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

    Research output: Contribution to journalArticle

    26 Citations (Scopus)

    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 ~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
    Volume63
    Issue number9
    DOIs
    Publication statusPublished - May 2002

    Keywords

    • 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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