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Imaging approaches to assess pharmacokinetics

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Non-invasive imaging approaches have had a significant impact on drug evaluation in humans. There are a host of end points in any clinical pharmacokinetic study and pharmacodynamic study that need to be measured during the drug development process. These end points are usually obtained invasively (or not at all) which limits the quality of data and in many cases, limits the population in which these data can be obtained. Imaging approaches can help fill this gap by obtaining the PK/PD end points non-invasively. Imaging techniques that are currently used in the pharmaceutical sciences include fluorescence imaging, magnetic resonance imaging (MRI), computed tomography (CT), single photon emission computed tomography (SPECT), and positron emission tomography (PET). Fluorescence imaging is relatively inexpensive and has good resolution but images are inherently two dimensional and the data is semi-quantitative in nature. Also, fluorescence emitters that are conjugated to drugs, especially small molecular drugs, may alter the pharmacokinetic properties of the drug itself. Furthermore, there are no reported studies of whole body human fluorescence imaging. Recently, with the advent of fluorescence molecular tomography, fluorescence imaging can now be used to quantitate whole body biodistribution, though this technology has been used only in small animals and has not been fully validated. MR and CT imaging is often used to visualize anatomy in both preclinical models and in humans. Also, in some cases, MR and CT imaging has been utilized to functional changes with drug treatment. The utility of these techniques to evaluate drug concentrations in blood or tissues is limited. The nuclear imaging techniques, PET and SPECT, are often used for functional imaging studies. PET and SPECT are highly sensitive, three dimensional imaging modalities that are currently being utilized in animal, as well as in human studies. An advantage of nuclear imaging techniques is that the same subject or animal can be used longitudinally over time, reducing the number of subjects or animals needed to obtain a rich dataset. PET imaging has the advantage that the radiotracers used (11C, 18F, 15O, 13N) are common elements that are part of the structure of many drugs and they have relatively short half-lives. Conversely, SPECT imaging requires drugs be labeled with 99Tc, a bulky radiotracer, which may have a greater potential to alter the pharmacokinetic profile of the drug itself. As a result, PET imaging is more suited for pharmacokinetic studies.

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