Study Overview: This study is testing a new drug, HDP-101, to see if it is safe and works well in people with a type of blood cancer called multiple myeloma, which affects plasma cells in the bone marrow. The study has two parts: first, they will try different doses to find the safest and most effective amount. Then, they will see how well the drug works on its own. * **Participation Details:** Participants must be at least 18 years old and have had certain previous treatments for multiple myeloma. They should have a life expectancy longer than 12 weeks and meet specific health criteria. * **Study Phases:** The first part involves finding the right dose, while the second part focuses on the drug’s effect and safety. * **Exclusions:** People who have had recent transplants, certain other health conditions, or specific infections cannot join this study. If you qualify and are interested, joining might involve regular hospital visits and health checks. This study could help find better treatments for multiple myeloma, but it's important to consider the time commitment and potential risks.