SPECT Imaging of SST2-Expressing Tumors with 99mTc-Based Somatostatin Receptor Antagonists: The Role of Tetraamine, HYNIC, and Spacers

Abstract

[99mTc]Tc-HYNIC-TOC is the most widely used 99mTc-labeled somatostatin receptor (SST) agonist for the SPECT imaging of SST-expressing tumors, such as neuroendocrine tumors. Recently, radiolabeled SST antagonists have shown improved diagnostic efficacy over agonists. 99mTc-labeled SST antagonists are lacking in clinical practice. Surprisingly, when [99mTc]Tc-HYNIC was conjugated to the SST2 antagonist SS01, SST2 imaging was not feasible. This was not the case when [99mTc]Tc-N4 was conjugated to SS01. Here, we assessed the introduction of different spacers (X: β-Ala, Ahx, Aun and PEG4) among HYNIC and SS01 with the aim of restoring the affinity of HYNIC conjugates. In addition, we used the alternative antagonist JR11 for determining the suitability of HYNIC with 99mTc-labeled SST2 antagonists. We performed a head-to-head comparison of the N4 conjugates of SS01 and JR11. [99mTc]Tc-HYNIC-TOC was used as a reference, and HEK-SST2 cells were used for in vitro and in vivo evaluation. EDDA was used as a co-ligand for all [99mTc]Tc-HYNIC conjugates. The introduction of Ahx restored, to a great extent, the SST2-mediated cellular uptake of the [99mTc]Tc-HYNIC-X conjugates (X: spacer), albeit lower than the corresponding [99mTc]Tc-N4-conjugates. SPECT/CT images showed that all 99mTc-labeled conjugates accumulated in the tumor and kidneys with [99mTc]Tc-HYNIC-PEG4-SS01, [99mTc]Tc-N4-SS01 and [99mTc]Tc-N4-JR11 having notably higher kidney uptake. Biodistribution studies showed similar or better tumor-to-non-tumor ratios for the [99mTc]Tc-HYNIC-Ahx conjugates, compared to the [99mTc]Tc-N4 counterparts. The [99mTc]Tc-HYNIC-Ahx conjugates of SS01 and JR11 were comparable to [99mTc]Tc-HYNIC-TOC as imaging agents. HYNIC is a suitable chelator for the development of 99mTc-labeled SST2 antagonists when a spacer of appropriate length, such as Ahx, is used.

Development and Evaluation of 99mTc Tricarbonyl Complexes Derived from Flutamide with Affinity for Androgen Receptor

“…When compared with flutamide, which has a high PPB (95 ± 1%), all the complexes presented clearly lower values. These results show the impact of the donor atoms set on the overall physicochemical behavior of Tc-labeled small biomolecules, a fact that has been corroborated by our group and other research groups before [ 14 , 15 , 17 ]. A comparison of C3 , which has the lowest PPB, with C4 , which has the highest value, also demonstrates that the length of the linker has, as expected, a considerable impact on the physicochemical properties of the labeled compounds.…”

“…Even if these early attempts to prepare 99m Tc-labeled derivatives were not successful, further research should be undertaken since the influences of the chelator and spacer in the biological properties of potential radiopharmaceuticals are well documented in the literature for a variety of molecular targets [ 12 , 13 , 14 , 15 , 16 ].…”

Radiolabeled Peptides for Cancer Imaging and Therapy: From Bench-to-Bedside

“…The well-established monodentate ligand hydrazinonicotinamide (HYNIC) was firstly conjugated to the antagonists SS01 and JR11 (similarly to [ 99m Tc] Tc-HYNIC-TOC (Tektrotyd)). [17] To our surprise, these conjugates massively lost their affinity for the SST2, which was possible to restore, to a different extent, by incorporating linkers of variable Fig. 2.…”

“…Aph(Hor) = 4-amino-l-hydroorotyl-phenylalanine; d-Aph(Cbm) = d-4-amino-carbamoyl-phenylalanine length and hydrophilicity, with aminohexanoic acid performing the best. [17] In parallel, we utilized the alternative chelating system 6-carboxy-1,4,8,11-tetraazaundecane (N4), in combination with SS01 and LM3. [18] [ 99m Tc]Tc-N4-LM3 ([ 99m Tc]Tc-TECANT-1) was selected, within the ERAPerMED project 'TECANT', to be the first 99m Tc-labeled SST2 antagonist for clinical translation, due to its favorable in vivo properties preclinically.…”

Radiolabeled Somatostatin Analogs—A Continuously Evolving Class of Radiopharmaceuticals

“…The first attempts to label SST 2 antagonists with 99m Tc via the monodentate ligand hydrazinonicotinamide (HYNIC) using ethylenediamine N , N ′ diacetic acid (EDDA) as a co-ligand (similarly to the clinically used agonist [ 99m Tc]Tc-HYNIC/EDDA-TOC) failed because the antagonist entirely lost its affinity for SST 2 [ 46 ], once more depicting the extreme sensitivity of the antagonists to N-terminal modifications. Further studies illustrated that the loss of affinity can be circumvented, to a certain extent, when a spacer of appropriate length and nature (e.g., aminohexanoic acid) is introduced between the antagonist and HYNIC [ 47 ]. Nevertheless, the alternative chelating system 6-carboxy-1,4,8,11-tetraazaundecane (N4) seems to be better suited to 99m Tc-based SST 2 antagonists.…”