Modeling the Action of an Antitumor Drug:  A Density Functional Theory Study of the Mechanism of Tirapazamine

Density functional theory methods are employed to investigate experimentally proposed mechanisms by which the antitumor drug tirapazamine may react with a DNA sugar-C₁‘ radical to give the sugar derivative deoxyribonolactone, with concomitant DNA strand breakage. For the previously proposed minor pathway, ionization of the sugar-C₁‘ radical by tirapazamine, the calculated ionization energy, and the electron affinity of the models of the sugar-C₁‘ radical of DNA and tirapazamine suggest that tirapazamine must be protonated to be able to oxidize the sugar-C₁‘ radical. The preferred mechanism for reaction of tirapazamine with a sugar-C₁‘ radical, in agreement with experimental observations, is found to proceed by direct attack of an N-oxide oxygen of tirapazamine at the sugar-C₁‘ position, followed by homolytic cleavage of the N−O bond of the drug moiety. Possible alternative mechanisms are also investigated.