title: Cytotoxicity of the bioreductive agent RH1 and its lack of interaction with radiation.
abstract: OBJECTIVE
  RH1 is a new bioreductive agent that was developed as a cytotoxic agent with selectivity for tumour cells expressing high levels of the enzyme DT-diaphorase (DTD). The aim of the present study was to investigate the cytotoxicity of RH1 in relation to cellular levels of reducing enzymes and any interaction of RH1 with ionizing radiation under oxic and hypoxic conditions.
  
  
  METHODS
  The MB-MDA231 human breast cancer cell line (WT) and WT cells transfected with the NQO1 gene encoding DTD (the D7 cell line) were used to examine the dependency of RH1's cytotoxicity on cellular DTD activity. The role of the 1-electron reducing enzyme P450 reductase was also studied using a P450 reductase-transfected isogenic cell line (R4). A clonogenic assay was used to investigate the cytotoxicity of RH1 with and without irradiation in air and in nitrogen. In all cases drug exposure was for 3 h.
  
  
  RESULTS
  DTD levels were around 300-fold higher in D7 compared to WT and R4 cells. RH1 was cytotoxic at nanomolar concentrations to all the cell lines, and was 2-3 times more toxic in the D7 cells with high DTD than in the other two cell lines. Doses of RH1 was around 2-fold more effective in hypoxic than in oxic WT cells, but not by as much in D7 cells. RH1 did not radiosensitise the cells but showed an additive effect when combined with irradiation under oxic and hypoxic conditions.
  
  
  CONCLUSIONS
  RH1 shows high clonogenic cytotoxicity to MDA231 cells with high DTD activity but its selectivity based on the presence of DTD is much less than as shown in previous reports. RH1 showed an additive cell killing effect when combined with irradiation under both oxic and hypoxic conditions.
authors.affiliation: Cancer Research UK Groups of Experimental Radiation Oncology, Paterson Institute for Cancer Research, Christie Hospital NHS Trust, Manchester, UK.
authors.forename: Joo-Young
authors.initials: JY
authors.lastname: Kim
authors.forename: C M L
authors.initials: CM
authors.lastname: West
authors.forename: H
authors.initials: H
authors.lastname: Valentine
authors.forename: T H
authors.initials: TH
authors.lastname: Ward
authors.forename: A V
authors.initials: AV
authors.lastname: Patterson
authors.forename: I J
authors.initials: IJ
authors.lastname: Stratford
authors.forename: S A
authors.initials: SA
authors.lastname: Roberts
authors.forename: J H
authors.initials: JH
authors.lastname: Hendry
mesh_terms: D000970:Antineoplastic Agents / Q000494:pharmacology*; D001388:Aziridines / Q000494:pharmacology*; D016227:Benzoquinones / Q000494:pharmacology*; D045744:Cell Line, Tumor / Q000187:drug effects / Q000528:radiation effects; D004353:Drug Evaluation, Preclinical; D006801:Humans; D008325:Mammary Neoplasms, Experimental / Q000201:enzymology / Q000473:pathology* / Q000532:radiotherapy*; D016660:NAD(P)H Dehydrogenase (Quinone) / Q000378:metabolism; D009251:NADPH-Ferrihemoprotein Reductase / Q000378:metabolism; D011838:Radiation-Sensitizing Agents / Q000494:pharmacology*; D014162:Transfection; D014410:Tumor Stem Cell Assay
chemical_list: C117776:2,5-diaziridinyl-3-(hydroxymethyl)-6-methyl-1,4-benzoquinone; D000970:Antineoplastic Agents; D001388:Aziridines; D016227:Benzoquinones; D011838:Radiation-Sensitizing Agents; D009251:NADPH-Ferrihemoprotein Reductase; D016660:NAD(P)H Dehydrogenase (Quinone)