Engineering membrane proteins for nuclear medicine: Applications for gene therapy and cell tracking

Nuclear Imaging techniques such as PET and SPECT imaging are expected to pl ay major roles in evaluating the efficacy of in vivo gene therapy. In parti cular, the quantification of vector delivery and imaging the efficacy of ge m expression are of key interests in testing new treatment paradigms and in designing novel vectors. In this review article we illustrate how nuclear imaging can be used to image novel cell-surface expressed fusion proteins a nd how this strategy can be used to probe for phenotypic changes Ln genetic ally manipulated cells. Since the described approach uses new fusion protei ns, typically not present on eukaryotic cells, such "artificial receptors" can be designed to bind radioisotopes currently In clinical use. The descri bed fusion proteins consist of 1) a binding domain such as a peptide based chelator that binds Tc-99m oxotechnetate and 2) a membrane anchoring domain . A variety of fusion proteins have been tested so far and the most promisi ng one to date consists of a metallothionein (Mr)-derived C-terminal peptid e fused to a type II membrane protein markers containing the N-terminal mem brane anchoring domain of neutral endopeptidase (PEP). Cell-surface express ion of MT in transfected cells has been demonstrated using monoclonal antib odies in vitro. Both in vitro and in vivo transchelation experiments have c onfirmed expression of Tc-99m-binding sites Ln eukaryotic cells. We expect the described approach to evolve into a useful strategy to "tag" transfecte d cells with Tc-99m and thus assessing efficiency of gene delivery and expr ession.