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First published on August 30, 2005; DOI: 10.1124/dmd.105.005017

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Received for publication April 25, 2005.
Revised August 28, 2005.
Accepted for publication August 29, 2005.

Unusual Dehydroxylation of Antimicrobial Amidoxime Prodrugs by Cytochrome b5 and NADH Cytochrome b5 Reductase

Janelle Saulter 1, Joseph Kurian 2, Lauren Trepanier 2, Richard Tidwell 1, Arlene Bridges 1, David Boykin 3, Chad Stephens 3, Mariappan Anbazhagan 3, James Edwin Hall 1*

1 University of North Carolina at Chapel Hill 2 University of Wisconsin-Madison 3 Georgia State University

* Address correspondence to: E-mail: je_hall{at}unc.edu

Abstract

Furamidine is an effective antimicrobial agent; however, oral potency of furamidine is poor. A prodrug of furamidine, DB289, has greatly improved oral potency. DB289 is transformed to furamidine via O-demethylation and N-dehydroxylation reactions with four intermediate metabolites formed. The O-demethylation reactions have been shown to be catalyzed by CYP450. The enzymes catalyzing the reductive N-dehydroxylation reactions have not been determined. The objective of this study was to identify the enzymes that catalyze N-dehydroxylation of metabolites M1, a monoamidoxime, and M2, a diamidoxime, formed during generation of furamidine. M1 and M2 metabolism was investigated using human liver microsomes and human soluble cytochrome b5 and NAD cytochrome b5 reductase, expressed in Escherichia coli. Kinetics of M1 and M2 reduction by human liver microsomes exhibited high affinity and moderate capacity. Metabolism was significantly inhibited by antibodies to cytochrome b5 and b5 reductase and by chemical inhibitors of b5 reductase. The amidoximes were efficiently metabolized by liver mitochondria, which contain cytochrome b5/b5 reductase, but not by liver cytosol, which contains minimal amounts of these proteins. Expressed cytochrome b5/b5 reductase, in the absence of any other proteins, efficiently catalyzed reduction of both amidoximes. Km values were similar to those for microsomes and Vmax values were 33-36-fold higher in the recombinant system, compared to microsomes. Minimal activity was seen with cytochrome b5 or b5 reductase alone, or with cytochrome P450 reductase alone or with cytochrome b5. These results indicate that cytochrome b5 and b5 reductase play a direct role in metabolic activation of DB289 to furamidine.


Key words: bioactivation, cytochrome b5, enzyme inhibitors, enzyme kinetics


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