Novel radiohybrid PET-Tracers for SST₂-targeted imaging of neuroendocrine tumors.

Abstract

⁶⁸Ga- and ¹⁷⁷Lu-labeled theranostic companion tracers have become a mainstay in the clinical management of SST₂ overexpressing neuroendocrine tumors. Despite the excellent radionuclide characteristics of [¹⁸F]fluorine for PET imaging, ¹⁸F-labeled SST₂-targeted tracers remain underrepresented. Novel radiohybrid SST₂-tracers with DOTA as a bridging unit were designed, allowing radiolabeling with either ¹⁸F or ⁶⁸Ga. Seven DOTA-TATE derivatives (rhTATE1/2: N-SiFAlin-N, N-Me₂-Gly-D-Dap/Lys(trans-DOTA-TATE)-OH and (rhTATE2.1-2.5: H-AA1-AA2-AA3-D-Dap(N-SiFAlin-N, N-Me₂-Gly)-D-Lys(trans-DOTA-TATE)-OH) with different linkers and hydrophilic modifiers (AA1-AA3) were synthesized and compared to [¹⁸F]SiTATE. Competitive binding studies (IC₅₀) were performed using hSST₂-CHO cells and [¹²⁵I]TOC. Internalization was investigated using AR42J cells. Biodistribution and PET/CT studies were performed using AR42J xenograft bearing CD1 nu/nu mice. SST₂ specificity was confirmed in a blocking study (+/- co-injection of octreotide). While the first-generation compounds showed good affinity (IC₅₀: [^natGa]rhTATE1: 5.6 ± 1.4 nM, [^natGa]rhTATE2: 5.7 ± 0.2 nM) but high lipophilicity (LogD_pH=7.4 = -1.03 and - 1.19), the inclusion of hydrophilic modifiers ([^natGa]rhTATE2.1-[^natGa]rhTATE2.5) improved affinity (IC₅₀: 2.6 to 3.7 nM) and hydrophilicity (LogD_pH=7.4 = -2.30 to -2.12). The compounds demonstrated efficient internalization (357% to 841% compared to [¹²⁵I]TOC), and variable human serum albumin affinity (84.8% to 98.8%). [¹⁸F][^natGa]rhTATE2.2 showed the highest tumor accumulation (27.9 ± 4.8%iD/g), while [¹⁸F][^natGa]rhTATE2.5 showed lower tumor uptake (18.6 ± 6.2%iD/g), but substantially lower background accumulation, providing improved tumor-to-organ ratios. This study demonstrates a SST₂-targeted radiohybrid concept using bifunctionalized DOTA as a bridging unit. N-terminal modifications with hydrophilic tripeptides improved the pharmacokinetic properties. [¹⁸F][^natGa]rhTATE2.5 (D-Glu-D-Glu-D-Glu) compares particularly well to [¹⁸F]SiTATE regarding background clearance and tumor accumulation, with the additional advantage of radiohybrid radiolabeling.