Role of the semi-quinone free radical of the anti-tumour agent etoposide (VP-16-213) in the inactivation of single- and double-stranded phi X174 DNA.

The mechanism of action of the anti-tumour agent etoposide (VP-16-213) could involve its bioactivation to metabolites which can damage DNA. Active metabolites of etoposide, generated in vitro, are the 3',4'-dihydroxy-derivative (catechol) and its oxidation product, the ortho-quinone. The conversion of the catechol into the ortho-quinone (and vice versa) proceeds via formation of a semi-quinone free radical. We investigated the role of this radical species in the inactivation of biologically active single- (ss) and double-stranded (RF) phi X174 DNA. Since the formation of semi-quinone free radicals from the ortho-quinone of etoposide is pH dependent, experiments were performed, in which the ortho-quinone was incubated at pH 4, 7.4 and greater than or equal to 9. ESR measurements showed no formation of radical species from the ortho-quinone at pH 4, but an increased rate of generation of the primary semi-quinone free radical at pH values 7.4 to 10; at still higher pH values a secondary semi-quinone free radical was produced. HPLC analyses demonstrated chemical stability of the ortho-quinone at pH 4, but an accelerated decay was observed when the pH was elevated from 7.4 to 9, with its concomitant conversion into more polar components and into the catechol of etoposide. Ss phi X174 DNA, exposed to the ortho-quinone, was inactivated at an increasing rate at pH values increasing from 4 to 7.4 and subsequently to 9. RF phi X174 DNA was only significantly inactivated in incubations with the ortho-quinone at pH 4, not at pH values 7.4 and 9. From these data it is concluded that the primary semi-quinone free radical of etoposide may to a great extent be responsible for the ortho-quinone-induced ss phi X174 DNA inactivation, but that this radical species is not lethal towards RF phi X174 DNA.