Direct labeling approach

Direct labelling with deuterium is another approach, and three promising methods have been reported. One, described by Koch and Stuart and coworkers,41 is based on the specific exchange of hydrogen attached to hydroxylated carbon atoms by deuterium atoms that occurs on treatment with Raney nickel-D20. Although no exchange was observed with soluble starch or glycogen, the H-2, H-3, and H-6 atoms of... 

The direct labeling technique may be applied to either unfixed cells or fixed tissue, provided that the antigen is stable to fixation. Often this must be determined empirically (see Chapter 8). The following protocol presents the simplest approach direct labeling of a surface antigen on unfixed cells in suspension. 

Aliphatic and aromatic nucleophilic substitutions with p Fjfluoride are usually performed either on an immediate precursor of the target molecule (direct labelling using a one-step process) or on an indirect precursor followed by one or more chemical steps leading to the target radiotracer. The first approach, if highly desirable, is in fact rarely practicable. The reaction conditions are often not compatible with the structure or with the various chemical functions borne by the radiopharmaceutical. It is therefore common that the radiosynthesis comprises at least two chemical steps first the introduction of fluorine-18 followed by what is often a (multi)deprotection step. It is not unusual either that fluorine-18 is first incorporated into a much simpler and chemically more robust molecule which is then coupled to a more sensitive entity under milder conditions, possibly still followed by a final deprotection step. Suchlike multi-step procedures are possible thanks to the favourable half-life of fluorine-18. However, the more complicated the process, the more chance of side reactions and complicated final purifications (see also Section 2.3), which may seriously hamper the automation of the process. 

The hazard classification should lead directly to labelling of acute health effects, environmental and physical hazards. The labelling approach that involves a risk assessment should only be applied to chronic health hazards, e.g. carcinogenicity, reproductive toxicity, or target organ systemic toxicity based on repeated exposure. The only chemicals it may be applied to are those in the consumer product setting where consumer exposures are generally limited in quantity and duration ... 

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