Radiolabeling/radiolabeled metabolic activation

The mechanism is believed to involve metabolic activation in the kidney itself. Thus, when radiolabeled chloroform was given to mice, in the kidney, the radiolabel was localized in... 

The elucidation of the mechanism has revealed that this specificity is due to a requirement for metabolic activation for which the Clara cell is particularly suited. Thus, early studies using radiolabeled 4-ipomeanol found that the compound was localized particularly in the lungs (when expressed as nmol g-1 wet weight of tissue), and was covalently bound to lung protein (Fig. 7.38). This binding was five times that seen in the liver. Furthermore, autoradiography revealed that the radiolabeled 4-ipomeanol was bound to the Clara cells,... 

It has been proposed that metabolic activation of diphenylhydantoin may be responsible for the teratogenicity. After the administration of radioactively labeled diphenylhydantoin to pregnant mice, radioactive drug or a metabolite was found to be covalently bound to protein in the embryo. It was shown that both the teratogenicity and embryolethality of diphenylhydantoin could be increased by using an inhibitor of epoxide hydrolase (see chap. 4), trichloropropene oxide. Similarly, the covalent binding of radiolabeled diphenylhydantoin to protein was also increased by this treatment. 

Ethylene dibromide induced gene mutations, sister chromatid exchanges, chromosomal aberrations and cell transformation in animal cells. It induced mutations in two human lymphoblastoid cell lines, AHH-1 and TK6 in the absence of exogenous metabolic activation. Administration of radiolabelled ethylene dibromide to Wistar rats and BALB/c mice resulted in binding to DNA, RNA and proteins. [The nature of the binding was not characterized.]... 

The mechanism is believed to involve metabolic activation in the kidney itself. Thus, when radiolabelled chloroform was given to mice, in the kidney the radiolabel was localized in the tubular cells which were necrotic. Certain microsomal enzyme inducers such as 3-methylcholanthrene decreased the nephrotoxicity but not hepatotoxicity of chloroform, and phenobarbital pretreatment had no effect on nephrotoxicity but increased hepatotoxicity. Pretreatment with polybrominated biphenyls, however, increased toxicity to both target organs and also increased mixed function oxidase activity in both. In vitro studies have shown that microsomal enzymemediated metabolism of chloroform to C02 occurs in... 

Direct evidence for accumulation of PAHs by species that are able to metabolize these compounds is often difficult to obtain because of the rapid rate of biotransformation to metabolites that are not routinely detected by standard analytical techniques. If appreciable metabolism is occurring, then the bioaccumulation study needs to be performed using radiolabeled compounds to allow a mass-balance accounting of parent compounds and metabolites. Hence, for metabolically active species that accumulate very little parent compound, it may be more appropriate to monitor the metabolic products for evidence of accumulation. Two techniques have been developed to measure PAH metabolites one utilizes HPLC fluorescence detection of metabolites in bile of teleosts and the other uses P-postlabeling for DNA adducts that occur as a result of the interaction of metabolites and cellular DNA. Both techniques have displayed a highly positive correlation with environmental concentrations, making them useful for monitoring populations in our coastal areas. 

It may be helpful to examine the main elimination pathways, whether hepatic or renal, for the product in question. Routine radiolabelled metabolism studies are, however, not generally appropriate. One of the principal aims of the ADME and pharmacokinetic data should be to aid in the interpretation of the toxicity studies by providing comparative exposure data to the pharmacologically active moiety in animal species and humans. 

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... 

Thomas et al. (43) showed that phenanthrene is actively metabolized in salmonTds and Lee et al. (41) have shown that benzo[a]pyrene is biodegraded in three species of marine fish. Varanasi et al. (VI ) demonstrated for the first time that the skin of fish exposed to aromatic hydrocarbons via either force-feeding, intraperitoneal injection, or in flowing seawater accumulate substantial concentrations of metabolic products. This is of particular interest since studies of mammalian systems have shown that some alkyl naphthalenes can be accelerators of skin carcinogenesis (32). Varanasi et al. (11) also demonstrated that the mucus of rainFow trout exposed to radiolabeled naphthalene... 

The retention of 1,2-dibromoethane in tissues and body fluids can be altered by concurrent exposure to modifiers of enzyme activity, such as disulfiram (Plotnick et al. 1979). The concentration of radiolabeled 1,2-dibromoethane in the liver, kidneys, spleen, testes, and brain increased significantly in rats fed disulfiram in the diet for 12 days before an oral dose of 15 mg C-1,2- dibromoethane/kg compared with rats not fed disulfiram. Disulfiram, an inhibitor of P-450 metabolism (via action on acetaldehyde dehydrogenase), was found to increase the uptake of C into liver nuclei. These observations correlate well with the results of chronic studies (Wong et al. 1982) that demonstrated enhanced tumorigenic effects in the liver and testes following combined 1,2-dibromoethane and disulfiram exposure. 

Soil. Residual activity in soil is limited to approximately 3-4 months (Hartley and Kidd, 1987). Plant. The penetration, translocation, and metabolism of radiolabeled MCPA in a cornland weed Galium aparine) was studied by Leafe (1962). Carbon dioxide was identified as a metabolite but this could only account for 7% of the applied MCPA. Though no additional compounds were identified, it was postulated that MCPA was detoxified in the weed via loss of both carbon atoms of the side chain. 

When a compound undergoes metabolism, the pharmacokinetics of major metabolites, particularly those that have pharmacological activity or are responsible for toxicities, should be examined. A long half-life of a metabolite may result in accumulation long after the concentration of the parent molecule has reached steady state. Much of the evaluation of the pharmacokinetics and the rates and routes of metabolism will be studied in animals using radiolabelled drug, but should be supported by cold assays. 

The principal mechanism for the deactivation of released catecholamines is, however, not enzymatic destmction but reuptake into the nerve ending. The presynaptic membrane contains an amine pump—a saturable, high-affinity, Na" -dependent active-transport system that requires energy for its function. The recycled neurotransmitter is capable of being released again, as experiments with radiolabelled [ H]NE have shown, and can be incorporated into chromaffin granules as well. Many drugs interfere with neurotransmitter reuptake and metabolism, as discussed in subsequent sections. 

---