A Study to Evaluate The Efficacy And Safety Of RO7198457 In Combination With Pembrolizumab Versus Pembrolizumab Alone In Participants With Previously Untreated Advanced Melanoma.

Inclusion Criteria:

Histologically confirmed metastatic (recurrent or de novo Stage IV) or unresectable locally advanced (Stage IIIC or IIID) cutaneous, acral, or mucosal melanoma;
Eastern Cooperative Oncology Group (ECOG) Performance Status of 0 or 1;
Life expectancy >/= 12 weeks;
Adequate hematologic and end-organ function;
Naive to prior systemic anti-cancer therapy for advanced melanoma with some exceptions;
Tumor specimen availability;
Measurable disease per RECIST v1.1.

Exclusion criteria:

Ocular/uveal melanoma;
Any anti-cancer therapy with the exceptions as specified in the protocol;
Symptomatic, untreated, or actively progressing central nervous system (CNS) metastases;
Previous splenectomy;
History of autoimmune disease;
Prior allogeneic bone marrow transplantation or prior solid organ transplantation;
Positive test for Human Immunodeficiency Virus (HIV) infection;
Active hepatitis B or C or tuberculosis;
Significant cardiovascular disease;
Known clinically significant liver disease.