A Study of Modakafusp Alfa on Adult Participants With Relapsed/Refractory Multiple Myeloma

The drug being tested in this study, and which will be given through a vein, is called modakafusp alfa (TAK-573 ) as single agent or in combination with dexamethasone. The study will determine the safety, tolerability, and efficacy of modakafusp alfa as single agent and in combination with dexamethasone in participants with relapsed/refractory multiple myeloma (RRMM). The study consists of 3 Parts: Part 1: Dose Escalation, Part 2: Dose Expansion, Part 3: Dose Extension The study will enroll approximately 65 participants in Part 1, 35 in Part 2, and 236 in Part 3. Participants will be assigned to one of the following treatment groups in Parts 1 and 2 of the study. Participants will be randomly assigned in Part 3 of the study as given below: * Part 1 (Dose Escalation) Schedule A: Modakafusp alfa 0.001 Up to 14 mg/kg * Part 1 (Dose Escalation) Schedule B: Modakafusp alfa TBD * Part 1 (Dose Escalation) Schedule C: Modakafusp alfa TBD * Part 1 (Dose Escalation) Schedule D: Modakafusp alfa TBD * Part 2 (Dose Expansion): Modakafusp alfa TBD + Dexamethasone 40 mg * Part 3 (Dose Extension): Modakafusp alfa 120 mg * Part 3 (Dose Extension): Modakafusp alfa 240 mg The Part 1 (Dose Escalation) portion of the study will follow a 3+3 dose escalation design to evaluate once-weekly up to 4 different schedules of administration of modakafusp alfa starting at 0.001 mg/kg for dose limiting toxicity (DLT) evaluation and to determine the maximum tolerated dose (MTD) or an optimal biological dose (OBD) for assessments in Part 2. The Part 2 (Dose Expansion) will further assess the safety profile of modakafusp alfa and its efficacy at MTD or OBD. For Part 3 (Dose Extension) participants will be randomized 1:1 to receive single-agent modakafusp alfa 120 mg or 240 mg Q4W. Parts 1 and 2 will be conducted at multiple centers in the United States. Part 3 will be conducted worldwide. The maximum treatment duration in this study is up to 12 months (Parts 1 and 2) or until disease progression (Part 3) and overall time to participate in the study is approximately up to 90 months. Participants with clinical benefit may continue treatment after sponsor approval.