Discontinuous oral absorption pharmacokinetic model and bioavailability of 1-(2-fluoro-5-methyl-beta-L-arabinofuranosyl)uracil (L-FMAU) in rats.

1-(2-fluoro-5-methyl-beta-L-arabinofuranosyl)uracil (L-FMAU), the L isomer of FMAU, has shown potent activity against hepatitis B virus and Epstein-Barr virus. L-FMAU showed double peaks in the plasma concentration versus time profiles following oral administration to rats, indicating discontinuous oral absorption. The objective of this study was to characterize the bioavailability and pattern of L-FMAU absorption using a pharmacokinetic model which incorporated two separate absorption processes following oral administration of the nucleoside in an animal model, the rat. Simultaneous fitting of differential equations to L-FMAU plasma concentrations following oral and intravenous administration was performed using PCNONLIN. Total clearance of L-FMAU was moderate, averaging 0.47 +/- 0.16 L h-1 (mean +/- SD). Distributional clearance averaged 0.18 +/- 0.14 L h-1. The volume of the central compartment averaged 0.30 +/- 0.09 L, and the volume of the peripheral compartment averaged 0.15 +/- 0.08 L. The first-order absorption rate constants describing the first and second absorption phases averaged 1.22 +/- 1.56 and 4.14 +/- 5.42 h-1, respectively. Oral bioavailability was calculated by three methods: AUC, urinary excretion data, and a discontinuous oral absorption pharmacokinetic model. Bioavailability averaged 0.59 +/- 0.16, 0.64 +/- 0.23, and 0.63 +/- 0.13, respectively, for the three methods. The discontinuous oral absorption pharmacokinetic model is a promising new method for estimating absorption from two phases and for calculating oral bioavailability.

Pharmacokinetics of Ara-CMP-Stearate (YNK01): phase I study of the oral Ara-C derivative.

Ara-CMP-Stearate (1-beta-D-arabinofuranosylcytosine-5'-stearylphosphate, YNK 01, Fosteabine) is the orally applicable prodrug of cytosine-arabinoside (Ara-C). During a phase I study in patients with advanced low-grade non-Hodgkin lymphomas or acute myeloid leukemia, the pharmacokinetic parameters of Ara-CMP-Stearate (kindly provided by ASTA Medica, Frankfurt, Germany) were determined by HPLC analysis. Seventy-two hours after a first starting dose which served for the determination of baseline pharmacokinetic parameters, Ara-CMP-Stearate was administered over 14 days by daily oral application. Ara-CMP-Stearate was started at a dose of 100 mg/day and was escalated in subsequent patients to 200 mg/day and 300 mg/day. Plasma and urine concentrations of Ara-CMP-Stearate, Ara-C and Ara-U were measured during the initial treatment phase and within 72 h after the end of the 14-day treatment cycle. So far six patients have been treated with 100 mg/day, three with 200 mg/day and another six with 300 mg/day. One patient was treated consecutively with 100 mg, 300 mg and 600 mg. Fitting the results of the plasma concentration measurements of Ara-CMP-Stearate to a one-compartment model, the following pharmacokinetic parameters were obtained (average and variation coefficient VC). Ara-CMP-Stearate dose-independent parameters: lag time = 1.04 h (0.57); tmax = 5.72 h (0.30); t1/2 = 9.4 h (0.36). Dose-dependent parameters: at 100 mg: AUC = 1099 ng/h/ml (0.31); concentration(max) = 53.8 ng/ml (0.28); at 200 mg: AUC = 2753 ng/h/ml (0.32); concentration(max) = 154.8 ng/ml (0.46); at 300 mg: AUC = 2940 ng/h/ml (0.66); concentration(max) = 160.0 ng/ml (0.59). The long lag time and late tmax can be explained by resorption in the distal part of the small intestine. No Ara-CMP-Stearate was detected in urine samples (limit of detection = 500 pg/ml). Pharmacokinetic parameters of Ara-C following Ara-CMP-Stearate application showed the following characteristics: t1/2 = 24.3 h (0.39); AUC (100 mg) = 262 ng/h/ml (0.93); AUC (200 mg) = 502 ng/h/ml (0.87); AUC (300 mg) = 898 ng/h/ml (1.07). Since Ara-CMP-Stearate causes intravascular hemolysis after intravenous administration, it was not possible to determine its bioavailability by comparing the AUC after oral and i.v. application. Instead, the renal elimination of Ara-U, as the main metabolite of Ara-C was measured during the first 72-h period and after the last application.(ABSTRACT TRUNCATED AT 400 WORDS)

Manual and automated determination of 1-beta-D-arabinofuranosyl-E-5-(2-bromovinyl)uracil and its metabolite (E)-5-(2-bromovinyl)uracil in urine.

This paper describes the determination of 1-beta-D-arabinofuranosyl-E-5-(2-bromovinyl)uracil in urine. The method involves sample clean-up by liquid-liquid extraction with ethyl acetate followed by high-performance liquid chromatographic (HPLC) analysis. The sample preparation may be performed either manually or automatically using a Zymark Py-robotic system. The chloro analog of the parent compound, CV-araU, is used as the internal standard. As low as 0.1 microgram of analyte per ml of urine can be measured. This sensitivity is adequate for pharmacokinetic studies but could be improved quite easily if necessary.

Automated sequential trace enrichment of dialysates combined with high-performance liquid chromatography and automated heart-cutting for the determination of the nucleoside 1-(beta-D-arabinofuranosyl)-5-(1-propynyl)uracil and its metabolite 5-propynyluracil in urine.

The use of a new configuration to control the automated sequential trace enrichment of dialysate (ASTED) system has been used to estimate 1-(beta-D-arabinofuranosyl)-5-(1-propynyl)uracil and its metabolite 5-propynyluracil in urine. The system employs "heart-cutting" as a means to improve the efficiency of sample preparation and reduce analysis time. Using this technique the mean within- and between-run imprecision (coefficient of variation) at three different urine analyte concentrations was found to be 1.6 and 3.6% and 1.7 and 3.3% for 5-propynyluracil and 1-(beta-D-arabinofuranosyl)-5-(1-propynyl)uracil, respectively.

Metabolism of 5'-ether prodrugs of 1-beta-D-arabinofuranosyl-E-5-(2-bromovinyl)uracil in rats.

1-beta-D-Arabinofuranosyl-E-5-(2-bromovinyl)uracil (BV-araU) is a selective antiherpesviral agent that has been shown to be metabolically stable in mice. However, E-5-(2-bromovinyl)uracil (BVU) is the major metabolite found after oral dosing in animals other than mice. When BV-araU was given orally to germ-free rats, only small amounts of BVU were found in the plasma, suggesting an important role of enterobacteria in the formation of BVU. Then, the metabolism of BV-araU prodrugs was studied in specific-pathogen free rats to select oral prodrugs of BV-araU with enhanced metabolic stability. 5'-O-Ethyl BV-araU (Et-BV-araU) gave about a 2-fold higher BV-araU blood concentration 3 and 6 hr after administration than after oral dosing of BV-araU, while the level of BVU was lower. Other aliphatic alkyl prodrugs also gave a lower level of BVU, but did not give the same elevation in blood concentration of BV-araU as did Et-BV-araU. Dosing of 5'-O-acetyl BV-araU resulted in blood concentrations of BV-araU and BVU similar to those after oral administration of BV-araU. 5'-O-Aromatic alkyl prodrugs showed poor bioavailability. A nearly 2-fold higher urinary recovery rate was seen for Et-BV-araU than for BV-araU or 5'-O-acetyl BV-araU. The conversion of Et-BV-araU to BV-araU was demonstrated in vitro using rat liver extract in the presence of co-factors, although the reaction was slow. The 5'-O-aliphatic alkyl prodrugs were completely resistant to degradation by enterobacteria, whereas the esters were partially degraded to BVU. Et-BV-araU may be a useful oral prodrug of BV-araU due to its increased metabolic stability and bioavailability.

High-performance liquid chromatographic analysis of cytosine arabinoside and metabolites in biological samples.

A rapid, non-radioactive method to quantitate therapeutically realistic levels of 1-beta-D-arabinofuranosylcytosine (Ara-C) and its metabolites would be useful both in the clinic, for monitoring drug levels, and in the laboratory for correlating drug levels with cellular and molecular perturbations. Liquid chromatographic analysis of arabinose-nucleoside analogs in biological samples is complicated by the presence of interfering nucleosides and nucleotides. We report the development of two analytic procedures to measure Ara-C and metabolite levels in biological samples. One method uses a quaternary ammonium type anion-exchange resin to achieve isocratic separation in less than one hour. The second method utilizes a boronate-derivatized polyacrylamide column which binds cis-diols to selectively retain cytosine and uridine, while arabinose compounds are eluted with recovery approaching 100%. The eluted compounds are then easily quantitated on a reversed-phase C18 column. The sensitivity of both procedures was sufficient to obtain pharmacokinetic data on Ara-C and uracil-arabinose levels in serum and urine and on Ara-C triphosphate levels in tumor cells.

Pharmacologic studies of continuous infusion of arabinosylcytosine by liquid infusion system.

Arabinosylcytosine (ara-C) was administered by prolonged intravenous infusion with a portable liquid infusion system (LIS) to patients with acute myelogenous leukemia. With the use of tritiated ara-C and this portable system, pharmacologic studies were performed in 8 patients. Most of the plasma radioactivity is in the deaminated product, arabinosyluracil (ara-U). After continuous intravenous infusion, a constant ara-C level is achieved slowly in the plasma. Unless a loading (priming) dose is administered immediately before beginning the infusion, a steady-state ara-C level cannot be achieved until 8 to 24 hr after the infusion. The infusion system has two mechanisms--one for giving a loading dose (3 ml/min) and the other for regular infusion at a rate of 0.5 to 2.0 ml/hr. If a loading dose is given before continuous infusion, a steady-state are-C level is achieved within an hour. The plasma ara-C disappearance curves are biphasic with a terminal half-life of 104 min, which is the same as that of a single injection. The cumulative urinary excretion after approximately 23 hr of infusion varied from 14% to 35% in different patients; more than 90% is ara-U and the remainder is ara-C. Our results have demonstrated that LIS can be used conveniently to sustain a constant plasma level of ara-C. The LIS increases mobility of both inpatients and outpatients and is particularly convenient for ambulatory patients.