Characterization of dark liver pigment observed in rats after subchronic dosing of the beta3-adrenergic receptor agonist LY368842.

Dark liver pigmentation was observed in F344 rats in a subchronic toxicology study after daily dosing of LY368842 glycolate. In addition, green-colored urine was observed in some animals. To identify the source of the pigment and its potential for toxic consequences, the liver pigment was isolated from the liver tissue of rats. The resulting material was a dark brown to black powder that was insoluble in water, organic solvents, or a tissue-solubilizing agent. Several techniques, such as chemical degradation, HPLC, tandem mass spectrometry (LC/MS/MS), (1)H NMR, and matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS), were employed to characterize the dark liver pigment. Following oxidative degradation of the isolated pigment, degradation products related to LY368842 were identified or tentatively identified using LC/MS/MS. Two degradation products had the same protonated molecular ion at m/z 505, which is 30 amu higher than that of LY368842. The major m/z 505 product has been identified as the indole-2,3-dione oxidative product based on (1)H NMR data and confirmed by an authentic standard. In addition, monohydroxylated product was also identified in the degradation mixture. These degradation products were consistent with the metabolites found in vivo in rats. MALDI-MS analyses of liver and urine pigment both identified a product with a protonated molecular ion at m/z 977, suggesting formation of indirubin-like and indigo-like pigments. The results obtained suggest that the oxidative metabolites of LY368842 played a key role in the formation of the liver and urine pigments.

Metabolism and disposition of the antihypertensive agent moxonidine in humans.

The metabolism and pharmacokinetics of moxonidine, a potent central-acting antihypertensive agent, were studied in four healthy subjects after a single oral administration of approximately 1 mg (approximately 60 muCi) of [(14)C(3)]moxonidine. Moxonidine was rapidly absorbed, with peak plasma concentration achieved between 0.5 to 2 h postdose. The maximal plasma concentration and the area under the curve of unchanged moxonidine are lower than those determined for radioactivity, indicating presence of circulating metabolite(s). The total recovery of radiocarbon over 120 h ranged from 99.6 to 105.2%, with 92.3 to 103.3% of the radioactivity excreted in the urine and only 1.9 to 7.3% of the dose excreted in the feces. Thus, renal elimination represented the principal route of excretion of radioactivity. Metabolites of moxonidine were identified in urine and plasma samples by high performance liquid chromatography and liquid chromatography-tandem mass spectrometry. Oxidation of moxonidine on the methyl group or on the imidazoline ring resulted in the formation of hydroxymethyl moxonidine, hydroxy moxonidine, dihydroxy moxonidine, and dehydrogenated moxonidine. Metabolite profiling results indicated that parent moxonidine was the most abundant component in the urine. The dehydrogenated moxonidine was the major urinary metabolite as well as the major circulating metabolite. Moxonidine also underwent phase II metabolism, generating a cysteine conjugate. In summary, moxonidine is well absorbed after oral administration. The major clearance pathway for moxonidine in humans is via renal elimination. Furthermore, seven metabolites were identified with three metabolites unique to humans.