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The feasibility of targeted imaging and therapy using radiolabeled albumin-binding domain-derived affinity proteins (ADAPTs) has been demonstrated. However, high renal uptake of radioactivity limits the maximum tolerated dose. Successful reduction of renal retention of radiolabeled Fab fragments has been demonstrated by incorporating a cleavable linker between the targeting agent and the radiometal chelator. The present study investigated if the introduction of a glycine-leucine-glycine-lysine (GLGK)-linker would reduce the kidney uptake of radiolabeled ADAPT6 and also compared it with the non-residualizing [125I]I-[(4-hydroxyphenyl)ethyl]maleimide ([125I]I-HPEM) labeling strategy. GLGK was site-specifically coupled to human epidermal growth factor receptor 2 (HER2)-targeting ADAPT6. Conjugates without the cleavable linker were used as controls and all constructs were labeled with lutetium-177 (177Lu). [125I]I-HPEM was coupled to ADAPT6 at the C-terminus. Biodistribution of all constructs was evaluated in NMRI mice 4 h after injection. Specific binding to HER2-expressing cells in vitro was demonstrated for all constructs. No significant difference in kidney uptake was observed between the [177Lu]Lu-2,2 ',2",2"'-(1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrayl)tetraacetic acid-GLGK-conjugates and the controls. The renal activity of [125I]I-HPEM-ADAPT6 was significantly lower compared with all other constructs. In conclusion, the incorporation of the cleavable GLGK-linker did not result in lower renal retention. Therefore, the present study emphasized that, in order to achieve a reduction of renal retention, alternative molecular design strategies may be required for different targeting agents., QC 20240201

Designed ankyrin repeat protein (DARPin) G3 is an engineered scaffold protein. This small (14.5 kDa) targeting protein binds with high affinity to human epidermal growth factor receptor 2 (HER2). HER2 is overexpressed in several cancers. The use of the DARPin G3 for radionuclide therapy is complicated by its high renal reabsorption after clearance via the glomeruli. We tested the hypothesis that a fusion of the DARPin G3 with an albumin-binding domain (ABD) would prevent rapid renal excretion and high renal reabsorption resulting in better tumour targeting. Two fusion proteins were produced, one with the ABD at the C-terminus (G3-ABD) and another at the N-terminus (ABD-G3). Both variants were labelled with Lu-177. The binding properties of the novel constructs were evaluated in vitro and their biodistribution was compared in mice with implanted human HER2-expressing tumours. Fusion with the ABD increased the retention time of both constructs in blood compared with the non-ABD-fused control. The effect of fusion with the ABD depended strongly on the order of the domains in the constructs, resulting in appreciably better targeting properties of [Lu-177]Lu-G3-ABD. Our data suggest that the order of domains is critical for the design of targeting constructs based on scaffold proteins., The two first authors contributed equally.Corresponding author: Vladimir Tolmachev

QC 20231211

QC 20231211

Overexpression of the human epidermal growth factor receptor 2 (HER2) in breast and gastric cancer is exploited for targeted therapy using monoclonal antibodies and antibody-drug conjugates. Small engineered scaffold proteins, such as the albumin binding domain (ABD) derived affinity proteins (ADAPTs), are a promising new format of targeting probes for development of drug conjugates with well-defined structure and tunable pharmacokinetics. Radiolabeled ADAPT6 has shown excellent tumor-targeting properties in clinical trials. Recently, we developed a drug conjugate based on the HER2-targeting ADAPT6 fused to an albumin binding domain (ABD) for increased bioavailability and conjugated to DM1 for cytotoxic action, designated as ADAPT6-ABD-mcDM1. In this study, we investigated the therapeutic efficacy of this conjugate in mice bearing HER2-expressing SKOV3 ovarian cancer xenografts. A secondary aim was to evaluate several formats of imaging probes for visualization of HER2 expression in tumors. Administration of ADAPT6-ABD-mcDM1 provided a significant delay of tumor growth and increased the median survival of the mice, in comparison with both a non-targeting homologous construct (ADAPT(Neg)-ABD-mcDM1) and the vehicle-treated groups, without inducing toxicity to liver or kidneys. Moreover, the evaluation of imaging probes showed that small scaffold proteins, such as Tc-99m(CO)(3)-ADAPT6 or the affibody molecule Tc-99m-Z(HER2:41071), are well suited as diagnostic companions for potential stratification of patients for ADAPT6-ABD-mcDM1-based therapy.

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 [Tc-99m]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 (G(3)C), and Gly-Gly-Gly-Ser-Cys connected via a (Gly-Gly-Gly-Ser)(3)-linker (designated as G3-(G(3)S)(3)C) would further improve the contrast of imaging using Tc-99m-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 [Tc-99m]Tc-G3-G(3)C, [Tc-99m]Tc-G3-(G(3)S)(3)C, and [Tc-99m]Tc-G3-E3C in mice was compared with the biodistribution of [Tc-99m]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, [Tc-99m]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.

QC 20221101

Epidermal growth factor receptor (EGFR) is overexpressed in many malignancies. EGFR-targeted therapy extends survival of patients with disseminated cancers. Radionuclide molecular imaging of EGFR expression would make EGFR-directed treatment more personalized and therefore more efficient. A previous study demonstrated that affibody molecule [68Ga]Ga-DFO-ZEGFR:2377 permits specific positron-emission tomography (PET) imaging of EGFR expression in xenografts at 3 h after injection. We anticipated that imaging at 24 h after injection would provide higher contrast, but this is prevented by the short half-life of 68Ga (67.6 min). Here, we therefore tested the hypothesis that the use of the non-conventional long-lived positron emitter 66Ga (T1/2 = 9.49 h, β+ = 56.5%) would permit imaging with higher contrast. 66Ga was produced by the 66Zn(p,n)66Ga nuclear reaction and DFO-ZEGFR:2377 was efficiently labelled with 66Ga with preserved binding specificity in vitro and in vivo. At 24 h after injection, [66Ga]Ga-DFO-ZEGFR:2377 provided 3.9-fold higher tumor-to-blood ratio and 2.3-fold higher tumor-to-liver ratio than [68Ga]Ga-DFO-ZEGFR:2377 at 3 h after injection. At the same time point, [66Ga]Ga-DFO-ZEGFR:2377 provided 1.8-fold higher tumor-to-blood ratio, 3-fold higher tumor-to-liver ratio, 1.9-fold higher tumor-to-muscle ratio and 2.3-fold higher tumor-to-bone ratio than [89Zr]Zr-DFO-ZEGFR:2377. Biodistribution data were confirmed by whole body PET combined with magnetic resonance imaging (PET/MRI). The use of the positron emitter 66Ga for labelling of DFO-ZEGFR:2377 permits PET imaging of EGFR expression at 24 h after injection and improves imaging contrast.

Radionuclide molecular imaging of human epidermal growth factor (HER2) expression may be helpful to stratify breast and gastroesophageal cancer patients for HER2-targeting therapies. ADAPTs (albumin-binding domain derived affinity proteins) are a new type of small (46-59 amino acids) proteins useful as probes for molecular imaging. The aim of this first-in-human study was to evaluate biodistribution, dosimetry, and safety of the HER2-specific 99mTc-ADAPT6. METHODS: Twenty-nine patients with primary breast cancerwere included. In 22 patients with HER2-positive (n = 11) or HER2-negative (n = 11) histopathology an intravenous injection with 385±125 MBq 99mTc-ADAPT6 was performed, randomized to an injected protein mass of either 500 µg (n = 11) or 1000 µg (n = 11). Planar scintigraphy followed by SPECT imaging was performed after 2, 4, 6 and 24 h. An additional cohort (n = 7) was injected with 165±29 MBq (injected protein mass 250 µg) and imaging was performed after 2 h only. RESULTS: Injections of 99mTc-ADAPT6 at all injected mass levels were well tolerated and not associated with adverse effects. 99mTc-ADAPT6 cleared rapidly from blood and most other tissues. The normal organs with the highest accumulation were kidney, liver and lung. Effective doses were 0.009±0.002 and 0.010±0.003 mSv/MBq for injected protein masses of 500 and 1000 µg, respectively. Injection of 500 µg resulted in excellent discrimination between HER2-positive and HER2-negative tumors already 2 h after injection (tumor-to-contralateral breast ratio was 37±19 vs 5±2, p<0.01). The tumor-to-contralateral breast ratios for HER2-positive tumors were significantly (p<0.05) higher for injected mass of 500 µg than for both 250 and 1000 µg. CONCLUSION: Injections of 99mTc-ADAPT6 are safe and associated with low absorbed and effective doses. Protein dose of 500 µg is preferable for discrimination between tumors with high and low expression of HER2. Further studies are justified to evaluate if 99mTc-ADAP

Simple Summary Metastasis-targeting therapy might improve outcomes in oligometastatic prostate cancer. Epithelial cell adhesion molecule (EpCAM) is overexpressed in 40-60% of prostate cancer cases and might be used as a target for specific delivery of toxins and drugs. Radionuclide molecular imaging could enable non-invasive detection of EpCAM and stratification of patients for targeted therapy. Designed ankyrin repeat proteins (DARPins) are scaffold proteins, which can be selected for specific binding to different targets. The DARPin Ec1 binds strongly to EpCAM. To determine an optimal design of Ec1-based probes, we labeled Ec1 at two different positions with four different nuclides (Ga-68, In-111, Co-57 and I-125) and investigated the impact on Ec1 biodistribution. We found that the C-terminus is the best position for labeling and that In-111 and I-125 provide the best imaging contrast. This study might be helpful for scientists developing imaging probes based on scaffold proteins. The epithelial cell adhesion molecule (EpCAM) is intensively overexpressed in 40-60% of prostate cancer (PCa) cases and can be used as a target for the delivery of drugs and toxins. The designed ankyrin repeat protein (DARPin) Ec1 has a high affinity to EpCAM (68 pM) and a small size (18 kDa). Radiolabeled Ec1 might be used as a companion diagnostic for the selection of PCa patients for therapy. The study aimed to investigate the influence of radiolabel position (N- or C-terminal) and composition on the targeting and imaging properties of Ec1. Two variants, having an N- or C-terminal cysteine, were produced, site-specifically conjugated to a DOTA chelator and labeled with cobalt-57, gallium-68 or indium-111. Site-specific radioiodination was performed using ((4-hydroxyphenyl)-ethyl)maleimide (HPEM). Biodistribution of eight radiolabeled Ec1-probes was measured in nude mice bearing PCa DU145 xenografts. In all cases, positioning of a label at the C-terminus provided the best tumor-to-org

Albumin binding domain-Derived Affinity ProTeins (ADAPTs) are small (5 kDa) engineered scaffold proteins that are promising targeting agents for radionuclide-based imaging. A recent clinical study has demonstrated that radiolabeled ADAPTs can efficiently visualize human epidermal growth factor receptor 2 (HER2) expression in breast cancer using SPECT imaging. However, the use of ADAPTs directly labeled with radiometals for targeted radionuclide therapy is limited by their high reabsorption and prolonged retention of activity in kidneys. In this study, we investigated whether a co-injection of lysine or gelofusin, commonly used for reduction of renal uptake of radiolabeled peptides in clinics, would reduce the renal uptake of [Tc-99m]Tc(CO)(3)-ADAPT6 in NMRI mice. In order to better understand the mechanism behind the reabsorption of [Tc-99m]Tc(CO)(3)-ADAPT6, we included several compounds that act on various parts of the reabsorption system in kidneys. Administration of gelofusine, lysine, probenecid, furosemide, mannitol, or colchicine did not change the uptake of [Tc-99m]Tc(CO)(3)-ADAPT6 in kidneys. Sodium maleate reduced the uptake of [Tc-99m]Tc(CO)(3)-ADAPT6 to ca. 25% of the uptake in the control, a high dose of fructose (50 mmol/kg) reduced the uptake by ca. two-fold. However, a lower dose (20 mmol/kg) had no effect. These results indicate that common clinical strategies are not effective for reduction of kidney uptake of [Tc-99m]Tc(CO)(3)-ADAPT6 and that other strategies for reduction of activity uptake or retention in kidneys should be investigated for ADAPT6.

Affibody molecules are the most studied class of engineered scaffold proteins (ESPs) in radionuclide molecular imaging. Attempts to use affibody molecules directly labelled with radiometals for targeted radionuclide therapy were hampered by the high uptake and retention of radioactivity in kidneys. Several promising strategies have been implemented to circumvent this problem. Here, we investigated whether a pharmacological approach targeting different components of the reabsorption system could be used to lower the uptake of [Tc-99m]Tc-Z(HER:2395) affibody molecule in kidneys. Pre-injection of probenecid, furosemide, mannitol or colchicine had no influence on activity uptake in kidneys compared to the control group. Mice pre-injected with maleate and fructose had 33% and 51% reduction in the kidney-associated activity, respectively, compared to the control group. Autoradiography images showed that the accumulation of activity after [Tc-99m]Tc-Z(HER2:2395) injection was in the renal cortex and that both maleate and fructose could significantly reduce it. Results from this study demonstrate that pharmacological intervention with maleate and fructose was effective in reducing the kidney uptake of affibody molecules. A presumable mechanism is the disruption of ATP-mediated cellular uptake and endocytosis processes of affibody molecules by tubular cells.

Epithelial cell adhesion molecule (EpCAM) is overexpressed in 55%-75% of ovarian carcinomas (OC). EpCAM might be used as a target for a treatment of disseminated OC. Designed ankyrin repeats protein (DARPin) Ec1 is a small (18 kDa) protein, which binds to EpCAM with subnanomolar affinity. We tested a hypothesis that Ec1 labeled with a non-residualizing label might serve as a companion imaging diagnostic for stratification of patients for EpCAM-targeting therapy. Ec1 was labeled with I-125 using N-succinimidyl-para-iodobenzoate. Binding affinity, specificity, and cellular processing of [I-125]I-PIB-Ec1 were evaluated using SKOV-3 and OVCAR-3 ovarian carcinoma cell lines. Biodistribution and tumor-targeting properties of [I-125]I-PIB-Ec1 were studied in Balb/c nu/nu mice bearing SKOV-3 and OVCAR-3 xenografts. EpCAM-negative Ramos lymphoma xenografts served as specificity control. Binding of [I-125]I-PIB-Ec1 to ovarian carcinoma cell lines was highly specific and had affinity in picomolar range. Slow internalization of [I-125]I-PIB-Ec1 by OC cells confirmed utility of non-residualizing label for in vivo imaging. [I-125]I-PIB-Ec1 provided 6 h after injection tumor-to-blood ratios of 30 +/- 11 and 48 +/- 12 for OVCAR-3 and SKOV-3 xenografts, respectively, and high contrast to other organs. Tumor targeting was highly specific. Saturation of tumor uptake at a high dose of Ec1 in SKOV-3 model provided a rationale for dose selection in further studies using therapeutic conjugates of Ec1 for targeted therapy. In conclusion, [I-125]I-PIB-Ec1 is a promising agent for visualizing EpCAM expression in OC.

Fusion toxins consisting of an affinity protein fused to toxic polypeptides derived from Pseudomonas exotoxin A (ETA) are promising agents for targeted cancer therapy. In this study, we examined whether fusion toxins consisting of an albumin binding domain-derived affinity protein (ADAPT) interacting with human epidermal growth factor receptor 2 (HER2), coupled to the ETA-derived polypeptides PE38X8 or PE25, with or without an albumin binding domain (ABD) for half-life extension, can be used for specific killing of HER2-expressing cells. The fusion toxins could easily be expressed in a soluble form in Escherichia coli and purified to homogeneity. All constructs had strong affinity for HER2 (K-D 10 to 26 nM) and no tendency for aggregation could be detected. The fusion toxins including the ABD showed strong interaction with human and mouse serum albumin [equilibrium dissociation constant (K-D) 1 to 3 nM and 2 to 10 nM, respectively]. The in vitro investigation of the cytotoxic potential revealed IC50-values in the picomolar range for cells expressing high levels of HER2. The specificity was also demonstrated, by showing that free HER2 receptors on the target cells are required for fusion toxin activity. In mice, the fusion toxins containing the ABD exhibited an appreciably longer time in circulation. The uptake was highest in liver and kidney. Fusion with PE25 was associated with the highest hepatic uptake. Collectively, the results suggest that fusion toxins consisting of ADAPTs and ETA-derivatives are promising agents for targeted cancer therapy.

Hypoxia-induced carbonic anhydrase IX (CAIX) expression is a prognostic marker in solid tumors. In recent years many radiotracers have been developed, but a fair comparison of these compounds is not possible because of the diversity in tumor models and other experimental parameters. In this study we performed a direct in vivo comparison of three promising CAIX targeting radiotracers in xenografted head and neck cancer models. The biodistribution of [In-111]In-DOTA-ZCAIX:2 was directly compared with [In-111]In-DTPA-G250-F(ab')(2) and [In-111] In-DTPA-G250 in female BALB/C nu/nu mice bearing two HNSCC xenografts with different levels of CAIX expression. In vivo biodistribution was quantified by means of microSPECT/CT scans and ex vivo biodistribution was determined with the use of gamma-counter. Tumors were snap frozen and sections were stained for CAIX expression, vessels, hypoxia (pimonidazole) and tumor blood perfusion. Tracer uptake was significantly higher in SSCNij153 tumors compared to SCCNij185 tumors for [In-111]In-DOTA-HE3-ZCAIX:2: 0.32 +/- 0.03 versus 0.18 +/- 0.01%ID/g,(p = 0.003) 4 h p.i., for [In-111]In-DTPA-girentuximab-F(ab')(2): 3.0 +/- 0.5%ID/g and 1.2 +/- 0.1%ID/g (p = 0.03), 24 h p.i. and for [In-111]In-DTPA-girentuximab: 30 +/- 2.1%ID/g and 7.0 +/- 1.0%ID/g (p = 0.0002) 72 h p.i. SPECT imaging with both [In-111]In-DTPA-girentuximab-F(ab') 2 and [111In]In-DTPA-girentuximab showed a clear difference in tracer distribution between the two tumor models. The whole IgG, i.e. [In-111]In-DTPA-girentuximab, showed the highest tumor-to-muscle ratio. We showed that different CAIX-targeting radiotracers can discriminate a low CAIX-expressing tumor from a high CAIX-expressing head and neck cancer xenografts model. In these hypoxic head and neck xenograft models [In-111]In-DTPA-girentuximab showed the most promising results.