This study looks at a new treatment called HDP-101 for patients with multiple myeloma, a type of blood cancer. The study has two parts: one to find the right dose (Part 1) and another to see how well it works (Part 2a). The study uses a special method to decide the safest dose. It will also check how the drug moves in the body, called pharmacokinetics (PK), and its effects. Participants must be 18 or older and have tried other treatments that didn’t work. They must not have had certain other treatments or conditions, like other cancers or infections.

• This study involves two phases and aims to find a safe and effective dose of HDP-101.
• Participants will need to have a confirmed diagnosis of multiple myeloma and a life expectancy of more than 12 weeks.
• People who have active infections or certain other medical conditions cannot join.