Site-specifically conjugated anti-HER2 Affibody molecules as one-step reagents for target expression analyses on cells and xenograft samples

A Simple and Low-cost Preliminary Quantification of Target Membrane Protein in Single Cells

To study the heterogeneity of target membrane proteins in single cells with cellular integrity, we proposed a simple and low-cost method to obtain the copy number of the membrane proteins. HeLa cells were labeled by FITC affinity bodies specifically targeting HER2 membrane proteins. The immunolabeled HeLa cells were quantified by a laboratory-built laser induced fluorescence detector. A series of fluorescent microspheres with known number of FITC molecules on the surface were used to establish the calibration curve, instead of the standard fluorescent solutions, because the morphology of the microspheres was similar to the cells, and the distribution of FITC on the spheres were similar to the distribution of HER2 on the HeLa. The fluorescence intensity of the cells was converted to the molecule number of HER2 by the calibration curve. A capillary electrophoresis system was used to drive the microspheres and cells through the detection window. The copy number of HER2 in HeLa cells ranged from 4,036 to 1,224,920 ± 100 (2.5-97.5%), and the median of copy numbers were 104,438 ± 100 per cell. This method for measuring low-abundance membrane proteins can be utilized for the initial exploration of proteomics in ordinary laboratories.

Affibody-derived drug conjugates: Potent cytotoxic molecules for treatment of HER2 over-expressing tumors

Patients with HER2-positive tumors often suffer resistance to therapy, warranting development of novel treatment modalities. Affibody molecules are small affinity proteins which can be engineered to bind to desired targets. They have in recent years been found to allow precise targeting of cancer specific molecular signatures such as the HER2 receptor. In this study, we have investigated the potential of an affibody molecule targeting HER2, Z_HER2:2891, conjugated with the cytotoxic maytansine derivate MC-DM1, for targeted cancer therapy. Z_HER2:2891 was expressed as a monomer (Z_HER2:2891), dimer ((Z_HER2:2891)₂) and dimer with an albumin binding domain (ABD) for half-life extension ((Z_HER2:2891)₂-ABD). All proteins had a unique C-terminal cysteine that could be used for efficient and site-specific conjugation with MC-DM1. The resulting affibody drug conjugates were potent cytotoxic molecules for human cells over-expressing HER2, with sub-nanomolar IC₅₀-values similar to trastuzumab emtansine, and did not affect cells with low HER2 expression. A biodistribution study of a radiolabeled version of (Z_HER2:2891)₂-ABD-MC-DM1, showed that it was taken up by the tumor. The major site of off-target uptake was the kidneys and to some extent the liver. (Z_HER2:2891)₂-ABD-MC-DM1 was found to have a half-life in circulation of 14 h. The compound was tolerated well by mice at 8.5 mg/kg and was shown to extend survival of mice bearing HER2 over-expressing tumors. The findings in this study show that affibody molecules are a promising class of engineered affinity proteins to specifically deliver small molecular drugs to cancer cells and that such conjugates are potential candidates for clinical evaluation on HER2-overexpressing cancers.

A novel HPV16 E7-affitoxin for targeted therapy of HPV16-induced human cervical cancer

Cervical cancer, the second most common cause of cancer death in women worldwide, is significantly associated with infection of high-risk human papillomaviruses (HPVs), especially the most common genotype, HPV 16. To date, there is no established noninvasive therapy to treat cervical cancer.

Methods: Here, we report a novel affitoxin that targets HPV16 E7 protein, one of the primary target proteins in molecular targeted therapy for HPV-induced cervical cancer. The affitoxin, Z_HPV16E7 affitoxin384 was generated by fusing the modified Pseudomonas Exotoxin A (PE38KDEL) to the HPV16 E7-specific affibody. The expressed and purified Z_HPV16E7 affitoxin384 was characterized using numerous methods. SPR assay, indirect immunofluorescence assay, and near-infrared (NIR) optical imaging were respectively performed to assess the targeting ability of Z_HPV16E7 affitoxin384 to HPV16 E7 protein both in vitro and in vivo. Cell viability assays and SiHa tumor-bearing nude mice were used to evaluate the efficacy of Z_{HPV16 E7} affitoxin384 in vitro and in vivo, respectively.

Results: Using in vitro methods the SPR assay and indirect immunofluorescence assay showed that Z_HPV16E7 affitoxin384 targeted HPV16 E7 with high binding affinity and specificity. Significant reduction of cell viability in HPV16 positive cells was observed in the presence of Z_{HPV16 E7} affitoxin384. By NIR optical imaging, Z_{HPV16 E7} affitoxin384 specifically targeted HPV16 positive tumors in vivo. Z_HPV16E7 affitoxin384 showed significant in vivo antitumor efficacy in two kinds of tumor-bearing nude mouse models.

Conclusions: Z_HPV16E7 affitoxin384 is a potent anti-cervical cancer therapeutic agent that could be effective against HPV16 positive tumors in humans.

3D Imaging Detection of HER2 Based in the Use of Novel Affibody-Quantum Dots Probes and Ratiometric Analysis

Patients with breast cancer (BC) overexpressing HER2 (HER2+) are selected for Trastuzumab treatment, which blocks HER2 and improves cancer prognosis. However, HER2+ diagnosis, by the gold standard, immunohistochemistry, could lead to errors, associated to: a) variability in sample manipulation (thin 2D sections), b) use of subjective algorithms, and c) heterogeneity of HER2 expression within the tissue. Therefore, we explored HER2 3D detection by multiplexed imaging of Affibody-Quantum Dots conjugates (Aff-QD), ratiometric analysis (RMAFI) and thresholding, using BC multicellular tumor spheroids (BC-MTS) (~120 μm of diameter) as 3D model of BC. HER2+, HER2- and hybrid HER2+/- BC-MTS (mimicking heterogeneous tissue) were incubated simultaneously with two Aff-QD probes (anti-HER2 and negative control (NC), respectively, (1:1)). Confocal XY sections were recorded along the Z distance, and processed by automatized RMAFI (anti-HER2 Aff-QD/ NC). Quantifying the NC fluorescence allowed to predict the fraction of non-specific accumulation of the anti-HER2 probe within the thick sample, and resolve the specific HER2 level. HER2 was detected up to 30μm within intact BC-MTS, however, permeabilization improved detection up to 70μm. Specific HER2 signal was objectively quantified, and HER2 3D-density of 9.2, 48.3 and 30.8% were obtained in HER2-, HER2+ and hybrid HER2+/- permeabilized BC-MTS, respectively. Therefore, by combining the multiplexing capacity of Aff-QD probes and RMAFI, we overcame the challenge of non-specific probe accumulation in 3D samples with minimal processing, yielding a fast, specific spatial HER2 detection and objective quantification.

Chemically-defined camelid antibody bioconjugate for the magnetic resonance imaging of Alzheimer's disease

Today, molecular imaging of neurodegenerative diseases is mainly based on small molecule probes. Alternatively, antibodies are versatile tools that may be developed as new imaging agents. Indeed, they can be readily obtained to specifically target any antigen of interest and their scaffold can be functionalized. One of the critical issues involved in translating antibody-based probes to the clinic is the design and synthesis of perfectly-defined conjugates. Camelid single-domain antibody-fragments (VHHs) are very small and stable antibodies that are able to diffuse in tissues and potentially cross the blood brain barrier (BBB). Here, we selected a VHH (R3VQ) specifically targeting one of the main lesions of Alzheimer's disease (AD), namely the amyloid-beta (Aß) deposits. It was used as a scaffold for the design of imaging probes for magnetic resonance imaging (MRI) and labeled with the contrastophore gadolinium using either a random or site-specific approach. In contrast to the random strategy, the site-specific conjugation to a single reduced cysteine in the C-terminal part of the R3VQ generates a well-defined bioconjugate in a high yield process. This new imaging probe is able to cross the BBB and label Aß deposits after intravenous injection. Also, it displays improved r1 and r2 relaxivities, up to 30 times higher than a widely used clinical contrast agent, and it allows MRI detection of amyloid deposits in post mortem brain tissue of a mouse model of AD. The ability to produce chemically-defined VHH conjugates that cross the BBB opens the way for future development of tailored imaging probes targeting intracerebral antigens.

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.

Cancer stratification by molecular imaging

The lack of specificity of traditional cytotoxic drugs has triggered the development of anticancer agents that selectively address specific molecular targets. An intrinsic property of these specialized drugs is their limited applicability for specific patient subgroups. Consequently, the generation of information about tumor characteristics is the key to exploit the potential of these drugs. Currently, cancer stratification relies on three approaches: Gene expression analysis and cancer proteomics, immunohistochemistry and molecular imaging. In order to enable the precise localization of functionally expressed targets, molecular imaging combines highly selective biomarkers and intense signal sources. Thus, cancer stratification and localization are performed simultaneously. Many cancer types are characterized by altered receptor expression, such as somatostatin receptors, folate receptors or Her2 (human epidermal growth factor receptor 2). Similar correlations are also known for a multitude of transporters, such as glucose transporters, amino acid transporters or hNIS (human sodium iodide symporter), as well as cell specific proteins, such as the prostate specific membrane antigen, integrins, and CD20. This review provides a comprehensive description of the methods, targets and agents used in molecular imaging, to outline their application for cancer stratification. Emphasis is placed on radiotracers which are used to identify altered expression patterns of cancer associated markers.

[99mTc(CO)3]+-(HE)3-ZIGF1R:4551, a new Affibody conjugate for visualization of insulin-like growth factor-1 receptor expression in malignant tumours

PURPOSE

Radionuclide imaging of insulin-like growth factor type 1 receptor (IGF-1R) expression in tumours might be used for selection of patients who would benefit from IGF-1R-targeted therapy. We have previously shown the feasibility of IGF-1R imaging using the Affibody molecule (111)In-DOTA-His(6)-Z(IGF1R:4551). The use of (99m)Tc instead of (111)In should improve sensitivity and resolution of imaging, and reduce the dose burden to patients. We hypothesized that inclusion of a HEHEHE tag instead of a His(6) tag in Z(IGF1R:4551) would permit its convenient purification using IMAC, enable labelling with [(99m)Tc(CO)(3)](+), and improve its biodistribution.

METHODS

Z(IGF1R:4551) was expressed with a HEHEHE tag in the N terminus. The resulting (HE)(3)-Z(IGF1R:4551) construct was labelled with [(99m)Tc(CO)(3)](+). Targeting of IGF-1R-expressing cells using [(99m)Tc(CO)(3)](+)-(HE)(3)-Z(IGF1R:4551) was evaluated in vitro and in vivo.

RESULTS

(HE)(3)-Z(IGF1R:4551) was stably labelled with (99m)Tc with preserved specific binding to IGF-1R-expressing DU-145 prostate cancer cells in vitro. In mice, [(99m)Tc(CO)(3)](+)-(HE)(3)-Z(IGF1R:4551) accumulated in IGF-1R-expressing organs (pancreas, stomach, lung and salivary gland). [(99m)Tc(CO)(3)](+)-(HE)(3)-Z(IGF1R:4551) demonstrated 3.6-fold lower accumulation in the liver and spleen than (111)In-DOTA-Z(IGF1R:4551). In NMRI nu/nu mice with DU-145 prostate cancer xenografts, the tumour uptake was 1.32 ± 0.11 %ID/g and the tumour-to-blood ratio was 4.4 ± 0.3 at 8 h after injection. The xenografts were visualized using a gamma camera 6 h after injection.

CONCLUSION

(99m)Tc(CO)(3)](+)-(HE)(3)-Z(IGF1R:4551) is a promising candidate for visualization of IGF-1R expression in malignant tumours.

Single molecule fluorescence detection and tracking in mammalian cells: the state-of-the-art and future perspectives

Insights from single-molecule tracking in mammalian cells have the potential to greatly contribute to our understanding of the dynamic behavior of many protein families and networks which are key therapeutic targets of the pharmaceutical industry. This is particularly so at the plasma membrane, where the method has begun to elucidate the mechanisms governing the molecular interactions that underpin many fundamental processes within the cell, including signal transduction, receptor recognition, cell-cell adhesion, etc. However, despite much progress, single-molecule tracking faces challenges in mammalian samples that hinder its general application in the biomedical sciences. Much work has recently focused on improving the methods for fluorescent tagging of target molecules, detection and localization of tagged molecules, which appear as diffraction-limited spots in charge-coupled device (CCD) images, and objectively establishing the correspondence between moving particles in a sequence of image frames to follow their diffusive behavior. In this review we outline the state-of-the-art in the field and discuss the advantages and limitations of the methods available in the context of specific applications, aiming at helping researchers unfamiliar with single molecules methods to plan out their experiments.

Affibody-DyLight conjugates for in vivo assessment of HER2 expression by near-infrared optical imaging

Purpose

Amplification of the HER2/neu gene and/or overexpression of the corresponding protein have been identified in approximately 20% of invasive breast carcinomas. Assessment of HER2 expression in vivo would advance development of new HER2-targeted therapeutic agents and, potentially, facilitate choice of the proper treatment strategy offered to the individual patient. We present novel HER2-specific probes for in vivo evaluation of the receptor status by near-infrared (NIR) optical imaging.

Experimental Design

Affibody molecules were expressed, purified, and labeled with NIR-fluorescent dyes. The binding affinity and specificity of the obtained probe were tested in vitro. For in vivo validation, the relationship of the measured NIR signal and HER2 expression was characterized in four breast cancer xenograft models, expressing different levels of HER2. Accumulation of Affibody molecules in tumor tissue was further confirmed by ex vivo analysis.

Results

Affibody-DyLight conjugates showed high affinity to HER2 (K_D = 3.66±0.26). No acute toxicity resulted from injection of the probes (up to 0.5 mg/kg) into mice. Pharmacokinetic studies revealed a relatively short (37.53±2.8 min) half-life of the tracer in blood. Fluorescence accumulation in HER2-positive BT-474 xenografts was evident as soon as a few minutes post injection and reached its maximum at 90 minutes. On the other hand, no signal retention was observed in HER2-negative MDA-MB-468 xenografts. Immunostaining of extracted tumor tissue confirmed penetration of the tracer into tumor tissue.

Conclusions

The results of our studies suggest that Affibody-DyLight-750 conjugate is a powerful tool to monitor HER2 status in a preclinical setting. Following clinical validation, it might provide complementary means for assessment of HER2 expression in breast cancer patients (assuming availability of proper NIR scanners) and/or be used to facilitate detection of HER2-positive metastatic lesions during NIR-assisted surgery.

Protein-based tumor molecular imaging probes

Molecular imaging is an emerging discipline which plays critical roles in diagnosis and therapeutics. It visualizes and quantifies markers that are aberrantly expressed during the disease origin and development. Protein molecules remain to be one major class of imaging probes, and the option has been widely diversified due to the recent advances in protein engineering techniques. Antibodies are part of the immunosystem which interact with target antigens with high specificity and affinity. They have long been investigated as imaging probes and were coupled with imaging motifs such as radioisotopes for that purpose. However, the relatively large size of antibodies leads to a half-life that is too long for common imaging purposes. Besides, it may also cause a poor tissue penetration rate and thus compromise some medical applications. It is under this context that various engineered protein probes, essentially antibody fragments, protein scaffolds, and natural ligands have been developed. Compared to intact antibodies, they possess more compact size, shorter clearance time, and better tumor penetration. One major challenge of using protein probes in molecular imaging is the affected biological activity resulted from random labeling. Site-specific modification, however, allows conjugation happening in a stoichiometric fashion with little perturbation of protein activity. The present review will discuss protein-based probes with focus on their application and related site-specific conjugation strategies in tumor imaging.

HER2-affitoxin: a potent therapeutic agent for the treatment of HER2-overexpressing tumors

PURPOSE

Cancers overexpressing the HER2/neu gene are usually more aggressive and are associated with poor prognosis. Although trastuzumab has significantly improved the outcome, many tumors do not respond or acquire resistance to current therapies. To provide an alternative HER2-targeted therapy, we have developed and characterized a novel recombinant protein combining an HER2-specific Affibody and modified Pseudomonas aeruginosa exotoxin A (PE 38), which, after binding to HER2, is internalized and delivered to the cytosol of the tumor cell, where it blocks protein synthesis by ADP ribosylation of eEF-2.

EXPERIMENTAL DESIGN

The effect of the Affitoxin on cell viability was assessed using CellTiter-Glo (Promega). To assess HER2-specific efficacy, athymic nude mice bearing BT-474 breast cancer, SK-OV-3 ovarian cancer, and NCI-N87 gastric carcinoma xenografts were treated with the Affitoxin (HER2- or Tag-specific), which was injected every third day. Affitoxin immunogenicity in female BALB/c mice was investigated using standard antibody production and splenocyte proliferation assays.

RESULTS

In vitro experiments proved that HER2-Affitoxin is a potent agent that eliminates HER2-overexpressing cells at low picomolar concentrations. Therapeutic efficacy studies showed complete eradication of relatively large BT-474 tumors and significant effects on SK-OV-3 and NCI-N87 tumors. HER2-Affitoxin cleared quickly from circulation (T(1/2) < 10 minutes) and was well tolerated by mice at doses of 0.5 mg/kg and below. Immunogenicity studies indicated that HER2-Affitoxin induced antibody development after the third injected dose.

CONCLUSIONS

Our findings showed that HER2-Affitoxin is an effective anticancer agent and a potential candidate for clinical studies.

Optimal specific radioactivity of anti-HER2 Affibody molecules enables discrimination between xenografts with high and low HER2 expression levels

PURPOSE

Overexpression of the HER2 receptor is a biomarker for predicting those patients who may benefit from trastuzumab therapy. Radiolabelled Affibody molecules can be used to visualize HER2 expression in tumour xenografts with high sensitivity. However, previous studies demonstrated that the difference in uptake in xenografts with high and low HER2 expression levels is not proportional to the difference in expression levels. We hypothesized that discrimination between tumours with high and low HER2 expression may be improved by increasing the injected dose (reducing the specific activity) of the tracer.

METHODS

The influence of injected dose of anti-HER2 (111)In-DOTA-Z(HER2 342) Affibody molecule on uptake in SKOV-3 (high HER2 expression) and LS174T (low expression) xenografts was investigated. The optimal range of injected doses enabling discrimination between xenografts with high and low expression was determined. To verify this, tumour uptake was measured in mice carrying both SKOV-3 and LS174T xenografts after injection of either 1 or 15 μg (111)In-DOTA-Z(HER2:342).

RESULTS

An increase in the injected dose caused a linear decrease in the radioactivity accumulation in the LS174T xenografts (low HER2 expression). For SKOV-3 xenografts, the dependence of the tumour uptake on the injected dose was less dramatic. The injection of 10-30 μg (111)In-DOTA-Z(HER2:342) per mouse led to the largest difference in uptake between the two types of tumour. Experiments in mice bearing two xenografts confirmed that the optimized injected dose enabled better discrimination of expression levels.

CONCLUSION

Careful optimization of the injected dose of Affibody molecules is required for maximum discrimination between xenografts with high and low levels of HER2 expression. This information has potential relevance for clinical imaging applications.

Design of an optimized scaffold for affibody molecules

Affibody molecules are non-immunoglobulin-derived affinity proteins based on a three-helical bundle protein domain. Here, we describe the design process of an optimized Affibody molecule scaffold with improved properties and a surface distinctly different from that of the parental scaffold. The improvement was achieved by applying an iterative process of amino acid substitutions in the context of the human epidermal growth factor receptor 2 (HER2)-specific Affibody molecule Z(HER2:342). Replacements in the N-terminal region, loop 1, helix 2 and helix 3 were guided by extensive structural modeling using the available structures of the parent Z domain and Affibody molecules. The effect of several single substitutions was analyzed followed by combination of up to 11 different substitutions. The two amino acid substitutions N23T and S33K accounted for the most dramatic improvements, including increased thermal stability with elevated melting temperatures of up to +12 degrees C. The optimized scaffold contains 11 amino acid substitutions in the nonbinding surface and is characterized by improved thermal and chemical stability, as well as increased hydrophilicity, and enables generation of identical Affibody molecules both by chemical peptide synthesis and by recombinant bacterial expression. A HER2-specific Affibody tracer, [MMA-DOTA-Cys61]-Z(HER2:2891)-Cys (ABY-025), was produced by conjugating MMA-DOTA (maleimide-monoamide-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) to the peptide produced either chemically or in Escherichia coli. ABY-025 showed high affinity and specificity for HER2 (equilibrium dissociation constant, K(D), of 76 pM) and detected HER2 in tissue sections of SKOV-3 xenograft and human breast tumors. The HER2-binding capacity was fully retained after three cycles of heating to 90 degrees C followed by cooling to room temperature. Furthermore, the binding surfaces of five Affibody molecules targeting other proteins (tumor necrosis factor alpha, insulin, Taq polymerase, epidermal growth factor receptor or platelet-derived growth factor receptor beta) were grafted onto the optimized scaffold, resulting in molecules with improved thermal stability and a more hydrophilic nonbinding surface.

HER2- and EGFR-specific affiprobes: novel recombinant optical probes for cell imaging

The human epidermal growth factor receptors, EGFR and HER2, are members of the EGFR family of cell-surface receptors/tyrosine kinases. EGFR- and HER2-positive cancers represent a more aggressive disease with greater likelihood of recurrence, poorer prognosis, and decreased survival rate, compared to EGFR- or HER2-negative cancers. The details of HER2 proto-oncogenic functions are not deeply understood, partially because of a restricted availability of tools for EGFR and HER2 detection (A. Sorkin and L. K. Goh, Exp. Cell Res. 2009, 315, 683-696). We have created photostable and relatively simple-to-produce imaging probes for in vitro staining of EGFR and HER2. These new reagents, called affiprobes, consist of a targeting moiety, a HER2- or EGFR-specific Affibody molecule, and a fluorescent moiety, mCherry (red) or EGFP (green). Our flow cytometry and confocal microscopy experiments demonstrated high specificity and signal/background ratio of affiprobes. Affiprobes are able to stain both live cells and frozen tumor xenograph sections. This type of optical probe can easily be extended for targeting other cell-surface antigens/ receptors.

Evaluation of a maleimido derivative of CHX-A'' DTPA for site-specific labeling of affibody molecules

Affibody molecules are a new class of small targeting proteins based on a common three-helix bundle structure. Affibody molecules binding a desired target may be selected using phage-display technology. An Affibody molecule Z HER2:342 binding with subnanomolar affinity to the tumor antigen HER2 has recently been developed for radionuclide imaging in vivo. Introduction of a single cysteine into the cysteine-free Affibody scaffold provides a unique thiol group for site-specific labeling of recombinant Affibody molecules. The recently developed maleimido-CHX-A'' DTPA was site-specifically conjugated at the C-terminal cysteine of Z HER2:2395-C, a variant of Z HER2:342, providing a homogeneous conjugate with a dissociation constant of 56 pM. The yield of labeling with (111)In was >99% after 10 min at room temperature. In vitro cell tests demonstrated specific binding of (111)In-CHX-A'' DTPA-Z 2395-C to HER2-expressing cell-line SKOV-3 and good cellular retention of radioactivity. In normal mice, the conjugate demonstrated rapid clearance from all nonspecific organs except kidney. In mice bearing SKOV-3 xenografts, the tumor uptake of (111)In-CHX-A'' DTPA-Z 2395-C was 17.3 +/- 4.8% IA/g and the tumor-to-blood ratio 86 +/- 46 (4 h postinjection). HER2-expressing xenografts were clearly visualized 1 h postinjection. In conclusion, coupling of maleimido-CHX-A'' DTPA to cysteine-containing Affibody molecules provides a well-defined uniform conjugate, which can be rapidly labeled at room temperature and provides high-contrast imaging of molecular targets in vivo.

Affibody molecules for in vivo characterization of HER2-positive tumors by near-infrared imaging

PURPOSE

HER2 overexpression has been associated with a poor prognosis and resistance to therapy in breast cancer patients. We are developing molecular probes for in vivo quantitative imaging of HER2 receptors using near-infrared (NIR) optical imaging. The goal is to provide probes that will minimally interfere with the studied system, that is, whose binding does not interfere with the binding of the therapeutic agents and whose effect on the target cells is minimal.

EXPERIMENTAL DESIGN

We used three different types of HER2-specific Affibody molecules [monomer ZHER2:342, dimer (ZHER2:477)2, and albumin-binding domain-fused-(ZHER2:342)2] as targeting agents and labeled them with Alexa Fluor dyes. Trastuzumab was also conjugated, using commercially available kits, as a standard control. The resulting conjugates were characterized in vitro by toxicity assays, Biacore affinity measurements, flow cytometry, and confocal microscopy. Semiquantitative in vivo NIR optical imaging studies were carried out using mice with s.c. xenografts of HER2-positive tumors.

RESULTS

The HER2-specific Affibody molecules were not toxic to HER2-overexpressing cells and their binding to HER2 did interfere with neither binding nor effectives of trastuzumab. The binding affinities and specificities of the Affibody-Alexa Fluor fluorescent conjugates to HER2 were unchanged or minimally affected by the modifications. Pharmacokinetics and biodistribution studies showed the albumin-binding domain-fused-(ZHER2:342)2-Alexa Fluor 750 conjugate to be an optimal probe for optical imaging of HER2 in vivo.

CONCLUSION

Our results suggest that Affibody-Alexa Fluor conjugates may be used as a specific NIR probe for the noninvasive semiquantitative imaging of HER2 expression in vivo.

SPECT imaging with 99mTc-labeled EGFR-specific nanobody for in vivo monitoring of EGFR expression

PURPOSE

Overexpression of the epidermal growth factor receptor (EGFR) occurs with high incidence in various carcinomas. The oncogenic expression of the receptor has been exploited for immunoglobulin-based diagnostics and therapeutics. We describe the use of a llama single-domain antibody fragment, termed Nanobody, for the in vivo radioimmunodetection of EGFR overexpressing tumors using single photon emission computed tomography (SPECT) in mice.

METHODS

Fluorescence-activated cell sorting (FACS) analysis was performed to evaluate the specificity and selectivity of 8B6 Nanobody to bind EGFR on EGFR overexpressing cells. The Nanobody was then labeled with (99m)Tc via its C-terminal histidine tail. Uptake in normal organs and tissues was assessed by ex vivo analysis. In vivo tumor targeting of (99m)Tc-8B6 Nanobody was evaluated via pinhole SPECT in mice bearing xenografts of tumor cells with either high (A431) or moderate (DU145) overexpression of EGFR.

RESULTS

FACS analysis indicated that the 8B6 Nanobody only recognizes cells overexpressing EGFR. In vivo blood clearance of (99m)Tc-8B6 Nanobody is relatively fast (half-life, 1.5 h) and mainly via the kidneys. At 3 h postinjection, total kidney accumulation is high (46.6+/-0.9%IA) compared to total liver uptake (18.9+/-0.6%IA). Pinhole SPECT imaging of mice bearing A431 xenografts showed higher average tumor uptake (5.2+/-0.5%IA/cm(3)) of (99m)Tc-8B6 Nanobody compared to DU145 xenografts (1.8+/-0.3%IA/cm(3), p<0.001).

CONCLUSION

The EGFR-binding Nanobody investigated in this study shows high specificity and selectivity towards EGFR overexpressing cells. Pinhole SPECT analysis with (99m)Tc-8B6 Nanobody enabled in vivo discrimination between tumors with high and moderate EGFR overexpression. The favorable biodistribution further corroborates the suitability of Nanobodies for in vivo tumor imaging.

Radionuclide therapy of HER2-positive microxenografts using a 177Lu-labeled HER2-specific Affibody molecule

A radiolabeled anti-HER2 Affibody molecule (Z(HER2:342)) targets HER2-expressing xenografts with high selectivity and gives good imaging contrast. However, the small size (approximately 7 kDa) results in rapid glomerular filtration and high renal accumulation of radiometals, thus excluding targeted therapy. Here, we report that reversible binding to albumin efficiently reduces the renal excretion and uptake, enabling radiometal-based nuclide therapy. The dimeric Affibody molecule (Z(HER2:342))(2) was fused with an albumin-binding domain (ABD) conjugated with the isothiocyanate derivative of CHX-A''-DTPA and labeled with the low-energy beta-emitter (177)Lu. The obtained conjugate [CHX-A''-DTPA-ABD-(Z(HER2:342))(2)] had a dissociation constant of 18 pmol/L to HER2 and 8.2 and 31 nmol/L for human and murine albumin, respectively. The radiolabeled conjugate displayed specific binding to HER2-expressing cells and good cellular retention in vitro. In vivo, fusion with ABD enabled a 25-fold reduction of renal uptake in comparison with the nonfused dimer molecule (Z(HER2:342))(2). Furthermore, the biodistribution showed high and specific uptake of the conjugate in HER2-expressing tumors. Treatment of SKOV-3 microxenografts (high HER2 expression) with 17 or 22 MBq (177)Lu-CHX-A''-DTPA-ABD-(Z(HER2:342))(2) completely prevented formation of tumors, in contrast to mice given PBS or 22 MBq of a radiolabeled non-HER2-binding Affibody molecule. In LS174T xenografts (low HER2 expression), this treatment resulted in a small but significant increase of the survival time. Thus, fusion with ABD improved the in vivo biodistribution, and the results highlight (177)Lu-CHX-A''-DTPA-ABD-(Z(HER2:342))(2) as a candidate for treatment of disseminated tumors with a high level of HER2 expression.