Tirapazamine

Photochemical Electron Transfer Reactions of Tirapazamine(para)

ABSTRACT

The absorption and fluorescence spectra of 3-aminobenzo-1,2,4-triazine di-N-oxide (tirapazamine) have been recorded and exhibit a dependence on solvent that correlates with the Dimroth E^sub T30^ parameter. Time-dependent density functional theory calculations reveal that the transition of tirapazamine in the visible region is pi > pi* in nature. The fluorescence lifetime is 98 +/- 2 ps in water. The fluorescence quantum yield is ~0.002 in water. The fluorescence of tirapazamine is efficiently quenched by electron donors via an electron-transfer process. Linear Stern-Volmer fluorescence quenching plots are observed with sodium azide, potassium thiocyanate, guanosine monophosphate and tryptophan (Trp) methyl ester hydrochloride. Guanosine monophosphate, tyrosine (Tyr) methyl ester hydrochloride and Trp methyl ester hydrochloride appear to quench the fluorescence at a rate greater than diffusion control implying that these substrates complex with tirapazamine in its ground state. This complexation was detected by absorption spectroscopy.

Abbreviations: AMP. adenosine 5' monophosphate; DABCO, 1,4diazabicyclo[2.2.21octane; GMP, guanosine monophosphate; 2MeTHF, 2-methyltetrahydrofuran; TCSPC, time-correlated single photon counting; TD-DFT, time-dependent density functional theory; tirapazamine, 3-aminobenzo-1,2,4-triazine di-N-oxide; Trp, tryptophan: Tyr, tyrosine.

INTRODUCTION

Since the late 1980s, a considerable amount of attention has been given to 3-aminobenzo-1,2,4-triazine di-N-oxide (tirapazamine; [11) as a potential antitumor agent, either alone or as part of combination therapies (1). Tirapazamine has been found to be capable of acting selectively in hypoxic environments found in rapidly growing tumors (2,3). One of the proposed mechanisms of action of this compound is shown in Scheme 1: enzymatic electron transfer and subsequent proton transfer to generate the radical 3, which undergoes beta-fission (4) to generate 3-aminobenzo- 1,2,4-triazine-N-oxide (4) and hydroxyl radical, which is well established as a promoter of nucleic acid cleavage (5). The fact that this reaction sequence may be quenched in the presence of oxygen gives rise to the potential in vivo selectivity of this agent.

The enzymatically triggered series of reactions can, in principle, be mimicked by light-induced electron-transfer reactions in aqueous solution. The present study was undertaken to investigate that possibility.

Acknowledgements-Support of this work by the NSF-Environmental Molecular Science Institute at The Ohio State University (NSF CHE-0089147 to M.S.P. and C.M.H.) and the American Cancer Society (RPG 00-028-01 to K.S.G.) is gratefully acknowledged, as are the computational resources made available from the Ohio Supercomputer Center. The authors would also like to thank Terry Gustafson and Frank DeLucia Jr. at OSU for performing the TCSPC experiments.

(para)Posted on the web site on January 28, 2002.

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James S. Poole1, Christopher M. Hadad1, Matthew S. Platz*1, Zachary P. Fredin1, Laura Pickard1, Elisa Levya Guerrero1, Margarita Kessler1, Goutam Chowdhury 2, Delshanee Kotandeniya2 and Kent S. Gates2

1Department of Chemistry, The Ohio State University, Columbus, OH and

2Department of Chemistry and Biochemistry, University of Missouri, Columbia, MO

Received 22 August 2001; accepted 17 January 2002

*To whom correspondence should be addressed at: Department of Chemistry, Ohio State University, 100W 18th Avenue. Columbus, OH 43210, USA. Fax: 614-292-5151: e-mail: platz.1@osu.edu

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