INCMGA00012 in Patients With Previously Treated Unresectable or Metastatic Adenosquamous Pancreatic or Ampullary Cancer

Inclusion Criteria: * Age ≥18 years. * Have histologically or cytologically - proven adenosquamous carcinoma of the pancreas or ampulla. * Has unresectable or metastatic measurable disease. * Has received (or been intolerant to or ineligible for) at least 1 prior line of cytotoxic chemotherapy and received no more than 2 prior systemic treatments. * Presence of at least one lesion with measurable disease. * Accept to have a tumor biopsy of an accessible lesion at baseline and on treatment. * Eastern Cooperative Oncology Group (ECOG) performance status 0 or 1. * If HIV-positive, then all of the following criteria must also be met: cluster of differentiation (CD) 4+ count ≥ 350/μL, undetectable viral load, and receiving highly active antiretroviral therapy. * Life expectancy of greater than 3 months. * Patients must have adequate organ and marrow function defined by study-specified laboratory tests prior to initial study drug. * Woman of childbearing potential must have a negative pregnancy test and follow contraceptive guidelines as defined per protocol. * Men must use acceptable form of birth control while on study. * Ability to understand and willingness to sign a written informed consent document. Exclusion Criteria * Known history or evidence of brain metastases. * Has had chemotherapy, radiation, or biological cancer therapy within 14 days prior to the first dose of study drug. * Has received an investigational agent or used an investigational device within 28 days of the first dose of study drug. * Expected to require any other form of systemic or localized antineoplastic therapy while on study. * Has had major surgery within 28 days of dosing of investigational agent, excluding minor procedures. * Has received a live vaccine within 28 days prior to the first dose of study drug. * Prior treatment with immunotherapy agents (including, anti-PD-1, anti-PD-L1, anti-PD-L2, anti-CTLA4, anti-OX40 and LAG-3 antibodies) * Have used any systemic steroids within 14 days of study treatment. * Hypersensitivity reaction to any monoclonal antibody. * Evidence of clinical or radiographic ascites. * Have clinically significant and/or malignant pleural effusion. * Uncontrolled intercurrent illness including, but not limited to, uncontrolled infection, symptomatic congestive heart failure, unstable angina, cardiac arrhythmia, metastatic cancer, or psychiatric illness/social situations that would limit compliance with study requirements. * History of autoimmune disease requiring systemic immunosuppression within the last 2 years. * Presence of any tissue or organ allograft, regardless of need for immunosuppression, including corneal allograft. Patients with a history of allogeneic hematopoeitic stem cell transplant will be excluded. * All toxicities attributed to prior anti-cancer therapy other than alopecia and fatigue must have resolved to a grade 1 or baseline before administration of study drug. * Infection with Hepatitis A, B or C. * Patient has a pulse oximetry of \<92% on room air. * Patient is on supplemental home oxygen. * Has an unhealed surgical wound or ulcer, or a bone fracture considered non-healing. * Patient has clinically significant heart disease. * Patient is, at the time of signing informed consent, a regular user (including "recreational use") of any illicit drugs or other substance abuse. * Unwilling or unable to follow the study schedule for any reason. * Patient has history of non-infectious pneumonitis. * Serum albumin level less than 2.8 g/dL.