Antitumor treatment regimens comprising: administering, to a patient diagnosed with cancer, an antitumor agent comprising 4-amino-1-(2-cyano-2-deoxy-β-D-arabinofuranosyl)-2(1H)-pyrimidinone, or a salt thereof, by way of continuous intravenous infusion, in an amount of 2.0 to 4.0 mg per m2 total body surface area of the patient, of 4-amino-1-(2-cyano-2-deoxy-β-D-arabinofuranosyl)-2(1H)-pyrimidinone equivalent, per day, for a duration of 168 to 336 hours. Compositions for providing the methods are also disclosed.