Evaluation of Radiolabeled Peptides for HER2 PET Imaging of Breast Cancer

72 Objectives: Breast cancer is the most frequently diagnosed cancer in women and ranks second among causes for cancer related death in women. Significantly, therapy targeting receptors overexpressed in breast cancer has led to successful treatment options for many women. In particular, treatments targeting the Human Epidermal Growth Factor Receptor 2 (HER2) such as Trastuzumab, Pertuzumab and T-DM1 have improved patient outcomes. As patient response relies heavily on the expression of HER2, the development of imaging strategies for HER2 and other receptors to aid in patient selection for targeted therapies is an urgent need. One potential solution is the use of peptides as radiolabeled imaging agents. Methods: HER2 binding peptides were designed with the conjugation of a DOTA chelator to bind 68Ga for PET imaging. A modified peptide reported by Huawei et al. from Journal of Fluorescence, (DOTA-Bn-SCN)-PEG2-GSGKCCYSL, was radiolabeled with 68Ga at 95 °C for 12 min at a pH of 4.5.1 The combination of labeled peptide and phosporamidon or PA (an enzyme inhibitor to prevent degradation described by Nock et al. in Journal of Nuclear Medicine) was added to three cell-lines: HER2-negative MDA-MB-231, HER2 positive SKBR3, and HER2 positive BT474 to confirm specificity and uptake of the designed peptide. Cells were plated on 24-well plates (8 per cell line) and incubated at 37 °C for 15 min in the presence of 300nM of peptide and 200nM of PA in 1mL of media per well. Cells were washed with ice-cold PBS and removed with 0.5M NaOH. Cells were collected and counted on a HIDEX Automatic Gamma Counter and the %uptake/mg of radioactivity/peptide per protein was determined. Results: Peptide radiolabeling resulted in greater than 98% radiochemical yields, with a specific activity of 49Ci/g, showing an efficient radiolabeling that is favorable for cell-studies and future investigations. Cell-studies showed that HER2 positive SKBR3 cells had significantly more %uptake/mg of the radiotracer (0.020±0.006s.e.) similar to HER2 positive BT474 cells (0.026±.008s.e.) compared to the non-HER2 expressing MDA-MB-231 (0.010±.004s.e.). All samples had n=16 and analysis was done using a one-way ANOVA with a Tukey’s post-hoc test. Conclusions: These preliminary studies show promising results for the development of Targeted peptides for HER2 imaging. Other peptide sequences are under investigation. With these preliminary results, tumor xenograft mice models will be planned to look at the in vivo stability, binding affinity, and biodistribution. Overall, we aim to optimize our peptide conjugates to fine tune pharmacokinetics and PET imaging attributes.Acknowledgements: I would like to thank the Lapi Lab and the Cyclotron Crew for their constant support and help. I would also like to thank UAB Radiology for funding and opportunity to do my research.