Imaging hNET reporter gene expression with 124I-MIBG

The norepinephrine transporter (NET) has recently been suggested as a useful reporter gene. We have extended this effort by constructing an internal ribosomal entry site (IRES)-linked hNET-green fluorescent protein (GFP) hybrid reporter gene for both nuclear and optical imaging. Methods: A retroviral vector pQCXhNET-IRES-GFP was constructed and used to generate several reporter cell lines and xenografts. Transduced cells were sorted by fluorescence-activated cell sorting based on GFP expression and used for both in vitro and in vivo imaging studies. Results: The transduced reporter cells accumulated I-123- or I-124-labeled metaiodobenzylguanidine (MIBG) to high levels compared with the wild-type parent cell lines. Differences in MIBG accumulation between cell lines were primarily due to differences in influx (K-1) rather than efflux (k(2)). The estimated MIBG distribution volumes (V-d) for transduced Jurkat, C6, and COS-7 cells were 572 +/- 13, 754 +/- 25, and 1,556 +/- 38 mL/g, respectively. A correlation between radiotracer accumulation (K1) and GFP fluorescence intensity was also demonstrated. Sequential imaging studies of mice bearing pQCXhNET-IRES-GFP transduced and wild-type C6 xenografts demonstrated several advantages of I-124-MIBG small-animal PET compared with I-123-MIBG gamma-camera/SPECT. This was primarily due to the longer half-life of 124 1 and to the retention and slow clearance (half-time, 63 +/- 6 h) of MIBG from transduced xenografts compared with that from wild-type xenografts (half-time, 12 +/- 1 h) and other organs (half-time, 2.6-21 h). Very high radioactivity ratios were observed at later imaging times; at 73 h after I-124-MIBG injection, the C6/hNET-IRES-GFP xenograft-to-muscle ratio was 293 +/- 48 whereas the C6 xenograft-to-muscle ratio was 0.71 +/- 0.19. Conclusion: These studies demonstrate the potential for a wider application of hNET reporter imaging and the future translation to patient studies using radiopharmaceuticals that are currently available for both SPECT and PET.