Subjects assigned to CC-220 monotherapy, who develop progressive disease (PD) will have the option to receive DEX in addition to CC-220 after consultation with the Medical Monitor. The dose of CC-220 will not be higher than the dose of CC-220 used in combination with dexamethasone in Cohort B that has been determined to be safe. Progressive disease must be confirmed in accordance with international myeloma working group (IMWG) criteria.

The starting dose of DEX will be 40 mg for subjects who are ≤75 years of age and 20 mg for subjects who are >75 years of age, given once weekly. This treatment will continue until PD, unacceptable toxicity or the subject withdraws consent.

For Cohorts A and B, the starting dose level of CC-220, dose level 1, is 0.3 mg. A dose level -1, of 0.15 mg, may also be evaluated if the starting dose level of 0.3 mg for 21 days of a 28-day cycle is not tolerated. For Cohorts E and F, the starting dose level of CC-220, dose level 1, is one dose level below the maximum dose for Cohort B that has been determined to be safe by the dose escalation committee (DEC) at the start of enrollment for both cohorts. For Cohort E in addition to CC-220 and DEX, daratumumab will be administered intravenously (IV) at a 16mg/kg dose. For Cohort F in addition to CC-220 and DEX, bortezomib will be administered subcutaneous (SC) at a 1.3mg/m2 dose.

All subjects with a minimal response (MR) or better who discontinue study treatment in Part 1 or Part 2 of the study for a reason other than PD or withdrawal of consent from the study will be followed for response assessment every 28 days (every 21 days for Cohort F) until PD.

The study will be conducted in compliance with the International Council for Harmonisation (ICH) of Technical Requirements for Registration of Pharmaceuticals for Human Use/Good Clinical Practice (GCP) and applicable regulatory requirements.

The initiation of Part 2 will begin when the RP2D is established in Part 1 in either Cohort A or Cohort B. Either cohort may begin once the RP2D is determined for each cohort independently during Part 1. All expansion decisions will be determined by the DEC after review of all safety, PK, biomarker and preliminary efficacy data, as applicable. During Part 2, the Independent Expert Reviewer will review safety data and any other data deemed relevant so that subject safety is ensured.