participants who have not been post-menopausal for at least 24 consecutive months, who have had menses within the preceding 24 months, or who have not undergone surgical sterilization, specifically hysterectomy and/or bilateral oophorectomy or bilateral salpingectomy), negative serum or urine pregnancy test at screening and on day 0 (study entry).
• Participants who can become pregnant must agree to use two methods of contraception.
• Partner sterilization with documentation of azoospermia prior to the participant's entry into the study, and this partner is the sole partner for that participant. The documentation of partner sterility can come from the site personnel's review of medical records or medical history interview provided by the participant or the partner. Self-reported documentation of reproductive potential should be entered in the source documents.
• Participants who can impregnate a partner and who are engaging in sexual activity that could lead to pregnancy must agree to use condoms from 10 days prior to study entry and during study follow up to avoid impregnating a partner who can get pregnant.

Exclusion Criteria:

- Clinical symptoms, imaging studies or liver histology suggestive of advanced fibrosis (exclude fibrosis grade 3 and 4 by FibroScan (Fibroscan®< 9 kpa) within 12 months from entry or done at the pre-infusion visit.

Note: If FibroScan results from within 12 months are not available, imaging will be performed at the pre-infusion visit.

• Presence of a LI-RADS4 or 5 liver lesion on imaging within 12 months from entry or done at pre-infusion visit, if prior results not available.
• Alpha fetoprotein > 20 ng/ml Note: AFP above normal but < 20 is acceptable for entry if earlier AFP levels (older than 6 months) are within normal range and imaging is negative in last 3 months).
• HIV-1, HCV or hepatitis delta virus infection within 12 months from entry or done at screen, if prior results not available.
• History of hematopoietic stem cell transplant or solid organ transplant;
• Any confirmed significant allergic reactions (urticaria or anaphylaxis) against any drug, monoclonal antibody or vaccine, or multiple drug allergies (non-active hay fever is acceptable);
• History of cardiovascular disease (e.g., cardiac insufficiency, coronary artery disease, cardiomyopathy, congestive heart failure, family history of congenital long QT syndrome, family history of sudden death);
• History or presence of clinically significant ECG abnormalities based on the average of the triplicate ECG recordings (e.g., QT corrected for heart rate using the Fridericia's correction factor [QTcF] > 450 ms for males and QTcF > 470 ms for females);
• History of systemic corticosteroids, immunosuppressive anti-cancer, systemic interferons or interleukins within the last 6 months;
• History of chronic liver disease from another cause, immune complex disease, or autoimmune diseases that in the opinion of the investigator would preclude participation.
• Any significant acute infection (e.g. influenza, COVID-19) or any other clinically significant illness within 2 weeks prior to Day 0.
• Laboratory abnormalities in the parameters listed below:
• Absolute neutrophil count < 1,000 /mm3
• Hemoglobin < 10 gm/dL
• Platelet count < 150,000 /mm3
• ALT > 2.0 x ULN
• AST > 2.0 x ULN
• Total bilirubin > 1.5 ULN (except individuals with known Gilbert's)
• Albumin < 3.5 gm/dL
• Calculated creatinine clearance < 70 mL/min (using the Cockcroft Gault formula).
• INR >/= 1.2
• Pregnancy or lactation;
• Any vaccination within 14 days prior to IP administration;
• Receipt of anti-HBV mAb therapy of any kind in the past (including HBIG);
• Participation in another clinical study of an investigational product currently or within past 12 weeks, or expected participation during this study.

In Groups 1-4, eligible participants will be randomized at a 3:1 ratio to receive a single intravenous infusion of HepB mAb19 or placebo (normal saline) at one of four increasing dose levels (1 mg/kg, 3 mg/kg, 10 mg/kg and 30 mg/kg). In Group 5 participants will receive HepB mAb19 at the maximum tolerated dose (MTD)