To Evaluate Safety, Tolerability, and Clinical Activity of the Antibody-drug Conjugate, GSK2857916 Administered in Combination With Lenalidomide Plus Dexamethasone (Arm A), or in Combination With Bortezomib Plus Dexamethasone (Arm B) in Subjects With Relapsed/Refractory Multiple Myeloma (RRMM)

Inclusion Criteria:

- Capable of giving signed informed consent.

- Male or female, 18 years or older (at the time consent is obtained).

- Have confirmed diagnosis of Multiple Myeloma as defined by the IMWG.

- Eastern Cooperative Oncology Group (ECOG) performance status of 0 to 2

- Have undergone stem cell transplant (SCT), or are considered transplant ineligible.

- Have been previously treated with at least 1 prior line of MM therapy, and must have
documented disease progression during or after their most recent therapy according to
the IMWG criteria.

- Must have at least ONE aspect of measurable disease, defined as one the following:
Urine M-protein excretion >=200 milligram (mg)/24 hours, or; Serum M-protein
concentration >=0.5 gram (g)/deciliter (dL) (>=5.0 g/Liter), or; Serum free light
chain (FLC) assay: involved FLC level >=10 mg/dL (>=100 mg/L) and an abnormal serum
FLC ratio (<0.26 or >1.65).

- Subjects with a history of autologous SCT, are eligible for study participation
provided the following eligibility criteria are met: Autologous SCT was >100 days
prior to study enrollment; No active bacterial, viral, or fungal infection(s) present;
Subject meets the remainder of the eligibility criteria.

- All prior treatment-related toxicities (defined by National Cancer Institute Common
Toxicity Criteria for Adverse Events [NCI-CTCAE], Version 4.03, 2010) must be <= Grade
1 at the time of enrollment, except for alopecia. Subjects with Grade 2 neuropathy can
be enrolled into Len/Dex treatment arm, but not into Bor/Dex treatment arm.

- Adequate organ system functions as defined by the laboratory assessments.

- The contraception's used by female subject's be consistent with local regulations,
regarding methods of contraception for those participating in clinical studies. A
female subject 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 child bearing
potential (WOCBP) or 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,
during the intervention period and for at least 120 days after the last dose of study
intervention and agrees not to donate eggs (ova, oocytes) for the purpose of
reproduction during this period. WOCBP must have 2 negative highly sensitive serum
pregnancy tests, as required by local regulations (first within 14 days of Cycle 1 Day
1 and the second one within 72 hours of dosing on Cycle1 Day1) and agree to use
effective contraception during the study and for 120 days after the last dose of study
medication; 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 subject's using contraception should be consistent with local regulations
regarding the methods of contraception for those participating in clinical studies.
Males are eligible to participate if they 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 <1% per year when having sexual intercourse with a WOCBP who is
not currently pregnant.

Exclusion Criteria:

- Systemic anti-myeloma therapy (including systemic steroids) within 14 days, or
plasmapheresis within 7 days prior to the first dose of study drug.

- Use of an investigational drug within 14 days or five half-lives (whichever is longer)
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 allogenic stem cell transplant.

- 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 subject's safety). Subjects with isolated proteinuria
resulting from MM are eligible, provided they fulfill criteria.

- Any serious and/or unstable pre-existing medical, psychiatric disorder or other
conditions (including lab abnormalities) that could interfere with subject's safety,
obtaining informed consent or compliance to the study procedures.

- Current active liver or biliary disease (with the exception of Gilbert's syndrome or
asymptomatic gallstones, or otherwise stable chronic liver disease per investigator's
assessment).

- Subjects with previous or concurrent malignancies are allowed only if the second tumor
is not contributing to the subject's illness. The subject must not be receiving active
therapy, other than hormonal therapy for this disease and the disease must be
considered medically stable for at least 2 years.

- Evidence of cardiovascular risk including any of the following:

a. Corrected QT (QTc) interval >=470 millisecond (msec); Evidence of current
clinically significant uncontrolled arrhythmias, including clinically significant ECG
abnormalities including 2nd degree (Type II) or 3rd degree atrioventricular (AV)
block; History of myocardial infarction, acute coronary syndromes (including unstable
angina), coronary angioplasty, or stenting or bypass grafting within six months of
Screening; Class III or IV heart failure as defined by the New York Heart Association
functional classification system; Uncontrolled hypertension.

- Known immediate or delayed hypersensitivity reaction or idiosyncratic reaction to
drugs chemically related to GSK2857916, or any of the components of the study
treatment.

- Pregnant or lactating female.

- Active infection requiring treatment.

- Known 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 treatment).

- Current corneal disease except for mild punctuate keratopathy.

- 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: Subjects
with positive Hepatitis C antibody due to prior resolved disease can be enrolled, only
if a confirmatory negative Hepatitis C RNA test is obtained.

- Current corneal disease except for mild punctuate keratopathy.

- Additional Exclusion Criteria for subjects Assigned to Treatment A: Subjects unable to
tolerate antithrombotic prophylaxis must be excluded; Discontinuation of prior
treatment with lenalidomide due to intolerable AEs.

- Additional Exclusion Criteria for Subjects Assigned to Treatment B: Unacceptable AEs
from previous bortezomib treatment; Ongoing Grade 2 or higher peripheral neuropathy or
neuropathic pain from previous bortezomib treatment; Intolerance or contraindications
to anti-viral prophylaxis.