Study of Pembrolizumab (MK-3475) in Participants With Relapsed or Refractory Primary Mediastinal Large B-cell Lymphoma or Relapsed or Refractory Richter Syndrome (MK-3475-170/KEYNOTE-170)

Treatment with pembrolizumab will continue for a maximum of 35 administrations (approximately
2 years) or until documented disease progression by investigator assessment, unacceptable
adverse event(s) (AEs), intercurrent illness that prevents further administration of
treatment, participant withdraws consent, pregnancy of the participant, noncompliance with
study treatment or procedure requirements, or administrative reasons.

Inclusion Criteria:

- PMBCL:

- Diagnosis of relapsed or refractory primary mediastinal large B-cell lymphoma AND

- Has relapsed after autologous stem cell transplant (auto-SCT) or has failed to achieve
a Complete Response or Partial Response within 60 days of auto-SCT. Participants may
have received intervening therapy after auto-SCT for relapsed or refractory disease,
in which case they must have relapsed after or be refractory to their last treatment.

OR

- For participants who are ineligible for auto-SCT, has received at least ≥2 lines of
prior therapy and has failed to respond to or relapsed after their last line of
treatment. For participants who received consolidative local radiotherapy after
systemic therapy, local radiotherapy will not be considered as a separate line of
treatment.

- Previously exposed to rituximab as part of prior lines of treatment.

- RS:

- Pathologic diagnosis per local institutional review of Richter syndrome that
transformed from chronic lymphocytic leukemia (CLL).

- Relapsed or refractory Richter syndrome and has received ≥1 previous treatment for RS.

- All Participants:

- Radiographically measureable disease.

- Performance status of 0 or 1 on the Eastern Cooperative Oncology Group (ECOG)
Performance Scale.

- Life expectancy >3 months.

- Adequate organ function.

- Female participants of childbearing potential must be willing to use an adequate
method of contraception for the course of the study through 120 days after the last
dose of study drug.

- Male participants of childbearing potential must agree to use an adequate method of
contraception, starting with the first dose of study drug through 120 days after the
last dose of study drug.

Exclusion Criteria:

- Is currently participating and receiving study therapy or has participated in a study
of an investigational agent and received study therapy or used an investigational
device within 4 weeks of the first dose of study drug.

- Is receiving systemic steroid therapy <3 days before the first dose of study drug or
receiving any other form of immunosuppressive medication.

- Prior monoclonal antibody within 4 weeks prior to study Day 1 (2 weeks for RS
participants) or who has not recovered (i.e. ≤ Grade 1 or at baseline) from adverse
events due to agents administered more than 4 weeks earlier (2 weeks for RS
participants).

- Prior chemotherapy or targeted small molecule therapy within 2 weeks prior to study
Day 1 or prior radiation therapy within 4 weeks prior to study Day 1.

- Allogeneic hematopoietic stem cell transplantation within the last 5 years.

- Has a known additional malignancy (except underlying CLL for RS) that is progressing
or requires active treatment. Exceptions include basal cell carcinoma of the skin,
squamous cell carcinoma of the skin, or in situ cervical cancer that has undergone
potentially curative therapy.

- Known clinically active central nervous system involvement.

- Active autoimmune disease requiring systemic treatment in past 2 years.

- History of (non-infectious) pneumonitis that required steroids, or current
pneumonitis.

- Active infection requiring intravenous systemic therapy.

- Is pregnant or breastfeeding, or expecting to conceive or father children within the
projected duration of the study, starting with the pre-screening or screening visit
through 120 days after the last dose of study drug.

- Has received prior therapy with an anti-programmed cell death 1 (anti-PD-1),
anti-programmed cell death ligand 1 (anti-PD-L1), anti-PD-L2, anti-CD137, or
anti-cytotoxic T-lymphocyte-associated antigen-4 (CTLA-4) antibody (including
ipilimumab or any other antibody or drug specifically targeting T-cell co-stimulation
or checkpoint pathways).

- Known human immunodeficiency virus (HIV), or Hepatitis B or C.

- Has received a live vaccine within 30 days prior to first dose of study drug.