Pharmacokinetics, toxicokinetics, distribution, metabolism and excretion of linezolid in mouse, rat and dog.

1. Linezolid (ZYVOX), the first of a new class of antibiotics, the oxazolidinones, is approved for treatment of Gram-positive bacterial infections. 2. The aim was to determine the absorption, distribution, metabolism and excretion (ADME) of linezolid in mouse, rat and dog in support of preclinical safety studies and clinical development. 3. Conventional replicate study designs were employed in animal experiments, and biofluids were assayed by HPLC or HPLC-MS. 4. Linezolid was rapidly absorbed after p.o. dosing with an p.o. bioavailability of > 95% in rat and dog, and > 70% in mouse. Twenty-eight-day i.v./p.o. toxicokinetic studies in rat (20-200mg kg(-1) day(-1)) and dog (10-80 mg kg(-1) day(-1)) revealed neither a meaningful increase in clearance nor accumulation upon multiple dosing. 5. Linezolid had limited protein binding (<35%) and was very well distributed to most extravascular sites, with a volume of distribution at steady-state (V(ss)) approximately equal to total body water. 6. Linezolid circulated mainly as parent drug and was excreted mainly as parent drug and two inactive carboxylic acids, PNU-142586 and PNU-142300. Minor secondary metabolites were also characterized. In all species, the clearance rate was determined by metabolism. 7. Radioactivity recovery was essentially complete within 24-48 h. Renal excretion of parent drug and metabolites was a major elimination route. Parent drug underwent renal tubular reabsorption, significantly slowing parent drug excretion and allowing a slow metabolic process to become rate-limiting in overall clearance. 8. It is concluded that ADME data were relatively consistent across species and supported the rat and dog as the principal non-clinical safety species.

Pharmacokinetics, metabolism, and excretion of linezolid following an oral dose of [(14)C]linezolid to healthy human subjects.

Linezolid (Zyvox), the first of a new class of antibiotics, the oxazolidinones, is approved for treatment of Gram-positive bacterial infections, including resistant strains. The disposition of linezolid in human volunteers was determined, after a 500-mg (100-microCi) oral dose of [(14)C]linezolid. Radioactive linezolid was administered as a single dose, or at steady-state on day 4 of a 10-day, 500-mg b.i.d. regimen of unlabeled linezolid (n = 4/sex/regimen). Mean recovery of radioactivity in excreta was 93.8 +/- 1.1% (range 91.2-95.2%, n = 15), of which 83.9 +/- 3.3% (range 76.7-88.4%) was in urine and 9.9 +/- 3.4% (range 5.3-16.9%) was in feces. There was no major difference in rate or route of excretion of radioactivity by dose regimen. Linezolid was excreted primarily intact, and as two inactive, morpholine ring-oxidized metabolites, PNU-142586 and PNU-142300. Other minor metabolites were characterized by high-performance liquid chromatography-atmospheric pressure chemical ionization-mass spectrometry and (19)F NMR spectroscopy. After the single radioactive dose, linezolid was the major circulating drug-related material accounting for about 78% (male) and 93% (female) of the radioactivity area under the curve (AUC). PNU-142586 (T(max) of 3-5 h) accounted for about 26% (male) and 9% (female) of the radioactivity AUC. PNU-142300 (T(max) of 2-3 h) accounted for about 7% (male) and 4% (female) of the radioactivity AUC. Overall, mean linezolid and PNU-142586 exposures at steady-state were similar across sex. In conclusion, linezolid circulates in plasma mainly as parent drug. Linezolid and two major, inactive metabolites account for the major portion of linezolid disposition, with urinary excretion representing the major elimination route. Formation of PNU-142586 was the rate-limiting step in the clearance of linezolid.

Pharmacokinetics, metabolism, and excretion of irinotecan (CPT-11) following I.V. infusion of [(14)C]CPT-11 in cancer patients.

This study determined the disposition of irinotecan hydrochloride trihydrate (CPT-11) after i.v. infusion of 125 mg/m(2) (100 microCi) [(14)C]CPT-11 in eight patients with solid tumors. Mean +/- S.D. recovery of radioactivity in urine and feces was 95.8 +/- 2.7% (range 92.2-100.3%, n = 7) of dose. Radioactivity in blood, plasma, urine, and feces was determined for at least 168 h after dosing. Fecal excretion accounted for 63.7 +/- 6.8 (range 54.2-74.9%, n = 7) of dose, whereas urinary excretion accounted for 32.1 +/- 6.9% (range 21.7-43.8%; n = 7) of dose. One patient with a biliary T-tube excreted 30.1% of dose in bile, 14.2% in feces, and 48.2% in urine. Quantitative radiometric HPLC revealed that CPT-11 was the major excretion product in urine, bile, and feces. Aminopentane carboxylic acid (APC) and SN-38 glucuronide (SN-38G) were the most significant metabolites in urine and bile, whereas SN-38 and NPC, a primary amine metabolite, were relatively minor excretion products. SN-38 and APC were the most significant metabolites in feces. The relatively higher amount of SN-38 in feces compared with bile is presumably due to hydrolysis of SN-38G to SN-38 by enteric bacterial beta-glucuronidases. There was close correspondence between quantitative fluorescence HPLC and mass balance findings. CPT-11 was the major circulating component in plasma (55% of the mean radiochemical area under the curve), and CPT-11, SN-38, SN-38G, and APC accounted for 93% of the mean radiochemical AUC. These results show that the parent drug and its three major metabolites account for virtually all CPT-11 disposition, with fecal excretion representing the major elimination pathway.

Metabolism of a bisphosphonate ester (PNU-91638) in rat.

1. Metabolites of the cyclic bisphosphonate ester, 4-[2,2'-bis-(5,5- dimethyl-1,3,2-dioxaphosphorinan-2-yl)] butanoyl-3-fluoro-benzene (PNU-91638) in bile or urine of the male Sprague-Dawley rat were characterized by mass spectrometry. The chronically bile duct/duodenal-cannulated male rats received a single oral dose of 100 mg/kg [13C] [14C]PNU-91638. Bile and urine samples were analysed for radioactivity and profiled by hplc with radiometric and UV detection. 2. The 0-28-h bile and urine accounted for 46.0 and 19.7% of dose respectively. The structures of radioactive peaks were investigated by ionspray and liquid secondary ion mass spectrometry (LSIMS) and LSIMS/MS analysis. 3. Major metabolites in urine included two regioisomeric phenol glucuronides, a gem methyl hydroxylated metabolite of the bisphosphonate heterocycle, a phenol metabolite, parent drug and a benzylic alcohol metabolite. Additional metabolites in bile included an unstable phenol/glutathione adduct (from a putative epoxide intermediate) with several minor isobaric regioisomers, and a carboxylic acid derived from the gem methyl hydroxylated bisphosphonate ring. 4. The structures proposed have not been confirmed by nmr due to discontinuation of the development of PNU-91638.

Biotransformation of lifibrol (U-83860) to mixed glyceride metabolites by rat and human hepatocytes in primary culture.

Lifibrol (U-83860), K12.148) is a lipid-lowering drug that has the potential to accumulate in the liver and induce hepatic peroxisome proliferation. To investigate the identity and potential human relevance of persistent lifibrol-related residues in rat liver, rat and human hepatocyte primary cultures were treated with 30 microM of [14C]lifibrol. After a steady uptake for 24 hr, cellular levels of radioactivity became stable for the next 24-48 hr. A nonradioactive lifibrol chase caused an efflux of intracellular radioactivity. Cellular autoradiography revealed the association of radioactivity with small lipid drops at 6 hr exposure and with large lipid drops at 24 hr exposure. HPLC analysis of media revealed that lifibrol acyl glucuronide and a t-butylcarboxylic acid metabolite (U-94613) were the major metabolites of rat and human hepatocytes, respectively. Using an HPLC method suitable for nonpolar metabolites, the analysis of rat and human cell extracts revealed a broad band of multiple, radioactive peaks that had a similar retention and UV spectrum to a synthetic standard of lifibrol cholesterol ester. Folch extracts of liver from rats treated with [14C]lifibrol or unlabeled lifibrol and [14C]acetate had a unique radioactive TLC band that had similar HPLC retention to hepatocyte residues. The group of nonpolar peaks from the hepatocytes was purified by HPLC. Conversion of the lifibrol sec-hydroxy group to a nicotinate ester afforded particle beam-electron impact mass spectra of the cholesterol ester standard and hepatocyte residues. The derivatized rat hepatocyte residue did not contain detectable lifibrol cholesterol ester (M+.816), but did contain molecular ion clusters corresponding to a mixed triglyceride of lifibrol and two fatty acids. Lifibrol-specific product ions of molecular ion clusters centered at M+.1021, 1047, and 1073 were observed at m/z 448, 430, and 310. The major lifibrol-containing triglycerides had a fatty acid composition of C16-C20 with 0-6 unsaturations.

Metabolism of the bisphosphonate ester U-91502 in rats.

The major metabolites of the bisphosphonate ester U-91502 were isolated from the bile and urine of male Sprague-Dawley rats and identified by NMR and MS. Bile duct-exteriorized and taurocholate-supplemented rats received single oral doses of 10-140 mg/kg of labeled U-91502. Analysis of radioactivity in bile, urine, and feces showed that U-91502-related radioactivity was rapidly excreted, predominantly in bile, achieving peak concentrations in bile of 1250 +/- 622 micrograms-eq/g during first 3 hr after a 10 mg/kg dose. The three major drug-related materials in bile and urine were separated by HPLC and designated in order of reversed-phase elution as metabolites A, B, and C. The least polar metabolite (C) was shown by HPLC/particle beam/MS and HPLC/electrospray/MS to be the triester, U-94532. Metabolite C cochromatographed with a synthesized standard of U-94532A. Metabolite B was the glucuronide conjugate of the 5-hydroxy pyrimidinone, U-97294. Metabolite A was a product of glutathione addition to a putative pyrimidinone 4,5-epoxide. Mechanisms for the formation of metabolites A, B, and C based on metabolite structure and stability were proposed.