Radiolabeled tracers

The methods for detection and quantitation of radiolabeled tracers are deterrnined by the type of emission, ie, y-, or x-rays, the tracer affords the energy of the emission and the efficiency of the system by which it is measured. Detection of radioactivity can be achieved in all cases using the Geiger counter. However, in the case of the radionucHdes that emit low energy betas such as H, large amounts of isotopes are required for detection and accurate quantitation of a signal. This is in most cases undesirable and impractical. Thus, more sensitive and reproducible methods of detection and quantitation have been developed. 

Other methods of sensitive detection of radiotracers have been developed more recently. Eourier transform nmr can be used to detect (nuclear spin 1/2), which has an efficiency of detection - 20% greater than that of H. This technique is useful for ascertaining the position and distribution of tritium in the labeled compound (14). Eield-desorption mass spectrometry (fdms) and other mass spectral techniques can be appHed to detection of nanogram quantities of radiolabeled tracers, and are weU suited for determining the specific activity of these compounds (15). 

Radiotracers have also been used extensively for the quantitative rnicrodeterrnination of blood semm levels of hormones (qv), proteins, neurotransmitters, and other physiologically important compounds. Radioimmunoassay, which involves the competition of a known quantity of radiolabeled tracer, usually I or H, with the unknown quantity of semm component for binding to a specific antibody that has been raised against the component to be deterrnined, is used in the rnicro deterrnination of physiologically active materials in biological samples (see Immunoassay). 

R. Haubner, H.J. Wester, Radiolabeled tracers for imaging of tumor angiogenesis and evaluation of anti-angiogenic therapies, Curr. Pharm. Des. 10(13) (2004) 1439-1455. 

The use of radiolabeled tracer analyte (quantity too small to affect the assay)... 

RIA relies on the use of radiolabelled tracers and the ability to separate the unbound or free radiolabelled antigen (or antibody) from the antigen-antibody complex. 

Competitive displacement of a radiolabeled tracer ligand (see Note 2) is the usual method to determine the receptor binding affinity of a GPCR ligand. Additional details of these methods are provided elsewhere (28). 

Finally, there has been speculation and recent experimental support for the involvement of a Nazarov-type cyclization in the biosynthesis of c/ s-jasmonic acid ° and marine-derived prostanoids. Radiolabel tracer studies have demonstrated Ae intermediacy of 8-HPETE (104) in the biosynthesis of prostanoid intermediate preclavulone A (lO ). This remarkable conversion was proposed to proceed by formation of allene oxide (105) followed by isomerization to (107) via the 2-oxidocyclopentadienyl cation (106 Scheme 41). To demonstrate the chemical feasibility of this proposal, Corey reported the transformation of epoxysilane (108) to, inter alia, the cyclopentenone (111 Scheme 42). The reaction is presumed to involve formation of the allene oxide (109) followed by isomerization to the 2-oxi-dopentadienylic cation (110). Conrotatory closure of (110) is expected to produce the cis isomer of (111) as observed. 

However, the study of red wine color benefits from many other different approaches, including the use of radiolabeled tracers, time-of-flight matrix-assisted laser desorption-ionization mass spectrometry, spectral analysis that reveal copigmentation, predictive analysis using... 

In general, the treatment of verum and calibration samples should be identical. If specific sample preparation (e.g., extraction or precipitation) as a separation or clean-up step is only used for samples but not always for cahbrators, the recovery of this separation or clean-up step must be determined and used to correct reported sample concentrations. Investigations to determine recovery may use radiolabeled tracer analyte or an internal standard not recognized by the antibody and measured using another technique. 

Another aspect is hydrolysis and colloid formation. During the reduction of pertechnetate with stannous salts, tin is oxidized and hydrolyzed to form highly polydispersed colloidal particles. In some cases, mainly when weak or unsuitable ligands are used in the " Tc labeling, interference of colloidal tin oxides on the biodistribution of " Tc-radiolabeled tracers may occur. Such effects and the biodistribution of Tc-Sn colloid in dependence of the preparation conditions were subject of detailed studies. 

III.C). Tissues are then ground and subjected to acid extraction and the metabolites are purified by HPLC. The cADPR fraction, identified by its coelution with radiolabeled tracer, is then collected, concentrated, and added to sea urchin egg homogenates or other cADPR-sensitive microsomes. A fluorimetric assay for Ca + release then forms the basis of the mass assay. The authenticity of cADPR in samples can then be validated by inhibition of Ca + release with the specific cADPR antagonist, 8-amino-cADPR (see Section IV. A), or by desensitization of the microsomes by prior release of microsomal Ca2+ by a maximal dose of cADPR (Dargie et ah, 1990). This assay has been used not only to quantify cADPR (Walseth et ah, 1991) in tissues but also to identify ADP-iibosyl cyclase activities (Howard et ah, 1993 Summerhill et ah, 1993 Lee et ah, 1993b). 

Keywords. Indene, Bioconversion, Rhodococcus, Crixivan, Flux analysis, Chemostat, Steady state. Radiolabeled tracers... 

The foremost requirement is the accurate determination of an observable bioreaction network structure that describes indene oxidation in Rhodococcus. Based on product accumulation profiles and induction studies, indene bioconversion networks have been proposed for several isolates [8]. To validate further these networks for our strains and employ them for flux analysis we developed an experimental system that can maintain cells at steady state while allowing accurate metabolite measurements for flux determination. The system comprised a chemostat with a regular feed of hquid medium and separate supply of the indene precursor through a gas phase hne. Indene uptake and metabolite production rates were easily measured in this system, leading to the calculation of the unknown bioconversion fluxes. Additional measurements for system closure and further validation were obtained by using radiolabeled tracers and measuring the products of their oxidation in Rhodococcus cultures [9,10]. 

In all of the radiolabeled tracer experiments conducted, the concentrations of the corresponding indene metabolites were found to be constant so that the linear model with respect to the radiolabeled tracer is justified. However, for the mass balance on radiolabeled indene, the total metabolite concentration is not constant and the first-order kinetic model is only satisfied when the total concentration is sufficiently low such that for that respective enzyme. 

Yanagimachi KS, Stafford DE, Dexter AF, Sinskey AJ, Drew SW, Stephanopoulos G (2001) Application of radiolabeled tracers to biocatalytic flirx analysis (submitted)... 

Covalent protein binding studies require the use of radiolabeled compounds. H-tracers are often used because they are relatively easy to synthesize and are of high specific radioactivity. However, possibilities exist that the H-label may be lost if oxygenation of the test compound occurs directly on the carbons where the H-label is attached. As a consequence, observed binding values could be underestimated. tracers, on the contrary, are less likely to lose the label, and therefore are preferred. However, possibilities of losing a radiolabel should be considered for any given radiolabeled tracer. Therefore, evaluation of the loss of radiolabels needs to be performed before covalent... 

Brady F, Luthra SK, Brown GD, Osman S, Aboagye E, Saleem A et al. Radiolabelled tracers and anticancer drugs for assessment of therapeutic efficacy using PET. Curr Pharm Des 2001 7 1863-1892. 

Some people worry about additional radiation, especially to the brain. But with whole-body PET, it does not work like that. Yes, the CT part of the scanner does deliver some extra radiation, but it can be a low dose. In fact, the next generation of PET scanners for the brain—a combination of PET/MRI—will use no radiation to take the structural pictures. The small amount of radiolabeled tracer compound you receive for a PET scan already circulates everywhere in the body, including the brain. So why not take pictures of that while the tracer is already in your body and your body is already in the scanner ... 

---