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Excerpt
Conventional nuclear medicine imaging is frequently overlooked in favour of the more sophisticated and more expensive modality of PET. However, single photon emitting tracers do offer a more widely available tool of considerable value to pharmaceutical academia and industry. At the 1999 BNMS autumn meeting, we had the privilege of organizing a session on nuclear medicine imaging in drug delivery. Application of nuclear medicine techniques to the evaluation of pharmaceutical formulations has been an interesting area of work for the authors for more than 20 years [1, 2]. Hopefully, those attending the session were able to gain a better understanding of the role of in-vivo imaging in the drug development process.
The physiological basis of radionuclide techniques, coupled with the ability for quantification of data, gives our speciality a unique edge over other imaging modalities. Studies are generally undertaken to assess drug delivery systems using a model drug or a radiolabelled carrier rather than the active drug itself. In many instances, commonly used radiopharmaceuticals such as 99Tcm-DTPA and 99Tcm-HMPAO, or simple physical models such as ion exchange resins, may be appropriate for study. However, a fundamental proviso is that the radiolabelled tracer has been thoroughly validated for its intended purpose prior to the clinical study. This may be achieved by a simple in-vitro study. The nature and stability of the labelling will depend upon whether the study is intended to examine release, deposition, retention or dispersion, or is being used to monitor the effects of a physiological process, such as the effect on gastrointestinal transit.
The diverse nature of pharmaceutical studies using gamma camera imaging is shown in Table 1. Typical examples include the in-vivo assessment of the efficiency of inhalers and spacers for pulmonary drug delivery, where it has been shown that only a very low proportion of the administered drug actually reaches the intended site [3]. Other examples include the evaluation of controlled release formulations designed to release drug in the distal small bowel or colon. Scintigraphy can be used to determine the position of drug release to assess the site-specific absorption of orally administered drugs [4]. In this way, imaging can provide new information on regional gastrointestinal physiology. Imaging also plays an important role in assessing drug targeting and pharmacokinetics, for example in cancer chemotherapy [5]. Other findings have been of a more incidental nature, such as the observation of altered biodistribution of radiolabelled PEG-liposomes on repeat administration [6].
In recent years, there has been a search for alternatives to the oral delivery route for dosing of pharmacologically active materials that may be destroyed by gut enzymes and hepatic ‘first-pass’ effects. Proteins and peptide-based formulations such as hormones and vaccines are now being delivered by inhalation and intra-nasal administration [7, 8]. Gamma scintigraphy has played an important role in the evaluation of the new delivery devices, by measuring deposition and clearance characteristics.
