ACT201 Injection in Healthy Participants and Participants With Chronic Hepatitis B (CHB)

This is a single-center, randomized, double-blind, placebo-controlled Phase I clinical trial consisting of two parts. Part 1 includes single ascending dose (SAD) and multiple ascending dose (MAD) cohorts conducted in healthy participants. Part 2 is a multiple ascending dose (MAD) trial enrolling HBeAg-negative chronic hepatitis B (CHB) participants with suppressed HBV DNA under stable nucleos(t)ide analog (NA) therapy.

Inclusion Criteria: Part 1: * Participants fully understand the purpose, nature and methods of the trial as well as potential adverse events, voluntarily participate in this trial and provide written informed consent; * Aged between 18 and 55 years old (inclusive) at the time of informed consent, male or female; * Body mass index meets specified criteria; * Physical examination, vital signs, laboratory tests, electrocardiogram and imaging examinations at screening are normal or abnormalities are considered clinically insignificant; * Participants (including their partners) are willing to use effective contraceptive measures from screening through 6 months after the last administration of investigational product, with no plans for pregnancy, sperm donation or oocyte donation. Part 2: * Participants fully understand the purpose, nature and methods of the trial as well as potential adverse events, voluntarily participate in this trial and provide written informed consent; * Aged between 18 and 65 years old (inclusive) at the time of informed consent, male or female; * Body mass index meets specified criteria; * HBsAg-positive or HBV DNA-positive for at least 6 months, or previous liver biopsy confirming chronic HBV infection; * Receiving stable nucleos(t)ide analogue (NA) therapy at screening, with no planned changes to NA regimen during the trial; * Serum ALT ≤ 2 × ULN at screening; HBeAg, HBV DNA and HBsAg levels meet protocol-specified criteria; * Participants (including their partners) are willing to use effective contraceptive measures from screening through 6 months after the last administration of investigational product, with no plans for pregnancy, sperm donation or oocyte donation. Exclusion Criteria: Part 1: * Known or suspected hypersensitivity to ACT201 or its excipients; or participants with allergic diathesis (multiple drug and food allergies judged clinically significant by the Investigator). * History of clinically significant diseases involving cardiovascular, hematologic and lymphatic, urinary, endocrine, immune, psychiatric, or nervous systems (e.g., epilepsy). * Vital signs or laboratory examinations at screening meet the exclusion cut-off values specified in the protocol. * Use of any prescription drugs, over-the-counter medications, vitamin products or herbal medicines within 2 weeks prior to the first dose (topical medications with local effects are excluded). * Positive HBsAg, hepatitis B core antibody, hepatitis C antibody, human immunodeficiency virus antibody, or Treponema pallidum antibody at screening. * QTcF interval (QT corrected by Fridericia's formula) \> 450 ms at screening. * Daily cigarette consumption exceeding 5 cigarettes within 3 months before screening. * History of drug abuse or illicit drug use within 1 year before screening, or positive urine drug screen at screening. * History of alcohol abuse within 6 months before screening (14 alcohol units per week: 1 unit = 285 mL beer with \~3.5% alcohol, or 25 mL spirits with \~40% alcohol, or 100 mL wine with \~10% alcohol), or positive breath alcohol test at screening. * Vaccination administered within 1 month before screening, or planned vaccination during the trial period. * Blood loss or blood donation ≥ 400 mL within 3 months before screening (menstrual bleeding in female participants excluded), or planned blood donation during the trial period. * Female participants who are breastfeeding or have a positive serum pregnancy test at screening. * Participation in any clinical trial with an investigational medicinal product/investigational device within 3 months before screening or within 5 half-lives (whichever is longer). * Any other condition deemed unsuitable for trial participation by the Investigator. Part 2： * Major trauma or major surgery within 3 months before screening; or planned surgery during the trial period that may impair trial compliance or safety assessment as assessed by the Investigator. * Uncontrolled and clinically significant abnormalities other than chronic HBV infection, such as acute cerebrovascular disease, severe or unstable cardiac disease, uncontrolled diabetes, uncontrolled hypertension, uncontrolled dyslipidemia, etc. * History of other clinically significant liver diseases. * History of liver cirrhosis or progressive liver fibrosis; or liver stiffness measurement (LSM) ≥ 8.5 kPa at screening. * Alpha-fetoprotein \> 50 ng/mL, or imaging suggestive of possible malignant hepatic lesions. * Past or current manifestations of hepatic decompensation. * History of extrahepatic diseases potentially related to HBV immune status. * History of vasculitis; or signs/symptoms suggestive of underlying vasculitis; or past/current other diseases potentially associated with vasculitic disorders. * Active infection requiring systemic antiviral or antibacterial treatment at screening, excluding HBV infection. * History of malignant tumors within 5 years before screening, except for specific curable cancers resected surgically. * Prior solid organ or bone marrow transplantation. * Use of systemic immunosuppressants within 3 months before the first dose of investigational product \[short-term (≤7 days) glucocorticoids for prophylaxis or treatment of non-autoimmune diseases excluded\]; use of immunomodulators or cytotoxic agents within 6 months before the first dose of investigational product. * Receipt of any oligonucleotide or small interfering RNA (siRNA) therapy within 12 months before the first dose of investigational product. * Coexisting indication for anticoagulant therapy or anticipated requirement for anticoagulation during the trial. * Any of the following laboratory results at screening, or other clinically significant abnormalities rendering the participant unsuitable for trial participation: Platelet count \< 125 × 10\^9/L Absolute neutrophil count \< 1.5 × 10\^9/L Hemoglobin \< 100 g/L Total bilirubin \> 1.25 × ULN Serum albumin \< 35 g/L Estimated glomerular filtration rate (eGFR) \< 60 mL/min/1.73 m\^2 (calculated using the CKD-EPI formula) Prothrombin time international normalized ratio (INR) \> 1.25 Positive hepatitis C antibody, human immunodeficiency virus antibody, or Treponema pallidum antibody * QTcF interval (QT corrected by Fridericia's formula) \> 450 ms at screening, or other clinically significant electrocardiogram abnormalities identified at screening. * Known or suspected hypersensitivity to ACT201 or its excipients; or participants with allergic diathesis (multiple drug and food allergies judged clinically significant by the Investigator). * Daily cigarette consumption exceeding 5 cigarettes within 3 months before screening. * History of drug abuse or illicit drug use within 1 year before screening, or positive urine drug screen at screening. * History of alcohol abuse within 6 months before screening, or positive breath alcohol test at screening. * Vaccination administered within 1 month before screening, or planned vaccination during the trial period. * Blood loss or blood donation ≥ 400 mL within 3 months before screening (menstrual bleeding in female participants excluded), or planned blood donation during the trial period. * Female participants who are breastfeeding or have a positive serum pregnancy test at screening. * Participation in any clinical trial with an investigational medicinal product/investigational device within 3 months before screening or within 5 half-lives (whichever is longer). * Any other condition deemed unsuitable for trial participation by the Investigator.