Study to Assess Safety of HDP-101 in Patients With Relapsed Refractory Multiple Myeloma

This study tests HDP-101, a new targeted antibody-drug that delivers a toxin directly to multiple myeloma cells, in patients whose myeloma has come back or stopped responding to multiple prior treatments. The drug targets a protein called BCMA found on myeloma cells. **You may be eligible if...** - You are 18 or older with confirmed active multiple myeloma - You have previously been treated with an immunomodulatory drug, a proteasome inhibitor, and an anti-CD38 antibody - You have had a stem cell transplant or are considered ineligible for one - You still have measurable disease and your organ function meets the study requirements - Your expected survival is more than 12 weeks and your performance status is acceptable (ECOG 0–2) **You may NOT be eligible if...** - You have previously received any BCMA-targeting therapy (for the Phase 2a portion) - Your myeloma has spread to the brain - You have plasma cell leukemia - You have a history of congestive heart failure - You have received a stem cell transplant within the past 12 weeks - You have had radiation therapy in the past 21 days Talk to your doctor to see if this trial is right for you.