title: Modifying rates of reductive elimination of leaving groups from indolequinone prodrugs: a key factor in controlling hypoxia-selective drug release.
abstract: 3-(4-Methylcoumarin-7-yloxy)methylindole-4,7-diones were synthesised as model prodrugs in order to investigate the correlation between rates of reductive elimination from the (indolyl-3-yl)methyl position with reductive metabolism by hypoxic tumor cells and NADPH: cytochrome P450. Rates of elimination of the chromophore/fluorophore (7-hydroxy-4-methylcoumarin) following one-electron reduction of indolequinones to their semiquinone radicals (Q*-) was measured by pulse radiolysis utilising spectrophotometric and fluorometric detection. Incorporation of a thienyl or methyl substituent at the (indol-3-yl)CHR-position (where R=thienyl or methyl adjacent to the phenolic ether linking bond) significantly shortened the half-life of reductive elimination from 87 to 6 and 2 ms, respectively. Elimination from the methyl substituted analogue can thus compete effectively with the reaction of the semiquinone radical with oxygen at levels typically present in tumours (half-life approximately 1.8 ms at 0.5% O2). Chemical kinetic predictions were confirmed by metabolism in breast tumour MCF-7 cells between 0-2.1% O2. Rates of reductive release of the fluorophore from the non-fluorescent parent indolequinones (R=H, Me, thienyl) were similar under anoxia ( approximately 1.7 nmol coumarinmin(-1)mg protein(-1)) reflecting the similarity in one-electron reduction potential. Whereas coumarin release from the indolequinone (R=H) was completely inhibited above 0.5% O2, the enhanced rate of reductive elimination when R=thienyl or Me increased the metabolic rate of release to approximately 0.35 and 0.7 nmol coumarinmin(-1)mg protein(-1), respectively at 0.5% O2; complete inhibition occurring by 2.1% O2. Similar 'oxygen profiles' of release were observed with NADPH: cytochrome P450 reductase. In conclusion, it is possible to modify rates of reductive elimination from indolequinones to control the release of drugs over a range of tumour hypoxia.
authors: Gray Cancer Institute, Mount Vernon Hospital, Middlesex HA6 2JR, Northwood, UK. everett@gci.ac.uk
authors: Steven A
authors: SA
authors: Everett
authors: Elizabeth
authors: E
authors: Swann
authors: Matthew A
authors: MA
authors: Naylor
authors: Michael R L
authors: MR
authors: Stratford
authors: Kantilal B
authors: KB
authors: Patel
authors: Natasha
authors: N
authors: Tian
authors: Robert G
authors: RG
authors: Newman
authors: Borivoj
authors: B
authors: Vojnovic
authors: Christopher J
authors: CJ
authors: Moody
authors: Peter
authors: P
authors: Wardman
mesh_terms: D003374:Coumarins / Q000378:metabolism* / Q000494:pharmacology; D016503:Drug Delivery Systems / Q000379:methods*; D006801:Humans; D000860:Hypoxia / Q000378:metabolism; D009251:NADPH-Ferrihemoprotein Reductase / Q000378:metabolism*; D010084:Oxidation-Reduction; D010100:Oxygen / Q000378:metabolism*; D011355:Prodrugs / Q000378:metabolism*; D014407:Tumor Cells, Cultured
chemical_list: D003374:Coumarins; D011355:Prodrugs; C030123:coumarin; D009251:NADPH-Ferrihemoprotein Reductase; D010100:Oxygen