PET Imaging of the Neurotensin Targeting Peptide NOTA-NT-20.3 Using Cobalt-55, Copper-64 and Gallium-68

Unveiling the potential of copper-61 vs. gallium-68 for SSTR PET imaging

PURPOSE

In recent years, copper-61 has attracted considerable attention from both physicists and radiochemists due to its favorable physical decay properties for PET imaging and its ease of production at any cyclotron center producing [18F]FDG. The aim of this study was to evaluate the potential of 61Cu-based radiopharmaceuticals for PET imaging of NETs, as an alternative to the commonly used gallium-68.

METHODS

Copper-61 was produced by irradiation of natural zinc liquid targets, followed by post-processing. In vitro evaluation of 61Cu- and 68Ga-labeled SST analogues was performed in SSTR positive AR42J tumor cells. PET/MRI was carried out in mice bearing AR42J subcutaneous tumors.

RESULTS

High molar activity [61Cu]Cu-DOTA-TATE and [61Cu]Cu-NOTA-TATE were successfully prepared with a radiochemical purity of over 95% and were shown to be stable for at least 6 h after the EOS. Both 61Cu- and 68Ga-labeled SST analogues exhibited high cellular uptake, with residual uptake when blocked with an excessive amount of peptide precursor. [61Cu]Cu-NOTA-TATE showed the highest tumor uptake at 1 h p.i. (13.25 ± 1.86%ID/g) and the tumor-to-non-tumor ratio increased from 1 h to 4 h p.i. At the later time point, tumor visualization improved compared to 1 h p.i. Moreover, preclinical PET/MR images demonstrated that [61Cu]Cu-NOTA-TATE has a more favorable biodistribution and imaging properties than [61Cu]Cu-DOTA-TATE, with the extended PET imaging window providing a clear advantage of [61Cu]Cu-NOTA-TATE over its gallium-68 analogues.

CONCLUSION

[61Cu]Cu-NOTA-TATE showed similar biodistribution and pharmacokinetics to [68Ga]Ga-DOTA-TATE at 1 h p.i., while demonstrating superior imaging characteristics for late PET imaging. These findings demonstrate that [61Cu]Cu-NOTA-TATE holds promising characteristics for improving the detection of NETs with increased translational potential.

Radiocobalt-Labeling of a Polypyridylamine Chelate Conjugated to GE11 for EGFR-Targeted Theranostics

The overexpression of the epidermal growth factor receptor (EGFR) in certain types of prostate cancers and glioblastoma makes it a promising target for targeted radioligand therapy. In this context, pairing an EGFR-targeting peptide with the emerging theranostic pair comprising the Auger electron emitter cobalt-58m (58mCo) and the Positron Emission Tomography-isotope cobalt-55 (55Co) would be of great interest for creating novel radiopharmaceuticals for prostate cancer and glioblastoma theranostics. In this study, GE11 (YHWYGYTPQNVI) was investigated for its EGFR-targeting potential when conjugated using click chemistry to N1-((triazol-4-yl)methyl)-N1,N2,N2-tris(pyridin-2-ylmethyl)ethane-1,2-diamine (TZTPEN). This chelator is suitable for binding Co2+ and Co3+. With cobalt-57 (57Co) serving as a surrogate radionuclide for 55/58mCo, the novel GE11-TZTPEN construct was successfully radiolabeled with a high radiochemical yield (99%) and purity (>99%). [57Co]Co-TZTPEN-GE11 showed high stability in PBS (pH 5) and specific uptake in EGFR-positive cell lines. Disappointingly, no tumor uptake was observed in EGFR-positive tumor-bearing mice, with most activity being accumulated predominantly in the liver, gall bladder, kidneys, and spleen. Some bone uptake was also observed, suggesting in vivo dissociation of 57Co from the complex. In conclusion, [57Co]Co-TZTPEN-GE11 shows poor pharmacokinetics in a mouse model and is, therefore, not deemed suitable as a targeting radiopharmaceutical for EGFR.

Radiolabeling Diaminosarcophagine with Cyclotron-Produced Cobalt-55 and [55Co]Co-NT-Sarcage as a Proof of Concept in a Murine Xenograft Model

Cobalt-sarcophagine complexes exhibit high kinetic inertness under various stringent conditions, but there is limited literature on radiolabeling and in vivo positron emission tomography (PET) imaging using no carrier added 55Co. To fill this gap, this study first investigates the radiolabeling of DiAmSar (DSar) with 55Co, followed by stability evaluation in human serum and EDTA, pharmacokinetics in mice, and a direct comparison with [55Co]CoCl2 to assess differences in pharmacokinetics. Furthermore, the radiolabeling process was successfully used to generate the NTSR1-targeted PET agent [55Co]Co-NT-Sarcage (a DSar-functionalized SR142948 derivative) and administered to HT29 tumor xenografted mice. The [55Co]Co-DSar complex can be formed at 37 °C with purity and stability suitable for preclinical in vivo radiopharmaceutical applications, and [55Co]Co-NT-Sarcage demonstrated prominent tumor uptake with a low background signal. In a direct comparison with [64Cu]Cu-NT-Sarcage, [55Co]Co-NT-Sarcage achieved a higher tumor-to-liver ratio but with overall similar biodistribution profile. These results demonstrate that Sar would be a promising chelator for constructing Co-based radiopharmaceuticals including 55Co for PET and 58mCo for therapeutic applications.

Preclinical evaluation of [58mCo]Co-DOTA-PSMA-617 for Auger electron therapy of prostate cancer

Prostate-specific membrane antigen (PSMA), highly expressed in prostate cancer, is a promising target for radionuclide therapy. Auger electron-emitting radionuclides are well suited for targeted radionuclide therapy if they can be delivered close to the DNA of the targeted cells. This preclinical study evaluated the theranostic pair [55/58mCo]Co-DOTA-PSMA-617 for PET imaging and Auger electron therapy of prostate cancer. [58mCo]Co-DOTA-PSMA-617 was successfully prepared with > 99% radiochemical yield and purity. In vitro, uptake and subcellular distribution assays in PSMA-positive prostate cancer cells showed PSMA-specific uptake with high cell-associated activity in the nucleus. Incubation with [58mCo]Co-DOTA-PSMA-617 reduced cell viability and clonogenic survival in a significant dose-dependent manner (p < 0.05). Biodistribution of xenografted mice showed high specific tumor uptake of the cobalt-labeled PSMA ligand for all time points with rapid clearance from normal tissues, which PET imaging confirmed. In vivo, therapy with [58mCo]Co-DOTA-PSMA-617 in tumor-bearing mice demonstrated significantly increased median survival for treated mice compared to control animals (p = 0.0014). In conclusion, [55/58mCo]Co-DOTA-PSMA-617 displayed excellent in vitro and in vivo properties, offering significant survival benefits in mice with no observed toxicities.

Separation of cyclotron-produced cobalt-55/58m from iron targets using cation exchange chromatography with non-aqueous solvents and extraction chromatography

Cobalt-55 and -58m form a theranostic pair that has relevant properties for cancer research. We report a cation exchange chromatography/extraction chromatography method that separates cyclotron-produced 55/58mCo from 54/57Fe in <1.5 h, recovers >85% Co and achieves [55Co]Co-NOTA and -DOTA AMA 89 ± 48 and 35 ± 7 MBq/nmol (EOB), respectively. Cobalt-55 and -58m were quantitatively labeled to functionalized NOTA at 106 and 50 MBq/nmol (EOB), respectively, corroborating measured AMA. This method is faster than previously published methods and achieves better [55/58mCo]Co-NOTA and -DOTA AMA.

Theranostic cobalt-55/58m for neurotensin receptor-mediated radiotherapy in vivo: A pilot study with dosimetry

Neurotensin receptor 1 (NTSR1) can stimulate tumor proliferation through neurotensin (NTS) activation and are overexpressed by a variety of cancers. The high binding affinity of NTS/NTSR1 makes radiolabeled NTS derivatives interesting for cancer diagnosis and staging. Internalization of NTS/NTSR1 also suggests therapeutic application with high LET alpha particles and low energy electrons. We investigated the therapeutic efficacy of [58mCo]Co-NOTA-NT-20.3 in vivo using murine models xenografted with NTSR1-positive HT29 human colorectal adenocarcinoma cells, and utilized [55Co]Co-NOTA-NT-20.3 for dosimetry.

METHODS

Targeting properties and cytotoxicity of [55/58mCo]Co-NOTA-NT-20.3 were assessed with HT29 cells. Female nude mice were xenografted with HT29 tumors and administered [55Co or 58mCo]Co-NOTA-NT-20.3 to evaluate pharmacokinetics or for therapy, respectively. Dosimetry calculations followed the Medical Internal Radiation Dose (MIRD) formalism and human absorbed dose rate per unit activity were obtained from OpenDose. The pilot therapy study consisted of two groups (each N = 3) receiving 110 ± 15 MBq and 26 ± 6 MBq [58mCo]Co-NOTA-NT-20.3 one week after tumor inoculation, and control (N = 3). Tumor sizes and masses were measured twice a week after therapy. Complete blood count and kidney histology were also performed to assess toxicity.

RESULTS

HPLC measured radiochemical purity of [55,58mCo]Co-NOTA-NT-20.3 > 99 %. Labeled compounds retained NTS targeting properties. [58mCo]Co-NOTA-NT-20.3 exhibited cytotoxicity for HT29 cells and was >15× more potent than [58mCo]CoCl2. Xenografted tumors responded modestly to administered doses, but mice showed no signs of radiotoxicity. Absorbed dose to tumor and kidney with 110 MBq [58mCo]Co-NOTA-NT-20.3 were 0.6 Gy and 0.8 Gy, respectively, and other organs received less than half of the absorbed dose to tumor. Off-target radiation dose from cobalt-58g was small but reduces the therapeutic window.

CONCLUSION

The enhanced in vitro cytotoxicity and high tumor-to-background led us to investigate the therapeutic efficacy of [58mCo]Co-NOTA-NT-20.3 in vivo. Although we were unable to induce tumor response commensurate with [177Lu]Lu-NT127 (NLys-Lys-Pro-Tyr-Tle-Leu) studies involving similar time-integrated activity, the absence of observed toxicity may constitute an opportunity for targeting vectors with improved uptake and/or retention to avoid the aftereffects of other high-LET radioactive emissions. Future studies with higher uptake, activity and/or multiple dosing regimens are warranted. The theranostic approach employed in this work was crucial for dosimetry analysis.