Metabolism of [(14)C]omapatrilat, a sulfhydryl-containing vasopeptidase inhibitor in humans.

Omapatrilat, a potent vasopeptidase inhibitor, is currently under development for the treatment of hypertension and congestive heart failure. This study describes the plasma profile along with isolation and identification of urinary metabolites of omapatrilat from subjects dosed orally with 50 mg of [(14)C]omapatrilat. Only a portion of the radioactivity in plasma was unextractable (40-43%). Prominent metabolites identified in plasma were S-methyl omapatrilat, acyl glucuronide of S-methyl omapatrilat, and S-methyl (S)-2-thio-3-phenylpropionic acid. Omapatrilat accounted for less than 3% of the radioactivity. However, after dithiothreitol reduction all of the radioactivity was extractable and was characterized to be omapatrilat and its hydrolysis product (S)-2-thio-3-phenylpropionic acid, both apparently bound to proteins via reversible disulfide bonds. Urinary profile of radioactivity showed no parent compound but the presence of several metabolites that can be grouped into three categories. 1) Three metabolites, accounting for 56% of the urinary radioactivity, resulted from the hydrolysis of the exocyclic amide bond of omapatrilat. Two metabolites were diastereomers of S-methyl sulfoxide of (S)-2-thio-3-phenylpropionic acid, and the third was the acyl glucuronide of S-methyl (S)-2-thio-3-phenylpropionic acid. 2) One disulfide, identified as the L-cysteine mixed disulfide of omapatrilat, accounted for 8% of the radioactivity in the urine. 3) Five metabolites, derived from omapatrilat, accounted for 30% of the radioactivity in the urine. Two of these metabolites were mixtures of diastereomers of S-methyl sulfoxide of omapatrilat and the third was the S-methyl omapatrilat ring sulfoxide. The other two metabolites were S-methyl omapatrilat and its acyl glucuronide. These results indicate that omapatrilat undergoes extensive metabolism in humans.

Metabolic studies on BMS-200475, a new antiviral compound active against hepatitis B virus.

BMS-200475 was recently shown to have potent antiviral activity against hepatitis B virus (50% effective concentration = 3.7 nM; 50% cytotoxic concentration = 30 microM). In metabolic studies in both HepG2 and hepatitis B virus-transfected 2.2.15 human hepatoma cell lines, the metabolism was similar, the primary products being the di- and triphosphates. The accumulation of triphosphate was rapid and detectable down to a 5 nM concentration of added drug. When cells were labeled at 25 microM, the intracellular triphosphate concentration attained 30 pmol/10(6) cells ( approximately 30 microM). The intracellular half-life of the triphosphate was about 15 h. Compared with five other nucleoside analogs of medical interest (lamivudine, penciclovir, ganciclovir, acyclovir, and lobucavir), BMS-200475 was most efficiently phosphorylated to the triphosphate in HepG2 cells.

Microsomal triglyceride transfer protein. Specificity of lipid binding and transport.

Microsomal triglyceride transfer protein (MTP) is a lipid transfer protein that is required for the assembly and secretion of very low density lipoproteins by the liver and chylomicrons by the intestine. To further elucidate the nature of the lipid molecule binding and transport site on MTP, we have studied the relative rates at which MTP transports different lipid species. Assay conditions were chosen in which there were minimal changes in the physical properties of the substrate membranes so that transfer rates would reflect MTP-lipid interactions at a membrane surface. Lipid transport rates decreased in order of triglyceride > cholesteryl ester > diglyceride > cholesterol > phosphatidylcholine. Changes in the hydrophobic nature of a lipid molecule by the addition of a fatty acid, modulated the ability of MTP to transport it. Addition of one acyl chain from diglyceride to triglyceride, lysophosphatidylcholine to phosphatidylcholine, or cholesterol to cholesteryl ester increased the rate of MTP-mediated transport 10-fold. In contrast, the lipid transport rate was insensitive to the changes in the structure or charge of the polar head group on phospholipid substrates. Zwitterionic, net negative, or net positive charged phospholipid molecules were all transported at a comparable rate. The ability of MTP to transport lipids is strongly correlated to the binding of these lipids to MTP. Thus, MTP has a specific preference for binding and transporting nonpolar lipid compared with phospholipids, and within a class of lipid molecules, a decrease in polarity increases its tendency to be transported.