Criteria for dose-limiting toxicity (DLT) included hematologic indicators such as Grade 3-5 febrile neutropenia and thrombocytopenia with bleeding. Non-hematologic criteria, excluding corneal toxicity, comprise Grade 3-5 toxicities, with exceptions for manageable nausea, vomiting, or diarrhea, controlled Grade 3 hypertension, and events linked to disease progression. Tumor lysis syndrome (TLS) of Grade 3 or 4, successfully managed within 7 days without end-organ damage, is considered. Corneal toxicity, assessed by the GSK corneal grading scale at Grade 4, is a DLT. Other organ-specific toxicities, notably liver toxicity meeting GSK stopping criteria, also qualify as DLTs. Severity was graded using National Cancer Institute - Common Terminology Criteria for Adverse Events (version 5.0).

DLT Evaluable Population included participants in DE who have received the first course of treatment containing both agents within a sub-study and followed up within cycle 1 (Each cycle is of 21 days) or withdrew within cycle 1 due to an AE meeting the definition of a DLT.

An AE is any untoward medical occurrence in a clinical investigation participant, temporally associated with the use of a medicinal product, whether or not considered related to the medicinal product. AEs were coded using the Medical Dictionary for Regulatory Activities (MedDRA) coding system.

Safety population included all participants who received at least 1 dose of any component of the combination therapy.

Blood samples were collected for the analysis of following hematology parameters: eosinophils, anemia, hemoglobin increased, lymphocyte count decreased, lymphocyte count increased, neutrophil count decreased, platelet count decreased, leukocytosis and white blood cell decreased. Grade 1 (G1): mild; Grade 2 (G2): moderate; Grade 3 (G3): severe; Grade 4 (G4) life-threatening or disabling. Higher grade indicates greater severity and an increase in CTCAE grade was defined relative to the Baseline grade. Any worst-case post baseline increase to G1, G2, G3, and G4 are presented. The laboratory parameters were graded according to Common Terminology Criteria for Adverse Events (CTCAE) version 5.

Safety Population.

Blood samples were collected for the analysis of following chemistry parameters: Hypoglycemia, hypoalbuminemia, alkaline phosphatase (ALP) increased, alanine aminotransferase (ALT) increased, aspartate aminotransferase (AST) increased, blood bilirubin increased, creatine kinase (CPK) increased, creatinine increased, gamma glutamyl transferase (GGT) increased, hyperkalemia, blood lactate dehydrogenase (LDH) increased, hypermagnesemia, hypomagnesemia, hypernatremia, hypercalcemia, hypocalcemia and chronic kidney disease. G1: mild; G2: moderate; G3: severe. Higher grade indicates greater severity and an increase in CTCAE grade was defined relative to the Baseline grade. Any worst-case post baseline increase to G1, G2 and G3 are presented. The laboratory parameters were graded according to CTCAE version 5.

Safety Population. Only those participants with data available at the specified categories were analyzed.

Overall Response Rate (ORR) was defined as the percentage of participants with a confirmed PR or better as the best overall response (i.e., Partial Response [PR], Very Good Partial Response [VGPR], Complete Response [CR], and stringent Complete Response [sCR]), as assessed by the investigator per international myeloma working group (IMWG) (2016). CR defined as negative immunofixation of serum and urine AND disappearance of any soft tissue plasmacytomas AND <5% plasmacytomas in the bone marrow; sCR defined as CR as above PLUS normal serum free light-chain (FLC) assay ratio and absence of clonal cells in bone marrow by immunohistochemistry or immunofluorescence; VGPR defined as serum and urine M-component detectable by immunofixation but not on electrophoresis OR ≥ 90% reduction in serum M-component plus urine M-component <100 mg/24 h; PR defined as ≥50% reduction of serum M-protein and reduction in 24-hour urinary M-protein by ≥90% or to <200 mg/24 h.

No participants were enrolled in CE Phase as study was terminated.

Overall Response Rate (ORR) was defined as the percentage of participants with a confirmed PR or better as the best overall response (i.e., Partial Response [PR], Very Good Partial Response [VGPR], Complete Response [CR], and stringent Complete Response [sCR]), as assessed by the investigator per international myeloma working group (IMWG) (2016). CR defined as negative immunofixation of serum and urine AND disappearance of any soft tissue plasmacytomas AND <5% plasmacytomas in the bone marrow; sCR defined as CR as above PLUS normal serum free light-chain (FLC) assay ratio and absence of clonal cells in bone marrow by immunohistochemistry or immunofluorescence; VGPR defined as serum and urine M-component detectable by immunofixation but not on electrophoresis OR ≥ 90% reduction in serum M-component plus urine M-component <100 mg/24 h; PR defined as ≥50% reduction of serum M-protein and reduction in 24-hour urinary M-protein by ≥90% or to <200 mg/24 h.

Safety Population

Clinical benefit rate was defined as the percentage of participants with a confirmed minimal response (MR) or better according to the IMWG Response Criteria. MR is >= 25% but < 49% reduction of serum M-protein and reduction in 24-hour urinary M-protein by 50-89%.

No participants were enrolled in CE Phase as study was terminated.

Partial Response [PR], Very Good Partial Response [VGPR], Complete Response [CR], and stringent Complete Response [sCR] as assessed by the investigator per IMWG (2016). CR defined as negative immunofixation of serum and urine AND disappearance of any soft tissue plasmacytomas AND <5% plasmacytomas in the bone marrow; sCR defined as CR as above PLUS normal serum free light-chain (FLC) assay ratio and absence of clonal cells in bone marrow by immunohistochemistry or immunofluorescence; VGPR defined as serum and urine M-component detectable by immunofixation but not on electrophoresis OR ≥ 90% reduction in serum M-component plus urine M-component <100 mg/24 h; PR defined as ≥50% reduction of serum M-protein and reduction in 24-hour urinary M-protein by ≥90% or to <200 mg/24 h.

Safety Population

Partial Response [PR], Very Good Partial Response [VGPR], Complete Response [CR], and stringent Complete Response [sCR] as assessed by the investigator per IMWG (2016). CR = negative immunofixation of serum and urine AND disappearance of any soft tissue plasmacytomas AND <5% plasmacytomas in the bone marrow; sCR=stringent complete response, CR as above PLUS normal serum free light-chain (FLC) assay ratio and absence of clonal cells in bone marrow by immunohistochemistry or immunofluorescence; VGPR = serum and urine M-component detectable by immunofixation but not on electrophoresis OR ≥ 90% reduction in serum M-component plus urine M-component <100 mg/24 h; PR = ≥50% reduction of serum M-protein and reduction in 24-hour urinary M-protein by ≥90% or to <200 mg/24 h.

No participants were enrolled in CE Phase as study was terminated.

Blood samples were collected for PK analysis of belantamab mafodotin Antibody-Drug Conjugate (ADC) when administered intravenously in combination with dostarlimab.

Pharmacokinetic population. Only those participants with data available at the specified data points were analyzed.

Blood samples were collected for PK analysis of belantamab mafodotin total antibody when administered intravenously in combination with dostarlimab.

Pharmacokinetic population. Only those participants with data available at the specified data points were analyzed.

Blood samples were collected for PK analysis of belantamab mafodotin cys-mcMMAF when administered intravenously in combination with dostarlimab.

Pharmacokinetic population. Only those participants with data available at the specified data points were analyzed.

Blood samples were to be collected for PK analysis of Belantamab Mafodotin Antibody-Drug Conjugate (ADC).

No participants were enrolled in CE Phase as study was terminated.

Blood samples were to be collected for PK analysis of Belantamab mafodotin plasma total antibody.

No participants were enrolled in CE Phase as study was terminated.

Blood samples were to be collected for PK analysis of Belantamab Mafodotin Cys- Monomethyl Auristatin-F (Cys-mcMMAF)

No participants were enrolled in CE Phase as study was terminated.

Blood samples were collected for PK analysis of dostarlimab when administered intravenously in combination with belantamab mafodotin.

Pharmacokinetic population included all participants in the Safety population who had at least 1 non-missing PK assessment. Only those participants with data available at the specified data points were analyzed.

Blood samples were to be collected for PK analysis of dostarlimab when administered intravenously in combination with belantamab mafodotin.

No participants were enrolled in CE Phase as study was terminated.

Serum samples were collected for the analysis of the presence of ADAs using validated immunoassays. All samples were tested in screening assay, and positive samples were further characterized for antibody titers.

Safety Population

Serum samples were to be collected for the analysis of the presence of ADAs using validated immunoassays. All samples were to be further tested in screening assay, and positive samples were further to be characterized for antibody titers.

No participants were enrolled in CE Phase as study was terminated.

Serum samples were collected for the analysis of the presence of ADAs using validated immunoassays. All samples were tested in screening assay, and positive samples were further characterized for antibody titers.

Safety Population.

Serum samples were to be collected for the analysis of the presence of ADAs using validated immunoassays. All samples were to be further tested in screening assay, and positive samples were further to be characterized for antibody titers.

No participants were enrolled in CE Phase as study was terminated.

Serum samples were collected for the analysis of the presence of ADAs using validated immunoassays. All samples were to be further tested in screening assay, and positive samples were further to be characterized for antibody titers.

Safety Population. No participants were found positive for ADAs, hence participants were not analyzed for concentration of ADAs.

Serum samples were to be collected for the analysis of the presence of ADAs using validated immunoassays. All samples were to be further tested in screening assay, and positive samples were further to be characterized for antibody titers.

No participants were enrolled in CE Phase as study was terminated.

Serum samples were collected for the analysis of the presence of ADAs using validated immunoassays. All samples were to be further tested in screening assay, and positive samples were further to be characterized for antibody titers.

Safety Population. Only those participants with positive post-baseline antibody against belantamab mafodotin were analyzed.

Serum samples were to be collected for the analysis of the presence of ADAs using validated immunoassays. All samples were to be further tested in screening assay, and positive samples were further to be characterized for antibody titers.

No participants were enrolled in CE Phase as study was terminated.

An adverse event (AE) is any untoward medical occurrence in a clinical study participant, temporally associated with the use of a medicinal product, whether or not considered related to the medicinal product. Adverse events of special interest (AESIs) were collected.

Safety Population.

An adverse event (AE) is any untoward medical occurrence in a clinical study participant, temporally associated with the use of a medicinal product, whether or not considered related to the medicinal product. Adverse events of special interest (AESIs) were to be collected.

No participants were enrolled in CE Phase as study was terminated.

The corneal events were graded according to CTCAE version 5. Grade 1: mild; Grade 2: moderate; Grade 3: severe or medically significant. Higher grade indicates greater severity and an increase in CTCAE grade was defined relative to the Baseline grade. Results are presented for number of participants with any corneal events by maximum grade as per CTCAE grade.

Safety Population

The corneal events were to be graded according to CTCAE version 5. Grade 1: mild; Grade 2: moderate; Grade 3: severe or medically significant. Higher grade indicates greater severity and an increase in CTCAE grade was defined relative to the Baseline grade. Corneal Events were to be examined.

No participants were enrolled in CE Phase as study was terminated.

PFS is defined as the time from randomization until the earliest date of confirmed progressive disease (PD) per IMWG, or death due to any cause.

No participants were enrolled in CE Phase as study was terminated.

DoR is defined as the time from first documented evidence or PR or better until progressive disease per IMWG or death due to progressive disease among participants who achieve confirmed partial response or better.

No participants were enrolled in CE Phase as study was terminated.

TTR is defined as the time between the date of randomization and the first documented evidence of response (PR or better), among participants who achieve a response (confirmed PR or better).

No participants were enrolled in CE Phase as study was terminated.

OS is defined as the time from randomization until death due to any cause.

No participants were enrolled in CE Phase as study was terminated.

An adverse event (AE) is any untoward medical occurrence in a clinical study participant, temporally associated with the use of a medicinal product, whether or not considered related to the medicinal product. SAE is defined as any untoward medical occurrence that; results in death, is life-threatening, requires hospitalization or prolongation of existing hospitalization, results in disability/incapacity, is a congenital anomaly/birth defect, other situations judged by physician, is associated with liver injury and impaired liver function. AEs and SAEs were to be coded using the Medical Dictionary for Regulatory Activities (MedDRA) coding system.

No participants were enrolled in CE Phase as study was terminated.

An adverse event (AE) is any untoward medical occurrence in a clinical study participant, temporally associated with the use of a medicinal product, whether or not considered related to the medicinal product. Number of participants with AEs leading to discontinuation were to be evaluated.

No participants were enrolled in CE Phase as study was terminated.

Number of participants with dose reduction or delay were to be evaluated.

No participants were enrolled in CE Phase as study was terminated.

Blood samples were to be collected for the analysis of following hematology parameters: eosinophils, anemia, hemoglobin increased, lymphocyte count decreased, lymphocyte count increased, neutrophil count decreased, platelet count decreased, leukocytosis and white blood cell decreased. The laboratory parameters were to be graded according to Common Terminology Criteria for Adverse Events (CTCAE) version 5. Grade 1 (G1): mild; Grade 2 (G2): moderate; Grade 3 (G3): severe or medically significant. Higher grade indicates greater severity and an increase in CTCAE grade was defined relative to the Baseline grade.

No participants were enrolled in CE Phase as study was terminated.

Blood samples were to be collected for the analysis of following chemistry parameters: Hypoglycemia, hypoalbuminemia, alkaline phosphatase (ALP) increased, alanine aminotransferase (ALT) increased, aspartate aminotransferase (AST) increased, blood bilirubin increased, creatine kinase (CPK) increased, creatinine increased, gamma glutamyl transferase (GGT) increased, hyperkalemia, blood lactate dehydrogenase (LDH) increased, hypermagnesemia, hypomagnesemia, hypernatremia, hypercalcemia, hypocalcemia and chronic kidney disease. The laboratory parameters were to be graded according to CTCAE version 5. G1: mild; G2: moderate; G3: severe or medically significant. Higher grade indicates greater severity and an increase in CTCAE grade was defined relative to the Baseline grade.

No participants were enrolled in CE Phase as study was terminated.