Reintroduction of copper-67 to radioimmunotherapy and evaluation of its imaging potential

Objectives: Copper-67 (Cu-67, t_1/2 = 2.58 days) β^- decays (β^-_max,562 keV) by low energy gamma rays (93 & 185 keV) potentiating this isotope to be used for both radiotherapy and single-photon emission computed tomography (SPECT). Previously, a pitfall for the theranostic use of ⁶⁷Cu had been the inability to obtain high specific activities and adequate quantities. Recently this problem has been partially overcome at Idaho Accelerator Center. Herein, we investigate the practicability of developing a ⁶⁷Cu labeled HER2 specific antibody as a theranostic agent by using ⁶⁷Cu with substantially improved quality.

Methods: Pertuzumab, a HER2 specific antibody, was conjugated with a commercially available bifunctional chelator, NOTA-NCS, and radiolabeled with ⁶⁷Cu. To provide proof-of-principle results, a pilot evaluation of ⁶⁷Cu-enabled radioimmunotherapy was performed in five groups of HER2⁺ HCC 1954 xenografted mice. All mice intravenously received 0.1 mg of trastuzumab (Herceptin®) 30 min prior to the tail vein injection of either the Pertuzumab only (Group 1: 13.3 µg) or ⁶⁷Cu-NOTA-Pertuzumab (Group 2: 0.1 mCi/3.3 µg; Group 3: 0.2 mCi/6.7 µg; Group 4: 0.2 mCi/133.3 µg; Group 5: 0.4 mCi/13.3 µg). SPECT/CT imaging was performed at 2 and 5 days post injection. The percent change in tumor volume over time was determined.

Results: The NOTA-Pertuzumab conjugate remained 80.6% immunoreactivity of the intact antibody. The specific activity of ⁶⁷Cu-NOTA-Pertuzumab was obtained in the range of 30 – 195 mCi/mg which is a significant increase from the previously reported range at ~ 1 mCi/mg of antibody. Shown in the pilot radioimmunotherapy evaluation, Groups 3, 4, and 5 with higher radioactivity doses demonstrated a significant decrease in tumor volume while Group 1 and 2 with no or lower radioactivity doses showed an increase. Treatment groups 3-5, however exhibited a radioactivity-dose-dependent toxic response to the ⁶⁷Cu-activity received with decreased survival rates. Under the same radioactivity doses, Group 3 demonstrated a faster decrease in tumor volume than Group 4 due to its higher specific activity. Higher specific activity is expected to make the ⁶⁷Cu-labeled radioimmunoconjugate accumulate more and gain better retention in tumors, thereby causing more damage to tumors. Tumors were clearly visualized on Day 2 and Day 5 by SPECT with a dose as low as 0.1 mCi per mouse, indicative of the anticipated targeted delivery of the radioimmunoconjugate, which is of clinical value to stratify patients or predict the therapy outcome.

Conclusion: The improved quality of ⁶⁷Cu was proved to significantly increase the specific activity of the radioimmunoconjugate. Our preliminary data with ⁶⁷Cu-NOTA-Pertuzumab demonstrated the practicability of using radioimmunoconjugates for targeted theranostics of cancer. Research Support: This project was partially supported by the Dr. Jack Krohmer Professorship Funds.