Reactions of glutathione with the catechol, the ortho-quinone and the semi-quinone free radical of etoposide. Consequences for DNA inactivation.

Etoposide [4'-demethylepipodophyllotoxin-9-(4,6-O-ethylidene-beta- D-glucopyranoside)] can be metabolized to DNA-inactivating catechol, ortho-quinone and semi-quinone free radical derivatives which may contribute to its cytotoxicity. In this paper, we examined in vitro whether glutathione (GSH), which is known to react easily with quinoid compounds, could interact with the active etoposide intermediates and in this way influence the cytotoxicity of the parent compound. To this end, reactions of GSH with the etoposide intermediates were studied, using HPLC and ESR measurements, together with the effects of GSH on the biological inactivation of single-stranded (ss) and double-stranded (RF) phi X174 DNA by these compounds. From the results it could be determined that: (a) GSH does not react with the catechol and, as a consequence, has no effect on the reaction of this intermediate of etoposide with ss and RF phi X174 DNA; (b) GSH reacts with the ortho-quinone most likely by formation of a conjugate and by two-electron reduction to the catechol, resulting in a partial protection of ss and RF phi X174 DNA against inactivation by this species; and (c) GSH protects ss phi X174 DNA against inactivation by the semi-quinone free radical of etoposide probably by conjugation with this species.