Carbetimer is a low molecular weight polymer derived from ethylene and maleic anhydride with potent antineoplastic activity. Preclinical studies have shown antitumor effects against many tumor cell lines including B 16 melanoma. Its antitumor actions are felt to include both reduction of uridine and cytidine nucleoside triphosphate as well as the diminution of pyrimidine salvage by inhibiting uridine, cytidine, and thymidine kinase. Besides direct antiproliferative effects, Carbetimer could act through immunomodulation, as suggested by the induction of cytotoxic T-cells in mice and the discordant in vitro and in vivo activities against the Lewis lung carcinoma model. Unfortunately, Carbetimer failed to demonstrate efficacy in Phase II trials