A Study of Venetoclax and ABBV-838 Combination Therapy With Dexamethasone in Participants With Multiple Myeloma Whose Cancer Has Come Back or Had No Response to Recent Cancer Treatment

This is an open-label, multicenter clinical trial designed to evaluate the safety and potential efficacy of venetoclax and ABBV-838 combination therapy with dexamethasone in participants with relapsed or refractory multiple myeloma (MM) who have received 2 or more prior lines of therapy for multiple myeloma (MM). The study will consist of 2 arms: Arm A and Arm B (if applicable). Each arm will have a dose escalation and dose expansion portion.

The study will consist of 2 arms: Arm A and Arm B (if applicable). Arm A dose escalation will investigate up to 3 doses of ABBV-838 at 3-week dosing intervals (Q3W) in combination with venetoclax and dexamethasone. Arm A dose expansion portion will investigate the ABBV-838 Q3W dosing interval with venetoclax and dexamethasone at the recommended phase two dose (RPTD) combination defined from the Dose Escalation portion.

Based on data from the ongoing ABBV-838 monotherapy study (Study M14-467) Arm B dose escalation may be conducted, if deemed necessary. If conducted, Arm B dose excalation will investigate up to 3 doses of ABBV-838 at either weekly (Q1W) or bi-weekly (Q2W) dosing intervals in combination with venetoclax and dexamethasone. Arm B dose expansion portion will investigate either the ABBV-838 Q1W or Q2W dosing interval in combination with venetoclax and dexamethasone at the RPTD combination defined from the Dose Escalation portion.

Eligibility Criteria:
Inclusion Criteria:

* Eastern Cooperative Oncology Group (ECOG) performance status less than or equal to 1 for participants in the dose escalation portion of the study and ECOG less than or equal to 2 in the dose expansion portion.
* Received at least 2 prior therapies including an Immunomodulatory Thalidomide Derivative Compounds (IMiD) and a proteasome inhibitor.
* Documented relapsed or progressive multiple myeloma on or after any regimen or is refractory to the most recent line of therapy.
* Received at least 2 prior therapies including an IMiD and a proteasome inhibitor.
* Documented relapsed or progressive multiple myeloma on or after any regimen or is refractory to the most recent line of therapy.
* Eligible for and agree to bone marrow (BM) aspirate prior to treatment start and at designated times per protocol.
* Measurable disease at Screening, defined as at least one of the following M component in serum (greater than or equal to 0.5 g/dL) and/or urine (greater than or equal to 0.2 g excreted in a 24 hour collection sample) or serum free light chain greater than or equal to 100 mg/dL with an abnormal κ/λ ratio of less than 0.26 or greater than 1.65.

Exclusion Criteria:

* Received any anti-myeloma therapy (other than monoclonal antibodies), including chemotherapy, radiotherapy, biological, immunotherapy or an investigational therapy, including targeted small molecule agents within 5 half-lives (or 14 days if half-live unknown) prior to first dose of first dose of venetoclax, ABBV-838, and dexamethasone.
* Received anti-myeloma monoclonal antibodies within 6 weeks prior to first dose of venetoclax, ABBV-838, and dexamethasone.
* Has a significant history of renal, neurologic (peripheral neuropathy), psychiatric, endocrinologic (diabetes mellitus), metabolic, immunologic, cardiovascular, pulmonary or hepatic disease within the last 6 months.
* Received corticosteroid therapy at a dose equivalent to greater than or equal to 4 mg/day of dexamethasone within 3 weeks prior to first dose.