ImmunoPET using engineered antibody fragments: fluorine-18 labeled diabodies for same-day imaging

Development of Anti-LRRC15 Small Fragments for Imaging Purposes Using a Phage-Display ScFv Approach

The human leucine-rich repeat-containing protein 15 (LRRC15) is a membrane protein identified as a marker of CAF (cancer-associated fibroblast) cells whose overexpression is positively correlated with cancer grade and outcome. Nuclear molecular imaging (i.e., SPECT and PET) to track LRRC15 expression could be very useful in guiding further therapeutic strategies. In this study, we developed an ScFv mouse phage-display library to obtain small fragment antibodies against human LRRC15 for molecular imaging purposes. Mice were immunized with recombinant human LRRC15 (hLRRC15), and lymph node cells were harvested for ScFv (single-chain variable fragment) phage-display analysis. The built library was used for panning on cell lines with constitutive or induced expression after transfection. The choice of best candidates was performed by screening various other cell lines, using flow cytometry. The selected candidates were reformatted into Cys-ScFv or Cys-diabody by addition of cysteine, and cloned in mammalian expression vectors to obtain batches of small fragments that were further used in site-specific radiolabeling tests. The obtained library was 1.2 × 107 cfu/µg with an insertion rate >95%. The two panning rounds performed on cells permittedenrichment of 2 × 10−3. Screening with flow cytometry allowed us to identify 28 specific hLRRC15 candidates. Among these, two also recognized murine LRCC15 and were reformatted into Cys-ScFv and Cys-diabody. They were expressed transiently in a mammalian system to obtain 1.0 to 4.5 mg of Cys fragments ready for bioconjugation and radiolabeling. Thus, in this paper, we demonstrate the relevance of the phage-display ScFv library approach for the fast-track development of small antibodies for imaging and/or immunotherapy purposes.

Effect of molecular size on interstitial pharmacokinetics and tissue catabolism of antibodies

Advances in antibody engineering have enabled the construction of novel molecular formats in diverse shapes and sizes, providing new opportunities for biologic therapies and expanding the need to understand how various structural aspects affect their distribution properties. To assess the effect of antibody size on systemic pharmacokinetics (PK) and tissue distribution with or without neonatal Fc receptor (FcRn) binding, we evaluated a series of non-mouse-binding anti-glycoprotein D monoclonal antibody formats, including IgG [~150 kDa], one-armed IgG [~100 kDa], IgG-HAHQ (attenuated FcRn binding) [~150 kDa], F(ab')₂ [~100 kDa], and F(ab) [~50 kDa]. Tissue-specific concentration-time profiles were corrected for blood content based on vascular volumes and normalized based on interstitial volumes to allow estimation of interstitial concentrations and interstitial:serum concentration ratios. Blood correction demonstrated that the contribution of circulating antibody on total uptake was greatest at early time points and for highly vascularized tissues. Tissue interstitial PK largely mirrored serum exposure profiles. Similar interstitial:serum ratios were obtained for the two FcRn-binding molecules, IgG and one-armed IgG, which reached pseudo-steady-state kinetics in most tissues. For non-FcRn-binding molecules, interstitial:serum ratios changed over time, suggesting that these molecules did not reach steady-state kinetics during the study. Furthermore, concentration-time profiles of both intact and catabolized molecule were measured by a dual tracer approach, enabling quantification of tissue catabolism and demonstrating that catabolism levels were highest for IgG-HAHQ. Overall, these data sets provide insight into factors affecting preclinical distribution and may be useful in estimating interstitial concentrations and/or catabolism in human tissues.

Imaging Reveals Importance of Shape and Flexibility for Glomerular Filtration of Biologics

Advances in antibody engineering have enabled the construction of novel molecular formats in diverse shapes and sizes, providing new opportunities for cancer immunotherapeutic drug discovery while also revealing limitations in knowledge of structure-activity relationships. The current understanding of renal filtration originates largely from data reported for dextrans, IgG, albumin, and selected globular proteins. For a one-armed IgG-based T-cell imaging agent, we observed higher renal signal than typically observed for bivalent IgGs, prompting us to explore the factors governing renal filtration of biologics. We constructed a small representative library of IgG-like formats with varied shapes and hinge flexibilities falling broadly into two categories: branched molecules including bivalent IgG and (scFv)2Fc, and nonbranched molecules including one-armed IgG, one-armed IgG with stacked Fab, and one-armed IgG with a rigid IgA2 hinge. Transmission electron microscopy revealed Y-shaped structures for the branched molecules and pseudo-linear structures for the nonbranched molecules. Single-photon emission CT imaging, autoradiography, and tissue harvest studies demonstrated higher renal uptake and catabolism for nonbranched molecules relative to branched molecules. Among the nonbranched molecules, the one-armed IgG with rigid IgA2 hinge molecule demonstrated higher kidney uptake and decreased systemic exposure relative to molecules with a more flexible hinge. Our results show that differences in shape and hinge flexibility drive the increased glomerular filtration of one-armed relative to bivalent antibodies and highlight the practical advantages of using imaging to assess renal filtration properties. These findings are particularly relevant for T-cell-dependent bispecific molecules, many of which have nonstandard antibody structures.

Labeling single domain antibody fragments with 18F using a novel residualizing prosthetic agent - N-succinimidyl 3-(1-(2-(2-(2-(2-[18F]fluoroethoxy)ethoxy)ethoxy)ethyl)-1H-1,2,3-triazol-4-yl)-5-(guanidinomethyl)benzoate

INTRODUCTION

Labeling single domain antibody fragments (sdAbs) with ¹⁸F is an attractive strategy for immunoPET. Earlier, we developed a residualizing label, N-succinimidyl 3-((4-(4-fluorobutyl)-1H-1,2,3-triazol-1-yl)methyl)-5-(guanidinomethyl)benzoate ([¹⁸F]RL-I), synthesized via a click reaction for labeling sdAbs with ¹⁸F, that has attractive features but suffered from modest radiochemical yields and suboptimal hydrophobicity. Herein, we have evaluated the potential utility of an analogous agent, N-succinimidyl 3-(1-(2-(2-(2-(2-[¹⁸F]fluoroethoxy)ethoxy)ethoxy)ethyl)-1H-1,2,3-triazol-4-yl)-5-(guanidinomethyl)benzoate ([¹⁸F]SFETGMB; [¹⁸F]RL-III) designed to address these limitations.

METHODS

[¹⁸F]RL-III was synthesized by the click reaction between 3-((2,3-bis(tert-butoxycarbonyl)guanidino)methyl)-5-ethynylbenzoate and 1-azido-2-(2-(2-(2-[¹⁸F]fluoroethoxy)ethoxy)ethoxy)ethane and subsequent deprotection. The anti-HER2 sdAbs 5F7 and 2Rs15d were labeled by conjugation with [¹⁸F]RL-III and compared in a paired-label fashion to the sdAbs labeled using N-succinimidyl 4-guanidinomethyl-3-[¹²⁵I]iodobenzoate ([¹²⁵I]SGMIB) or N-succinimidyl 3-guanidinomethyl-5-[¹²⁵I]iodobenzoate (iso-[¹²⁵I]SGMIB). The ¹⁸F-labeled sdAbs were evaluated in vitro using HER2-expressing breast and ovarian carcinoma cells (BT474/BT474M1 and SKOV-3) and in vivo in athymic mice bearing subcutaneous SKOV-3 or BT474 xenografts. PET imaging of athymic mice bearing either subcutaneous BT474 or intracranial BT474M1Br-Fluc xenografts after administration of [¹⁸F]RL-III-5F7 also was performed.

RESULTS

Radiochemical yields for the synthesis of Boc₂-[¹⁸F]RL-III (21.5 ± 3.4%) were significantly higher than reported for Boc₂-[¹⁸F]RL-I. The overall radiochemical yields for the synthesis of [¹⁸F]RL-III-2Rs15d and [¹⁸F]RL-III-5F7 from aqueous [¹⁸F]fluoride were 1.7 ± 0.7% and 3.8 ± 2.3%, respectively. Both sdAbs, labeled using [¹⁸F]RL-III, retained affinity and immunoreactivity to HER2. Uptake and internalization of [¹⁸F]RL-III-5F7 in HER2-expressing cells was higher than that seen for [¹⁸F]RL-III-2Rs15d. Although different xenograft models were used, [¹⁸F]RL-III-2Rs15d showed relatively high uptake in a number of normal tissues, while uptake of [¹⁸F]RL-III-5F7 was mainly in tumor and kidneys with minimal background activity. Concordant with the necropsy experiments, microPET imaging with [¹⁸F]RL-III-5F7 in the BT474 subcutaneous model demonstrated clear delineation of the tumor (12.2 ± 5.1% ID/g) with minimal background activity except in kidneys. A tumor uptake (max) of 0.98%ID/g and a tumor-to-normal brain ratio of 9.8:1 were observed for [¹⁸F]RL-III-5F7 in the intracranial model.

CONCLUSIONS

Although higher radiochemical yields than that reported for [¹⁸F]RL-I were obtained, considerable improvements are needed for this method to be of practical utility. Despite clear tumor delineation with [¹⁸F]RL-III-5F7 as early as 1 h, high activity levels in the kidneys remain a concern.

HER2-directed antibodies, affibodies and nanobodies as drug-delivery vehicles in breast cancer with a specific focus on radioimmunotherapy and radioimmunoimaging

PURPOSE

The aim of the present paper is to review the role of HER2 antibodies, affibodies and nanobodies as vehicles for imaging and therapy approaches in breast cancer, including a detailed look at recent clinical data from antibody drug conjugates and nanobodies as well as affibodies that are currently under development.

RESULTS

Clinical and preclinical studies have shown that the use of monoclonal antibodies in molecular imaging is impaired by slow blood clearance, associated with slow and low tumor uptake and with limited tumor penetration potential. Antibody fragments, such as nanobodies, on the other hand, can be radiolabelled with short-lived radioisotopes and provide high-contrast images within a few hours after injection, allowing early diagnosis and reduced radiation exposure of patients. Even in therapy, the small radioactively labeled nanobodies prove to be superior to radioactively labeled monoclonal antibodies due to their higher specificity and their ability to penetrate the tumor.

CONCLUSION

While monoclonal antibodies are well established drug delivery vehicles, the current literature on molecular imaging supports the notion that antibody fragments, such as affibodies or nanobodies, might be superior in this approach.

Two Patient Studies of a Companion Diagnostic Immuno-Positron Emission Tomography (PET) Tracer for Measuring Human CA6 Expression in Cancer for Antibody Drug Conjugate (ADC) Therapy

An antigen binding fragment (BFab) derived from a tumor-associated mucin 1–sialoglycotope antigen (CA6) targeting antibody (huDS6) was engineered. We synthesized a companion diagnostic positron emission tomography (PET) tracer by radiolabeling BFab with [⁶⁴Cu] to measure CA6 expression on cancer tissues prior to anti-human CA6 (huDS6-DM4 antibody-drug conjugate) therapy for ovarian and breast cancer patients. After chemotherapy, the ovarian patient received PET scan with ¹⁸F-2-fluoro-2-deoxyglucose ([¹⁸F]FDG: 10 mCi), followed by [⁶⁴Cu]-DOTA-BFab ([⁶⁴Cu]BFab; 5.5 mCi) 1 week later for PET scanning of CA6 expression and subsequent surgery. The breast cancer patient was treated with chemotherapy before primary tumor resection and subsequent [¹⁸F]FDG-PET scan. 4 weeks later the patient received of [⁶⁴Cu]BFab (11.7 mCi) for CA6 PET scan. Whole body [¹⁸F]FDG-PET of the breast cancer patient indicated FDG-avid tumor metastases to the liver, bilateral hila and thoracic spine, but no uptake was observed for the ovarian patient. Each patient was also imaged by PET/CT with [⁶⁴Cu]BFab at 1 and 24 hours after tracer administration. The [⁶⁴Cu]BFab tracer was well tolerated by both patients without adverse effects, and no significant tracer uptake was observed in both patients. Immunohistochemistry (IHC) data indicated CA6 expressions were weak to intermediate and matched with the [⁶⁴Cu]BFab-PET signals.

Immune-Directed Molecular Imaging Biomarkers

Imaging has played a critical role in the management of patients with cancer. Novel therapies are emerging rapidly; however, they are effective only in some patients. With the advent of new targeted therapeutics and immunotherapy, the limitations of conventional imaging methods are becoming more evident. FDG-PET imaging is restricted to the optimal assessment of immune therapies. There is a critical unmet need for pharmacodynamic and prognostic imaging biomarkers. Radiolabeled antibodies or small molecules can allow for specific assessment of targets in expression and concentration. Several such imaging agents have been under preclinical development. Early human studies with radiolabeled monoclonal antibodies or small molecules targeted to the epidermal growth factor receptor pathway have shown potential; targeted imaging of CA19.9 and CA-IX and are being further explored. Immune-directed imaging agents are highly desirable as biomarkers and preliminary studies with radiolabeled antibodies targeting immune mechanisms appear promising. While novel agents are being developed, larger well-designed studies are needed to validate the role of these agents as biomarkers in the clinical management of patients.

Nanobodies as non-invasive imaging tools

Antibodies and antibody fragments have found wide application for therapeutic and diagnostic purposes. Single-domain antibody fragments, also known as ‘heavy-chain variable domains’ or ‘nanobodies’, are a recent addition to the toolbox. Discovered some 30 years ago, nanobodies are the smallest antibody-derived fragments that retain antigen-binding properties. Their small size, stability, specificity, affinity and ease of manufacture make them appealing for use as imaging agents in the laboratory and the clinic. With the recent surge in immunotherapeutics and the success of cancer immunotherapy, it is important to be able to image immune responses and cancer biomarkers non-invasively to allocate resources and guide the best possible treatment of patients with cancer. This article reviews recent advances in the application of nanobodies as cancer imaging agents. While much work has been done in preclinical models, first-in-human applications are beginning to show the value of nanobodies as imaging agents.

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Imaging is essential to make the right clinical decisions for many diseases, including cancer.

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Nanobodies have desirable properties as imaging agents, such as high specificity, affinity and a short blood half-life.

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Several nanobodies have found application in preclinical and clinical studies as imaging agents.

Effect of Modulating FcRn Binding on Direct and Pretargeted Tumor Uptake of Full-length Antibodies

Full-length antibodies lack ideal pharmacokinetic properties for rapid targeted imaging, prompting the pursuit of smaller peptides and fragments. Nevertheless, studying the disposition properties of antibody-based imaging agents can provide critical insight into the pharmacology of their therapeutic counterparts, particularly for those coupled with potent payloads. Here, we evaluate modulation of binding to the neonatal Fc receptor (FcRn) as a protein engineering-based pharmacologic strategy to minimize the overall blood pool background with directly labeled antibodies and undesirable systemic click reaction of radiolabeled tetrazine with circulating pretargeted trans-cyclooctene (TCO)-modified antibodies. Noninvasive SPECT imaging of mice bearing HER2-expressing xenografts was performed both directly (¹¹¹In-labeled antibody) and indirectly (pretargeted TCO-modified antibody followed by ¹¹¹In-labeled tetrazine). Pharmacokinetic modulation of antibodies was achieved by two distinct methods: Fc engineering to reduce binding affinity to FcRn, and delayed administration of an antibody that competes with binding to FcRn. Tumor imaging with directly labeled antibodies was feasible in the absence of FcRn binding, rapidly attaining high tumor-to-blood ratios, but accompanied by moderate liver and spleen uptake. Pretargeted imaging of tumors with non-FcRn-binding antibody was also feasible, but systemic click reaction still occurred, albeit at lower levels than with parental antibody. Our findings demonstrate that FcRn binding impairment of full-length IgG antibodies moderately lowers tumor accumulation of radioactivity, and shifts background activity from blood pool to liver and spleen. Furthermore, reduction of FcRn binding did not eliminate systemic click reaction, but yielded greater improvements in tumor-to-blood ratio when imaging with directly labeled antibodies than with pretargeting.

Tri-functional platform for construction of modular antibody fragments for in vivo 18F-PET or NIRF molecular imaging

Positron emission tomography (PET) molecular imaging is a powerful tool for interrogating physiological and biochemical processes to understand the biology of disease and advance therapeutic developments. Near-infrared fluorescence (NIRF) optical imaging has become increasingly popular for intraoperative staging to enable cellular resolution imaging of tumor margins during surgical resection. In addition, engineered antibody fragments have emerged as promising molecular imaging agents given their exquisite target selectivity, rapid systemic clearance and site-selective chemical modification. We report a tri-functional platform for construction of a modular antibody fragment that can rapidly be labeled with radionuclides or fluorophores for PET or NIRF molecular imaging of prostate stem cell antigen (PSCA).

To provide a universal approach towards the targeted delivery of PET and optical imaging agents, we have developed a tri-functional platform (TFP) for the facile construction of modular, target-specific tracers.

First-in-Humans Imaging with 89Zr-Df-IAB22M2C Anti-CD8 Minibody in Patients with Solid Malignancies: Preliminary Pharmacokinetics, Biodistribution, and Lesion Targeting

Immunotherapy is becoming the mainstay for treatment of a variety of malignancies, but only a subset of patients responds to treatment. Tumor-infiltrating CD8-positive (CD8+) T lymphocytes play a central role in antitumor immune responses. Noninvasive imaging of CD8+ T cells may provide new insights into the mechanisms of immunotherapy and potentially predict treatment response. We are studying the safety and utility of ⁸⁹Zr-IAB22M2C, a radiolabeled minibody against CD8+ T cells, for targeted imaging of CD8+ T cells in patients with cancer. Methods: The initial dose escalation phase of this first-in-humans prospective study included 6 patients (melanoma, 1; lung, 4; hepatocellular carcinoma, 1). Patients received approximately 111 MBq (3 mCi) of ⁸⁹Zr-IAB22M2C (at minibody mass doses of 0.2, 0.5, 1.0, 1.5, 5, or 10 mg) as a single dose, followed by PET/CT scans at approximately 1-2, 6-8, 24, 48, and 96-144 h after injection. Biodistribution in normal organs, lymph nodes, and lesions was evaluated. In addition, serum samples were obtained at approximately 5, 30, and 60 min and later at the times of imaging. Patients were monitored for safety during infusion and up to the last imaging time point. Results: ⁸⁹Zr-IAB22M2C infusion was well tolerated, with no immediate or delayed side effects observed after injection. Serum clearance was typically biexponential and dependent on the mass of minibody administered. Areas under the serum time-activity curve, normalized to administered activity, ranged from 1.3 h/L for 0.2 mg to 8.9 h/L for 10 mg. Biodistribution was dependent on the minibody mass administered. The highest uptake was always in spleen, followed by bone marrow. Liver uptake was more pronounced with higher minibody masses. Kidney uptake was typically low. Prominent uptake was seen in multiple normal lymph nodes as early as 2 h after injection, peaking by 24-48 h after injection. Uptake in tumor lesions was seen on imaging as early as 2 h after injection, with most ⁸⁹Zr-IAB22M2C-positive lesions detectable by 24 h. Lesions were visualized early in patients receiving treatment, with SUV ranging from 5.85 to 22.8 in 6 target lesions. Conclusion: ⁸⁹Zr-IAB22M2C imaging is safe and has favorable kinetics for early imaging. Biodistribution suggests successful targeting of CD8+ T-cell-rich tissues. The observed targeting of tumor lesions suggests this may be informative for CD8+ T-cell accumulation within tumors. Further evaluation is under way.

A Dual-Modality Linker Enables Site-Specific Conjugation of Antibody Fragments for 18F-Immuno-PET and Fluorescence Imaging

Antibody-based dual-modality (PET/fluorescence) imaging enables both presurgery antigen-specific immuno-PET for noninvasive whole-body evaluation and intraoperative fluorescence for visualization of superficial tissue layers for image-guided surgery. Methods: We developed a universal dual-modality linker (DML) that facilitates site-specific conjugation to a cysteine residue-bearing antibody fragment, introduction of a commercially available fluorescent dye (via an amine-reactive prosthetic group), and rapid and efficient radiolabeling via click chemistry with ¹⁸F-labeled trans-cyclooctene (¹⁸F-TCO). To generate a dual-modality antibody fragment-based imaging agent, the DML was labeled with the far-red dye sulfonate cyanine 5 (sCy5), site-specifically conjugated to the C-terminal cysteine of the anti-prostate stem cell antigen (PSCA) cys-diabody A2, and subsequently radiolabeled by click chemistry with ¹⁸F-TCO. The new imaging probe was evaluated in a human PSCA-positive prostate cancer xenograft model by sequential immuno-PET and optical imaging. Uptake in target tissues was confirmed by ex vivo biodistribution. Results: We successfully synthesized a DML for conjugation of a fluorescent dye and ¹⁸F. The anti-PSCA cys-diabody A2 was site-specifically conjugated with either DML or sCy5 and radiolabeled via click chemistry with ¹⁸F-TCO. Immuno-PET imaging confirmed in vivo antigen-specific targeting of prostate cancer xenografts as early as 1 h after injection. Rapid renal clearance of the 50-kDa antibody fragment enables same-day imaging. Optical imaging showed antigen-specific fluorescent signal in PSCA-positive xenografts and high contrast to surrounding tissue and PSCA-negative xenografts. Conclusion: The DML enables site-specific conjugation away from the antigen-binding site of antibody fragments, with a controlled linker-to-protein ratio, and combines signaling moieties for 2 imaging systems into 1 molecule. Dual-modality imaging could provide both noninvasive whole-body imaging with organ-level biodistribution and fluorescence image-guided identification of tumor margins during surgery.

Antibody Fragment and Affibody ImmunoPET Imaging Agents: Radiolabelling Strategies and Applications

Antibodies have long been recognised as potent vectors for carrying diagnostic medical radionuclides, contrast agents and optical probes to diseased tissue for imaging. The area of ImmunoPET combines the use of positron emission tomography (PET) imaging with antibodies to improve the diagnosis, staging and monitoring of diseases. Recent developments in antibody engineering and PET radiochemistry have led to a new wave of experimental ImmunoPET imaging agents that are based on a range of antibody fragments and affibodies. In contrast to full antibodies, engineered affibody proteins and antibody fragments such as minibodies, diabodies, single-chain variable region fragments (scFvs), and nanobodies are much smaller but retain the essential specificities and affinities of full antibodies in addition to more desirable pharmacokinetics for imaging. Herein, recent key developments in the PET radiolabelling strategies of antibody fragments and related affibody molecules are highlighted, along with the main PET imaging applications of overexpressed antigen-associated tumours and immune cells.

Labeling Single Domain Antibody Fragments with Fluorine-18 Using 2,3,5,6-Tetrafluorophenyl 6-[18F]Fluoronicotinate Resulting in High Tumor-to-Kidney Ratios

ImmunoPET agents are being investigated to assess the status of epidermal growth factor receptor 2 (HER2) in breast cancer patients with the goal of selecting those likely to benefit from HER2-targeted therapies and monitoring their progress after these treatments. We have been exploring the use of single domain antibody fragments (sdAbs) labeled with ¹⁸F using residualizing prosthetic agents for this purpose. In this study, we have labeled two sdAbs that bind to different domains on the HER2 receptor, 2Rs15d and 5F7, using 2,3,5,6-tetrafluorophenyl 6-[¹⁸F]fluoronicotinate ([¹⁸F]TFPFN) and evaluated their HER2 targeting properties in vitro and in vivo. The overall decay-corrected radiochemical yield for the synthesis of [¹⁸F]TFPFN-2Rs15d and [¹⁸F]TFPFN-5F7 was 5.7 ± 3.6 and 4.0 ± 2.0%, respectively. The radiochemical purity of labeled sdAbs was >95%, immunoreactive fractions were about 60%, and affinity was in the low nanomolar range. Intracellularly trapped activity from [¹⁸F]TFPFN-2Rs15d and [¹⁸F]TFPFN-5F7 in HER2-expressing SKOV-3 ovarian and BT474M1 breast carcinoma cells were similar to the sdAbs labeled using the previously validated radioiodination residualizing prosthetic agents N-succinimidyl 4-guanidinomethyl-3-[¹²⁵I]iodobenzoate ([¹²⁵I]SGMIB) and N-succinimidyl 3-guanidinomethyl-5-[¹²⁵I]iodobenzoate ( iso-[¹²⁵I]SGMIB). Intracellular activity was about 2-fold higher for radiolabeled 5F7 compared with 2Rs15d for both ¹⁸F and ¹²⁵I. While tumor uptake of both [¹⁸F]TFPFN-2Rs15d and [¹⁸F]TFPFN-5F7 was comparable to those for the coadministered ¹²⁵I-labeled sdAb, renal uptake of the ¹⁸F-labeled sdAbs was substantially lower. In microPET images, the tumor was clearly delineated in SKOV-3 and BT474 xenograft-bearing athymic mice with low levels of background activity in normal tissues, except the bladder. These results indicate that the [¹⁸F]TFPFN prosthetic group could be a valuable reagent for developing sdAb-based immunoPET imaging agents.

Fluorine-18 Labeling of the HER2-Targeting Single-Domain Antibody 2Rs15d Using a Residualizing Label and Preclinical Evaluation

PURPOSE

Our previous studies with F-18-labeled anti-HER2 single-domain antibodies (sdAbs) utilized 5F7, which binds to the same epitope on HER2 as trastuzumab, complicating its use for positron emission tomography (PET) imaging of patients undergoing trastuzumab therapy. On the other hand, sdAb 2Rs15d binds to a different epitope on HER2 and thus might be a preferable vector for imaging in these patients. The aim of this study was to evaluate the tumor targeting of F-18 -labeled 2Rs15d in HER2-expressing breast carcinoma cells and xenografts.

PROCEDURES

sdAb 2Rs15d was labeled with the residualizing labels N-succinimidyl 3-((4-(4-[¹⁸F]fluorobutyl)-1H-1,2,3-triazol-1-yl)methyl)-5-(guanidinomethyl)benzoate ([¹⁸F]RL-I) and N-succinimidyl 4-guanidinomethyl-3-[¹²⁵I]iodobenzoate ([¹²⁵I]SGMIB), and the purity and HER2-specific binding affinity and immunoreactivity were assessed after labeling. The biodistribution of I-125- and F-18-labeled 2Rs15d was determined in SCID mice bearing subcutaneous BT474M1 xenografts. MicroPET/x-ray computed tomograph (CT) imaging of [¹⁸F]RL-I-2Rs15d was performed in this model and compared to that of nonspecific sdAb [¹⁸F]RL-I-R3B23. MicroPET/CT imaging was also done in an intracranial HER2-positive breast cancer brain metastasis model after administration of 2Rs15d-, 5F7-, and R3B23-[¹⁸F]RL-I conjugates.

RESULTS

[¹⁸F]RL-I was conjugated to 2Rs15d in 40.8 ± 9.1 % yield and with a radiochemical purity of 97-100 %. Its immunoreactive fraction (IRF) and affinity for HER2-specific binding were 79.2 ± 5.4 % and 7.1 ± 0.4 nM, respectively. [¹²⁵I]SGMIB was conjugated to 2Rs15d in 58.4 ± 8.2 % yield and with a radiochemical purity of 95-99 %; its IRF and affinity for HER2-specific binding were 79.0 ± 12.9 % and 4.5 ± 0.8 nM, respectively. Internalized radioactivity in BT474M1 cells in vitro for [¹⁸F]RL-I-2Rs15d was 43.7 ± 3.6, 36.5 ± 2.6, and 21.7 ± 1.2 % of initially bound radioactivity at 1, 2, and 4 h, respectively, and was similar to that seen for [¹²⁵I]SGMIB-2Rs15d. Uptake of [¹⁸F]RL-I-2Rs15d in subcutaneous xenografts was 16-20 %ID/g over 1-3 h. Subcutaneous tumor could be clearly delineated by microPET/CT imaging with [¹⁸F]RL-I-2Rs15d but not with [¹⁸F]RL-I-R3B23. Intracranial breast cancer brain metastases could be visualized after intravenous administration of both [¹⁸F]RL-I-2Rs15d and [¹⁸F]RL-I-5F7.

CONCLUSIONS

Although radiolabeled 2Rs15d conjugates exhibited lower tumor cell retention both in vitro and in vivo than that observed previously for 5F7, given that it binds to a different epitope on HER2 from those targeted by the clinically utilized HER2-targeted therapeutic antibodies trastuzumab and pertuzumab, F-18-labeled 2Rs15d has potential for assessing HER2 status by PET imaging after trastuzumab and/or pertuzumab therapy.

Fluorine-18 labeling of an anti-HER2 VHH using a residualizing prosthetic group via a strain-promoted click reaction: Chemistry and preliminary evaluation

In a previous study, we evaluated a HER2-specific single domain antibody fragment (sdAb) 2Rs15d labeled with ¹⁸F via conjugation of a residualizing prosthetic agent that was synthesized by copper-catalyzed azide-alkyne cycloaddition (CuAAC). In order to potentially increase overall efficiency and decrease the time required for labeling, we now investigate the use of a strain-promoted azide-alkyne cycloaddition (SPAAC) between the 2Rs15d sdAb, which had been pre-derivatized with an azide-containing residualizing moiety, and an ¹⁸F-labeled aza-dibenzocyclooctyne derivative. The HER2-targeted sdAb 2Rs15d and a nonspecific sdAb R3B23 were pre-conjugated with a moiety containing both azide- and guanidine functionalities. The thus derivatized sdAbs were radiolabeled with ¹⁸F using an ¹⁸F-labeled aza-dibenzocyclooctyne derivative ([¹⁸F]F-ADIBO) via SPAAC, generating the desired conjugate ([¹⁸F]RL-II-sdAb). For comparison, unmodified 2Rs15d was labeled with N-succinimidyl 4-guanidinomethyl-3-[¹²⁵I]iodobenzoate ([¹²⁵I]SGMIB), the prototypical residualizing agent for radioiodination. Radiochemical purity (RCP), immunoreactive fraction (IRF), HER2-binding affinity and cellular uptake of [¹⁸F]RL-II-2Rs15d were assessed in vitro. Paired label biodistribution of [¹⁸F]RL-II-2Rs15d and [¹²⁵I]SGMIB-2Rs15d, and microPET/CT imaging of [¹⁸F]RL-II-2Rs15d and the [¹⁸F]RL-II-R3B23 control sdAb were performed in nude mice bearing HER2-expressing SKOV-3 xenografts. A radiochemical yield of 23.9 ± 6.9% (n = 8) was achieved for the SPAAC reaction between [¹⁸F]F-ADIBO and azide-modified 2Rs15d and the RCP of the labeled sdAb was >95%. The affinity (K_d) and IRF for the binding of [¹⁸F]RL-II-2Rs15d to HER2 were 5.6 ± 1.3 nM and 73.1 ± 22.5% (n = 3), respectively. The specific uptake of [¹⁸F]RL-II-2Rs15d by HER2-expressing BT474M1 breast carcinoma cells in vitro was 14-17% of the input dose at 1, 2, and 4 h, slightly higher than seen for co-incubated [¹²⁵I]SGMIB-2Rs15d. The uptake of [¹⁸F]RL-II-2Rs15d in SKOV-3 xenografts at 1 h and 2 h p.i. were 5.54 ± 0.77% ID/g and 6.42 ± 1.70% ID/g, respectively, slightly higher than those for co-administered [¹²⁵I]SGMIB-2Rs15d (4.80 ± 0.78% ID/g and 4.78 ± 1.39% ID/g). MicroPET/CT imaging with [¹⁸F]RL-II-2Rs15d at 1-3 h p.i. clearly delineated SKOV-3 tumors while no significant accumulation of activity in tumor was seen for [¹⁸F]RL-II-R3B23. With the exception of kidneys, normal tissue levels for [¹⁸F]RL-II-2Rs15d were low and cleared rapidly. To our knowledge, this is the first time SPAAC method has been used to label an sdAb with ¹⁸F, especially with residualizing functionality.

Synthesis and Biologic Evaluation of a Novel 18F-Labeled Adnectin as a PET Radioligand for Imaging PD-L1 Expression

The programmed death protein (PD-1) and its ligand (PD-L1) play critical roles in a checkpoint pathway cancer cells exploit to evade the immune system. A same-day PET imaging agent for measuring PD-L1 status in primary and metastatic lesions could be important for optimizing drug therapy. Herein, we have evaluated the tumor targeting of an anti-PD-L1 adnectin after ¹⁸F-fluorine labeling. Methods: An anti-PD-L1 adnectin was labeled with ¹⁸F in 2 steps. This synthesis featured fluorination of a novel prosthetic group, followed by a copper-free click conjugation to a modified adnectin to generate ¹⁸F-BMS-986192. ¹⁸F-BMS-986192 was evaluated in tumors using in vitro autoradiography and PET with mice bearing bilateral PD-L1-negative (PD-L1(-)) and PD-L1-positive (PD-L1(+)) subcutaneous tumors. ¹⁸F-BMS-986192 was evaluated for distribution, binding, and radiation dosimetry in a healthy cynomolgus monkey. Results:¹⁸F-BMS-986192 bound to human and cynomolgus PD-L1 with a dissociation constant of less than 35 pM, as measured by surface plasmon resonance. This adnectin was labeled with ¹⁸F to yield a PET radioligand for assessing PD-L1 expression in vivo. ¹⁸F-BMS-986192 bound to tumor tissues as a function of PD-L1 expression determined by immunohistochemistry. Radioligand binding was blocked in a dose-dependent manner. In vivo PET imaging clearly visualized PD-L1 expression in mice implanted with PD-L1(+), L2987 xenograft tumors. Two hours after dosing, a 3.5-fold-higher uptake (2.41 ± 0.29 vs. 0.82 ± 0.11 percentage injected dose per gram, P < 0.0001) was observed in L2987 than in control HT-29 (PD-L1(-)) tumors. Coadministration of 3 mg/kg ADX_5322_A02 anti-PD-L1 adnectin reduced tumor uptake at 2 h after injection by approximately 70%, whereas HT-29 uptake remained unchanged, demonstrating PD-L1-specific binding. Biodistribution in a nonhuman primate showed binding in the PD-L1-rich spleen, with rapid blood clearance through the kidneys and bladder. Binding in the PD-L1(+) spleen was reduced by coadministration of BMS-986192. Dosimetry estimates indicate that the kidney is the dose-limiting organ, with an estimated human absorbed dose of 2.20E-01 mSv/MBq. Conclusion:¹⁸F-BMS-986192 demonstrated the feasibility of noninvasively imaging the PD-L1 status of tumors by small-animal PET studies. Clinical studies with ¹⁸F-BMS-986192 are under way to measure PD-L1 expression in human tumors.

Evaluation of Castration-Resistant Prostate Cancer with Androgen Receptor-Axis Imaging

Castration-resistant prostate cancer (CRPC) is the lethal form of prostate cancer, and more than 26,000 men will die from this disease in 2016. The pathophysiology of CRPC is clearly multifactorial, but most often, androgen receptor (AR) upregulation is associated with its earliest beginnings and the AR increase is part of the multimolecular complex including downstream effector proteins linked to AR (AR-axis) responsible for rapid proliferation and malignant features of the malignant cell. In both animal models and patients, glycolysis (Warburg effect) is also an early manifestation of CRPC transformation. At Memorial Sloan Kettering Cancer Center, we have focused our energies on imaging studies of the AR-axis in CRPC, using ¹⁸F-FDG, ¹⁸F-16β-fluoro-5α-dihydrotestosterone (¹⁸F-FDHT), and a variety of radiolabeled antibodies targeting downstream effectors, such as prostate-specific membrane antigen (PSMA). Small-molecular-weight PSMA-targeting agents are not part of this review. In this review, we will focus on molecular imaging of the AR-axis in metastatic CRPC (mCRPC) and discuss our personal experience with these tracers. Our goal is to put these radiopharmaceuticals in the context of mCRPC biology and diagnosis (e.g., ¹⁸F-FDHT).

High-Throughput Approaches to the Development of Molecular Imaging Agents

Molecular imaging allows for the visualization of changes at the cellular level in diseases such as cancer. A successful molecular imaging agent must rely on disease-selective targets and ligands that specifically interact with those targets. Unfortunately, the translation of novel target-specific ligands into the clinic has been frustratingly slow with limitations including the complex design and screening approaches for ligand identification, as well as their subsequent optimization into useful imaging agents. This review focuses on combinatorial library approaches towards addressing these two challenges, with particular focus on phage display and one-bead one-compound (OBOC) libraries. Both of these peptide-based techniques have proven successful in identifying new ligands for cancer-specific targets and some of the success stories will be highlighted. New developments in screening methodology and sequencing technology have pushed the bounds of phage display and OBOC even further, allowing for even faster and more robust discovery of novel ligands. The combination of multiple high-throughput technologies will not only allow for more accurate identification, but also faster affinity maturation, while overall streamlining the process of translating novel ligands into clinical imaging agents.

First-in-Human Imaging with 89Zr-Df-IAB2M Anti-PSMA Minibody in Patients with Metastatic Prostate Cancer: Pharmacokinetics, Biodistribution, Dosimetry, and Lesion Uptake

We conducted a phase I dose-escalation study with ⁸⁹Zr-desferrioxamine-IAB2M (⁸⁹Zr-IAB2M), an anti-prostate-specific membrane antigen minibody, in patients with metastatic prostate cancer.

METHODS

Patients received 185 MBq (5 mCi) of ⁸⁹Zr-IAB2M and Df-IAB2M at total mass doses of 10 (n = 6), 20 (n = 6), and 50 mg (n = 6). Whole-body and serum clearance, normal-organ and lesion uptake, and radiation absorbed dose were estimated, and the effect of mass escalation was analyzed.

RESULTS

Eighteen patients were injected and scanned without side effects. Whole-body clearance was monoexponential, with a median biologic half-life of 215 h, whereas serum clearance showed biexponential kinetics, with a median biologic half-life of 3.7 (12.3%/L) and 33.8 h (17.9%/L). The radiation absorbed dose estimates were 1.67, 1.36, and 0.32 mGy/MBq to liver, kidney, and marrow, respectively, with an effective dose of 0.41 mSv/MBq (1.5 rem/mCi). Both skeletal and nodal lesions were detected with ⁸⁹Zr-IAB2M, most visualized by 48-h imaging.

CONCLUSION

⁸⁹Zr-IAB2M is safe and demonstrates favorable biodistribution and kinetics for targeting metastatic prostate cancer. Imaging with 10 mg of minibody mass provides optimal biodistribution, and imaging at 48 h after injection provides good lesion visualization. Assessment of lesion targeting is being studied in detail in an expansion cohort.

Site-Specific Radiofluorination of Biomolecules with 8-[(18)F]-Fluorooctanoic Acid Catalyzed by Lipoic Acid Ligase

New methodologies for site-specifically radiolabeling proteins with (18)F are required to generate high quality radiotracers for preclinical and clinical applications with positron emission tomography. Herein, we report an approach by which we use lipoic acid ligase (LplA) to conjugate [(18)F]-fluorooctanoic acid to an antibody fragment bearing the peptide substrate of LplA. The mild conditions of the reaction preserve antibody immunoreactivity, and the efficiency of LplA allows for >90% yield even with very small amounts of peptidic precursor (1-10 nmol). These features are advantageous compared to the current gold standard in the field. Moreover, the methodology introduces a new application for an important tool in chemical biology.

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.

Enzyme-Mediated Modification of Single-Domain Antibodies for Imaging Modalities with Different Characteristics

Antibodies are currently the fastest-growing class of therapeutics. Although naked antibodies have proven valuable as pharmaceutical agents, they have some limitations, such as low tissue penetration and a long circulatory half-life. They have been conjugated to toxic payloads, PEGs, or radioisotopes to increase and optimize their therapeutic efficacy. Although nonspecific conjugation is suitable for most in vitro applications, it has become evident that site specifically modified antibodies may have advantages for in vivo applications. Herein we describe a novel approach in which the antibody fragment is tagged with two handles: one for the introduction of a fluorophore or (18)F isotope, and the second for further modification of the fragment with a PEG moiety or a second antibody fragment to tune its circulatory half-life or its avidity. Such constructs, which recognize Class II MHC products and CD11b, showed high avidity and specificity. They were used to image cancers and could detect small tumors.

Mechanisms of action of therapeutic antibodies for cancer

The therapeutic utility of antibodies and their derivatives is achieved by various means. The FDA has approved several targeted antibodies that disrupt signaling of various growth factor receptors for the treatment of a number of cancers. Rituximab, and other anti-CD20 monoclonal antibodies are active in B cell malignancies. As more experience has been gained with anti-CD20 monoclonal antibodies, the multifactorial nature of their anti-tumor mechanisms has emerged. Other targeted antibodies function to dampen inhibitory checkpoints. These checkpoint inhibitors have recently achieved dramatic results in several cancers, including melanoma. These and related antibodies continue to be investigated in the clinical and pre-clinical settings. Novel antibody structures that target two or more antigens have also made their way into clinical use. Tumor targeted antibodies can also be conjugated to chemo- or radiotherapeutic agents, or catalytic toxins, as a means to deliver toxic payloads to cancer cells. Here we provide a review of these mechanisms and a discussion of their relevance to current and future clinical applications.

Development and characterization of an αvβ6-specific diabody and a disulfide-stabilized αvβ6-specific cys-diabody

INTRODUCTION

This work describes the development and characterization of two antibody fragments that specifically target the α(v)β(6) integrin, a non-covalent diabody and a disulfide-stabilized cys-diabody. The diabodies were analyzed for their ability to bind both immobilized and cell surface-bound α(v)β(6). Radiolabeling was done using non-site-specific and site-specific conjugation approaches with N-succinimidyl 4-[(18)F]fluorobenzoate ([(18)F]-SFB) and the bifunctional chelator 1,4,7-triazacyclononane-triacetic acid maleimide (NOTA-maleimide) and copper-64 ([(64)Cu]), respectively. The affects of each radiolabeling method on RCY, RCP, and immunoreactivity were analyzed for the [(18)F]-FB-α(v)β(6) diabody, [(18)F]-FB-α(v)β(6) cys-diabody, and the [(64)Cu]-NOTA-α(v)β(6) cys-diabody.

METHODS

Diabodies were constructed from the variable domains of the humanized 6.3G9 anti-α(v)β(6) intact antibody. The anti-α(v(β(6) cys-diabody was engineered with C-terminal cysteines to enable covalent dimerization and site-specific modification. Biochemical characterization included SDS-PAGE, Western blot, and electrospray ionization to confirm MW, and flow cytometry and ELISA experiments were used to determine binding affinity and specificity to α(v)β(6). The diabodies were radiolabeled with [(18)F]-SFB and in addition, the anti-α(v)β(6) cys-diabody was also radiolabeled site-specifically using NOTA-maleimide and [(64)Cu]. Immunoreactivities were confirmed using in vitro cell binding to DX3Puroβ(6) (α(v)β(6)+) and DX3Puro (α(v)β(6)-)cell lines.

RESULTS

The diabodies were purified from cell culture supernatants with purities >98%. Subnanomolar binding affinity towards αvβ6 was confirmed by ELISA (diabody IC(50)=0.8 nM, cys-diabody IC(50)=0.6 nM) and flow cytometry revealed high specificity only to the DX3Puroβ(6) cell line for both diabodies. RCYs were 22.6%±3.6% for the [(18)F]-FB-α(v)β(6) diabody, 8.3%±1.7% for the [(18)F]-FB-α(v)β(6) cys-diabody and 43.5%±5.5% for the [(64)Cu]-NOTA-α(v)β(6) cys-diabody. In vitro cell binding assays revealed excellent specificity and retention of immunoreactivity ([(18)F]-FB-α(v)β(6) diabody=58.7%±6.7%, [(18)F]-FB-α(v)β(6) cys-diabody=80.4%±4.4%, [(64)Cu]-NOTA-α(v)β(6) cys-diabody=59.4%±0.6%) regardless of the radiolabeling method used.

CONCLUSIONS

Two novel diabodies with excellent binding affinity and specificity for the α(v)β(6) integrin in vitro were developed. Radiolabeling of the diabodies with fluorine-18 ([(18)F]) and [(64)Cu] revealed advantages and disadvantages with regards to methodologies and RCYs, however immunoreactivities were well preserved regardless of radiolabeling approach.

PET Imaging of Macrophage Mannose Receptor-Expressing Macrophages in Tumor Stroma Using 18F-Radiolabeled Camelid Single-Domain Antibody Fragments

Tumor-associated macrophages constitute a major component of the stroma of solid tumors, encompassing distinct subpopulations with different characteristics and functions. We aimed to identify M2-oriented tumor-supporting macrophages within the tumor microenvironment as indicators of cancer progression and prognosis, using PET imaging. This can be realized by designing (18)F-labeled camelid single-domain antibody fragments (sdAbs) specifically targeting the macrophage mannose receptor (MMR), which has been identified as an important biomarker on this cell population.

METHODS

Cross-reactive anti-MMR sdAbs were generated after immunization of an alpaca with the extracellular domains of both human and mouse MMR. The lead binder was chosen on the basis of comparisons of binding affinity and in vivo pharmacokinetics. The PET tracer (18)F-fluorobenzoate (FB)-anti-MMR sdAb was developed using the prosthetic group N-succinimidyl-4-(18)F-fluorobenzoate ((18)F-SFB), and its biodistribution, tumor-targeting potential, and specificity in terms of macrophage and MMR targeting were evaluated in mouse tumor models.

RESULTS

Four sdAbs were selected after affinity screening, but only 2 were found to be cross-reactive for human and mouse MMR. The lead anti-MMR 3.49 sdAb, bearing an affinity of 12 and 1.8 nM for mouse and human MMR, respectively, was chosen for its favorable in vivo biodistribution profile and tumor-targeting capacity. (18)F-FB-anti-MMR 3.49 sdAb was synthesized with a 5%-10% radiochemical yield using an automated and optimized protocol. In vivo biodistribution analyses showed fast clearance via the kidneys and retention in MMR-expressing organs and tumor. The kidney retention of the fluorinated sdAb was 20-fold lower than a (99m)Tc-labeled counterpart. Compared with MMR- and C-C chemokine receptor 2-deficient mice, significantly higher uptake was observed in tumors grown in wild-type mice, demonstrating the specificity of the (18)F tracer for MMR and macrophages, respectively.

CONCLUSION

Anti-MMR 3.49 was denoted as the lead cross-reactive MMR-targeting sdAb. (18)F radiosynthesis was optimized, providing an optimal probe for PET imaging of the tumor-promoting macrophage subpopulation in the tumor stroma.

Anti-MET immunoPET for non-small cell lung cancer using novel fully human antibody fragments

MET, the receptor of hepatocyte growth factor, plays important roles in tumorigenesis and drug resistance in numerous cancers, including non-small cell lung cancer (NSCLC). As increasing numbers of MET inhibitors are being developed for clinical applications, antibody fragment-based immunopositron emission tomography (immunoPET) has the potential to rapidly quantify in vivo MET expression levels for drug response evaluation and patient stratification for these targeted therapies. Here, fully human single-chain variable fragments (scFvs) isolated from a phage display library were reformatted into bivalent cys-diabodies (scFv-cys dimers) with affinities to MET ranging from 0.7 to 5.1 nmol/L. The candidate with the highest affinity, H2, was radiolabeled with (89)Zr for immunoPET studies targeting NSCLC xenografts: low MET-expressing Hcc827 and the gefitinib-resistant Hcc827-GR6 with 4-fold MET overexpression. ImmunoPET at as early as 4 hours after injection produced high-contrast images, and ex vivo biodistribution analysis at 20 hours after injection showed about 2-fold difference in tracer uptake levels between the parental and resistant tumors (P < 0.01). Further immunoPET studies using a larger fragment, the H2 minibody (scFv-CH3 dimer), produced similar results at later time points. Two of the antibody clones (H2 and H5) showed in vitro growth inhibitory effects on MET-dependent gefitinib-resistant cell lines, whereas no effects were observed on resistant lines lacking MET activation. In conclusion, these fully human antibody fragments inhibit MET-dependent cancer cells and enable rapid immunoPET imaging to assess MET expression levels, showing potential for both therapeutic and diagnostic applications.

The development of immunoconjugates for targeted cancer therapy

Immunoconjugates are specific, highly effective, minimally toxic anticancer therapies that are beginning to show promise in the clinic. Immunoconjugates consist of three separate components: an antibody that binds to a cancer cell antigen with high specificity, an effector molecule that has a high capacity to kill the cancer cell, and a linker that will ensure the effector does not separate from the antibody during transit and will reliably release the effector to the cancer cell or tumour stroma. The high affinity antibody-antigen interaction allows specific and selective delivery of a range of effectors, including pharmacologic agents, radioisotopes, and toxins, to cancer cells. Some anticancer molecules are not well tolerated when administered systemically owing to unacceptable toxicity to the host. However, this limitation can be overcome through the linking of such cytotoxins to specific antibodies, which mask the toxic effects of the drug until it reaches its target. Conversely, many unconjugated antibodies are highly specific for a cancer target, but have low therapeutic potential and can be repurposed as delivery vehicles for highly potent effectors. In this Review, we summarize the successes and shortcomings of immunoconjugates, and discuss the future potential for the development of these therapies.

Compact microfluidic device for rapid concentration of PET tracers

HPLC purification and reformulation of positron emission tomography (PET) tracers can lead to significant dilution of the final product, making it difficult to produce a sufficiently high radioactivity concentration for some applications (e.g. small animal imaging, in vitro assays, and labelling of proteins with prosthetic groups). This is especially true for molecules with lengthy or low-yield syntheses. Starting the synthesis with more radioactivity increases the final radioactivity concentration but increases hazards and complexity of handling. An alternative is to concentrate the final product by a process such as rotary evaporation prior to downstream use. Because a rotovap requires significant space within a hot cell that could be put to more productive use, we developed a compact microfluidic system for concentration of PET tracers. This system also provides advantages in terms of repeatability, interfacing and potential for automation. We present here the design and performance characterization of the system, and demonstrate the concentration of several tracers in aqueous-based HPLC mobile phases.

Imaging preclinical tumour models: improving translational power

Recent developments and improvements of multimodal imaging methods for use in animal research have substantially strengthened the options of in vivo visualization of cancer-related processes over time. Moreover, technological developments in probe synthesis and labelling have resulted in imaging probes with the potential for basic research, as well as for translational and clinical applications. In addition, more sophisticated cancer models are available to address cancer-related research questions. This Review gives an overview of developments in these three fields, with a focus on imaging approaches in animal cancer models and how these can help the translation of new therapies into the clinic.

Minibody-indocyanine green based activatable optical imaging probes: the role of short polyethylene glycol linkers

Minibodies show rapider blood clearance than IgGs due to smaller size that improves target-to-background ratio (TBR) in in vivo imaging. Additionally, the ability to activate an optical probe after binding to the target greatly improves the TBR. An optical imaging probe based on a minibody against prostate-specific membrane antigen (PSMA-MB) and conjugated with an activatable fluorophore, indocyanine green (ICG), was designed to fluoresce only after binding to cell-surface PSMA. To further reduce background signal, short polyethylene glycol (PEG) linkers were employed to improve the covalent bonding ratio of ICG. New PSMA-MBs conjugated with bifunctional ICG derivatives specifically visualized PSMA-positive tumor xenografts in mice bearing both PSMA-positive and -negative tumors within 6 h postinjection. The addition of short PEG linkers significantly improved TBRs; however, it did not significantly alter the biodistribution. Thus, minibody-ICG conjugates could be a good alternative to IgG-ICG in the optical cancer imaging for further clinical applications.

Design and preclinical evaluation of a 99mTc-labelled diabody of mAb J591 for SPECT imaging of prostate-specific membrane antigen (PSMA)

BACKGROUND

Sensitive and specific detection of nodal status, sites of metastases and low-volume recurrent disease could greatly improve management of patients with advanced prostate cancer. Prostate-specific membrane antigen (PSMA) is a well-established marker for prostate carcinoma with increased levels of expression in high-grade, hormone-refractory and metastatic disease. The monoclonal antibody (mAb) J591 is directed against an extracellular epitope of PSMA and has been shown to efficiently target disseminated disease including metastases in lymph nodes and bone. Its use as a diagnostic imaging agent however is limited due to its slow pharmacokinetics. In this study a diabody derived from mAb J591 was developed as a single photon emission computed tomography (SPECT) tracer with improved pharmacokinetics for the detection of PSMA expression in prostate cancer.

METHODS

A diabody in VH-VL orientation and with a C-terminal cysteine was expressed in HEK293T cells and purified by a combination of metal ion affinity and size exclusion chromatography. Specificity and affinity were determined in cell binding studies. For SPECT imaging, the diabody was site-specifically labelled with [99mTc(CO)3]+ via the C-terminal His tag and evaluated in a subcutaneous DU145/DU145-PSMA prostate carcinoma xenograft model.

RESULTS

J591C diabody binds to PSMA-expressing cells with low nanomolar affinity (3.3 ± 0.2 nM). SPECT studies allowed imaging of tumour xenografts with high contrast from 4 h post injection (p.i.). Ex vivo biodistribution studies showed peak tumour uptake of the tracer of 12.1% ± 1.7% injected dose (ID)/g at 8 h p.i. with a tumour to blood ratio of 8.0. Uptake in PSMA-negative tumours was significantly lower with 6.3% ± 0.5% at 8 h p.i. (p < 0.001).

CONCLUSION

The presented diabody has favourable properties required to warrant its further development for antibody-based imaging of PSMA expression in prostate cancer, including PSMA-specific uptake, favourable pharmacokinetics compared to the parental antibody and efficient site-specific radiolabelling with 99mTc.

Quantitative immunoPET of prostate cancer xenografts with 89Zr- and 124I-labeled anti-PSCA A11 minibody

Prostate stem cell antigen (PSCA) is expressed on the cell surface in 83%-100% of local prostate cancers and 87%-100% of prostate cancer bone metastases. In this study, we sought to develop immunoPET agents using (124)I- and (89)Zr-labeled anti-PSCA A11 minibodies (scFv-CH3 dimer, 80 kDa) and evaluate their use for quantitative immunoPET imaging of prostate cancer.

METHODS

A11 anti-PSCA minibody was alternatively labeled with (124)I- or (89)Zr-desferrioxamine and injected into mice bearing either matched 22Rv1 and 22Rv1×PSCA or LAPC-9 xenografts. Small-animal PET data were obtained and quantitated with and without recovery coefficient-based partial-volume correction, and the results were compared with ex vivo biodistribution.

RESULTS

Rapid and specific localization to PSCA-positive tumors and high-contrast imaging were observed with both (124)I- and (89)Zr-labeled A11 anti-PSCA minibody. However, the differences in tumor uptake and background uptake of the radiotracers resulted in different levels of imaging contrast. The nonresidualizing (124)I-labeled minibody had lower tumor uptake (3.62 ± 1.18 percentage injected dose per gram [%ID/g] 22Rv1×PSCA, 3.63 ± 0.59 %ID/g LAPC-9) than the residualizing (89)Zr-labeled minibody (7.87 ± 0.52 %ID/g 22Rv1×PSCA, 9.33 ± 0.87 %ID/g LAPC-9, P < 0.0001 for each), but the (124)I-labeled minibody achieved higher imaging contrast because of lower nonspecific uptake and better tumor-to-soft-tissue ratios (22Rv1×PSCA:22Rv1 positive-to-negative tumor, 13.31 ± 5.59 (124)I-A11 and 4.87 ± 0.52 (89)Zr-A11, P = 0.02). Partial-volume correction was found to greatly improve the correspondence between small-animal PET and ex vivo quantification of tumor uptake for immunoPET imaging with both radionuclides.

CONCLUSION

Both (124)I- and (89)Zr-labeled A11 anti-PSCA minibody showed high-contrast imaging of PSCA expression in vivo. However, the (124)I-labeled A11 minibody was found to be the superior imaging agent because of lower nonspecific uptake and higher tumor-to-soft-tissue contrast. Partial-volume correction was found to be essential for robust quantification of immunoPET imaging with both (124)I- and (89)Zr-labeled A11 minibody.

Camelid single-domain antibody-fragment engineering for (pre)clinical in vivo molecular imaging applications: adjusting the bullet to its target

INTRODUCTION

Molecular imaging is a fast developing field and there is a growing need for specific imaging tracers in the clinic. Camelid single-domain antibody-fragments (sdAbs) recently emerged as a new class of molecular imaging tracers.

AREAS COVERED

We review the importance of molecular imaging in the clinic and the use of camelid sdAbs as in vivo molecular imaging tracers. Interest in imaging tracers based on antibody fragments or man-made protein scaffolds expanded over the last years. Camelid sdAbs are small, monomeric binding fragments that are derived from unique heavy-chain-only antibodies. In vivo imaging studies with sdAbs targeting various cell membrane receptors in different disease models have been reported and more sdAb imaging tracers are under development. The first clinical trial with a camelid sdAb as a molecular imaging tracer targeting the breast cancer marker Human Epidermal growth factor Receptor 2 is currently ongoing.

EXPERT OPINION

We expect that the development and use of sdAbs as tracers for both preclinical and clinical molecular imaging applications will become widespread.

PET imaging of CD105/endoglin expression with a ⁶¹/⁶⁴Cu-labeled Fab antibody fragment

PURPOSE

The goal of this study was to generate and characterize the Fab fragment of TRC105, a monoclonal antibody that binds with high affinity to human and murine CD105 (i.e., endoglin), and investigate its potential for PET imaging of tumor angiogenesis in a small-animal model after (61/64)Cu labeling.

METHODS

TRC105-Fab was generated by enzymatic papain digestion. The integrity and CD105 binding affinity of TRC105-Fab was evaluated before NOTA (1,4,7-triazacyclononane-1,4,7-triacetic acid) conjugation and (61/64)Cu labeling. Serial PET imaging and biodistribution studies were carried out in the syngeneic 4T1 murine breast cancer model to quantify tumor targeting efficiency and normal organ distribution of (61/64)Cu-NOTA-TRC105-Fab. Blocking studies with unlabeled TRC105 were performed to confirm CD105 specificity of the tracer in vivo. Immunofluorescence staining was also conducted to correlate tracer uptake in the tumor and normal tissues with CD105 expression.

RESULTS

TRC105-Fab was produced with high purity through papain digestion of TRC105, as confirmed by SDS-PAGE, HPLC analysis, and mass spectrometry. (61/64)Cu labeling of NOTA-TRC105-Fab was achieved with about 50 % yield (specific activity about 44 GBq/μmol). PET imaging revealed rapid uptake of (64)Cu-NOTA-TRC105-Fab in the 4T1 tumor (3.6 ± 0.4, 4.2 ± 0.5, 4.9 ± 0.3, 4.4 ± 0.7, and 4.6 ± 0.8 %ID/g at 0.5, 2, 5, 16, and 24 h after injection, respectively; n = 4). Since tumor uptake peaked soon after tracer injection, (61)Cu-labeled TRC105-Fab was also able to provide tumor contrast at 3 and 8 h after injection. CD105 specificity of the tracer was confirmed with blocking studies and histological examination.

CONCLUSION

We report PET imaging of CD105 expression using (61/64)Cu-NOTA-TRC105-Fab, which exhibited prominent and target-specific uptake in the 4T1 tumor. The use of a Fab fragment led to much faster tumor uptake (which peaked at a few hours after tracer injection) compared to radiolabeled intact antibody, which may be translated into same-day immunoPET imaging for clinical investigation.

Positron emission tomography imaging of tumor angiogenesis with a (61/64)Cu-labeled F(ab')(2) antibody fragment

The objective of this study was to characterize the in vitro and in vivo properties of the F(ab')(2) fragment of TRC105, a human/murine chimeric IgG1 monoclonal antibody that binds with high avidity to human and murine CD105 (i.e., endoglin), and investigate its potential for positron emission tomography (PET) imaging of tumor angiogenesis after (61/64)Cu-labeling. TRC105-F(ab')(2) of high purity was produced by pepsin digestion of TRC105, which was confirmed by SDS-PAGE, HPLC analysis, and mass spectrometry. (61/64)Cu-labeling of NOTA-TRC105-F(ab')(2) (NOTA denotes 1,4,7-triazacyclononane-1,4,7-triacetic acid) was achieved with yields of >75% (specific activity: ∼115 GBq/μmol). PET imaging revealed rapid tumor uptake of (64)Cu-NOTA-TRC105-F(ab')(2) in the 4T1 murine breast cancer model (5.8 ± 0.8, 7.6 ± 0.6, 5.6 ± 0.4, 5.0 ± 0.6, and 3.8 ± 0.7% ID/g at 0.5, 3, 16, 24, and 48 h postinjection respectively; n = 4). Since tumor uptake peaked at 3 h postinjection, (61)Cu-NOTA-TRC105-F(ab')(2) also gave good tumor contrast at 3 and 8 h postinjection. CD105 specificity of the tracers was confirmed by blocking studies and histopathology. In conclusion, the use of a F(ab')(2) fragment led to more rapid tumor uptake (which peaked at 3 h postinjection) than radiolabeled intact antibody (which often peaked after 24 h postinjection), which may allow for same day immunoPET imaging in future clinical studies.

Advances in immuno-positron emission tomography: antibodies for molecular imaging in oncology

Identification of cancer cell-surface biomarkers and advances in antibody engineering have led to a sharp increase in the development of therapeutic antibodies. These same advances have led to a new generation of radiolabeled antibodies and antibody fragments that can be used as cancer-specific imaging agents, allowing quantitative imaging of cell-surface protein expression in vivo. Immuno-positron emission tomography (immunoPET) imaging with intact antibodies has shown success clinically in diagnosing and staging cancer. Engineered antibody fragments, such as diabodies, minibodies, and single-chain Fv (scFv) -Fc, have been successfully employed for immunoPET imaging of cancer cell-surface biomarkers in preclinical models and are poised to bring same-day imaging into clinical development. ImmunoPET can potentially provide a noninvasive approach for obtaining target-specific information useful for titrating doses for radioimmunotherapy, for patient risk stratification and selection of targeted therapies, for evaluating response to therapy, and for predicting adverse effects, thus contributing to the ongoing development of personalized cancer treatment.