Introduction: We have previously developed a pretargeting approach for affibody-mediated cancer therapy based on PNA-PNA hybridization. In this article we have further developed this approach by optimizing the production of the primary agent, Z(HER2.342)-SR-HP1, and labeling the secondary agent, HP2, with the therapeutic radionuclide Lu-177. We also studied the biodistribution profile of Lu-177-HP2 in mice, and evaluated pretargeting with Lu-177-HP2 in vitro and in vivo.

Methods: The biodistribution profile of Lu-177-HP2 was evaluated in NMRI mice and compared to the previously studied In-111-HP2. Pretargeting using Lu-177-HP2 was studied in vitro using the HER2-expressing cell lines BT-474 and SKOV-3, and in vivo in mice bearing SKOV-3 xenografts.

Results and conclusion: Using an optimized production protocol for Z(HER2:342)-SR-HP1 the ligation time was reduced from 15 h to 30 min, and the yield increased from 45% to 70%. Lu-177-labeled HP2 binds specifically in vitro to BT474 and SKOV-3 cells pre-treated with Z(HER2:342)-SR-HP1.Lu-177-HP2 was shown to have a more rapid blood clearance compared to In-111-HP2 in NMRI mice, and the measured radioactivity in blood was 0.22 +/- 0.1 and 0.68 +/- 0.07%ID/g for Lu-177- and In-111-HP2, respectively, at 1 h p.i. In contrast, no significant difference in kidney uptake was observed (4.47 +/- 1.17 and 3.94 +/- 0.58%ID/g for Lu-177- and In-111-HP2, respectively, at I h p.i.). Co-injection with either Gelofusine or lysine significantly reduced the kidney uptake for Lu-177-HP2 (1.0 +/- 0.1 and 1.6 +/- 0.2, respectively, vs. 2.97 +/- 0.87%ID/g in controls at 4 h p.i.). Lu-177-HP2 accumulated in SKOV-3 xenografts in BALB/C nu/nu mice when administered after injection of Z(HER2:342)-SR-HP1. Without pre-injection of Z(HER2:342)-SR-HP1, the uptake of Lu-177-HP2 was about 90-fold lower in tumor (0.23 +/- 0.08 vs. 20.7 +/- 3.5%ID/g). The tumor-to-kidney radioactivity accumulation ratio was almost 5-fold higher in the group of mice pre-injected with Z(HER2:342)-SR-HP1. In conclusion, (177)LuHP2 was shown to be a promising secondary agent for affibody-mediated tumor pretargeting in vivo.