Indolequinone Antitumor Agents:  Reductive Activation and Elimination from (5-Methoxy-1-methyl-4,7-dioxoindol-3-yl)methyl Derivatives and Hypoxia-Selective Cytotoxicity in Vitro

Abstract

A series of indolequinones bearing a variety of leaving groups at the (indol-3-yl)methyl position was synthesized by functionalization of the corresponding 3-(hydroxymethyl)indolequinone, and the resulting compounds were evaluated in vitro as bioreductively activated cytotoxins. The elimination of a range of functional groups-carboxylate, phenol, and thiol-was demonstrated upon reductive activation under both chemical and quantitative radiolytic conditions. Only those compounds which eliminated such groups under both sets of conditions exhibited significant hypoxia selectivity, with anoxic:oxic toxicity ratios in the range 10-200. With the exception of the 3-hydroxymethyl derivative, radiolytic generation of semiquinone radicals and HPLC analysis indicated that efficient elimination of the leaving group occurred following one-electron reduction of the parent compound. The active species in leaving group elimination was predominantly the hydroquinone rather than the semiquinone radical. The resulting iminium derivative acted as an alkylating agent and was efficiently trapped by added thiol following chemical reduction and by either water or 2-propanol following radiolytic reduction. A chain reaction in the radical-initiated reduction of these indolequinones (not seen in a simpler benzoquinone) in the presence of a hydrogen donor (2-propanol) was observed. Compounds that were unsubstituted at C-2 were found to be up to 300 times more potent as cytotoxins than their 2-alkyl-substituted analogues in V79-379A cells, but with lower hypoxic cytotoxicity ratios.

Antitumour Quinones

“…This enhanced activity may be due to the hydroxymethyl group, which allows formation of a reactive quinone methide intermediate (72) after two-electron reduction (Scheme 7) [215,216].…”

Mechanisms of action of quinone-containing alkylating agents i: NQO1-directed drug development

“…These observations were confirmed by Phillips and coworkers in a later study (61). Interestingly, Naylor et al determined that leaving group elimination at the indole-3-position occurred predominantly from the hydroquinone (2-electron reduction product) rather than the semiquinone (1-electron reduction product) (62).…”

Labelled compounds of interest as antitumour agents – VIII. Synthesis of 2H‐isotopomers of pentamethylmelamine and of a potential prodrug thereof

“…In the 13 C NMR spectrum of 3b, the CD 3 carbon resonated at d 26.5 as a septet ( 1 J C-D =20.8 Hz). Although the anion derived from pentamethylmelamine 3a can be alkylated with simple electrophiles, 16 all attempts to alkylate the anions from 3a and 3b with 3-chloromethyl-1,2-dimethyl-5-methoxy indole-4,7-dione 15 failed. The indoledione unit was destroyed under the basic reaction conditions; the melamine anion is clearly much more basic than are the phenolate anions which are reported 15 to react smoothly with this reagent.…”

“…Although the anion derived from pentamethylmelamine 3a can be alkylated with simple electrophiles, 16 all attempts to alkylate the anions from 3a and 3b with 3-chloromethyl-1,2-dimethyl-5-methoxy indole-4,7-dione 15 failed. The indoledione unit was destroyed under the basic reaction conditions; the melamine anion is clearly much more basic than are the phenolate anions which are reported 15 to react smoothly with this reagent. However, we have very recently shown 12 that the corresponding alcohol 6 can act as an alkylating electrophile with isoquinolin-1-ones under Mitsunobu conditions.…”

“…Further, we required the potential prodrug 7, in which the labelled pentamethylmelamine is attached to an indole-4,7-dione moiety. The latter quinone unit can be bioreduced selectively in hypoxic solid tumours; 15 this reduction is designed to trigger the release of the drug selectively in these sites. The labelled prodrug 7 was required for NMR studies on the conformation of the prodrug and for mass spectrometric studies on release of the drug after reduction.…”