Shorter Peptide Nucleic Acid Probes Improve Affibody-Mediated Peptide Nucleic Acid-Based Pretargeting.

Affibody-mediated PNA-based pretargeting shows promise for HER2-expressing tumor radiotherapy. In our recent study, a 15-mer ZHER2:342-HP15 affibody-PNA conjugate, in combination with a shorter 9-mer [177Lu]Lu-HP16 effector probe, emerged as the most effective pretargeting strategy. It offered a superior tumor-to-kidney uptake ratio and more efficient tumor targeting compared to longer radiolabeled effector probes containing 12 or 15 complementary PNA bases. To enhance the production efficiency of our pretargeting system, we here introduce even shorter 6-, 7-, and 8-mer secondary probes, designated as HP19, HP21, and HP20, respectively. We also explore the replacement of the original 15-mer Z-HP15 primary probe with shorter 12-mer Z-HP12 and 9-mer Z-HP9 alternatives. This extended panel of shorter PNA-based probes was synthesized using automated microwave-assisted methods and biophysically screened in vitro to identify shorter probe combinations with the most effective binding properties. In a mouse xenograft model, we evaluated the biodistribution of these probes, comparing them to the Z-HP15:[177Lu]Lu-HP16 combination. Tumor-to-kidney ratios at 4 and 144 h postinjection of the secondary probe showed no significant differences among the Z-HP9:[177Lu]Lu-HP16, Z-HP9:[177Lu]Lu-HP20, and the Z-HP15:[177Lu]Lu-HP16 pairs. Importantly, tumor uptake significantly exceeded, by several hundred-fold, that of most normal tissues, with kidney uptake being the critical organ for radiation therapy. This suggests that using a shorter 9-mer primary probe, Z-HP9, in combination with 9-mer HP16 or 8-mer HP20 secondary probes effectively targets tumors while minimizing the dose-limiting kidney uptake of radionuclide. In conclusion, the Z-HP9:HP16 and Z-HP9:HP20 probe combinations offer good prospects for both cost-effective production and efficient in vivo pretargeting of HER2-expressing tumors.

Biocatalysis in Drug Design: Engineered Reductive Aminases (RedAms) Are Used to Access Chiral Building Blocks with Multiple Stereocenters.

Novel building blocks are in constant demand during the search for innovative bioactive small molecule therapeutics by enabling the construction of structure-activity-property-toxicology relationships. Complex chiral molecules containing multiple stereocenters are an important component in compound library expansion but can be difficult to access by traditional organic synthesis. Herein, we report a biocatalytic process to access a specific diastereomer of a chiral amine building block used in drug discovery. A reductive aminase (RedAm) was engineered following a structure-guided mutagenesis strategy to produce the desired isomer. The engineered RedAm (IR-09 W204R) was able to generate the (S,S,S)-isomer 3 in 45% conversion and 95% ee from the racemic ketone 2. Subsequent palladium-catalyzed deallylation of 3 yielded the target primary amine 4 in a 73% yield. This engineered biocatalyst was used at preparative scale and represents a potential starting point for further engineering and process development.

Biotransformation of the Novel Myeloperoxidase Inhibitor AZD4831 in Preclinical Species and Humans.

We report herein an in-depth analysis of the metabolism of the novel myeloperoxidase inhibitor AZD4831 ((R)-1-(2-(1-aminoethyl)-4-chlorobenzyl)-2-thioxo-2,3-dihydro-1H-pyrrolo[3,2-d]pyrimidin-4(5H)-one) in animals and human. Quantitative and qualitative metabolite profiling were performed on samples collected from mass balance studies in rats and humans. Exposure of circulating human metabolites with comparable levels in animal species used in safety assessment were also included. Structural characterization of 20 metabolites was performed by liquid chromatography high-resolution mass spectrometry, and quantification was performed by either 14C analysis using solid phase scintillation counting or accelerator mass spectrometry and, where available, authentication with synthesized metabolite standards. A complete mass balance study in rats is presented, while data from dogs and human are limited to metabolite profiling and characterization. The metabolism of AZD4831 is mainly comprised of reactions at the primary amine nitrogen and the thiourea sulfur, resulting in several conjugated metabolites with or without desulfurization. A carbamoyl glucuronide metabolite of AZD4831 (M7) was the most abundant plasma metabolite in both human healthy volunteers and heart failure patients after single and repeated dose administration of AZD4831, accounting for 75%-80% of the total drug-related exposure. Exposures to M7 and other human circulating metabolites were covered in rats and/or dogs, the two models most frequently used in the toxicology studies, and were also highly abundant in the mouse, the second model other than rat used in carcinogenicity studies. The carbamoyl glucuronide M7 was the main metabolite in rat bile, while a desulfurized and cyclized metabolite (M5) was abundant in rat plasma and excreta. SIGNIFICANCE STATEMENT: The biotransformation of AZD4831, a novel myeloperoxidase inhibitor inhibiting xanthine derivative bearing thiourea and primary aliphatic amine functions, is described. Twenty characterized metabolites demonstrate the involvement of carbamoylation with glucuronidation, desulfurization, and cyclization as main biotransformation reactions. The carbamoyl glucuronide was the main metabolite in human plasma, likely governed by a significant species difference in plasma protein binding for this metabolite, but this and other human plasma metabolites were covered in animals used in the toxicity studies.

Discovery of AZD4831, a Mechanism-Based Irreversible Inhibitor of Myeloperoxidase, As a Potential Treatment for Heart Failure with Preserved Ejection Fraction.

Myeloperoxidase is a promising therapeutic target for treatment of patients suffering from heart failure with preserved ejection fraction (HFpEF). We aimed to discover a covalent myeloperoxidase inhibitor with high selectivity for myeloperoxidase over thyroid peroxidase, limited penetration of the blood-brain barrier, and pharmacokinetics suitable for once-daily oral administration at low dose. Structure-activity relationship, biophysical, and structural studies led to prioritization of four compounds for in-depth safety and pharmacokinetic studies in animal models. One compound (AZD4831) progressed to clinical studies on grounds of high potency (IC50, 1.5 nM in vitro) and selectivity (>450-fold vs thyroid peroxidase in vitro), the mechanism of irreversible inhibition, and the safety profile. Following phase 1 studies in healthy volunteers and a phase 2a study in patients with HFpEF, a phase 2b/3 efficacy study of AZD4831 in patients with HFpEF started in 2021.

Affibody-Mediated PNA-Based Pretargeted Cotreatment Improves Survival of Trastuzumab-Treated Mice Bearing HER2-Expressing Xenografts.

Treatment of patients with human epidermal growth factor receptor 2 (HER2)-expressing tumors using the monoclonal antibody trastuzumab increases survival. The Affibody-based peptide nucleic acid (PNA)-mediated pretargeted radionuclide therapy has demonstrated efficacy against HER2-expressing xenografts in mice. Structural studies suggest that Affibody molecules and trastuzumab bind to different epitopes on HER2. The aim of this study was to test the hypothesis that a combination of PNA-mediated pretargeted radionuclide therapy and trastuzumab treatment of HER2-expressing xenografts can extend survival compared with monotherapies. Methods: Mutual interference of the primary pretargeting probe ZHER2:342-SR-HP1 and trastuzumab in binding to HER2-expressing cell lines was investigated in vitro. Experimental therapy evaluated the survival of mice bearing HER2-expressing SKOV-3 xenografts after treatment with vehicle, trastuzumab only, pretargeting using Affibody-PNA chimera ZHER2:342-SR-HP1 and complementary probe 177Lu-HP2, and combination of trastuzumab and pretargeting. The ethical permit limited the study to 90 d. The animals' weights were monitored during the study. After study termination, samples of liver and kidneys were evaluated by a veterinary pathologist for toxicity signs. Results: The presence of a large molar excess of trastuzumab had no influence on the affinity of ZHER2:342-SR-HP1 binding to HER2-expressing cells in vitro. The affinity of trastuzumab was not affected by a large excess of ZHER2:342-SR-HP1 The median survival of mice treated with trastuzumab (75.5 d) was significantly longer than the survival of mice treated with a vehicle (59.5 d). Median survival of mice treated with pretargeting was not reached by day 90. Six mice of 10 in this group survived, and 2 had complete remission. All mice in the combination treatment group survived, and tumors in 7 mice had disappeared at study termination. There was no significant difference between animal weights in the different treatment groups. No significant pathologic alterations were detected in livers and kidneys of treated animals. Conclusion: Treatment of mice bearing HER2-expressing xenografts with the combination of trastuzumab and Affibody-mediated PNA-based radionuclide pretargeting significantly increased survival compared with monotherapies. Cotreatment was not toxic for normal tissues.

Stability Enhancement of a Dimeric HER2-Specific Affibody Molecule through Sortase A-Catalyzed Head-to-Tail Cyclization.

Natural backbone-cyclized proteins have an increased thermostability and resistance towards proteases, characteristics that have sparked interest in head-to-tail cyclization as a method to stability-enhance proteins used in diagnostics and therapeutic applications, for example. In this proof-of principle study, we have produced and investigated a head-to-tail cyclized and HER2-specific ZHER2:342 Affibody dimer. The sortase A-mediated cyclization reaction is highly efficient (>95%) under optimized conditions, and renders a cyclic ZHER3:342-dimer with an apparent melting temperature, Tm, of 68 °C, which is 3 °C higher than that of its linear counterpart. Circular dichroism spectra of the linear and cyclic dimers looked very similar in the far-UV range, both before and after thermal unfolding to 90 °C, which suggests that cyclization does not negatively impact the helicity or folding of the cyclic protein. The cyclic dimer had an apparent sub-nanomolar affinity (Kd ~750 pM) to the HER2-receptor, which is a ~150-fold reduction in affinity relative to the linear dimer (Kd ~5 pM), but the anti-HER2 Affibody dimer remained a high-affinity binder even after cyclization. No apparent difference in proteolytic stability was detected in an endopeptidase degradation assay for the cyclic and linear dimers. In contrast, in an exopeptidase degradation assay, the linear dimer was shown to be completely degraded after 5 min, while the cyclic dimer showed no detectable degradation even after 60 min. We further demonstrate that a site-specifically DyLight 594-labeled cyclic dimer shows specific binding to HER2-overexpressing cells. Taken together, the results presented here demonstrate that head-to-tail cyclization can be an effective strategy to increase the stability of an Affibody dimer.

Comparative Evaluation of Novel 177Lu-Labeled PNA Probes for Affibody-Mediated PNA-Based Pretargeting.

Affibody-mediated PNA-based pretargeting is a promising approach to radionuclide therapy of HER2-expressing tumors. In this study, we test the hypothesis that shortening the PNA pretargeting probes would increase the tumor-to-kidney dose ratio. The primary probe ZHER2:342-SR-HP15 and the complementary secondary probes HP16, HP17, and HP18, containing 9, 12, and 15 nucleobases, respectively, and carrying a 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) chelator were designed, synthesized, characterized in vitro, and labeled with 177Lu. In vitro pretargeting was studied in HER2-expressing SKOV3 and BT474 cell lines. The biodistribution of these novel probes was evaluated in immunodeficient mice bearing SKOV3 xenografts and compared to the previously studied [177Lu]Lu-HP2. Characterization confirmed the formation of high-affinity duplexes between HP15 and the secondary probes, with the affinity correlating with the length of the complementary PNA sequences. All the PNA-based probes were bound specifically to HER2-expressing cells in vitro. In vivo studies demonstrated HER2-specific uptake of all 177Lu-labeled probes in xenografts in a pretargeting setting. The ratio of cumulated radioactivity in the tumor to the radioactivity in kidneys was dependent on the secondary probe's size and decreased with an increased number of nucleobases. The shortest PNA probe, [177Lu]Lu-HP16, showed the highest tumor-to-kidney ratio. [177Lu]Lu-HP16 is the most promising secondary probe for affibody-mediated tumor pretargeting.

Evaluation of an antibody-PNA conjugate as a clearing agent for antibody-based PNA-mediated radionuclide pretargeting.

Radionuclide molecular imaging of cancer-specific targets is a promising method to identify patients for targeted antibody therapy. Radiolabeled full-length antibodies however suffer from slow clearance, resulting in high background radiation. To overcome this problem, a pretargeting system based on complementary peptide nucleic acid (PNA) probes has been investigated. The pretargeting relies on sequential injections of primary, PNA-tagged antibody and secondary, radiolabeled PNA probe, which are separated in time, to allow for clearance of non-bound primary agent. We now suggest to include a clearing agent (CA), designed for removal of primary tumor-targeting agent from the blood. The CA is based on the antibody cetuximab, which was conjugated to PNA and lactosaminated by reductive amination to improve hepatic clearance. The CA was evaluated in combination with PNA-labelled trastuzumab, T-ZHP1, for radionuclide HER2 pretargeting. Biodistribution studies in normal mice demonstrated that the CA cleared ca. 7 times more rapidly from blood than unmodified cetuximab. Injection of the CA 6 h post injection of the radiolabeled primary agent [131I]I-T-ZHP1 gave a moderate reduction of the radioactivity concentration in the blood after 1 h from 8.5 ± 1.8 to 6.0 ± 0.4%ID/g. These proof-of-principle results could guide future development of a more efficient CA.

Preclinical Evaluation of the GRPR-Targeting Antagonist RM26 Conjugated to the Albumin-Binding Domain for GRPR-Targeting Therapy of Cancer.

The targeting of gastrin-releasing peptide receptors (GRPR) was recently proposed for targeted therapy, e.g., radiotherapy. Multiple and frequent injections of peptide-based therapeutic agents would be required due to rapid blood clearance. By conjugation of the GRPR antagonist RM26 (D-Phe-Gln-Trp-Ala-Val-Gly-His-Sta-Leu-NH2) to an ABD (albumin-binding domain), we aimed to extend the blood circulation of peptides. The synthesized conjugate DOTA-ABD-RM26 was labelled with indium-111 and evaluated in vitro and in vivo. The labelled conjugate was stable in PBS and retained specificity and its antagonistic function against GRPR. The half-maximal inhibitory concentration (IC50) of natIn-DOTA-ABD-RM26 in the presence of human serum albumin was 49 ± 5 nM. [111In]In-DOTA-ABD-RM26 had a significantly longer residence time in blood and in tumors (without a significant decrease of up to 144 h pi) than the parental RM26 peptide. We conclude that the ABD-RM26 conjugate can be used for GRPR-targeted therapy and delivery of cytotoxic drugs. However, the undesirable elevated activity uptake in kidneys abolishes its use for radionuclide therapy. This proof-of-principle study justified further optimization of the molecular design of the ABD-RM26 conjugate.

Preparation of Conjugates for Affibody-Based PNA-Mediated Pretargeting.

Affibody molecules are small engineered scaffold proteins suitable for in vivo tumor targeting. Radionuclide molecular imaging using directly radiolabelled affibody molecules provides excellent imaging. However, affibody molecules have a high renal reabsorption, which complicates their use for radionuclide therapy. The high renal reabsorption is a common problem for the use of engineered scaffold proteins for radionuclide therapy. Affibody-based PNA-mediated pretargeting reduces dramatically the absorbed dose to the kidneys and makes affibody-based radionuclide therapy possible. This methodology might, hopefully, solve the problem of high renal reabsorption for radionuclide therapy mediated by other engineered scaffold proteins.

Fast and Efficient Fc-Specific Photoaffinity Labeling To Produce Antibody-DNA Conjugates.

Antibody-DNA conjugates are powerful tools for DNA-assisted protein analysis. Growing usage of these methods demands efficient production of high-quality conjugates. We developed an easy and fast synthesis route yielding covalent antibody-DNA conjugates with a defined conjugation site and low batch-to-batch variability. We utilize the Z domain from protein A, containing the unnatural amino acid 4-benzoylphenylalanine (BPA) for photoaffinity labeling of the antibodies' Fc region. Z(xBPA) domains are C-terminally modified with triple-glycine (G3)-modified DNA-oligonucleotides via enzymatic Sortase A coupling. We show reliable modification of the most commonly used IgG's. To prove our conjugates' functionality, we detected antibody-antigen binding events in an assay called Droplet Barcode Sequencing for Protein analysis (DBS-Pro). It confirms not only retained functionality for both conjugate parts but also the potential of using DBS-Pro for quantifying protein abundances. As intermediates are easily storable and our approach is modular, it offers a convenient strategy for screening various antibody-DNA conjugates using the same starting material.

Site-specific conjugation of recognition tags to trastuzumab for peptide nucleic acid-mediated radionuclide HER2 pretargeting.

Pretargeting is a promising strategy to reach high imaging contrast in a shorter time than by targeting with directly radiolabeled monoclonal antibodies (mAbs). One of problems in pretargeting is a site-specific, reproducible and uniform conjugation of recognition tags to mAbs. To solve this issue we propose a photoconjugation to covalently couple a recognition tag to a mAb via a photoactivatable Z domain. The Z-domain, a 58-amino acid protein derived from the IgG-binding B-domain of Staphylococcus aureus protein A, has a well-characterized binding site in the Fc portion of IgG. We tested the feasibility of this approach using pretargeting based on hybridization between peptide nucleic acids (PNAs). We have used photoconjugation to couple trastuzumab with the PNA-based hybridization probe, HP1. A complementary [57Co]Co-labeled PNA hybridization probe ([57Co]Co-HP2) was used as the secondary targeting probe. In vitro studies demonstrated that trastuzumab-ZHP1 bound specifically to human epidermal growth factor receptor 2 (HER2)-expressing cells with nanomolar affinity. The binding of the secondary [57Co]Co-HP2 probe to trastuzumab-PNA-pretreated cells was in the picomolar affinity range. A two-fold increase in SKOV-3 tumor targeting was achieved when [57Co]Co-HP2 (0.7 nmol) was injected 48 h after injection of trastuzumab-ZHP1 (0.5 nmol) compared with trastuzumab-ZHP1 alone (0.8 ±â€¯0.2 vs. 0.33 ±â€¯0.06 %ID/g). Tumor accumulation of [57Co]Co-HP2 was significantly reduced by pre-saturation with trastuzumab or when no trastuzumab-ZHP1 was preinjected. A tumor-to-blood uptake ratio of 1.5 ±â€¯0.3 was achieved resulting in a clear visualization of HER2-expressing xenografts as confirmed by SPECT imaging. In conclusion, the feasibility of stable site-specific coupling of a PNA-based recognition tag to trastuzumab and successful pretargeting has been demonstrated. This approach can hopefully be used for a broad range of mAbs and recognition tags.

Development of an optimal imaging strategy for selection of patients for affibody-based PNA-mediated radionuclide therapy.

Affibody molecules are engineered scaffold proteins, which demonstrated excellent binding to selected tumor-associated molecular abnormalities in vivo and highly sensitive and specific radionuclide imaging of Her2-expressing tumors in clinics. Recently, we have shown that peptide nucleic acid (PNA)-mediated affibody-based pretargeted radionuclide therapy using beta-emitting radionuclide 177Lu extended significantly survival of mice bearing human Her2-expressing tumor xenografts. In this study, we evaluated two approaches to use positron emission tomography (PET) for stratification of patients for affibody-based pretargeting therapy. The primary targeting probe ZHER2:342-SR-HP1 and the secondary probe HP2 (both conjugated with DOTA chelator) were labeled with the positron-emitting radionuclide 68Ga. Biodistribution of both probes was measured in BALB/C nu/nu mice bearing either SKOV-3 xenografts with high Her2 expression or DU-145 xenografts with low Her2 expression. 68Ga-HP2 was evaluated in the pretargeting setting. Tumor uptake of both probes was compared with the uptake of pretargeted 177Lu-HP2. The uptake of both 68Ga-ZHER2:342-SR-HP1 and 68Ga-HP2 depended on Her2-expression level providing clear discrimination of between tumors with high and low Her2 expression. Tumor uptake of 68Ga-HP2 correlated better with the uptake of 177Lu-HP2 than the uptake of 68Ga-ZHER2:342-SR-HP1. The use of 68Ga-HP2 as a theranostics counterpart would be preferable approach for clinical translation.

Radionuclide Therapy of HER2-Expressing Human Xenografts Using Affibody-Based Peptide Nucleic Acid-Mediated Pretargeting: In Vivo Proof of Principle.

Affibody molecules are small proteins engineered using a nonantibody scaffold. Radiolabeled Affibody molecules are excellent imaging probes, but their application to radionuclide therapy has been prevented by high renal reabsorption. The aim of this study was to test the hypothesis that Affibody-based peptide nucleic acid (PNA)-mediated pretargeted therapy of human epidermal growth factor receptor 2 (HER2)-expressing cancer extends survival without accompanying renal toxicity. Methods: A HER2-targeting Affibody molecule ligated with an AGTCGTGATGTAGTC PNA hybridization probe (ZHER2:342-SR-HP1) was used as the primary pretargeting agent. A complementary AGTCGTGATGTAGTC PNA conjugated to the chelator DOTA and labeled with the radionuclide 177Lu (177Lu-HP2) was used as the secondary agent. The influence of different factors on pretargeting was investigated. Experimental radionuclide therapy in mice bearing SKOV-3 xenografts was performed in 6 cycles separated by 7 d. Results: Optimal tumor targeting was achieved when 16 MBq/3.5 µg (0.65 nmol) of 177Lu-HP2 was injected 16 h after injection of 100 µg (7.7 nmol) of ZHER2:342-SR-HP1. The calculated absorbed dose to tumors was 1,075 mGy/MBq, whereas the absorbed dose to kidneys was 206 mGy/MBq and the absorbed dose to blood (surrogate of bone marrow) was 4 mGy/MBq. Survival of mice was significantly longer (P < 0.05) in the treatment group (66 d) than in the control groups treated with the same amount of ZHER2:342-SR-HP1 only (37 d), the same amount and activity of 177Lu-HP2 only (32 d), or phosphate-buffered saline (37 d). Conclusion: The studied pretargeting system can deliver an absorbed dose to tumors appreciably exceeding absorbed doses to critical organs, making Affibody-based PNA-mediated pretargeted radionuclide therapy highly attractive.

Evaluation of affibody molecule-based PNA-mediated radionuclide pretargeting: Development of an optimized conjugation protocol and 177Lu labeling.

INTRODUCTION: We have previously developed a pretargeting approach for affibody-mediated cancer therapy based on PNA-PNA hybridization. In this article we have further developed this approach by optimizing the production of the primary agent, ZHER2:342-SR-HP1, and labeling the secondary agent, HP2, with the therapeutic radionuclide 177Lu. We also studied the biodistribution profile of 177Lu-HP2 in mice, and evaluated pretargeting with 177Lu-HP2 in vitro and in vivo. METHODS: The biodistribution profile of 177Lu-HP2 was evaluated in NMRI mice and compared to the previously studied 111In-HP2. Pretargeting using 177Lu-HP2 was studied in vitro using the HER2-expressing cell lines BT-474 and SKOV-3, and in vivo in mice bearing SKOV-3 xenografts. RESULTS AND CONCLUSION: Using an optimized production protocol for ZHER2:342-SR-HP1 the ligation time was reduced from 15h to 30min, and the yield increased from 45% to 70%. 177Lu-labeled HP2 binds specifically in vitro to BT474 and SKOV-3 cells pre-treated with ZHER2:342-SR-HP1. 177Lu-HP2 was shown to have a more rapid blood clearance compared to 111In-HP2 in NMRI mice, and the measured radioactivity in blood was 0.22±0.1 and 0.68±0.07%ID/g for 177Lu- and 111In-HP2, respectively, at 1h p.i. In contrast, no significant difference in kidney uptake was observed (4.47±1.17 and 3.94±0.58%ID/g for 177Lu- and 111In-HP2, respectively, at 1h p.i.). Co-injection with either Gelofusine or lysine significantly reduced the kidney uptake for 177Lu-HP2 (1.0±0.1 and 1.6±0.2, respectively, vs. 2.97±0.87%ID/g in controls at 4h p.i.). 177Lu-HP2 accumulated in SKOV-3 xenografts in BALB/C nu/nu mice when administered after injection of ZHER2:342-SR-HP1. Without pre-injection of ZHER2:342-SR-HP1, the uptake of 177Lu-HP2 was about 90-fold lower in tumor (0.23±0.08 vs. 20.7±3.5%ID/g). The tumor-to-kidney radioactivity accumulation ratio was almost 5-fold higher in the group of mice pre-injected with ZHER2:342-SR-HP1. In conclusion, 177Lu-HP2 was shown to be a promising secondary agent for affibody-mediated tumor pretargeting in vivo.

Evaluation of the first 44Sc-labeled Affibody molecule for imaging of HER2-expressing tumors.

INTRODUCTION: Affibody molecules are small (58 amino acids) high-affinity proteins based on a tri-helix non-immunoglobulin scaffold. A clinical study has demonstrated that PET imaging using Affibody molecules labeled with 68Ga (T½=68min) can visualize metastases of breast cancer expressing human epidermal growth factor receptor type 2 (HER2) and provide discrimination between tumors with high and low expression level. This may help to identify breast cancer patients benefiting from HER2-targeting therapies. The best discrimination was at 4h post injection. Due to longer half-life, a positron-emitting radionuclide 44Sc (T½=4.04h) might be a preferable label for Affibody molecules for imaging at several hours after injection. METHODS: A synthetic second-generation anti-HER2 Affibody molecule ZHER2:2891 was labeled with 44Sc via a DOTA-chelator conjugated to the N-terminal amino group. Binding specificity, affinity and cellular processing 44Sc-DOTA-ZHER2:2891 and 68Ga-DOTA-ZHER2:2891 were compared in vitro using HER2-expressing cells. Biodistribution and imaging properties of 44Sc-DOTA-ZHER2:2891 and 68Ga-DOTA-ZHER2:2891 were evaluated in Balb/c nude mice bearing HER2-expression xenografts. RESULTS: The labeling yield of 98±2% and specific activity of 7.8GBq/µmol were obtained. The conjugate demonstrated specific binding to HER2-expressing SKOV3.ip cells in vitro and to SKOV3.ip xenografts in nude mice. The distribution of radioactivity at 3h post injection was similar for 44Sc-DOTA-ZHER2:2891 and 68Ga-DOTA-ZHER2:2891, but the blood clearance of the 44Sc-labeled variant was slower and the tumor-to-blood ratio was reduced (15±2 for 44Sc-DOTA-ZHER2:2891 vs 46±9 for 68Ga-DOTA-ZHER2:2891). At 6h after injection of 44Sc-DOTA-ZHER2:2891 the tumor uptake was 8±2% IA/g and the tumor-to-blood ratio was 51±8. Imaging using small-animal PET/CT demonstrated that 44Sc-DOTA-ZHER2:2891 provides specific and high-contrast imaging of HER2-expressing xenografts. CONCLUSION: The 44Sc- DOTA-ZHER2:2891 Affibody molecule is a promising probe for imaging of HER2-expression in malignant tumors.

Increasing the Net Negative Charge by Replacement of DOTA Chelator with DOTAGA Improves the Biodistribution of Radiolabeled Second-Generation Synthetic Affibody Molecules.

A promising strategy to enable patient stratification for targeted therapies is to monitor the target expression in a tumor by radionuclide molecular imaging. Affibody molecules (7 kDa) are nonimmunoglobulin scaffold proteins with a 25-fold smaller size than intact antibodies. They have shown an apparent potential as molecular imaging probes both in preclinical and clinical studies. Earlier, we found that hepatic uptake can be reduced by the incorporation of negatively charged purification tags at the N-terminus of Affibody molecules. We hypothesized that liver uptake might similarly be reduced by positioning the chelator at the N-terminus, where the chelator-radionuclide complex will provide negative charges. To test this hypothesis, a second generation synthetic anti-HER2 ZHER2:2891 Affibody molecule was synthesized and labeled with (111)In and (68)Ga using DOTAGA and DOTA chelators. The chelators were manually coupled to the N-terminus of ZHER2:2891 forming an amide bond. Labeling DOTAGA-ZHER2:2891 and DOTA-ZHER2:2891 with (68)Ga and (111)In resulted in stable radioconjugates. The tumor-targeting and biodistribution properties of the (111)In- and (68)Ga-labeled conjugates were compared in SKOV-3 tumor-bearing nude mice at 2 h postinjection. The HER2-specific binding of the radioconjugates was verified both in vitro and in vivo. Using the DOTAGA chelator gave significantly lower radioactivity in liver and blood for both radionuclides. The (111)In-labeled conjugates showed more rapid blood clearance than the (68)Ga-labeled conjugates. The most pronounced influence of the chelators was found when they were labeled with (68)Ga. The DOTAGA chelator gave significantly higher tumor-to-blood (61 ± 6 vs 23 ± 5, p < 0.05) and tumor-to-liver (10.4 ± 0.6 vs 4.5 ± 0.5, p < 0.05) ratios than the DOTA chelator. This study demonstrated that chelators may be used to alter the uptake of Affibody molecules, and most likely other scaffold-based imaging probes, for improvement of imaging contrast.

Feasibility of Affibody Molecule-Based PNA-Mediated Radionuclide Pretargeting of Malignant Tumors.

Affibody molecules are small (7 kDa), non-immunoglobulin scaffold proteins with a potential as targeting agents for radionuclide imaging of cancer. However, high renal re-absorption of Affibody molecules prevents their use for radionuclide therapy with residualizing radiometals. We hypothesized that the use of Affibody-based peptide nucleic acid (PNA)-mediated pretargeting would enable higher accumulation of radiometals in tumors than in kidneys. To test this hypothesis, we designed an Affibody-PNA chimera ZHER2:342-SR-HP1 containing a 15-mer HP1 PNA recognition tag and a complementary HP2 hybridization probe permitting labeling with both (125)I and (111)In. (111)In-ZHER2:342-SR-HP1 bound specifically to HER2-expressing BT474 and SKOV-3 cancer cells in vitro, with a KD of 6±2 pM for binding to SKOV-3 cells. Specific high affinity binding of the radiolabeled complementary PNA probe (111)In-/(125)I-HP2 to ZHER2:342-SR-HP1 pre-treated cells was demonstrated. (111)In-ZHER2:342-SR-HP1 demonstrated specific accumulation in SKOV-3 xenografts in BALB/C nu/nu mice and rapid clearance from blood. Pre-saturation of SKOV-3 with non-labeled anti-HER2 Affibody or the use of HER2-negative Ramos xenografts resulted in significantly lower tumor uptake of (111)In-ZHER2:342-SR-HP1. The complementary PNA probe (111)In/(125)I-HP2 accumulated in SKOV-3 xenografts when ZHER2:342-SR-HP1 was injected 4 h earlier. The tumor accumulation of (111)In/(125)I-HP2 was negligible without ZHER2:342-SR-HP1 pre-injection. The uptake of (111)In-HP2 in SKOV-3 xenografts was 19±2 %ID/g at 1 h after injection. The uptake in blood and kidneys was approximately 50- and 2-fold lower, respectively. In conclusion, we have shown that the use of Affibody-based PNA-mediated pretargeting enables specific delivery of radiometals to tumors and provides higher radiometal concentration in tumors than in kidneys.

Design, Preparation, and Characterization of PNA-Based Hybridization Probes for Affibody-Molecule-Mediated Pretargeting.

In radioimmunotherapy, the contrast between tumor and normal tissue can be improved by using a pretargeting strategy with a primary targeting agent, which is conjugated to a recognition tag, and a secondary radiolabeled molecule binding specifically to the recognition tag. The secondary molecule is injected after the targeting agent has accumulated in the tumor and is designed to have a favorable biodistribution profile, with fast clearance from blood and low uptake in normal tissues. In this study, we have designed and evaluated two complementary peptide nucleic acid (PNA)-based probes for specific and high-affinity association in vivo. An anti-HER2 Affibody-PNA chimera, Z(HER2:342)-SR-HP1, was produced by a semisynthetic approach using sortase A catalyzed ligation of a recombinantly produced Affibody molecule to a PNA-based HP1-probe assembled using solid-phase chemistry. A complementary HP2 probe carrying a DOTA chelator and a tyrosine for dual radiolabeling was prepared by solid-phase synthesis. Circular dichroism (CD) spectroscopy and UV thermal melts showed that the probes can hybridize to form a structured duplex with a very high melting temperature (T(m)), both in HP1:HP2 and in Z(HER2:342)-SR-HP1:HP2 (T(m) = 86-88 °C), and the high binding affinity between Z(HER2:342)-SR-HP1 and HP2 was confirmed in a surface plasmon resonance (SPR)-based binding study. Following a moderately fast association (1.7 × 10(5) M(-1) s(-1)), the dissociation of the probes was extremely slow and <5% dissociation was observed after 17 h. The equilibrium dissociation constant (K(D)) for Z(HER2:342)-SR-HP1:HP2 binding to HER2 was estimated by SPR to be 212 pM, suggesting that the conjugation to PNA does not impair Affibody binding to HER2. The biodistribution profiles of (111)In- and (125)I-labeled HP2 were measured in NMRI mice, showing very fast blood clearance rates and low accumulation of radioactivity in kidneys and other organs. The measured radioactivity in blood was 0.63 ± 0.15 and 0.41 ± 0.15%ID/g for (125)I- and (111)In-HP2, respectively, at 1 h p.i., and at 4 h p.i., the kidney accumulation of radioactivity was 0.17 ± 0.04%ID/g for (125)I-HP2 and 3.83 ± 0.39%ID/g for (111)In-HP2. Taken together, the results suggest that a PNA-based system has suitable biophysical and in vivo properties and is a promising approach for pretargeting of Affibody molecules.