A Study of Belantamab Mafodotin to Investigate Safety, Tolerability, Pharmacokinetics, Immunogenicity and Clinical Activity in Participants With Relapsed/Refractory Multiple Myeloma (RRMM)

Inclusion Criteria:

* Provide signed written informed consent, which includes compliance with requirements and restrictions listed in the consent form.
* Male or female, 18 years or older (at the time consent is obtained).
* Eastern Cooper Oncology Group (ECOG) performance status of 0-2.
* Histological or cytologically confirmed diagnosis of MM as defined according to IMWG criteria and a) Has undergone stem cell transplant or transplant is considered not feasible by local assessment, b) Has failed at least 2 prior lines of anti-myeloma treatments, containing all of the following classes of drugs: alkylating agent, IMID and PI, c) In addition, eligible participant needs to be refractory to an IMiD (i.e.,lenalidomide or pomalidomide), and to a proteasome inhibitor (i.e., bortezomib, ixazomib or carfizomib) as defined by International Myeloma Working Group (IMWG) criteria, d) Participants who failed with cluster of differentiation38 (CD38) antibody (i.e., daratumumab) in previous clinical trials can also be considered to include if they meet the remainder of inclusion criteria in this protocol.
* Has measurable disease with at least one of the following: a) Serum M-protein greater than or equal to 0.5 grams per deciliter (g/dL) (5 g/L) , b) Urine M-protein greater than or equal to 200 mg/24hour, c) Serum Free light chain (FLC) assay: Involved FLC level greater than or equal to10 mg/dL (greater than or equal to 100 mg/L) and an abnormal serum free light chain ratio (less than 0.26 or greater than 1.65)
* Participants with a history of autologous stem cell transplant are eligible for study participation provided the following eligibility criteria are met: a) Transplant was greater than 100 days prior to study enrolment, b) No active infection(s), c) Participant meets the remainder of the eligibility criteria outlined in this protocol.
* Adequate organ system functions.
* 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 at least one of the following conditions applies: Is 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 less than 1 percent per year), preferably with low user dependency, during the intervention period and for at least 80 days after the last dose of study intervention and agrees not to donate eggs (ova, oocytes) for the purpose of reproduction during this period. The investigator should evaluate the effectiveness of the contraceptive method in relationship to the first dose of study intervention. A WOCBP must have a negative highly sensitive serum pregnancy test (as required by local regulations) within 72 hours before the first dose of study intervention. The investigator is responsible for review of medical history, menstrual history, and recent sexual activity to decrease the risk for inclusion of a woman with an early undetected pregnancy.
* Male Participants: Contraceptive use by men or women should be consistent with local regulations regarding the methods of contraception for those participating in clinical studies. Male participants are eligible to participate if they agree to the following during the intervention period and for at least 140 days: Refrain from donating sperm, plus either be abstinent from heterosexual intercourse as their preferred and usual lifestyle (abstinent on a long term and persistent basis) and agree to remain abstinent. or Must agree to use contraception/barrier as detailed below: Agree to use a male condom and female partner to use an additional highly effective contraceptive method with a failure rate of less than 1 percent per year as when having sexual intercourse with a woman of childbearing potential who is not currently pregnant.
* All prior treatment-related toxicities (defined by National Cancer Institute-Common Toxicity Criteria for Adverse Events (NCI-CTCAE), version 5.0 must be less than or equal to Grade 1 at the time of enrolment except for alopecia and Grade 2 peripheral neuropathy.

Exclusion Criteria:

* Systemic anti-myeloma therapy within less than 14 days, or plasmapheresis within 7 days prior to the first dose of study drug.
* 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 (SCT).
* 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, provided they fulfil criteria.
* 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 GSK Medical Monitor, will not affect the evaluation of the effects of this clinical trial treatment on the currently targeted malignancy (MM).
* Evidence of cardiovascular risk including any of the following: a) corrected QT internal Fridericia (QTcF) interval greater than equal to 480 milliseconds (msecs), b) Evidence of current clinically significant untreated arrhythmias, including clinically significant ECG abnormalities such as 2nd degree (Mobitz Type II) or 3rd degree atrioventricular (AV) block, c) History of myocardial infarction, acute coronary syndromes (including unstable angina), coronary angioplasty, or stenting or bypass grafting within three months of Screening, d) Class III or IV heart failure as defined by the New York Heart Association functional classification system, e) Uncontrolled severe hypertension, e.g. 170/110.
* Known immediate or delayed hypersensitivity reaction or idiosyncrasy to drugs chemically related to belantamab mafodotin, or any of the components of the study intervention.
* Pregnant or lactating female.
* Active infection requiring antibiotic, antiviral, or antifungal treatment.
* Known human immunodeficiency virus (HIV) infection.
* Presence of hepatitis B surface antigen (HBsAg), or hepatitis B core antibody (HBcAb at screening or within 3 months prior to first dose of study intervention).
* Positive hepatitis C antibody test result or positive hepatitis C ribonucleic acid (RNA) test result at screening or within 3 months prior to first dose of study intervention.