Immuno-Positron Emission Tomography: Shedding Light on Clinical Antibody Therapy

Clinical Applications of Immuno-PET in Lymphoma: A Systematic Review

Simple Summary

A systematic review of the published literature was performed to assess current clinicalapplications of immuno-PET in patients diagnosed with any histological type of lymphoma. The initial search yielded 1407 articles from PubMed/Medline and Scopus databases, but only 2 articles were found to comply with the inclusion criteria and 2 more were found during the cross-reference check. All four articles were deemed of sufficient methodological quality according to the QUADAS-2 assessment and were included in the review. Among the four included articles, three described the use of ⁸⁹Zr-labelled antibodies targeting CD20+ relapsed/refractory B-cell lymphomas and one concerned the use of ⁶⁸Ga-labelled mAb targeting CXCR4 in patients with non-Hodgkin lymphomas. Very limited literature data are currently available on the use of iPET in patients with lymphoma. However, iPET may represent a useful tool to non-invasively visualize the heterogeneous individual immunological environment, thus potentially guiding treatment-planning in lymphoma patients, and hence deserves further exploitation.

Abstract

Objective: Immuno-positron emission tomography (iPET) combines the sensitivity of the PET imaging technique and the targeting specificity of radio-labelled monoclonal antibodies (mAb). Its first clinical applications in humans were described in the late 1990s, and several pathologies have benefitted from this molecular imaging modality since then. Our scope was to assess current clinical applications of immuno-PET in patients with lymphoma. Therefore, a systematic review of the published literature was performed. Methods: PubMed/Medline and Scopus databases were independently searched by two nuclear medicine physicians, to identify studies describing the clinical use of immuno-PET in patients with lymphoma. Methodological quality of the included articles was assessed by using the Quality Assessment of Diagnostic Accuracy Studies criteria. The studies were then analyzed concerning the molecular target of interest. Results: The initial search yielded 1407 articles. After elimination of duplicates, 1339 titles/abstracts were evaluated. Only two articles were found to comply with the inclusion criteria and two more were found during the cross-reference check. Among the four included articles, three described the use of ⁸⁹Zr-labelled antibodies targeting CD20+ relapsed/refractory B-cell lymphomas and one concerned the use of ⁶⁸Ga-labelled mAb targeting CXCR4 in patients with non-Hodgkin lymphomas. Conclusions: Very limited literature data are currently available on the clinical use of iPET in patients with lymphoma. This technique is encountering obstacles in its wider use, possibly because of the need of specific facilities, unfavorable dosimetry, and unclear correlation of immuno-tracer biodistribution with patients’ clinical and tumors’ molecular characteristics. However, iPET may represent a useful tool to non-invasively visualize the heterogenous individual immunological environment, thus potentially guiding treatment-planning in lymphoma patients, and hence deserves further exploitation.

Review: PET imaging with macro- and middle-sized molecular probes

Recent progress in radiolabeling of macro- and middle-sized molecular probes has been extending possibilities to use PET molecular imaging for dynamic application to drug development and therapeutic evaluation. Theranostics concept also accelerated the use of macro- and middle-sized molecular probes for sharpening the contrast of proper target recognition even the cellular types/subtypes and proper selection of the patients who should be treated by the same molecules recognition. Here, brief summary of the present status of immuno-PET, and then further development of advanced technologies related to immuno-PET, peptidic PET probes, and nucleic acids PET probes are described.

Cross-species/cross-modality physiologically based pharmacokinetics for biologics: 89Zr-labelled albumin-binding domain antibody GSK3128349 in humans

Two-pore physiologically-based pharmacokinetics (PBPK) for biologics describes the tissue distribution and elimination kinetics of soluble proteins as a function of their hydrodynamic radius and the physiological properties of the organs. Whilst many studies have been performed in rodents to parameterize the PBPK framework in terms of organ-specific lymph flow rates, similar validation in humans has been limited. This is mainly due to the paucity of the tissue distribution time course data for biologics that is not distorted by target-related binding. Here, we demonstrate that a PBPK model based on rodent data provided good to satisfactory extrapolation to the tissue distribution time course of ⁸⁹Zr-labeled albumin-binding domain antibody (AlbudAb™) GSK3128349 in healthy human volunteers, including correct prediction of albumin-like plasma half-life, volume of distribution, and extravasation half-life. The AlbudAb™ used only binds albumin, and hence it also provides information about the tissue distribution kinetics and turnover of that ubiquitous and multifunctional plasma protein.

Total-Body PET and Highly Stable Chelators Together Enable Meaningful 89Zr-Antibody PET Studies up to 30 Days After Injection

The use of ⁸⁹Zr-antibody PET imaging to measure antibody biodistribution and tissue pharmacokinetics is well established, but current PET systems lack the sensitivity needed to study ⁸⁹Zr-labeled antibodies beyond 2-3 isotope half-lives (7-10 d), after which a poor signal-to-noise ratio is problematic. However, studies across many weeks are desirable to better match antibody circulation half-life in human and nonhuman primates. These studies investigated the technical feasibility of using the primate mini-EXPLORER PET scanner, making use of its high sensitivity and 45-cm axial field of view, for total-body imaging of ⁸⁹Zr-labeled antibodies in rhesus monkeys up to 30 d after injection. Methods: A humanized monoclonal IgG antibody against the herpes simplex viral protein glycoprotein D (gD) was radiolabeled with ⁸⁹Zr via 1 of 4 chelator-linker combinations (benzyl isothiocyanate-DFO [DFO-Bz-NCS], where DFO is desferrioxamine B; DFO-squaramide; DFO*-Bz-NCS, where DFO* is desferrioxamine*; and DFO*-squaramide). The pharmacokinetics associated with these 4 chelator-linker combinations were compared in 12 healthy young male rhesus monkeys (∼1-2 y old, ∼3 ± 1 kg). Each animal was initially injected intravenously with unlabeled antibody in a peripheral vessel in the right arm (10 mg/kg, providing therapeutic-level antibody concentrations), immediately followed by approximately 40 MBq of one of the ⁸⁹Zr-labeled antibodies injected intravenously in a peripheral vessel in the left arm. All animals were imaged 6 times over a period of 30 d, with an initial 60-min dynamic scan on day 0 (day of injection) followed by static scans of 30-45 min on approximately days 3, 7, 14, 21, and 30, with all acquired using a single bed position and images reconstructed using time-of-flight list-mode ordered-subsets expectation maximization. Activity concentrations in various organs were extracted from the PET images using manually defined regions of interest. Results: Excellent image quality was obtained, capturing the initial distribution phase in the whole-body scan; later time points showed residual ⁸⁹Zr mainly in the liver. Even at 30 d after injection, representing approximately 9 half-lives of ⁸⁹Zr and with a total residual activity of only 20-40 kBq in the animal, the image quality was sufficient to readily identify activity in the liver, kidneys, and upper and lower limb joints. Significant differences were noted in late time point liver uptake, bone uptake, and whole-body clearance between chelator-linker types, whereas little variation (±10%) was observed within each type. Conclusion: These studies demonstrate the ability to image ⁸⁹Zr-radiolabeled antibodies up to 30 d after injection while maintaining satisfactory image quality, as provided by the primate mini-EXPLORER with high sensitivity and long axial field of view. Quantification demonstrated potentially important differences in the behavior of the 4 chelators. This finding supports further investigation.

Enhancement of the Inelastic Nuclear Interaction Rate in Crystals via Antichanneling

The interaction rate of a charged particle beam with the atomic nuclei of a target varies significantly if the target has a crystalline structure. In particular, under specific orientations of the target with respect to the incident beam, the probability of inelastic interaction with nuclei can be enhanced with respect to the unaligned case. This effect, which can be named antichanneling, can be advantageously used in the cases where the interaction between beam and target has to be maximized. Here we propose to use antichanneling to increase the radioisotope production yield via cyclotron. A dedicated set of experimental measurements was carried out at the INFN Legnaro Laboratories with the AN2000 and CN accelerators to prove the existence of the antichanneling effect. The variation of the interaction yield at hundreds of keV to MeV energies was observed by means of sapphire and indium phosphide crystals, achieving an enhancement of the interaction rate up to 73% and 25%, respectively. Such a result may pave the way to the development of a novel type of nozzle for the existing cyclotrons, which can exploit crystalline materials as targets for radioisotope production, especially to enhance the production rate for expensive prime materials with minor upgrades of the current instrumentation.

Application of Immuno-PET in Antibody-Drug Conjugate Development

Targeted therapies hold great promise for cancer treatment and may exhibit even greater efficacy when combined with patient selection tools. The clinical impact of identifying likely responders includes reducing the number of unnecessary and ineffective therapies as well as more accurately determining drug effects. Positron emission tomography (PET) imaging using zirconium-89 radiolabeled monoclonal antibodies (mAbs), also referred to as zirconium-89 (⁸⁹Zr)-immuno-PET, provides a potential biomarker to measure target expression and verify optimal delivery of targeted agents to tumors. Antibody-drug conjugates (ADCs) combine the high affinity and specificity of mAbs with the potency of cytotoxic drugs to target tumor-expressing antigen and destroy cancer cells. Thus, ⁸⁹Zr-immuno-PET of whole-body biodistribution, pharmacokinetics, and tumor targeting of antibodies and ADCs to predict toxicity and efficacy could help guide individualized treatment. Here, we review how ⁸⁹Zr-immuno-PET is being used as a companion diagnostic with the development of ADCs. Furthermore, we discuss how ⁸⁹Zr-immuno-PET may be utilized in future clinical trials as an adjunct tool with novel ADCs to select cancer patients who have the greatest potential to benefit from treatment and improve ADC dosing regimens.

Molecular Imaging of Hydrolytic Enzymes Using PET and SPECT

Hydrolytic enzymes are a large class of biological catalysts that play a vital role in a plethora of critical biochemical processes required to maintain human health. However, the expression and/or activity of these important enzymes can change in many different diseases and therefore represent exciting targets for the development of positron emission tomography (PET) and single-photon emission computed tomography (SPECT) radiotracers. This review focuses on recently reported radiolabeled substrates, reversible inhibitors, and irreversible inhibitors investigated as PET and SPECT tracers for imaging hydrolytic enzymes. By learning from the most successful examples of tracer development for hydrolytic enzymes, it appears that an early focus on careful enzyme kinetics and cell-based studies are key factors for identifying potentially useful new molecular imaging agents.

Recent Advances in Zirconium-89 Chelator Development

The interest in zirconium-89 (⁸⁹Zr) as a positron-emitting radionuclide has grown considerably over the last decade due to its standardized production, long half-life of 78.2 h, favorable decay characteristics for positron emission tomography (PET) imaging and its successful use in a variety of clinical and preclinical applications. However, to be utilized effectively in PET applications it must be stably bound to a targeting ligand, and the most successfully used ⁸⁹Zr chelator is desferrioxamine B (DFO), which is commercially available as the iron chelator Desferal^®. Despite the prevalence of DFO in ⁸⁹Zr-immuno-PET applications, the development of new ligands for this radiometal is an active area of research. This review focuses on recent advances in zirconium-89 chelation chemistry and will highlight the rapidly expanding ligand classes that are under investigation as DFO alternatives.

In Vitro and In Vivo Comparison of Selected Ga-68 and Zr-89 Labelled Siderophores

Purpose

Some [⁶⁸Ga]siderophores show promise in specific and sensitive imaging of infection. Here, we compare the in vitro and in vivo behaviour of selected Ga-68 and Zr-89 labelled siderophores.

Procedures

Radiolabelling was performed in HEPES or sodium acetate buffer systems. Radiochemical purity of labelled siderophores was determined using chromatography. Partition coefficients, in vitro stability and protein binding affinities were determined. Ex vivo biodistribution and animal imaging was studied in mice.

Results

Certain differences among studied siderophores were observed in labelling efficiency. Protein binding and stability tests showed highest stabilities and lowest protein binding affinities for Ga-68 and [⁸⁹Zr]triacetylfusarinine C (TAFC). All studied Ga-68 and [⁸⁹Zr]siderophores exhibited a similar biodistribution and pharmacokinetics in mice with the exception of [⁸⁹Zr]ferrioxamine E (FOXE).

Conclusions

Zr-89 and [⁶⁸Ga]siderophores showed analogous in vitro and in vivo behaviour. Tested [⁸⁹Zr]siderophores could be applied for longitudinal positron emission tomography (PET) studies of fungal infections and especially TAFC for the development of novel bioconjugates.

Electronic supplementary material

The online version of this article (doi:10.1007/s11307-015-0897-6) contains supplementary material, which is available to authorized users.

Immuno-PET imaging based radioimmunotherapy in head and neck squamous cell carcinoma model

The epidermal growth factor receptor (EGFR) is one of the most comprehensively studied molecular targets in head and neck squamous cell carcinoma (HNSCC). However, inherent and acquired resistance are serious problems and are responsible for limited clinical efficacy and tumor recurrence. In this study, we evaluated the feasibility of immuno-positron emission tomography (PET) imaging and radioimmunotherapy (RIT) with ⁶⁴Cu-/¹⁷⁷Lu-PCTA-cetuximab in cetuximab-resistant SNU-1066 HNSCC xenografted model. The cellular uptake of ⁶⁴Cu/¹⁷⁷Lu-3,6,9,15-tetraazabicyclo[9.3.1]-pentadeca-1(15),11,13-triene-3,6,9,-triacetic acid (PCTA)-cetuximab showed good correlation with western blot and flow cytometry analysis in EGFR expression level of various HNSCC cells. ¹⁷⁷Lu-PCTA-cetuximab selectively killed cetuximab-resistant SNU-1066 cells in vitro. ⁶⁴Cu-/¹⁷⁷Lu-PCTA-cetuximab specifically accumulated in SNU-1066 tumor and those uptakes were peaked at 48 h and 7 day, respectively in biodistribution, PET and single-photon emission computed tomography/computed tomography (SPECT/CT) imaging. RIT with single dose of ¹⁷⁷Lu-PCTA-cetuximab exhibited significant tumor regression and markedly reduced 2-[¹⁸F]fluoro-2-deoxy-D-glucose (¹⁸F-FDG) uptake, compared to other groups. Proliferation index were dramatically decreased and apoptotic index increased in RIT group. These results suggest that a diagnostic and therapeutic convergence radiopharmaceutical, ⁶⁴Cu-/¹⁷⁷Lu-PCTA-cetuximab has the potential of target selection using immuno-PET imaging and targeted therapy by RIT in EGFR expressing cetuximab-resistant HNSCC tumors.

Radiosynthesis and preclinical PET evaluation of 89Zr-nivolumab (BMS-936558) in healthy non-human primates

Cancer immunotherapy, unlike traditional cytotoxic chemotherapeutic treatments, engages the immune system to identify cancer cells and stimulate immune responses. The Programmed Death-1 (PD-1) protein is an immunoinhibitory receptor expressed by activated cytotoxic T-lymphocytes (CTL) that seek out and destroy cancer cells. Multiple cancer types express and upregulate the Programmed Death-Ligand 1 (PD-L1) and 2 (PD-L2) which bind to PD-1 as an immune escape mechanism. Nivolumab is a fully human IgG4 anti-PD-1 monoclonal antibody (mAb) approved for treatment of multiple cancer types. This study reports the preparation and in vivo evaluation of ⁸⁹Zr labeled nivolumab in healthy non-human primates (NHP) as a preliminary study of biodistribution and clearance. The radiochemical and in vivo stabilities of the ⁸⁹Zr complex were shown to be acceptable for imaging. Three naïve NHPs were intravenously injected with tracer only or tracer co-injected with nivolumab followed by co-registered by positron emission tomography (PET) and magnetic resonance imaging (MRI), acquired for eight days following injection. Image-derived standardized uptake values (SUV) were quantified by region of interest (ROI) analysis. Radioactivity in the spleen was significantly reduced by addition of excess nivolumab compared to the tracer only study at all imaging time points. Liver uptake of the radiotracer was consistent as a clearance organ with minimal signal from other tissues: lung, muscle, brain, heart, and kidney. The results indicate specific biodistribution to the spleen, which can be blocked by co-administration of excess nivolumab. Distribution to other organs is consistent with elimination pathways of antibodies, with primary clearance through the liver.

Clinical Applications of Small-molecule PET Radiotracers: Current Progress and Future Outlook

Radiotracers, or radiopharmaceuticals, are bioactive molecules tagged with a radionuclide used for diagnostic imaging or radiotherapy and, when a positron-emitting radionuclide is chosen, the radiotracers are used for PET imaging. The development of novel PET radiotracers in many ways parallels the development of new pharmaceuticals, and small molecules dominate research and development pipelines in both disciplines. The 4 decades since the introduction of [¹⁸F]FDG have seen the development of many small molecule PET radiotracers. Ten have been approved by the US Food and Drug Administration as of 2016, whereas hundreds more are being evaluated clinically. These radiotracers are being used in personalized medicine and to support drug discovery programs where they are greatly improving our understanding of and ability to treat diseases across many areas of medicine including neuroscience, cardiovascular medicine, and oncology.

The Landscape of Clinical Trials Evaluating the Theranostic Role of PET Imaging in Oncology: Insights from an Analysis of ClinicalTrials.gov Database

In the war on cancer marked by personalized medicine, positron emission tomography (PET)-based theranostic strategy is playing an increasingly important role. Well-designed clinical trials are of great significance for validating the PET applications and ensuring evidence-based cancer care. This study aimed to provide a comprehensive landscape of the characteristics of PET clinical trials using the substantial resource of ClinicalTrials.gov database. We identified 25,599 oncology trials registered with ClinicalTrials.gov in the last ten-year period (October 2005-September 2015). They were systematically reviewed to validate classification into 519 PET trials and 25,080 other oncology trials used for comparison. We found that PET trials were predominantly phase 1-2 studies (86.2%) and were more likely to be single-arm (78.9% vs. 57.9%, P <0.001) using non-randomized assignment (90.1% vs. 66.7%, P <0.001) than other oncology trials. Furthermore, PET trials were small in scale, generally enrolling fewer than 100 participants (20.3% vs. 25.7% for other oncology trials, P = 0.014), which might be too small to detect a significant theranostic effect. The funding support from industry or National Institutes of Health shrunk over time (both decreased by about 5%), and PET trials were more likely to be conducted in only one region lacking international collaboration (97.0% vs. 89.3% for other oncology trials, P <0.001). These findings raise concerns that clinical trials evaluating PET imaging in oncology are not receiving the attention or efforts necessary to generate high-quality evidence. Advancing the clinical application of PET imaging will require a concerted effort to improve the quality of trials.

Affinity enhancement of nanobody binding to EGFR: in silico site-directed mutagenesis and molecular dynamics simulation approaches

Epidermal growth factor receptor (EGFR), a transmembrane glycoprotein, is overexpressed in many cancers such as head-neck, breast, prostate, and skin cancers for this reason it is a good target in cancer therapy and diagnosis. In nanobody-based cancer diagnosis and treatment, nanobodies with high affinity toward receptor (e.g. EGFR) results in effective treatment or diagnosis of cancer. In this regard, the main aim of this study is to develop a method based on molecular dynamic (MD) simulations for designing of 7D12 based nanobody with high affinity compared with wild-type nanobody. By surveying electrostatic and desolvation interactions between different residues of 7D12 and EGFR, the critical residues of 7D12 that play the main role in the binding of 7D12 to EGFR were elucidated and based on these residues, five logical variants were designed. Following the 50 ns MD simulations, pull and umbrella sampling simulation were performed for 7D12 and all its variants in complex with EGFR. Binding free energy of 7D12 (and all its variants) with EGFR was obtained by weighted histogram analysis method. According to binding free energy results, GLY101 to GLU mutation showed the highest binding affinity but this variant is unstable after 50 ns MD simulations. ALA100 to GLU mutation shows suitable binding enhancement with acceptable structural stability. Suitable position and orientation of GLU in residue 100 of 7D12 against related amino acids of EGFR formed some extra hydrogen and electrostatic interactions which resulted in binding enhancement.

Direct flow separation strategy, to isolate no-carrier-added 90Nb from irradiated Mo or Zr targets

90Nb has an intermediate half-life of 14.6 h, a high positron branching of 53% and optimal β + emission energy of only E mean 0.35 MeV per decay. These favorable characteristics suggest it may be a potential candidate for application in immuno-PET. Our recent aim was to conduct studies on distribution coefficients for ZrIV and NbV in mixtures of HCl/H2O2 and HCl/oxalic acid for anion exchange resin (AG 1 × 8) and UTEVA resin to develop a “direct flow” separation strategy for 90Nb. The direct flow concept refers to a separation accomplished using a single eluent on multiple columns, effectively streamlining the separation process and increasing the time efficiency. Finally, we also demonstrated that this separation strategy is applicable to the production of the positron emitter 90Nb via the irradiation of molybdenum targets and isolation of 90Nb from the irradiated molybdenum target.

Advances in PET Detection of the Antitumor T Cell Response

Positron emission tomography (PET) is a powerful noninvasive imaging technique able to measure distinct biological processes in vivo by administration of a radiolabeled probe. Whole-body measurements track the probe accumulation providing a means to measure biological changes such as metabolism, cell location, or tumor burden. PET can also be applied to both preclinical and clinical studies providing three-dimensional information. For immunotherapies (in particular understanding T cell responses), PET can be utilized for spatial and longitudinal tracking of T lymphocytes. Although PET has been utilized clinically for over 30 years, the recent development of additional PET radiotracers have dramatically expanded the use of PET to detect endogenous or adoptively transferred T cells in vivo. Novel probes have identified changes in T cell quantity, location, and function. This has enabled investigators to track T cells outside of the circulation and in hematopoietic organs such as spleen, lymph nodes, and bone marrow, or within tumors. In this review, we cover advances in PET detection of the antitumor T cell response and areas of focus for future studies.

Microfluidic Preparation of a 89Zr-Labeled Trastuzumab Single-Patient Dose

(89)Zr-labeled antibodies are being investigated in several clinical trials; however, the time requirement for synthesis of clinical doses can hinder patient throughput because of scheduling difficulties. Additionally, low specific activity due to poor labeling efficiency can require larger amounts of the radiopharmaceutical to be administered, possibly leading to adverse side effects. Here, we describe the design and evaluation of a microfluidic reactor capable of synthesizing a single clinical dose of (89)Zr-labeled antibody. (89)Zr-labeled trastuzumab was chosen for this validation because it is currently being evaluated in clinical trials for imaging human epidermal growth factor receptor 2-positive cancer patients.

METHODS

A microreactor fabricated from polydimethylsiloxane/glass was silanated with trimethoxy(octadecyl) silane to reduce antibody adsorption. Desferrioxamine-p-benzyl-isothiocyanate (DFO-Bz-NCS) was conjugated to trastuzumab in an 8:1 molar ratio following the literature procedures using aseptic techniques. Radiolabeling was performed by pumping (89)Zr-oxalate and DFO-Bz-trastuzumab into the microfluidic reactor at a total rate of 20 μL/min in ratios varying from 1:37 to 1:592 mg:MBq at 37°C to achieve optimal labeling.

RESULTS

Silanated reactors showed low antibody adsorption in comparison to unmodified reactors (95% monoclonal antibody recovered vs. 0% recovered). Labeling of the modified trastuzumab was shown to be achievable at a specific activity above the reported literature value of 220 MBq/mg. A high radiochemical purity was achieved without an incubation period at specific activities of less than 148 MBq/mg; however, specific activities up to 592 MBq/mg could be achieved with an incubation period. Clinical doses were able to be prepared and passed all quality control guidelines set by the Food and Drug Administration. Samples were sterile, colorless, and radiochemically pure (100%); maintained the ability to bind to the intended receptor; formed a minimal amount of aggregates (1%-4%); and were completed within 45-60 min.

CONCLUSION

(89)Zr-labeled trastuzumab for use in a clinical setting was synthesized in a microfluidic reactor in under an hour while reducing the amount of handling required by a technician. Use of this compact platform not only could enable the use of radiolabeled antibodies to become a common practice, but also could spread the use of radiolabeled antibodies beyond locations with cyclotron facilities.

A solid target system with remote handling of irradiated targets for PET cyclotrons

A solid target system was developed for a PET cyclotron. The system is compatible with many different target materials in the form of foils and electroplated/sputtered targets which makes it useful for production of a wide variety of different PET radionuclides. The target material is manually loaded into the system. Remote handling of irradiated target material is managed with a pneumatic piston and a vacuum technique which allows the targets to be dropped into a shielded transport container. To test the target performance, proton irradiations (12.8 MeV, 45 μA) of monoisotopic yttrium foils (0.64 mm, direct water cooling) were performed to produce 89Zr. The yields were 2200±200 MBq (1 h, n=13) and 6300±65 MBq (3 h, n=3).

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.

A pretargeting system for tumor PET imaging and radioimmunotherapy

Labeled antibodies, as well as their fragments and antibody-derived recombinant constructs, have long been proposed as general vectors to target radionuclides to tumor lesions for imaging and therapy. They have indeed shown promise in both imaging and therapeutic applications, but they have not fulfilled the original expectations of achieving sufficient image contrast for tumor detection or sufficient radiation dose delivered to tumors for therapy. Pretargeting was originally developed for tumor immunoscintigraphy. It was assumed that directly-radiolabled antibodies could be replaced by an unlabeled immunoconjugate capable of binding both a tumor-specific antigen and a small molecular weight molecule. The small molecular weight molecule would carry the radioactive payload and would be injected after the bispecific immunoconjugate. It has been demonstrated that this approach does allow for both antibody-specific recognition and fast clearance of the radioactive molecule, thus resulting in improved tumor-to-normal tissue contrast ratios. It was subsequently shown that pretargeting also held promise for tumor therapy, translating improved tumor-to-normal tissue contrast ratios into more specific delivery of absorbed radiation doses. Many technical approaches have been proposed to implement pretargeting, and two have been extensively documented. One is based on the avidin-biotin system, and the other on bispecific antibodies binding a tumor-specific antigen and a hapten. Both have been studied in preclinical models, as well as in several clinical studies, and have shown improved targeting efficiency. This article reviews the historical and recent preclinical and clinical advances in the use of bispecific-antibody-based pretargeting for radioimmunodetection and radioimmunotherapy of cancer. The results of recent evaluation of pretargeting in PET imaging also are discussed.

Tripodal tris(hydroxypyridinone) ligands for immunoconjugate PET imaging with (89)Zr(4+): comparison with desferrioxamine-B

Due to its long half-life (78 h) and decay properties (77% electron capture, 23% β(+), Emax = 897 keV, Eav = 397 keV, Eγ = 909 keV, Iγ = 100%) (89)Zr is an appealing radionuclide for immunoPET imaging with whole IgG antibodies. Derivatives of the siderophore desferrioxamine-B (H3DFO) are the most widely used bifunctional chelators for coordination of (89)Zr(4+) because the radiolabeling of the resulting immunoconjugates is rapid under mild conditions. (89)Zr-DFO complexes are reportedly stable in vitro but there is evidence that (89)Zr(4+) is released in vivo, and subsequently taken up by the skeleton. We have evaluated a novel tripodal tris(hydroxypyridinone) chelator, H3CP256 and its bifunctional maleimide derivative, H3YM103, for coordination of Zr(4+) and compared the NMR spectra, and the (89)Zr(4+) radiolabeling, antibody conjugation, serum stability and in vivo distribution of radiolabelled immunoconjugates with those of H3DFO and its analogues. H3CP256 coordinates (89)Zr(4+) at carrier-free concentrations forming [(89)Zr(CP256)](+). Both H3DFO and H3CP256 were efficiently radiolabelled using [(89)Zr(C2O4)4](4-) at ambient temperature in quantitative yield at pH 6-7 at millimolar concentrations of chelator. Competition experiments demonstrate that (89)Zr(4+) dissociates from [(89)Zr(DFO)](+) in the presence of one equivalent of H3CP256 (relative to H3DFO) at pH 6-7, resulting largely in [(89)Zr(CP256)](+). To assess the stability of H3DFO and H3YM103 immunoconjugates radiolabelled with (89)Zr, maleimide derivatives of the chelators were conjugated to the monoclonal antibody trastuzumab via reduced cysteine side chains. Both immunoconjugates were labelled with (89)Zr(4+) in >98% yield at high specific activities and the labeled immunoconjugates were stable in serum with respect to dissociation of the radiometal. In vivo studies in mice indicate that (89)Zr(4+) dissociates from YM103-trastuzumab with significant amounts of activity becoming associated with bones and joints (25.88 ± 0.58% ID g(-1) 7 days post-injection). In contrast, <8% ID g(-1) of (89)Zr activity becomes associated with bone in animals administered (89)Zr-DFO-trastuzumab over the course of 7 days. The tris(hydroxypyridinone) chelator, H3CP256, coordinates (89)Zr(4+) rapidly under mild conditions, but the (89)Zr-labelled immunoconjugate, (89)Zr-YM103-trastuzumab was observed to release appreciable amounts of (89)Zr(4+)in vivo, demonstrating inferior stability when compared with (89)Zr-DFO-trastuzumab. The significantly lower in vivo stability is likely to be a result of lower kinetic stability of the Zr(4+) tris(hydroxypyridinone complex) relative to that of DFO and its derivatives.

Tumour targeting and radiation dose of radioimmunotherapy with (90)Y-rituximab in CD20+ B-cell lymphoma as predicted by (89)Zr-rituximab immuno-PET: impact of preloading with unlabelled rituximab

Purpose

To compare using immuno-PET/CT the distribution of ⁸⁹Zr-labelled rituximab without and with a preload of unlabelled rituximab to assess the impact of preloading with unlabelled rituximab on tumour targeting and radiation dose of subsequent radioimmunotherapy with ⁹⁰Y-labelled rituximab in CD20+ B-cell lymphoma.

Methods

Five patients with CD20+ B-cell lymphoma and progressive disease were prospectively enrolled. All patients underwent three study phases: initial dosimetric phase with baseline ⁸⁹Zr-rituximab PET/CT imaging without a cold preload, followed 3 weeks later by a second dosimetric phase with administration of a standard preload (250 mg/m²) of unlabelled rituximab followed by injection of ⁸⁹Zr-rituximab, and a therapeutic phase 1 week later with administration of unlabelled rituximab followed by ⁹⁰Y-rituximab. PET/CT imaging and tracer uptake by organs and lesions were assessed.

Results

With a cold rituximab preload, the calculated whole-body dose of ⁹⁰Y-rituximab was similar (mean 0.87 mSv/MBq, range 0.82–0.99 mSv/MBq) in all patients. Without a preload, an increase in whole-body dose of 59 % and 87 % was noted in two patients with preserved circulating CD20+ B cells. This increase in radiation dose was primarily due to a 12.4-fold to 15-fold higher dose to the spleen without a preload. No significant change in whole-body dose was noted in the three other patients with B-cell depletion. Without a preload, consistently higher tumour uptake was noticed in patients with B-cell depletion.

Conclusion

Administration of the standard preload of unlabelled rituximab impairs radioconjugate tumour targeting in the majority of patients eligible for radioimmunotherapy, that is patients previously treated with rituximab-containing therapeutic regimens. This common practice may need to be reconsidered and further evaluated as the rationale for this high preload has its origin in the “prerituximab era”.

Clinical Trial Application: CTA 2011-005474-38

Trial Registry: EudraCT

Electronic supplementary material

The online version of this article (doi:10.1007/s00259-015-3025-6) contains supplementary material, which is available to authorized users.

Spatial resolution and image qualities of Zr-89 on Siemens Biograph TruePoint PET/CT

PURPOSE

Zirconium-89 (t(1/2)=78.41 hours) is an ideal metallic radioisotope for immuno-positron emission tomography (PET), given that its physical half-life closely matches the biological half-life of monoclonal antibodies. In this study, the authors measured the spatial resolution and image quality of Zr-89 PET and compared the results against those obtained using F-18 PET, which is widely regarded as the gold standard for comparison of imaging characteristics.

MATERIALS AND METHODS

The spatial resolution and image qualities of Zr-89 were measured on the Siemens Biograph Truepoint TrueV PET/CT scanner, partly according to NEMA NU2-2007 standards. For spatial resolution measurement, the Zr-89 point source was located at the center of the axial field of view (FOV) and offset 1/4 axial FOV from the center. For image quality measurements, an NEMA IEC Phantom was used. The NEMA IEC Phantom consists of six hot spheres that were filled with Zr-89 solution. Spatial resolution and image quality (%contrast, %background variability [BV], and source to background ratio [SBR]) were assessed to compare the imaging characteristics of F-18 with those of Siemens Biograph Truepoint TrueV.

RESULTS

The transverse and axial spatial resolutions at 1 cm were 4.5 and 4.7 mm for Zr-89, respectively. The %contrast of Zr-89 was 25.5% for the smallest 10 mm sized sphere and 89.8% for the largest 37 mm sized sphere, and for F-18, it was 32.5% for the smallest 10 mm sized sphere and 103.9% for the largest 37 mm sized sphere using the ordered subset expectation maximization (OSEM) reconstruction method. The %BV of F-18 PET was 6.4% for the smallest 10 mm sized sphere and 3.5% for the largest 37 mm sized sphere using the OSEM reconstruction. The SBR of Zr-89 was 1.8 for the smallest 10 mm sized sphere and 3.7 for the largest 37 mm sized sphere, and for F-18, it was 2.0 for the smallest 10 mm sized sphere and 4.1 for the largest 37 mm sized sphere using the OSEM reconstruction method.

CONCLUSIONS

This study assessed Zr-89 imaging characteristics using a Siemens Biograph Truepoint TrueV PET/CT scanner and compared the results with those obtained for F-18 PET. Although spatial resolution and image quality of Zr-89 PET were lower compared with F-18 PET, due to longer positron range and low positron branching ratio, Zr-89 is advantageous for immuno-PET due to well-matched half-life with monoclonal antibodies.

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.

Molecular imaging of targeted therapies with positron emission tomography: the visualization of personalized cancer care

INTRODUCTION

Molecular imaging has been defined as the visualization, characterization and measurement of biological processes at the molecular and cellular level in humans and other living systems. In oncology it enables to visualize (part of) the functional behaviour of tumour cells, in contrast to anatomical imaging that focuses on the size and location of malignant lesions. Available molecular imaging techniques include single photon emission computed tomography (SPECT), positron emission tomography (PET) and optical imaging. In PET, a radiotracer consisting of a positron emitting radionuclide attached to the biologically active molecule of interest is administrated to the patient. Several approaches have been undertaken to use PET for the improvement of personalized cancer care. For example, a variety of radiolabelled ligands have been investigated for intratumoural target identification and radiolabelled drugs have been developed for direct visualization of the biodistibution in vivo, including intratumoural therapy uptake. First indications of the clinical value of PET for target identification and response prediction in oncology have been reported. This new imaging approach is rapidly developing, but uniformity of scanning processes, standardized methods for outcome evaluation and implementation in daily clinical practice are still in progress. In this review we discuss the available literature on molecular imaging with PET for personalized targeted treatment strategies.

CONCLUSION

Molecular imaging with radiolabelled targeted anticancer drugs has great potential for the improvement of personalized cancer care. The non-invasive quantification of drug accumulation in tumours and normal tissues provides understanding of the biodistribution in relation to therapeutic and toxic effects.

Imaging of CD47 expression in xenograft and allograft tumor models

CD47 functions as a marker of "self" by inhibiting phagocytosis of autologous cells. CD47 has been shown to be overexpressed by various tumor types as a means of escaping the antitumor immune response. The goal of this research was to investigate the utility of CD47 imaging using positron emission tomography (PET) in both human xenograft and murine allograft tumor models. Anti-CD47 antibodies were conjugated with p-isothiocyanatobenzyldesferrioxamine (Df-Bz-NCS) and labeled with 89Zr. We employed xenograft and allograft small-animal models of cancer in biodistribution and PET imaging studies to investigate the specificity and PET imaging robustness of CD47. Ab-Df-Bz-NCS conjugates were labeled with 89Zr with specific activity of 0.9 to 1.6 μCi/μg. Biodistribution studies in the xenograft and allograft model showed similar specific tumor uptake of the antihuman and antimouse CD47 antibodies. However, the tracer retention in the liver, spleen, and kidneys was significantly higher in the allograft-bearing animals, suggesting uptake mediated by the CD47 normally expressed throughout the reticular endothelial system. CD47, a marker of "self," was evaluated as a diagnostic PET biomarker in xenograft and allograft cancer animal models. CD47 imaging is feasible, warranting further studies and immunoPET tracer development.

Development of novel ADCs: conjugation of tubulysin analogues to trastuzumab monitored by dual radiolabeling

Tubulysins are highly toxic tubulin-targeting agents with a narrow therapeutic window that are interesting for application in antibody-drug conjugates (ADC). For full control over drug-antibody ratio (DAR) and the effect thereof on pharmacokinetics and tumor targeting, a dual-labeling approach was developed, wherein the drug, tubulysin variants, and the antibody, the anti-HER2 monoclonal antibody (mAb) trastuzumab, are radiolabeled. (131)I-radioiodination of two synthetic tubulysin A analogues, the less potent TUB-OH (IC50 > 100 nmol/L) and the potent TUB-OMOM (IC50, ~1 nmol/L), and their direct covalent conjugation to (89)Zr-trastuzumab were established. Radioiodination of tubulysins was 92% to 98% efficient and conversion to N-hydroxysuccinimide (NHS) esters more than 99%; esters were isolated in an overall yield of 68% ± 5% with radiochemical purity of more than 99.5%. Conjugation of (131)I-tubulysin-NHS esters to (89)Zr-trastuzumab was 45% to 55% efficient, resulting in ADCs with 96% to 98% radiochemical purity after size-exclusion chromatography. ADCs were evaluated for their tumor-targeting potential and antitumor effects in nude mice with tumors that were sensitive or resistant to trastuzumab, using ado-trastuzumab emtansine as a reference. ADCs appeared stable in vivo. An average DAR of 2 and 4 conferred pharmacokinetics and tumor-targeting behavior similar to parental trastuzumab. Efficacy studies using single-dose TUB-OMOM-trastuzumab (DAR 4) showed dose-dependent antitumor effects, including complete tumor eradications in trastuzumab-sensitive tumors in vivo. TUB-OMOM-trastuzumab (60 mg/kg) displayed efficacy similar to ado-trastuzumab emtansine (15 mg/kg) yet more effective than trastuzumab. Our findings illustrate the potential of synthetic tubulysins in ADCs for cancer treatment.

Why are we failing to implement imaging studies with radiolabelled new molecular entities in early oncology drug development?

In early drug development advanced imaging techniques can help with progressing new molecular entities (NME) to subsequent phases of drug development and thus reduce attrition. However, several organizational, operational, and regulatory hurdles pose a significant barrier, potentially limiting the impact these techniques can have on modern drug development. Positron emission tomography (PET) of radiolabelled NME is arguably the best example of a complex technique with a potential to deliver unique decision-making data in small cohorts of subjects. However, to realise this potential the impediments to timely inclusion of PET into the drug development process must be overcome. In the present paper, we discuss the value of PET imaging with radiolabelled NME during early anticancer drug development, as exemplified with one such NME. We outline the multiple hurdles and propose options on how to streamline the organizational steps for future studies.

(89)Zr-labeled compounds for PET imaging guided personalized therapy

(89)Zr-immuno-PET is an attractive option for the in vivo evaluation of monoclonal antibodies (mAbs). For the coupling of (89)Zr to monoclonal antibodies several conjugation strategies are available all using desferrioxamine as chelate. Here we discuss the production of (89)Zr, the available methods for coupling of (89)Zr via desferrioxamine to mAbs, and the evaluation of (89)Zr-mAb conjugates in preclinical and clinical studies.:

Antibody-based imaging of HER-2: moving into the clinic

Human epidermal growth factor receptor-2 (HER-2) mediates a number of important cellular activities, and is up-regulated in a diverse set of cancer cell lines, especially breast cancer. Accordingly, HER-2 has been regarded as a common drug target in cancer therapy. Antibodies can serve as ideal candidates for targeted tumor imaging and drug delivery, due to their inherent affinity and specificity. Advanced by the development of a wide variety of imaging techniques, antibody-based imaging of HER-2 can allow for early detection and localization of tumors, as well as monitoring of drug delivery and tissue's response to drug treatment. In this review article, antibody-based imaging of HER-2 are summarized and discussed, with an emphasis on the involved imaging methods.

ImmunoPET and biodistribution with human epidermal growth factor receptor 3 targeting antibody ⁸⁹Zr-RG7116

The humanized monoclonal antibody with high affinity for the human epidermal growth factor receptor (HER) 3, RG7116, is a glycoengineered, IgG1 class antibody. By labeling RG7116 with zirconium-89 ((89)Zr) we aimed to visualize in vivo HER3 expression and study the biodistribution of this antibody in human tumor-bearing mice. Biodistribution of (89)Zr-RG7116 was studied in subcutaneously xenografted FaDu tumor cells (HER3-positive). Dose-dependency of (89)Zr-RG7116 organ distribution and specific tumor uptake was assessed by administering doses ranging from 0.05 to 10 mg/kg RG7116 to SCID/Beige mice. Biodistribution was analyzed at 24 and 144 h after injection. MicroPET imaging was performed at 1, 3, and 6 days after injection of 1.0 mg/kg (89)Zr-RG7116 in the FaDu, H441, QG-56 and Calu-1 xenografts with varying HER3 expression. The excised tumors were analyzed for HER3 expression. Biodistribution analyses showed a dose- and time-dependent (89)Zr-RG7116 tumor uptake in FaDu tumors. The highest tumor uptake of (89)Zr-RG7116 was observed in the 0.05 mg/kg dose group with 27.5%ID/g at 144 h after tracer injection. MicroPET imaging revealed specific tumor uptake of (89)Zr-RG7116 in FaDu and H441 models with an increase in tumor uptake over time. Biodistribution data was consistent with the microPET findings in FaDu, H441, QG56 and Calu-1 xenografts, which correlated with HER3 expression levels. In conclusion, (89)Zr-RG7116 specifically accumulates in HER3 expressing tumors. PET imaging with this tracer provides real-time non-invasive information about RG7116 distribution, tumor targeting and tumor HER3 expression levels.

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.

Production of 89Zr via the 89Y(p,n)89Zr reaction in aqueous solution: effect of solution composition on in-target chemistry

OBJECTIVE

The existing solid target production method of radiometals requires high capital and operational expenditures, which limit the production of radiometals to the small fraction of cyclotron facilities that are equipped with solid target systems. Our objective is to develop a robust solution target method, which can be applicable to a wide array of radiometals and would be simply and easily adopted by existing cyclotron facility for the routine production of radiometals.

METHOD

We have developed a simplified, solution target approach for production of (89)Zr using a niobium target by 14 MeV energy proton bombardment of aqueous solutions of yttrium salts via the (89)Y(p,n)(89)Zr nuclear reaction. The production conditions were optimized, following a detailed mechanistic study of the gas evolution.

RESULTS

Although the solution target approach avoided the expense and complication of solid target processing, rapid radiolytic formation of gases in the target represents a major impediment in the success of solution target. To address this challenge we performed a systematic mechanistic study of gas evolution. Gas evolution was found to be predominantly due to decomposition of water to molecular hydrogen and oxygen. The rate of gas evolutions varied >40-fold depending on solution composition even under the same irradiation condition. With chloride salts, the rate of gas evolution increased in the order rank Na<Ca<Y. However, the trend was reversed with the corresponding nitrate salts, and further addition of nitric acid to the irradiating solution minimized gas evolution. At optimized condition, (89)Zr was produced in moderate yield (4.36 ± 0.48 MBq/μA • h) and high effective specific activity (464 ± 215 MBq/μg) using the solution target approach (2.75 M yttrium nitrate, 1.5 N HNO3, 2h irradiation at 20 μA).

CONCLUSION

The novel findings on substrate dependent, radiation-induced water decomposition provide fundamental data for the development and optimization of conditions for solution targets. The developed methodology of irradiation of nitrate salts in dilute nitric acid solutions can be translated to the production of a wide array of radiometals like (64)Cu, (68)Ga and (86)Y, and is well suited for short-lived isotopes.

Preclinical evaluation of 89Zr-labeled anti-CD44 monoclonal antibody RG7356 in mice and cynomolgus monkeys: Prelude to Phase 1 clinical studies

RG7356 is a humanized antibody targeting the constant region of CD44. RG7356 was radiolabeled with (89)Zr for preclinical evaluations in tumor xenograft-bearing mice and normal cynomolgus monkeys to enable study of its biodistribution and the role of CD44 expression on RG7356 uptake.   Studies with (89)Zr-RG7356 were performed in mice bearing tumor xenografts that differ in the level of CD44 expression (CD44(+) or CD44(-)) and RG7356 responsiveness (resp or non-resp): MDA-MB-231 (CD44(+), resp), PL45 (CD44(+), non-resp) and HepG2 (CD44(-), non-resp). Immuno-PET whole body biodistribution studies were performed in normal cynomolgus monkeys to determine normal organ uptake after administration of a single dose. At 1, 2, 3, and 6 days after injection, (89)Zr-RG7356 uptake in MDA-MB-231 (CD44(+), resp) xenografts was nearly constant and about 9 times higher than in HepG2 (CD44(-), non-resp) xenografts (range 27.44 ± 12.93 to 33.13 ± 7.42% ID/g vs. 3.25 ± 0.38 to 3.90 ± 0.58% ID/g). Uptake of (89)Zr-RG7356 was similar in MDA-MB-231 (CD44(+), resp) and PL45 (CD44(+), non-resp) xenografts. Studies in monkeys revealed antibody uptake in spleen, salivary glands and bone marrow, which might be related to the level of CD44 expression. (89)Zr-RG7356 uptake in these normal organs decreased with increasing dose levels of unlabeled RG7356. (89)Zr-RG7356 selectively targets CD44(+) responsive and non-responsive tumors in mice and CD44(+) tissues in monkeys. These studies indicate the importance of accurate antibody dosing in humans to obtain optimal tumor targeting. Moreover, efficient binding of RG7356 to CD44(+) tumors may not be sufficient in itself to drive an anti-tumor response.

Clinical relevance of novel imaging technologies for sentinel lymph node identification and staging

The sentinel lymph node (SLN) concept has become a standard of care for patients with breast cancer and melanoma, yet its clinical application to other cancer types has been somewhat limited. This is mainly due to the reduced accuracy of conventional SLN mapping techniques (using blue dye and/or radiocolloids as lymphatic tracers) in cancer types where lymphatic drainage is more complex, and SLNs are within close proximity to other nodes or the tumour site. In recent years, many novel techniques for SLN mapping have been developed including fluorescence, x-ray, and magnetic resonant detection. Whilst each technique has its own advantages/disadvantages, the role of targeted contrast agents (for enhanced retention in the SLN, or for immunostaging) is increasing, and may represent the new standard for mapping the SLN in many solid organ tumours. This review article discusses current limitations of conventional techniques, limiting factors of nanoparticulate based contrast agents, and efforts to circumvent these limitations with modern tracer architecture.

Designing the magic bullet? The advancement of immuno-PET into clinical use

The development of noninvasive imaging techniques using monoclonal antibodies (mAbs) is a quickly evolving field. Immuno-PET uses positron-emitting isotopes to track the localization of mAbs with excellent image quality. Procedures for labeling mAbs with (89)Zr or (124)I using good manufacturing procedures have been established, and therefore these radiopharmaceuticals are being investigated for clinical use. This short review will focus on immuno-PET with full mAbs using long-lived positron-emitting isotopes ((89)Zr and (124)I) over the past 5 y and discuss their progress into clinical use.

New strategy for monitoring targeted therapy: molecular imaging

Targeted therapy is becoming an increasingly important component in the treatment of cancer. How to accurately monitor targeted therapy has been crucial in clinical practice. The traditional approach to monitor treatment through imaging has relied on assessing the change of tumor size by refined World Health Organization criteria, or more recently, by the Response Evaluation Criteria in Solid Tumors. However, these criteria, which are based on the change of tumor size, show some limitations for evaluating targeted therapy. Currently, genetic alterations are identified with prognostic as well as predictive potential concerning the use of molecularly targeted drugs. Conversely, considering the limitations of invasiveness and the issue of expression heterogeneity, molecular imaging is better able to assay in vivo biologic processes noninvasively and quantitatively, and has been a particularly attractive tool for monitoring treatment in clinical cancer practice. This review focuses on the applications of different kinds of molecular imaging including positron emission tomography-, magnetic resonance imaging-, ultrasonography-, and computed tomography-based imaging strategies on monitoring targeted therapy. In addition, the key challenges of molecular imaging are addressed to successfully translate these promising techniques in the future.

Extracellular matrix domain formation as an indicator of chondrocyte dedifferentiation and hypertrophy

Cartilage injury represents one of the most significant clinical conditions. Implantation of expanded autologous chondrocytes from noninjured compartments of the joint is a typical strategy for repairing cartilage. However, two-dimensional culture causes dedifferentiation of chondrocytes, making them functionally inferior for cartilage repair. We hypothesized that functional exclusion of dedifferentiated chondrocytes can be achieved by the selective mapping of collagen molecules deposited by chondrogenic cells in a three-dimensional environment. Freshly isolated and in vitro expanded human fetal or adult articular chondrocytes were cultured in a thermoreversible hydrogel at density of 1 × 10(7) cells/mL for 24 h. Chondrocytes were released from the gel, stained with antibodies against collagen type 2 (COL II) or COL I or COL X and sorted by fluorescence activated cell sorting. Imaging flow cytometry, immunohistochemistry, quantitative polymerase chain reaction, and glycosaminoglycan (GAG) assays were performed to evaluate the differences between COL II domain forming and COL II domain-negative cells. Freshly dissected periarticular chondrocytes robustly formed domains that consisted of the extracellular matrix surrounding cells in the hydrogel as a capsule clearly detectable by imaging flow cytometry (ImageStream) and confocal microscopy. These domains were almost exclusively formed by COL II. In contrast to that, a significant percentage of freshly isolated growth plate pre-hypertrophic and hyperdrophic chondrocytes deposited matrix domains positive for COL II, COL I, and COL X. The proportion of the cells producing COL II domains decreased with the increased passage of in vitro expanded periarticular fetal or adult articular chondrocytes. Sorted COL II domain forming cells deposited much higher levels of COL II and GAGs in pellet assays than COL II domain-negative cells. COL II domain forming cells expressed chondrogenic genes at higher levels than negative cells. We report a novel method that allows separation of functionally active chondrogenic cells, which deposit high levels of COL II from functionally inferior dedifferentiated cells or hypertrophic chondrocytes producing COL X. This approach may significantly improve current strategies used for cartilage repair.

(89)Zr, a radiometal nuclide with high potential for molecular imaging with PET: chemistry, applications and remaining challenges

Molecular imaging-and especially Positron Emission Tomography (PET)-is of increasing importance for the diagnosis of various diseases and thus is experiencing increasing dissemination. Consequently, there is a growing demand for appropriate PET tracers which allow for a specific accumulation in the target structure as well as its visualization and exhibit decay characteristics matching their in vivo pharmacokinetics. To meet this demand, the development of new targeting vectors as well as the use of uncommon radionuclides becomes increasingly important. Uncommon nuclides in this regard enable the utilization of various selectively accumulating bioactive molecules such as peptides, antibodies, their fragments, other proteins and artificial structures for PET imaging in personalized medicine. Among these radionuclides, 89Zr (t1/2 = 3.27 days and mean Eβ+ = 0.389 MeV) has attracted increasing attention within the last years due to its favorably long half-life, which enables imaging at late time-points, being especially favorable in case of slowly-accumulating targeting vectors. This review outlines the recent developments in the field of 89Zr-labeled bioactive molecules, their potential and application in PET imaging and beyond, as well as remaining challenges.

Inert coupling of IRDye800CW and zirconium-89 to monoclonal antibodies for single- or dual-mode fluorescence and PET imaging

IRDye800CW and zirconium-89 ((89)Zr) have very attractive properties for optical imaging and positron emission tomography (PET) imaging, respectively. Here we describe a procedure for dual labeling of mAbs with IRDye800CW and (89)Zr in a current good manufacturing practice (cGMP)-compliant way. IRDye800CW and (89)Zr are coupled inertly, without impairment of immunoreactivity and pharmacokinetics of the mAb. Organ and whole-body distribution of the final product can be assessed by optical and PET imaging, respectively. For this purpose, a minimal amount of the chelate N-succinyldesferrioxamine (N-sucDf) is first conjugated to the mAb. Next, N-sucDf-mAb is conjugated with IRDye800CW, after which the N-sucDf-mAb-IRDye800CW is labeled with (89)Zr. After each of these three steps, the product is purified by gel filtration. The sequence of this process avoids unnecessary radiation exposure to personnel and takes about 5 h. The process can be scaled up by the production of large batches of premodified mAbs that can be dispensed and stored until they are labeled with (89)Zr.

Imaging tumor angiogenesis in breast cancer experimental lung metastasis with positron emission tomography, near-infrared fluorescence, and bioluminescence

The goal of this study was to develop a molecular imaging agent that can allow for both positron emission tomography (PET) and near-infrared fluorescence (NIRF) imaging of CD105 expression in metastatic breast cancer. TRC105, a chimeric anti-CD105 monoclonal antibody, was labeled with both a NIRF dye (i.e., IRDye 800CW) and (64)Cu to yield (64)Cu-NOTA-TRC105-800CW. Flow cytometry analysis revealed no difference in CD105 binding affinity/specificity between TRC105 and NOTA-TRC105-800CW. Serial bioluminescence imaging (BLI) was carried out to non-invasively monitor the lung tumor burden in BALB/c mice, after intravenous injection of firefly luciferase-transfected 4T1 (i.e., fLuc-4T1) murine breast cancer cells to establish the experimental lung metastasis model. Serial PET imaging revealed that fLuc-4T1 lung tumor uptake of (64)Cu-NOTA-TRC105-800CW was 11.9 ± 1.2, 13.9 ± 3.9, and 13.4 ± 2.1 %ID/g at 4, 24, and 48 h post-injection respectively (n = 3). Biodistribution studies, blocking fLuc-4T1 lung tumor uptake with excess TRC105, control experiments with (64)Cu-NOTA-cetuximab-800CW (which served as an isotype-matched control), ex vivo BLI/PET/NIRF imaging, autoradiography, and histology all confirmed CD105 specificity of (64)Cu-NOTA-TRC105-800CW. Successful PET/NIRF imaging of tumor angiogenesis (i.e., CD105 expression) in the breast cancer experimental lung metastasis model warrants further investigation and clinical translation of dual-labeled TRC105-based agents, which can potentially enable early detection of small metastases and image-guided surgery for tumor removal.

Imaging of the β-cells of the islets of Langerhans

The major aim of this paper is to review the present status of the techniques for the non-invasive imaging and quantification of insulin-producing pancreatic islet β-cells. Emphasis is placed on both the expansion of prior work already considered in a prior review and novel achievements. Thus, the use of d-mannoheptulose analogs, hypoglycemic sulfonylureas and glinides, neural imaging agents, neuro-hormonal receptor ligands and nanoparticles is first dealt with. Thereafter, consideration is given on optical imaging technologies, the identification of new β-cells specific binding and target proteins, the functional imaging of islets transplanted into the eye anterior chamber and in vivo manganese-enhanced magnetic resonance imaging.

PET Tracers for Clinical Imaging of Breast Cancer

Molecular imaging of breast cancer has undoubtedly permitted a substantial development of the overall diagnostic accuracy of this malignancy in the last years. Accurate tumour staging, design of individually suited therapies, response evaluation, early detection of recurrence and distant lesions have also evolved in parallel with the development of novel molecular imaging approaches. In this context, positron emission tomography (PET) can be probably seen as the most interesting molecular imaging technology with straightforward clinical application for such purposes. Dozens of radiotracers for PET imaging of breast cancer have been tested in laboratory animals. However, in this review we shall focus mainly in the smaller group of PET radiopharmaceuticals that have lead through into the clinical setting. PET imaging can be used to target general metabolic phenomena related to tumoural transformation, including glucose metabolism and cell proliferation, but can also be directed to specific hormone receptors that are characteristic of the breast cancer cell. Many other receptors and transport molecules present in the tumour cells could also be of interest for imaging. Furthermore, molecules related with the tumour microenvironment, tumour induced angiogenesis or even hypoxia could also be used as molecular biomarkers for breast cancer imaging.

Multimodality imaging of breast cancer experimental lung metastasis with bioluminescence and a monoclonal antibody dual-labeled with 89Zr and IRDye 800CW

Metastatic breast cancer is incurable. The goal of this study was to develop a positron emission tomography (PET)/near-infrared fluorescent (NIRF) probe for imaging CD105 expression in breast cancer experimental lung metastasis. TRC105, a chimeric anti-CD105 antibody, was dual-labeled with a NIRF dye (IRDye 800CW) and (89)Zr to yield (89)Zr-Df-TRC105-800CW. Luciferase-transfected 4T1 murine breast cancer cells were injected intravenously into female mice to establish the tumor model. Bioluminescence imaging (BLI) was carried out to noninvasively monitor the lung tumor burden. PET imaging revealed that 4T1 lung tumor uptake of (89)Zr-Df-TRC105-800CW was 8.7 ± 1.4, 10.9 ± 0.5, and 9.7 ± 1.1% ID/g at 4, 24, and 48 h postinjection (n = 4), with excellent tumor contrast. Biodistribution studies, blocking, control studies with (89)Zr-Df-cetuximab-800CW, ex vivo BLI/PET/NIRF imaging, and histology all confirmed CD105 specificity of the tracer. Broad clinical potential of TRC105-based agents was shown in many tumor types, which also enabled early detection of small metastasis and intraoperative guidance for tumor removal.