Procedures and treatment
tumor, intercurrent illness, PD‐L1
CPS, POSITIVE, TUMOR, METASTATIC DISEASE
The patients received preoperative treatment with the combination of mFOLFOX (oxaliplatin 85 mg/mRepeat PET and CT C/A/P were obtained approximately 2–3 weeks after completion of neoadjuvant combination therapy. Those patients with no evidence of metastatic disease on PET & CT C/A/P and physically fit for operation underwent potentially curative surgical resection 4–8 weeks after completion of neoadjuvant combination therapy. If there was evidence of metastatic disease on PET &CT C/A/P, or physically not fit for operation, the participant came off the study (i.e., did not undergo surgical resection and adjuvant therapy) and managed with the standard care.Postoperative adjuvant treatment (started 6–8 weeks after surgery) consisted of mFOLFOX every 2 weeks for additional 4 doses plus pembrolizumab every 3 weeks for additional 12 doses (total 1 year of therapy). Patients continued to receive treatment until either one of the following occurred: completion of adjuvant therapy, development of radiographically confirmed progression, the participant withdrew consent, intercurrent illness that prevented further administration of treatment, or the sponsor‐investigator decided to withdraw the participant. Efficacy outcomes during the adjuvant chemotherapy phase were determined by radiologic measurements through CT C/A/P. Assessment of response were performed every 3 months for the first year and as per the standard institutional guidelines thereafter.Tissue analysis: Pretreatment biopsies and surgical specimens were analyzed for the immunohistochemical expression of ICI treatment related markers: PD‐L1, CD8, and CD20 before and after preoperative pembrolizumab + mFOLFOX therapy.Formalin‐fixed paraffin‐embedded 4‐μm thick whole tissue sections were utilized for immunohistochemical staining. Staining for PD‐L1 (clone 22C3) was performed in the standardized manner using the EnVision FLEX visualization system on the Autostainer Link 48 (all: Dako/Agilent Tecnologies, Inc.). Staining for CD8 (clone C8/144B, Biocare Medical) at 1:150 dilution following antigen retrieval using DIVA Decloaker (DV2004, Biocare Medical), and for CD20 at 1:100 dilution following antigen retrieval using Target Retrieval Solution, Citrate pH 6.1 (10x) (Dako/Agilent Tecnologies, Inc.) were run on the Intellipath FLX autostainer (Biocare Medical) with the polymer refine detection Anti‐Mouse Envision + System HRP (K400, Dako/Agilent Tecnologies, Inc.).PD‐L1 staining was scored utilizing the standardized Combined Positive Score (CPS). CD8 and CD20 were each assessed as the percentage of all cells in the tumor area.