Preclinical Evaluation of [^155/161Tb]Tb-Crown-TATE—A Novel SPECT Imaging Theranostic Agent Targeting Neuroendocrine Tumours

Terbium radioisotopes (¹⁴⁹Tb, ¹⁵²Tb, ¹⁵⁵Tb, ¹⁶¹Tb) offer a unique class of radionuclides which encompass all four medicinally relevant nuclear decay modalities (α, β⁺, γ, β⁻/e⁻), and show high potential for the development of element-matched theranostic radiopharmaceuticals. The goal of this study was to design, synthesise, and evaluate the suitability of crown-TATE as a new peptide-conjugate for radiolabelling of [¹⁵⁵Tb]Tb³⁺ and [¹⁶¹Tb]Tb³⁺, and to assess the imaging and pharmacokinetic properties of each radiotracer in tumour-bearing mice. [¹⁵⁵Tb]Tb-crown-TATE and [¹⁶¹Tb]Tb-crown-TATE were prepared efficiently under mild conditions, and exhibited excellent stability in human serum (>99.5% RCP over 7 days). Longitudinal SPECT/CT images were acquired for ¹⁵⁵Tb- and ¹⁶¹Tb- labelled crown-TATE in male NRG mice bearing AR42J tumours. The radiotracers, [¹⁵⁵Tb]Tb-crown-TATE and [¹⁶¹Tb]Tb-crown-TATE, showed high tumour targeting (32.6 and 30.0 %ID/g, respectively) and minimal retention in non-target organs at 2.5 h post-administration. Biodistribution studies confirmed the SPECT/CT results, showing high tumour uptake (38.7 ± 8.0 %ID/g and 38.5 ± 3.5 %ID/g, respectively) and favourable tumour-to-background ratios. Blocking studies further confirmed SSTR2-specific tumour accumulation. Overall, these findings suggest that crown-TATE has great potential for element-matched molecular imaging and radionuclide therapy using ¹⁵⁵Tb and ¹⁶¹Tb.