title: Bis-bioreductive agents as hypoxia-selective cytotoxins: nitracrine N-oxide.
abstract: Drugs with two reducible centers, both of which must be metabolized by oxygen-inhibitable processes for full activation ("bis-bioreductive agents"), offer potential for the development of hypoxia-selective cytotoxins with improved oxygen sensitivity. The sidechain N-oxide (1-NCO) of the (mono)bioreductive agent nitracrine (1-NC) has been synthesized and evaluated as a potential example of such an approach. The association constant for reversible DNA binding of 1-NCO was 15-fold lower than that of 1-NC, as measured by equilibrium dialysis in a low ionic strength buffer, indicating that the N-oxide has the potential to act as a less toxic pro-drug of 1-NC. Cell uptake and aerobic cytotoxicity of 1-NCO were much lower than for 1-NC whereas its hypoxic selectivity as a cytotoxin was greatly increased. In stirred suspension cultures of AA8 cells, pure (less than 0.02% 1-NC) 1-NCO was 1000-1500 times more potent under hypoxia than in 20% O2. For 1-NC the corresponding ratio was 10 +/- 1. 1-NCO had greater hypoxic selectivity in this system than misonidazole (ratio 11), RSU 1069 (ratio 25), 8Me-5NQ (ratio 60), or SR 4233 (ratio 80). Studies of 1-NCO metabolism indicate rapid, O2-inhibited reduction to 1-NC. The data are consistent with a two-step bioactivation mechanism, with reduction of the N-oxide generating a DNA intercalator of increased binding affinity, followed by reduction of the nitro group of this DNA-targeted cytotoxin to form reactive cytotoxic metabolites.
authors.affiliation: Department of Pathology, University of Auckland School of Medicine, New Zealand.
authors.forename: W R
authors.initials: WR
authors.lastname: Wilson
authors.forename: P
authors.initials: P
authors.lastname: Van Zijl
authors.forename: W A
authors.initials: WA
authors.lastname: Denny
mesh_terms: D000818:Animals; D000970:Antineoplastic Agents / Q000138:chemical synthesis / Q000494:pharmacology*; D015687:Cell Hypoxia; D002470:Cell Survival / Q000187:drug effects; D006224:Cricetinae; D066298:In Vitro Techniques; D009564:Nitracrine / Q000031:analogs & derivatives* / Q000138:chemical synthesis / Q000494:pharmacology; D011355:Prodrugs / Q000138:chemical synthesis / Q000494:pharmacology*; D013997:Time Factors
chemical_list: D000970:Antineoplastic Agents; D011355:Prodrugs; C001815:C 684; D009564:Nitracrine