Affibody-mediated PNA-based pretargeted co-treatment improves survival of trastuzumab-treated mice bearing HER2-expressing xenografts.

Treatment of patients with HER2-expressing tumors using the monoclonal antibody trastuzumab increases survival. The affibody-based peptide nucleic acid (PNA)-mediated pretargeted radionuclide therapy has demonstrated efficacy against HER2-expressing xenografts in mice. Structural studies suggest that affibody molecules and trastuzumab bind to different epitopes on HER2. The aim of this study was to test the hypothesis that a combination of PNA-mediated pretargeted radionuclide therapy and trastuzumab treatment of HER2-expressing xenografts can extend the survival compared to monotherapies. Methods: Mutual interference of the primary pretargeting probe ZHER2:342-SR-HP1 and trastuzumab in binding to HER2-expressing cell lines was investigated in vitro. Experimental therapy evaluated the survival of mice bearing HER2-expressing SKOV-3 xenografts after treatment with a) vehicle; b) trastuzumab only; c) pretargeting using affibody-PNA chimera ZHER2:342-SR-HP1 and complementary probe 177Lu-HP2; and d) combination of trastuzumab and pretargeting. The ethical permit limited the study to 90 days. The animals' weights were monitored during the study. After the study termination, samples of liver and kidneys were evaluated by a veterinary pathologist for toxicity signs. Results: The presence of a large molar excess of trastuzumab had no influence on the affinity of ZHER2:342-SR-HP1 binding to HER2-expressing cells in vitro. The affinity of trastuzumab was not affected by a large excess of ZHER2:342-SR-HP1. The median survival of mice treated with trastuzumab (75.5 d) was significantly longer than the survival of mice treated with a vehicle (59.5 d). Median survival of mice treated with pretargeting was not reached by the day 90. Six mice of ten in this group survived, and two had complete remission. All mice in the combination treatment group survived, and tumors in seven mice had disappeared at the study termination. There was no significant difference between animal weights in the different treatment groups. No significant pathologic alterations were detected in livers and kidneys of treated animals. Conclusion: Treatment of mice bearing HER2-expressing xenografts with the combination of trastuzumab and affibody-mediated PNA-based radionuclide pretargeting significantly increased the survival compared to monotherapies. Co-treatment was not toxic for normal tissues.