title: Inhibition by atovaquone of CYP2C9-mediated sulphamethoxazole hydroxylamine formation.
abstract: OBJECTIVE
  To determine whether the antiprotozoal drug atovaquone inhibits the cytochrome P(450) (CYP)2C9-mediated metabolism of sulphamethoxazole (SMX) to its potentially harmful hydroxylamine metabolite (SMX-HA) in vitro.
  
  
  METHODS
  Generation of SMX-HA from SMX was measured directly using high-performance liquid chromatography in human liver microsomes or expressed CYP2C9*1, with or without preincubation with reduced nicotinamide adenine dinucleotide phosphate, and the inhibition constant (K(i)) for atovaquone was determined. To determine the effect of protein binding in vitro, in some experiments, atovaquone was pre-incubated with serum proteins, followed by filtration.
  
  
  RESULTS
  The K(i) for inhibition of SMX-HA formation by atovaquone was 15 microM, which is within clinically attainable total plasma atovaquone concentrations of 45-55 microM. Atovaquone (45 microM) inhibited SMX-HA formation by 39% in human liver microsomes. However, following preincubation of atovaquone with serum proteins, no inhibitory effect by atovaquone was observed, consistent with previous reports of high plasma protein binding for atovaquone. Compared with human liver microsomes, CYP2C9*1 showed an eightfold greater specific activity for SMX-HA generation; as for liver microsomes, CYP2C9*1 activity was inhibited by atovaquone.
  
  
  CONCLUSIONS
  Atovaquone is a relatively weak inhibitor of CYP2C9-mediated SMX-HA formation in vitro. However, the effect is not observed in the presence of serum proteins. It is therefore unlikely that atovaquone would significantly inhibit SMX-HA formation in vivo.
authors.affiliation: Department of Medical Sciences, School of Veterinary Medicine, University of Wisconsin-Madison, 2015 Linden Drive West, Madison, WI 53706-1102, USA.
authors.forename: Jackie L
authors.initials: JL
authors.lastname: Miller
authors.forename: Lauren A
authors.initials: LA
authors.lastname: Trepanier
mesh_terms: D000328:Adult; D000981:Antiprotozoal Agents / Q000494:pharmacology*; D001189:Aryl Hydrocarbon Hydroxylases; D053626:Atovaquone; D001798:Blood Proteins / Q000378:metabolism; D002478:Cells, Cultured; D065729:Cytochrome P-450 CYP2C9; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D004347:Drug Interactions; D004791:Enzyme Inhibitors / Q000494:pharmacology*; D005260:Female; D006801:Humans; D008297:Male; D008862:Microsomes, Liver / Q000201:enzymology; D008875:Middle Aged; D009285:Naphthoquinones / Q000494:pharmacology*; D011485:Protein Binding; D039181:Steroid 16-alpha-Hydroxylase; D013250:Steroid Hydroxylases / Q000378:metabolism*; D013420:Sulfamethoxazole / Q000031:analogs & derivatives* / Q000378:metabolism* / Q000493:pharmacokinetics*
chemical_list: D000981:Antiprotozoal Agents; D001798:Blood Proteins; D004791:Enzyme Inhibitors; D009285:Naphthoquinones; C059873:sulfamethoxazole hydroxylamine; D003577:Cytochrome P-450 Enzyme System; D013250:Steroid Hydroxylases; C450260:CYP2C9 protein, human; D065729:Cytochrome P-450 CYP2C9; D001189:Aryl Hydrocarbon Hydroxylases; D039181:Steroid 16-alpha-Hydroxylase; D013420:Sulfamethoxazole; D053626:Atovaquone