A Study on the Safety, Efficacy and Immune Response Following Sequential Treatment with an Anti-sense Oligonucleotide Against Chronic Hepatitis B (CHB) and Chronic Hepatitis B Targeted Immunotherapy (CHB-TI) in CHB Patients Receiving Nucleos(t)ide Analogue (NA) Therapy

Participants, who, in the opinion of the investigator, can and will comply with the requirements of the protocol.

• Written or witnessed/thumb printed informed consent obtained from the participant prior to performance of any study-specific procedure.
• A male or female between, and including, 18 and 65 years of age at the time of signing of the informed consent (except for South Korea, where a male or female between, and including, 19 and 65 years of age at the time of signing of the informed consent can participate in the study).
• Participants who are Hepatitis B envelop antigen (HBeAg) positive or negative.
• Participants who have documented chronic HBV infection >=6 months prior to screening and currently stable on NA therapy defined as no changes to their nucleos(t)ide regimen from at least 6 months prior to screening and with no planned changes to the stable regimen over the duration of the study.
• CHB patient, under and adherent to treatment with a NA with high barrier to resistance (e.g. entecavir, tenofovir disoproxil fumarate and tenofovir alafenamide).
• Participants with ALT <=2x upper limit of normal (ULN) (i.e., no ALT >2x ULN) documented in approximately the last 6 months.
• Participants with plasma or serum HBsAg concentration >100 IU/mL.
• Participants must be adequately suppressed, defined as plasma or serum HBV DNA <90 IU/mL.
• A male participant is eligible if he agrees to the following during the intervention period and for at least 90 days after the last dose of study intervention:

Agree to use a male condom [and should also be advised of the benefit for a female partner to use a highly effective method of contraception as a condom may break or leak] when having sexual intercourse with a woman of childbearing potential (WOCBP) who is not currently pregnant.

• A female participant is eligible:

Is a WOCBP and using a contraceptive method that is highly effective (with a failure rate of <1% per year), preferably with low user dependency during the intervention period and for at least 90 days after the last dose of study treatment.

• A WOCBP must have a negative highly sensitive pregnancy test (urine or serum as required by local regulations) within 24 hours before the first dose of study intervention.

Medical conditions

• Clinically significant abnormalities, aside from chronic HBV infection.
• Co-infection with:

HDV

• History of or suspected liver cirrhosis and/or evidence of cirrhosis as determined by:

Liver biopsy (METAVIR Score F4) or Liver stiffness >12 kPa

• FibroScan TE score >9.6 kPa and FibroTest score >0.59 at Screening.
• Diagnosed or suspected HCC.
• History of:

extrahepatic disorders possibly related to HBV immune conditions

• Positive (or borderline positive) ANCA at screening.
• Low C3/C4 at screening AND evidence of past history or current manifestations of vasculitic/inflammatory/autoimmune conditions.
• History of alcohol or drug abuse/dependence.
• QTcF >=450 msec.
• Laboratory results as follows:

ACR >=0.03 mg/mg

• Medical history of hepatic decompensation.
• Planned or previous liver transplantation.
• Documented evidence of other currently active cause of hepatitis.
• Any other clinical condition that might pose additional risk to the participant due to participation in the study.
• Major congenital defects.
• Recurrent history or uncontrolled neurological disorders or seizures.
• History of any reaction or hypersensitivity likely to be exacerbated by any component of the study intervention(s). Prior/Concomitant therapy
• Use of any investigational or non-registered product other than the study interventions within 30 days before the first dose of study interventions, or their planned use during the study.
• Use of systemic cytotoxic agents, chronic antiviral agents or Chinese herbal medicines which may have activity against HBV within 6 months prior the study.
• Currently taking, or took within 12 months of screening, any interferon-containing therapy.
• Administration of adenovirus/adenovector-based or MVA-based vaccine within the last 12 months, except for adenovirus/adenovector-based COVID-19 vaccines that could be administered up to 30 days prior to the first study vaccine dose (applicable for all patients except for the patients in France) OR Administration of adenovirus/adenovector-based or MVA-based vaccine within the last 12 months (applicable for the patients in France only).
• Planned administration/administration of a vaccine not foreseen by the study protocol within 14 days before the first dose and/or 30 days after the last dose of study intervention administration, with the exception of influenza vaccine that may be given at any time except within a 7-day period before or after each dose and COVID-19 vaccine that may be given at any time except within a 30-day period before or after each vaccine dose apart from COVID-19 mRNA based-vaccines that may be administered any time except for the period of 14 days before and 30 days after each study vaccine dose.
• Administration of:

immunoglobulins and/or any blood products or plasma derivatives within 3 months before the first dose of study interventions or planned administration during the study

• Chronic administration of immunosuppressants or other immune-modifying drugs within 3 months prior to the first study intervention (e.g. prednisone equivalent >=20 mg/day; >=10 mg/day applicable in Germany only). Inhaled and topical steroids are allowed.
• Participants for whom immunosuppressive treatment is not advised.
• Treatment with nephrotoxic drugs or competitors of renal excretion within 2 months prior to Screening or planned during the study.
• Participants requiring anti-coagulation therapies. Prior/Concurrent clinical study experience
• Concurrently participating in another clinical study, at any time during the study period, in which the participant has been/will be exposed to an investigational/a non-investigational intervention.
• Previous participation in clinical trials with administration of either GSK3228836 or GSK3528869A.
• Previous participation in a clinical study in which he/she has received an investigational product within the following time period prior to the first dosing day in the current study: 5 half-lives or twice the duration of the biological effect of the study treatment or 90 days.
• Prior treatment with any other oligonucleotide/siRNA within 12 months prior to the first dosing day. Other exclusions:
• Pregnant or lactating female.
• Female planning to become pregnant/to discontinue contraceptive precautions.
• Any study personnel or their immediate dependents, family, or household members.
• History of/sensitivity to GSK3228836, or components thereof, or a history of drug or other allergy that contraindicates their participation.