The study has two parts. In Part A, healthy volunteers will receive a single dose of GS-4321 given either as an injection under the skin or into a vein. This part will help researchers understand how the medication moves through the body and whether it causes any unwanted effects. In Part B, people who have chronic hepatitis delta infection will receive multiple doses of GS-4321 over time. These participants must already be taking certain medications for hepatitis B, such as entecavir, tenofovir alafenamide, or tenofovir disoproxil fumarate. Researchers will check the virus levels in the blood and monitor liver function through blood tests and measurements of liver stiffness using a special imaging method called elastography.

Throughout the study, participants will have regular visits where blood samples will be taken to measure virus levels, check how the liver is working, and look for any side effects. The study will also check whether participants develop immune responses to the medication by looking for antidrug antibodies in the blood. For those in Part B, the treatment period will last up to 96 weeks, with regular monitoring to see how well the medication works in reducing virus levels and improving liver health markers such as alanine aminotransferase, which is a substance in the blood that can indicate liver inflammation.