Comparative metabolism and nucleic acids incorporation of 5-FC and 5-FU in MCF-7 BCCs exposed to the 5-FC/Ad.CMV-CD gene therapy system and 5-FU. MCF-7 cells were incubated for 90 minutes with 100 MOI of Ad.CMV-CD followed by 48 hours of exposure to 500 μmol/L 5-FC with 20 μCi of [6-³H]5-FC or for 24 hours with 5 μmol/L 5-FU in the presence of 25 μCi of [6-³H]5-FU. The conversion of 5-FC into 5-FU was determined after the collection of 1 mL of medium by HPLC as described in the Materials and Methods. Formation of fluorinated nucleotides (FUXP) was determined in the neutralized TCA cell extract by HPLC analysis using a Spherisorb SAX column eluted with a gradient of sodium phosphate, pH 3.3. The incorporation into nucleic acids was determined in the TCA pellets after digestion with NCS tissue solubilizer. The numbers above the histograms represent the micromoles of 5-FU or FUXP measured.