title: Yeast cytosine deaminase improves radiosensitization and bystander effect by 5-fluorocytosine of human colorectal cancer xenografts.
abstract: The efficacy of cancer gene therapy using bacterial cytosine deaminase (bCD)/5-fluorocytosine (5-FC) enzyme/prodrug strategy is limited by the inefficiency of cytosine deaminase (CD)-catalyzed conversion of 5-FC into 5-fluorouracil (5-FU). We have shown previously that yeast CD (yCD) is more efficient at the conversion of 5-FC than bCD. In the current study, we hypothesized that the increased production of 5-FU by yCD would enhance the efficacy of the CD/5-FC treatment strategy by increasing the bystander effect as well as the efficacy of radiotherapy because of the radiosensitizing capacity of 5-FU. To test this hypothesis, we generated stable HT29 human colon cancer cell lines expressing either bCD (HT29/bCD) or yCD (HT29/yCD). The amount of 5-FU produced in HT29/yCD tumors after a single injection of 5-FC (1000 mg/kg, i.p.) was 15-fold higher than that produced in HT29/bCD tumors. In tumor-bearing nude mice, the average minimum relative tumor size (compared with pretreatment values) of HT29/bCD tumors treated with 5-FC and radiation (500 mg/kg i.p. and 3 Gy, 5 days a week for 2 weeks) was 0.55+/-0.1, compared with 0.01+/-0.01 in HT29/yCD tumors (P = 0.002). Moreover, an increased cytotoxic and radiosensitizing effect of 5-FC on bystander cells was observed in vitro and in vivo when yCD was expressed in HT29 cells instead of bCD. In mice bearing HT29 tumors containing 10% HT29/yCD cells, the combined treatment resulted in a minimum tumor size of 0.20+/-0.07 compared with 0.60+/-0.1 in 10% HT29/bCD cells (P < 0.001). These results demonstrate that the use of yCD in the CD/5-FC strategy has a high potential to improve the therapeutic outcome of combined gene therapy and radiotherapy in cancer patients.
authors: Department of Radiation Oncology, University of Michigan, Ann Arbor 48109-0010, USA.
authors: E
authors: E
authors: Kievit
authors: M K
authors: MK
authors: Nyati
authors: E
authors: E
authors: Ng
authors: L D
authors: LD
authors: Stegman
authors: J
authors: J
authors: Parsels
authors: B D
authors: BD
authors: Ross
authors: A
authors: A
authors: Rehemtulla
authors: T S
authors: TS
authors: Lawrence
mesh_terms: D000818:Animals; D000964:Antimetabolites, Antineoplastic / Q000493:pharmacokinetics / Q000494:pharmacology; D001419:Bacteria / Q000201:enzymology; D001711:Biotransformation; D043525:Cytosine Deaminase; D005260:Female; D005437:Flucytosine / Q000493:pharmacokinetics / Q000494:pharmacology*; D005472:Fluorouracil / Q000493:pharmacokinetics / Q000494:pharmacology; D015316:Genetic Therapy; D019073:HT29 Cells / Q000187:drug effects* / Q000201:enzymology / Q000528:radiation effects*; D006801:Humans; D051379:Mice; D008819:Mice, Nude; D009700:Nucleoside Deaminases / Q000235:genetics / Q000378:metabolism*; D011355:Prodrugs / Q000493:pharmacokinetics; D011838:Radiation-Sensitizing Agents / Q000493:pharmacokinetics / Q000494:pharmacology*; D012441:Saccharomyces cerevisiae / Q000201:enzymology; D014161:Transduction, Genetic; D023041:Xenograft Model Antitumor Assays
chemical_list: D000964:Antimetabolites, Antineoplastic; D011355:Prodrugs; D011838:Radiation-Sensitizing Agents; D005437:Flucytosine; D009700:Nucleoside Deaminases; D043525:Cytosine Deaminase; D005472:Fluorouracil