Crystal structure of 3-amino-5-methyl-1,2,4-benzotriazine 1-oxide: Evidence for formation of a covalent attachment between a carbon-centered radical and the antitumor agent tirapazamine

Abstract

The compound 3-amino-1,2,4-benzotriazine 1,4-dioxide (tirapazamine, 1) is a potent antitumor agent. We isolated 3-amino-5-methyl-1,2,4-benzotriazine 1-oxide (5) from the reaction tirapazamine with methyl radical and characterized this compound by X-ray crystallography. The crystal structure of 5 provides clear evidence that this carbon-centered radical forms a covalent attachment with the drug tirapazamine. Given that tirapazamine generates carbon-centered DNA radicals as part of its mechanism of action, this finding may provide a chemical basis for understanding the small but significant levels of drug that become covalently associated with DNA during the damage process. The title compound C₈H₈N₄O crystallized in the monoclinic space group P2₁/c with unit cell parameters: a = 15.670(8), b = 7.381(4), c = 13.491(7) Å, α = 90(1), β = 95.623(9), γ = 90(1)°, and Z = 8.