Quantitative Whole Body Autoradiography QWBA

Distribution studies with radiolabeled test substances in animals are an important part of the drag development process. Traditional routine methods used for these studies are quantitative tissue distribution studies (QTD) and alternatively whole-body autoradiography (WBA) with detection of the radioactivity in whole-body sections on X-ray film (John R. J. Baker 1989). WBA is a qualitative detection method with a very high local resolution which includes all organs an many small substructures. 

Radioluminography (RLG) is an alternative method of radiation detection based on the phosphorus imaging technique. RLG is much more sensitive than the WBA technique, its exposure time is much shorter and it has a much wider linear measure range. Because of the latter property RLG enables a quantification of drag concentrations in whole-body sections. 

Quantitative Whole-Body Autoradiography (QWBA) is based on the RLG technique and the use of standards obtained from dilution series containing known concentrations of radioactivity. Isotopes used in QWBA are mainly 14C and 3H. These standards were cut together with the whole-body sections to ensure an identical thickness and used for the internal calibration. The information of the calibration curve allows the determination of the concentrations in the organs and tissues of interest which can be derived from the measured area and the section thickness. 

The results of distribution studies form the principle basis for the assessment of exposure and the elimination of residues in human organs and tissues. 

Direct determinations of exposure in man are generally limited to measurements in blood and plasma, which are easily accessible. Distribution patterns are determined in animals, usually rats, instead. Apart from the standard study design described below all kind of animals can be used up to the size of rabbits. The results obtained with QWBA provide pharmacokinetic data on the test substance and/or its metabolites, and evidence for interpretations regarding potential toxicological and pharmacological target organs. 

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The majority of the 14C-human ADME studies are conducted with a small number of healthy adult subjects (often between 6-8) and if bile collection is needed, a small group of additional subjects are included [228], Traditionally, due to ethical reasons, male subjects are selected for the 14C-ADME studies. Before the start of the 14C-ADME studies, study sponsors have the responsibility to determine stability of the radiolabel, purity of the radiolabel (distinguishing degradants from metabolites is very important), and conduct tissue distribution studies in nonclinical species preferably using quantitative whole-body autoradiography (QWBA) to detect radioactivity in tissues, organs, and excreta to determine the safe radioactivity dose. Nonclinical tissue distribution study data are extrapolated and used to show that radioactivity exposure of a specific tissue/organ will be well below the allowable limits to humans [229,230], Most of the 14C-human ADME studies consider a total radioactivity dose of 100[tCi or less to be safe [231],... 

Richter, W.F., Starke, V., and Whitby, B., The distribution pattern of radioactivity across different tissues in quantitative whole-body autoradiography (QWBA) studies, Eur. J. Pharm. Sci., 28(1-2), 155, 2006. 

Tissue distribution studies in pigmented Long-Evans rats to provide dosimetry to various tissues and organs are required to support human ADME with radiolabeled compound. Typically, these studies are limited to a single dose by the intended route of administration (PO, IV, etc.). The radioactivity levels in various tissues at different time points can be measured by quantitative whole-body autoradiography (QWBA) where sections of the... 

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