Pembrolizumab With Talimogene Laherparepvec or Placebo in Unresected MelanomaMASTERKEY-265

- Age ≥ 18 years with histologically confirmed diagnosis of melanoma and stage IIIB to IVM1c for whom surgery is not recommended. - Subjects must have measurable disease and be a candidate for intralesional therapy administration into cutaneous, subcutaneous, or nodal lesions. - Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1. - Adequate hematologic, hepatic, renal, and coagulation function. - Subjects with serine/threonine protein kinase B-Raf V600 (BRAFV600) wild-type tumors must not have received any prior systemic anticancer treatment consisting of chemotherapy, immunotherapy, or targeted therapy given in a non-adjuvant setting for unresectable stage IIIB to IVM1c melanoma. - Subjects with B-Raf V600 (BRAFV600) mutated tumors who have received prior BRAF inhibitor therapy either alone or in combination with mitogen-activated protein kinase kinase (MEK) inhibitor as their only prior systemic therapy are eligible. - Subjects who received prior adjuvant therapy for melanoma will not be excluded (including, but not limited to, interferon, ipilimumab, limb infusion/perfusion, or use of investigational agents in the adjuvant setting) with the exception that prior adjuvant therapy with inhibitors of programmed cell death-1 (PD-1) or programmed cell death ligand 1 (PD-L1) is not allowed. However, if the subject received adjuvant therapy, the subject must have completed therapy at least 28 days prior to enrollment. - Subjects must have a tumor sample that is adequate for PD-L1 assessment prior to randomization.

- Subjects must not have clinically active cerebral metastases. - Subjects must not have primary uveal or mucosal melanoma, history or evidence of melanoma associated with immunodeficiency states or history of other malignancy within the past 3 years. - Subjects may not have been previously treated with talimogene laherparepvec, any other oncolytic virus, pembrolizumab, or any other inhibitor of PD-1, PD-L1, or PD-L2. - Subjects must not have history or evidence of symptomatic autoimmune pneumonitis, glomerulonephritis, vasculitis, other symptomatic autoimmune disease, documented history of autoimmune disease or syndrome requiring systemic treatment in the past 2 years (ie, with use of disease modifying agents, steroids or immunosuppressive agents) except vitiligo or resolved childhood asthma/atopy, or evidence of clinically significant immunosuppression. - Subjects must not have active herpetic skin lesions or prior complications of herpetic infection and must not require intermittent or chronic treatment with an antiherpetic drug (eg, acyclovir), other than intermittent topical use.