The invention relates to 5-fluoro-2'-deoxyuridine-5'-O-[1-naphthyl (benzoxy-L-alaninyl)] phosphate (NUC-3373), or a pharmaceutically acceptable salt thereof, for use in the treatment of cancer, in particular by intravenous infusion for a continuous period of up to 10 hours. The invention also relates to methods of treating cancer by administration of NUC-3373 to particular sub-groups of cancer patient. The invention further relates to methods for selecting a patient for treatment with NUC-3373.
