Comparative biodistribution of imaging agents for in vivo molecular profiling of disseminated prostate cancer in mice bearing prostate cancer xenografts: focus on 111In- and 125I-labeled anti-HER2 humanized monoclonal trastuzumab and ABY-025 Affibody

The emerging role of radionuclide molecular imaging of HER2 expression in breast cancer

Targeting of human epidermal growth factor type 2 (HER2) using monoclonal antibodies, antibody-drug conjugates and tyrosine kinase inhibitors extends survival of patients with HER2-expressing metastatic breast cancer. High expression of HER2 is a predictive biomarker for such specific treatment. Accurate determination of HER2 expression level is necessary for stratification of patients to targeted therapy. Non-invasive in vivo radionuclide molecular imaging of HER2 has a potential of repetitive measurements, addressing issues of heterogeneous expression and conversion of HER2 status during disease progression or in response to therapy. Imaging probes based of several classes of targeting proteins are currently in preclinical and early clinical development. Both preclinical and clinical data suggest that the most promising are imaging agents based on small proteins, such as single domain antibodies or engineered scaffold proteins. These agents permit a very specific high-contrast imaging at the day of injection.

Imaging using radiolabelled targeted proteins: radioimmunodetection and beyond

The use of radiolabelled antibodies was proposed in 1970s for staging of malignant tumours. Intensive research established chemistry for radiolabelling of proteins and understanding of factors determining biodistribution and targeting properties. The use of radioimmunodetection for staging of cancer was not established as common practice due to approval and widespread use of [¹⁸F]-FDG, which provided a more general diagnostic use than antibodies or their fragments. Expanded application of antibody-based therapeutics renewed the interest in radiolabelled antibodies. RadioimmunoPET emerged as a powerful tool for evaluation of pharmacokinetics of and target engagement by biotherapeutics. In addition to monoclonal antibodies, new radiolabelled engineered proteins have recently appeared, offering high-contrast imaging of expression of therapeutic molecular targets in tumours shortly after injection. This creates preconditions for noninvasive determination of a target expression level and stratification of patients for targeted therapies. Radiolabelled proteins hold great promise to play an important role in development and implementation of personalised targeted treatment of malignant tumours. This article provides an overview of biodistribution and tumour-seeking features of major classes of targeting proteins currently utilized for molecular imaging. Such information might be useful for researchers entering the field of the protein-based radionuclide molecular imaging.

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

Affibody molecules are the most studied class of engineered scaffold proteins (ESPs) in radionuclide molecular imaging. Attempts to use affibody molecules directly labelled with radiometals for targeted radionuclide therapy were hampered by the high uptake and retention of radioactivity in kidneys. Several promising strategies have been implemented to circumvent this problem. Here, we investigated whether a pharmacological approach targeting different components of the reabsorption system could be used to lower the uptake of [^99mTc]Tc-Z_HER:2395 affibody molecule in kidneys. Pre-injection of probenecid, furosemide, mannitol or colchicine had no influence on activity uptake in kidneys compared to the control group. Mice pre-injected with maleate and fructose had 33% and 51% reduction in the kidney-associated activity, respectively, compared to the control group. Autoradiography images showed that the accumulation of activity after [^99mTc]Tc-Z_HER2:2395 injection was in the renal cortex and that both maleate and fructose could significantly reduce it. Results from this study demonstrate that pharmacological intervention with maleate and fructose was effective in reducing the kidney uptake of affibody molecules. A presumable mechanism is the disruption of ATP-mediated cellular uptake and endocytosis processes of affibody molecules by tubular cells.

Site-Specific 89Zr- and 111In-Radiolabeling and In Vivo Evaluation of Glycan-free Antibodies by Azide-Alkyne Cycloaddition with a Non-natural Amino Acid

Antibody-drug conjugates (ADCs) are a class of targeted therapeutics consisting of a monoclonal antibody coupled to a cytotoxic payload. Various bioconjugation methods for producing site-specific ADCs have been reported recently, in efforts to improve immunoreactivity and pharmacokinetics and minimize batch variance-potential issues associated with first-generation ADCs prepared via stochastic peptide coupling of lysines or reduced cysteines. Recently, cell-free protein synthesis of antibodies incorporating para-azidomethyl phenylalanine (pAMF) at specific locations within the protein sequence has emerged as a means to generate antibody-drug conjugates with strictly defined drug-antibody-ratio, leading to ADCs with markedly improved stability, activity, and specificity. The incorporation of pAMF enables the conjugation of payloads functionalized for strain-promoted azide-alkyne cycloaddition. Here, we introduce two dibenzylcyclooctyne-functionalized bifunctional chelators that enable the incorporation of radioisotopes for positron emission tomography with ⁸⁹Zr (t_1/2 = 78.4 h, β⁺ = 395 keV (22%), γ = 897 keV) or single photon emission computed tomography with ¹¹¹In (t_1/2 = 67.3 h, γ = 171 keV (91%), 245 keV (94%)) under physiologically compatible conditions. We show that the corresponding radiolabeled conjugates with site-specifically functionalized antibodies targeting HER2 are amenable to targeted molecular imaging of HER2+ expressing tumor xenografts in mice and exhibit a favorable biodistribution profile in comparison with conventional, glycosylated antibody conjugates generated by stochastic bioconjugation.

Affibody Molecules as Targeting Vectors for PET Imaging

Affibody molecules are small (58 amino acids) engineered scaffold proteins that can be selected to bind to a large variety of proteins with a high affinity. Their small size and high affinity make them attractive as targeting vectors for molecular imaging. High-affinity affibody binders have been selected for several cancer-associated molecular targets. Preclinical studies have shown that radiolabeled affibody molecules can provide highly specific and sensitive imaging on the day of injection; however, for a few targets, imaging on the next day further increased the imaging sensitivity. A phase I/II clinical trial showed that ⁶⁸Ga-labeled affibody molecules permit an accurate and specific measurement of HER2 expression in breast cancer metastases. This paper provides an overview of the factors influencing the biodistribution and targeting properties of affibody molecules and the chemistry of their labeling using positron emitters.

Influence of Residualizing Properties of the Radiolabel on Radionuclide Molecular Imaging of HER3 Using Affibody Molecules

Human epidermal growth factor receptor type 3 (HER3) is an emerging therapeutic target in several malignancies. To select potential responders to HER3-targeted therapy, radionuclide molecular imaging of HER3 expression using affibody molecules could be performed. Due to physiological expression of HER3 in normal organs, high imaging contrast remains challenging. Due to slow internalization of affibody molecules by cancer cells, we hypothesized that labeling (HE)₃-Z_HER3:08698-DOTAGA affibody molecule with non-residualizing [¹²⁵I]-N-succinimidyl-4-iodobenzoate (PIB) label would improve the tumor-to-normal organs ratios compared to previously reported residualizing radiometal labels. The [¹²⁵I]I-PIB-(HE)₃-Z_HER3:08698-DOTAGA was compared side-by-side with [¹¹¹In]In-(HE)₃-Z_HER3:08698-DOTAGA. Both conjugates demonstrated specific high-affinity binding to HER3-expressing BxPC-3 and DU145 cancer cells. Biodistribution in mice bearing BxPC-3 xenografts at 4 and 24 h pi showed faster clearance of the [¹²⁵I]I-PIB label compared to the indium-111 label from most tissues, except blood. This resulted in higher tumor-to-organ ratios in HER3-expressing organs for [¹²⁵I]I-PIB-(HE)₃-Z_HER3:08698-DOTAGA at 4 h, providing the tumor-to-liver ratio of 2.4 ± 0.3. The tumor uptake of both conjugates was specific, however, it was lower for the [¹²⁵I]I-PIB label. In conclusion, the use of non-residualizing [¹²⁵I]I-PIB label for HER3-targeting affibody molecule provided higher tumor-to-liver ratio than the indium-111 label, however, further improvement in tumor uptake and retention is needed.

Integrating molecular nuclear imaging in clinical research to improve anticancer therapy

Effective patient selection before or early during treatment is important to increasing the therapeutic benefits of anticancer treatments. This selection process is often predicated on biomarkers, predominantly biospecimen biomarkers derived from blood or tumour tissue; however, such biomarkers provide limited information about the true extent of disease or about the characteristics of different, potentially heterogeneous tumours present in an individual patient. Molecular imaging can also produce quantitative outputs; such imaging biomarkers can help to fill these knowledge gaps by providing complementary information on tumour characteristics, including heterogeneity and the microenvironment, as well as on pharmacokinetic parameters, drug-target engagement and responses to treatment. This integrative approach could therefore streamline biomarker and drug development, although a range of issues need to be overcome in order to enable a broader use of molecular imaging in clinical trials. In this Perspective article, we outline the multistage process of developing novel molecular imaging biomarkers. We discuss the challenges that have restricted the use of molecular imaging in clinical oncology research to date and outline future opportunities in this area.

[Diagnosis of pelvic inflammatory disease: Clinical, paraclinical, imaging and laparoscopy criteria. CNGOF and SPILF Pelvic Inflammatory Diseases Guidelines]

The objective of this literature review is to update the recommendations for clinical practice about the diagnosis of pelvic inflammatory disease (PID), microbiologic diagnosis excluded. An adnexal pain or cervical motion tenderness are the signs that allow a positive diagnosis of PID (LE2). Associated signs (fever, leucorrhoea, metrorrhagia) reinforce clinical diagnosis (LE2). In a woman consulting for symptoms compatible with PID, a pelvic clinical examination is recommended (grade B). In cases of suspected PID, hyperleukocytosis associated with a high C-reactive protein suggests a complicated PID or a differential diagnosis such as acute appendicitis (LE3). The absence of hyperleukocytosis or normal CRP does not rule out the diagnosis of PID (LE1). When PID is suspected, a blood test with a blood count and a CRP test is recommended (grade C). Pelvic ultrasound scan does not contribute to the positive diagnosis of uncomplicated PID because it is insensitive and unspecific (LE3). However, ultrasound scan is recommended to look for signs of complicated PID (polymorphic collection) or differential diagnosis (grade C). Waiting for an ultrasound scan to be performed should not delay the start-up of antibiotic therapy. In case of diagnostic uncertainty, an abdominal-pelvic CT scan with contrast injection is useful for differential diagnosis of urinary, digestive or gynaecological origin (LE2). Laparoscopy is not recommended for the unique purpose of the positive diagnosis of PID (grade B).

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.

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

INTRODUCTION

We have previously developed a pretargeting approach for affibody-mediated cancer therapy based on PNA-PNA hybridization. In this article we have further developed this approach by optimizing the production of the primary agent, Z_HER2:342-SR-HP1, and labeling the secondary agent, HP2, with the therapeutic radionuclide ¹⁷⁷Lu. We also studied the biodistribution profile of ¹⁷⁷Lu-HP2 in mice, and evaluated pretargeting with ¹⁷⁷Lu-HP2 in vitro and in vivo.

METHODS

The biodistribution profile of ¹⁷⁷Lu-HP2 was evaluated in NMRI mice and compared to the previously studied ¹¹¹In-HP2. Pretargeting using ¹⁷⁷Lu-HP2 was studied in vitro using the HER2-expressing cell lines BT-474 and SKOV-3, and in vivo in mice bearing SKOV-3 xenografts.

RESULTS AND CONCLUSION

Using an optimized production protocol for Z_HER2:342-SR-HP1 the ligation time was reduced from 15h to 30min, and the yield increased from 45% to 70%. ¹⁷⁷Lu-labeled HP2 binds specifically in vitro to BT474 and SKOV-3 cells pre-treated with Z_HER2:342-SR-HP1. ¹⁷⁷Lu-HP2 was shown to have a more rapid blood clearance compared to ¹¹¹In-HP2 in NMRI mice, and the measured radioactivity in blood was 0.22±0.1 and 0.68±0.07%ID/g for ¹⁷⁷Lu- and ¹¹¹In-HP2, respectively, at 1h p.i. In contrast, no significant difference in kidney uptake was observed (4.47±1.17 and 3.94±0.58%ID/g for ¹⁷⁷Lu- and ¹¹¹In-HP2, respectively, at 1h p.i.). Co-injection with either Gelofusine or lysine significantly reduced the kidney uptake for ¹⁷⁷Lu-HP2 (1.0±0.1 and 1.6±0.2, respectively, vs. 2.97±0.87%ID/g in controls at 4h p.i.). ¹⁷⁷Lu-HP2 accumulated in SKOV-3 xenografts in BALB/C nu/nu mice when administered after injection of Z_HER2:342-SR-HP1. Without pre-injection of Z_HER2:342-SR-HP1, the uptake of ¹⁷⁷Lu-HP2 was about 90-fold lower in tumor (0.23±0.08 vs. 20.7±3.5%ID/g). The tumor-to-kidney radioactivity accumulation ratio was almost 5-fold higher in the group of mice pre-injected with Z_HER2:342-SR-HP1. In conclusion, ¹⁷⁷Lu-HP2 was shown to be a promising secondary agent for affibody-mediated tumor pretargeting in vivo.

Preclinical Evaluation of 18F-Labeled Anti-HER2 Nanobody Conjugates for Imaging HER2 Receptor Expression by Immuno-PET

The human growth factor receptor type 2 (HER2) is overexpressed in breast as well as other types of cancer. Immuno-PET, a noninvasive imaging procedure that could assess HER2 status in both primary and metastatic lesions simultaneously, could be a valuable tool for optimizing application of HER2-targeted therapies in individual patients. Herein, we have evaluated the tumor-targeting potential of the 5F7 anti-HER2 Nanobody (single-domain antibody fragment; ∼13 kDa) after (18)F labeling by 2 methods.

METHODS

The 5F7 Nanobody was labeled with (18)F using the novel residualizing label N-succinimidyl 3-((4-(4-(18)F-fluorobutyl)-1H-1,2,3-triazol-1-yl)methyl)-5-(guanidinomethyl)benzoate ((18)F-SFBTMGMB; (18)F-RL-I) and also via the most commonly used (18)F protein-labeling prosthetic agent N-succinimidyl 3-(18)F-fluorobenzoate ((18)F-SFB). For comparison, 5F7 Nanobody was also labeled using the residualizing radioiodination agent N-succinimidyl 4-guanidinomethyl-3-(125)I-iodobenzoate ((125)I-SGMIB). Paired-label ((18)F/(125)I) internalization assays and biodistribution studies were performed on HER2-expressing BT474M1 breast carcinoma cells and in mice with BT474M1 subcutaneous xenografts, respectively. Small-animal PET/CT imaging of 5F7 Nanobody labeled using (18)F-RL-I also was performed.

RESULTS

Internalization assays indicated that intracellularly retained radioactivity for (18)F-RL-I-5F7 was similar to that for coincubated (125)I-SGMIB-5F7, whereas that for (18)F-SFB-5F7 was lower than coincubated (125)I-SGMIB-5F7 and decreased with time. BT474M1 tumor uptake of (18)F-RL-I-5F7 was 28.97 ± 3.88 percentage injected dose per gram of tissue (%ID/g) at 1 h and 36.28 ± 14.10 %ID/g at 2 h, reduced by more than 90% on blocking with trastuzumab, indicating HER2 specificity of uptake, and was also 26%-28% higher (P < 0.05) than that of (18)F-SFB-5F7. At 2 h, the tumor-to-blood ratio for (18)F-RL-I-5F7 (47.4 ± 13.1) was significantly higher (P < 0.05) than for (18)F-SFB-5F7 (25.4 ± 10.3); however, kidney uptake was 28-36-fold higher for (18)F-RL-I-5F7.

CONCLUSION

(18)F-RL-I-5F7 is a promising tracer for evaluating HER2 status by immuno-PET; however, in settings in which renal background is problematic, strategies for reducing its kidney uptake may be needed.

Affibody-mediated PET imaging of HER3 expression in malignant tumours

Human epidermal growth factor receptor 3 (HER3) is involved in the progression of various cancers and in resistance to therapies targeting the HER family. In vivo imaging of HER3 expression would enable patient stratification for anti-HER3 immunotherapy. Key challenges with HER3-targeting are the relatively low expression in HER3-positive tumours and HER3 expression in normal tissues. The use of positron-emission tomography (PET) provides advantages of high resolution, sensitivity and quantification accuracy compared to SPECT. Affibody molecules, imaging probes based on a non-immunoglobulin scaffold, provide high imaging contrast shortly after injection. The aim of this study was to evaluate feasibility of PET imaging of HER3 expression using (68)Ga-labeled affibody molecules. The anti-HER3 affibody molecule HEHEHE-Z08698-NOTA was successfully labelled with (68)Ga with high yield, purity and stability. The agent bound specifically to HER3-expressing cancer cells in vitro and in vivo. At 3 h pi, uptake of (68)Ga-HEHEHE-Z08698-NOTA was significantly higher in xenografts with high HER3 expression (BT474, BxPC-3) than in xenografts with low HER3 expression (A431). In xenografts with high expression, tumour-to-blood ratios were >20, tumour-to-muscle >15, and tumour-to-bone >7. HER3-positive xenografts were visualised using microPET 3 h pi. In conclusion, PET imaging of HER3 expression is feasible using (68)Ga-HEHEHE-Z08698-NOTA shortly after administration.

Diabody Pretargeting with Click Chemistry In Vivo

Radioimmunotherapy and nuclear imaging (immuno-PET/SPECT) of cancer with radiometal-labeled antibody fragments or peptides is hampered by low tumor-to-kidney ratios because of high renal radiometal retention. Therefore, we developed and evaluated a pretargeting strategy using click chemistry in vivo to reduce kidney uptake and avoid unwanted radiation toxicity. We focused on the bioorthogonal reaction between a trans-cyclooctene (TCO)-functionalized TAG72 targeting diabody, AVP04-07, and a low-molecular-weight radiolabeled tetrazine probe that was previously shown to have low kidney retention and relatively fast renal clearance.

METHODS

AVP04-07 diabodies were functionalized with TCO tags, and in vitro immunoreactivity toward bovine submaxillary mucin and tetrazine reactivity were assessed. Next, pretargeting biodistribution studies were performed in LS174T tumor-bearing mice with AVP04-07-TCO(n) (where n indicates the number of TCO groups per diabody) and radiolabeled tetrazine to optimize the TCO modification grade (0, 1.8, or 4.7 TCO groups per diabody) and the (177)Lu-tetrazine dose (0.1, 1.0, or 10 Eq with respect to the diabody). Radiolabeled tetrazine was injected at 47 h after diabody injection, and mice were euthanized 3 h later. A pretargeting SPECT/CT study with (111)In-tetrazine was performed with the optimized conditions.

RESULTS

Immunoreactivity for native AVP04-07 was similar to that for TCO-functionalized AVP04-07, and the latter reacted efficiently with radiolabeled tetrazine in vitro. The combination of the pretargeting component AVP04-07 functionalized with 4.7 TCO groups and 1 Eq of (177)Lu-tetrazine with respect to the diabody showed the most promising biodistribution. Specifically, high (177)Lu-tetrazine tumor uptake (6.9 percentage injected dose/g) was observed with low renal retention, yielding a tumor-to-kidney ratio of 5.7. SPECT/CT imaging confirmed the predominant accumulation of radiolabeled tetrazine in the tumor and low nontumor retention.

CONCLUSION

Pretargeting provides an alternative radioimmunotherapy and nuclear imaging strategy by overcoming the high renal retention of low-molecular-weight radiometal tumor-homing agents through the separate administration of a tumor-homing agent and a radioactive probe with fast clearance.

Antibody Positron Emission Tomography Imaging in Anticancer Drug Development

More than 50 monoclonal antibodies (mAbs), including several antibody–drug conjugates, are in advanced clinical development, forming an important part of the many molecularly targeted anticancer therapeutics currently in development. Drug development is a relatively slow and expensive process, limiting the number of drugs that can be brought into late-stage trials. Development decisions could benefit from quantitative biomarkers, enabling visualization of the tissue distribution of (potentially modified) therapeutic mAbs to confirm effective whole-body target expression, engagement, and modulation and to evaluate heterogeneity across lesions and patients. Such biomarkers may be realized with positron emission tomography imaging of radioactively labeled antibodies, a process called immunoPET. This approach could potentially increase the power and value of early trials by improving patient selection, optimizing dose and schedule, and rationalizing observed drug responses. In this review, we summarize the available literature and the status of clinical trials regarding the potential of immunoPET during early anticancer drug development.

Development of a 124I-labeled version of the anti-PSMA monoclonal antibody capromab for immunoPET staging of prostate cancer: Aspects of labeling chemistry and biodistribution

Correct staging of prostate cancer is an unmet clinical need. Radionuclide targeting of prostate-specific membrane antigen (PSMA) with 111In-labeled capromab pendetide (ProstaScint) is a clinical option for prostate cancer staging. We propose the use of 124I-labeled capromab to decrease the retention of radioactivity in healthy organs (due to the non-residualizing properties of the radiolabel). The use of 124I as a label should increase imaging sensitivity due to the advantages of PET as an imaging modality. Capromab targets the intracellular domain of PSMA; accumulation of radioactivity in the tumor should not depend on internalization of the antigen/antibody complex. Capromab was iodinated, and its targeting properties were compared with indium labeled counterpart in LNCaP xenografts in dual isotope mode. PSMA-negative xenografts (PC3) were used as a negative control. Radioiodinated capromab bound to PSMA specifically. Biodistribution of 125I/111In-capromab showed a more rapid clearance of iodine radioactivity from liver, spleen, kidneys, bones, colon tissue, as well as tumors. Maximum tumor uptake (13±8% ID/g for iodine and 29±9% ID/g for indium) and tumor-to-non-tumor ratios for both agents were measured 5 days post-injection (pi). High tumor accumulation and low uptake of radioactivity in normal organs were confirmed using microPET/CT 5 days pi of 124I-capromab.

Positron emission tomography imaging with 18F-labeled ZHER2:2891 affibody for detection of HER2 expression and pharmacodynamic response to HER2-modulating therapies

PURPOSE

Expression of HER2 has profound implications on treatment strategies in various types of cancer. We investigated the specificity of radiolabeled HER2-targeting ZHER2:2891 Affibody, [(18)F]GE-226, for positron emission tomography (PET) imaging.

EXPERIMENTAL DESIGN

Intrinsic cellular [(18)F]GE-226 uptake and tumor-specific tracer binding were assessed in cells and xenografts with and without drug treatment. Specificity was further determined by comparing tumor localization of a fluorescently labeled analogue with DAKO HercepTest.

RESULTS

[(18)F]GE-226 uptake was 11- to 67-fold higher in 10 HER2-positive versus HER2-negative cell lines in vitro independent of lineage. Uptake in HER2-positive xenografts was rapid with net irreversible binding kinetics making possible the distinction of HER2-negative [MCF7 and MCF7-p95HER2: NUV60 (%ID/mL) 6.1 ± 0.7; Ki (mL/cm(3)/min) 0.0069 ± 0.0014] from HER2-positive tumors (NUV60 and Ki: MCF7-HER2, 10.9 ± 1.5 and 0.015 ± 0.0035; MDA-MB-361, 18.2 ± 3.4 and 0.025 ± 0.0052; SKOV-3, 18.7 ± 2.4 and 0.036 ± 0.0065) within 1 hour. Tumor uptake correlated with HER2 expression determined by ELISA (r(2) = 0.78), and a fluorophore-labeled tracer analogue colocalized with HER2 expression. Tracer uptake was not influenced by short-term or continuous treatment with trastuzumab in keeping with differential epitope binding, but reflected HER2 degradation by short-term NVP-AUY922 treatment in SKOV-3 xenografts (NUV60: 13.5 ± 2.1 %ID/mL vs. 9.0 ± 0.9 %ID/mL for vehicle or drug, respectively).

CONCLUSIONS

[(18)F]GE-226 binds with high specificity to HER2 independent of cell lineage. The tracer has potential utility for HER2 detection, irrespective of prior trastuzumab treatment, and to discern HSP90 inhibitor-mediated HER2 degradation.

Methods for radiolabelling of monoclonal antibodies

The use of radionuclide labels allows to study the pharmacokinetics of monoclonal antibodies, to control the specificity of their targeting and to monitor the response to an antibody treatment with high accuracy. Selection of label depends on the processing of an antibody after binding to an antigen, and on the character of information to be derived from the study (distribution of antibody in the extracellular space, target occupancy or determination of sites of metabolism). This chapter provides protocols for labelling of antibodies with iodine-125 (suitable also for other radioisotopes of iodine) and with indium-111. Since radiolabelling might damage and reduce the immunoreactive fraction and/or affinity of an antibody, the methods for assessment of these characteristics of an antibody are provided for control.

Influence of macrocyclic chelators on the targeting properties of (68)Ga-labeled synthetic affibody molecules: comparison with (111)In-labeled counterparts

Affibody molecules are a class of small (7 kDa) non-immunoglobulin scaffold-based affinity proteins, which have demonstrated substantial potential as probes for radionuclide molecular imaging. The use of positron emission tomography (PET) would further increase the resolution and quantification accuracy of Affibody-based imaging. The rapid in vivo kinetics of Affibody molecules permit the use of the generator-produced radionuclide ⁶⁸Ga (T_1/2 = 67.6 min). Earlier studies have demonstrated that the chemical nature of chelators has a substantial influence on the biodistribution properties of Affibody molecules. To determine an optimal labeling approach, the macrocyclic chelators 1,4,7,10-tetraazacylododecane-1,4,7,10-tetraacetic acid (DOTA), 1,4,7-triazacyclononane-N,N,N-triacetic acid (NOTA) and 1-(1,3-carboxypropyl)-1,4,7- triazacyclononane-4,7-diacetic acid (NODAGA) were conjugated to the N-terminus of the synthetic Affibody molecule Z_HER2:S1 targeting HER2. Affibody molecules were labeled with ⁶⁸Ga, and their binding specificity and cellular processing were evaluated. The biodistribution of ⁶⁸Ga-DOTA-Z_HER2:S1,⁶⁸Ga-NOTA-Z_HER2:S1 and ⁶⁸Ga-NODAGA-Z_HER2:S1, as well as that of their ¹¹¹In-labeled counterparts, was evaluated in BALB/C nu/nu mice bearing HER2-expressing SKOV3 xenografts. The tumor uptake for ⁶⁸Ga-DOTA-Z_HER2:S1 (17.9±0.7%IA/g) was significantly higher than for both ⁶⁸Ga-NODAGA-Z_HER2:S1(16.13±0.67%IA/g) and ⁶⁸Ga-NOTA-Z_HER2:S1 (13±3%IA/g) at 2 h after injection. ⁶⁸Ga-NODAGA-Z_HER2:S1 had the highest tumor-to-blood ratio (60±10) in comparison with both ⁶⁸Ga-DOTA-Z_HER2:S1 (28±4) and ⁶⁸Ga-NOTA-Z_HER2:S1 (42±11). The tumor-to-liver ratio was also higher for ⁶⁸Ga-NODAGA-Z_HER2:S1 (7±2) than the DOTA and NOTA conjugates (5.5±0.6 vs.3.3±0.6). The influence of chelator on the biodistribution and targeting properties was less pronounced for ⁶⁸Ga than for ¹¹¹In. The results of this study demonstrate that macrocyclic chelators conjugated to the N-terminus have a substantial influence on the biodistribution of HER2-targeting Affibody molecules labeled with ⁶⁸Ga.This can be utilized to enhance the imaging contrast of PET imaging using Affibody molecules and improve the sensitivity of molecular imaging. The study demonstrated an appreciable difference of chelator influence for ⁶⁸Ga and ¹¹¹In.

Influence of DOTA chelator position on biodistribution and targeting properties of (111)In-labeled synthetic anti-HER2 affibody molecules

Affibody molecules are a class of affinity proteins. Their small size (7 kDa) in combination with the high (subnanomolar) affinity for a number of cancer-associated molecular targets makes them suitable for molecular imaging. Earlier studies demonstrated that the selection of radionuclide and chelator may substantially influence the tumor-targeting properties of affibody molecules. Moreover, the placement of chelators for labeling of affibody molecules with (99m)Tc at different positions in affibody molecules influenced both blood clearance rate and uptake in healthy tissues. This introduces an opportunity to improve the contrast of affibody-mediated imaging. In this comparative study, 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) was conjugated to the synthetic affibody molecule Z(HER2:S1) at three different positions: DOTA-A1-Z(HER2:S1) (N-terminus), DOTA-K58-Z(HER2:S1) (C-terminus), and DOTA-K50-Z(HER2:S1) (middle of helix 3). The affinity for HER2 differed slightly among the variants and the K(D) values were determined to be 133 pM, 107 pM and 94 pM for DOTA-A1-Z(HER2:S1), DOTA-K50-Z(HER2:S1), and DOTA-K58-Z(HER2:S1), respectively. Z(HER2:S1)-K50-DOTA showed a slightly lower melting point (57 °C) compared to DOTA-A1-Z(HER2:S1) (64 °C) and DOTA-K58-Z(HER2:S1) (62 °C), but all variants showed good refolding properties after heat treatment. All conjugates were successfully labeled with (111)In resulting in a radiochemical yield of 99% with preserved binding capacity. In vitro specificity studies using SKOV-3 and LS174T cell lines showed that the binding of the radiolabeled compounds was HER2 receptor-mediated, which also was verified in vivo using BALB/C nu/nu mice with LS174T and Ramos lymphoma xenografts. The three conjugates all showed specific uptake in LS174T xenografts in nude mice, where DOTA-A1-Z(HER2:S1)and DOTA-K58-Z(HER2:S1) showed the highest uptake. Overall, DOTA-K58-Z(HER2:S1) provided the highest tumor-to-blood ratio, which is important for a high-contrast imaging. In conclusion, the positioning of the DOTA chelator influences the cellular processing and the biodistribution pattern of radiolabeled affibody molecules, creating preconditions for imaging optimization.