title: Photodynamic therapy enhances the efficacy of gene-directed enzyme prodrug therapy.
abstract: BACKGROUND
  Gene-directed enzyme prodrug therapy (GDEPT) employing the cytosine deaminase (CD) gene, which encodes an enzyme that converts the nontoxic agent 5-fluorocytosine (5-FC) into the chemotherapeutic drug 5-fluorouracil (5-FU), has shown promise both in experimental animals and in clinical trials. Nevertheless, with the transfection systems available presently the percentage of tumor cells incorporating the desired gene is usually too low for successful therapy. We have examined the ability of photodynamic therapy (PDT) to enhance the efficacy of the metabolites, converted from 5-FC by CD gene transfected rat glioma cells.
  
  
  METHODS
  Hybrid tumor cell spheroids consisting of CD poitive and CD negative F98 glioma cells in varying ratios were used as in vitro tumor models. PDT was performed with the photosensitizer AlPcS2aand λ=670nm laser irradiance, both before and after confrontation with 5-FC.
  
  
  RESULTS
  PDT increased the toxicity of 5-FU either as pure drug or derived from monolayers of CD positive cells chalanged with 5-FC. PDT in combination with 5-FC resulted in a significantly enhanced inhibition of hybrid spheroid growth compared to non light treated controls. This was the case even at tumor to producer cell ratios as high as 40:1.
  
  
  CONCLUSIONS
  The results of the present study show that GDEPT and PDT interact in a synergistic manner over a range of prodrug concentration and tumor to transfected cell ratios. The degree of synergy was significant regardless if PDT treatment was given before or after 5-FC administration. The highest degree of interaction was observed though, when PDT was delivered prior to prodrug exposure.
authors.affiliation: Beckman Laser Institute and Medical Clinic, University of California, Irvine 1002 Health Sciences Rd Irvine, CA 92617, USA.
authors.forename: Catherine
authors.initials: C
authors.lastname: Christie
authors.affiliation: Beckman Laser Institute and Medical Clinic, University of California, Irvine 1002 Health Sciences Rd Irvine, CA 92617, USA.
authors.forename: Aftin
authors.initials: A
authors.lastname: Pomeroy
authors.affiliation: Beckman Laser Institute and Medical Clinic, University of California, Irvine 1002 Health Sciences Rd Irvine, CA 92617, USA.
authors.forename: Rohit
authors.initials: R
authors.lastname: Nair
authors.affiliation: Dept. of Radiation Biology, The Norwegian Radium Hospital, Oslo University Hospital, Oslo, Norway.
authors.forename: Kristian
authors.initials: K
authors.lastname: Berg
authors.affiliation: Beckman Laser Institute and Medical Clinic, University of California, Irvine 1002 Health Sciences Rd Irvine, CA 92617, USA. Electronic address: hhirschb@uci.edu.
authors.forename: Henry
authors.initials: H
authors.lastname: Hirschberg
mesh_terms: D045744:Cell Line, Tumor; D003131:Combined Modality Therapy / Q000379:methods; D043525:Cytosine Deaminase / Q000235:genetics; D004357:Drug Synergism; D057487:Enzyme Therapy / Q000379:methods*; D005437:Flucytosine / Q000008:administration & dosage*; D015316:Genetic Therapy / Q000379:methods*; D006801:Humans; D009374:Neoplasms, Experimental / Q000188:drug therapy* / Q000235:genetics* / Q000473:pathology; D010778:Photochemotherapy / Q000379:methods*; D017319:Photosensitizing Agents / Q000008:administration & dosage; D011355:Prodrugs / Q000627:therapeutic use*; D016896:Treatment Outcome
chemical_list: D017319:Photosensitizing Agents; D011355:Prodrugs; D005437:Flucytosine; D043525:Cytosine Deaminase
keywords: 5-FU; Cytosine deaminase gene; Glioblastoma multiforme; Photochemical internalization; Photodynamic therapy; Suicide gene therapy