Biochem Pharmacol. 2002 May 1;63(9):1629-39.  

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

Everett SA, Swann E, Naylor MA, Stratford MR, Patel KB, Tian N, Newman RG,
Vojnovic B, Moody CJ, Wardman P.

Gray Cancer Institute, Mount Vernon Hospital, Middlesex HA6 2JR, Northwood, UK.
everett@gci.ac.uk

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.

PMID: 12007566 [PubMed - indexed for MEDLINE]