A 2-helix small protein labeled with 68Ga for PET imaging of HER2 expression

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.

Diagnosis of Glioblastoma by Immuno-Positron Emission Tomography

Simple Summary

Neuroimaging has transformed the way brain tumors are diagnosed and treated. Although different non-invasive modalities provide very helpful information, in some situations, they present a limited value. By merging the specificity of antibodies with the resolution, sensitivity, and quantitative capabilities of positron emission tomography (PET), “Immuno-PET” allows us to conduct the non-invasive diagnosis and monitoring of patients over time using antibody-based probes as an in vivo, integrated, quantifiable, 3D, full-body “immunohistochemistry”, like a “virtual biopsy”. This review provides and focuses on immuno-PET applications and future perspectives of this promising imaging approach for glioblastoma.

Abstract

Neuroimaging has transformed neuro-oncology and the way that glioblastoma is diagnosed and treated. Magnetic Resonance Imaging (MRI) is the most widely used non-invasive technique in the primary diagnosis of glioblastoma. Although MRI provides very powerful anatomical information, it has proven to be of limited value for diagnosing glioblastomas in some situations. The final diagnosis requires a brain biopsy that may not depict the high intratumoral heterogeneity present in this tumor type. The revolution in “cancer-omics” is transforming the molecular classification of gliomas. However, many of the clinically relevant alterations revealed by these studies have not yet been integrated into the clinical management of patients, in part due to the lack of non-invasive biomarker-based imaging tools. An innovative option for biomarker identification in vivo is termed “immunotargeted imaging”. By merging the high target specificity of antibodies with the high spatial resolution, sensitivity, and quantitative capabilities of positron emission tomography (PET), “Immuno-PET” allows us to conduct the non-invasive diagnosis and monitoring of patients over time using antibody-based probes as an in vivo, integrated, quantifiable, 3D, full-body “immunohistochemistry” in patients. This review provides the state of the art of immuno-PET applications and future perspectives on this imaging approach for glioblastoma.

Targeting and imaging of HER2 overexpression tumor with a new peptide-based 68Ga-PET radiotracer

Human epidermal growth factor receptor 2 (HER2) overexpression, as a predictive biomarker, is associated with more tumor aggressiveness and worse clinical outcomes in cancer, whereas it's accurate identification has led to the choice of effective treatments in many patients. In this study, a peptide-based PET probe (⁶⁸Ga-DOTA-(Ser)₃-LTVSPWY) was developed for imaging HER2 expression in tumors. The DOTA-(Ser)₃-LTVSPWY was labeled with ⁶⁸Ga and then was evaluated in vitro with HER2-positive SKOV-3 cell line; moreover, the in vivo biodistribution and PET/CT imaging were performed in xenografted tumor-bearing nude mice. The ⁶⁸Ga-DOTA-(Ser)₃-LTVSPWY displayed the high radiochemical purity greater than 95% and good stability in normal saline and human serum. The cellular binding experiments showed that the cell uptake in HER2-positive ovarian cancer cells could be effectively blocked by non-labeled peptide. The K_d and B_max values for radiolabeled peptide were obtained at 2.5 ± 0.6 nM and (3.4 ± 0.2) × 10⁵ sites per cell, respectively. Biodistribution study demonstrated that tumor-to-blood and tumor-to-muscle ratios were about 1.73 ± 0.36 and 3.78 ± 0.17 at 120 min after the injection of the radiolabeled peptide, respectively. Tumor imaging by PET/CT exhibited high contrast tumor image at 60 min after injection in animal models. Consequently, the results were indicative of the specific accumulation of ⁶⁸Ga-DOTA-(Ser)₃-LTVSPWY peptide in HER2-positive tumors and the suitability of its application as a PET probe for the diagnosis of HER2-overexpression tumor.

Ovarian Cancer Targeted Theranostics

Ovarian cancer is a leading cause of death from gynecological malignancies. Although the prognosis is quite favorable if detected at an early stage, the vast majority of cases are diagnosed at an advanced stage, when 5-year survival rates are only 30–40%. Most recurrent ovarian tumors are resistant to traditional therapies underscoring the need for new therapeutic options. Theranostic agents, that combine diagnostic and therapeutic capabilities, are being explored to better detect, diagnose and treat ovarian cancer. To minimize morbidity, improve survival rates, and eventually cure patients, new strategies are needed for early detection and for delivering specifically anticancer therapies to tumor sites. In this review we will discuss various molecular imaging modalities and targets that can be used for imaging, therapeutic and theranostic agent development for improved diagnosis and treatment of ovarian cancer.

Optimized Molecular Design of ADAPT-Based HER2-Imaging Probes Labeled with 111In and 68Ga

Radionuclide molecular imaging is a promising tool for visualization of cancer associated molecular abnormalities in vivo and stratification of patients for specific therapies. ADAPT is a new type of small engineered proteins based on the scaffold of an albumin binding domain of protein G. ADAPTs have been utilized to select and develop high affinity binders to different proteinaceous targets. ADAPT6 binds to human epidermal growth factor 2 (HER2) with low nanomolar affinity and can be used for its in vivo visualization. Molecular design of ¹¹¹In-labeled anti-HER2 ADAPT has been optimized in several earlier studies. In this study, we made a direct comparison of two of the most promising variants, having either a DEAVDANS or a (HE)₃DANS sequence at the N-terminus, conjugated with a maleimido derivative of DOTA to a GSSC amino acids sequence at the C-terminus. The variants (designated DOTA-C⁵⁹-DEAVDANS-ADAPT6-GSSC and DOTA-C⁶¹-(HE)₃DANS-ADAPT6-GSSC) were stably labeled with ¹¹¹In for SPECT and ⁶⁸Ga for PET. Biodistribution of labeled ADAPT variants was evaluated in nude mice bearing human tumor xenografts with different levels of HER2 expression. Both variants enabled clear discrimination between tumors with high and low levels of HER2 expression. ¹¹¹In-labeled ADAPT6 derivatives provided higher tumor-to-organ ratios compared to ⁶⁸Ga-labeled counterparts. The best performing variant was DOTA-C⁶¹-(HE)₃DANS-ADAPT6-GSSC, which provided tumor-to-blood ratios of 208 ± 36 and 109 ± 17 at 3 h for ¹¹¹In and ⁶⁸Ga labels, respectively.

Gallium-68-labeled anti-HER2 single-chain Fv fragment: development and in vivo monitoring of HER2 expression

PURPOSE

We aimed to develop a gallium-68 (Ga-68)-labeled single-chain variable fragment (scFv) targeting the human epidermal growth factor receptor 2 (HER2) to rapidly and noninvasively evaluate the status of HER2 expression.

PROCEDURES

Anti-HER2 scFv was labeled with Ga-68 by using deferoxamine (Df) as a bifunctional chelate. Biodistribution of [(68)Ga]Df-anti-HER2 scFv was examined with tumor-bearing mice and positron emission tomography (PET) imaging was performed. The changes in HER2 expression after anti-HER2 therapy were monitored by PET imaging.

RESULTS

[(68)Ga]Df-anti-HER2 scFv was obtained with high radiochemical yield after only a 5-min reaction at room temperature. The probe showed high accumulation in HER2-positive xenografts and the intratumoral distribution of radioactivity coincided with HER2-positive regions. Furthermore, [(68)Ga]Df-anti-HER2 scFv helped visualize HER2-positive xenografts and monitor the changes in HER2 expression after anti-HER2 therapy.

CONCLUSION

[(68)Ga]Df-anti-HER2 scFv could be a promising probe to evaluate HER2 status by in vivo PET imaging, unless trastuzumab is prescribed as part of the therapy.

Comparison of two site-specifically (18)F-labeled affibodies for PET imaging of EGFR positive tumors

The epidermal growth factor receptor (EGFR) serves as an attractive target for cancer molecular imaging and therapy. Our previous positron emission tomography (PET) studies showed that the EGFR-targeting affibody molecules ⁶⁴Cu-DOTA-Z_EGFR:1907 and ¹⁸F-FBEM-Z_EGFR:1907 can discriminate between high and low EGFR-expression tumors and have the potential for patient selection for EGFR-targeted therapy. Compared with ⁶⁴Cu, ¹⁸F may improve imaging of EGFR-expression and is more suitable for clinical application, but the labeling reaction of ¹⁸F-FBEM-Z_EGFR:1907 requires a long synthesis time. The aim of the present study is to develop a new generation of ¹⁸F labeled affibody probes (Al¹⁸F-NOTA-Z_EGFR:1907 and ¹⁸F-CBT-Z_EGFR:1907) and to determine whether they are suitable agents for imaging of EGFR expression. The first approach consisted of conjugating Z_EGFR:1907 with NOTA and radiolabeling with Al¹⁸F to produce Al¹⁸F-NOTA-Z_EGFR:1907. In a second approach the prosthetic group ¹⁸F-labeled-2-cyanobenzothiazole (¹⁸F-CBT) was conjugated to Cys-Z_EGFR:1907 to produce ¹⁸F-CBT-Z_EGFR:1907. Binding affinity and specificity of Al¹⁸F-NOTA-Z_EGFR:1907 and ¹⁸F-CBT-Z_EGFR:1907 to EGFR were evaluated using A431 cells. Biodistribution and PET studies were conducted on mice bearing A431 xenografts after injection of Al¹⁸F-NOTA-Z_EGFR:1907 or ¹⁸F-CBT-Z_EGFR:1907 with or without coinjection of unlabeled affibody proteins. The radiosyntheses of Al¹⁸F-NOTA-Z_EGFR:1907 and ¹⁸F-CBT-Z_EGFR:1907 were completed successfully within 40 and 120 min with a decay-corrected yield of 15% and 41% using a 2-step, 1-pot reaction and 2-step, 2-pot reaction, respectively. Both probes bound to EGFR with low nanomolar affinity in A431 cells. Although ¹⁸F-CBT-Z_EGFR:1907 showed instability in vivo, biodistribution studies revealed rapid and high tumor accumulation and quick clearance from normal tissues except the bones. In contrast, Al¹⁸F-NOTA-Z_EGFR:1907 demonstrated high in vitro and in vivo stability, high tumor uptake, and relative low uptake in most of the normal organs except the liver and kidneys at 3 h after injection. The specificity of both probes for A431 tumors was confirmed by their lower uptake on coinjection of unlabeled affibody. PET studies showed that Al¹⁸F-NOTA-Z_EGFR:1907 and ¹⁸F-CBT-Z_EGFR:1907 could clearly identify EGFR positive tumors with good contrast. Two strategies for ¹⁸F-labeling of affibody molecules were successfully developed as two model platforms using NOTA or CBT coupling to affibody molecules that contain an N-terminal cysteine. Al¹⁸F-NOTA-Z_EGFR:1907 and ¹⁸F-CBT-Z_EGFR:1907 can be reliably obtained in a relatively short time. Biodistribution and PET studies demonstrated that Al¹⁸F-NOTA-Z_EGFR:1907 is a promising PET probe for imaging EGFR expression in living mice.

Improved tumor targeting of anti-HER2 nanobody through N-succinimidyl 4-guanidinomethyl-3-iodobenzoate radiolabeling

Nanobodies are approximately 15-kDa proteins based on the smallest functional fragments of naturally occurring heavy chain-only antibodies and represent an attractive platform for the development of molecularly targeted agents for cancer diagnosis and therapy. Because the human epidermal growth factor receptor type 2 (HER2) is overexpressed in breast and ovarian carcinoma, as well as in other malignancies, HER2-specific Nanobodies may be valuable radiodiagnostics and therapeutics for these diseases. The aim of the present study was to evaluate the tumor-targeting potential of anti-HER2 5F7GGC Nanobody after radioiodination with the residualizing agent N-succinimidyl 4-guanidinomethyl 3-(125/131)I-iodobenzoate (*I-SGMIB).

METHODS

The 5F7GGC Nanobody was radiolabeled using *I-SGMIB and, for comparison, with N(ε)-(3-*I-iodobenzoyl)-Lys(5)-N(α)-maleimido-Gly(1)-GEEEK (*I-IB-Mal-d-GEEEK), another residualizing agent, and by direct radioiodination using IODO-GEN ((125)I-Nanobody). The 3 labeled Nanobodies were evaluated in affinity measurements, and paired-label internalization assays were performed on HER2-expressing BT474M1 breast carcinoma cells and in paired-label tissue distribution measurements in mice bearing subcutaneous BT474M1 xenografts.

RESULTS

*I-SGMIB-Nanobody was produced in 50.4% ± 3.6% radiochemical yield and exhibited a dissociation constant of 1.5 ± 0.5 nM. Internalization assays demonstrated that intracellular retention of radioactivity was up to 1.5-fold higher for *I-SGMIB-Nanobody than for coincubated (125)I-Nanobody or *I-IB-Mal-d-GEEEK-Nanobody. Peak tumor uptake for *I-SGMIB-Nanobody was 24.50% ± 9.89% injected dose/g at 2 h, 2- to 4-fold higher than observed with other labeling methods, and was reduced by 90% with trastuzumab blocking, confirming the HER2 specificity of localization. Moreover, normal-organ clearance was fastest for *I-SGMIB-Nanobody, such that tumor-to-normal-organ ratios greater than 50:1 were reached by 24 h in all tissues except lungs and kidneys, for which the values were 10.4 ± 4.5 and 5.2 ± 1.5, respectively.

CONCLUSION

Labeling anti-HER2 Nanobody 5F7GGC with *I-SGMIB yields a promising new conjugate for targeting HER2-expressing malignancies. Further research is needed to determine the potential utility of *I-SGMIB-5F7GGC labeled with (124)I, (123)I, and (131)I for PET and SPECT imaging and for targeted radiotherapy, respectively.

Prospective of ⁶⁸Ga-radiopharmaceutical development

Positron Emission Tomography (PET) experienced accelerated development and has become an established method for medical research and clinical routine diagnostics on patient individualized basis. Development and availability of new radiopharmaceuticals specific for particular diseases is one of the driving forces of the expansion of clinical PET. The future development of the ⁶⁸Ga-radiopharmaceuticals must be put in the context of several aspects such as role of PET in nuclear medicine, unmet medical needs, identification of new biomarkers, targets and corresponding ligands, production and availability of ⁶⁸Ga, automation of the radiopharmaceutical production, progress of positron emission tomography technologies and image analysis methodologies for improved quantitation accuracy, PET radiopharmaceutical regulations as well as advances in radiopharmaceutical chemistry. The review presents the prospects of the ⁶⁸Ga-based radiopharmaceutical development on the basis of the current status of these aspects as well as wide range and variety of imaging agents.

Alternative Non-Antibody Protein Scaffolds for Molecular Imaging of Cancer

The development of improved methods for early detection and characterization of cancer presents a major clinical challenge. One approach that has shown excellent potential in preclinical and clinical evaluation is molecular imaging with small-scaffold, non-antibody based, engineered proteins. These novel diagnostic agents produce high contrast images due to their fast clearance from the bloodstream and healthy tissues, can be evolved to bind a multitude of cancer biomarkers, and are easily functionalized by site-specific bioconjugation methods. Several small protein scaffolds have been verified for in vivo molecular imaging including affibodies and their two-helix variants, knottins, fibronectins, DARPins, and several natural ligands. Further, the biodistribution of these engineered ligands can be optimized through rational mutation of the conserved regions, careful selection and placement of chelator, and modification of molecular size.

Microfluidics for synthesis of peptide-based PET tracers

Positron emission tomography (PET) is a powerful noninvasive tool for acquisition of the physiological parameters in human and animals with the help of PET tracers. Among all the PET tracers, radiolabeled peptides have been widely explored for cancer-related receptor imaging due to their high affinity and specificity to receptors. But radiochemistry procedures for production of peptide-based PET tracers are usually complex, which makes large-scale clinical studies relatively challenging. New radiolabeling technologies which could simplify synthesis and purification procedures, are extremely needed. Over the last decade, microfluidics and lab-on-a-chip (LOC) technology have boomed as powerful tools in the field of organic chemistry, which potentially provide significant help to the PET chemistry. In this minireview, microfluidic radiolabeling technology is described and its application for synthesis of peptide-based PET tracers is summarized and discussed.

Imaging EGFR and HER2 by PET and SPECT: a review

In an effort to discover a noninvasive method for predicting which cancer patients will benefit from therapy targeting the EGFR and HER2 proteins, a large body of the research has been conducted toward the development of PET and SPECT imaging agents, which selectively target these receptors. We provide a general overview of the advances made toward imaging EGFR and HER2, detailing the investigation of PET and SPECT imaging agents ranging in size from small molecules to monoclonal antibodies.

Design, synthesis and characterization of peptidomimetic conjugate of BODIPY targeting HER2 protein extracellular domain

Among the EGFRs, HER2 is a major heterodimer partner and also has important implications in the formation of particular tumors. Interaction of HER2 protein with other EGFR proteins can be modulated by small molecule ligands and, hence, these protein-protein interactions play a key role in biochemical reactions related to control of cell growth. A peptidomimetic (compound 5-1) that binds to HER2 protein extracellular domain and inhibits protein-protein interactions of EGFRs was conjugated with BODIPY (4,4-difluoro-5,7-dimethyl-4-bora-3a,4a-diaza-s-indacene). Conjugation of BODIPY to the peptidomimetic was investigated by different approaches. The conjugate was characterized for its ability to bind to HER2 overexpressing SKBR-3 and BT-474 cells. Furthermore, cellular uptake of conjugate of BODIPY was studied in the presence of membrane tracker and Lyso tracker using confocal microscopy. Our results suggested that fluorescently labeled compound 5-7 binds to the extracellular domain and stays in the membrane for nearly 24 h. After 24 h there is an indication of internalization of the conjugate. Inhibition of protein-protein interaction and downstream signaling effect of compound 5-1 was also studied by proximity ligation assay and Western blot analysis. Results suggested that compound 5-1 inhibit protein-protein interactions of HER2-HER3 and phosphorylation of HER2 in a time-dependent manner.

The diversity of (68)Ga-based imaging agents

Development of new radiopharmaceuticals and their availability are crucial factors influencing the expansion of clinical nuclear medicine. The number of new (68)Ga-based imaging agents for positron emission tomography (PET) is increasing greatly. (68)Ga has been used for labeling of a broad range of molecules (small organic molecules, peptides, proteins, and oligonucleotides) as well as particles, thus demonstrating its potential to become a PET analog of the legendary generator-produced gamma-emitting (99m)Tc but with added value of higher sensitivity and resolution as well as quantitation and dynamic scanning. Further, the availability of technology for GMP-compliant automated tracer production can facilitate the introduction of new radiopharmaceuticals and enable standardized, harmonized multicenter studies to be conducted for regulatory approval. This chapter presents some examples of tracers for targeted, pretargeted, and nontargeted imaging with emphasis on the potential of (68)Ga to facilitate clinically practical PET development and to promote the PET technique worldwide for earlier and better diagnostics, and personalized medicine with the ultimate objective of improved therapeutic outcome.

A novel radiofluorinated agouti-related protein for tumor angiogenesis imaging

A novel protein scaffold based on the cystine knot domain of the agouti-related protein (AgRP) has been used to engineer mutants that can bind to the α(v)β(3) integrin receptor with high affinity and specificity. In the current study, an (18)F-labeled AgRP mutant (7C) was prepared and evaluated as a positron emission tomography (PET) probe for imaging tumor angiogenesis. AgRP-7C was synthesized by solid phase peptide synthesis and site-specifically conjugated with 4-nitrophenyl 2-(18/19)F-fluoropropionate ((18/19)F-NFP) to produce the fluorinated peptide, (18/19)F-FP-AgRP-7C. Competition binding assays were used to measure the relative affinities of AgRP-7C and (19)F-FP-AgRP-7C to human glioblastoma U87MG cells that overexpress α(v)β(3) integrin. In addition, biodistribution, metabolic stability, and small animal PET imaging studies were conducted with (18)F-FP-AgRP-7C using U87MG tumor-bearing mice. Both AgRP-7C and (19)F-FP-AgRP-7C specifically competed with (125)I-echistatin for binding to U87MG cells with half maximal inhibitory concentration (IC(50)) values of 9.40 and 8.37 nM, respectively. Non-invasive small animal PET imaging revealed that (18)F-FP-AgRP-7C exhibited rapid and good tumor uptake (3.24 percentage injected dose per gram [% ID/g] at 0.5 h post injection [p.i.]). The probe was rapidly cleared from the blood and from most organs, resulting in excellent tumor-to-normal tissue contrasts. Tumor uptake and rapid clearance were further confirmed with biodistribution studies. Furthermore, co-injection of (18)F-FP-AgRP-7C with a large molar excess of blocking peptide c(RGDyK) significantly inhibited tumor uptake in U87MG xenograft models, demonstrating the integrin-targeting specificity of the probe. Metabolite assays showed that the probe had high stability, making it suitable for in vivo applications. (18)F-FP-AgRP-7C exhibits promising in vivo properties such as rapid tumor targeting, good tumor uptake, and excellent tumor-to-normal tissue ratios, and warrants further investigation as a novel PET probe for imaging tumor angiogenesis.

A molecular imaging primer: modalities, imaging agents, and applications

Molecular imaging is revolutionizing the way we study the inner workings of the human body, diagnose diseases, approach drug design, and assess therapies. The field as a whole is making possible the visualization of complex biochemical processes involved in normal physiology and disease states, in real time, in living cells, tissues, and intact subjects. In this review, we focus specifically on molecular imaging of intact living subjects. We provide a basic primer for those who are new to molecular imaging, and a resource for those involved in the field. We begin by describing classical molecular imaging techniques together with their key strengths and limitations, after which we introduce some of the latest emerging imaging modalities. We provide an overview of the main classes of molecular imaging agents (i.e., small molecules, peptides, aptamers, engineered proteins, and nanoparticles) and cite examples of how molecular imaging is being applied in oncology, neuroscience, cardiology, gene therapy, cell tracking, and theranostics (therapy combined with diagnostics). A step-by-step guide to answering biological and/or clinical questions using the tools of molecular imaging is also provided. We conclude by discussing the grand challenges of the field, its future directions, and enormous potential for further impacting how we approach research and medicine.

In vivo targeting of HER2-positive tumor using 2-helix affibody molecules

Molecular imaging of human epidermal growth factor receptor type 2 (HER2) expression has drawn significant attention because of the unique role of the HER2 gene in diagnosis, therapy and prognosis of human breast cancer. In our previous research, a novel cyclic 2-helix small protein, MUT-DS, was discovered as an anti-HER2 Affibody analog with high affinity through rational protein design and engineering. MUT-DS was then evaluated for positron emission tomography (PET) of HER2-positive tumor by labeling with two radionuclides, 68Ga and 18F, with relatively short half-life (t1/2<2 h). In order to fully study the in vivo behavior of 2-helix small protein and demonstrate that it could be a robust platform for labeling with a variety of radionuclides for different applications, in this study, MUT-DS was further radiolabeled with 64Cu or 111In and evaluated for in vivo targeting of HER2-positive tumor in mice. Design 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) conjugated MUT-DS (DOTA-MUT-DS) was chemically synthesized using solid phase peptide synthesizer and I2 oxidation. DOTA-MUT-DS was then radiolabeled with 64Cu or 111In to prepare the HER2 imaging probe (64Cu/111In-DOTA-MUT-DS). Both biodistribution and microPET imaging of the probe were evaluated in nude mice bearing subcutaneous HER2-positive SKOV3 tumors. DOTA-MUT-DS could be successfully synthesized and radiolabeled with 64Cu or 111In. Biodistribution study showed that tumor uptake value of 64Cu or 111In-labeled DOTA-MUT-DS was 4.66±0.38 or 2.17±0.15%ID/g, respectively, in nude mice bearing SKOV3 xenografts (n=3) at 1 h post-injection (p.i.). Tumor-to-blood and tumor-to-muscle ratios for 64Cu-DOTA-MUT-DS were attained to be 3.05 and 3.48 at 1 h p.i., respectively, while for 111In-DOTA-MUT-DS, they were 2.04 and 3.19, respectively. Co-injection of the cold Affibody molecule ZHER2:342 with 64Cu-DOTA-MUT-DS specifically reduced the SKOV3 tumor uptake of the probe by 48%. 111In-DOTA-MUT-DS displayed lower liver uptake at all the time points investigated and higher tumor to blood ratios at 4 and 20 h p.i., when compared with 64Cu-DOTA-MUT-DS. This study demonstrates that the 2-helix protein based probes, 64Cu/111In DOTA-MUT-DS, are promising molecular probes for imaging HER2-positive tumor. Two-helix small protein scaffold holds great promise as a novel and robust platform for imaging and therapy applications.

Designed hydrophilic and charge mutations of the fibronectin domain: towards tailored protein biodistribution

Engineered proteins are attractive affinity scaffolds for molecular imaging and drug delivery. Although exquisite binding specificity and affinity can be engineered, many proteins exhibit off-target uptake, particularly in the kidneys and liver, from physiologic effects. We quantified the ability to alter renal and hepatic uptake via hydrophilic and charge mutations. As a model protein, we used the 10th type III domain of human fibronectin, which has been engineered to bind many targets and has been validated for molecular imaging. We screened rational mutants, identified by structural and phylogenetic analyses, to yield eight mutations that collectively substantially increase protein hydrophilicity. Mutation of two parental clones yielded four domains with a range of hydrophilicity. These proteins were labeled with (64)Cu, injected intravenously into nu/nu mice (n = 3-5 each) and evaluated by positron emission tomography. Renal uptake strongly correlated with hydrophilicity (Pearson's correlation coefficient = 0.97), ranging from 29 ± 11 to 100 ± 22% ID/g at 1 h. Hepatic uptake inversely correlated with hydrophilicity (Pearson's correlation coefficient = -0.92), ranging from 30 ± 7 to 3 ± 1% ID/g. Thus, renal and hepatic uptake are directly tunable through hydrophilic mutation, identifiable by structural and phylogenetic analyses. To investigate charge, we mutated acidic and basic residues in both parental clones and evaluated (64)Cu-labeled mutants in nu/nu mice (n = 5-7). Selected charge removal reduced kidney signal: 78 ± 13 to 51 ± 8%ID/g (P < 0.0001) for the hydrophilic clone and 32 ± 10 to 21 ± 3 (P = 0.0005) for the hydrophobic clone. Elucidation of hydrophilicity and charge enabled modulation of background signal thereby enhancing the utility of protein scaffolds as translatable targeting agents for molecular imaging and therapy.

A phosphoramidate-based prostate-specific membrane antigen-targeted SPECT agent

BACKGROUND

Prostate-specific membrane antigen (PSMA) remains an active target for imaging and therapeutic applications for prostate cancer.

METHODS

In the present study, an irreversible phosphoramidate inhibitor, CTT-54 (IC50 = 14 nM), has been modified to deliver 99mTc-(CO)3-DTPA as a SPECT imaging payload to PSMA+ cells in vivo and in vitro. Percent uptake, competitive binding, and internalization will evaluate the imaging agent in vitro. Preliminary biodistribution and imaging will be utilized for in vivo evaluation.

RESULTS

In vitro studies demonstrate that the radiotracer 99mTc-(CO)3-DTPA-CTT-54 exhibits increasing cellular uptake in the PSMA+ LNCaP cells over time. More importantly, it was found that 99mTc-(CO)3-DTPA-CTT-54 is rapidly internalized into LNCaP cells, presumably through the PSMA enzyme-inhibitor complex. In a pilot biodistribution study, increasing accumulation of the radiotracer in LNCaP xenografts was observed from 2 to 4 hr and significant clearance from non-target tissues.

CONCLUSIONS

While DTPA may not represent the ideal chelate structure for 99mTc(CO)3, the data provides proof-of-concept support for the development of a next-generation phosphoramidate-based PSMA inhibitor-conjugates for use as SPECT imaging agents.

177Lu-DO3A-HSA-Z EGFR:1907: characterization as a potential radiopharmaceutical for radionuclide therapy of EGFR-expressing head and neck carcinomas

Epidermal growth factor receptor 1 (EGFR) is an attractive target for radionuclide therapy of head and neck carcinomas. Affibody molecules against EGFR (Z(EGFR)) show excellent tumor localizations in imaging studies. However, one major drawback is that radiometal-labeled Affibody molecules display extremely high uptakes in the radiosensitive kidneys which may impact their use as radiotherapeutic agents. The purpose of this study is to further explore whether radiometal-labeled human serum albumin (HSA)-Z(EFGR) bioconjugates display desirable profiles for the use in radionuclide therapy of EGFR-positive head and neck carcinomas. The Z(EFGR) analog, Ac-Cys-Z(EGFR:1907), was site-specifically conjugated with HSA. The resulting bioconjugate 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid (DO3A)-HSA-Z(EGFR:1907) was then radiolabeled with either (64)Cu or (177)Lu and subjected to in vitro cell uptake and internalization studies using the human oral squamous carcinoma cell line SAS. Positron emission tomography (PET), single photon emission computed tomography (SPECT), and biodistribution studies were conducted using SAS-tumor-bearing mice. Cell studies revealed a high (8.43 ± 0.55 % at 4 h) and specific (0.95 ± 0.09 % at 4 h) uptake of (177)Lu-DO3A-HSA-Z(EGFR:1907) as determined by blocking with nonradioactive Z(EGFR:1907). The internalization of (177)Lu-DO3A-HSA-Z(EGFR:1907) was verified in vitro and found to be significantly higher than that of (177)Lu-labeled Z(EFGR) at 2-24 h of incubation. PET and SPECT studies showed good tumor imaging contrasts. The biodistribution of (177)Lu-DO3A-HSA-Z(EGFR:1907) in SAS-tumor-bearing mice displayed high tumor uptake (5.1 ± 0.44 % ID/g) and liver uptake (31.5 ± 7.66 % ID/g) and moderate kidney uptake (8.5 ± 1.08 % ID/g) at 72 h after injection. (177)Lu-DO3A-HSA-Z(EGFR:1907) shows promising in vivo profiles and may be a potential radiopharmaceutical for radionuclide therapy of EGFR-expressing head and neck carcinomas.

Optical imaging with her2-targeted affibody molecules can monitor hsp90 treatment response in a breast cancer xenograft mouse model

PURPOSE

To determine whether optical imaging can be used for in vivo therapy response monitoring as an alternative to radionuclide techniques. For this, we evaluated the known Her2 response to 17-dimethylaminoethylamino-17-demethoxygeldanamycin hydrochloride (17-DMAG) treatment, an Hsp90 inhibitor.

EXPERIMENTAL DESIGN

After in vitro 17-DMAG treatment response evaluation of MCF7 parental cells and 2 HER2-transfected clones (clone A medium, B high Her2 expression), we established human breast cancer xenografts in nude mice (only parental and clone B) for in vivo evaluation. Mice received 120 mg/kg of 17-DMAG in 4 doses at 12-hour intervals intraperitonially (n = 14) or PBS as carrier control (n = 9). Optical images were obtained both pretreatment (day 0) and posttreatment (day 3, 6, and 9), always 5 hours postinjection of 500 pmol of anti-Her2 Affibody-AlexaFluor680 via tail vein (with preinjection background subtraction). Days 3 and 9 in vivo optical imaging signal was further correlated with ex vivo Her2 levels by Western blot after sacrifice.

RESULTS

Her2 expression decreased with 17-DMAG dose in vitro. In vivo optical imaging signal was reduced by 22.5% in clone B (P = 0.003) and by 9% in MCF7 parental tumors (P = 0.23) 3 days after 17-DMAG treatment; optical imaging signal recovered in both tumor types at days 6 to 9. In the carrier group, no signal reduction was observed. Pearson correlation of in vivo optical imaging signal with ex vivo Her2 levels ranged from 0.73 to 0.89.

CONCLUSIONS

Optical imaging with an affibody can be used to noninvasively monitor changes in Her2 expression in vivo as a response to treatment with an Hsp90 inhibitor, with results similar to response measurements in positron emission tomography imaging studies.

68Ga-labeling and in vivo evaluation of a uPAR binding DOTA- and NODAGA-conjugated peptide for PET imaging of invasive cancers

INTRODUCTION

The urokinase-type plasminogen activator receptor (uPAR) is a well-established biomarker for tumor aggressiveness and metastatic potential. DOTA-AE105 and DOTA-AE105-NH(2) labeled with (64)Cu have previously been demonstrated to be able to noninvasively monitor uPAR expression using positron emission tomography (PET) in human cancer xenograft mice models. Here we introduce (68)Ga-DOTA-AE105-NH(2) and (68)Ga-NODAGA-AE105-NH(2) and evaluate their imaging properties using small-animal PET.

METHODS

Synthesis of DOTA-AE105-NH(2) and NODAGA-AE105-NH(2) was based on solid-phase peptide synthesis protocols using the Fmoc strategy. (68)GaCl(3) was eluted from a (68)Ge/(68)Ga generator. The eluate was either concentrated on a cation-exchange column or fractionated and used directly for labeling. For in vitro characterization of both tracers, partition coefficient, buffer and plasma stability, uPAR binding affinity and cell uptake were determined. To characterize the in vivo properties, dynamic microPET imaging was carried out in nude mice bearing human glioma U87MG tumor xenograft.

RESULTS

In vitro experiments revealed uPAR binding affinities in the lower nM range for both conjugated peptides and identical to AE105. Labeling of DOTA-AE105-NH(2) and NODAGA-AE105-NH(2) with (68)Ga was done at 95°C and room temperature, respectively. The highest radiochemical yield and purity were obtained using fractionated elution, whereas a negative effect of acetone on labeling efficiency for NODAGA-AE105-NH(2) was observed. Good stability in phosphate-buffered saline and mouse plasma was observed. High cell uptake was found for both tracers in U87MG tumor cells. Dynamic microPET imaging demonstrated good tumor-to-background ratio for both tracers. Tumor uptake was 2.1% ID/g and 1.3% ID/g 30 min postinjection and 2.0% ID/g and 1.1% ID/g 60 min postinjection for (68)Ga-NODAGA-AE105-NH(2) and (68)Ga-DOTA-AE105-NH(2), respectively. A significantly higher tumor-to-muscle ratio (P<.05) was found for (68)Ga-NODAGA-AE105-NH(2) 60 min postinjection.

CONCLUSIONS

The use of (68)Ga-DOTA-AE105-NH(2) and (68)Ga-NODAGA-AE105-NH(2) as the first gallium-68 labeled uPAR radiotracers for noninvasive PET imaging is reported, which combine versatility with good imaging properties. These new tracers thus constitute an interesting alternative to the (64)Cu-labeled version ((64)Cu-DOTA-AE105 and 64Cu-DOTA-AE105-NH(2)) for detecting uPAR expression in tumor tissue. In our hands, the fractionated elution approach was superior for labeling of peptides, and (68)Ga-NODAGA-AE105-NH(2) is the favored tracer as it provides the highest tumor-to-background ratio.

The development of peptide-based tools for the analysis of angiogenesis

Limitations to the application of molecularly targeted cancer therapies are the inability to accurately match patient with effective treatment and the absence of a prompt readout of posttreatment response. Noninvasive agents that rapidly report vascular endothelial growth factor (VEGF) levels using positron emission tomography (PET) have the potential to enhance anti-angiogenesis therapies. Using phage display, two distinct classes of peptides were identified that bind to VEGF with nanomolar affinity and high selectivity. Co-crystal structures of these different peptide classes demonstrate that both bind to the receptor-binding region of VEGF. (18)F-radiolabelling of these peptides facilitated the acquisition of PET images of tumor VEGF levels in a HM7 xenograph model. The images obtained from one 59-residue probe, (18)F-Z-3B, 2 hr postinjection are comparable to those obtained with anti-VEGF antibody B20 72 hr postinjection. Furthermore, VEGF levels in growing SKOV3 tumors were followed using (18)F-Z-3B as a PET probe with VEGF levels increasing with tumor size.

68Ga-DOTA-affibody molecule for in vivo assessment of HER2/neu expression with PET

PURPOSE

Overexpression of HER2/neu in breast cancer is correlated with a poor prognosis. It may vary between primary tumors and metastatic lesions and change during the treatment. Therefore, there is a need for a new means to assess HER2/neu expression in vivo. In this work, we used (68)Ga-labeled DOTA-Z(HER2:2891)-Affibody to monitor HER2/neu expression in a panel of breast cancer xenografts.

METHODS

DOTA-Z(HER2:2891)-Affibody molecules were labeled with (68)Ga. In vitro binding was characterized by a receptor saturation assay. Biodistribution and PET imaging studies were conducted in athymic nude mice bearing subcutaneous human breast cancer tumors with three different levels of HER2/neu expression. Nonspecific uptake was analyzed using non-HER2-specific Affibody molecules. Signal detected by PET was compared with ex vivo assessment of the tracer uptake and HER2/neu expression.

RESULTS

The (68)Ga-DOTA-Z(HER2:2891)-Affibody probe showed high binding affinity to MDA-MB-361 cells (K (D) = 1.4 ± 0.19 nM). In vivo biodistribution and PET imaging studies demonstrated high radioactivity uptake in HER2/neu-positive tumors. Tracer was eliminated quickly from the blood and normal tissues, resulting in high tumor-to-blood ratios. The highest concentration of radioactivity in normal tissue was seen in the kidneys (227 ± 14%ID/g). High-contrast PET images of HER2/neu-overexpressing tumors were recorded as soon as 1 h after tracer injection. A good correlation was observed between PET imaging, biodistribution estimates of tumor tracer concentration, and the receptor expression.

CONCLUSION

These results suggest that PET imaging using (68)Ga-DOTA-Z(HER2:2891)-Affibody is sensitive enough to detect different levels of HER2/neu expression in vivo.

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.

Protein scaffold-based molecular probes for cancer molecular imaging

Protein scaffold molecules are powerful reagents for targeting various cell signal receptors, enzymes, cytokines and other cancer-related molecules. They belong to the peptide and small protein platform with distinct properties. For the purpose of development of new generation molecular probes, various protein scaffold molecules have been labeled with imaging moieties and evaluated both in vitro and in vivo. Among the evaluated probes Affibody molecules and analogs, cystine knot peptides, and nanobodies have shown especially good characteristics as protein scaffold platforms for development of in vivo molecular probes. Quantitative data obtained from positron emission tomography, single photon emission computed tomography/CT, and optical imaging together with biodistribution studies have shown high tumor uptakes and high tumor-to-blood ratios for these probes. High tumor contrast imaging has been obtained within 1 h after injection. The success of those molecular probes demonstrates the adequacy of protein scaffold strategy as a general approach in molecular probe development.

A novel 18F-labeled two-helix scaffold protein for PET imaging of HER2-positive tumor

PURPOSE

Two-helix scaffold proteins (~ 5 kDa) against human epidermal growth factor receptor type 2 (HER2) have been discovered in our previous work. In this research we aimed to develop an (18)F-labeled two-helix scaffold protein for positron emission tomography (PET) imaging of HER2-positive tumors.

METHODS

An aminooxy-functionalized two-helix peptide (AO-MUT-DS) with high HER2 binding affinity was synthesized through conventional solid phase peptide synthesis. The purified linear peptide was cyclized by I(2) oxidation to form a disulfide bridge. The cyclic peptide was then conjugated with a radiofluorination synthon, 4-(18)F-fluorobenzyl aldehyde ((18)F-FBA), through the aminooxy functional group at the peptide N terminus (30% yield, non-decay corrected). The binding affinities of the peptides were analyzed by Biacore analysis. Cell uptake assay of the resulting PET probe, (18)F-FBO-MUT-DS, was performed at 37°C. (18)F-FBO-MUT-DS with high specific activity (20-32 MBq/nmol, 88-140 μCi/μg, end of synthesis) was injected into mice xenograft model bearing SKOV3 tumor. MicroPET and biodistribution and metabolic stability studies were then conducted.

RESULTS

Cell uptake assays showed high and specific cell uptake (~12% applied activity at 1 h) by incubation of (18)F-FBO-MUT-DS with HER2 high-expressing SKOV3 ovarian cancer cells. The affinities (K(D)) of AO-MUT-DS and FBO-MUT-DS as tested by Biacore analysis were 2 and 1 nM, respectively. In vivo small animal PET demonstrated fast tumor targeting, high tumor accumulation, and good tumor to normal tissue contrast of (18)F-FBO-MUT-DS. Biodistribution studies further revealed that the probe had excellent tumor uptake (6.9%ID/g at 1 h post-injection) and was cleared through both liver and kidneys. Co-injection of the probe with 500 μg of HER2 Affibody protein reduced the tumor uptake (6.9 vs 1.8%ID/g, p < 0.05).

CONCLUSION

F-FBO-MUT-DS displays excellent HER2 targeting ability and tumor PET imaging quality. The two-helix scaffold proteins are suitable for development of (18)F-based PET probes.

Phage display and molecular imaging: expanding fields of vision in living subjects

In vivo molecular imaging enables non-invasive visualization of biological processes within living subjects, and holds great promise for diagnosis and monitoring of disease. The ability to create new agents that bind to molecular targets and deliver imaging probes to desired locations in the body is critically important to further advance this field. To address this need, phage display, an established technology for the discovery and development of novel binding agents, is increasingly becoming a key component of many molecular imaging research programs. This review discusses the expanding role played by phage display in the field of molecular imaging with a focus on in vivo applications. Furthermore, new methodological advances in phage display that can be directly applied to the discovery and development of molecular imaging agents are described. Various phage library selection strategies are summarized and compared, including selections against purified target, intact cells, and ex vivo tissue, plus in vivo homing strategies. An outline of the process for converting polypeptides obtained from phage display library selections into successful in vivo imaging agents is provided, including strategies to optimize in vivo performance. Additionally, the use of phage particles as imaging agents is also described. In the latter part of the review, a survey of phage-derived in vivo imaging agents is presented, and important recent examples are highlighted. Other imaging applications are also discussed, such as the development of peptide tags for site-specific protein labeling and the use of phage as delivery agents for reporter genes. The review concludes with a discussion of how phage display technology will continue to impact both basic science and clinical applications in the field of molecular imaging.

Radiolabeled affibody-albumin bioconjugates for HER2-positive cancer targeting

Affibody molecules have received significant attention in the fields of molecular imaging and drug development. However, Affibody scaffolds display an extremely high renal uptake, especially when modified with chelators and then labeled with radiometals. This unfavorable property may impact their use as radiotherapeutic agents in general and as imaging probes for the detection of tumors adjacent to kidneys in particular. Herein, we present a simple and generalizable strategy for reducing the renal uptake of Affibody molecules while maintaining their tumor uptake. Human serum albumin (HSA) was consecutively modified by 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid mono-N-hydroxysuccinimide ester (DOTA-NHS ester) and the bifunctional cross-linker sulfosuccinimidyl 4-[N-maleimidomethyl]cyclohexane-1-carboxylate (Sulfo-SMCC). The HER2 Affibody analogue, Ac-Cys-Z(HER2:342), was covalently conjugated with HSA, and the resulting bioconjugate DOTA-HSA-Z(HER2:342) was further radiolabeled with ⁶⁴Cu and ¹¹¹In and evaluated in vitro and in vivo. Radiolabeled DOTA-HSA-Z(HER2:342) conjugates displayed a significant and specific cell uptake into SKOV3 cell cultures. Positron emission tomography (PET) investigations using ⁶⁴Cu-DOTA-HSA-Z(HER2:342) were performed in SKOV3 tumor-bearing nude mice. High tumor uptake values (>14% ID/g at 24 and 48 h) and high liver accumulations but low kidney accumulations were observed. Biodistribution studies and single-photon emission computed tomography (SPECT) investigations using ¹¹¹In-DOTA-HSA-Z(HER2:342) validated these results. At 24 h post injection, the biodistribution data revealed high tumor (16.26% ID/g) and liver (14.11% ID/g) uptake but relatively low kidney uptake (6.06% ID/g). Blocking studies with coinjected, nonlabeled Ac-Cys-Z(HER2:342) confirmed the in vivo specificity of HER2. Radiolabeled DOTA-HSA-Z(HER2:342) Affibody conjugates are promising SPECT and PET-type probes for the imaging of HER2 positive cancer. More importantly, DOTA-HSA-Z(HER2:342) is suitable for labeling with therapeutic radionuclides (e.g., ⁹⁰Y or ¹⁷⁷Lu) for treatment studies. The approach of using HSA to optimize the pharmacokinetics and biodistribution profile of Affibodies may be extended to the design of many other targeting molecules.

Affibody-functionalized gold-silica nanoparticles for Raman molecular imaging of the epidermal growth factor receptor

The affibody functionalization of fluorescent surface-enhanced Raman scattering gold-silica nanoparticles as multimodal contrast agents for molecular imaging specific to epidermal growth factor receptor (EGFR) is reported. This nanoparticle bioconjugate reports EGFR-positive A431 tumors with a signal nearly 35-fold higher than EGFR-negative MDA-435S tumors. The low-level EGFR expression in adjacent healthy tissue is 7-fold lower than in the positive tumors. Validation via competitive inhibition reduces the signal by a factor of six, and independent measurement of EGFR via flow cytometry correlates at R(2) = 0.92.

Affibody-functionalized gold-silica nanoparticles for Raman molecular imaging of the epidermal growth factor receptor

The affibody functionalization of fluorescent surface-enhanced Raman scattering gold-silica nanoparticles as multimodal contrast agents for molecular imaging specific to epidermal growth factor receptor (EGFR) is reported. This nanoparticle bioconjugate reports EGFR-positive A431 tumors with a signal nearly 35-fold higher than EGFR-negative MDA-435S tumors. The low-level EGFR expression in adjacent healthy tissue is 7-fold lower than in the positive tumors. Validation via competitive inhibition reduces the signal by a factor of six, and independent measurement of EGFR via flow cytometry correlates at R(2) = 0.92.

Affibody-based nanoprobes for HER2-expressing cell and tumor imaging

This article reports the affibody-based nanoprobes specifically target and image human epidermal growth factor receptor type 2 (HER2)-expressing cells and tumors. The affibody molecules are a promising class of targeting ligands with simple, robust, and precise structure and high affinity. Using near-infrared (NIR) quantum dots (QDs) and iron oxide (IO) nanoparticles as two representative nanomaterials, we designed anti-HER2 affibody molecules with a N-terminus cysteine residue (Cysteine-Z(HER2:342)) and precisely conjugated with maleimide-functionalized nanoparticles to make nanoparticle-affibody conjugates. The in vitro and in vivo study showed the conjugates are highly specific to target and image HER2-expressing cells and tumors. This work indicated the nanoparticle-affibody conjugates may be excellent candidates as targeting probes for molecular imaging and diagnosis.

Chemical synthesis and evaluation of a backbone-cyclized minimized 2-helix Z-domain

The Z-molecule is a small, engineered IgG-binding affinity protein derived from the immunoglobulin-binding domain B of Staphylococcus aureus protein A. The Z-domain consists of 58 amino acids forming a well-defined antiparallel three-helix structure. Two of the three helices are involved in ligand binding, whereas the third helix provides structural support to the three-helix bundle. The small size and the stable three-helix structure are two attractive properties comprised in the Z-domain, but a further reduction in size of the protein is valuable for several reasons. Reduction in size facilitates synthetic production of any protein-based molecule, which is beneficial from an economical viewpoint. In addition, a smaller protein is easier to manipulate through chemical modifications. By omitting the third stabilizing helix from the Z-domain and joining the N- and C-termini by a native peptide bond, the affinity protein obtains the advantageous properties of a smaller scaffold and in addition becomes resistant to exoproteases. We here demonstrate the synthesis and evaluation of a novel cyclic two-helix Z-domain. The molecule has retained affinity for its target protein, is resistant to heat treatment, and lacks both N- and C-termini.

Facile labelling of an anti-epidermal growth factor receptor Nanobody with 68Ga via a novel bifunctional desferal chelate for immuno-PET

Purpose

The ∼15 kDa variable domains of camelid heavy-chain-only antibodies (called Nanobodies®) have the flexibility to be formatted as monovalent, monospecific, multivalent or multispecific single chain proteins with either fast or slow pharmacokinetics. We report the evaluation of the fast kinetic anti-epidermal growth factor receptor (EGFR) Nanobody 7D12, labelled with ⁶⁸Ga via the novel bifunctional chelate (BFC) p-isothiocyanatobenzyl-desferrioxamine (Df-Bz-NCS). Df-Bz-NCS has recently been introduced as the chelate of choice for ⁸⁹Zr immuno-positron emission tomography (PET).

Methods

Nanobody 7D12 was premodified with Df-Bz-NCS at pH 9. Radiolabelling with purified ⁶⁸Ga was performed at pH 5.0–6.5 for 5 min at room temperature. For in vitro stability measurements in storage buffer (0.25 M NaOAc with 5 mg ml⁻¹ gentisic acid, pH 5.5) at 4°C or in human serum at 37°C, a mixture of ⁶⁷Ga and ⁶⁸Ga was used. Biodistribution and immuno-PET studies of ⁶⁸Ga-Df-Bz-NCS-7D12 were performed in nude mice bearing A431 xenografts using ⁸⁹Zr-Df-Bz-NCS-7D12 as the reference conjugate.

Results

The Df-Bz-NCS chelate was conjugated to Nanobody 7D12 with a chelate to Nanobody molar substitution ratio of 0.2:1. The overall ⁶⁸Ga radiochemical yield was 55–70% (not corrected for decay); specific activity was 100–500 MBq/mg. Radiochemical purity of the conjugate was >96%, while the integrity and immunoreactivity were preserved. ^68/67Ga-Df-Bz-NCS-7D12 was stable in storage buffer as well as in human serum during a 5-h incubation period (<2% radioactivity loss). In biodistribution studies the ⁶⁸Ga-labelled Nanobody 7D12 showed high uptake in A431 tumours (ranging from 6.1 ± 1.3 to 7.2 ± 1.5%ID/g at 1–3 h after injection) and high tumour to blood ratios, which increased from 8.2 to 14.4 and 25.7 at 1, 2 and 3 h after injection, respectively. High uptake was also observed in the kidneys. Biodistribution was similar to that of the reference conjugate ⁸⁹Zr-Df-Bz-NCS-7D12. Tumours were clearly visualized in a PET imaging study.

Conclusion

Via a rapid procedure under mild conditions a ⁶⁸Ga-Nanobody was obtained that exhibited high tumour uptake and tumour to normal tissue ratios in nude mice bearing A431 xenografts. Fast kinetic ⁶⁸Ga-Nanobody conjugates can be promising tools for tumour detection and imaging of target expression.

Molecular imaging of HER2-positive breast cancer: a step toward an individualized 'image and treat' strategy

PURPOSE OF REVIEW

HER2 overexpression is correlated with aggressive tumor behavior and poor clinical outcome. Therefore, HER2 has become an important prognostic and predictive factor, as well as a target for molecular therapies. The article reviews recent advances in molecular imaging of HER2 that could facilitate individual approaches to targeted therapy of HER2-positive breast cancers.

RECENT FINDINGS

Because of the heterogeneity of breast cancer and possible discordance in HER2 status between primary tumors and distant metastases, assessment of HER2 expression by noninvasive imaging may become an important complement to immunohistochemistry or fluorescence in-situ hybridization analyses of biopsied tissue. Monoclonal antibodies such as trastuzumab and pertuzumab, or small scaffold proteins such as affibody molecules are used as HER2-targeting agents. For imaging purposes, these agents are labeled with positron or gamma-emitting radionuclides, optical dyes, or paramagnetic contrast molecules for positron emission tomography single photon emission tomography optical, and magnetic resonance imaging, respectively. HER2-specific molecular probes, combined with modern imaging techniques to provide information on HER2 expression not only in primary tumors but also in distant metastases not amenable to biopsy, may reduce problems with false negative results and, thereby, influence patient management by selecting patients that would benefit from HER2-targeted therapies.

SUMMARY

The new 'image and treat' strategy, involving assessment of target presence and distribution in an individual patient followed by optimized, target-specific drug delivery, may potentially improve efficacy of cancer treatment while reducing side effects.

Comparison of bifunctional chelates for (64)Cu antibody imaging

PURPOSE

Improved bifunctional chelates (BFCs) are needed to facilitate efficient (64)Cu radiolabeling of monoclonal antibodies (mAbs) under mild conditions and to yield stable, target-specific agents. The utility of two novel BFCs, 1-Oxa-4,7,10-triazacyclododecane-5-S-(4-isothiocyanatobenzyl)-4,7,10-triacetic acid (p-SCN-Bn-Oxo-DO3A) and 3,6,9,15-tetraazabicyclo[9.3.1]pentadeca-1(15),11,13-triene-4-S-(4-isothiocyanatobenzyl)-3,6,9-triacetic acid (p-SCN-Bn-PCTA), for mAb imaging with (64)Cu were compared to the commonly used S-2-(4-isothiocyanatobenzyl)-1,4,7,10-tetraazacyclododecane-tetraacetic acid (p-SCN-Bn-DOTA).

METHODS

The BFCs were conjugated to trastuzumab, which targets the HER2/neu receptor. (64)Cu radiolabeling of the conjugates was optimized. Receptor binding was analyzed using flow cytometry and radioassays. Finally, PET imaging and biodistribution studies were done in mice bearing either HER2/neu-positive or HER2/neu-negative tumors.

RESULTS

(64)Cu-Oxo-DO3A- and PCTA-trastuzumab were prepared at room temperature in >95% radiochemical yield (RCY) in <30 min, compared to only 88% RCY after 2 h for the preparation of (64)Cu-DOTA-trastuzumab under the same conditions. Cell studies confirmed that the immunoreactivity of the mAb was retained for each of the bioconjugates. In vivo studies showed that (64)Cu-Oxo-DO3A- and PCTA-trastuzumab had higher uptake than the (64)Cu-DOTA-trastuzumab at 24 h in HER2/neu-positive tumors, resulting in higher tumor to background ratios and better tumor images. By 40 h all three of the (64)Cu-BFC-trastuzumab conjugates allowed for clear visualization of the HER2/neu-positive tumors but not the negative control tumor.

CONCLUSION

The antibody conjugates of PCTA and Oxo-DO3A were shown to have superior (64)Cu radiolabeling efficiency and stability compared to the analogous DOTA conjugate. In addition, (64)Cu-PCTA and Oxo-DO3A antibody conjugates may facilitate earlier imaging with greater target to background ratios than the analogous (64)Cu-DOTA antibody conjugates.

Histo-proteomic profiling of formalin-fixed, paraffin-embedded tissue

In the functional proteome era, the proteomic profiling of clinicopathologic-annotated tissues is an essential step for mining and evaluating candidate biomarkers for disease. For many diseases, but especially cancer, the development of predictive biomarkers requires performing assays directly on the diseased tissue. The last decade has seen the explosion of both prognostic and predictive biomarkers in the research setting but few of these biomarkers have entered widespread clinical use. Previously, application of routine proteomic methodologies to clinical formalin-fixed and paraffin-embedded tissue specimens has provided unsatisfactory results. In this paper, we will discuss recent advancements in proteomic profiling technology for clinical applications. These approaches focus on the retention of histomorphologic information as an element of the proteomic analysis.

Targeting prostate cancer cells in vivo using a rapidly internalizing novel human single-chain antibody fragment

Human antibodies targeting prostate cancer cell surface epitopes may be useful for imaging and therapy. The objective of this study was to evaluate the tumor targeting of an internalizing human antibody fragment, a small-size platform, to provide high contrast in a mouse model of human prostate carcinoma.

METHODS

A prostate tumor-targeting single-chain antibody fragment (scFv), UA20, along with a nonbinding control scFv, N3M2, were labeled with (99m)Tc and evaluated for binding and rapid internalization into human prostate tumor cells in vitro and tumor homing in vivo using xenograft models. For the in vitro studies, the labeled UA20 scFv was incubated at 37 degrees C for 1 h with metastatic prostate cancer cells (DU145) to assess the total cellular uptake versus intracellular uptake. For the animal studies, labeled UA20 and N3M2 scFvs were administered to athymic mice implanted subcutaneously with DU145 cells. Mice were imaged with small-animal SPECT/CT with concomitant biodistribution at 1 and 3 h after injection.

RESULTS

The UA20 scFv was labeled in 55%-65% yield and remained stable in phosphate buffer within 24 h. The labeled UA20 scFv was taken up specifically by prostate tumor cells. Internalization was rapid, because incubation at 37 degrees C for less than 1 h resulted in 93% internalization of total cell-associated scFvs. In animal studies, SPECT/CT showed significant tumor uptake as early as 1 h after injection. At 3 h after injection, tumor uptake was 4.4 percentage injected dose per gram (%ID/g), significantly greater than all organs or tissues studied (liver, 2.7 %ID/g; other organs or tissues, <1 %ID/g), except the kidneys (81.4 %ID/g), giving tumor-to-blood and tumor-to-muscle ratios of 12:1 and 70:1, respectively. In contrast, the control antibody exhibited a tumor uptake of only 0.26 %ID/g, similar to that of muscle and fat. Tumor-specific targeting was evidenced by reduced tumor uptake of nearly 70% on administration of a 10-fold excess of unlabeled UA20 scFv. Kidney uptake was nonspecific, consistent with the route of excretion by scFvs.

CONCLUSION

The UA20 scFv showed rapid and specific internalization in prostate tumor cells in vitro and accumulation in prostate tumor xenografts in vivo, demonstrating the potential for future development for prostate cancer imaging and targeted therapy.