Longitudinal Immune-phenotyping of HCC Following MK-3475

Be willing and able to provide written informed consent for the trial. The subject may also provide consent for donation of tissue to the SingHealth Tissue Repository (STR). However, the subject may participate in the trial without participating in STR.Be male or female subject, who is at least 21 years of age on the date of first signed written informed consent Have diagnosis of HCC by AASLD imaging criteria or by cytology/histology.

Have technically resectable HCC, with complete extirpation of HCC at end of surgery by resection +/- intra-operative radiofrequency ablation (RFA) at time of enrolment:

Without extrahepatic metastases

Regional lymph nodes (LN) <2 cmLung lesions <1 cm Without invasion of main portal vein (PV3), or major left and right branches (PV2) Future liver remnant of >40% <5 hepatic lesions in total Have tumor larger than 1.5 cm in size

Demonstrate adequate organ function as defined in Table 3, all screening labs should be performed within 14 days of neoadjuvant treatment initiation.

Absolute neutrophil count (ANC): ≥1,200/mcLPlatelets: ≥80,000/ mcL Hemoglobin: ≥9 g/dL or ≥5.6 mmol/L without transfusion or EPO dependency (within 7 days of assessment) Serum creatinine: ≤1.5 X upper limit of normal (ULN) OR creatinine ≥60mL/min for subjects with creatinine >1.5X institutional ULN Serum total bilirubin: ≤ 1.5 X ULN AST (SGOT) and ALT (SGPT): ≤5.0 X ULN Albumin: ≥3.0 g/dL International Normalized Ratio (INR) or Prothrombin Time (PT) AND Activated Partial Thromboplastin Time (aPTT): ≤1.5 X ULN OR within therapeutic range for subjects on anticoagulant therapy. Abnormalities will be corrected prior to liver resection, if necessary according to institution practice Have Child-Pugh score ≤ 6. Be scheduled for liver resection within 5-6 weeks. Female subject of childbearing potential should have a negative urine or serum pregnancy within 72 hours prior to receiving the first dose of study medication. If the urine test is positive or cannot be confirmed as negative, a serum pregnancy test will be required. Female subjects 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 medication. Male subjects of childbearing potential must agree to use an adequate method of contraception starting with the first dose of study therapy through 120 days after the last dose of study therapy. Have a performance status of 0-1 using the ECOG Performance Scale. Have not had treatment for HCV or are not on current anti-HCV treatment for subjects with chronic infection by HCV. For subjects who have had anti-HCV therapy, the last dose of anti-HCV medication should be at least 4 weeks before first dose of pembrolizumab. Have HBV viral load under 100 IU/mL with or without treatment, for subjects with chronic hepatitis B infection. HBV viral load must be less than 100 IU/mL on at least 4 weeks of anti-viral therapy prior to first dose of pembrolizumab. Subjects on active HBV therapy with viral loads under 100 IU/mL should stay on the same therapy throughout study treatment. Subjects who are anti-HBc (+), negative for HBs Ag, negative for anti-HBs, and have an HBV viral load under 100 IU/mL do not require HBV anti-viral prophylaxis.