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

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 P 21/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)o, and Z = 8.