Study of GS-4321 in Healthy Participants and Participants With Chronic Hepatitis Delta Virus

The goals of this clinical study are to first learn more about safety and dosing of the study drug GS-4321 in healthy participants. The study will then learn about the safety and effectiveness of GS-4321 in participants with chronic hepatitis delta (CHD). The primary objective of Phase 1 of this study is to evaluate the safety, tolerability and Pharmacokinetics (PK) of the escalating single doses of GS-4321 administered in healthy participants. The primary objective of Phase 2 of this study is to evaluate the efficacy and safety of the multiple escalating doses of GS-4321 in participants with CHD.

Key Inclusion Criteria: Part A: * Participants assigned male or female at birth who are of childbearing potential and engage in heterosexual intercourse must agree to use protocol-specified method(s) of contraception. * Have a body mass index (BMI) of ≤ 30.0 kg/m2 at screening and at admission. Part B: * Participants assigned male or female at birth who are of childbearing potential and engage in heterosexual intercourse must agree to use protocol-specified method(s) of contraception. * Chronic hepatitis delta (CHD) for ≥ 6 months prior to screening, documented by prior medical history. * Must be receiving a commercially available entecavir, TAF, or TDF for the treatment of hepatitis B virus (HBV) infection at or prior to enrollment. Coformulation as part of a fixed-dose combination for the treatment of HIV is permitted. * Non-cirrhotic or compensated cirrhosis. * Hepatitis delta virus ribonucleic acid (HDV RNA ) \> 500 IU/mL at screening. * Alanine aminotransferase (ALT) level \> 1 × Upper limit of normal (ULN), but \< 10 × ULN at screening. Key Exclusion Criteria: Part A: * Positive serum or urine pregnancy test. * Participants with plans to breastfeed during the study period. Part B: * Positive serum or urine pregnancy test. * Participants with plans to breastfeed during the study period. * Current or previous clinically decompensated liver disease, including coagulopathy, hepatic encephalopathy, and esophageal varices hemorrhage due to HDV or HBV. * Child-Turcotte-Pugh (CTP)-B or -C or a CTP score of ≥ 7. Note: Other protocol defined Inclusion/Exclusion criteria may apply.