A Collaborative Community Effort Using Belantamab Mafodotin in Relapsed/Refractory Myeloma (COSTA)

Inclusion Criteria: 1. Participant must have histologically or cytologically confirmed diagnosis of MM as defined by IMWG, 2016 criteria, have had at least 4 prior therapies, and is relapsed or refractory to an anti-CD38 antibody, an IMID, and a proteasome inhibitor. Refractory myeloma is defined as disease that is nonresponsive while on therapy or progresses within 60 days of last therapy. Nonresponsive disease is defined as either failure to achieve at least minimal response or development of progressive disease (PD) while on any therapy. 2. Participant must have an Eastern Cooperative Oncology Group (ECOG) performance status of ≤ 2 3. Participant must be ≥ 18 years of age 4. Participant must have adequate organ function, defined as follows: 5. Female participants: a. A female participant is eligible to participate if she is not pregnant or breastfeeding and at least one of the following conditions applies: i. Is not a woman of childbearing potential (WOCBP) OR ii. Is a WOCBP and using a contraceptive method that is highly effective (with a failure rate of <1% per year), preferably with low user dependency (as described in Appendix 3), during the intervention period and for at least 4 months 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/urine pregnancy test (as required by the protocol) within 72 hours before the first dose of study intervention. WOCBP will have pregnancy testing within 72 hours on day 1 of each cycle. The investigator is responsible for review of medical history, menstrual history, and recent sexual activity. b. Nonchildbearing potential is defined as follows: i. Premenarchal ii. Premenopausal female with ONE of the following: 1. Documented hysterectomy 2. Documented bilateral salpingectomy 3. Documented bilateral oophorectomy 4. Documented post-tubal ligation surgery 5. For individuals with permanent infertility due to an alternate medical cause other than the above, (e.g., mullerian agenesis, androgen insensitivity), investigator discretion should be applied to determining study entry. a. Note: Documentation can come from the site personnel's: review of participant's medical records, medical examination, or medical history review iii. Postmenopausal female A postmenopausal state is define as no menses for 12 months without an alternative medical cause. A high follicle stimulating hormone (FSH) level in the postmenopausal range may be used to confirm a postmenopausal state in women not using hormonal contraception or hormonal replacement therapy (HRT). However, in the absence of 12 months of amenorrhea, confirmation with more than one FSH measurement is required. Females on HRT and whose menopausal status is in doubt will be required to use of the non-estrogen hormonal highly effective contraception methods if they wish to continue their HRT during the study. Otherwise, they must discontinue HRT to allow confirmation of postmenopausal status before study enrollment. 6. Male participants: Male participants are eligible to participate if they agree to the following during the intervention period and for 6 months after the last dose of study treatment to allow for clearance of any altered sperm: 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 c. Must agree to use contraception/barrier as detailed below: i. Agree to use a male condom, even if they have undergone a successful vasectomy, and female partner to use an additional highly effective contraceptive method with a failure rate of <1% per year as when having sexual intercourse with a woman of childbearing potential (including pregnant females). 7. All prior treatment-related toxicities (defined by National Cancer Institute- Common Toxicity Criteria for Adverse Events (NCI-CTCAE), version 5.0) must be ≤ Grade 1 at the time of enrollment except for alopecia or at a stable baseline for at least 6 weeks. 8. Participant must be able to understand the study procedures and agree to participate in the study by providing written informed consent 9. Participant must agree to not wear contact lenses while on the study 10. Participant must agree to have a detailed eye exam by an eye care specialist prior to each treatment of Belantamab mafodotin and agree to use preservative-free lubricant eye drops at least 4 times every day while on treatment Exclusion Criteria: 1. Participant must not have current corneal epithelial disease except mild changes in corneal epithelium 2. Participant must not have current unstable liver or biliary disease defined by the presence of ascites, encephalopathy, coagulopathy, hypoalbuminemia, esophageal or gastric varices, persistent jaundice, or cirrhosis. NOTE: Stable non-cirrhotic chronic liver disease (including Gilbert's syndrome or asymptomatic gallstones) or hepatobiliary involvement of malignancy is acceptable if otherwise meets entry criteria 3. Participant must not have 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 inclusion criteria 4. Participant unwilling to forego use of contact lenses while participating in this study 5. Participant must not be simultaneously in any other therapeutic clinical trial without permission of the Sponsor 6. Participant must not have used an investigational drug or approved systemic anti-myeloma therapy (including systemic steroids) within 14 days or five half-lives, whichever is shorter, preceding the first dose of study drug and to have recovered from any drug-related drug toxicity to at least Grade 1 7. Participant must not have had plasmapheresis within 7 days prior to first dose of study treatment 8. Participant must not have received prior treatment with a monoclonal antibody within 30 days of receiving the first dose of study drugs 9. Participant must not have had major surgery ≤ 4 weeks prior to initiating study treatment 10. Participant must not have any evidence of spontaneous mucosal or internal bleeding 11. Participant must not have had prior allogenic stem cell transplant. NOTE: Participants who have undergone syngeneic transplant will be allowed only if no currently active GvHD. 12. Participant must not have evidence of significant cardiovascular risk including any of the following: a. Evidence of current clinically significant uncontrolled arrhythmias, including clinically significant ECG abnormalities such as 2nd degree (Mobitz Type II) or 3rd degree atrioventricular (AV) block. b. History of recent myocardial infarction, acute coronary syndromes (including unstable angina), coronary angioplasty, or stenting or bypass grafting within three (3) months of initiating therapy on this study. c. Class III or IV heart failure as defined by the New York Heart Association functional classification system [NYHA, 1994] d. Uncontrolled hypertension 13. Participant must not have known immediate or delayed hypersensitivity reaction or idiosyncratic reactions to belantamab mafodotin or drugs chemically related to belantamab mafodotin, or dexamethasone, or any components of the study drug. 14. Participant must not have an active infection requiring treatment 15. Known HIV infection, unless the participant can meet all of the following criteria: Established anti-retroviral therapy (ART) for at least 4 weeks and HIV viral load <400 copies/mL CD4+ T-cell (CD4+) counts ≥350 cells/uL No history of AIDS-defining opportunistic infections within the last 12 months NOTE: consideration must be given to ART and prophylactic antimicrobials that may have a drug: drug interaction and/or overlapping toxicities with belantamab mafodotin or other combination products as relevant (See section 4.3, Concomitant Therapy) 16. Participant must not have 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 treatment. Note: presence of Hep B surface antibody (HBsAb) indicating previous vaccination will not exclude a participant. 17. Participant must not have positive hepatitis C antibody test result or positive hepatitis C RNA test result at screening or within 3 months prior to first dose of study treatment. NOTE: Participants with positive Hepatitis C antibody due to prior resolved disease can be enrolled, if a confirmatory negative Hepatitis C RNA test is obtained. NOTE: Hepatitis RNA testing is optional and participants with negative Hepatitis C antibody test are not required to also undergo Hepatitis C RNA testing. 18. Participant must not have invasive malignancies other than disease under study, unless the second malignancy has been medically stable for at least 2 years and, in the opinion of the principal investigators, will not affect the evaluation of the effects of clinical trial treatments on the currently targeted malignancy. Participants with curatively treated non-melanoma skin cancer, prostate cancer or ductal carcinoma in-situ breast cancer not requiring ongoing therapy may be enrolled without a 2-year restriction. 19. Participant must not have 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