3p-C-NETA-TATE: A Potential Somatostatin Analogue for Diagnostic and Therapeutic SSTR2 Targeting Radiopharmaceuticals

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Introduction: Somatostatin-based radiopharmaceuticals (e.g., [⁶⁸Ga] Ga-DOTATATE and [¹⁷⁷Lu] Lu-DOTATATE) have been used successfully to diagnose, monitor, and treat patients with neuroendocrine tumors. [¹⁸F] AlF-NOTA-octreotide, a promising ¹⁸F-labeled somatostatin analogue and potential replacement for ⁶⁸Ga-DOTA-peptides, is currently being studied in clinical trials.¹ However, ideally, the same precursor (combination of chelator-linker-vector) can be used to produce both diagnostic and therapeutic radioprobes with pharmacokinetic properties that are very similar (e.g., Al¹⁸F/²¹³Bi/¹⁷⁷Lu) or identical (e.g., complementary Tb-radionuclides), allowing for accurate personalized dosimetry estimation and radionuclide therapy of NET patients.² We evaluated the potentially versatile and effective chelator 3p-C-NETA in this study and present preliminary results of radiosynthesis and preclinical evaluation of [¹⁸F]AlF-3p-C-NETA-TATE.³

Methods: At varying temperatures, 3p-C-NETA was radiolabeled with diagnostic (⁶⁸Ga, Al¹⁸F) or therapeutic (¹⁷⁷Lu, ¹⁶¹Tb, and ²¹³Bi) radionuclides. The in vitro stability of the corresponding radiocomplexes in human serum at 37°C was determined. 3p-C-NETA-TATE was produced via standard solid/liquid-phase peptide synthesis and purified via HPLC. [¹⁸F]AlF-3p-C-NETA-TATE was synthesized in an automated AllinOne synthesis module (Trasis, Belgium), radio-HPLC was used to analyze it, and the in vitro stability of [¹⁸F]AlF-3p-C-NETA-TATE was investigated. At 37°C, [¹⁸F] AlF-3p-C-NETA-TATE was tested in formulation buffer, PBS, and human serum. [¹⁸F] AlF-3p-C-NETA-TATE was used to perform in vitro cell binding and internalization. The pharmacokinetics of [¹⁸F] AlF-3p-C-NETA-TATE were evaluated using PET/MRI and PET/CT in healthy rats and mice bearing BON1.SSTR2 xenografts, respectively, with [¹⁸F] AlF-NOTA-octreotide as a benchmark.¹

Results: At 25°C, 3p-C-NETA was efficiently labeled with ¹⁷⁷Lu and ²¹³Bi (RCY > 95%), and at 55°C, with ¹⁶¹Tb (> 95%) and ⁶⁸Ga (> 90%). To achieve good yields (> 85%), Al¹⁸F-labeling required 95°C. Over an 8-day period, the ¹⁷⁷Lu- and ¹⁶¹Tb-3p-C-NETA-complexes demonstrated excellent in vitro stability in human serum (97% intact). We also found that [¹⁸F] AlF-3p-C-NETA has a high in vitro stability (> 93% intact in human serum) for up to 2 hours. [⁶⁸Ga] Ga-3p-C-NETA demonstrated low stability in human serum (80% intact after 2 hours). [¹⁸F] AlF-3p-C-NETA-TATE was synthesized with high RCY (56%) and radiochemical purity (> 98%). In all tested conditions, [¹⁸F] AlF-3p-C-NETA-TATE demonstrated excellent in vitro stability with greater than 95% intact tracer after 4 hours. After 60 minutes of incubation with [¹⁸F] AlF-3p-C-NETA-TATE, high SSTR2-specific cell binding and internalization (16.3 ± 1.9% of which 80.1 ± 1.9% is internalized) was observed. Finally, [¹⁸F] AlF-3p-C-NETA-TATE demonstrated excellent pharmacokinetic properties and tumor accumulation comparable to [¹⁸F] AlF-NOTA-Octreotide.

Conclusions: 3p-C-NETA is a versatile chelator that can be used for both targeted radionuclide therapy (¹⁷⁷Lu, ²¹³Bi, ¹⁶¹Tb) and diagnostic applications (Al¹⁸F), and it has the potential to replace DOTA analogues currently in clinical use. [¹⁸F] AlF-3p-C-NETA-TATE and [²¹³Bi]-Bi-3p-C-NETA-TATE will be studied further as a potential theranostic pair in SSTR2 tumor-bearing mice.

• References

• 1 Pauwels E, Cleeren F, Tshibangu T, et al. [¹⁸F]AlF-NOTA-octreotide PET imaging: biodistribution, dosimetry and first comparison with [⁶⁸Ga]Ga-DOTATATE in neuroendocrine tumour patients. Eur J Nucl Med Mol Imaging 2020;47(13):3033–3046
• 2 Ahenkorah S, Cassells I, Deroose CM, et al. Bismuth-213 for targeted radionuclide therapy: from atom to bedside. Pharmaceutics 2021;13(5):599
• 3 Ahenkorah S, Murce E, Cawthorne C, et al. 3p-C-NETA: a versatile and effective chelator for development of Al¹⁸F-labeled and therapeutic radiopharmaceuticals. Theranostics 2022;12(13):5971–5985