Pharmacokinetics metabolite profiling

Ian N. Acworth, Vice President, ESA Biosciences, Inc., Chelmsford, MA Kevin B. Alton, Senior Director, Schering-Plough Research Institute, Department of Drug Metabolism and Pharmacokinetics, Kenilworth, NJ Jose M. Castro-Perez, Laboratory Manager-Metabolite Profiling, Waters Corporation, Milford, MA... 

Transdermai administration of high doses of selegiline may improve the pharmacokinetic and active metabolite profile of this drug s use as an antidepressant... 

All four udder quarters of twelve dairy cows in mid-lactation were treated at 4 times the therapeutic dose of 50 mg/ quarter by intramammary infusion of an aqueous gel containing 200 mg of pirlimycin free base equivalents, including the labeled " C-pirlimycin hydrochloride. The pharmacokinetic parameters for total pirlimycin residue in blood and milk were determined. Three cows were sacrificed at each of four post-treatment intervals (4, 6, 14, and 28 days) to establish tissue residue depletion kinetics. Metabolite profiles of the residues in milk, liver, urine and feces were obtained and the unknown radiolabeled components identified. 

Mass Balance Studies. Pharmacokinetic mass balance studies apply unlabeled, stable isotopes or radiolabeled compounds to study the extent of absorption and first-pass metabolism, distribution, and excretion of a given compound. In the microdosing approach, a C-labeled compound is administered to human volunteers at doses from as low as one microgram blood, urine, and fecal samples are collected over time and analyzed for C content by accelerator mass spectroscopy to determine half-life, plasma AUC, and maximal concentration (Cmax)- However, these methods are not very popular even when very low doses of radioactivity are involved. Highly sensitive, and more readily available, tech-niques for separation and analysis (e.g., LC-MS, LC-MS/MS) are frequently used alternatives that enable pharmacokinetic investigations and metabolite profiling of nonradiolabeled compounds. 

Accelerator mass spectrometry (AMS) is an ultrasensitive analytical method for radioactivity analysis. AMS offers 10 -10 -fold increases in sensitivity over LSC or other decay counting methods so that levels as low as 0.0001 DPM can be detected (Brown et al., 2005, 2006). AMS has been applied to mass balance determination, pharmacokinetic studies of total radioactivity, and measurement of chemically modified DNA and proteins in humans after the administration of a low radioisotope dose (approximately lOnCi/person for mass balance and drug metabolism studies) (Buchholz et al., 1999 Garner, 2000 Garner et al., 2002 Liberman et al., 2004 White and Brown, 2004). In addition, off-line HPLC-AMS has been explored for metabolite profiling after... 

Wikberg, T. Vuorela, A. Metabolite profiles of two [ C]-labelled catechol 0-methyltransferase inhibitors, nitecapone and entacapone, in rat and mouse urine and rat bile, Eur.J.Drug Metab.Pharmacokinet., 1994,19, 125-135. [HPLC of metabolites only]... 

Also, if conversion of drug to active metabolite shows significant departure from linear pharmacokinetics, it is possible that small differences in the rate of absorption of the parent drug (even within the 80-125% range for log transformed data) could result in clinically significant differences in the concentration/ time profiles for the active metabolite. When reliable data indicate that this situation may exist, a requirement of quantification of active metabolites in a bioequivalency study would seem to be fully justified. 

In terms of pharmacokinetics, LVX has an excellent profile. With an oral dose of 500 mg, LVX has a bioavailability of >99%, an AUC range of 41.9-47.7 mgh/mL, Cmax of 4.5-6.2 mg/mL, a clearance of 10.5-11.9 L/h, and a volume of distribution (Vd) of 1.3 L/kg (Hurst et al., 2002). In terms of protein bound material, only 24-38% is affected (Fish, 2003). Like other fluoroquinolones, there is a 19-44% AUC reduction when co-administered with an aluminum or magnesium antacid or iron sulfate (Qaqish and Polk, 2003). The major metabolite of levofloxacin arises from glucuronidation (Brysk-ier, 2005). 

TABLE 7.1. The Pharmacokinetic Profile of Oseltamivir (1) and Its Active Metabolite GS-4071 (7)... 

Ibtal body accumulation reflects both total intake and the rate of elimination. Factors important in the rate of elimination include pharmacokinetics, lipid solubility, metabolism of the parent compound, profile of metabolites formed, rate of formation of reactive intermediates, degree of enzyme induction, amount of relevant covalent binding with subceUular macromolecules, and the rate of removal from the cell... 

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