The disclosure provides methods of treating cancer in a subject or preventing a relapse or reducing the incidence of relapse of cancer in a subject in remission, comprising administering to the subject: (a) a therapeutically effective amount of an immunotherapeutic agent, e.g., an immune checkpoint blockade therapy, an adoptive cellular therapy, a marrow-infiltrating lymphocytes, an adenosine A2aR inhibitor, or an antibody; and (b) a compound having formula (I):




and the pharmaceutically acceptable salts thereof, wherein R1, R2, R2′, and X are as defined as set forth in the specification. Compounds having formula (I) are prodrugs that release glutamine analogs, e.g., 6-diazo-5-oxo-L-norleucine (DON).