• Men and women, 18 years of age and older on day of signing written informed consent

• Histologically or cytologically documented locally-advanced and/or metastatic squamous cell carcinoma of the skin (stage III/IV AJCC 2010) that is incurable

• Archival tumor tissue available for evaluation of PD-L1 and LAG-3 expression

• Measurable disease based on Response Criteria in Solid Tumors Version 1.1 (RECIST 1.1)

• Life expectancy of at least 12 weeks

• Eastern Cooperative Oncology Group (ECOG) Performance status of 0-2

• Screening laboratory values must meet the following criteria and should be obtained within 14 days prior to registration:

‣ WBC ≥ 2000/μl

⁃ Neutrophils ≥ 1500/μL

⁃ Platelets ≥ 100 x103/μL

⁃ Hemoglobin \> 9.0 g/dL

⁃ Serum creatinine ≤ 1.5 x ULN or creatinine clearance (CrCl) ≥ 40 mL/min (if using the Cockcroft-Gault formula below):

• Female CrCl = (140 - age in years) x weight in kg x 0.85/72 x serum creatinine in mg/dL Male CrCl = (140- age in years) x weight in kg x 1.00/72 x serum creatinine in mg/dL

⁃ AST/ALT ≤ 3 x ULN

⁃ Total Bilirubin ≤ 1.5 x ULN (except subjects with Gilbert Syndrome, who can have total bilirubin \< 3.0 mg/dL)

⁃ Negative pregnancy test and effective contraception (Pearl-Index \<1) for for women of childbearing potential (WOCBP) if the risk of conception exists

• Prior radiotherapy must have been completed at least 2 weeks prior to study drug administration

• Prior systemic antibiotic treatment must have been completed at least 30 days prior to stool sample collection