NeoBOMB1, a GRPR-Antagonist for Breast Cancer Theragnostics: First Results of a Preclinical Study with [67Ga]NeoBOMB1 in T-47D Cells and Tumor-Bearing Mice

Abstract

The GRPR-antagonist-based radioligands [^67/68Ga/¹¹¹In/¹⁷⁷Lu]NeoBOMB1 have shown excellent theragnostic profiles in preclinical prostate cancer models, while [⁶⁸Ga]NeoBOMB1 effectively visualized prostate cancer lesions in patients. We were further interested to explore the theragnostic potential of NeoBOMB1 in GRPR-positive mammary carcinoma, by first studying [⁶⁷Ga]NeoBOMB1 in breast cancer models; Methods: We investigated the profile of [⁶⁷Ga]NeoBOMB1, a [⁶⁸Ga]NeoBOMB1 surrogate, in GRPR-expressing T-47D cells and animal models; Results: NeoBOMB1 (IC₅₀s of 2.2 ± 0.2 nM) and [^natGa]NeoBOMB1 (IC₅₀s of 2.5 ± 0.2 nM) exhibited high affinity for the GRPR. At 37 °C [⁶⁷Ga]NeoBOMB1 strongly bound to the T-47D cell-membrane (45.8 ± 0.4% at 2 h), internalizing poorly, as was expected for a radioantagonist. [⁶⁷Ga]NeoBOMB1 was detected >90% intact in peripheral mouse blood at 30 min pi. In mice bearing T-47D xenografts, [⁶⁷Ga]NeoBOMB1 specifically localized in the tumor (8.68 ± 2.9% ID/g vs. 0.6 ± 0.1% ID/g during GRPR-blockade at 4 h pi). The unfavorably high pancreatic uptake could be considerably reduced (206.29 ± 17.35% ID/g to 42.46 ± 1.31% ID/g at 4 h pi) by increasing the NeoBOMB1 dose from 10 pmol to 200 pmol, whereas tumor uptake remained unaffected. Notably, tumor values did not decline from 1 to 24 h pi; Conclusions: [⁶⁷Ga]NeoBOMB1 can successfully target GRPR-positive breast cancer in animals with excellent prospects for clinical translation.

Keywords: GRPR-antagonist; PET-imaging; breast cancer; targeted tumor imaging; theragnostics.

Figure 1

Radiolabeling of NeoBOMB1 with ⁶⁷Ga: (a) Chemical structure of [⁶⁷Ga]NeoBOMB1 radioligand; (b) Radiochromatogram of HPLC analysis of [⁶⁷Ga]NeoBOMB1 labeling reaction mixture, showing a quantitative formation of high purity radioligand eluting at t_R = 20.0 min.

Figure 2

(a) [¹²⁵I-Tyr⁴]BBN displacement curves from GRPR-sites on T-47D cells after 3 h incubation at 4 °C by ◆ [^natGa]NeoBOMB1 (IC₅₀ 2.5 ± 0.2 nM, n = 3), ◇ NeoBOMB1 (IC₅₀ 2.2 ± 0.2 nM, n = 3) and ✴ [Tyr⁴]BBN (IC₅₀ 1.33 ± 0.09 nM, n = 5); (b) Curves of time-dependent association of [⁶⁷Ga]NeoBOMB1 to T-47D cells at 37 °C. Results represent average specific cell binding ± sd (◆ MB+I: ☐ membrane bound + ◇ internalized) vs. total added activity for each time point (n = 3, in triplicate); non-specific values were retrieved in the presence of 1 μM NeoBOMB1 and were subtracted from totals; the study was conducted with T-47D cells at 80–85% confluency.

Figure 3

Radiochromatogram of HPLC analysis of mouse blood sample collected 30 min pi of [⁶⁷Ga]NeoBOMB1, showing the presence of 90% intact [⁶⁷Ga]NeoBOMB1 in peripheral mouse blood at t_R = 33.9 min, as determined by co-injection of a [⁶⁷Ga]NeoBOMB1 sample in the HPLC.