title: Pharmacokinetics of a novel N-methyl-D-aspartate receptor antagonist (SM-18400): identification of an N-acetylated metabolite and pre-clinical assessment of N-acetylation polymorphism.
abstract: (S)-9-chloro-5-[p-aminomethyl-o-(carboxymethoxy)phenylcarbamoylmethyl]-6,7-dihydro-1 H,5 H-pyrido[1,2,3-de]quinoxaline-2,3-dione hydrochloride trihydrate (SM-18400) was given intravenously to rats and dogs and its pharmacokinetics was investigated. By LC/MS/MS analysis, the major metabolite in the rat serum was identified as N-acetylated SM-18400 (SM-NAc). In rats, AUC ratio of SM-NAc to SM-18400 was approximately 50%. However, 71% of the dose was excreted as unchanged SM-18400 and only 9.8% as SM-NAc in the urine and bile, indicating that the contribution of N-acetylation clearance (CL(NAc)) to the total clearance (CL(tot)) is limited to 10-30% in rats. No SM-NAc or other metabolites were detected in the dog serum, urine or bile. The in vitro intrinsic clearance (CL(int), ml/min/mg cytosolic protein) of N-acetyltransferase (NAT) activities of dog liver cytosol towards SM-18400 and hepatic N-acetylation clearance (CL(NAc), ml/min/kg body weight) estimated by well-stirred model were both only 5% of the respective rat value, well reflecting the relative in vivo CL(NAc)/CL(tot) ratios. CL(int) values for human liver cytosol samples (n = 4) and estimated CL(NAc) were all less than 18% and 7% of the rat, respectively. Based on these results, we concluded that the CL(NAc)/CL(tot) of human would be small enough to avoid major inter-individual variance in SM-18400 pharmacokinetics due to N-acetylation polymorphism. In addition, even a human liver cytosol sample lacking polymorphic NAT2 activity as determined by sulfamethazine (SMZ) N-acetylation analysis, proved capable of acetylating SM-18400, suggesting that NAT2 is not the major enzyme responsible for N-acetylation of SM-18400 in human. This fact would also reduce the risk of N-acetylation polymorphism playing a role in clinical use of this drug.
mesh_terms: D000107:Acetylation; D000123:Acetyltransferases / Q000378:metabolism; D000818:Animals; D019540:Area Under Curve; D001711:Biotransformation; D002851:Chromatography, High Pressure Liquid; D003600:Cytosol / Q000378:metabolism; D004285:Dogs; D018691:Excitatory Amino Acid Antagonists / Q000493:pharmacokinetics*; D006801:Humans; D066298:In Vitro Techniques; D007202:Indicators and Reagents; D007700:Kinetics; D008099:Liver / Q000378:metabolism; D008297:Male; D011110:Polymorphism, Genetic; D011810:Quinoxalines / Q000493:pharmacokinetics*; D051381:Rats; D017207:Rats, Sprague-Dawley; D016194:Receptors, N-Methyl-D-Aspartate / Q000037:antagonists & inhibitors*; D021241:Spectrometry, Mass, Electrospray Ionization; D013347:Subcellular Fractions / Q000378:metabolism
chemical_list: D018691:Excitatory Amino Acid Antagonists; D007202:Indicators and Reagents; D011810:Quinoxalines; D016194:Receptors, N-Methyl-D-Aspartate; C113033:SM 18400; D000123:Acetyltransferases