Preparation and evaluation of 89Zr-Zevalin for monitoring of 90Y-Zevalin biodistribution with positron emission tomography

The bioconjugation and radiosynthesis of 89Zr-DFO-labeled antibodies

The exceptional affinity, specificity, and selectivity of antibodies make them extraordinarily attractive vectors for tumor-targeted PET radiopharmaceuticals. Due to their multi-day biological half-life, antibodies must be labeled with positron-emitting radionuclides with relatively long physical decay half-lives. Traditionally, the positron-emitting isotopes 124I (t1/2=4.18 d), 86Y (t1/2=14.7 hr), and 64Cu (t1/2=12.7 hr) have been used to label antibodies for PET imaging. More recently, however, the field has witnessed a dramatic increase in the use of the positron-emitting radiometal 89Zr in antibody-based PET imaging agents. 89Zr is a nearly ideal radioisotope for PET imaging with immunoconjugates, as it possesses a physical half-life (t1/2=78.4 hr) that is compatible with the in vivo pharmacokinetics of antibodies and emits a relatively low energy positron that produces high resolution images. Furthermore, antibodies can be straightforwardly labeled with 89Zr using the siderophore-derived chelator desferrioxamine (DFO). In this protocol, the prostate-specific membrane antigen targeting antibody J591 will be used as a model system to illustrate (1) the bioconjugation of the bifunctional chelator DFO-isothiocyanate to an antibody, (2) the radiosynthesis and purification of a 89Zr-DFO-mAb radioimmunoconjugate, and (3) in vivo PET imaging with an 89Zr-DFO-mAb radioimmunoconjugate in a murine model of cancer.

Radiometals: towards a new success story in nuclear imaging?

The use of radiometal isotopes in positron emission tomography: a new success story in nuclear imaging?

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.

Pilot study of (89)Zr-bevacizumab positron emission tomography in patients with advanced non-small cell lung cancer

BACKGROUND

The aim of this pilot study was to evaluate whether the uptake of (89)Zr-bevacizumab in non-small cell lung cancer (NSCLC) tumors could be visualized and quantified. The correlation between tumor (89)Zr-bevacizumab uptake and tumor response to antitumor therapy with a bevacizumab-based regimen was explored.

METHODS

Seven NSCLC patients underwent static PET scans at days 4 and 7 after injection of 36.4 ± 0.9 MBq (mean ± SD) (89)Zr-bevacizumab, prior to commencing carboplatin-paclitaxel-bevacizumab chemotherapy (CPB). Overall survival (OS) and progression-free survival (PFS) to CPB followed by bevacizumab maintenance therapy was correlated to tumor tracer uptake, quantified using peak standardized uptake values (SUVpeak).

RESULTS

Zr-bevacizumab uptake (SUVpeak) was approximately four times higher in tumor tissues (primary tumor and metastases) than in non-tumor tissues (healthy muscle, lung, and fat) on days 4 and 7. A positive trend but no significant correlation could be found between SUVpeak and OS or PFS.

CONCLUSIONS

This pilot study shows that (89)Zr-bevacizumab PET imaging in NSCLC is feasible. Further investigation to validate this technique as a predictive biomarker for selecting patients for bevacizumab treatment is warranted.

Preparation and radiolabeling of a lyophilized (kit) formulation of DOTA-rituximab with ⁹⁰Y and ¹¹¹In for domestic radioimmunotherapy and radioscintigraphy of non-Hodgkin's lymphoma

Background

On the basis of results of our previous investigations on ⁹⁰Y-DTPA-rituximab and in order to fulfil national demands to radioimmunoconjugates for radioscintigraphy and radioimmunotherapy of Non-Hodgkin’s Lymphoma (NHL), preparation and radiolabeling of a lyophilized formulation (kit) of DOTA-rituximab with ¹¹¹In and ⁹⁰Y was investigated.

Methods

¹¹¹In and ⁹⁰Y with high radiochemical and radionuclide purity were prepared by ¹¹²Cd (p,2n)¹¹¹In nuclear reaction and a locally developed ⁹⁰Sr/⁹⁰Y generator, respectively. DOTA-rituximab immunoconjugates were prepared by the reaction of solutions of p-SCN-Bz-DOTA and rituximab in carbonate buffer (pH = 9.5) and the number of DOTA per molecule of conjugates were determined by transchelation reaction between DOTA and arsenaso yttrium(III) complex. DOTA-rituximab immunoconjugates were labeled with ¹¹¹In and ⁹⁰Y and radioimmunoconjugates were checked for radiochemical purity by chromatography methods and for immunoreactivity by cell-binding assay using Raji cell line. The stability of radiolabeled conjugate with the approximate number of 7 DOTA molecules per one rituximab molecule which was prepared in moderate yield and showed moderate immunoreactivity, compared to two other prepared radioimmunoconjugates, was determined at different time intervals and against EDTA and human serum by chromatography methods and reducing SDS-polyacrylamide gel electrophoresis, respectively. The biodistribution of the selected radioimmunoconjugate in rats was determined by measurement of the radioactivity of different organs after sacrificing the animals by ether asphyxiation.

Results

The radioimmunoconjugate with approximate DOTA/rituximab molar ratio of 7 showed stability after 24 h at room temperature, after 96 h at 4°C, as the lyophilized formulation after six months storage and against EDTA and human serum. This radioimmunoconjugate had a biodistribution profile similar to that of ⁹⁰Y-ibritumomab, which is approved by FDA for radioimmunotherapy of NHL, and showed low brain and lung uptakes and low yttrium deposition into bone.

Conclusion

Findings of this study suggest that further investigations may result in a lyophilized (kit) formulation of DOTA-rituximab which could be easily radiolabeled with ⁹⁰Y and ¹¹¹In in order to be used for radioimmunotherapy and radioscintigraphy of B-cell lymphoma in Iran.

Tandem myeloablative 131I-rituximab radioimmunotherapy and high-dose chemotherapy in refractory/relapsed non-Hodgkin lymphoma patients

This Phase I/II study investigated myeloablative (131)I-rituximab radioimmunotherapy (RIT) and high-dose chemotherapy supported by one or two autologous stem cell transplantations in heavily pretreated patients with relapsed or refractory B cell non-Hodgkin lymphoma. Myeloablative RIT was safe and feasible when followed by autologous stem cell transplantation with low incidence of secondary late effects and could be a reasonable alternative regimen especially in elderly patients and in patients who have concerns about high-dose chemotherapy. Tandem myeloablative (131)I-rituximab RIT and high-dose chemotherapy supported by two autologous stem cell transplantations was also feasible. However, the toxicity was higher than after myeloablative RIT, therefore it might be recommended to restrict the tandem approach to lymphoma with poor prognosis.

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.

(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.:

Zirconium-89 labeled antibodies: a new tool for molecular imaging in cancer patients

Antibody based positron emission tomography (immuno-PET) imaging is of increasing importance to visualize and characterize tumor lesions. Additionally, it can be used to identify patients who may benefit from a particular therapy and monitor the therapy outcome. In recent years the field is focused on ⁸⁹Zr, a radiometal with near ideal physical and chemical properties for immuno-PET. In this review we will discuss the production of  ⁸⁹Zr, the bioconjugation strategies, and applications in (pre-)clinical studies of  ⁸⁹Zr-based immuno-PET in oncology. To date, ⁸⁹Zr-based PET imaging has been investigated in a wide variety of cancer-related targets. Moreover, clinical studies have shown the feasibility for ⁸⁹Zr-based immuno-PET to predict and monitor treatment, which could be used to tailor treatment for the individual patient. Further research should be directed towards the development of standardized and robust conjugation methods and improved chelators to minimize the amount of released Zr⁴⁺ from the antibodies. Additionally, further validation of the imaging method is required. The ongoing development of new ⁸⁹Zr-labeled antibodies directed against novel tumor targets is expected to expand applications of  ⁸⁹Zr-labeled immuno-PET to a valuable method in the medical imaging.

Alternative chelator for ⁸⁹Zr radiopharmaceuticals: radiolabeling and evaluation of 3,4,3-(LI-1,2-HOPO)

Zirconium-89 is an effective radionuclide for antibody-based positron emission tomography (PET) imaging because its physical half-life (78.41 h) matches the biological half-life of IgG antibodies. Desferrioxamine (DFO) is currently the preferred chelator for ⁸⁹Zr⁴⁺; however, accumulation of ⁸⁹Zr in the bones of mice suggests that ⁸⁹Zr⁴⁺ is released from DFO in vivo. An improved chelator for ⁸⁹Zr⁴⁺ could eliminate the release of osteophilic ⁸⁹Zr⁴⁺ and lead to a safer PET tracer with reduced background radiation dose. Herein, we present an octadentate chelator 3,4,3-(LI-1,2-HOPO) (or HOPO) as a potentially superior alternative to DFO. The HOPO ligand formed a 1:1 Zr-HOPO complex that was evaluated experimentally and theoretically. The stability of ⁸⁹Zr-HOPO matched or surpassed that of ⁸⁹Zr-DFO in every experiment. In healthy mice, ⁸⁹Zr-HOPO cleared the body rapidly with no signs of demetalation. Ultimately, HOPO has the potential to replace DFO as the chelator of choice for ⁸⁹Zr-based PET imaging agents.

Radiolabeled Cetuximab Conjugates for EGFR Targeted Cancer Diagnostics and Therapy

The epidermal growth factor receptor (EGFR) has evolved over years into a main molecular target for the treatment of different cancer entities. In this regard, the anti-EGFR antibody cetuximab has been approved alone or in combination with: (a) chemotherapy for treatment of colorectal and head and neck squamous cell carcinoma and (b) with external radiotherapy for treatment of head and neck squamous cell carcinoma. The conjugation of radionuclides to cetuximab in combination with the specific targeting properties of this antibody might increase its therapeutic efficiency. This review article gives an overview of the preclinical studies that have been performed with radiolabeled cetuximab for imaging and/or treatment of different tumor models. A particularly promising approach seems to be the treatment with therapeutic radionuclide-labeled cetuximab in combination with external radiotherapy. Present data support an important impact of the tumor micromilieu on treatment response that needs to be further validated in patients. Another important challenge is the reduction of nonspecific uptake of the radioactive substance in metabolic organs like liver and radiosensitive organs like bone marrow and kidneys. Overall, the integration of diagnosis, treatment and monitoring as a theranostic approach appears to be a promising strategy for improvement of individualized cancer treatment.

Evaluation of 89Zr-rituximab tracer by Cerenkov luminescence imaging and correlation with PET in a humanized transgenic mouse model to image NHL

PURPOSE

This research aimed to study the use of Cerenkov luminescence imaging (CLI) for non-Hodgkin's lymphoma (NHL) using 89Zr-rituximab positron emission tomography (PET) tracer with a humanized transgenic mouse model that expresses human CD20 and the correlation of CLI with PET.

PROCEDURES

Zr-rituximab (2.6 MBq) was tail vein-injected into transgenic mice that express the human CD20 on their B cells (huCD20TM). One group (n=3) received 2 mg/kg pre-dose (blocking) of cold rituximab 2 h prior to tracer; a second group (n=3) had no pre-dose (non-blocking). CLI was performed using a cooled charge-coupled device optical imager. We also performed PET imaging and ex vivo studies in order to confirm the in vivo CLI results. At each time point (4, 24, 48, 72, and 96 h), two groups of mice were imaged in vivo and ex vivo with CLI and PET, and at 96 h, organs were measured by gamma counter.

RESULTS

huCD20 transgenic mice injected with 89Zr-rituximab demonstrated a high-contrast CLI image compared to mice blocked with a cold dose. At various time points of 4-96 h post-radiotracer injection, the in vivo CLI signal intensity showed specific uptake in the spleen where B cells reside and, hence, the huCD20 biomarker is present at very high levels. The time-activity curve of dose decay-corrected CLI intensity and percent injected dose per gram of tissue of PET uptake in the spleen were increased over the time period (4-96 h). At 96 h, the 89Zr-rituximab uptake ratio (non-blocking vs blocking) counted (mean±standard deviation) for the spleen was 1.5±0.6 for CLI and 1.9±0.3 for PET. Furthermore, spleen uptake measurements (non-blocking and blocking of all time points) of CLI vs PET showed good correlation (R2=0.85 and slope=0.576), which also confirmed the corresponding correlations parameter value (R2=0.834 and slope=0.47) obtained for ex vivo measurements.

CONCLUSIONS

CLI and PET of huCD20 transgenic mice injected with 89Zr-rituximab demonstrated that the tracer was able to target huCD20-expressing B cells. The in vivo and ex vivo tracer uptake corresponding to the CLI radiance intensity from the spleen is in good agreement with PET. In this report, we have validated the use of CLI with PET for NHL imaging in huCD20TM.

H6phospa-trastuzumab: bifunctional methylenephosphonate-based chelator with 89Zr, 111In and 177Lu

The acyclic chelator H6phospa and the bifunctional derivative p-SCN-Bn-H6phospa have been synthesized using nosyl protection chemistry and evaluated with (89)Zr, (111)In, and (177)Lu. The p-SCN-Bn-H6phospa derivative was successfully conjugated to trastuzumab with isotopic dilution assays indicating 3.3 ± 0.1 chelates per antibody and in vitro cellular binding assays indicating an immunoreactivity value of 97.9 ± 2.6%. Radiolabeling of the H6phospa-trastuzumab immunoconjugate was achieved with (111)In in 70-90% yields at room temperature in 30 minutes, while (177)Lu under the same conditions produced more inconsistent yields of 40-80%. Stability experiments in human serum revealed the (111)In-phospa-trastuzumab complex to be 52.0 ± 5.3% intact after 5 days at 37 °C, while the (177)Lu-phospa-trastuzumab to be only 2.0 ± 0.3% intact. Small animal SPECT/CT imaging using mice bearing subcutaneous SKOV-3 ovarian cancer xenografts was performed, and it was found that (111)In-phospa-trastuzumab successfully identified and delineated small (~2 mm in diameter) tumors from surrounding tissues, despite visible uptake in the kidneys and bone due to moderate chelate instability. As predicted from stability assays in serum, the (177)Lu-phospa-trastuzumab conjugate served as a negative control and displayed no tumor uptake, with high uptake in bones indicating rapid and complete radiometal dissociation and suggesting a potential application of H6phospa in transient lanthanide chelation for bone-delivery. Radiolabeling with (89)Zr was attempted, but even with elevated temperatures of 37 °C, the maximum observed radiometal incorporation over 18 hours was 12%. It can be concluded from this work that H6phospa is not superior to the previously studied H4octapa for use with (111)In and (177)Lu, but improvements in (89)Zr radiolabeling were observed over H4octapa, suggesting H6phospa to be an excellent starting point for elaboration of (89)Zr-based radiopharmaceutical development. To our knowledge, H6phospa is the best desferrioxamine alternative for (89)Zr radiolabeling to be studied to date.

Charting the mechanism and reactivity of zirconium oxalate with hydroxamate ligands using density functional theory: implications in new chelate design

DFT studies on Zr-oxalate reactivity with hydroxamic acids reveals new insight into the mechanism and coordination requirements of⁸⁹Zr⁴⁺ions.

Matching chelators to radiometals for radiopharmaceuticals

Radiometals comprise many useful radioactive isotopes of various metallic elements. When properly harnessed, these have valuable emission properties that can be used for diagnostic imaging techniques, such as single photon emission computed tomography (SPECT, e.g.(67)Ga, (99m)Tc, (111)In, (177)Lu) and positron emission tomography (PET, e.g.(68)Ga, (64)Cu, (44)Sc, (86)Y, (89)Zr), as well as therapeutic applications (e.g.(47)Sc, (114m)In, (177)Lu, (90)Y, (212/213)Bi, (212)Pb, (225)Ac, (186/188)Re). A fundamental critical component of a radiometal-based radiopharmaceutical is the chelator, the ligand system that binds the radiometal ion in a tight stable coordination complex so that it can be properly directed to a desirable molecular target in vivo. This article is a guide for selecting the optimal match between chelator and radiometal for use in these systems. The article briefly introduces a selection of relevant and high impact radiometals, and their potential utility to the fields of radiochemistry, nuclear medicine, and molecular imaging. A description of radiometal-based radiopharmaceuticals is provided, and several key design considerations are discussed. The experimental methods by which chelators are assessed for their suitability with a variety of radiometal ions is explained, and a large selection of the most common and most promising chelators are evaluated and discussed for their potential use with a variety of radiometals. Comprehensive tables have been assembled to provide a convenient and accessible overview of the field of radiometal chelating agents.

Radioimmunotherapy of B-Cell Non-Hodgkin's Lymphoma

This manuscript reviews current advances in the use of radioimmunotherapy (RIT) for the treatment of B-cell non-Hodgkin's lymphoma (NHL). RIT has been in use for more than 20 years and has progressed significantly with the discovery of new molecular targets, the development of new stable chelates, the humanization of monoclonal antibodies (MAbs), and the use of pretargeting techniques. Today, two products targeting the CD20 antigen are approved: (131)I-tositumomab (Bexxar(®)), and (90)Y-ibritumomab tiuxetan (Zevalin(®)). (131)I-tositumomab is available in the United States, and (90)Y-ibritumumab tiuxetan in Europe, the United States, Asia, and Africa. RIT can be integrated in clinical practice using non-ablative activities for treatment of patients with relapsed or refractory follicular lymphoma (FL) or as consolidation after induction chemotherapy in front-line treatment in FL patients. Despite the lack of phase III studies to clearly define the efficacy of RIT in the management of B lymphoma in the era of rituximab-based therapy, RIT efficacy in NHL has been demonstrated. In relapsing refractory FL and transformed NHL, RIT as a monotherapy induces around 30% complete response with a possibility of durable remissions. RIT consolidation after induction therapy significantly improves the quality of the response. Dose-limiting toxicity of RIT is hematological, depending on bone marrow involvement and prior treatment. Non-hematological toxicity is generally low. Different studies have been published assessing innovative protocols of RIT or new indications, in particular treatment in patients with aggressive lymphomas. High-dose treatment, RIT as consolidation after different therapeutic induction modalities, RIT in first-line treatment or fractionated RIT showed promising results. New MAbs, in particular humanized MAbs, or combinations of naked and radiolabeled MAbs, also appear promising. Personalized dosimetry protocols should be developed to determine injected activity.

Managing lymphoma with non-FDG radiotracers: current clinical and preclinical applications

Nuclear medicine imaging modalities such as positron emission tomography (PET) and single-photon emission computed tomography (SPECT) have played a prominent role in lymphoma management. PET with [(18)F]Fluoro-2-deoxy-D-glucose (FDG) is the most commonly used tool for lymphoma imaging. However, FDG-PET has several limitations that give the false positive or false negative diagnosis of lymphoma. Therefore, development of new radiotracers with higher sensitivity, specificity, and different uptake mechanism is in great demand in the management of lymphoma. This paper reviews non-FDG radiopharmaceuticals that have been applied for PET and SPECT imaging in patients with different types of lymphoma, with attention to diagnosis, staging, therapy response assessment, and surveillance for disease relapse. In addition, we introduce three radiolabeled anti-CD20 antibodies for radioimmunotherapy, which is another important arm for lymphoma treatment and management. Finally, the relatively promising radiotracers that are currently under preclinical development are also discussed in this paper.

(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.

Zirconium-89 labeled panitumumab: a potential immuno-PET probe for HER1-expressing carcinomas

INTRODUCTION

Anti-HER1 monoclonal antibody (mAb), panitumumab (Vectibix) is a fully human mAb approved by the FDA for the treatment of epidermal growth factor receptor (EGFR, HER1)-expressing colorectal cancers. By combining the targeted specificity of panitumumab with the quantitative in vivo imaging capabilities of PET, we evaluated the potential of (89)Zr-DFO-panitumumab PET/CT imaging and performed non-invasive, in vivo imaging of HER1 expression and estimated human dosimetry.

METHODS

Panitumumab was radiolabeled with (89)Zr using a derivative of desferrioxamine (DFO-Bz-NCS) and with (111)In using CHX-A" DTPA as bifunctional chelators. Comparative biodistribution/dosimetry of both radiotracers was performed in non-tumor bearing athymic nude mice (n=2 females and n=2 males) over 1-week following i.v. injection of either using (89)Zr-DFO-panitumumab or (111)In-CHX-A"-DTPA-panitumumab. Micro-PET/CT imaging of female athymic nude mice bearing human breast cancer tumors (n=5 per tumor group) with variable HER1-expression very low (BT-474), moderate (MDA-MB-231), and very high (MDA-MB-468) was performed at over 1 week following i.v. injection of (89)Zr-DFO-panitumumab.

RESULTS

Radiochemical yield and purity of (89)Zr-Panitumumab was >70% and >98% respectively with specific activity 150 ± 10 MBq/mg of panitumumab in a ~4 hr synthesis time. Biodistribution of (111)In-CHX-A" DTPA -panitumumab and (89)Zr-DFO-panitumumab in athymic non-tumor bearing nude mice displayed similar percent injected dose per gram of tissue with prominent accumulation of both tracers in the lymph nodes, a known clearance mechanism of panitumumab. Also exhibited was prolonged blood pool with no evidence of targeted accumulation in any organ. Human radiation dose estimates showed similar biodistributions with estimated human effective doses of 0.578 and 0.183 mSv/MBq for (89)Zr-DFO-panitumumab and (111)In-CHX-A"-DTPA-panitumumab, respectively. Given the potential quantitative and image quality advantages of PET, imaging of tumor bearing mice was only performed using (89)Zr-DFO-panitumumab. Immuno-PET imaging of (89)Zr-DFO-panitumumab in mice bearing breast cancer xenograft tumors with variable HER1 expression showed high tumor uptake (SUV >7) in the MDA-MB-468 high HER1-expressing mice and a strong correlation between HER1-expression level and tumor uptake (R(2)= 0.857, P < .001).

CONCLUSIONS

(89)Zr-DFO-panitumumab can prepared with high radiochemical purity and specific activity. (89)Zr-DFO-panitumumab microPET/CT showed uptake corresponding to HER-1 expression. Due to poor clearance, initial dosimetry estimates suggest that only a low dose (89)Zr-DFO-panitumumab shows favorable human dosimetry; however due to high tumor uptake, the use of (89)Zr-DFO-panitumumab is expected to be clinically feasible.

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

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

PET imaging with ⁸⁹Zr: from radiochemistry to the clinic

The advent of antibody-based cancer therapeutics has led to the concomitant rise in the development of companion diagnostics for these therapies, particularly nuclear imaging agents. A number of radioisotopes have been employed for antibody-based PET and SPECT imaging, notably ⁶⁴Cu, ¹²⁴I, ¹¹¹In, and (99m)Tc; in recent years, however, the field has increasingly focused on ⁸⁹Zr, a radiometal with near ideal physical and chemical properties for immunoPET imaging. In the review at hand, we seek to provide a comprehensive portrait of the current state of ⁸⁹Zr radiochemical and imaging research, including work into the production and purification of the isotope, the synthesis of new chelators, the development of new bioconjugation strategies, the creation of novel ⁸⁹Zr-based agents for preclinical imaging studies, and the translation of ⁸⁹Zr-labeled radiopharmaceuticals to the clinic. Particular attention will also be dedicated to emerging trends in the field, ⁸⁹Zr-based imaging applications using vectors other than antibodies, the comparative advantages and limitations of ⁸⁹Zr-based imaging compared to that with other isotopes, and areas that would benefit from more extensive investigation. At bottom, it is hoped that this review will provide both the experienced investigator and new scientist with a full and critical overview of this exciting and fast-developing field.

FDG-PET in Follicular Lymphoma Management

18-Fluoro-deoxyglucose positron emission tomography/computerised tomography (FDG PET/CT) is commonly used in the management of patients with lymphomas and is recommended for both initial staging and response assessment after treatment in patients with diffuse large B-cell lymphoma and Hodgkin lymphoma. Despite the FDG avidity of follicular lymphoma (FL), FDG PET/CT is not yet applied in standard clinical practice for patients with FL. However, FDG PET/CT is more accurate than conventional imaging for initial staging, often prompting significant management change, and allows noninvasive characterization to guide assessment of high-grade transformation. For restaging, FDG PET/CT assists in distinguishing between scar tissue and viable tumors in residual masses and a positive PET after induction treatment would seem to predict a shorter progression-free survival.

Development of a novel long-lived immunoPET tracer for monitoring lymphoma therapy in a humanized transgenic mouse model

Positron emission tomography (PET) is an attractive imaging tool to localize and quantify tracer biodistribution. ImmunoPET with an intact mAb typically requires two to four days to achieve optimized tumor-to-normal ratios. Thus, a positron emitter with a half-life of two to four days such as zirconium-89 [(89)Zr] (t1/2: 78.4 h) is ideal. We have developed an antibody-based, long-lived immunoPET tracer (89)Zr-Desferrioxamine-p-SCN (Df-Bz-NCS)-rituximab (Zr-iPET) to image tumor for longer durations in a humanized CD20-expressing transgenic mouse model. To optimize the radiolabeling efficiency of (89)Zr with Df-Bz-rituximab, multiple radiolabelings were performed. Radiochemical yield, purity, immunoreactivity, and stability assays were carried out to characterize the Zr-iPET for chemical and biological integrity. This tracer was used to image transgenic mice that express the human CD20 on their B cells (huCD20TM). Each huCD20TM mouse received a 7.4 MBq/dose. One group (n = 3) received a 2 mg/kg predose (blocking) of cold rituximab 2 h prior to (89)Zr-iPET; the other group (n = 3) had no predose (nonblocking). Small animal PET/CT was used to image mice at 1, 4, 24, 48, 72, and 120 h. Quality assurance of the (89)Zr-iPET demonstrated NCS-Bz-Df: antibody ratio (c/a: 1.5 ± 0.31), specific activity (0.44-1.64 TBq/mol), radiochemical yield (>70%), and purity (>98%). The Zr-iPET immunoreactivity was >80%. At 120 h, Zr-iPET uptake (% ID/g) as mean ± STD for blocking and nonblocking groups in spleen was 3.2 ± 0.1% and 83.3 ± 2.0% (p value <0.0013.). Liver uptake was 1.32 ± 0.05% and 0.61 ± 0.001% (p value <0.0128) for blocking and nonblocking, respectively. The small animal PET/CT image shows the spleen specific uptake of Zr-iPET in mice at 120 h after tracer injection. Compared to the liver, the spleen specific uptake of Zr-iPET is very high due to the expression of huCD20. We optimized the radiolabeling efficiency of (89)Zr with Df-Bz-rituximab. These radioimmunoconjugate lots were stable up to 5 days in serum in vitro. The present study showed that (89)Zr is well-suited for mAbs to image cancer over an extended period of time (up to 5 days).

90Nb – a potential PET nuclide: production and labeling of monoclonal antibodies

Fast progressing immuno-PET gives reasons to develop new potential medium-long and long-lived radioisotopes. One of the promising candidates is 90Nb. It has a half-life of 14.6 h, which allows visualizing and quantifying processes with medium and slow kinetics, such as tumor accumulation of antibodies and antibodies fragments or polymers and other nanoparticles. 90Nb exhibits a high positron branching of 53% and an optimal energy of β + emission of E mean=0.35 MeV only. Consequently, efficient radionuclide production routes and NbV labeling techniques are required.

90Nb was produced by the 90Zr(p,n) 90Nb nuclear reaction on natural zirconium targets. No-carrier-added (n.c.a.) 90Nb was separated from the zirconium target via a multi-step separation procedure including extraction steps and ion-exchange chromatography. Protein labeling was exemplified using the bifunctional chelator desferrioxamine attached to the monoclonal antibody rituximab. Desferrioxamine was coupled to rituximab via two different routes, by the use of N-succinyl-desferrioxamine (N-suc-Df) and by means of the bifunctional derivative p-isothiocyanatobenzyl-desferrioxamine B (Df-Bz-NCS), respectively. Following antibody modification, labeling with 90Nb was performed in HEPES buffer at pH 7 at room temperature. In vitro stability of the radiolabeled conjugates was tested in saline buffer at room temperature and in fetal calf serum (FCS) at 37 ºC.

The selected production route led to a high yield of 145 ± 10 MBq/μA h of 90Nb with high radioisotopic purity of >97%. This yield may allow for large scale production of about 10 GBq 90Nb. The separation procedure resulted in 76–81% yield. The Zr/90Nb decontamination factor reaches 107. Subsequent radiolabeling of the two different conjugates with 90Nb gave high yields; after one hour incubation at room temperature, more than 90% of 90Nb-Df-mAb was formed in both cases. At room temperature in aqueous solution, both 90Nb-Df-mAb constructs were more than 99% stable over a period of 18 d.

The developed production and separation strategy provided 90Nb with purity appropriate for radiolabeling applications. Labeling and stability studies proved the applicability of 90Nb as a potential positron emitter for immuno-PET.

Nanobodies targeting the hepatocyte growth factor: potential new drugs for molecular cancer therapy

Hepatocyte growth factor (HGF) and its receptor c-Met are associated with increased aggressiveness of tumors and poor prognostic outcome of patients with cancer. Here, we report the development and characterization of therapeutic anti-HGF (αHGF)-Nanobodies and their potential for positron emission tomographic (PET) imaging to assess HGF expression in vivo. Two αHGF-Nanobodies designated 1E2 and 6E10 were identified, characterized, and molecularly fused to an albumin-binding Nanobody unit (Alb8) to obtain serum half-life extension. The resulting Nanobody formats were radiolabeled with the positron emitter zirconium-89 ((89)Zr, t(1/2;) = 78 hours), administered to nude mice bearing U87 MG glioblastoma xenografts, and their biodistribution was assessed. In addition, their therapeutic effect was evaluated in the same animal model at doses of 10, 30, or 100 μg per mouse. The (89)Zr-Nanobodies showed similar biodistribution with selective tumor targeting. For example, 1E2-Alb8 showed decreased blood levels of 12.6%ID/g ± 0.6%ID/g, 7.2%ID/g ± 1.0%ID/g, 3.4%ID/g ± 0.3%ID/g, and 0.3%ID/g ± 0.1%ID/g at 1, 2, 3, and 7 days after injection, whereas tumor uptake levels remained relatively stable at these time points: 7.8%ID/g ± 1.1%ID/g, 8.9%ID/g ± 1.0%ID/g, 8.7%ID/g ± 1.5%ID/g, and 7.2%ID/g ±1.6%ID/g. Uptake in normal tissues was lower than in tumor, except for kidneys. In a therapy study, all Nanobody-treated mice showed tumor growth delay compared with the control saline group. In the 100-μg group, four of six mice were cured after treatment with 1E2-Alb8 and 73 days follow-up, and three of six mice when treated with 6E10-Alb8. These results provide evidence that Nanobodies 1E2-Alb8 and 6E10-Alb8 have potential for therapy and PET imaging of HGF-expressing tumors.

PET imaging with radiolabeled antibodies and tyrosine kinase inhibitors: immuno-PET and TKI-PET

During the last decade, the discovery of critical tumor targets has boosted the design of targeted therapeutic agents with monoclonal antibodies (mAbs) and tyrosine kinase inhibitors (TKIs) receiving most of the attention. Immuno-positron emission tomography (immuno-PET) and TKI-PET, the in vivo tracking and quantification of mAbs and TKIs biodistribution with PET, are exciting novel options for better understanding of the in vivo behavior and efficacy of these targeted drugs in individual patients and for more efficient drug development. Very recently, current good manufacturing practice compliant procedures for labeling of mAbs with positron emitters have been described, as well as the preparation of some radiolabeled TKIs, while the first proof of principle studies has been performed in patients. In this review, technical developments in immuno-PET and TKI-PET are described, and their clinical potential is discussed. An overview is provided for the most appealing preclinical immuno-PET and TKI-PET studies, as well as the first clinical achievements with these emerging technologies.

Biodistribution, radiation dosimetry and scouting of 90Y-ibritumomab tiuxetan therapy in patients with relapsed B-cell non-Hodgkin's lymphoma using 89Zr-ibritumomab tiuxetan and PET

Purpose

Positron emission tomography (PET) with ⁸⁹Zr-ibritumomab tiuxetan can be used to monitor biodistribution of ⁹⁰Y-ibritumomab tiuxetan as shown in mice. The aim of this study was to assess biodistribution and radiation dosimetry of ⁹⁰Y-ibritumomab tiuxetan in humans on the basis of ⁸⁹Zr-ibritumomab tiuxetan imaging, to evaluate whether co-injection of a therapeutic amount of ⁹⁰Y-ibritumomab tiuxetan influences biodistribution of ⁸⁹Zr-ibritumomab tiuxetan and whether pre-therapy scout scans with ⁸⁹Zr-ibritumomab tiuxetan can be used to predict biodistribution of ⁹⁰Y-ibritumomab tiuxetan and the dose-limiting organ during therapy.

Methods

Seven patients with relapsed B-cell non-Hodgkin’s lymphoma scheduled for autologous stem cell transplantation underwent PET scans at 1, 72 and 144 h after injection of ~70 MBq ⁸⁹Zr-ibritumomab tiuxetan and again 2 weeks later after co-injection of 15 MBq/kg or 30 MBq/kg ⁹⁰Y-ibritumomab tiuxetan. Volumes of interest were drawn over liver, kidneys, lungs, spleen and tumours. Ibritumomab tiuxetan organ absorbed doses were calculated using OLINDA. Red marrow dosimetry was based on blood samples. Absorbed doses to tumours were calculated using exponential fits to the measured data.

Results

The highest ⁹⁰Y absorbed dose was observed in liver (3.2 ± 1.8 mGy/MBq) and spleen (2.9 ± 0.7 mGy/MBq) followed by kidneys and lungs. The red marrow dose was 0.52 ± 0.04 mGy/MBq, and the effective dose was 0.87 ± 0.14 mSv/MBq. Tumour absorbed doses ranged from 8.6 to 28.6 mGy/MBq. Correlation between predicted pre-therapy and therapy organ absorbed doses as based on ⁸⁹Zr-ibritumomab tiuxetan images was high (Pearson correlation coefficient r = 0.97). No significant difference between pre-therapy and therapy tumour absorbed doses was found, but correlation was lower (r = 0.75).

Conclusion

Biodistribution of ⁸⁹Zr-ibritumomab tiuxetan is not influenced by simultaneous therapy with ⁹⁰Y-ibritumomab tiuxetan, and ⁸⁹Zr-ibritumomab tiuxetan scout scans can thus be used to predict biodistribution and dose-limiting organ during therapy. Absorbed doses to spleen were lower than those previously estimated using ¹¹¹In-ibritumomab tiuxetan. The dose-limiting organ in patients undergoing stem cell transplantation is the liver.

89Zr radiochemistry for positron emission tomography

The positron emitting isotope (89)Zr is an ideal radionuclide for use in positron emission tomography (PET) imaging with monoclonal antibodies (mAbs). This article reviews the cyclotron physics of (89)Zr production, and the chemical separation methods for isolating it from yttrium target material. (89)Zr coordination with the bifunctional chelate desferrioxamine B is discussed, along with the common procedures for attaching the chelate to mAbs. The review is intended to detail the procedure for creating (89)Zr labeled mAbs, going from cyclotron to PET.

Metallic radionuclides in the development of diagnostic and therapeutic radiopharmaceuticals

Metallic radionuclides are the mainstay of both diagnostic and therapeutic radiopharmaceuticals. Therapeutic nuclear medicine is less advanced but has tremendous potential if the radionuclide is accurately targeted. Great interest exists in the field of inorganic chemistry for developing target specific radiopharmaceuticals based on radiometals for non-invasive disease detection and cancer radiotherapy. This perspective will focus on the nuclear properties of a few important radiometals and their recent applications to developing radiopharmaceuticals for imaging and therapy. Other topics for discussion will include imaging techniques, radiotherapy, analytical techniques, and radiation safety. The ultimate goal of this perspective is to introduce inorganic chemists to the field of nuclear medicine and radiopharmaceutical development, where many applications of fundamental inorganic chemistry can be found.

A practical guide to the construction of radiometallated bioconjugates for positron emission tomography

Positron emission tomography (PET) has become a vital imaging modality in the diagnosis and treatment of disease, most notably cancer. A wide array of small molecule PET radiotracers have been developed that employ the short half-life radionuclides (11)C, (13)N, (15)O, and (18)F. However, PET radiopharmaceuticals based on biomolecular targeting vectors have been the subject of dramatically increased research in both the laboratory and the clinic. Typically based on antibodies, oligopeptides, or oligonucleotides, these tracers have longer biological half-lives than their small molecule counterparts and thus require labeling with radionuclides with longer, complementary radioactive half-lives, such as the metallic isotopes (64)Cu, (68)Ga, (86)Y, and (89)Zr. Each bioconjugate radiopharmaceutical has four component parts: biomolecular vector, radiometal, chelator, and covalent link between chelator and biomolecule. With the exception of the radiometal, a tremendous variety of choices exists for each of these pieces, and a plethora of different chelation, conjugation, and radiometallation strategies have been utilized to create agents ranging from (68)Ga-labeled pentapeptides to (89)Zr-labeled monoclonal antibodies. Herein, the authors present a practical guide to the construction of radiometal-based PET bioconjugates, in which the design choices and synthetic details of a wide range of biomolecular tracers from the literature are collected in a single reference. In assembling this information, the authors hope both to illuminate the diverse methods employed in the synthesis of these agents and also to create a useful reference for molecular imaging researchers both experienced and new to the field.

The rise of metal radionuclides in medical imaging: copper-64, zirconium-89 and yttrium-86

Positron emission tomography, with its high sensitivity and resolution, is growing rapidly as an imaging technology for the diagnosis of many disease states. The success of this modality is reliant on instrumentation and the development of effective and novel targeted probes. Initially, research in this area was focused on what we will define in this article as ‘standard’ PET isotopes (carbon-11, nitrogen-13, oxygen-15 and fluorine-18), but the short half-lives of these isotopes limit radiopharmaceutical development to those that probe rapid biological processes. To overcome these limitations, there has been a rise in nonstandard isotope probe development in recent years. This review focuses on the biological probes and processes that have been examined, in additiom to the preclinical and clinical findings with nonstandard radiometals: copper-64, zirconium-89, and yttrium-86.

PET imaging of tumor angiogenesis in mice with VEGF-A-targeted (86)Y-CHX-A″-DTPA-bevacizumab

Bevacizumab is a humanized monoclonal antibody that binds to tumor-secreted vascular endothelial growth factor (VEGF)-A and inhibits tumor angiogenesis. In 2004, the antibody was approved by the US Food and Drug Administration (FDA) for the treatment of metastatic colorectal carcinoma in combination with chemotherapy. This report describes the preclinical evaluation of a radioimmunoconjugate, (86)Y-CHX-A″-DTPA-bevacizumab, for potential use in Positron Emission Tomography (PET) imaging of VEGF-A tumor angiogenesis and as a surrogate marker for (90)Y-based radioimmunotherapy. Bevacizumab was conjugated to CHX-A″-DTPA and radiolabeled with (86)Y. In vivo biodistribution and PET imaging studies were performed on mice bearing VEGF-A-secreting human colorectal (LS-174T), human ovarian (SKOV-3) and VEGF-A-negative human mesothelioma (MSTO-211H) xenografts. Biodistribution and PET imaging studies demonstrated highly specific tumor uptake of the radioimmunoconjugate. In mice bearing VEGF-A-secreting LS-174T, SKOV-3 and VEGF-A-negative MSTO-211H tumors, the tumor uptake at 3 days postinjection was 13.6 ± 1.5, 17.4 ± 1.7 and 6.8 ± 0.7 % ID/g, respectively. The corresponding tumor uptake in mice coinjected with 0.05 mg cold bevacizumab were 5.8 ± 1.3, 8.9 ± 1.9 and 7.4 ± 1.0 % ID/g, respectively at the same time point, demonstrating specific blockage of the target in VEGF-A-secreting tumors. The LS-174T and SKOV3 tumors were clearly visualized by PET imaging after injecting 1.8-2.0 MBq (86)Y-CHX-A″-DTPA-bevacizumab. Organ uptake quantified by PET closely correlated (r(2) = 0.87, p = 0.64, n = 18) to values determined by biodistribution studies. This preclinical study demonstrates the potential of the radioimmunoconjugate, (86)Y-CHX-A″-DTPA-bevacizumab, for noninvasive assessment of the VEGF-A tumor angiogenesis status and as a surrogate marker for (90)Y-CHX-A″-DTPA-bevacizumab radioimmunotherapy.

Positive progress in immunoPET--not just a coincidence

The identification of tumor tissue biomarkers has led to the production, validation, and Food and Drug Administration-approval of a number of antibody-based targeted therapeutics in the past two decades. As a result of the significant role that these immunotherapeutics play in the management of cancer, and the potential utility of complementary imaging agents, immunoPET imaging has generated considerable interest. This update discusses the important factors to consider when designing a PET (positron emission tomography) imaging agent from the molecular target to the biological targeting molecule and radionuclide combination and also reviews recent preclinical and clinical findings in the immunoPET field. Although there are a variety of radionuclides that are currently utilized in PET studies, this update focuses on four of the positron emitters commonly used in labeling proteins: iodine-124, zirconium-89, copper-64, and fluorine-18. Notable advances in the preclinical setting include the continued development of immunoPET probes to predict the biodistribution of related radioimmunotherapeutics, the success of nontraditional radionuclide and antibody fragment combinations, the broader use of zirconium-89, and the recent emergence of (18)F-labeled diabodies for same-day imaging. Antibody-based PET probes constitute a valuable class of molecular imaging agents, and the progress made preclinically should expedite the transition of these targeted diagnostics to clinical applications.

Multimodal radio- (PET/SPECT) and fluorescence imaging agents based on metallo-radioisotopes: current applications and prospects for development of new agents

This perspective focuses on complexes of radioactive metal ions applied in multimodal radio- and optical imaging. The application of metal ions in radioimaging techniques such as Positron Emission Tomography (PET) and Single Photon Emission Computed Tomography (SPECT) has advantages over lighter nuclei in terms of half-life, but there are particular issues related to their speciation (particularly leaching from complexes) and with the combination of certain ions with fluorescent systems. The basic coordination chemistry of the ions involved and issues relating to biological conditions and their compatibility with optical imaging techniques are reviewed, the current literature presented in context, and the prospect of exploiting the intrinsic luminescence of certain metal-ligand complexes is discussed.

Prediction of (89)Zr production using the Monte Carlo code FLUKA

The widely used Monte Carlo simulation code FLUKA has been utilized to prototype a solid target for the production of (89)Zr by irradiation of a metallic (89)Y target foil in a 16.5MeV proton biomedical cyclotron, through the reaction (89)Y(p, n)(89)Zr. Simulations were performed with and without an Al energy degrader. In the setup of the geometry of the target, state of the art support tools, like SimpleGeo, were used for accurate, detailed modeling. The results permitted a quick assessment of all possible radionuclidic contaminants and confirmed that the use of an energy degrader avoids production of the most important impurity, (88)Zr. The estimated value for the activity produced in one hour of irradiation at 20μA is 384 ± 42MBq; this is encouraging, indicating possible production of clinically significant amounts of activity with the relatively simple target setup adopted. Initial experimental tests gave results in excellent agreement with simulations, confirming the usefulness and accuracy of FLUKA as a tool for the design and optimization of targets for the production of PET radionuclides.

89Zr-DFO-J591 for immunoPET of prostate-specific membrane antigen expression in vivo

(89)Zr (half-life, 78.41 h) is a positron-emitting radionuclide that displays excellent potential for use in the design and synthesis of radioimmunoconjugates for immunoPET. In the current study, we report the preparation of (89)Zr-desferrioxamine B (DFO)-J591, a novel (89)Zr-labeled monoclonal antibody (mAb) construct for targeted immunoPET and quantification of prostate-specific membrane antigen (PSMA) expression in vivo.

METHODS

The in vivo behavior of (89)Zr-chloride, (89)Zr-oxalate, and (89)Zr-DFO was studied using PET. High-level computational studies using density functional theory calculations have been used to investigate the electronic structure of (89)Zr-DFO and probe the nature of the complex in aqueous conditions. (89)Zr-DFO-J591 was characterized both in vitro and in vivo. ImmunoPET in male athymic nu/nu mice bearing subcutaneous LNCaP (PSMA-positive) or PC-3 (PSMA-negative) tumors was conducted. The change in (89)Zr-DFO-J591 tissue uptake in response to high- and low-specific-activity formulations in the 2 tumor models was measured using acute biodistribution studies and immunoPET.

RESULTS

The basic characterization of 3 important reagents-(89)Zr-chloride, (89)Zr-oxalate, and the complex (89)Zr-DFO-demonstrated that the nature of the (89)Zr species dramatically affects the biodistribution and pharmacokinetics. Density functional theory calculations provide a rationale for the observed high in vivo stability of (89)Zr-DFO-labeled mAbs and suggest that in aqueous conditions, (89)Zr-DFO forms a thermodynamically stable, 8-coordinate complex by coordination of 2 water molecules. (89)Zr-DFO-J591 was produced in high radiochemical yield (>77%) and purity (>99%), with a specific activity of 181.7 +/- 1.1 MBq/mg (4.91 +/- 0.03 mCi/mg). In vitro assays demonstrated that (89)Zr-DFO-J591 had an initial immunoreactive fraction of 0.95 +/- 0.03 and remained active for up to 7 d. In vivo biodistribution experiments revealed high, target-specific uptake of (89)Zr-DFO-J591 in LNCaP tumors after 24, 48, 96, and 144 h (34.4 +/- 3.2 percentage injected dose per gram [%ID/g], 38.0 +/- 6.2 %ID/g, 40.4 +/- 4.8 %ID/g, and 45.8 +/- 3.2 %ID/g, respectively). ImmunoPET studies also showed that (89)Zr-DFO-J591 provides excellent image contrast, with tumor-to-muscle ratios greater than 20, for the delineation of LNCaP xenografts between 48 and 144 h after administration.

CONCLUSION

These studies demonstrate that (89)Zr-DFO-labeled mAbs show exceptional promise as radiotracers for immunoPET of human cancers. (89)Zr-DFO-J591 displays high tumor-to-background tissue contrast in immunoPET and can be used to delineate and quantify PSMA-positive prostate tumors in vivo.

Cerenkov luminescence imaging of medical isotopes

The development of novel multimodality imaging agents and techniques represents the current frontier of research in the field of medical imaging science. However, the combination of nuclear tomography with optical techniques has yet to be established. Here, we report the use of the inherent optical emissions from the decay of radiopharmaceuticals for Cerenkov luminescence imaging (CLI) of tumors in vivo and correlate the results with those obtained from concordant immuno-PET studies.

METHODS

In vitro phantom studies were used to validate the visible light emission observed from a range of radionuclides including the positron emitters (18)F, (64)Cu, (89)Zr, and (124)I; beta-emitter (131)I; and alpha-particle emitter (225)Ac for potential use in CLI. The novel radiolabeled monoclonal antibody (89)Zr-desferrioxamine B [DFO]-J591 for immuno-PET of prostate-specific membrane antigen (PSMA) expression was used to coregister and correlate the CLI signal observed with the immuno-PET images and biodistribution studies.

RESULTS

Phantom studies confirmed that Cerenkov radiation can be observed from a range of positron-, beta-, and alpha-emitting radionuclides using standard optical imaging devices. The change in light emission intensity versus time was concordant with radionuclide decay and was also found to correlate linearly with both the activity concentration and the measured PET signal (percentage injected dose per gram). In vivo studies conducted in male severe combined immune deficient mice bearing PSMA-positive, subcutaneous LNCaP tumors demonstrated that tumor-specific uptake of (89)Zr-DFO-J591 could be visualized by both immuno-PET and CLI. Optical and immuno-PET signal intensities were found to increase over time from 24 to 96 h, and biodistribution studies were found to correlate well with both imaging modalities.

CONCLUSION

These studies represent the first, to our knowledge, quantitative assessment of CLI for measuring radiotracer uptake in vivo. Many radionuclides common to both nuclear tomographic imaging and radiotherapy have the potential to be used in CLI. The value of CLI lies in its ability to image radionuclides that do not emit either positrons or gamma-rays and are, thus, unsuitable for use with current nuclear imaging modalities. Optical imaging of Cerenkov radiation emission shows excellent promise as a potential new imaging modality for the rapid, high-throughput screening of radiopharmaceuticals.

Immuno-PET quantitation of de2-7 epidermal growth factor receptor expression in glioma using 124I-IMP-R4-labeled antibody ch806

Overexpression, activation, and mutations of the epidermal growth factor receptor (EGFR) are commonly found in solid tumors. The aim of this study was to develop a PET-based method for detecting the constitutively active mutant de2-7 EGFR, which is associated with disease progression and resistance to chemotherapy and radiotherapy in glioma.

METHODS

The chimeric antibody ch806, which selectively binds an epitope of the EGFR that is exposed only on overexpressed, mutant, or ligand-activated forms of the receptor, was conjugated to the radiohalogen (124)I via the residualizing ligand IMP-R4, and in vitro properties were characterized. In vivo biodistribution and small-animal PET studies were performed in BALB/c nude mice bearing U87MG.de2-7 glioma xenografts. Imaging results were correlated with measured tumor uptake of the radioconjugate.

RESULTS

(124)I-IMP-R4-ch806 had an immunoreactivity of 78.3% and was stable for 7 d when incubated in serum in vitro. The biodistribution analysis of (124)I-IMP-R4-ch806 demonstrated a maximal uptake of 30.95 +/- 6.01 percentage injected dose per gram (%ID/g) in U87MG.de2-7 xenografts at 48 h after injection, with prolonged tumor retention (6.07 +/- 0.80 %ID/g at 216 h after injection). The tumor-to-blood ratio increased from 0.44 at 4 h after injection to a maximum of 4.70 at 168 h after injection. PET of (124)I-IMP-R4-ch806 biodistribution was able to clearly detect the U87MG.de2-7 tumors at 24 h after injection and for at least 168 h after injection. Correlation between tumor PET image quantitation of (124)I-IMP-R4-ch806 and %ID/g determined from resected tissues (r = 0.9350) was excellent.

CONCLUSION

These results show that immuno-PET with (124)I-IMP-R4-ch806 is feasible and allows noninvasive quantitation of de2-7 EGFR expression in vivo.

Site-specifically 89Zr-labeled monoclonal antibodies for ImmunoPET

Three thiol reactive reagents were developed for the chemoselective conjugation of desferrioxamine (Df) to a monoclonal antibody via engineered cysteine residues (thio-trastuzumab). The in vitro stability and in vivo imaging properties of site-specifically radiolabeled (89)Zr-Df-thio-trastuzumab conjugates were investigated.

METHODS

The amino group of desferrioxamine B was acylated by bromoacetyl bromide, N-hydroxysuccinimidyl iodoacetate, or N-hydroxysuccinimidyl 4-[N-maleimidomethyl]cyclohexane-1-carboxylate to obtain thiol reactive reagents bromoacetyl-desferrioxamine (Df-Bac), iodoacetyl-desferrioxamine (Df-Iac) and maleimidocyclohexyl-desferrioxamine (Df-Chx-Mal), respectively. Df-Bac and Df-Iac alkylated the free thiol groups of thio-trastuzumab by nucleophilic substitution forming Df-Ac-thio-trastuzumab, while the maleimide reagent Df-Chx-Mal reacted via Michael addition to provide Df-Chx-Mal-thio-trastuzumab. The conjugates were radiolabeled with (89)Zr and evaluated for serum stability, and their positron emission tomography (PET) imaging properties were investigated in a BT474M1 (HER2-positive) breast tumor mouse model.

RESULTS

The chemoselective reagents were obtained in 14% (Df-Bac), 53% (Df-Iac) and 45% (Df-Chx-Mal) yields. Site-specific conjugation of Df-Chx-Mal to thio-trastuzumab was complete within 1 h at pH 7.5, while Df-Iac and Df-Bac respectively required 2 and 5 h at pH 9. Each Df modified thio-trastuzumab was chelated with (89)Zr in yields exceeding 75%. (89)Zr-Df-Ac-thio-trastuzumab and (89)Zr-Df-Chx-Mal-thio-trastuzumab were stable in mouse serum and exhibited comparable PET imaging capabilities in a BT474M1 (HER2-positive) breast cancer model reaching 20-25 %ID/g of tumor uptake and a tumor to blood ratio of 6.1-7.1.

CONCLUSIONS

The new reagents demonstrated good reactivity with engineered thiol groups of trastuzumab and very good chelation properties with (89)Zr. The site-specifically (89)Zr-labeled thio-antibodies were stable in serum and showed PET imaging properties comparable to lysine conjugates.

Measuring the pharmacodynamic effects of a novel Hsp90 inhibitor on HER2/neu expression in mice using Zr-DFO-trastuzumab

BACKGROUND

The positron-emitting radionuclide (89)Zr (t(1/2) = 3.17 days) was used to prepare (89)Zr-radiolabeled trastuzumab for use as a radiotracer for characterizing HER2/neu-positive breast tumors. In addition, pharmacodynamic studies on HER2/neu expression levels in response to therapeutic doses of PU-H71 (a specific inhibitor of heat-shock protein 90 [Hsp90]) were conducted.

METHODOLOGY/PRINCIPAL FINDINGS

Trastuzumab was functionalized with desferrioxamine B (DFO) and radiolabeled with [(89)Zr]Zr-oxalate at room temperature using modified literature methods. ImmunoPET and biodistribution experiments in female, athymic nu/nu mice bearing sub-cutaneous BT-474 (HER2/neu positive) and/or MDA-MB-468 (HER2/neu negative) tumor xenografts were conducted. The change in (89)Zr-DFO-trastuzumab tissue uptake in response to high- and low-specific-activity formulations and co-administration of PU-H71 was evaluated by biodistribution studies, Western blot analysis and immunoPET. (89)Zr-DFO-trastuzumab radiolabeling proceeded in high radiochemical yield and specific-activity 104.3+/-2.1 MBq/mg (2.82+/-0.05 mCi/mg of mAb). In vitro assays demonstrated >99% radiochemical purity with an immunoreactive fraction of 0.87+/-0.07. In vivo biodistribution experiments revealed high specific BT-474 uptake after 24, 48 and 72 h (64.68+/-13.06%ID/g; 71.71+/-10.35%ID/g and 85.18+/-11.10%ID/g, respectively) with retention of activity for over 120 h. Pre-treatment with PU-H71 was followed by biodistribution studies and immunoPET of (89)Zr-DFO-trastuzumab. Expression levels of HER2/neu were modulated during the first 24 and 48 h post-administration (29.75+/-4.43%ID/g and 41.42+/-3.64%ID/g, respectively). By 72 h radiotracer uptake (73.64+/-12.17%ID/g) and Western blot analysis demonstrated that HER2/neu expression recovered to baseline levels.

CONCLUSIONS/SIGNIFICANCE

The results indicate that (89)Zr-DFO-trastuzumab provides quantitative and highly-specific delineation of HER2/neu positive tumors, and has potential to be used to measure the efficacy of long-term treatment with Hsp90 inhibitors, like PU-H71, which display extended pharmacodynamic profiles.