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Cross-Species Quantitative Benchmarking of SSTR2 Radioligands in Fresh-Frozen Brain Sections: An Adaptable Framework
Caitlin McCutcheon1, Zachary Lawrence1, Marie James1
1Translational Sciences, Ratio Therapeutics, 19 Drydock Ave, Boston, MA, 02210, USA.
Rationale:
Somatostatin receptor subtype 2 (SSTR2) is a G-protein coupled receptor overexpressed in multiple cancers, including neuroendocrine tumors, small cell lung cancer, and hepatocellular carcinoma. Clinical validation of SSTR2-targeted radioligand therapy (Lutathera™) has driven development of next-generation ligands. Progress in next-generation SSTR2 ligand development is strengthened by quantitative platforms that preserve native receptor context and enable quantitative, cross-species binding data to drive translation and radiotherapeutic advancement.
Methods:
Fresh-frozen tissue sections from mouse, rat, cynomolgus monkey and human brain were assessed for SSTR2 binding using the high affinity antagonist [177Lu]Lu-DOTA-LM3. Non-equilibrium kinetics and equilibrium binding methods were used to derive apparent affinity, target density, and affinity measurements. Section-wipe/gamma counting enabled absolute quantification without requiring isotope standards. Autoradiography and SSTR2 immunohistochemistry confirmed anatomic localization. Characterization of unlabeled DOTA-LM3 and its unlabeled lutetium- and gallium-complexed derivatives were compared to assess the effects of radiometal conjugation.
Results:
Across all species, [1⁷⁷Lu]Lu-DOTA-LM3 exhibited specific, saturable binding characteristic of a single class of high-affinity sites. Autoradiography signals co-localized with SSTR2-rich regions identified by immunohistochemistry, confirming expected distribution patterns. Affinities derived from unlabeled and radiolabeled DOTA-LM3 were consistent, validating assay reproducibility. Whilst affinities of agonists and antagonists were conserved across species, radiometal substitution altered ligand properties: the lutetium-complexed form preserved affinity comparable to the chelator-only ligand, while the gallium-complexed variant showed markedly reduced affinity.
Conclusions:
This native-tissue platform provides efficient, translatable, reproducible and absolute quantification of SSTR2 ligand binding without tissue standards, supporting early selection of chemotypes, radiometals, and species-bridging expectations before in vivo studies.
Insights
A new method accurately quantifies somatostatin receptor subtype 2 (SSTR2) binding across species. This platform aids in developing targeted radioligand therapies for cancers like neuroendocrine tumors.
Area of Science:
- Pharmacology
- Oncology
- Radiochemistry
Background:
- Somatostatin receptor subtype 2 (SSTR2) is overexpressed in various cancers, including neuroendocrine tumors, driving interest in SSTR2-targeted radioligand therapies.
- Clinical success of SSTR2 radioligand therapy necessitates advanced platforms for developing next-generation ligands.
- Quantitative cross-species binding data are crucial for translating preclinical findings to clinical radiotherapeutic applications.
Purpose of the Study:
- To develop and validate a quantitative platform for assessing SSTR2 ligand binding in native tissues across multiple species.
- To enable precise characterization of novel SSTR2 ligands and radiometal conjugates for radiotherapeutic development.
- To establish a reproducible method for comparing ligand affinities and densities, facilitating early-stage drug development decisions.
Main Methods:
- Utilized fresh-frozen tissue sections from mouse, rat, cynomolgus monkey, and human brain for SSTR2 binding assays.
- Employed high-affinity antagonist [177Lu]Lu-DOTA-LM3 with non-equilibrium kinetics and equilibrium binding methods.
- Applied section-wipe/gamma counting for absolute quantification without isotope standards; confirmed localization via autoradiography and immunohistochemistry.
Main Results:
- [177Lu]Lu-DOTA-LM3 demonstrated specific, saturable binding to a single class of high-affinity SSTR2 sites across all tested species.
- Autoradiography signals precisely co-localized with SSTR2-rich regions identified by immunohistochemistry.
- Radiometal conjugation impacted ligand properties, with lutetium preserving affinity while gallium significantly reduced it.
Conclusions:
- The developed native-tissue platform offers efficient, translatable, and reproducible absolute quantification of SSTR2 ligand binding.
- This method eliminates the need for tissue standards, streamlining the selection of chemotypes and radiometals.
- The platform supports informed species-bridging expectations, crucial for advancing SSTR2-targeted radiotherapies before in vivo studies.
