Modifying rates of reductive elimination of leaving groups from indolequinone prodrugs: a key factor in controlling hypoxia-selective drug release

Steven A. Everett, Elizabeth Swann, Matthew A. Naylor, Michael R. L. Stratford, Kantilal B. Patel, Natasha Tian, Robert G. Newman, Borivoj Vojnovic, Christopher J. Moody, Peter Wardman

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    27 Citations (Scopus)


    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 ~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 (~1.7 nmol coumarin min?¹mg 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 ~0.35 and 0.7 nmol coumarin min?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.
    Original languageEnglish
    Pages (from-to)1629-1639
    Number of pages11
    JournalBiochemical Pharmacology
    Issue number9
    Publication statusPublished - May 2002


    • Indolequinone
    • 7-Hydroxy-4-methylcoumarin
    • Reduction
    • Chemical kinetics
    • Semiquinone radical
    • Pulse radiolysis
    • MCF-7 breast tumour cells
    • NADPH
    • Cytochrome P450 reductase
    • Hypoxia


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