Figure 3 .

Positron emission tomography permits visualisation of transgene expression, when the transgene is an enzyme, by administration of a radioactive labelled substrate, as the substrate will be selectively incorporated into the tissue(s) expressing the transgene. This figure shows the positron emission tomographic image of two rats: one had received intravenous injection of 10¹¹ PFU of an adenovirus encoding the reporter gene LacZ (Ad.LacZ) and the other the same dose of an adenovirus encoding HSV-tk (Ad.tk). Adenoviruses have strong liver tropism and a great proportion of the injected dose transduces liver cells. Two days after vector injection animals received a dose of ¹⁸FHBG, a substrate of HSV-tk. It can be seen that the radioactive label accumulates in the liver only in the animal injected with Ad.tk, while in the one that received Ad.LacZ most of the radioactivity is excreted by the kidneys and accumulates in the bladder. This study shows that PET represents a useful method to trace transgene expression.