A Study to Investigate the Efficacy and Safety of Two Doses of GSK2857916 in Participants With Multiple Myeloma Who Have Failed Prior Treatment With an Anti-CD38 Antibody

Inclusion Criteria

• Participants who provided signed written informed consent, which includes compliance with the requirements and restrictions listed in the consent form.
• Male or female, 18 years or older.
• Participants with Eastern Cooperative Oncology Group (ECOG) performance status of 0-2.
• Participants with histologically or cytologically confirmed diagnosis of MM as defined in IMWG, 2014 criteria, and participant has undergone stem cell transplant or is considered transplant ineligible and has failed at least 3 prior lines of anti-myeloma treatments, including an anti-CD38 antibody (example [e.g.], daratumumab) alone or in combination, and is refractory to an Immunomodulatory drug (IMiD) (that is [i.e.], lenalidomide or pomalidomide), and to a proteasome inhibitor (e.g., bortezomib, ixazomib or carfilzomib).
• The participant has measurable disease with at least one of the following: Serum M-protein >=0.5 grams per deciliter (g/dL) (>=5 grams per Liter [g/L]); Urine M-protein >=200 milligram per 24 hours (mg/24h); Serum Free light chain (FLC) assay: Involved FLC level >=10 mg/dL (>=100 mg/Liter) and an abnormal serum FLC ratio ( 1.65).
• Participants with a history of autologous stem cell transplant are eligible for study participation provided the following eligibility criteria are met: transplant was >100 days prior to study enrollment; no active infection(s); participants meet the remainder of the eligibility criteria outlined in the protocol.
• Participants with adequate organ system functions as defined follows: Absolute neutrophil count (ANC) >=1.0 X 10^9/L; Hemoglobin >=8.0 g/dL; Platelets>= 50 X 10^9/L; Total bilirubin =1.5xULN is acceptable if bilirubin is fractionated and direct bilirubin =30 milliliter per minute per 1.73 meter square (mL/min/m^2); Spot urine (albumin/creatinine ratios [spot urine]) =45 percent.
• Female participants: Contraceptive use by men or women should be consistent with local regulations regarding the methods of contraception for those participating in clinical studies. A female participant is eligible to participate if she is not pregnant or breastfeeding, and not a woman of childbearing potential (WOCBP) or is a WOCBP and using a contraceptive method that is highly effective (with a failure rate of =60 mg prednisone daily for >=4 days) within the past 14 days if administered to treat MM or non-MM disease.
• Symptomatic amyloidosis, active 'polyneuropathy, organomegaly, endocrinopathy, myeloma protein, and skin changes' (POEMS) syndrome, active plasma cell leukemia at the time of screening.
• Prior allogeneic stem cell transplant.
• Current corneal epithelial disease except mild punctate keratopathy.
• Use of an investigational drug within 14 days or five half-lives, whichever is shorter, preceding the first dose of study drug. Prior treatment with a monoclonal antibody within 30 days of receiving the first dose of study drugs. Prior B-cell maturation antigen (BCMA) targeted therapy.
• Evidence of active mucosal or internal bleeding.
• Any major surgery within the last four weeks.
• Presence of active renal condition (infection, requirement for dialysis or any other condition that could affect participant's safety). Participants with isolated proteinuria resulting from MM are eligible.
• Any serious and/or unstable pre-existing medical, psychiatric disorder or other conditions (including lab abnormalities) that could interfere with participant's safety, obtaining informed consent or compliance to the study procedures.
• Current unstable liver or biliary disease per investigator assessment defined by the presence of ascites, encephalopathy, coagulopathy, hypoalbuminemia, esophageal or gastric varices, persistent jaundice, or cirrhosis.
• Malignancies other than disease under study are excluded, except for any other malignancy from which the participant has been disease-free for more than 2 years and, in the opinion of the principal investigators and GlaxoSmithKline Medical Monito