Preparation, biodistribution, and small animal PET of 45Ti-transferrin

Investigation of 45Ti-transferrin was pursued to provide insight into the mechanism of action of titanocene dichloride, a chemotherapeutic agent currently in clinical trials.

Plasma protein-binding studies of processed 45Ti were performed by solubilizing the 45Ti residue in 0.05N HCl, of which 1.22 MBq (33 microCi) in 10 microL were added to 250 microL of dog plasma. 45Ti-Transferrin was prepared by redissolving the processed 45Ti in 25 micromol/L apotransferrin or by in vivo incorporation through preparation and introduction of 45Ti-citrate. Biodistribution studies were performed on normal Sprague-Dawley rats and EMT-6 tumor-bearing BALB/c mice with 45Ti-transferrin coinjected with 67Ga-citrate for direct comparison. microPET was performed on mice bearing EMT-6 tumors and the images were analyzed for tumor-to-muscle uptake ratios.

Direct labeling of apotransferrin in situ with 45Ti was achieved as well as in vivo incorporation by 2 h after injection with 45Ti-citrate. The biodistribution of 45Ti-transferrin and 67Ga-citrate showed similar trends. In Sprague-Dawley rats, initial blood uptake was higher for the 45Ti-transferrin, whereas bone uptake increased more for the 67Ga-citrate. EMT-6 tumor uptake in both cases was relatively high (14.6 +/- 1.83 %ID/g for 45Ti and 8.72 +/- 0.98 %ID/g for 67Ga [%ID/g = percentage injected dose per gram]) and remained elevated even out to 24 h after injection. The tumor-to-muscle ratio of the 67Ga-citrate reached 6.7 at 24 h, whereas the ratio of the 45Ti-transferrin increased to 4.3 at this time point. Uptake of 45Ti-transferrin was visualized in the EMT-6 murine mammary carcinoma tumor with microPET. In all cases, the tumor was clearly delineated from the surrounding tissue with tumor-to-muscle ratios on the order of 1.6.

45Ti forms a complex with apotransferrin that remains intact in vivo. Results of the biodistribution in mice showed that the tumor had increased uptake compared with nontarget organs (e.g., muscle). The microPET images of tumor-bearing mice clearly delineate the tumors from the surrounding tissue. Comparison of the data suggests that tissue uptake is similar whether injecting 45Ti-transferrin directly or as 45Ti-citrate, which transchelates to transferrin before the time of imaging.