Metabolites radiolabeled

Excretion. Some -hexane is exhaled following cessation of exposure. This could amount to approximately 10% of that absorbed (Mutti et al. 1984 Veulemans et al. 1982). Excretion is rapid and biphasic with half-lives of 0.2 hours and 1.7 hours. Most -hexane is excreted in the urine as metabolites. Radiolabeled 14C02 in exhaled air has been detected after animal exposure to l4C] -hexane (Bus et al. 1982), indicating that intermediary metabolism of some metabolites takes place. 2,5-Hexanedione and 4,5-dihydroxy-2-hexanone are the major urinary metabolites of -hexane in humans. Half-lives of excretion have been estimated to be 13-14 hours (Perbellini et al. 1981, 1986). 

EoUowing po administration moricizine is completely absorbed from the GI tract. The dmg undergoes considerable first-pass hepatic metabolism so that only 30—40% of the dose is bioavailable. Moricizine is extensively (95%) bound to plasma protein, mainly albumin and a -acid glycoprotein. The time to peak plasma concentrations is 0.42—3.90 h. Therapeutic concentrations are 0.06—3.00 ]l/niL. Using radiolabeled moricizine, more than 30 metabolites have been noted but only 12 have been identified. Eight appear in urine. The sulfoxide metabolite is equipotent to the parent compound as an antiarrhythmic. Elimination half-life is 2—6 h for the unchanged dmg and known metabolites, and 84 h for total radioactivity of the labeled dmg (1,2). 

Hyphenation of HPLC with NMR combines the power of sepai ation with a maximum of stiaictural information by NMR. HPLC-NMR has been used in the detection and identification of diaig metabolites in human urine since 1992. The rapid and unambiguous determination of the major metabolites of diaigs without any pretreatment of the investigated fluid represents the main advantage of this approach. Moreover the method is non-destmctive and without the need to use radiolabelled compounds. 

Fluorouracil (5-fluorouracil, 5-FU, Fig. 5) represents an early example of rational drag design in that it originated from the observation that tumor cells, especially from gut, incorporate radiolabeled uracil more efficiently into DNA than normal cells. 5-FU is a fluorinated pyrimidine analog that must be activated metabolically. In the cells 5-FU is converted to 5-fluoro-2>deoxyuridine-monophosphate (FdUMP). This metabolite inhibits thymidilate synthase which catalyses the conversion of uridylate (dUMP) to thymidilate (dTMP) whereby methylenetetrahydrofo-late plays the role of the carbon-donating cofactor. The reduced folate cofactor occupies an allosteric site of... 

The results of metabolism studies with laboratory animals and livestock indicate that endosulfan does not bioconcentrate in fatty tissues and milk. Lactating sheep administered radiolabeled endosulfan produced milk containing less than 2% of the label. Endosulfan sulfate was the major metabolite in milk (Gorbach et al. 1968). A half-life of about 4 days was reported for endosulfan metabolites in milk from survivors of a dairy herd accidentally exposed to acutely toxic concentrations of endosulfan endosulfan sulfate accounted for the bulk of the residues detected in the milk (Braun and Lobb 1976). No endosulfan residues were detected in the fatty tissue of beef cattle grazed on endosulfan-treated pastures for 31-36 days (detection limits of 10 ppm for endosulfan, 40 ppm for endosulfan diol) the animals began grazing 7 days after treatment of the pastures. Some residues were detected in the fatty tissue of one animal administered 1.1 mg/kg/day of endosulfan in the diet for 60 days. No endosulfan residues were... 

Of particular interest in brevetoxin research are the diagnosis of intoxication and identification of brevetoxins and their metabolites in biological fluids. We are investigating the distribution and fate of radiolabeled PbTx-3 in rats. Three model systems were used to study the toxicokinetics and metabolism of PbTx-3 1) rats injected intravenously with a bolus dose of toxin, 2) isolated rat livers perfused with toxin, and 3) isolated rat hepatocytes exposed to the toxin in vitro. 

To further investigate the role of the liver in brevetoxin metabolism, PbTx-3 was studied in the isolated perfused rat liver model (27, 28). Radiolabeled PbTx-3 was added to the reservoir of a recirculating system and allowed to mix thoroughly with the perfusate. Steady-state conditions were reached within 20 min. At steady-state, 55-65% of the delivered PbTx-3 was metabolized and/or extracted by the liver 26% remained in the effluent perfusate. Under a constant liver perfusion rate of 4 ml/min, the measured clearance rate was 0.11 ml/min/g liver. The calculated extraction ratio of 0.55 was in excellent agreement with the in vivo data. Radioactivity in the bile accounted for 7% of the total radiolabel perfused through the liver. PbTx-3 was metabolized and eliminated into bile as parent toxin plus four more-polar metabolites (Figure 3). Preliminary results of samples stained with 4-(p-nitrobenzyl)-pyridine (29) indicated the most polar metabolite was an epoxide. 

By applying an extension of the clearance concept 30, 31), in vitro metabolism was used to predict in vivo toxin elimination. Hepatocytes were incubated with 0.5 to 10 pg unlabeled PbTx-3 containing 0.1 pg radiolabeled toxin as tracer. Disappearance of parent compound and the appearance of metabolites were measured by HPLC equipped with a Radiomatic isotope detector. (1.6 nmol/min/g liver)... 

Absorption kinetic studies on fasted rats dosed by lipid-emulsion gavage revealed rapid appearance of triehloroethylene in the blood (typieally peaking at 15 minutes post-exposure) followed by rapid disappearance (Templin et al. 1993). Rats similarly dosed with radiolabelled trichloroethylene showed rapid serum albumin adduction which peaked at 4-8 hours, then decayed with a half-life consistent with that of albumin itself (Stevens et al. 1992). However, some of the detected radioactivity may have been due to trichloroethylene metabolites rather than the parent compound. 

It is appropriate at this stage to evaluate procedures that will be used to assess the effectiveness of bioremediation, which have been discussed in Chapter 13. These may include (a) use of radiolabeled substrates (although these will not generally be permitted in field operations) and the application of C-labeled substrates, (b) evaluation of the occurrence of metabolites, and (c) evaluation of markers such as specific enzymes. 

The researeh on dehydroepiandrosterone (DHEA) is limited beeause of the laek of radiolabeled metabolites. Robinzon et al. [126] showed that, using pig liver mierosomes, the radiolabeled metabolites of DHEA can be prepared in stable, pure form for bioehemical smdies. They utilized pig liver microsomal (PLM) fractions to prepare pH]-labeled 7a-hydroxy-DHEA (7a-OH-DHEA), 7[3-hydroxy-DHEA (7P-OH-DHEA), and 7-oxo-DHEA substrates from 50 pM [1,2,6,7-3H]DHEA. The metabolites were separated by silica gel PLC plates using ethyl aeetate-hexane-gla-eial aeetic acid (18 8 , v/v) as the mobile phase, extracted with ethyl aeetate, and dried under a stream of nitrogen. The purity of markers was determined with the use of TLC and GC/MS. 

Validation of true extraction efficiency normally requires the identification and quantitation of field-applied radiolabeled analyte(s), including resulting metabolites and all other degradation products. The manufacturer of a new pesticide has to perform such experiments and is able to determine the extraction efficiency of aged residues. Without any identification of residue components the calculation of the ratio between extracted radioactivity and total radioactivity inside the sample before extraction gives a first impression of the extraction efficiency of solvents. At best, this ratio is nearly 1 (i.e., a traceability of about 100%) and no further information is required. Such an efficient extraction solvent may serve as a reference solvent for any comparison with other extraction procedures. 

A monoclonal antibody-based ELISA has been utilized to determine ceftiofur levels in milk. The authors noted that matrix interference occurred, but a 1 100 dilution lowered the interference, and a 1 1000 dilution eliminated the matrix interference. Because of the high dilution, samples could not be measured below l.Opgkg The assay measured ceftiofur, its major metabolite desfuroylceftiofur, and ceftiofur protein conjugates and has been utilized to measure residues in milk from cows treated with therapeutic doses of the drug. The results from the incurred residue correlated well with a previous study using radiolabeled ceftiofur, confirming the detection of a metabolite that was not detected by HPLC. 

Fat tissues do not appear to highly concentrate diisopropyl methylphosphonate or its metabolites. Tissue blood ratios for adipose deposits range from 1.3 to 3.6 in the species studied (Hart 1976). There was a surprisingly high concentration of radiolabel in the skin for mice with a tissue blood ratio of 14.6 (Hart 1976). It has been suggested, however, that the skin samples were contaminated with urine. 

In a single-dose oral study in male and female rats (Bucci et al. 1992), only 0.5% of the radioactivity from a dose of 660 mg/kg [14C]-radiolabeled diisopropyl methylphosphonate was found in the tissues 120 hours after dosing. The investigators indicated that no important tissue depot for diisopropyl methylphosphonate or its metabolites could be identified from the data obtained. 

At low doses, the metabolism of diisopropyl methylphosphonate to IMPA in the body is rapid and nearly complete. After oral exposure to diisopropyl methylphosphonate, the principal metabolite isolated from both urine (93-99%) and feces ( 97%) in mink, mice, rats, dogs, and cattle is IMPA (Bucci et al. 1992 Hart 1976 Ivie 1980). Less than 0.5% of the radiolabel was detected in the exhaled air of rats and mice as carbon dioxide after diisopropyl methylphosphonate ingestion (Hart 1976). Thus, complete metabolism of diisopropyl methylphosphonate occurs only to a minor extent. 

Another refinement, that avoids the necessity of developing suitable fecal extraction and chromatographic methods, is to dose the radiolabeled compound by both the i.v. and p.o. routes in two separate studies. Knowing that, by definition, the whole of the i.v. dose must have been bioavailable, a comparison of the proportion of the dose in the urine after the two different routes allows estimation of the percent absorbed. An analogous approach can be used without the use of a radiolabel, when the urine from the two studies is analyzed either for the parent compound or, more usually, for a major common metabolite. Assuming quantitatively identical clearance after both the i.v. and p.o. doses, the ratio of the amounts of analyte in the two experiments gives the absorption. 

Ward et al. [125] investigated the disposition of 14C-radiolabeled primaquine in the isolated perfused rat liver preparation, after the administration of 0.5, 1.5, and 5 mg doses of the drug. The pharmacokinetics of primaquine in the experimental model was dependent on dose size. Increasing the dose from 0.5 to 5 mg produced a significant reduction in clearance from 11.6 to 2.9 mL/min. This decrease was accompanied by a disproportionate increase in the value of the area under the curve from 25.4 to 1128.6 pg/mL, elimination half-life from 33.2 to 413 min, and volume of distribution from 547.7 to 1489 mL. Primaquine exhibited dose dependency in its pattern of metabolism. While the carboxylic acid derivative of primaquine was not detected perfusate after the 0.5 mg dose, it was the principal perfusate metabolite after 5 mg dose. Primaquine was subject to extensive biliary excretion at all doses, the total amount of 14C-radioactivity excreted in the bile decreased from 60 to 30%i as the dose of primaquine was increased from 0.5 to 5 mg. 

Recently, Voogt et al. [91] have reported on the d5-pathway in steroid metabolism of Asterias rubens. These workers established the existence of the d5-pathway (Scheme 20), analogous to the pathway found in mammals this conclusion was based on the observation that radiolabeled cholesterol (1) was converted to pregnenolone (112), 17a-hydroxypregnenolone (141), and androstenediol (142). Labeled pregnenolone was converted additionally to progesterone (129). Androstenediol (142) was the main metabolite of de-hydroepiandrosterone (143), a reaction catalyzed by 17/i-hydroxysteroid dehydrogenase (17/1-HSD). The metabolic conversion of androstenedione (131) to testosterone (132) is also mediated by 17/J-HSD and is related to... 

The significance of mirex residues in various tissues is unresolved, as is the exact mode of action of mirex and its metabolites. Minchew et al. (1980) and others indicated that mirex is a neurotoxic agent, with a mode of action similar to that of other chlorinated hydrocarbon insecticides, such as DDT. In studies with crayfish and radiolabeled mirex, mirex toxicosis was associated with neurotoxic effects that included hyperactivity, uncoordinated movements, loss of equilibrium, and... 

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