Comparative Preclinical Evaluation of HYNIC-Modified Designed Ankyrin Repeat Proteins G3 for the 99mTc-Based Imaging of HER2-Expressing Malignant Tumors.

HER2 status determination is a necessary step for the proper choice of therapy and selection of patients for the targeted treatment of cancer. Targeted radiotracers such as radiolabeled DARPins provide a noninvasive and effective way for the molecular imaging of HER2 expression. This study aimed to evaluate tumor-targeting properties of three 99mTc-labeled DARPin G3 variants containing Gly-Gly-Gly-Cys (G3C), (Gly-Gly-Gly-Ser)3-Cys ((G3S)3C), or Glu-Glu-Glu-Cys (E3C) amino acid linkers at the C-terminus and conjugated to the HYNIC chelating agent, as well as to compare them with the clinically evaluated DARPin G3 labeled with 99mTc(CO)3 using the (HE)3-tag at the N-terminus. The labeling of DARPin G3-HYNIC variants provided radiochemical yields in the range of 50-80%. Labeled variants bound specifically to human HER2-expressing cancer cell lines with affinities in the range of 0.5-3 nM. There was no substantial influence of the linker and HYNIC chelator on the binding of 99mTc-labeled DARPin G3 variants to HER2 in vitro; however, [99mTc]Tc-G3-(G3S)3C-HYNIC had the highest affinity. Comparative biodistribution of [99mTc]Tc-G3-G3C-HYNIC, [99mTc]Tc-G3-(G3S)3C-HYNIC, [99mTc]Tc-G3-E3C-HYNIC, and [99mTc]Tc-(HE)3-G3 in healthy CD1 mice showed that there was a strong influence of the linkers on uptake in normal tissues. [99mTc]Tc-G3-E3C-HYNIC had an increased retention of activity in the liver and the majority of other organs compared to the other conjugates. The tumor uptake of [99mTc]Tc-G3-(G3S)3C-HYNIC and [99mTc]Tc-(HE)3-G3 in Nu/j mice bearing SKOV-3 xenografts was similar. The specificity of tumor targeting in vivo was demonstrated for both tracers. [99mTc]Tc-G3-(G3S)3C-HYNIC provided comparable, although slightly lower tumor-to-lung, tumor-to spleen and tumor-to-liver ratios than [99mTc]Tc-(HE)3-G3. Radiolabeling of DARPin G3-HYNIC conjugates with 99mTc provided the advantage of a single-step radiolabeling procedure; however, the studied HYNIC conjugates did not improve imaging contrast compared to the 99mTc-tricarbonyl-labeled DARPin G3. At this stage, [99mTc]Tc-(HE)3-G3 remains the most promising candidate for the clinical imaging of HER2-overexpressing cancers.

Influence of Molecular Design on the Tumor Targeting and Biodistribution of PSMA-Binding Tracers Labeled with Technetium-99m.

Previously, we designed the EuK-based PSMA ligand BQ0413 with an maE3 chelator for labeling with technetium-99m. It showed efficient tumor targeting, but our preclinical data and preliminary clinical results indicated that the renal excretion levels need to be decreased. We hypothesized that this could be achieved by a decrease in the ligand's total negative charge, achieved by substituting negatively charged glutamate residues in the chelator with glycine. The purpose of this study was to evaluate the tumor targeting and biodistribution of two new PSMA inhibitors, BQ0411 and BQ0412, compared to BQ0413. Conjugates were radiolabeled with Tc-99m and characterized in vitro, using PC3-pip cells, and in vivo, using NMRI and PC3-pip tumor-bearing mice. [99mTc]Tc-BQ0411 and [99mTc]Tc-BQ0412 demonstrated PSMA-specific binding to PC3-pip cells with picomolar affinity. The biodistribution pattern for the new conjugates was characterized by rapid excretion. The tumor uptake for [99mTc]Tc-BQ0411 was 1.6-fold higher compared to [99mTc]Tc-BQ0412 and [99mTc]Tc-BQ0413. [99mTc]Tc-BQ0413 has demonstrated predominantly renal excretion, while the new conjugates underwent both renal and hepatobiliary excretion. In this study, we have demonstrated that in such small targeting ligands as PSMA-binding EuK-based pseudopeptides, the structural blocks that do not participate in binding could have a crucial role in tumor targeting and biodistribution. The presence of a glycine-based coupling linker in BQ0411 and BQ0413 seems to optimize biodistribution. In conclusion, the substitution of amino acids in the chelating sequence is a promising method to alter the biodistribution of [99mTc]Tc-labeled small-molecule PSMA inhibitors. Further improvement of the biodistribution properties of BQ0413 is needed.

Preclinical Evaluation of a Novel High-Affinity Radioligand [99mTc]Tc-BQ0413 Targeting Prostate-Specific Membrane Antigen (PSMA).

Radionuclide imaging using radiolabeled inhibitors of prostate-specific membrane antigen (PSMA) can be used for the staging of prostate cancer. Previously, we optimized the Glu-urea-Lys binding moiety using a linker structure containing 2-napththyl-L-alanine and L-tyrosine. We have now designed a molecule that contains mercaptoacetyl-triglutamate chelator for labeling with Tc-99m (designated as BQ0413). The purpose of this study was to evaluate the imaging properties of [99mTc]Tc-BQ0413. PSMA-transfected PC3-pip cells were used to evaluate the specificity and affinity of [99mTc]Tc-BQ0413 binding in vitro. PC3-pip tumor-bearing BALB/C nu/nu mice were used as an in vivo model. [99mTc]Tc-BQ0413 bound specifically to PC3-pip cells with an affinity of 33 ± 15 pM. In tumor-bearing mice, the tumor uptake of [99mTc]Tc-BQ0413 (38 ± 6 %IA/g in PC3-pip 3 h after the injection of 40 pmol) was dependent on PSMA expression (3 ± 2 %IA/g and 0.9 ± 0.3 %IA/g in PSMA-negative PC-3 and SKOV-3 tumors, respectively). We show that both unlabeled BQ0413 and the commonly used binder PSMA-11 enable the blocking of [99mTc]Tc-BQ0413 uptake in normal PSMA-expressing tissues without blocking the uptake in tumors. This resulted in an appreciable increase in tumor-to-organ ratios. At the same injected mass (5 nmol), the use of BQ0413 was more efficient in suppressing renal uptake than the use of PSMA-11. In conclusion, [99mTc]Tc-BQ0413 is a promising probe for the visualization of PSMA-positive lesions using single-photon emission computed tomography (SPECT).

Evaluation of affinity matured Affibody molecules for imaging of the immune checkpoint protein B7-H3.

B7-H3 (CD276), an immune checkpoint protein, is a promising molecular target for immune therapy of malignant tumours. Sufficient B7-H3 expression level is a precondition for successful therapy. Radionuclide molecular imaging is a powerful technique for visualization of expression levels of molecular targets in vivo. Use of small radiolabelled targeting proteins would enable high-contrast radionuclide imaging of molecular targets if adequate binding affinity and specificity of an imaging probe could be provided. Affibody molecules, small engineered affinity proteins based on a non-immunoglobulin scaffold, have demonstrated an appreciable potential in radionuclide imaging. Proof-of principle of radionuclide visualization of expression levels of B7-H3 in vivo was demonstrated using the [99mTc]Tc-AC12-GGGC Affibody molecule. We performed an affinity maturation of AC12, enabling selection of clones with higher affinity. Three most promising clones were expressed with a -GGGC (triglycine-cysteine) chelating sequence at the C-terminus and labelled with technetium-99m (99mTc). 99mTc-labelled conjugates bound to B7-H3-expressing cells specifically in vitro and in vivo. Biodistribution in mice bearing B7-H3-expressing SKOV-3 xenografts demonstrated improved imaging properties of the new conjugates compared with the parental variant [99mTc]Tc-AC12-GGGC. [99mTc]Tc-SYNT-179 provided the strongest improvement of tumour-to-organ ratios. Thus, affinity maturation of B7-H3 Affibody molecules could improve biodistribution and targeting properties for imaging of B7-H3-expressing tumours.

Comparative Preclinical Evaluation of Peptide-Based Chelators for the Labeling of DARPin G3 with 99mTc for Radionuclide Imaging of HER2 Expression in Cancer.

Non-invasive radionuclide imaging of human epidermal growth factor receptor type 2 (HER2) expression in breast, gastroesophageal, and ovarian cancers may stratify patients for treatment using HER2-targeted therapeutics. Designed ankyrin repeat proteins (DARPins) are a promising type of targeting probe for radionuclide imaging. In clinical studies, the DARPin [99mTc]Tc-(HE)3-G3 labeled using a peptide-based chelator His-Glu-His-Glu-His-Glu ((HE)3), provided clear imaging of HER2 expressing breast cancer 2-4 h after injection. The goal of this study was to evaluate if the use of cysteine-containing peptide-based chelators Glu-Glu-Glu-Cys (E3C), Gly-Gly-Gly-Cys (G3C), and Gly-Gly-Gly-Ser-Cys connected via a (Gly-Gly-Gly-Ser)3-linker (designated as G3-(G3S)3C) would further improve the contrast of imaging using 99mTc-labeled derivatives of G3. The labeling of the new variants of G3 provided a radiochemical yield of over 95%. Labeled G3 variants bound specifically to human HER2-expressing cancer cell lines with affinities in the range of 1.9-5 nM. Biodistribution of [99mTc]Tc-G3-G3C, [99mTc]Tc-G3-(G3S)3C, and [99mTc]Tc-G3-E3C in mice was compared with the biodistribution of [99mTc]Tc-(HE)3-G3. It was found that the novel variants provide specific accumulation in HER2-expressing human xenografts and enable discrimination between tumors with high and low HER2 expression. However, [99mTc]Tc-(HE)3-G3 provided better contrast between tumors and the most frequent metastatic sites of HER2-expressing cancers and is therefore more suitable for clinical applications.

Evaluation of an Affibody-Based Binder for Imaging of Immune Check-Point Molecule B7-H3.

Radionuclide molecular imaging could provide an accurate assessment of the expression of molecular targets in disseminated cancers enabling stratification of patients for specific therapies. B7-H3 (CD276) is a transmembrane protein belonging to the B7 superfamily. This protein is overexpressed in different types of human malignancies and such upregulation is generally associated with a poor clinical prognosis. In this study, targeting properties of an Affibody-based probe, AC12, containing a -GGGC amino acid sequence as a chelator (designated as AC12-GGGC) labelled with technetium-99m (99mTc) were evaluated for imaging of B7-H3-expressing tumours. AC12-GGGC was efficiently labelled with 99mTc. [99mTc]Tc-AC12-GGGC bound specifically to B7-H3 expressing cells in vitro with affinities in nanomolar range. In mice bearing B7-H3-expressing xenografts, [99mTc]Tc-AC12-GGGC showed tumour uptake of 2.1 ± 0.5 %ID/g at 2 h after injection. Its clearance from blood, normal organs and tissues was very rapid. This new targeting agent, [99mTc]Tc-AC12-GGGC, provided high tumour-to-blood ratio already at 2 h (8.2 ± 1.9), which increased to 11.0 ± 0.5 at 4 h after injection. Significantly (p < 0.05) higher tumour-to-liver and higher tumour-to-bone ratios at 2 h in comparison with 4 h after injection were observed. Thus, [99mTc]Tc-AC12-GGGC could be a promising candidate for further development.

Radionuclide Molecular Imaging of EpCAM Expression in Triple-Negative Breast Cancer Using the Scaffold Protein DARPin Ec1.

Efficient treatment of disseminated triple-negative breast cancer (TNBC) remains an unmet clinical need. The epithelial cell adhesion molecule (EpCAM) is often overexpressed on the surface of TNBC cells, which makes EpCAM a potential therapeutic target. Radionuclide molecular imaging of EpCAM expression might permit selection of patients for EpCAM-targeting therapies. In this study, we evaluated a scaffold protein, designed ankyrin repeat protein (DARPin) Ec1, for imaging of EpCAM in TNBC. DARPin Ec1 was labeled with a non-residualizing [125I]I-para-iodobenzoate (PIB) label and a residualizing [99mTc]Tc(CO)3 label. Both imaging probes retained high binding specificity and affinity to EpCAM-expressing MDA-MB-468 TNBC cells after labeling. Internalization studies showed that Ec1 was retained on the surface of MDA-MB-468 cells to a high degree up to 24 h. Biodistribution in Balb/c nu/nu mice bearing MDA-MB-468 xenografts demonstrated specific uptake of both [125I]I-PIB-Ec1 and [99mTc]Tc(CO)3-Ec1 in TNBC tumors. [125I]I-PIB-Ec1 had appreciably lower uptake in normal organs compared with [99mTc]Tc(CO)3-Ec1, which resulted in significantly (p < 0.05) higher tumor-to-organ ratios. The biodistribution data were confirmed by micro-Single-Photon Emission Computed Tomography/Computed Tomography (microSPECT/CT) imaging. In conclusion, an indirectly radioiodinated Ec1 is the preferable probe for imaging of EpCAM in TNBC.