Patients with progressive disease receive pembrolizumab IV over 30 minutes on days 1 and 22, oxaliplatin IV over 2-6 hours, leucovorin calcium over 15 minutes to 2 hours, and fluorouracil IV over 46-48 hours on days 1, 15, and 29. Treatment repeats every 41 days (6 weeks) for 1-3 courses in the absence of disease progression or unacceptable toxicity.