Metabolism of the influenza neuraminidase inhibitor prodrug oseltamivir in the rat

The metabolism of [2-acetyl-(14)C]oseltamivir (GS4104, Ro 64-0796), the prodrug of the novel influenza neuraminidase inhibitor GS4071 (Ro 64-0802), was examined in rats after oral dosing. Intact oseltamivir was observed only in lung and urine, accounting for 37 and 15% of the total radioactivity in these samples, respectively. GS4071 was the major metabolite in plasma, tissues, and urine, and accounted for 32 to 56% of the radioactivity present in these samples. The second most abundant peak in these samples (13-24% of radioactivity) was a novel metabolite (M3). This metabolite was purified from urine of rats dosed orally with oseltamivir and was identified by liquid chromatography-mass spectrometry and NMR as the (R)-omega-carboxylic acid metabolite of oseltamivir. The omega-carboxylic acid metabolite of oseltamivir could not be produced in vitro. However, omega-hydroxylated products of oseltamivir were produced by rat liver microsomes. Both the (R)- and (S)-omega-hydroxylated products were observed, but formation of the (R)-isomer predominated. These data indicated that in the rat, oseltamivir was primarily metabolized to the active influenza neuraminidase inhibitor GS4071 and, to a lesser extent, to an (R)-omega-carboxylic acid metabolite.

Isolation and identification of a metabolite of cidofovir from rat kidney

Cidofovir is an acyclic nucleotide analog with potent and broad-spectrum antiviral activity against adenoviruses and herpesviruses including cytomegalovirus (CMV). Cidofovir undergoes intracellular phosphorylation by host enzymes to cidofovir phosphate and cidofovir diphosphate (the active form). An unidentified metabolite has been observed previously in rat tissues and in urine of rabbits, rats and monkeys dosed with cidofovir. In the present study, this metabolite was isolated from rat kidney following an intravenous dose of 100 mg kg-1 cidofovir. The metabolite (metabolite I) was separated from cidofovir and impurities using extraction on anion-exchange resin followed by preparative normal and reversed-phase high-performance liquid chromatography (HPLC). The isolated metabolite I was subjected to proton, 13C and phosphorus nuclear magnetic resonance (NMR) and matrix-assisted laser desorption/ionization mass spectroscopy, and confirmed to be cidofovir-phosphocholine. The uptake of cidofovir by rat kidney was saturated at an intravenous dose of 100 mg kg-1, probably as a result of saturation of the renal tubular secretion pathway. However, the relative abundance of cidofovir phosphocholine was not affected by dose.

Pharmacokinetics and metabolism of selected prodrugs of PMEA in rats

The oral bioavailability of PMEA [9-[2-(phosphonomethoxy)ethyl]adenine; adefovir) has been determined in rats from three bisester prodrugs of PMEA: bis-(pivaloyloxymethyl) PMEA (bis-POM PMEA), bis-(phenyl) PMEA, and bis-(o-ethoxyphenyl) PMEA. The prodrugs were each administered to 9 male rats as solutions in PEG 400 at a dose of 10 mg-equivalent of PMEA per kg. Plasma samples were obtained over the course of 12 hr and concentrations of PMEA were determined by fluorescence derivatization and analysis by HPLC. Concentrations of PMEA observed in plasma following oral administration of PMEA prodrugs were compared with levels observed for intravenous PMEA. The observed oral bioavailabilities of PMEA from bis-POM PMEA, bis-(phenyl) PMEA, and bis-(o-ethoxyphenyl) PMEA were 38.2%, 2.46%, and 40.1%, respectively. PMEA was the only metabolite formed after oral administration of bis-POM PMEA. Three metabolites were detected after oral administration of either bis-(phenyl) PMEA or bis-(o-ethoxyphenyl) PMEA to rats: PMEA, the corresponding monoester, and 2-adenylacetic acid. The major metabolite of bis-(phenyl) PMEA was 2-adenylacetic acid following oral administration. 2-Adenylacetic acid appears to have been formed from the intact prodrugs by a P450 mediated oxidation of the ethyl side chain.

Pharmacokinetics, bioavailability, metabolism, and tissue distribution of cidofovir (HPMPC) and cyclic HPMPC in rats

The pharmacokinetics, distribution, and metabolism of [14C]cidofovir and its cyclic analog {[2-14C]-1-[((S)-2-hydroxy-2-oxo-1,4, 2-dioxaphosphorinan-5-yl)methyl]cytosine; [14C]cyclic HPMPC} were compared after administration to Sprague-Dawley rats. After intravenous (iv) administration of [14C]cidofovir (3 or 5 mg/kg) or [14C]cyclic HPMPC (5 mg/kg), plasma concentrations of total radioactivity declined in a multiexponential manner for both drugs with terminal half-lives of 7-11 hr. No metabolites of cidofovir were observed in plasma. However, iv [14C]cyclic HPMPC was converted in vivo to cidofovir (3.9% of the AUC of total radioactivity). The corrected half-life of cyclic HPMPC was 0.53 +/- 0.05 hr. The clearance of iv cidofovir (0.60-0.79 liter/hr/kg) was significantly less than that of cyclic HPMPC (1.10 +/- 0.12 liter/hr/kg). Cyclic HPMPC was more efficiently secreted by the kidney than cidofovir. The steady-state volume of distribution of cyclic HPMPC (0.32 +/- 0.5 liter/kg) was substantially less than that of cidofovir (0.9-3.0 liter/ kg). Concentrations in kidney at 24 hr after iv administration of 5 mg/kg were approximately 20-fold higher for cidofovir (6.6 micrograms-eq/g) than for cyclic HPMPC, whereas levels of radioactivity in the remaining tissues were similar for both drugs. This is consistent with the improved therapeutic index observed for cyclic HPMPC in animals. After iv administration of [14C]cidofovir (3 mg/kg), concentrations in most tissues declined with half-lives > 12 hr, presumably reflecting the long intracellular half-life of phosphorylated drug. Kidneys and liver of animals given iv [14C]cidofovir (5 mg/kg) contained unchanged cidofovir (35-65%), metabolite I (attributed to cidovofir phosphocholine) (29-60%), and a peak coeluting with cyclic HPMPC on three different HPLC systems (metabolite II) (5-7%). Kidneys, liver, and lung of animals given iv [14C]cyclic HPMPC contained cyclic HPMPC (1-6%), cidofovir (44-50%), and metabolite I (44-54%). The subcutaneous bioavailability of cidofovir was 91.5%. The subcutaneous and oral bioavailabilities of cyclic HPMPC were 82.7% and 3.50 +/- 1.07%, respectively, based on total radioactivity.

An in vitro comparison of the permeability of prednisolone, prednisolone sodium phosphate, and prednisolone acetate across the NZW rabbit cornea

Controversy and ambiguity in the literature concerning the corneal penetration of prednisolone acetate over Prednisolone Sodium phosphate in NZW rabbits has recently prompted comparative studies using specific chromatographic assays. In vitro, corneal penetration studies were performed using the Ussing Chambers to compare the permeability and flux of both esters and prednisolone at 0.5% using a reversed phase HPLC-UV assay. Chromatograms of samples from the receiver chambers show primarily the presence of prednisolone from both esters; only prednisolone phosphate penetrated the cornea intact. Flux measurements were similar for prednisolone and both salt forms in terms of the metabolite prednisolone. Permeability coefficient calculations give the relative comparison: prednisolone acetate greater than prednisolone greater than prednisolone sodium phosphate.

Assay methodology for prednisolone, prednisolone acetate and prednisolone sodium phosphate in rabbit aqueous humor and ocular physiological solutions

Prednisolone, prednisolone acetate and prednisolone sodium phosphate are glucocorticoids used for ocular, anti-inflammatory therapy. A reversed-phase high-performance liquid chromatographic assay using ultraviolet detection has been developed that affords baseline resolution of the above analytes in balanced salt solutions and rabbit aqueous humor. The drugs can be quantified at 0.025-0.05 micrograms/ml in the above matrices; 6 alpha-methylprednisolone is used as the internal standard. Both esters of prednisolone are vulnerable to chemical and enzymatic hydrolysis giving prednisolone. Analysis of aqueous humor samples shows prednisolone acetate penetrating/metabolizing primarily to prednisolone; prednisolone sodium phosphate penetrates the cornea giving the ester and alcohol.

Comparative corneal penetration of prednisolone sodium phosphate and prednisolone acetate in NZW rabbits

Comparative corneal penetration studies in the literature with prednisolone sodium phosphate solution and prednisolone acetate suspension administered to rabbit eyes give conflicting results concerning the greater bioavailability of prednisolone acetate. A recent in vitro penetration study shows similar fluxes for both salt forms in terms of prednisolone, quantified by a specific HPLC assay. An in vivo comparison has been conducted in NZW rabbit eyes and shows similar bioavailability for both drugs. Prednisolone sodium phosphate, prednisolone acetate and the primary metabolite prednisolone are quantified by a specific HPLC assay in aqueous humor.