Evaluation of 99mTc-Z IGF1R:4551-GGGC affibody molecule, a new probe for imaging of insulin-like growth factor type 1 receptor expression

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.

Novel small 99mTc-labeled affibody molecular probe for PD-L1 receptor imaging

OBJECTIVE

The in vivo imaging of programmed death ligand 1 (PD-L1) can monitor changes in PD-L1 expression and guide programmed death 1 (PD-1) or PD-L1-targeted immune checkpoint therapy. A 99mTc-labeled affibody molecular probe targeting the PD-L1 receptor was prepared and evaluated its tracing effect in PD-L1-overexpressing colon cancer.

METHODS

The PD-L1 affibody was prepared by genetic recombineering. The 99mTc labeling of the affibody was achieved by sodium glucoheptonate and an SnCl2 labeling system. The labeling rate, radiochemical purity, and stability in vitro were determined by instant thin-layer chromatography; MC38-B7H1 (PD-L1-positive) and MC38 (PD-L1-negative) colon cancer cells were used to evaluate its affinity to PD-L1 by cell-binding experiments. The biodistribution of the 99mTc-labeled affibody molecular probe was then determined in C57BL/6J mice bearing MC38-B7H1 tumors, and tumor targeting was assessed in C57BL/6J mice with MC38-B7H1, MC38 double xenografts.

RESULT

The nondecayed corrected yield of the 99mTc-PD-L1 affibody molecular probe was 95.95% ± 1.26%, and showed good stability both in phosphate-buffered saline (PBS) and fetal bovine serum within 6 h. The affinity of the 99mTc-PD-L1 affibody molecular probe for cell-binding assays was 10.02 nmol/L. Single photon emission-computed tomography imaging showed a rapid uptake of the tracer in PD-L1-positive tumors and very little tracer retention in PD-L1-negative control tumors. The tracer was significantly retained in the kidneys and bladder, suggesting that it is mainly excreted through the urinary system. Heart, liver, lung, and muscle tissue showed no significant radioactive retention. The biodistribution in vitro also showed significant renal retention, a small amount of uptake in the thyroid and gastrointestinal tract, and rapid blood clearance, and the tumor-to-blood radioactivity uptake ratio peaked 120 min after drug injection.

CONCLUSION

The 99mTc-PD-L1 affibody molecular probe that we prepared can effectively target to PD-L1-positive tumors imaging in vivo, and clear in blood quickly, with no obvious toxic side effects, which is expected to become a new type of tracer for detecting PD-L1 expression in tumors.

Preclinical Evaluation of a New Format of 68Ga- and 111In-Labeled Affibody Molecule ZIGF-1R:4551 for the Visualization of IGF-1R Expression in Malignant Tumors Using PET and SPECT

The Insulin-like growth factor-1 receptor (IGF-1R) is a molecular target for several monoclonal antibodies undergoing clinical evaluation as anticancer therapeutics. The non-invasive detection of IGF-1R expression in tumors might enable stratification of patients for specific treatment and improve the outcome of both clinical trials and routine treatment. The affibody molecule Z_IGF-1R:4551 binds specifically to IGF-1R with subnanomolar affinity. The goal of this study was to evaluate the ⁶⁸Ga and ¹¹¹In-labeled affibody construct NODAGA-(HE)₃-Z_IGF-1R:4551 for the imaging of IGF-1R expression, using PET and SPECT. The labeling was efficient and provided stable coupling of both radionuclides. The two imaging probes, [⁶⁸Ga]Ga-NODAGA-(HE)₃-Z_IGF-1R:4551 and [¹¹¹In]In-NODAGA-(HE)₃-Z_IGF-1R:4551, demonstrated specific binding to IGF-1R-expressing human cancer cell lines in vitro and to IGF-1R-expressing xenografts in mice. Preclinical PET and SPECT/CT imaging demonstrated visualization of IGF-1R-expressing xenografts already one hour after injection. The tumor-to-blood ratios at 3 h after injection were 7.8 ± 0.2 and 8.0 ± 0.6 for [⁶⁸Ga]Ga-NODAGA-(HE)₃-Z_IGF-1R:4551 and [¹¹¹In]In-NODAGA-(HE)₃-Z_IGF-1R:4551, respectively. In conclusion, a molecular design of the Z_IGF-1R:4551 affibody molecule, including placement of a (HE)₃-tag on the N-terminus and site-specific coupling of a NODAGA chelator on the C-terminus, provides a tracer with improved imaging properties for visualization of IGF-1R in malignant tumors, using PET and SPECT.

Insights on the emerging biotechnology of histidine-rich peptides

In the late 70's, the discovery of the restriction enzymes made possible the biological production of functional proteins by recombinant DNA technologies, a fact that largely empowered both biotechnological and pharmaceutical industries. Short peptides or small protein domains, with specific molecular affinities, were developed as purification tags in downstream processes to separate the target protein from the culture media or cell debris, upon breaking the producing cells. Among these tags, and by exploiting the interactivity of the imidazole ring of histidine residues, the hexahistidine peptide (H6) became a gold standard. Although initially used almost exclusively in protein production, H6 and related His-rich peptides are progressively proving a broad applicability in novel utilities including enzymatic processes, advanced drug delivery systems and diagnosis, through a so far unsuspected adaptation of their binding capabilities. In this context, the coordination of histidine residues and metals confers intriguing functionalities to His-rich sequences useable in the forward-thinking design of protein-based nano- and micro-materials and devices, through strategies that are comprehensively presented here.

Novel Receptor Tyrosine Kinase Pathway Inhibitors for Targeted Radionuclide Therapy of Glioblastoma

Glioblastoma (GB) remains the most fatal brain tumor characterized by a high infiltration rate and treatment resistance. Overexpression and/or mutation of receptor tyrosine kinases is common in GB, which subsequently leads to the activation of many downstream pathways that have a critical impact on tumor progression and therapy resistance. Therefore, receptor tyrosine kinase inhibitors (RTKIs) have been investigated to improve the dismal prognosis of GB in an effort to evolve into a personalized targeted therapy strategy with a better treatment outcome. Numerous RTKIs have been approved in the clinic and several radiopharmaceuticals are part of (pre)clinical trials as a non-invasive method to identify patients who could benefit from RTKI. The latter opens up the scope for theranostic applications. In this review, the present status of RTKIs for the treatment, nuclear imaging and targeted radionuclide therapy of GB is presented. The focus will be on seven tyrosine kinase receptors, based on their central role in GB: EGFR, VEGFR, MET, PDGFR, FGFR, Eph receptor and IGF1R. Finally, by way of analyzing structural and physiological characteristics of the TKIs with promising clinical trial results, four small molecule RTKIs were selected based on their potential to become new therapeutic GB radiopharmaceuticals.

An affibody-conjugated nanoprobe for IGF-1R targeted cancer fluorescent and photoacoustic dual-modality imaging

Dual-modal molecular imaging that combines photoacoustic imaging with near-infrared fluorescence imaging integrates the benefits of both imaging modalities and may achieve more precise detection of disease. In this study, silver sulfide quantum dots (Ag₂S QDs) with superior photoacoustic properties and a strong fluorescent emission in the NIR region were successfully synthesized. They were further modified with the insulin-like growth factor 1 receptor (IGF-1R) targeted small scaffold protein, Affibody (Z_IGF-1) to achieved targeted photoacoustic/fluorescent dual-modal imaging of cancer. Our results showed that the prepared nanoprobe had good tumor targeting properties in vivo, and the probe also showed good biocompatibility without any significant toxicity.

Alpha-Particle Therapy for Multifocal Osteosarcoma: A Hypothesis

Osteosarcoma (OST) is the most common bone tumor in children and adolescents with a second peak of incidence in elderly adults usually diagnosed as secondary tumors in Paget's disease or irradiated bone. Subjects with metastatic disease or whose disease relapses after the initial therapy have a poor prognosis. Moreover, multifocal OST contains tumor-initiating cells that are resistant to chemotherapy. The use of aggressive therapies in an attempt to eradicate these cells can have long-term negative consequences in these vulnerable patient populations. ²²⁷Th-labeled molecular probes based on ligands to OST-associated receptors such as IGF-1R (insulin-like growth factor receptor 1), HER2 (human epidermal growth factor receptor 2), and PSMA (prostate-specific membrane antigen) are expected to detect and treat osseous and nonosseous sites of multifocal OST. Published reports indicate that ²²⁷Th has limited myelotoxicity, can be stably chelated to its carriers and, as it decays at targeted sites, ²²⁷Th produces ²²³Ra that is subsequently incorporated into the areas of increased osteoblastic activity, that is, osseous metastatic lesions. Linear energy transfer of α particles emitted by ²²⁷Th and its daughter ²²³Ra is within the range of the optimum relative biological effectiveness. The radiotoxicity of α particles is virtually independent of the phase in the cell cycle, oxygenation, and the dose rate. For these reasons, even resistant OST cells remain susceptible to killing by high-energy α particles, which can also kill adjacent quiescent OST cells or cells with low expression of targeted receptors. Systemic side effects are minimized by the limited range of these intense radiations. Quantitative single-photon emission computed tomography of ²²⁷Th and ²²³Ra is feasible. Additionally, the availability of radionuclide pairs, for example, ⁸⁹Zr for positron emission tomography and ²²⁷Th for therapy, establish a strong basis for the theranostic use of ²²⁷Th in the individualized treatment of multifocal OST.

Novel EBV LMP-2-affibody and affitoxin in molecular imaging and targeted therapy of nasopharyngeal carcinoma

Epstein-Barr virus (EBV) infection is closely linked to several human malignancies including endemic Burkitt’s lymphoma, Hodgkin’s lymphoma and nasopharyngeal carcinomas (NPC). Latent membrane protein 2 (LMP-2) of EBV plays a pivotal role in pathogenesis of EBV-related tumors and thus, is a potential target for diagnosis and targeted therapy of EBV LMP-2⁺ malignant cancers. Affibody molecules are developing as imaging probes and tumor-targeted delivery of small molecules. In this study, four EBV LMP-2-binding affibodies (Z_{EBV LMP-2}12, Z_{EBV LMP-2}132, Z_{EBV LMP-2}137, and Z_{EBV LMP-2}142) were identified by screening a phage-displayed LMP-2 peptide library for molecular imaging and targeted therapy in EBV xenograft mice model. Z_{EBV LMP-2} affibody has high binding affinity for EBV LMP-2 and accumulates in mouse tumor derived from EBV LMP-2⁺ xenografts for 24 h after intravenous (IV) injection. Subsequent fusion of Pseudomonas exotoxin PE38KDEL to the Z_{EBV LMP-2} 142 affibody led to production of Z142X affitoxin. This fused Z142X affitoxin exhibits high cytotoxicity specific for EBV⁺ cells in vitro and significant antitumor effect in mice bearing EBV⁺ tumor xenografts by IV injection. The data provide the proof of principle that EBV LMP-2-speicifc affibody molecules are useful for molecular imaging diagnosis and have potentials for targeted therapy of LMP-2-expressing EBV malignancies.

Molecular imaging diagnosis and targeted therapy have been successfully used for several types of tumors, but not yet applied to diagnose or treat EBV-associated NPC. Affibody molecules are small proteins engineered to bind to a large number of target proteins with high affinity, and therefore, can be developed as potential biopharmaceutical drugs for molecular diagnosis and therapeutic applications. In the present study, we screened and characterized EBV LMP-2-specific affibodies and evaluated their usage in molecular imaging of LMP-2 expressing cells and EBV LMP-2 tumor-bearing mice. Subsequently, we engineered and obtained an EBV LMP-2 affitoxin based on EBV LMP-2-binding affibodies and demonstrated its targeted cytotoxicity for EBV⁺ cell lines in vitro and in vivo. Our data indicate that the EBV LMP-2-specific affibody and its derived affitoxin are useful for diagnosis of LMP-2 expressing cells and targeted therapy of EBV-derived, LMP-2⁺ malignancies.

Targeted Delivery to Tumors: Multidirectional Strategies to Improve Treatment Efficiency

Malignant tumors are characterized by structural and molecular peculiarities providing a possibility to directionally deliver antitumor drugs with minimal impact on healthy tissues and reduced side effects. Newly formed blood vessels in malignant lesions exhibit chaotic growth, disordered structure, irregular shape and diameter, protrusions, and blind ends, resulting in immature vasculature; the newly formed lymphatic vessels also have aberrant structure. Structural features of the tumor vasculature determine relatively easy penetration of large molecules as well as nanometer-sized particles through a blood⁻tissue barrier and their accumulation in a tumor tissue. Also, malignant cells have altered molecular profile due to significant changes in tumor cell metabolism at every level from the genome to metabolome. Recently, the tumor interaction with cells of immune system becomes the focus of particular attention, that among others findings resulted in extensive study of cells with preferential tropism to tumor. In this review we summarize the information on the diversity of currently existing approaches to targeted drug delivery to tumor, including (i) passive targeting based on the specific features of tumor vasculature, (ii) active targeting which implies a specific binding of the antitumor agent with its molecular target, and (iii) cell-mediated tumor targeting.

Molecular Imaging of IGF-1R in Cancer

The important role of insulin-like growth factor 1 receptor (IGF-1R) in malignant tumors has been well established. Increased IGF-1R activity promotes cancer cell proliferation, migration, and invasion and is associated with tumor metastasis, treatment resistance, poor prognosis, and shortened survival in patients with cancer. However, while IGF-1R has become a promising target for cancer therapy, IGF-1R-targeted therapy is ineffective in unselected patients. It is therefore essential to evaluate IGF-1R expression before treatment in order to identify responsive patients, monitor therapy efficacy, and estimate prognosis. Insulin-like growth factor 1 receptor molecular imaging is an optimal method for assessing the expression of IGF-1R in vivo accurately and noninvasively. In this review, we will summarize the current status of IGF-1R molecular imaging in cancer, in which 5 major classes of ligands that have been developed for noninvasive IGF-1R molecular imaging will be discussed: natural ligands, monoclonal antibodies, antibody fragments, affibodies, and small molecules. For decades, IGF-1R molecular imaging is studied in full swing and more effort is needed in the future.

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.

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.

Alternative reagents to antibodies in imaging applications

Antibodies have been indispensable tools in molecular biology, biochemistry and medical research. However, a number of issues surrounding validation, specificity and batch variation of commercially available antibodies have prompted research groups to develop novel non-antibody binding reagents. The ability to select highly specific monoclonal non-antibody binding proteins without the need for animals, the ease of production and the ability to site-directly label has enabled a wide variety of applications to be tested, including imaging. In this review, we discuss the success of a number of non-antibody reagents in imaging applications, including the recently reported Affimer.

Affibody Molecules in Biotechnological and Medical Applications

Affibody molecules are small (6.5-kDa) affinity proteins based on a three-helix bundle domain framework. Since their introduction 20 years ago as an alternative to antibodies for biotechnological applications, the first therapeutic affibody molecules have now entered clinical development and more than 400 studies have been published in which affibody molecules have been developed and used in a variety of contexts. In this review, we focus primarily on efforts over the past 5 years to explore the potential of affibody molecules for medical applications in oncology, neurodegenerative, and inflammation disorders, including molecular imaging, receptor signal blocking, and delivery of toxic payloads. In addition, we describe recent examples of biotechnological applications, in which affibody molecules have been exploited as modular affinity fusion partners.

Affibody molecules as engineered protein drugs

Affibody molecules can be used as tools for molecular recognition in diagnostic and therapeutic applications. There are several preclinical studies reported on diagnostic and therapeutic use of this molecular class of alternative scaffolds, and early clinical evidence is now beginning to accumulate that suggests the Affibody molecules to be efficacious and safe in man. The small size and ease of engineering make Affibody molecules suitable for use in multispecific constructs where AffiMabs is one such that offers the option to potentiate antibodies for use in complex disease.

Influence of the N-Terminal Composition on Targeting Properties of Radiometal-Labeled Anti-HER2 Scaffold Protein ADAPT6

Radionuclide-imaging-based stratification of patients to targeted therapies makes cancer treatment more personalized and therefore more efficient. Albumin-binding domain derived affinity proteins (ADAPTs) constitute a novel group of imaging probes based on the scaffold of an albumin-binding domain (ABD). To evaluate how different compositions of the N-terminal sequence of ADAPTs influence their biodistribution, a series of human epidermal growth factor receptor type 2 (HER2)-binding ADAPT6 derivatives with different N-terminal sequences were created: GCH₆DANS (2), GC(HE)₃DANS (3), GCDEAVDANS (4), and GCVDANS(5). These were compared with the parental variant: GCSS(HE)₃DEAVDANS (1). All variants were site-specifically conjugated with a maleimido-derivative of a DOTA chelator and labeled with ¹¹¹In. Binding to HER2-expressing cells in vitro, in vivo biodistribution as well as targeting properties of the new variants were compared with properties of the ¹¹¹In-labeled parental ADAPT variant 1 (¹¹¹In-DOTA-1). The composition of the N-terminal sequence had an apparent influence on biodistribution of ADAPT6 in mice. The use of a hexahistidine tag in ¹¹¹In-DOTA-2 was associated with elevated hepatic uptake compared to the (HE)₃-containing counterpart, ¹¹¹In-DOTA-3. All new variants without a hexahistidine tag demonstrated lower uptake in blood, lung, spleen, and muscle compared to uptake in the parental variant. The best new variants, ¹¹¹In-DOTA-3 and ¹¹¹In-DOTA-5, provided tumor uptakes of 14.6 ± 2.4 and 12.5 ± 1.3% ID/g at 4 h after injection, respectively. The tumor uptake of ¹¹¹In-DOTA-3 was significantly higher than the uptake of the parental ¹¹¹In-DOTA-1 (9.1 ± 2.0% ID/g). The tumor-to-blood ratios of 395 ± 75 and 419 ± 91 at 4 h after injection were obtained for ¹¹¹In-DOTA-5 and ¹¹¹In-DOTA-3, respectively. In conclusion, the N-terminal sequence composition affects the biodistribution and targeting properties of ADAPT-based imaging probes, and its optimization may improve imaging contrast.

Feasibility of imaging of epidermal growth factor receptor expression with ZEGFR:2377 affibody molecule labeled with 99mTc using a peptide-based cysteine-containing chelator

The epidermal growth factor receptor (EGFR) is overexpressed in a number of malignant tumors and is a molecular target for several specific anticancer antibodies and tyrosine kinase inhibitors. The overexpression of EGFR is a predictive biomarker for response to several therapy regimens. Radionuclide molecular imaging might enable detection of EGFR overexpression by a non-invasive procedure and could be used repeatedly. Affibody molecules are engineered scaffold proteins, which could be selected to have a high affinity and selectivity to predetermined targets. The anti-EGFR ZEGFR:2377 affibody molecule is a potential imaging probe for EGFR detection. The use of the generator-produced radionuclide ^99mTc should facilitate clinical translation of an imaging probe due to its low price, availability and favorable dosimetry of the radionuclide. In the present study, we evaluated feasibility of ZEGFR:2377 labeling with ^99mTc using a peptide-based cysteine-containing chelator expressed at the C-terminus of ZEGFR:2377. The label was stable in vitro under cysteine challenge. In addition, ^99mTc-ZEGFR:2377 was capable of specific binding to EGFR-expressing cells with high affinity (274 pM). Studies in BALB/C nu/nu mice bearing A431 xenografts demonstrated that ^99mTc-ZEGFR:2377 accumulates in tumors in an EGFR-specific manner. The tumor uptake values were 3.6±1 and 2.5±0.4% ID/g at 3 and 24 h after injection, respectively. The corresponding tumor-to-blood ratios were 1.8±0.4 and 8±3. The xenografts were clearly visualized at both time-points. This study demonstrated the potential of ^99mTc-labeled ZEGFR:2377 for imaging of EGFR in vivo.

Comparative Evaluation of Affibody Molecules for Radionuclide Imaging of in Vivo Expression of Carbonic Anhydrase IX

Overexpression of the enzyme carbonic anhydrase IX (CAIX) is documented for chronically hypoxic malignant tumors as well as for normoxic renal cell carcinoma. Radionuclide molecular imaging of CAIX would be useful for detection of hypoxic areas in malignant tumors, for patients' stratification for CAIX-targeted therapies, and for discrimination of primary malignant and benign renal tumors. Earlier, we have reported feasibility of in vivo radionuclide based imaging of CAIX expressing tumors using Affibody molecules, small affinity proteins based on a nonimmunoglobulin scaffold. In this study, we compared imaging properties of several anti-CAIX Affibody molecules having identical scaffold parts and competing for the same epitope on CAIX, but having different binding paratopes. Four variants were labeled using residualizing ^99mTc and nonresidualizing ¹²⁵I labels. All radiolabeled variants demonstrated high-affinity detection of CAIX-expressing cell line SK-RC-52 in vitro and specific accumulation in SK-RC-52 xenografts in vivo. ¹²⁵I-labeled conjugates demonstrated much lower radioactivity uptake in kidneys but higher radioactivity concentration in blood compared with ^99mTc-labeled counterparts. Although all variants cleared rapidly from blood and nonspecific compartments, there was noticeable difference in their biodistribution. The best variant for imaging of expression of CAIX in disseminated cancer was ^99mTc-(HE)₃-ZCAIX:2 providing tumor uptake of 16.3 ± 0.9% ID/g and tumor-to-blood ratio of 44 ± 7 at 4 h after injection. For primary renal cell carcinoma, the most promising imaging candidate was ¹²⁵I-ZCAIX:4 providing tumor-kidney ratio of 2.1 ± 0.5. In conclusion, several clones of scaffold proteins should be evaluated to select the best variant for development of an imaging probe with optimal sensitivity for the intended application.

PET imaging of insulin-like growth factor type 1 receptor expression with a 64Cu-labeled Affibody molecule

The insulin-like growth factor 1 receptor (IGF-1R) serves as an attractive target for cancer molecular imaging and therapy. Previous single photon emission computerized tomography (SPECT) studies showed that the IGF-1R-targeting Affibody molecules (99m)Tc-ZIGF1R:4551-GGGC, [(99m)Tc(CO)3](+)-(HE)3-ZIGF1R:4551 and (111)In-DOTA-ZIGF1R:4551 can discriminate between high and low IGF-1R-expression tumors and have the potential for patient selection for IGF-1R-targeted therapy. Compared with SPECT, positron emission tomography (PET) may improve imaging of IGF-1R-expression, because of its high sensitivity, high spatial resolution, strong quantification ability. The aim of the present study was to develop the (64)Cu-labeled NOTA-conjugated Affibody molecule ZIGF-1R:4:40 as a PET probe for imaging of IGF-1R-positive tumor. An Affibody analogue (Ac-Cys-ZIGF-1R:4:40) binding to IGF-1R was site-specifically conjugated with NOTA and labeled with (64)Cu. Binding affinity and specificity of (64)Cu-NOTA-ZIGF-1R:4:40 to IGF-1R were evaluated using human glioblastoma U87MG cells. Small-animal PET, biodistribution, and metabolic stability studies were conducted on mice bearing U87MG xenografts after the injection of (64)Cu-NOTA-ZIGF-1R:4:40 with or without co-injection of unlabeled Affibody proteins. The radiosynthesis of (64)Cu-NOTA-ZIGF-1R:4:40 was completed successfully within 60 min with a decay-corrected yield of 75 %. (64)Cu-NOTA-ZIGF-1R:4:40 bound to IGF-1R with low nanomolar affinity (K D = 28.55 ± 3.95 nM) in U87MG cells. (64)Cu-NOTA-ZIGF-1R:4:40 also displayed excellent in vitro and in vivo stability. In vivo biodistribution and PET studies demonstrated targeting of U87MG gliomas xenografts was IGF-1R specific. The tumor uptake was 5.08 ± 1.07 %ID/g, and the tumor to muscle ratio was 11.89 ± 2.16 at 24 h after injection. Small animal PET imaging studies revealed that (64)Cu-NOTA-ZIGF-1R:4:40 could clearly identify U87MG tumors with good contrast at 1-24 h after injection. This study demonstrates that (64)Cu-NOTA-ZIGF-1R:4:40 is a promising PET probe for imaging IGF-1R positive tumor.