188Re-ZHER2:V2, a Promising Affibody-Based Targeting Agent Against HER2-Expressing Tumors: Preclinical Assessment

First-in-human validation of enzymolysis clearance strategy for decreasing renal radioactivity using modified [68Ga]Ga-HER2 Affibody

PURPOSE

Enzymolysis clearance strategy, characterized by releasing the non-reabsorbable radioactive fragment under the specific cleavage of enzymes, is confirmed to be a safe and effective way to reduce the renal radioactivity accumulation in mice. However, the effectiveness of this strategy in humans remains unknown. Human epidermal growth factor receptor 2 (HER2) is overexpressed in various types of tumors, and radiolabeled HER2 Affibody is believed to be an attractive tool for HER2-targeted theranostics. However, its wide application is limited by the high and persistent renal uptake. In this study, we intend to validate the effectiveness of enzymolysis clearance strategy in reducing renal accumulation by using a modified HER2 Affibody.

MATERIALS AND METHODS

A new HER2 Affibody ligand, NOTA-MVK-ZHER2:2891, containing a cleavable Met-Val-Lys (MVK) linker was synthesized and labeled with 68Ga. The microPET imaging study was performed in SKOV-3 tumor mice to assess the uptakes of the control ligand and the MVK one in tumors and kidneys. Seven healthy volunteers were included for biodistribution and dosimetric studies with both the control and MVK ligands performed 1 week apart. Urine and blood samples from healthy volunteers were collected for in vivo metabolism study of the two ligands. Four HER2-positive and two HER2-negative patients were recruited for [68Ga]Ga-NOTA-MVK-ZHER2:2891 PET/CT imaging at 2 and 4 h post-injection (p.i.).

RESULTS

[68Ga]Ga-NOTA-MVK-ZHER2:2891 was stable both in PBS and in mouse serum. MicroPET images showed that the tumor uptake of [68Ga]Ga-NOTA-MVK-ZHER2:2891 was comparable to that of [68Ga]Ga-NOTA-ZHER2:2891 at all the time points, while the kidney uptake was significantly reduced 40 min p.i. (P < 0.05). The biodistribution study in healthy volunteers showed that the kidney uptake of MVK ligand was significantly lower than that of the control ligand at 1 h p.i. (P < 0.05), with the SUVmean of 34.3 and 45.8, respectively, while the uptakes of the two ligands in the other organs showed negligible difference. The effective doses of the MVK ligand and the control one were 26.1 and 28.7 µSv/MBq, respectively. The enzymolysis fragment of [68Ga]Ga-NOTA-Met-OH was observed in the urine samples of healthy volunteers injected with the MVK ligand, indicating that the enzymolysis clearance strategy worked in humans. The PET/CT study of patients showed that the range of SUVmax of HER2-positive lesions was 9.4-21, while that of HER2-negative lesions was 2.7-6.2, which suggested that the MVK modification did not affect the ability of ZHER2:2891 structure to bind with HER2.

CONCLUSION

We for the first time demonstrated that enzymolysis clearance strategy can effectively reduce renal radioactivity accumulation in humans. This strategy is expected to decrease renal radiation dose of peptide and small protein-based radiotracers, especially in the field of radionuclide therapy.

Advances in the Application of Radionuclide-Labeled HER2 Affibody for the Diagnosis and Treatment of Ovarian Cancer

Human epidermal growth factor receptor 2 (HER2) is a highly expressed tumor marker in epithelial ovarian cancer, and its overexpression is considered to be a potential factor of poor prognosis. Therefore, monitoring the expression of HER2 receptor in tumor tissue provides favorable conditions for accurate localization, diagnosis, targeted therapy, and prognosis evaluation of cancer foci. Affibody has the advantages of high affinity, small molecular weight, and stable biochemical properties. The molecular probes of radionuclide-labeled HER2 affibody have recently shown broad application prospects in the diagnosis and treatment of ovarian cancer; the aim is to introduce radionuclides into the cancer foci, display systemic lesions, and kill tumor cells through the radioactivity of the radionuclides. This process seamlessly integrates the diagnosis and treatment of ovarian cancer. Current research and development of new molecular probes of radionuclide-labeled HER2 affibody should focus on overcoming the deficiencies of non-specific uptake in the kidney, bone marrow, liver, and gastrointestinal tract, and on reducing the background of the image to improve image quality. By modifying the amino acid sequence; changing the hydrophilicity, surface charge, and lipid solubility of the affibody molecule; and using different radionuclides, chelating agents, and labeling conditions to optimize the labeling method of molecular probes, the specific uptake of molecular probes at tumor sites will be improved, while reducing radioactive retention in non-target organs and obtaining the best target/non-target value. These measures will enable the clinical use of radionuclide-labeled HER2 affibody molecular probes as soon as possible, providing a new clinical path for tumor-specific diagnosis, targeted therapy, and efficacy evaluation. The purpose of this review is to describe the application of radionuclide-labeled HER2 affibody in the imaging and treatment of ovarian cancer, including its potential clinical value and dilemmas.

Experimental Therapy of HER2-Expressing Xenografts Using the Second-Generation HER2-Targeting Affibody Molecule 188Re-ZHER2:41071

HER2-targeted radionuclide therapy might be helpful for the treatment of breast, gastric, and ovarian cancers which have developed resistance to antibody and antibody-drug conjugate-based therapies despite preserved high HER2-expression. Affibody molecules are small targeting proteins based on a non-immunoglobulin scaffold. The goal of this study was to test in an animal model a hypothesis that the second-generation HER2-targeting Affibody molecule 188Re-ZHER2:41071 might be useful for treatment of HER2-expressing malignant tumors. ZHER2:41071 was efficiently labeled with a beta-emitting radionuclide rhenium-188 (188Re). 188Re-ZHER2:41071 demonstrated preserved specificity and high affinity (KD = 5 ± 3 pM) of binding to HER2-expressing cells. In vivo studies demonstrated rapid washout of 188Re from kidneys. The uptake in HER2-expressing SKOV-3 xenografts was HER2-specific and significantly exceeded the renal uptake 4 h after injection and later. The median survival of mice, which were treated by three injections of 16 MBq 188Re-ZHER2:41071 was 68 days, which was significantly longer (<0.0001 in the log-rank Mantel-Cox test) than survival of mice in the control groups treated with vehicle (29 days) or unlabeled ZHER2:41071 (27.5 days). In conclusion, the experimental radionuclide therapy using 188Re-ZHER2:41071 enabled enhancement of survival of mice with human tumors without toxicity to the kidneys, which is the critical organ.

Cytotoxicity evaluation of curcumin-loaded affibody-decorated liposomes against breast cancerous cell lines

Curcumin is known as an effective anticancer herbal medicine but unfortunately, its bioavailability is poor which necessitate efforts for developing more efficient and specific delivery systems. Human epidermal growth factor receptor 2 (HER 2) due to its overexpression in various types of cancers, is demonstrated to be a good candidate as a target for anticancer therapy. In this study, cytotoxicity of curcumin encapsulated in ZHER2:342 Affibody-decorated liposome was investigated against SKBR3 and MCF-7 cancerous cell lines. Curcumin-containing liposomes were prepared from soybeans lecetin and cholesterol by thin-film hydration method. Affibody ZHER2:342 molecules via C-terminal cysteine residue were conjugated covalently to the prepared liposomes. Particle size analysis was performed using atomic force microscopy (AFM) and dynamic light scattering (DLS). Curcumin loading was measured using UV-Vis spectrophotometry and cytotoxic activity of curcumin formulations against cancerous cell lines was investigated by MTT assay. Induction of apoptosis was investigated using flow cytometry through Annexin V staining. Particle analysis showed the formation of spherical liposomes with a mean diameter of about 150 nm. Cytotoxic activity of curcumin was improved by its encapsulation in both liposomes and affibody-decorated liposomes. The Annexin V staining indicated the induction of apoptosis by affibody-decorated liposomes in both MCF-7 and SKBR3 cells. Decoration of curcumin-loaded liposomes with affibody ZHER2:342 may improve curcumin apoptotic function independently of HER2 expression level.

Influence of Several Compounds and Drugs on the Renal Uptake of Radiolabeled Affibody Molecules

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 [^99mTc]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 [^99mTc]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.

Preclinical Imaging in Targeted Cancer Therapies

Preclinical imaging with radiolabeled probes can provide noninvasive tools to test the efficacy of targeted agents in tumors harboring specific genetic alterations and to identify imaging parameters that can be used as pharmacodynamics markers in cancer patients. The present review will primarily focus on preclinical imaging studies that can accelerate the clinical approval of targeted agents and promote the development of imaging biomarkers for clinical applications. Since only subgroups of patients may benefit from treatment with targeted anticancer agents, the identification of a patient population expressing the target is of primary importance for the success of clinical trials. Preclinical imaging studies tested the ability of new radiolabeled compounds to recognize mutant, amplified, or overexpressed targets and some of these tracers were transferred to the clinical setting. More common tracers such as ¹⁸F-Fluorothymidine and ¹⁸F-Fluorodeoxyglucose were employed in animal models to test the inhibition of the target and downstream pathways through the evaluation of early changes of proliferation and glucose metabolism allowing the identification of sensitive and resistant tumors. Furthermore, since the majority of patients treated with targeted anticancer agents will invariably develop resistance, preclinical imaging studies were performed to test the efficacy of reversal agents to overcome resistance. These studies provided consistent evidence that imaging with radiolabeled probes can monitor the reversal of drug resistance by newly designed alternative compounds. Finally, despite many difficulties and challenges, preclinical imaging studies targeting the expression of immune checkpoints proved the principle that it is feasible to select patients for immunotherapy based on imaging findings. In conclusion, preclinical imaging can be considered as an integral part of the complex translational process that moves a newly developed targeted agent from laboratory to clinical application intervening in all clinically relevant steps including patient selection, early monitoring of drug effects and reversal of drug resistance.

In vivo imaging characterization and anticancer efficacy of a novel HER2 affibody and pemetrexed conjugate in lung cancer model

INTRODUCTION

In this study, a new agent consisting of HER2-specific affibody Z_HER2:V2 and chemotherapy drug pemetrexed was synthesized to develop a new targeted drug. Its biological characteristics and anticancer efficacy were assessed in cells level and xenografts models by radiolabeling with technetium-99m.

METHODS

After the Z_HER2:V2-pemetrexed conjugate was synthesized, radiolabeling of the conjugate was performed using its C-terminal 4 amino acids (Gly-Gly-Gly-Cys) as the chelating moiety. The radiochemical yield of the [^99mTc]Tc-Z_HER2:V2-pemetrexed was identified by instant thin-layer chromatography (ITLC). Stability of the radiolabeled conjugate was investigated both in vitro and in vivo. In vitro binding affinity and cell internalization study of the probe were performed in A549 cells (HER2-positive). Tumor uptake was evaluated by in vitro uptake assay in A549 cells and H23 cells (HER2-negative), and by in vivo biodistribution and SPECT imaging in A549 and H23 tumor-bearing mice. The antitumor efficacy of the Z_HER2:V2-pemetrexed conjugate was evaluated in cells and xenograft models.

RESULTS

The Z_HER2:V2-pemetrexed was successfully synthesized and conjugated with technetium-99 m, and acquired the radiochemical yield of 97.0 ± 0.3%. The stability of [^99mTc]Tc-Z_HER2:V2-pemetrexed was good in both physiological saline and human serum. The radiolabeled agent displayed excellent HER2-binding specificity and affinity in vitro, and was gradually internalized into the cells. Biodistribution study revealed obvious tumor uptake in A549 xenografts (percentage injected dose per gram, 2.6 ± 1.0%ID/g at 4 h postinjection), while the uptake in HER2-negative H23 tumors was much lower (0.2 ± 0.1%ID/g at 4 h postinjection, P < 0.01). SPECT imaging exhibited an intensity in the A549 xenograft which could be blocked by excess Z_HER2:V2-pemetrexed. Treatment with Z_HER2:V2-pemetrexed significantly impaired the tumor growth (P < 0.05), with less weight loss than pemetrexed.

CONCLUSION

[^99mTc]Tc-Z_HER2:V2-pemetrexed showed desirable property and HER2-specificity. The Z_HER2:V2-pemetrexed conjugate could inhibit tumor growth of HER2-positive lung adenocarcinoma and may have the potential to become a targeted drug for lung cancer.

ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE

The compound described herein performs HER2-targeting with favorable anticancer efficacy and offers the potential of novel targeting strategies for further tumor therapy.

Influence of composition of cysteine-containing peptide-based chelators on biodistribution of 99mTc-labeled anti-EGFR affibody molecules

Epidermal growth factor receptor (EGFR) is overexpressed in a number of cancers and is the molecular target for several anti-cancer therapeutics. Radionuclide molecular imaging of EGFR expression should enable personalization of anti-cancer treatment. Affibody molecule is a promising type of high-affinity imaging probes based on a non-immunoglobulin scaffold. A series of derivatives of the anti-EGFR affibody molecule ZEGFR:2377, having peptide-based cysteine-containing chelators for conjugation of ^99mTc, was designed and evaluated. It was found that glutamate-containing chelators Gly-Gly-Glu-Cys (GGEC), Gly-Glu-Glu-Cys (GEEC) and Glu-Glu-Glu-Cys (EEEC) provide the best labeling stability. The glutamate containing conjugates bound to EGFR-expressing cells specifically and with high affinity. Specific targeting of EGFR-expressing xenografts in mice was demonstrated. The number of glutamate residues in the chelator had strong influence on biodistribution of radiolabeled affibody molecules. Increase of glutamate content was associated with lower uptake in normal tissues. The ^99mTc-labeled variant containing the EEEC chelator provided the highest tumor-to-organ ratios. In conclusion, optimizing the composition of peptide-based chelators enhances contrast of imaging of EGFR-expression using affibody molecules.

Same-Day Imaging Using Small Proteins: Clinical Experience and Translational Prospects in Oncology

Imaging of expression of therapeutic targets may enable stratification of patients for targeted treatments. The use of small radiolabeled probes based on the heavy-chain variable region of heavy-chain-only immunoglobulins or nonimmunoglobulin scaffolds permits rapid localization of radiotracers in tumors and rapid clearance from normal tissues. This makes high-contrast imaging possible on the day of injection. This mini review focuses on small proteins for radionuclide-based imaging that would allow same-day imaging, with the emphasis on clinical applications and promising preclinical developments within the field of oncology.

A Novel Affibody-Auristatin E Conjugate With a Potent and Selective Activity Against HER2+ Cell Lines

Targeted therapy is a new type of cancer treatment that most often uses biologically active drugs attached to a monoclonal antibody. This so called antibody-drug conjugate strategy allows the use of highly toxic substances that target tumor cells specifically, leaving healthy tissues largely unaffected. Over the last few years, antibody-drug conjugates have become a powerful tool in cancer treatment. We developed and characterized a novel cytotoxic conjugate against HER2 tumors in which the antibody has been substituted with a much smaller molecule: the affibody. The conjugate is composed of the ZHER2:2891 affibody that recognizes HER2 and a highly potent cytotoxic drug auristatin E. The ZHER2:2891 molecule does not contain cysteine(s) in its amino acid sequence. We generated 3 variants of ZHER2:2891, each containing a single cysteine to allow conjugation through the maleimide group that is present in the cytotoxic component. In 2 variants, we introduced single S46C and D53C substitutions. In the third variant, a short Drug Conjugation Sequence (DCS) containing a single cysteine was introduced at the C-terminus of ZHER2:2891, resulting in ZHER2:2891-DCS. The latter variant exhibited a significantly higher conjugation yield, and therefore its cytotoxicity has been studied more thoroughly. The ZHER2:2891-DCS-MMAE conjugate killed the HER2-overexpressing SK-BR-3 and MDA-MB-453 cells efficiently (IC50 values of 5.2 and 24.8 nM, respectively). The T-47-D and MDA-MB-231 cells that express normal levels of HER2 were significantly less sensitive to the conjugate (IC50 values of 135.6 and 161.5 nM, respectively). Overall, we have demonstrated for the first time that proteins other than antibodies/antibody fragments can be successfully combined with a linker-drug module, resulting in conjugates that eliminate cancer cells selectively.

Targeting HER2 in Nuclear Medicine for Imaging and Therapy

Since its discovery, the human epidermal growth factor 2 (HER2) has been extensively studied. Presently, there are 2 standard diagnostic techniques to assess HER2 status in biopsies: immunohistochemistry and fluorescence in situ hybridization. While these techniques have played an important role in the treatment of patients with HER2-positive cancer, they both require invasive biopsies for analysis. Moreover, the expression of HER2 is heterogeneous in breast cancer and can change over the course of the disease. Thus, the degree of HER2 expression in the small sample size of biopsied tumors at the time of analysis may not represent the overall status of HER2 expression in the whole tumor and in between tumor foci in the metastatic setting as the disease progresses. Unlike biopsy, molecular imaging using probes against HER2 allows for a noninvasive, whole-body assessment of HER2 status in real time. This technique could potentially select patients who may benefit from HER2-directed therapy and offer alternative treatments to those who may not benefit. Several antibodies and small molecules against HER2 have been labeled with different radioisotopes for nuclear imaging and/or therapy. This review presents the most recent advances in HER2 targeting in nuclear medicine focusing on preclinical and clinical studies.

Comparative evaluation of tumor targeting using the anti-HER2 ADAPT scaffold protein labeled at the C-terminus with indium-111 or technetium-99m

ABD-Derived Affinity Proteins (ADAPTs) is a novel class of engineered scaffold proteins derived from an albumin-binding domain of protein G. The use of ADAPT6 derivatives as targeting moiety have provided excellent preclinical radionuclide imaging of human epidermal growth factor 2 (HER2) tumor xenografts. Previous studies have demonstrated that selection of nuclide and chelator for its conjugation has an appreciable effect on imaging properties of scaffold proteins. In this study we performed a comparative evaluation of the anti-HER2 ADAPT having an aspartate-glutamate-alanine-valine-aspartate-alanine-asparagine-serine (DEAVDANS) N-terminal sequence and labeled at C-terminus with ^99mTc using a cysteine-containing peptide based chelator, glycine-serine-serine-cysteine (GSSC), and a similar variant labeled with ¹¹¹In using a maleimido derivative of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) chelator. Both ^99mTc-DEAVDANS-ADAPT6-GSSC and ¹¹¹In-DEAVDANS-ADAPT6-GSSC-DOTA accumulated specifically in HER2-expressing SKOV3 xenografts. The tumor uptake of both variants did not differ significantly and average values were in the range of 19–21%ID/g. However, there was an appreciable variation in uptake of conjugates in normal tissues that resulted in a notable difference in the tumor-to-organ ratios. The ¹¹¹In-DOTA label provided 2–6 fold higher tumor-to-organ ratios than ^99mTc-GSSC and is therefore the preferable label for ADAPTs.

Radioimmunotherapy for delivery of cytotoxic radioisotopes: current status and challenges

INTRODUCTION

Radioimmunotherapy (RIT) with monoclonal antibodies and their fragments labelled with radionuclides emitting α -particles, β-particles or Auger electrons have been used for many years in the development of anticancer strategies. While RIT has resulted in approved radiopharmaceuticals for the treatment of hematological malignancies, its use in solid tumors still remains challenging.

AREAS COVERED

In this review, we discuss the exciting progress towards elucidating the potential of current and novel radioimmunoconjugates and address the challenges for translation into clinical practice.

EXPERT OPINION

There are still technical and logistical challenges associated with the use of RIT in routine clinical practice, including development of novel and more specific targeting moieties, broader access α to α-emitters and better tailoring of pre-targeting approaches. Moreover, improved understanding of the heterogeneous nature of solid tumors and the critical role of tumor microenvironments will help to optimize clinical response to RIT by delivering sufficient radiation doses to even more radioresistant tumor cells.

Radionuclide Tumor Targeting Using ADAPT Scaffold Proteins: Aspects of Label Positioning and Residualizing Properties of the Label

Visualization of cancer-associated alterations of molecular phenotype using radionuclide imaging is a noninvasive approach to stratifying patients for targeted therapies. The engineered albumin-binding domain-derived affinity protein (ADAPT) is a promising tracer for radionuclide molecular imaging because of its small size (6.5 kDa), which satisfies the precondition for efficient tumor penetration and rapid clearance. Previous studies demonstrated that the human epidermal growth factor receptor type 2 (HER2)-targeting ADAPT6 labeled with radiometals at the N terminus is able to image HER2 expression in xenografts a few hours after injection. The aim of this study was to evaluate whether the use of a nonresidualizing label or placement of the labels at the C terminus would further improve the targeting properties of ADAPT6. Methods: Two constructs, Cys²-ADAPT6 and Cys⁵⁹-ADAPT6, having the (HE)₃DANS sequence at the N terminus were produced and site-specifically labeled using ¹¹¹In-DOTA or ¹²⁵I-iodo-((4-hydroxyphenyl)ethyl) maleimide (HPEM). The conjugates were compared in vitro and in vivo. HER2-targeting properties and biodistribution were evaluated in BALB/C nu/nu mice bearing ovarian carcinoma cell (SKOV-3) xenografts. Results: Specific HER2 binding and high affinity were preserved after labeling. Both Cys²-ADAPT6 and Cys⁵⁹-ADAPT6 were internalized slowly by HER2-expressing cancer cells. Depending on the label position, uptake at 4 h after injection varied from 10% to 22% of the injected dose per gram of tumor tissue. Regardless of terminus position, the ¹²⁵I-HPEM label provided more than 140-fold lower renal uptake than the ¹¹¹In-DOTA label at 4 after injection. The tumor-to-organ ratios were, in contrast, higher for both of the ¹¹¹In-DOTA-labeled ADAPT variants in other organs. Tumor-to-blood ratios for ¹¹¹In-labeled Cys²-ADAPT6 and Cys⁵⁹-ADAPT6 did not differ significantly (250-280), but ¹¹¹In-DOTA-Cys⁵⁹-ADAPT6 provided significantly higher tumor-to-lung, tumor-to-liver, tumor-to-spleen, and tumor-to-muscle ratios. Radioiodinated variants had similar tumor-to-organ ratios, but ¹²⁵I-HPEM-Cys⁵⁹-ADAPT6 had significantly higher tumor uptake and a higher tumor-to-kidney ratio. Conclusion: Residualizing properties of the label strongly influence the targeting properties of ADAPT6. The position of the radiolabel influences targeting as well, although to a lesser extent. Placement of a label at the C terminus yields the best biodistribution features for both radiometal and radiohalogen labels. Low renal retention of the radioiodine label creates a precondition for radionuclide therapy using ¹³¹I-labeled HPEM-Cys⁵⁹-ADAPT6.

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, Z_HER2:342-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 Z_HER2:342-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 Z_HER2:342-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 Z_HER2:342-SR-HP1. Without pre-injection of Z_HER2:342-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 Z_HER2:342-SR-HP1. In conclusion, ¹⁷⁷Lu-HP2 was shown to be a promising secondary agent for affibody-mediated tumor pretargeting in vivo.

Radioimmunotherapy for Prostate Cancer--Current Status and Future Possibilities

Prostate cancer (PCa) is one of the most common cancers in men and is the second leading cause of cancer-related deaths in the USA. In the United States, it is the second most frequently diagnosed cancer after skin cancer, and in Europe it is number one. According to the American Cancer Society, approximately 221,000 men in the United States would be diagnosed with PCa during 2015, and approximately 28,000 would die of the disease. According to the International Agency for Research on Cancer, approximately 345,000 men were diagnosed with PCa in Europe during 2012, and despite more emphasis placed on early detection through routine screening, 72,000 men died of the disease. Hence, the need for improved therapy modalities is of utmost importance. And targeted therapies based on radiolabeled specific antibodies or peptides are a very interesting and promising alternative to increase the therapeutic efficacy and overall chance of survival of these patients. There are currently several preclinical and some clinical studies that have been conducted, or are ongoing, to investigate the therapeutic efficacy and toxicity of radioimmunotherapy (RIT) against PCa. One thing that is lacking in a lot of these published studies is the dosimetry data, which are needed to compare results between the studies and the study locations. Given the complicated tumor microenvironment and overall complexity of RIT to PCa, old and new targets and targeting strategies like combination RIT and pretargeting RIT are being improved and assessed along with various therapeutic radionuclides candidates. Given alone or in combination with other therapies, these new and improved strategies and RIT tools further enhance the clinical response to RIT drugs in PCa, making RIT for PCa an increasingly practical clinical tool.

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 Z_HER2:342-SR-HP1 containing a 15-mer HP1 PNA recognition tag and a complementary HP2 hybridization probe permitting labeling with both ¹²⁵I and ¹¹¹In. ¹¹¹In-Z_HER2:342-SR-HP1 bound specifically to HER2-expressing BT474 and SKOV-3 cancer cells in vitro, with a K_D of 6±2 pM for binding to SKOV-3 cells. Specific high affinity binding of the radiolabeled complementary PNA probe ¹¹¹In-/¹²⁵I-HP2 to Z_HER2:342-SR-HP1 pre-treated cells was demonstrated. ¹¹¹In-Z_HER2:342-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 ¹¹¹In-Z_HER2:342-SR-HP1. The complementary PNA probe ¹¹¹In/¹²⁵I-HP2 accumulated in SKOV-3 xenografts when Z_HER2:342-SR-HP1 was injected 4 h earlier. The tumor accumulation of ¹¹¹In/¹²⁵I-HP2 was negligible without Z_HER2:342-SR-HP1 pre-injection. The uptake of ¹¹¹In-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.

Feasibility of Affibody-Based Bioorthogonal Chemistry-Mediated Radionuclide Pretargeting

Affibody molecules constitute a new class of probes for radionuclide tumor targeting. The small size of Affibody molecules is favorable for rapid localization in tumors and clearance from circulation. However, high renal reabsorption of Affibody molecules prevents the use of residualizing radiometals, including several promising low-energy β- and α-emitters, for radionuclide therapy. We tested a hypothesis that Affibody-based pretargeting mediated by a bioorthogonal interaction between trans-cyclooctene (TCO) and tetrazine would provide higher accumulation of radiometals in tumor xenografts than in the kidneys.

METHODS

TCO was conjugated to the anti-human epidermal growth factor receptor 2 (HER2) Affibody molecule Z2395. DOTA-tetrazine was labeled with (111)In and (177)Lu. In vitro pretargeting was studied in HER2-expressing SKOV-3 and BT474 cell lines. In vivo studies were performed on BALB/C nu/nu mice bearing SKOV-3 xenografts.

RESULTS

(125)I-Z2395-TCO bound specifically to HER2-expressing cells in vitro with an affinity of 45 ± 16 pM. (111)In-tetrazine bound specifically and selectively to Z2395-TCO pretreated cells. In vivo studies demonstrated HER2-specific (125)I-Z2395-TCO accumulation in xenografts. TCO-mediated (111)In-tetrazine localization was shown in tumors, when the radiolabeled tracer was injected 4 h after an injection of Z2395-TCO. At 1 h after injection, the tumor uptake of (111)In-tetrazine and (177)Lu-tetrazine was approximately 2-fold higher than the renal uptake. Pretargeting provided more than a 56-fold reduction of renal uptake of (111)In in comparison with direct targeting.

CONCLUSION

The feasibility of Affibody-based bioorthogonal chemistry-mediated pretargeting was demonstrated. The use of pretargeting provides a substantial reduction of radiometal accumulation in kidneys, creating preconditions for palliative radionuclide therapy.

Site-Specific Radioiodination of HER2-Targeting Affibody Molecules using 4-Iodophenethylmaleimide Decreases Renal Uptake of Radioactivity

Affibody molecules are small scaffold-based affinity proteins with promising properties as probes for radionuclide-based molecular imaging. However, a high reabsorption of radiolabeled Affibody molecules in kidneys is an issue. We have shown that the use of ¹²⁵I-3-iodo-((4-hydroxyphenyl)ethyl)maleimide (IHPEM) for site-specific labeling of cysteine-containing Affibody molecules provides high tumor uptake but low radioactivity retention in kidneys. We hypothesized that the use of 4-iodophenethylmaleimide (IPEM) would further reduce renal retention of radioactivity because of higher lipophilicity of radiometabolites. An anti-human epidermal growth factor receptor type 2 (HER2) Affibody molecule (Z_HER2:2395) was labeled using ¹²⁵I-IPEM with an overall yield of 45±3 %. ¹²⁵I-IPEM-Z_HER2:2395 bound specifically to HER2-expressing human ovarian carcinoma cells (SKOV-3 cell line). In NMRI mice, the renal uptake of ¹²⁵I-IPEM-Z_HER2:2395 (24±2 and 5.7±0.3 % IA g⁻¹at 1 and 4 h after injection, respectively) was significantly lower than uptake of ¹²⁵I-IHPEM-Z_HER2:2395 (50±8 and 12±2 % IA g⁻¹at 1 and 4 h after injection, respectively). In conclusion, the use of a more lipophilic linker for the radioiodination of Affibody molecules reduces renal radioactivity.