Notes: Abstract Background: Experimental studies have demonstrated that 5-fluorouracil(5-FU) enhances zidovudine (AZT)-induced DNA strand breaks and cytotoxicity.Phase I studies have demonstrated that the maximum tolerable dose (MTD) of AZTis 8000 mg/sqm when administered i.v. over two hours after weekly 5-FU +l-leucovorin (LV), and that this combination has promising antitumor activity.The purpose of this study was therefore to evaluate the antitumor activity ofweekly bolus 5-FU + LV + AZT, administered at its MTD, and to determinewhether 5-FU enhances AZT-induced DNA strand breaks in blood nuclear cells. Patients and methods: Twenty-nine chemotherapy-naïve metastaticcolorectalcancer patients with measurable disease entered the study to evaluate theactivity of a weekly 5-FU 500 mg/m2 i.v. bolus + LV 250mg/m2 i.v. two-hour infusion + AZT 8000mg/m2 i.v. two-hour infusion. In 10 different patients, whoduring three different weeks received 5-FU + LV, AZT and 5-FU + LV + AZT, DNAstrand breaks in blood nuclear cells were determined by a fluorescent analysisof DNA unwinding. Results: Treatment was generally well tolerated and WHO grades III–IVtoxicities, consisting mostly of diarrhea (17%), were uncommon. Onepatient died of severe diarrhea with consequent hypokalemia and cardiacarrhythmia. All patients were considered evaluable for response, and 3(10%) complete and 10 (35%) partial responses were observed, foran objective response rate of 45% (95% confidence limit interval26%–64%). Both 5-FU + LV and AZT decreased the percentageof double stranded DNA in nuclear blood cells. The greatest effect wasobserved with 5-FU + LV + AZT, which reduced the percentage of double strandedDNA to 50% and 36% after 24 and 48 hours, respectively, and thisinteraction between 5-FU + LV and AZT was found to be cumulative. Conclusions: These studies demonstrate that the present dose and scheduleof AZT in combination with 5-FU + LV has significant activity in metastaticcolorectal cancer and that the combination of 5-FU + LV with AZT increases theamount of DNA damage. Therefore, AZT in combination with 5-FU + LV warrantsfurther study in colorectal cancer.