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Doxifluridine, a new fluorouracil analog with a low myelosuppressive effect, has recently been subject to various disease-oriented, Phase II trials. For the present evaluation of drug tolerance, the Phase II data of 114 patients having received 376 doxifluridine cycles has been used. The treatment cycles consisted of 5 daily intravenous injections of 4,000 mg/m2 non-pretreated patients, and 3,000 mg/m2 in pretreated patients. Previous observations showing that doxifluridine is less myelotoxic than fluorouracil have been confirmed. 54% of the patients had no leucopenia (maintaining WBC counts over 3,000/mm3 and 90% had no thrombopenia (platelets not lower than 100,000/mm3) throughout treatment. However, a WHO grade 4 hematologic toxicity was observed in 9 patients, and 2 toxic deaths were related to severe granulocytopenia and sepsis. Digestive tract toxicity was similar and equally frequent as the one observed with fluorouracil: mucositis with oral ulcerations (19%), nausea and vomiting requiring specific treatment (8%) and severe but never hemorrhagic diarrhoea (5%). Neurologic toxicity was frequent, with 20% of patients complaining of somnolence and/or peripheral neuropathy, 7% of impaired consciousness and 1% of WHO grade 4 cerebellar ataxia. Among the 10% of patients with cardiac symptoms, 6% were benign and transient arrhythmias, and 4% were severe, including 1 myocardial infarction, 1 spontaneously reversible cardiac arrest and 2 ventricular fibrillations successfully treated with cardioversion. In spite of its encouraging antitumor activity and its good hematologic tolerance, intravenous doxifluridine, as used in this study, cannot be recommended because of the observed neurologic and cardiac toxicity. Oral doxifluridine is presently under investigation with preliminary results suggesting a lack of neuro- or cardiotoxicity.