The One-Electron Reduction Potential of 3-Amino-1, 2, 4-benzotriazine 1, 4-dioxide (Tirapazamine): A Hypoxia-Selective Bioreductive Drug

Abstract

The one-electron reduction potential of 3-amino-l, 2, 4-benzotriazine 1, 4-dioxide, tirapazamine (SR 4233) in aqueous solution has been determined by pulse radiol-ysis. Reversible electron transfer was achieved between radiolytically-generated one-electron reduced radicals of tirapazamine (T), and quinones or benzyl viologen as redox standards. The reduction potential E_m7(T/T^±) was -0.45 ± 0.01 V vs. NHE at pH 7. From the pH dependence of the reduction potential, pK_a = 5.6 ± 0.2 was estimated for the tirapazamine radical, a value similar to the pK_a determined by other methods.

Key Words:

• Tirapazamine
• reduction potential
• pK_a
• pulse radiolysis.