Immuno-PET of Cancer: A Revival of Antibody Imaging

89Zr-Immuno-PET with Immune Checkpoint Inhibitors: Measuring Target Engagement in Healthy Organs

INTRODUCTION

89Zr-immuno-PET (positron emission tomography with zirconium-89-labeled monoclonal antibodies ([89Zr]Zr-mAbs)) can be used to study the biodistribution of mAbs targeting the immune system. The measured uptake consists of target-specific and non-specific components, and it can be influenced by plasma availability of the tracer. To find evidence for target-specific uptake, i.e., target engagement, we studied five immune-checkpoint-targeting [89Zr]Zr-mAbs to (1) compare the uptake with previously reported baseline values for non-specific organ uptake (ns-baseline) and (2) look for saturation effects of increasing mass doses.

METHOD

89Zr-immuno-PET data from five [89Zr]Zr-mAbs, i.e., nivolumab and pembrolizumab (anti-PD-1), durvalumab (anti-PD-L1), BI 754,111 (anti-LAG-3), and ipilimumab (anti-CTLA-4), were analysed. For each mAb, 2-3 different mass doses were evaluated. PET scans and blood samples from at least two time points 24 h post injection were available. In 35 patients, brain, kidneys, liver, spleen, lungs, and bone marrow were delineated. Patlak analysis was used to account for differences in plasma activity concentration and to quantify irreversible uptake (Ki). To identify target engagement, Ki values were compared to ns-baseline Ki values previously reported, and the effect of increasing mass doses on Ki was investigated.

RESULTS

All mAbs, except ipilimumab, showed Ki values in spleen above the ns-baseline for the lowest administered mass dose, in addition to decreasing Ki values with higher mass doses, both indicative of target engagement. For bone marrow, no ns-baseline was established previously, but a similar pattern was observed. For kidneys, most mAbs showed Ki values within the ns-baseline for both low and high mass doses. However, with high mass doses, some saturation effects were seen, suggestive of a lower ns-baseline value. Ki values were near zero in brain tissue for all mass doses of all mAbs.

CONCLUSION

Using Patlak analysis and the established ns-baseline values, evidence for target engagement in (lymphoid) organs for several immune checkpoint inhibitors could be demonstrated. A decrease in the Ki values with increasing mass doses supports the applicability of Patlak analysis for the assessment of target engagement for PET ligands with irreversible uptake behavior.

Recent Advances in the Development of Tetrazine Ligation Tools for Pretargeted Nuclear Imaging

Tetrazine ligation has gained interest as a bio-orthogonal chemistry tool within the last decade. In nuclear medicine, tetrazine ligation is currently being explored for pretargeted approaches, which have the potential to revolutionize state-of-the-art theranostic strategies. Pretargeting has been shown to increase target-to-background ratios for radiopharmaceuticals based on nanomedicines, especially within early timeframes. This allows the use of radionuclides with short half-lives which are more suited for clinical applications. Pretargeting bears the potential to increase the therapeutic dose delivered to the target as well as reduce the respective dose to healthy tissue. Combined with the possibility to be applied for diagnostic imaging, pretargeting could be optimal for theranostic approaches. In this review, we highlight efforts that have been made to radiolabel tetrazines with an emphasis on imaging.

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.

Improved Quantification of 18F-FDG PET during 131I-Rituximab Therapy on Mouse Lymphoma Models after 131I Prompt Emission Correction

¹⁸F-FDG Positron Emission Tomography (PET) is used to monitor tumor response to ¹³¹I-therapy, but is confounded by prompt emissions (284, 364, 637, and 723 keV) from ¹³¹I, particularly in animal PET imaging. We propose a method for correcting this emission in ¹⁸F-FDG PET. The ¹³¹I prompt emission effect was assessed within various energy windows and various activities. We applied a single gamma correction method to a phantom and in vivo mouse model. The ¹³¹I prompt emission fraction was 12% when 300 µCi of ¹³¹I and 100 µCi of FDG were administered, and increased exponentially with escalating ¹³¹I activity for all energy windows. The difference in spill-over ratio was reduced to <5% after ¹³¹I prompt emission correction. In the mouse model, the standard uptake value (SUV) did not differ significantly between FDG PET only (gold standard) and FDG PET after ¹³¹I prompt emission-correction, whereas it was overestimated by 38% before correction. Contrast was improved by 18% after ¹³¹I prompt emission correction. We first found that count contamination on ¹⁸F-FDG follow-up scans due to ¹³¹I spilled-over count after ¹³¹I rituximab tumor targeted therapy. Our developed ¹³¹I prompt emission-correction method increased accuracy during measurement of standard uptake values on ¹⁸F-FDG PET.

Clinical Results in Medullary Thyroid Carcinoma Suggest High Potential of Pretargeted Immuno-PET for Tumor Imaging and Theranostic Approaches

Monoclonal antibody (mAb)-based therapies have experienced considerable growth in cancer management. When labeled with radionuclides, mAbs also represent promising probes for imaging or theranostic approaches. Initially, mAbs have been radiolabeled with single-photon emitters, such as 131I, 99mTc, or 111In, for diagnostic purposes or to improve radioimmunotherapy (RIT) using dosimetry estimations. Today, more accurate imaging is achieved using positron- emission tomography (PET). Thanks to the important technical advances in the production of PET emitters and their related radiolabeling methods, the last decade has witnessed the development of a broad range of new probes for specific PET imaging. Immuno-PET, which combines the high sensitivity and resolution of a PET camera with the specificity of a monoclonal antibody, is fully in line with this approach. As RIT, immuno-PET can be performed using directly radiolabeled mAbs or using pretargeting to improve imaging contrast. Pretargeted immuno-PET has been developed against different antigens, and promising results have been reported in tumor expressing carcinoembryonic antigen (CEA; CEACAM5) using a bispecific mAb (BsmAb) and a radiolabeled peptide. Medullary thyroid carcinoma (MTC) is an uncommon thyroid cancer subtype which accounts for <10% of all thyroid neoplasms. Characterized by an intense expression of CEA, MTC represents a relevant tumor model for immuno-PET. High sensitivity of pretargeted immunoscintigraphy using murine or chimeric anti-CEA BsMAb and pretargeted haptens-peptides labeled with 111In or 131I were reported in metastatic MTC patients 20 years ago. Recently, an innovative clinical study reported high tumor uptake and contrast using pretargeted anti-CEA immuno-PET in relapsed MTC patients. This review focuses on MTC as an example, but the same pretargeting technique has been applied with success for clinical PET imaging of other CEA-expressing tumors and other pretargeting systems. In particular, those exploiting bioorthogonal chemistry also appear interesting in preclinical animal models, suggesting the high potential of pretargeting for diagnostic and theranostic applications.

Selection Criteria for Determination of Optimal Reconstruction Method for Cu-64 Trastuzumab Dosimetry on Siemens Inveon PET Scanner

The goal of this study was to suggest criteria for the determination of the optimal image reconstruction algorithm for image-based dosimetry of Cu-64 trastuzumab PET in a mouse model. Image qualities, such as recovery coefficient (RC), spill-over ratio (SOR), and non-uniformity (NU), were measured according to National Electrical Manufacturers Association (NEMA) NU4-2008. Mice bearing a subcutaneous tumor ( 200 &nbsp; mm 3 , HER2 NCI N87) were injected with monoclonal antibodies (trastuzumab) with Cu-64. Preclinical mouse PET images were acquired at 4 time points after injection (2, 15, 40 and 64 h). Phantom and Cu-64 trastuzumab PET images were reconstructed using various reconstruction algorithms (filtered back projection (FBP), 3D reprojection algorithm (FBP-3DRP), 2D ordered subset expectation maximization (OSEM 2D), and OSEM 3D maximum a posteriori (OSEM3D-MAP)) and filters. The absorbed dose for the tumor and the effective dose for organs for Cu-64 trastuzumab PET were calculated using the OLINDA/EXM program with various reconstruction algorithms. Absorbed dose for the tumor ranged from 923 mGy/MBq to 1830 mGy/MBq with application of reconstruction algorithms and filters. When OSEM2D was used, the effective osteogenic dose increased from 0.0031 to 0.0245 with an increase in the iteration number (1 to 10). In the region of kidney, the effective dose increased from 0.1870 to 1.4100 when OSEM2D was used with iteration number 1 to 10. To determine the optimal reconstruction algorithms and filters, a correlation between RC and NU was plotted and selection criteria (0.9 < RC < 1.0 and < 10% of NU) were suggested. According to the selection criteria, OSEM2D (iteration 1) was chosen for the optimal reconstruction algorithm. OSEM2D (iteration 10) provided 154.7% overestimated effective dose and FBP with a Butterworth filter provided 20.9% underestimated effective dose. We suggested OSEM2D (iteration 1) for the calculation of the effective dose of Cu-64 trastuzumab on an Inveon PET scanner.

Clinical Potential of Human Epidermal Growth Factor Receptor 2 and Human Epidermal Growth Factor Receptor 3 Imaging in Breast Cancer

Increased expression of the human epidermal growth factor receptor (HER) protein family are targets in breast cancer for imaging and therapy. Imaging modalities targeting HER2 and HER3 can diagnose breast cancer with a specific, biologically relevant target. Repeat biopsies do not address heterogeneity intratumorally or between primary disease and metastasis. HER2- and HER3-targeted PET is an important tool to diagnose disease in breast cancer and evaluate response to targeted therapies. PET and single photon emission computed tomography with radiolabeled biomolecules can be used to detect and quantify specific targets, conferring a better understanding of the behavior and effectiveness of treatments.

Human Epidermal Growth Factor Receptor 2-Targeted PET/Single- Photon Emission Computed Tomography Imaging of Breast Cancer: Noninvasive Measurement of a Biomarker Integral to Tumor Treatment and Prognosis

Increased human epidermal growth factor receptor 2 (HER2) expression is a hallmark of aggressive breast cancer. Imaging modalities have the potential to diagnose HER2-positive breast cancer and detect distant metastases. The heterogeneity of HER2 expression between primary and metastatic disease sites limits the value of tumor biopsies. Molecular imaging is a noninvasive tool to assess HER2-positive primary lesions and metastases. Radiolabeled antibodies, antibody fragments, and affibody molecules devise a reliable and quantitative method for detecting HER2-positive cancer using PET. HER2-targeted PET imaging is a valuable clinical tool with respect to both the care and maintenance of patients with breast cancer.

Quantitative Imaging for Targeted Radionuclide Therapy Dosimetry - Technical Review

Targeted radionuclide therapy (TRT) is a promising technique for cancer therapy. However, in order to deliver the required dose to the tumor, minimize potential toxicity in normal organs, as well as monitor therapeutic effects, it is important to assess the individualized internal dosimetry based on patient-specific data. Advanced imaging techniques, especially radionuclide imaging, can be used to determine the spatial distribution of administered tracers for calculating the organ-absorbed dose. While planar scintigraphy is still the mainstream imaging method, SPECT, PET and bremsstrahlung imaging have promising properties to improve accuracy in quantification. This article reviews the basic principles of TRT and discusses the latest development in radionuclide imaging techniques for different theranostic agents, with emphasis on their potential to improve personalized TRT dosimetry.

Multifunctional Desferrichrome Analogues as Versatile 89Zr(IV) Chelators for ImmunoPET Probe Development

New bifunctional hexa- and octadentate analogues of the hydroxamate-containing siderophore desferrichrome (DFC) have been synthesized and evaluated as ⁸⁹Zr-chelating agents for immunoPET applications. The in vitro and in vivo inertness of these new ligands, Orn3-hx (hexadentate) and Orn-4hx derivatives (octadentate), was compared to the gold standard hexadentate, hydroxamate-containing chelator for ⁸⁹Zr desferrioxamine (DFO). Density functional theory was employed to model the geometries of the resulting Zr(IV) complexes and to predict their relative stabilities as follows: Zr(Orn4-hx) > Zr(DFC) > Zr(Orn3-hx). Transchelation challenge experiments of the corresponding radiochemical complexes with excess ethylenediaminetetraacetate (EDTA) indicated complex stability in accordance with DFT calculations. Radiolabeling of these ligands with ⁸⁹Zr was quantitative (0.25 μmol of ligand, pH 7.4, room temperature, 20 min). For antibody conjugation, the isothiocyanate (NCS) functional group was introduced to the N terminus of Orn3-hx and Orn-4hx. An additional trifunctional derivative that bears a silicon-rhodamine fluorophore on the C-terminus and NCS on the N terminus was also furnished. As proof of concept, all NCS derivatives were conjugated to the HER2-targeting antibody, trastuzumab. Radiolabeling of immunoconjugates with ⁸⁹Zr was accomplished with radiochemical yields of 16 ± 2% to 95 ± 2%. These constructs were administered to naive mice (male, C57BL/6J, n = 4) to assess in vivo inertness, which is inversely correlated with uptake of ⁸⁹Zr in bone, after 96 h circulation time. We found bone uptake to range from 7.0 ± 2.2 to 10.7 ± 1.3% ID/g, values that compare well to the corresponding DFO conjugate (7.1 ± 0.8% ID/g). In conclusion, we have rationally designed linear, bifunctional and trifunctional desferrichrome analogues suitable for the mild and inert radiolabeling of antibodies with the radionuclide ⁸⁹Zr.

In Vivo Pretargeted Imaging of HER2 and TAG-72 Expression Using the HaloTag Enzyme

A novel pretargeted SPECT imaging strategy based on the HaloTag enzyme has been evaluated for the first time in a living system. To determine the efficacy of this approach, two clinically relevant cancer biomarkers, HER2 and TAG-72, were selected to represent models of internalizing and noninternalizing antigens, respectively. In MDA-MB-231/H2N (HER2-expressing) and LS174T (TAG-72-expressing) xenograft tumors in mice, pretargeting experiments were performed in which HaloTag-conjugated derivatives of the antibodies trastuzumab (anti-HER2) or CC49 (anti-TAG-72) were utilized as primary agents, and the small molecule HaloTag ligands 111In-HTL-1, -2, and -3 were evaluated as secondary agents. While this approach was not sufficiently sensitive to detect the internalizing HER2 antigen, pretargeting experiments involving the most optimal secondary agent, 111In-HTL-3, were successful in detecting the noninternalizing antigen TAG-72 and provided high-contrast SPECT images at 4 and 24 h postinjection.

Immuno-PET for Clinical Theranostic Approaches

Recent advances in molecular characterization of tumors have allowed identification of new molecular targets on tumor cells or biomarkers. In medical practice, the identification of these biomarkers slowly but surely becomes a prerequisite before any treatment decision, leading to the concept of personalized medicine. Immuno-positron emission tomography (PET) fits perfectly with this approach. Indeed, monoclonal antibodies (mAbs) labelled with radionuclides represent promising probes for theranostic approaches, offering a non-invasive solution to assess in vivo target expression and distribution. Immuno-PET can potentially provide useful information for patient risk stratification, diagnosis, selection of targeted therapies, evaluation of response to therapy, prediction of adverse effects or for titrating doses for radioimmunotherapy. This paper reviews some aspects and recent developments in labelling methods, biological targets, and clinical data of some novel PET radiopharmaceuticals.

Immuno-PET Using Anticarcinoembryonic Antigen Bispecific Antibody and 68Ga-Labeled Peptide in Metastatic Medullary Thyroid Carcinoma: Clinical Optimization of the Pretargeting Parameters in a First-in-Human Trial

Earlier clinical studies reported a high sensitivity of pretargeted immunoscintigraphy using murine or chimeric anticarcinoembryonic antigen (CEA) bispecific antibody (BsMAb) and peptides labeled with ¹¹¹In or ¹³¹I in medullary thyroid carcinoma (MTC). Preclinical studies showed that new-generation humanized recombinant anti-CEA × antihistamine-succinyl-glycine (HSG) trivalent BsMAb TF2 and radiolabeled HSG peptide (IMP288) present good features for PET. This study aimed at optimizing molar doses and pretargeting interval of TF2 and ⁶⁸Ga-labeled IMP288 for immuno-PET in relapsed MTC patients with calcitonin serum levels greater than 150 pg/mL.

METHODS

Five cohorts (C1-C5) of 3 patients received variable molar doses of TF2 and approximately 150 MBq of ⁶⁸Ga-IMP288 after different pretargeting time intervals (C1: 120 nmol TF2, 6 nmol IMP288, 24 h; C2: 120 nmol TF2, 6 nmol IMP288, 30 h; C3: 120 nmol TF2, 6 nmol IMP288, 42 h; C4: 120 nmol TF2, 3 nmol IMP288, 30 h; and C5: 60 nmol TF2, 3 nmol IMP288, 30 h). TF2 and ⁶⁸Ga-IMP288 pharmacokinetics were monitored. Whole-body PET was recorded 60 and 120 min after ⁶⁸Ga-IMP288 injection. Tumor maximal SUV (T-SUV_max) and T-SUV_max-to-mediastinum blood-pool (MBP) SUV_mean ratios (T/MBP) were determined.

RESULTS

In C1, T-SUV_max and T/MBP ranged from 4.09 to 8.93 and 1.39 to 3.72 at 60 min and 5.14 to 11.25 and 2.73 to 5.38 at 120 min, respectively. Because of the high MBP, the delay was increased to 30 h in C2, increasing T-SUV_max and T/MBP. Further increasing the delay to 42 h in C3 decreased T-SUV_max and T/MBP, showing that 30 h was the most favorable delay. In C4, the TF2-to-peptide mole ratio was increased to 40 (delay 30 h), resulting in high T-SUV_max but with higher MBP than in C2. In C5, the molar dose of TF2 was reduced, resulting in lower imaging performance. Pharmacokinetics demonstrated a fast TF2 clearance and a clear relationship between blood activity clearance and the ratio between the molar amount of injected peptide to the molar amount of circulating TF2 at the time of peptide injection.

CONCLUSION

High tumor uptake and contrast can be obtained with pretargeted anti-CEA immuno-PET in relapsed MTC patients, especially using optimized pretargeting parameters: a BsMAb-to-peptide mole ratio of 20 and 30 h pretargeting delay.

Combination Radioimmunotherapy Approaches and Quantification of Immuno-PET

Monoclonal antibodies (mAbs), which play a prominent role in cancer therapy, can interact with specific antigens on cancer cells, thereby enhancing the patient's immune response via various mechanisms, or mAbs can act against cell growth factors and, thereby, arrest the proliferation of tumor cells. Radionuclide-labeled mAbs, which are used in radioimmunotherapy (RIT), are effective for cancer treatment because tumor associated-mAbs linked to cytotoxic radionuclides can selectively bind to tumor antigens and release targeted cytotoxic radiation. Immunological positron emission tomography (immuno-PET), which is the combination of PET with mAb, is an attractive option for improving tumor detection and mAb quantification. However, RIT remains a challenge because of the limited delivery of mAb into tumors. The transport and uptake of mAb into tumors is slow and heterogeneous. The tumor microenvironment contributed to the limited delivery of the mAb. During the delivery process of mAb to tumor, mechanical drug resistance such as collagen distribution or physiological drug resistance such as high intestinal pressure or absence of lymphatic vessel would be the limited factor of mAb delivery to the tumor at a potentially lethal mAb concentration. When α-emitter-labeled mAbs were used, deeper penetration of α-emitter-labeled mAb inside tumors was more important because of the short range of the α emitter. Therefore, combination therapy strategies aimed at improving mAb tumor penetration and accumulation would be beneficial for maximizing their therapeutic efficacy against solid tumors.

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.

Quantitative in vivo cell-surface receptor imaging in oncology: kinetic modeling and paired-agent principles from nuclear medicine and optical imaging

The development of methods to accurately quantify cell-surface receptors in living tissues would have a seminal impact in oncology. For example, accurate measures of receptor density in vivo could enhance early detection or surgical resection of tumors via protein-based contrast, allowing removal of cancer with high phenotype specificity. Alternatively, accurate receptor expression estimation could be used as a biomarker to guide patient-specific clinical oncology targeting of the same molecular pathway. Unfortunately, conventional molecular contrast-based imaging approaches are not well adapted to accurately estimating the nanomolar-level cell-surface receptor concentrations in tumors, as most images are dominated by nonspecific sources of contrast such as high vascular permeability and lymphatic inhibition. This article reviews approaches for overcoming these limitations based upon tracer kinetic modeling and the use of emerging protocols to estimate binding potential and the related receptor concentration. Methods such as using single time point imaging or a reference-tissue approach tend to have low accuracy in tumors, whereas paired-agent methods or advanced kinetic analyses are more promising to eliminate the dominance of interstitial space in the signals. Nuclear medicine and optical molecular imaging are the primary modalities used, as they have the nanomolar level sensitivity needed to quantify cell-surface receptor concentrations present in tissue, although each likely has a different clinical niche.

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.

Whole-body immunoPET reveals active SIV dynamics in viremic and antiretroviral therapy-treated macaques

The detection of viral dynamics and localization in the context of controlled HIV infection remains a challenge and is limited to blood and biopsies. We developed a method to capture total-body simian immunodeficiency virus (SIV) replication using immunoPET (antibody-targeted positron emission tomography). The administration of a poly(ethylene glycol)-modified, (64)Cu-labeled SIV Gp120-specific antibody led to readily detectable signals in the gastrointestinal and respiratory tract, lymphoid tissues and reproductive organs of viremic monkeys. Viral signals were reduced in aviremic antiretroviral-treated monkeys but detectable in colon, select lymph nodes, small bowel, nasal turbinates, the genital tract and lung. In elite controllers, virus was detected primarily in foci in the small bowel, select lymphoid areas and the male reproductive tract, as confirmed by quantitative reverse-transcription PCR (qRT-PCR) and immunohistochemistry. This real-time, in vivo viral imaging method has broad applications to the study of immunodeficiency virus pathogenesis, drug and vaccine development, and the potential for clinical translation.

Development of an enzymatic pretargeting strategy for dual-modality imaging

A pretargeted imaging strategy based on the HaloTag dehalogenase enzyme is described. Here, a HaloTag-Trastuzumab conjugate has been used as the primary agent targeting HER2 expression, and three new radiolabelled HaloTag ligands have been used as secondary agents, two of which offer dual-modality (SPECT/optical) imaging capability.

Tumor immunotargeting using innovative radionuclides

This paper reviews some aspects and recent developments in the use of antibodies to target radionuclides for tumor imaging and therapy. While radiolabeled antibodies have been considered for many years in this context, only a few have reached the level of routine clinical use. However, alternative radionuclides, with more appropriate physical properties, such as lutetium-177 or copper-67, as well as alpha-emitting radionuclides, including astatine-211, bismuth-213, actinium-225, and others are currently reviving hopes in cancer treatments, both in hematological diseases and solid tumors. At the same time, PET imaging, with short-lived radionuclides, such as gallium-68, fluorine-18 or copper-64, or long half-life ones, particularly iodine-124 and zirconium-89 now offers new perspectives in immuno-specific phenotype tumor imaging. New antibody analogues and pretargeting strategies have also considerably improved the performances of tumor immunotargeting and completely renewed the interest in these approaches for imaging and therapy by providing theranostics, companion diagnostics and news tools to make personalized medicine a reality.

Radioimmunoconjugates for the treatment of cancer

Radioimmunotherapy (RIT) has been developed for more than 30 years. Two products targeting the CD20 antigen are approved in the treatment of non-Hodgkin B-cell lymphoma (NHBL): iodine 131-tositumomab and yttrium 90-ibritumomab tiuxetan. RIT can be integrated in clinical practice for the treatment of patients with relapsed or refractory follicular lymphoma (FL) or as consolidation after induction chemotherapy. High-dose treatment, RIT in first-line treatment, fractionated RIT, and use of new humanized monoclonal antibodies (MAbs), in particular targeting CD22, showed promising results in NHBL. In other hemopathies, such as multiple myeloma, efficacy has been demonstrated in preclinical studies. In solid tumors, more resistant to radiation and less accessible to large molecules such as MAbs, clinical efficacy remains limited. However, pretargeting methods have shown clinical efficacy. Finally, new beta emitters such as lutetium 177, with better physical properties will further improve the safety of RIT and alpha emitters, such as bismuth 213 or astatine 211, offer the theoretical possibility to eradicate the last microscopic clusters of tumor cells, in the consolidation setting. Personalized treatments, based on quantitative positron emission tomography (PET), pre-therapeutic imaging, and dosimetry procedures, also could be applied to adapt injected activity to each patient.

Preclinical evaluation of a radioiodinated fully human antibody for in vivo imaging of vascular adhesion protein-1-positive vasculature in inflammation

Vascular adhesion protein-1 (VAP-1) is an endothelial glycoprotein mediating leukocyte trafficking from blood to sites of inflammation. BTT-1023 is a fully human monoclonal anti-VAP-1 antibody developed to treat inflammatory diseases. In this study, we preclinically evaluated radioiodinated BTT-1023 for inflammation imaging.

METHODS

Rabbits were intravenously injected with radioiodinated BTT-1023. Distribution and pharmacokinetics were assessed by PET/CT up to 72 h after injection. Human radiation dose estimates for (124)I-BTT-1023 were extrapolated. Additionally, rabbits with chemically induced synovitis were imaged with (123)I-BTT-1023 SPECT/CT.

RESULTS

Radioiodinated BTT-1023 cleared rapidly from blood circulation and distributed to liver and thyroid. Inflamed joints were delineated by SPECT/CT. The estimated human effective dose due to (124)I-BTT-1023 was 0.55 mSv/MBq, if blockage of thyroid uptake is assumed.

CONCLUSION

The radioiodinated BTT-1023 was able to detect mild inflammation in vivo. Clinical (124)I-BTT-1023 PET studies with injected radioactivity of 0.5-0.7 MBq/kg may be justified.

New frontiers in the design and synthesis of imaging probes for PET oncology: current challenges and future directions

Despite being developed over 30 years ago, 2-deoxy-2-[(18)F]fluoro-D-glucose remains the most frequently used radiotracer in PET oncology. In the last decade, interest in new and more specific radiotracers for imaging biological processes of oncologic interest has increased exponentially. This review summarizes the strategies underlying the development of those probes together with their validation and status of clinical translation; a brief summary of new radiochemistry strategies applicable to PET imaging is also included. The article finishes with a consideration of the challenges imaging scientists must overcome to bring about increased adoption of PET as a diagnostic or pharmacologic tool.

Carcinoembryonic antigen is the preferred biomarker for in vivo colorectal cancer targeting

Background:

Colorectal cancer-specific biomarkers have been used as molecular targets for fluorescent intra-operative imaging, targeted PET/MRI, and selective cytotoxic drug delivery yet the selection of biomarkers used is rarely evidence-based. We evaluated sensitivities and specificites of four of the most commonly used markers: carcinoembryonic antigen (CEA), tumour-associated glycoprotein-72 (TAG-72), folate receptor-α (FRα) and Epithelial growth factor receptor (EGFR).

Methods:

Marker expression was evaluated semi-quantitatively in matched mucosal and colorectal cancer tissues from 280 patients using immunohistochemistry (scores of 0–15). Matched positive and negative lymph nodes from 18 patients were also examined.

Results:

Markers were more highly expressed in tumour tissue than in matched normal tissue in 98.8%, 79.0%, 37.1% and 32.8% of cases for CEA, TAG-72, FRα and EGFR, respectively. Carcinoembryonic antigen showed the greatest differential expression, with tumours scoring a mean of 10.8 points higher than normal tissues (95% CI 10.31–11.21, P<0.001). Similarly, CEA showed the greatest differential expression between positive and negative lymph nodes. Receiver operating characteristic analyses showed CEA to have the best sensitivity (93.7%) and specificity (96.1%) for colorectal cancer detection.

Conclusion:

Carcinoembryonic antigen has the greatest potential to allow highly specific tumour imaging and drug delivery; future translational research should aim to exploit this.

Cancer stem cell targeting: the next generation of cancer therapy and molecular imaging

Cancer stem cells (CSCs) have the capacity to generate the heterogeneous lineages of all cancer cells comprising a tumor and these populations of cells are likely to be more relevant in determining prognosis. However, these cells do not operate in isolation, but instead rely upon signals co-opted from their microenvironment, making the targeting and imaging of CSCs within a cancer mass a daunting task. A better understanding of the molecular cell biology underlying CSC pathology will facilitate the development of new therapeutic targets and novel strategies for the successful eradication of cancer. In addition, the continued investigation of sensitive molecular-imaging modalities will enable more accurate staging, treatment planning and the ability to monitor the effectiveness of CSC-targeted therapies in vivo. In this review, we explore the possibilities and limitations of CSC-directed therapies and molecular imaging modalities.

Molecular imaging in the management of cervical cancer

Positron emission tomography (PET), magnetic resonance imaging (MRI), and integrated 18-fluorodeoxyglucose ((18)F-FDG) PET/computed tomography are valuable techniques for assessing prognosis, treatment response after the completion of concurrent chemoradiation, suspicious or documented recurrence, unexplained post therapy elevations in tumor markers, and the response to salvage treatment when managing cervical cancer. However, PET plays a limited role in the primary staging of MRI-defined node-negative patients. Currently, (18)F-FDG is still the only tracer approved for routine use, but several novel targeting PET compounds, high-Tesla MRI machines, diffusion-weighted imaging without contrast, and dynamic nuclear polarized-enhanced (13)C-MR spectroscopic imaging may hold promising applications.