Novel 85Rb-enriched rubidium salt compounds of general formula 1, below and novel compounds of general formula 2, below. Compositions that contain at least one of the novel compounds and optionally further contain an antitumor drug. Methods that entail administering such compounds and compositions to treat cancer, optionally in combination with a conventional form of cancer therapy, such as chemotherapy and radiation treatment. When administered with a conventional form of cancer therapy, the compounds and compositions of the invention may be administered before, simultaneously with, or after administration of the conventional form of cancer therapy.