Radiolabeled compounds

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. 

We would like to thank A. E. Pohland, Food and Drug Administration, Washington, for supplying us with the dioxins and G. E. Lynn, Dow Chemical Co., Michigan for providing us with the radiolabeled compound. We also thank L. L. Whitta and J. R. Tanner for their technical assistance and H. L. Trenholm and C. J. Paul for their cooperation. 

Diisopropyl methylphosphonate was absorbed from the gastrointestinal tract of mink, rats, mice, dogs, and cattle after oral administration of the [14C]-radiolabeled compound as indicated by the appearance of the radiolabel in the blood after exposure (Bucci et al. 1992 Hart 1976 Ivie 1980). Quantitative measurements of absorption can only be approximated (Hart 1976 Ivie 1980). 

The study of diisopropyl methylphosphonate distribution in a lactating Jersey cow was the only study that used multiple doses of diisopropyl methylphosphonate (Ivie 1980). In this single cow, radioactivity was detected in the blood 2 hours after dosing with [14C]-radiolabeled compound but not in the tissues. The animal had received diisopropyl methylphosphonate in one gelatin capsule for 5 days before the radiolabeled dose was administered. If tissue uptake in the cow was similar to that in dogs, measurements made 2 hours after dosing may not have provided an opportunity to measure tissue uptake of label. After 24 hours, 0.1% of the administered label was found in the cow s milk (Ivie 1980). 

One of the first decisions to be made when designing an experiment is the method of detection to be used with a particular solute. If radiolabeled material is available, a simple method of analysis is to count the radiolabel appearing in the receiver compartment as a function of time. While convenient, this can be a dangerous practice. Depending upon the type of radioisotope, its position in the molecule, and its specific activity, radiolabeled compounds can be subject to a variety of chemical and solution-catalyzed degradation pathways. If the stock solution contains a significant amount of radioactive impurities or generates them as a result of solution instability, then the possibility for preferential transport of... 

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. 

The trialkylstannyl intermediates required in this synthetic sceme to prepare labelled compounds can be obtained in several ways. One method is the addition of the organotin hydride to the carbon-carbon triple bond of an alkyne (equation 93). These reactions have already been discussed in detail above. A second approach is to add a trialkylstan-nylvinyllithium to a ketone (equation 95), and a third method involves adding trialkylstan-nyllithium to a /J-halo, a, /J-unsaturated ester (equation 96). Although this last reaction gives a suitable trialkylstannane, these stannanes have proven to be inert in the destanny-lation reaction and, therefore, have not been used extensively to prepare radiolabelled compounds. 

The value of light microscope autoradiography is the cellular/tissue locations of either the radiolabeled compound or its metabolic products are easily ascertained. 

A useful adjunct is the well-known pulse-chase technique, in which exposure to a radiolabeled compound for a specified period of time (pulse) is followed by the administration of the same compound that is not labeled (chase). 

A radioactivity detector is used to measure radioactivity in the HPLC eluent, using a flow cell. The detection principle is based on liquid scintillation technology to detect phosphors caused by radiation, though a solid-state scintillator is often used around the flow cell [17,31]. This detector is very specific and can be extremely sensitive. It is often used for conducting experiments using tritium or C-14 radiolabeled compounds in toxicological, metabolic, or degradation studies. 

In the early years of analysis data on the bioconcentration of LAS in biota were obtained using radiolabeled compounds and LSC, not distinguishing between parent compounds and metabolites [1,27,30]. These methods were non-specific and the data produced were unreliable due to overestimation. Therefore, it was important to develop accurate and specific analytical techniques in order to isolate and quantify these separately. 

Radiolabeling experiments showed that mirex is not metabolized in humans, rodents, cows, or minipigs the parent compound was the only radiolabeled compound present in the plasma, fat, and feces (Dorough and Ivie 1974 Gibson et al. 1972 Kutz et al. 1974 Mehendale et al. 1972 Morgan et al. 1979). Flowever, a monohydro derivative of mirex was identified in the feces, but not the fat or plasma, of rhesus monkeys given an oral or intravenous dose of mirex (Pittman et al. 1976 Stein et al. 1976 Wener et al. 1976). It is believed that the suspected metabolite may have arisen as a result of bacterial action in the lower gut or in the feces (Stein et al. 1976). 

Scintigraphic imaging is a noninvasive imaging technique commonly applied in nuclear medicine. Radiolabeled compounds (called radiopharmaceuticals or radiotracers) are administered intravenously to patients for diagnostic or, in certain cases, therapeutic purposes. The in vivo distribution can provide important physiological information about tissue function. 

It is a broad-spectrum herbicide active both pre- and post-emergent. Studies with radiolabelled compounds ( ) show it to be rapidly translocated particularly to the meristematic regions. Translocation to the underground storage organs of perennial weeds prevents regrowth of these weeds. Most herbaceous and woody plants are controlled in the field at 0.4-1.0 kg a.e./ha whereas most woody plants require 0.7-3.0 kg a.e./ha (10). 

The high-throughput purification in a discovery environment and the removal of transition metals using adsorption on or crystallization in the presence of activated carbon, glass-bead sponges, polymeric fibers, or silica-bound scavengers and the preparative isolation of radiolabeled compounds are out of the scope of this contribution. 

In the development of most new active substances, it is required to investigate the disposition of the compound and its metabolite(s) and their rates and routes of elimination. This is generally carried out with radiolabelled compound, usually In the United Kingdom, approval of the Administration of Radioactive Substances Advisory Committee (ARSAC) is required for administration of radiolabelled compound to man. The purpose of the submission is to demonstrate that the dose of absorbed radiation is minimised by administration of the lowest dose that is consistent with meeting the objectives of the study. In general, the estimated absorbed radiation dose should be less than 500 xSv, but higher amoimts are permissible if they can be justified. The estimate is based on tissue distribution of radioactivity in animals and the pharmacokinetics in animals and man. 

Know the physical properties of the substances with which you are working. Keep in mind that some compounds (such as acetaldehyde and tritiated water) have low boiling points. Again, keep in mind that some gloves do not offer an adequate barrier to certain chemicals. Some compounds enter the body with such facility that special care must be exercised when they are in use. One example is dimethyl sulfoxide, which as a solvent facilitates the entry of many solutes into the body. There are many known cases where radiolabeled compounds contaminated individuals who failed to consider this power of DMSO as a solute vehicle. 

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