Evaluation of Chelator-to-Antibody Ratio on Development of 89Zr-iPET Tracer for Imaging of PD-L1 Expression on Tumor

89Zr-iPET has been widely used for preclinical and clinical immunotherapy studies to predict patient stratification or evaluate therapeutic efficacy. In this study, we prepared and evaluated 89Zr-DFO-anti-PD-L1-mAb tracers with varying chelator-to-antibody ratios (CARs), including 89Zr-DFO-anti-PD-L1-mAb_3X (tracer_3X), 89Zr-DFO-anti-PD-L1-mAb_10X (tracer_10X), and 89Zr-DFO-anti-PD-L1-mAb_20X (tracer_20X). The DFO-anti-PD-L1-mAb conjugates with varying CARs were prepared using a random conjugation method and then subjected to quality control. The conjugates were radiolabeled with 89Zr and evaluated in a PD-L1-expressing CT26 tumor-bearing mouse model. Next, iPET imaging, biodistribution, pharmacokinetics, and ex vivo pathological and immunohistochemical examinations were conducted. LC-MS analysis revealed that DFO-anti-PD-L1-mAb conjugates were prepared with CARs ranging from 0.4 to 2.0. Radiochemical purity for all tracer groups was >99% after purification. The specific activity levels of tracer_3X, tracer_10X, and tracer_20X were 2.2 ± 0.6, 8.2 ± 0.6, and 10.5 ± 1.6 μCi/μg, respectively. 89Zr-iPET imaging showed evident tumor uptake in all tracer groups and reached the maximum uptake value at 24 h postinjection (p.i.). Biodistribution data at 168 h p.i. revealed that the tumor-to-liver, tumor-to-muscle, and tumor-to-blood uptake ratios for tracer_3X, tracer_10X, and tracer_20X were 0.46 ± 0.14, 0.58 ± 0.33, and 1.54 ± 0.51; 4.7 ± 1.3, 7.1 ± 3.9, and 14.7 ± 1.1; and 13.1 ± 5.8, 19.4 ± 13.8, and 41.3 ± 10.6, respectively. Significant differences were observed between tracer_3X and tracer_20X in the aforementioned uptake ratios at 168 h p.i. The mean residence time and elimination half-life for tracer_3X, tracer_10X, and tracer_20X were 25.4 ± 4.9, 24.2 ± 6.1, and 25.8 ± 3.3 h and 11.8 ± 0.5, 11.1 ± 0.7, and 11.7 ± 0.6 h, respectively. No statistical differences were found between-tracer in the aforementioned pharmacokinetic parameters. In conclusion, 89Zr-DFO-anti-PD-L1-mAb tracers with a CAR of 1.4-2.0 may be better at imaging PD-L1 expression in tumors than are traditional low-CAR 89Zr-iPET tracers.

Characterization of chimeric antigen receptor modified T cells expressing scFv-IL-13Rα2 after radiolabeling with 89Zirconium oxine for PET imaging

BACKGROUND

Chimeric antigen receptor (CAR) T cell therapy is an exciting cell-based cancer immunotherapy. Unfortunately, CAR-T cell therapy is associated with serious toxicities such as cytokine release syndrome (CRS) and neurotoxicity. The mechanism of these serious adverse events (SAEs) and how homing, distribution and retention of CAR-T cells contribute to toxicities is not fully understood. Enabling in vitro methods to allow meaningful, sensitive in vivo biodistribution studies is needed to better understand CAR-T cell disposition and its relationship to both effectiveness and safety of these products.

METHODS

To determine if radiolabelling of CAR-T cells could support positron emission tomography (PET)-based biodistribution studies, we labeled IL-13Rα2 targeting scFv-IL-13Rα2-CAR-T cells (CAR-T cells) with 89Zirconium-oxine (89Zr-oxine) and characterized and compared their product attributes with non-labeled CAR-T cells. The 89Zr-oxine labeling conditions were optimized for incubation time, temperature, and use of serum for labeling. In addition, T cell subtype characterization and product attributes of radiolabeled CAR-T cells were studied to assess their overall quality including cell viability, proliferation, phenotype markers of T-cell activation and exhaustion, cytolytic activity and release of interferon-γ upon co-culture with IL-13Rα2 expressing glioma cells.

RESULTS

We observed that radiolabeling of CAR-T cells with 89Zr-oxine is quick, efficient, and radioactivity is retained in the cells for at least 8 days with minimal loss. Also, viability of radiolabeled CAR-T cells and subtypes such as CD4 + , CD8 + and scFV-IL-13Rα2 transgene positive T cell population were characterized and found similar to that of unlabeled cells as determined by TUNEL assay, caspase 3/7 enzyme and granzyme B activity assay. Moreover, there were no significant changes in T cell activation (CD24, CD44, CD69 and IFN-γ) or T cell exhaustion (PD-1, LAG-3 and TIM3) markers expression between radiolabeled and unlabeled CAR-T cells. In chemotaxis assays, migratory capability of radiolabeled CAR-T cells to IL-13Rα2Fc was similar to that of non-labeled cells.

CONCLUSIONS

Importantly, radiolabeling has minimal impact on biological product attributes including potency of CAR-T cells towards IL-13Rα2 positive tumor cells but not IL-13Rα2 negative cells as measured by cytolytic activity and release of IFN-γ. Thus, IL-13Rα2 targeting CAR-T cells radiolabeled with 89Zr-oxine retain critical product attributes and suggest 89Zr-oxine radiolabeling of CAR-T cells may facilitate biodistribution and tissue trafficking studies in vivo using PET.

Calcium Carbonate Core-Shell Particles for Incorporation of 225Ac and Their Application in Local α-Radionuclide Therapy

Actinium-225 (225Ac) radiolabeled submicrometric core-shell particles (SPs) made of calcium carbonate (CaCO3) coated with biocompatible polymers [tannic acid-human serum albumin (TA/HSA)] have been developed to improve the efficiency of local α-radionuclide therapy in melanoma models (B16-F10 tumor-bearing mice). The developed 225Ac-SPs possess radiochemical stability and demonstrate effective retention of 225Ac and its daughter isotopes. The SPs have been additionally labeled with zirconium-89 (89Zr) to perform the biodistribution studies using positron emission tomography-computerized tomography (PET/CT) imaging for 14 days after intratumoral injection. According to the PET/CT analysis, a significant accumulation of 89Zr-SPs in the tumor area is revealed for the whole investigation period, which correlates with the direct radiometry analysis after intratumoral administration of 225Ac-SPs. The histological analysis has revealed no abnormal changes in healthy tissue organs after treatment with 225Ac-SPs (e.g., no acute pathologic findings are detected in the liver and kidneys). At the same time, the inhibition of tumor growth has been observed as compared with control samples [nonradiolabeled SPs and phosphate-buffered saline (PBS)]. The treatment of mice with 225Ac-SPs has resulted in prolonged survival compared to the control samples. Thus, our study validates the application of 225Ac-doped core-shell submicron CaCO3 particles for local α-radionuclide therapy.

CD38-Targeted Theranostics of Lymphoma with 89Zr/177Lu-Labeled Daratumumab

Lymphoma is a heterogeneous disease with varying clinical manifestations and outcomes. Many subtypes of lymphoma, such as Burkitt's lymphoma and diffuse large B cell lymphoma, are highly aggressive with dismal prognosis even after conventional chemotherapy and radiotherapy. As such, exploring specific biomarkers for lymphoma is of high clinical significance. Herein, a potential marker, CD38, is investigated for differentiating lymphoma. A CD38-targeting monoclonal antibody (mAb, daratumumab) is then radiolabeled with Zr-89 and Lu-177 for theranostic applications. As the diagnostic component, the Zr-89-labeled mAb is highly specific in delineating CD38-positive lymphoma via positron emission tomography (PET) imaging, while the Lu-177-labeled mAb serves well as the therapeutic component to suppress tumor growth after a one-time administration. These results strongly suggest that CD38 is a lymphoma-specific marker and prove that 89Zr/177Lu-labeled daratumumab facilitates immunoPET imaging and radioimmunotherapy of lymphoma in preclinical models. Further clinical evaluation and translation of this CD38-targeted theranostics may be of significant help in lymphoma patient stratification and management.

Biodistribution PET/CT Study of Hemoglobin-DFO-89Zr Complex in Healthy and Lung Tumor-Bearing Mice

Proteins, as a major component of organisms, are considered the preferred biomaterials for drug delivery vehicles. Hemoglobin (Hb) has been recently rediscovered as a potential drug carrier, but its use for biomedical applications still lacks extensive investigation. To further explore the possibility of utilizing Hb as a potential tumor targeting drug carrier, we examined and compared the biodistribution of Hb in healthy and lung tumor-bearing mice, using for the first time ⁸⁹Zr labelled Hb in a positron emission tomography (PET) measurement. Hb displays a very high conjugation yield in its fast and selective reaction with the maleimide-deferoxamine (DFO) bifunctional chelator. The high-resolution X-ray structure of the Hb-DFO complex demonstrated that cysteine β93 is the sole attachment moiety to the αβ-protomer of Hb. The Hb-DFO complex shows quantitative uptake of ⁸⁹Zr in solution as determined by radiochromatography. Injection of 0.03 mg of Hb-DFO-⁸⁹Zr complex in healthy mice indicates very high radioactivity in liver, followed by spleen and lungs, whereas a threefold increased dosage results in intensification of PET signal in kidneys and decreased signal in liver and spleen. No difference in biodistribution pattern is observed between naïve and tumor-bearing mice. Interestingly, the liver Hb uptake did not decrease upon clodronate-mediated macrophage depletion, indicating that other immune cells contribute to Hb clearance. This finding is of particular interest for rapidly developing clinical immunology and projects aiming to target, label or specifically deliver agents to immune cells.

Integration of a porous coordination network and black phosphorus nanosheets for improved photodynamic therapy of tumor

Selectively attenuating the protection offered by heat shock protein 90 (HSP90), which is indispensable for the stabilization of the essential regulators of cell survival and works as a cell guardian under oxidative stress conditions, is a potential approach to improve the efficiency of cancer therapy. Here, we designed a biodegradable nanoplatform (APCN/BP-FA) based on a Zr(iv)-based porphyrinic porous coordination network (PCN) and black phosphorus (BP) sheets for efficient photodynamic therapy (PDT) by enhancing the accumulation of the nanoplatforms in the tumor area and attenuating the protection of cancer cells. Owing to the favorable degradability of BP, the nanosystem exhibited accelerated the release of the HSP90 inhibitor tanespimycin (17-AAG) and an apparent promotion in the reactive oxygen species (ROS) yield of PCN as well as expedited the degradation of the PCN-laden BP nanoplatforms. Both in vitro and in vivo results revealed that the elevated amounts of ROS and reduced cytoprotection in tumor cells were caused by the nanoplatforms. This strategy may provide a promising method for attenuating cytoprotection to aid efficient photodynamic therapy.

Effect of Hydrothermal Aging and Beverages on Color Stability of Lithium Disilicate and Zirconia Based Ceramics

Background and Objectives: All-ceramic prosthesis is widely used in modern dental practice because of its improved physico-mechanical and optical properties. These restorations are exposed to coloring agents from various nutrition and beverages in the oral cavity. Long-term color stability is critical for the success of these restorative materials. The purpose of this in vitro study was to assess the effect of common beverages and mouthwash on the color stability of lithium disilicate (LD), monolithic zirconia (MZ) and bilayer zirconia (BZ) surfaces. Materials and Method: Thirty disc-shaped specimens from each material were fabricated; each group was subdivided (n = 10) according to coffee, green tea and chlorhexidine immersion solutions. The baseline color of ceramic discs was recorded according to the CIE L*a*b* system with a portable spectrophotometer. The second measurement was recorded after 3000 thermocycling and immersion in coloring agents for 7 days. The mean color difference was calculated and data were compared with Kruskal-Wallis and Mann-Whitney post hoc tests (0.05). Results: ΔE values for LD with the immersion of coffee, tea, and Chlorhexidine gluconate (CHG) were 1.78, 2.241 and 1.58, respectively. Corresponding ΔE values for MZ were 5.60, 5.19, and 4.86; marginally higher than the clinically acceptable level of 3.5. Meanwhile, BZ showed better color stability compared to MZ with ΔE values of 4.22, 2.11 and 1.43. Conclusions: Among the ceramics evaluated, LD ceramic was found to be more color stable, while MZ ceramics displayed a higher susceptibility to discoloration. MZ and BZ ceramic colors were significantly altered with coffee immersion, while LD ceramics were more affected by green tea.

Quantitative and correlative biodistribution analysis of 89Zr-labeled mesoporous silica nanoparticles intravenously injected into tumor-bearing mice

The biodistribution of ⁸⁹Zr-labeled mesoporous silica nanoparticles (MSNs) was evaluated in detail using a prostate cancer mouse model bearing LNCaP C4-2 and PC-3 tumor xenografts with focus on passive targeting. PEGylation of radiolabeled MSNs significantly improved the blood circulation times and radically enhanced the accumulation in tumors comparable to the accumulation levels previously reported for similar but actively targeted particles. The distribution of the passively targeted MSNs was related to the degree of vascularization of the tumors and did not follow the trends observed in vitro. Correlative analyses of organ-to-blood ratios revealed that little or no accumulation of the particles is observed in the lungs, heart, and brain, and that the particles detected were present in the blood pool. On the other hand, clear accumulation was observed in the liver and spleen, in addition to the uptake in the tumors. The accumulation of particles in the kidney did not correlate with the MSN concentration in the blood, but indicated a rather steady level of particles in the kidney. The results, which partly contradict previous studies, highlight the importance of correlative analyses in order to evaluate the organ accumulation of particles.

A generic 89Zr labeling method to quantify the in vivo pharmacokinetics of liposomal nanoparticles with positron emission tomography

Liposomal nanoparticles are versatile drug delivery vehicles that show great promise in cancer therapy. In an effort to quantitatively measure their in vivo pharmacokinetics, we developed a highly efficient 89Zr liposome-labeling method based on a rapid ligand exchange reaction between the membrane-permeable 89Zr(8-hydroxyquinolinate)4 complex and the hydrophilic liposomal cavity-encapsulated deferoxamine (DFO). This novel 89Zr-labeling strategy allowed us to prepare radiolabeled forms of a folic acid (FA)-decorated active targeting 89Zr-FA-DFO-liposome, a thermosensitive 89Zr-DFO-liposome, and a renal avid 89Zr-PEG-DFO-liposome at room temperature with near-quantitative isolated radiochemical yields of 98%±1% (n=6), 98%±2% (n=5), and 97%±1% (n=3), respectively. These 89Zr-labeled liposomal nanoparticles showed remarkable stability in phosphate-buffered saline and serum at 37°C without leakage of radioactivity for 48 h. The uptake of 89Zr-FA-DFO-liposome by the folate receptor-overexpressing KB cells was almost 15-fold higher than the 89Zr-DFO-liposome in vitro. Positron emission tomography imaging and ex vivo biodistribution studies enabled us to observe the heterogeneous distribution of the 89Zr-FA-DFO-liposome and 89Zr-DFO-liposome in the KB tumor xenografts, the extensive kidney accumulation of the 89Zr-FA-DFO-liposome and 89Zr-PEG-DFO-liposome, and the different metabolic fate of the free and liposome-encapsulated 89Zr-DFO. It also unveiled the poor resistance of all three liposomes against endothelial uptake resulting in their catabolism and high uptake of free 89Zr in the skeleton. Thus, this technically simple 89Zr-labeling method would find widespread use to guide the development and clinical applications of novel liposomal nanomedicines.

Heat-Induced Radiolabeling of Nanoparticles for Monocyte Tracking by PET

Heat-induced radiolabeling (HIR) yielded (89) Zr-Feraheme (FH) nanoparticles (NPs) that were used to determine NP pharmacokinetics (PK) by positron emission tomography (PET). Standard uptake values indicated a fast hepatic uptake that corresponded to blood clearance, and a second, slow uptake process by lymph nodes and spleen. By cytometry, NPs were internalized by circulating monocytes and monocytes in vitro. Using an IV injection of HIR (89) Zr-FH (rather than in vitro cell labeling), PET/PK provided a view of monocyte trafficking, a key component of the immune response.

In Vivo Integrity and Biological Fate of Chelator-Free Zirconium-89-Labeled Mesoporous Silica Nanoparticles

Traditional chelator-based radio-labeled nanoparticles and positron emission tomography (PET) imaging are playing vital roles in the field of nano-oncology. However, their long-term in vivo integrity and potential mismatch of the biodistribution patterns between nanoparticles and radio-isotopes are two major concerns for this approach. Here, we present a chelator-free zirconium-89 ((89)Zr, t1/2 = 78.4 h) labeling of mesoporous silica nanoparticle (MSN) with significantly enhanced in vivo long-term (>20 days) stability. Successful radio-labeling and in vivo stability are demonstrated to be highly dependent on both the concentration and location of deprotonated silanol groups (-Si-O(-)) from two types of silica nanoparticles investigated. This work reports (89)Zr-labeled MSN with a detailed labeling mechanism investigation and long-term stability study. With its attractive radio-stability and the simplicity of chelator-free radio-labeling, (89)Zr-MSN offers a novel, simple, and accurate way for studying the in vivo long-term fate and PET image-guided drug delivery of MSN in the near future.

[(89)Zr]oxinate4 for long-term in vivo cell tracking by positron emission tomography

PURPOSE

(111)In (typically as [(111)In]oxinate3) is a gold standard radiolabel for cell tracking in humans by scintigraphy. A long half-life positron-emitting radiolabel to serve the same purpose using positron emission tomography (PET) has long been sought. We aimed to develop an (89)Zr PET tracer for cell labelling and compare it with [(111)In]oxinate3 single photon emission computed tomography (SPECT).

METHODS

[(89)Zr]Oxinate4 was synthesised and its uptake and efflux were measured in vitro in three cell lines and in human leukocytes. The in vivo biodistribution of eGFP-5T33 murine myeloma cells labelled using [(89)Zr]oxinate4 or [(111)In]oxinate3 was monitored for up to 14 days. (89)Zr retention by living radiolabelled eGFP-positive cells in vivo was monitored by FACS sorting of liver, spleen and bone marrow cells followed by gamma counting.

RESULTS

Zr labelling was effective in all cell types with yields comparable with (111)In labelling. Retention of (89)Zr in cells in vitro after 24 h was significantly better (range 71 to >90%) than (111)In (43-52%). eGFP-5T33 cells in vivo showed the same early biodistribution whether labelled with (111)In or (89)Zr (initial pulmonary accumulation followed by migration to liver, spleen and bone marrow), but later translocation of radioactivity to kidneys was much greater for (111)In. In liver, spleen and bone marrow at least 92% of (89)Zr remained associated with eGFP-positive cells after 7 days in vivo.

CONCLUSION

[(89)Zr]Oxinate4 offers a potential solution to the emerging need for a long half-life PET tracer for cell tracking in vivo and deserves further evaluation of its effects on survival and behaviour of different cell types.

The impact of weakly bound ⁸⁹Zr on preclinical studies: non-specific accumulation in solid tumors and aspergillus infection

Preclinical studies involving (89)Zr often report significant bone accumulation, which is associated with dissociation of the radiometal from the tracer. However, experiments determining the uptake of unbound (89)Zr in disease models are not performed as routine controls. The purpose of the present study was to investigate the impact of free or weakly bound (89)Zr on PET quantifications in disease models, in order to determine if such control experiments are warranted.

METHODS

Chemical studies were carried out to find a (89)Zr compound that would solubilize the (89)Zr as a weak chelate, thus mimicking free or weakly bound (89)Zr released in circulation. (89)Zr oxalate had the desired characteristics, and was injected into mice bearing FaDu and HT29 solid tumor xenografts, and mice infected in the lungs with the mold Aspergillus fumigatus, as well as in healthy controls (naïve). PET/CT or PET/MR imaging followed to quantify the distribution of the radionuclide in the disease models.

RESULTS

(89)Zr oxalate was found to have a plasma half-life of 5.1 ± 2.3 h, accumulating mainly in the bones of all animals. Both tumor types accumulated (89)Zr on the order of 2-4 %ID/cm(3), which is comparable to EPR-mediated accumulation of certain species. In the aspergillosis model, the concentration of (89)Zr in lung tissue of the naïve animals was 6.0 ± 1.1 %ID/g. This was significantly different from that of the animals with advanced disease, showing 11.6 ± 1.8 %ID/g.

CONCLUSIONS

Given the high levels of (89)Zr accumulation in the disease sites in the present study, we recommend control experiments mapping the biodistribution of free (89)Zr in any preclinical study employing (89)Zr where bone uptake is observed. Aqueous (89)Zr oxalate appears to be a suitable compound for such studies. This is especially relevant in studies where the tracer accumulation is based upon passive targeting, such as EPR.

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.

Development and characterization of 89Zr-labeled panitumumab for immuno-positron emission tomographic imaging of the epidermal growth factor receptor

The epidermal growth factor receptor (EGFR) is overexpressed in the majority of malignancies and has been associated with poor outcomes. Panitumumab, an anti-EGFR monoclonal antibody that binds to the extracellular binding domain of EGFR, is increasingly used with radiotherapy and chemotherapy but has associated toxicities. The purpose of this study was to develop and characterize a novel targeted imaging agent for the EGFR using radiolabeled panitumumab. Flow cytometry studies were performed to evaluate EGFR expression in several cell lines. Desferrioxamine-Bz-NCS (DFO) was conjugated to panitumumab and labeled with (89)Zr. Cell uptake studies were performed in four cell lines. For biodistribution studies and micro-positron emission tomography/computed tomography (PET/CT), mouse xenograft models were generated using the same cell lines. PET was performed, and tumors and select organs were harvested for biodistribution studies. Panitumumab was radiolabeled with (89)Zr with high radiochemical purity and specific activity and was found to be stable in serum. Cell binding studies demonstrated that radiotracer uptake in cells correlated with the degree of EGFR expression. MicroPET/CT imaging studies demonstrated a high intensity of (89)Zr-panitumumab in A431 and HCT 116 tumors in comparison with the EGFR-negative tumors. Biodistribution studies confirmed the results from the imaging studies. (89)Zr-panitumumab imaging of EGFR-positive tumors demonstrated levels of radiotracer uptake associated with EGFR expression.

Behavior and impact of zirconium in the soil-plant system: plant uptake and phytotoxicity

Zirconium (Zr) is a transition metal that has both stable and radioactive isotopes.This metal has gained significant attention as a major pollutant of concern, partly because it has been prominent in the debate concerning the growing anthropogenic pressure on the environment. Its numerous past and present uses have induced significant soil and water pollution. Zr is generally considered to have low mobility in soils. The behavior of Zr particularly depends on the characteristics of the media in which it exists, and even its presence in the biosphere as a contaminate may affect its behavior. In this chapter, we describe the relationship between the behavior of Zrand its speciation in soils, its uptake and accumulation by plants, its translocation and toxicity inside plants, and mechanisms by which plants detoxify it.Zr is abundant and occurs naturally in the earth's crust. Zr emissions to the atmosphere are increasing from anthropogenic activities such as its use in industry and nuclear reactors. Zr forms various complexes with soil components, which reduces its soil mobility and phytoavailabilty. The mobility and phytoavailabilty of Zr in soil depend on its speciation and the physicochemical properties of soil that include soil pH, texture, and organic contents. Despite having low soil mobility and phytoavailability,amounts of Zr are absorbed by plants, mainly through the root system and can thereby enter the food chain.After plant uptake, Zr mainly accumulates in root cells. Zr does not have any known essential function in plant or animal metabolism. Although little published data are available, we conclude that the phytotoxicity of Zr is generally low.Notwithstanding, Zr can significantly reduce plant growth and can affect plantenzyme activity. When exposed to Zr-induced toxicity, plants possess numerous defense mechanisms to cope with the toxicity. Such strategies include Zr sequestration in plant roots and activation of various antioxidants. Because Zr may have impact on the biosphere, we believe it deserves to be evaluated in supplementary studies that will enhance the understanding of its behavior in soil-plant systems.

The osseointegration of zirconia dental implants

Zirconia is currently extensively used in medicine, especially in orthopedic surgery for various joint replacement appliances. Its outstanding mechanical and chemical properties have made it the "material of choice" for various types of prostheses. Its color in particular makes it a favored material to manufacture dental implants. A literature search through Medline enables one to see zirconia's potential but also to point out and identify its weaknesses. The search shows that zirconia is a biocompatible, osteoconductive material that has the ability to osseointegrate. Its strength of bonding to bone depends on the surface structure of the implant. Although interesting, the studies do not allow for the recommendation of the use of zirconia implants in daily practice. The lack of studies examining the chemical and structural composition of zirconia implants does not allow for a "gold standard" to be established in the implant manufacturing process. Randomized clinical trials (RCT) are urgently needed on surface treatments of zirconia implants intended to achieve the best possible osseointegration.

Bayesian model selection validates a biokinetic model for zirconium processing in humans

BACKGROUND

In radiation protection, biokinetic models for zirconium processing are of crucial importance in dose estimation and further risk analysis for humans exposed to this radioactive substance. They provide limiting values of detrimental effects and build the basis for applications in internal dosimetry, the prediction for radioactive zirconium retention in various organs as well as retrospective dosimetry. Multi-compartmental models are the tool of choice for simulating the processing of zirconium. Although easily interpretable, determining the exact compartment structure and interaction mechanisms is generally daunting. In the context of observing the dynamics of multiple compartments, Bayesian methods provide efficient tools for model inference and selection.

RESULTS

We are the first to apply a Markov chain Monte Carlo approach to compute Bayes factors for the evaluation of two competing models for zirconium processing in the human body after ingestion. Based on in vivo measurements of human plasma and urine levels we were able to show that a recently published model is superior to the standard model of the International Commission on Radiological Protection. The Bayes factors were estimated by means of the numerically stable thermodynamic integration in combination with a recently developed copula-based Metropolis-Hastings sampler.

CONCLUSIONS

In contrast to the standard model the novel model predicts lower accretion of zirconium in bones. This results in lower levels of noxious doses for exposed individuals. Moreover, the Bayesian approach allows for retrospective dose assessment, including credible intervals for the initially ingested zirconium, in a significantly more reliable fashion than previously possible. All methods presented here are readily applicable to many modeling tasks in systems biology.

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.

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.

Reliability of a new biokinetic model of zirconium in internal dosimetry: part II, parameter sensitivity analysis

The reliability of biokinetic models is essential for the assessment of internal doses and a radiation risk analysis for the public and occupational workers exposed to radionuclides. In the present study, a method for assessing the reliability of biokinetic models by means of uncertainty and sensitivity analysis was developed. In the first part of the paper, the parameter uncertainty was analyzed for two biokinetic models of zirconium (Zr); one was reported by the International Commission on Radiological Protection (ICRP), and one was developed at the Helmholtz Zentrum München-German Research Center for Environmental Health (HMGU). In the second part of the paper, the parameter uncertainties and distributions of the Zr biokinetic models evaluated in Part I are used as the model inputs for identifying the most influential parameters in the models. Furthermore, the most influential model parameter on the integral of the radioactivity of Zr over 50 y in source organs after ingestion was identified. The results of the systemic HMGU Zr model showed that over the first 10 d, the parameters of transfer rates between blood and other soft tissues have the largest influence on the content of Zr in the blood and the daily urinary excretion; however, after day 1,000, the transfer rate from bone to blood becomes dominant. For the retention in bone, the transfer rate from blood to bone surfaces has the most influence out to the endpoint of the simulation; the transfer rate from blood to the upper larger intestine contributes a lot in the later days; i.e., after day 300. The alimentary tract absorption factor (fA) influences mostly the integral of radioactivity of Zr in most source organs after ingestion.

Reliability of a new biokinetic model of zirconium in internal dosimetry: part I, parameter uncertainty analysis

The reliability of biokinetic models is essential in internal dose assessments and radiation risk analysis for the public, occupational workers, and patients exposed to radionuclides. In this paper, a method for assessing the reliability of biokinetic models by means of uncertainty and sensitivity analysis was developed. The paper is divided into two parts. In the first part of the study published here, the uncertainty sources of the model parameters for zirconium (Zr), developed by the International Commission on Radiological Protection (ICRP), were identified and analyzed. Furthermore, the uncertainty of the biokinetic experimental measurement performed at the Helmholtz Zentrum München-German Research Center for Environmental Health (HMGU) for developing a new biokinetic model of Zr was analyzed according to the Guide to the Expression of Uncertainty in Measurement, published by the International Organization for Standardization. The confidence interval and distribution of model parameters of the ICRP and HMGU Zr biokinetic models were evaluated. As a result of computer biokinetic modelings, the mean, standard uncertainty, and confidence interval of model prediction calculated based on the model parameter uncertainty were presented and compared to the plasma clearance and urinary excretion measured after intravenous administration. It was shown that for the most important compartment, the plasma, the uncertainty evaluated for the HMGU model was much smaller than that for the ICRP model; that phenomenon was observed for other organs and tissues as well. The uncertainty of the integral of the radioactivity of Zr up to 50 y calculated by the HMGU model after ingestion by adult members of the public was shown to be smaller by a factor of two than that of the ICRP model. It was also shown that the distribution type of the model parameter strongly influences the model prediction, and the correlation of the model input parameters affects the model prediction to a certain extent depending on the strength of the correlation. In the case of model prediction, the qualitative comparison of the model predictions with the measured plasma and urinary data showed the HMGU model to be more reliable than the ICRP model; quantitatively, the uncertainty model prediction by the HMGU systemic biokinetic model is smaller than that of the ICRP model. The uncertainty information on the model parameters analyzed in this study was used in the second part of the paper regarding a sensitivity analysis of the Zr biokinetic models.

Human biokinetic data and a new compartmental model of zirconium--a tracer study with enriched stable isotopes

Biokinetic models describing the uptake, distribution and excretion of trace elements are an essential tool in nutrition, toxicology, or internal dosimetry of radionuclides. Zirconium, especially its radioisotope (95)Zr, is relevant to radiation protection due to its production in uranium fission and neutron activation of nuclear fuel cladding material. We present a comprehensive set of human data from a tracer study with stable isotopes of zirconium. The data are used to refine a biokinetic model of zirconium. Six female and seven male healthy adult volunteers participated in the study. It includes 16 complete double tracer investigations with oral ingestion and intravenous injection, and seven supplemental investigations. Tracer concentrations were measured in blood plasma and urine collected up to 100 d after tracer administration. The four data sets (two chemical tracer forms in plasma and urine) each encompass 105-240 measured concentration values above detection limits. Total fractional absorption of ingested zirconium was found to be 0.001 for zirconium in citrate-buffered drinking solution and 0.007 for zirconium oxalate solution. Biokinetic models were developed based on the linear first-order kinetic compartmental model approach used by the International Commission on Radiological Protection (ICRP). The main differences of the optimized systemic model of zirconium to the current ICRP model are (1) recycling into the transfer compartment made necessary by the observed tracer clearance from plasma, (2) different parameters related to fractional absorption for each form of the ingested tracer, and (3) a physiologically based excretion pathway to urine. The study considerably expands the knowledge on the biokinetics of zirconium, which was until now dominated by data from animal studies. The proposed systemic model improves the existing ICRP model, yet is based on the same principles and fits well into the ICRP radiation protection approach.

Dissolution rate and biokinetic model of zirconium tritide particles in rat lungs

Metal tritide has been used for different areas such as research, purification, compression, and storage of tritium. Current radiation protection guidelines for tritium compounds describe the behavior of the metal tritide as the same as that of organically bound tritium. However, the biokinetic behavior of metal tritide varies according to materials. This study evaluated the behavior of zirconium tritide (ZrT) particles in rat lungs. The dissolution rate of the ZrT particles in simulated lung fluid was obtained from a specific setup. The ZrT was classified as type-S material according to its low dissolution rate. The ZrT particles were then instilled into rat lungs. The tritium retention time in different rat organs and tritium clearance from rats was obtained by sacrificing a total of 44 rats in a 6 mo period. A biokinetic model for ZrT particles in rat lungs was developed. The predicted retention curves with various phases of tritium in each organ agreed very well with the experimental data. The result can be used to estimate the human annual limit of intake and derived air concentration of ZrT particles.

Measurement techniques for tracer kinetic studies with stable isotopes of zirconium

Biokinetic models are used in radiation protection to assess internal radiation doses. Experiments with stable isotopes as tracers can be performed to obtain characteristic parameters of these models. Two methods for the measurement of zirconium isotopes in human biological samples are presented--thermal ionisation mass spectrometry (TIMS) and proton nuclear activation analysis (PNA). Descriptions include sample preparation, operating conditions, relative uncertainties and method detection limits as well as important properties of both methods.

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

PURPOSE

To evaluate whether (89)Zr can be used as a PET surrogate label for quantification of (90)Y-ibritumomab tiuxetan ((90)Y-Zevalin) biodistribution and dosimetry before myeloablative radioimmunotherapy.

METHODS

Zevalin was labelled with (89)Zr by introducing N-succinyldesferal (N-sucDf) as a second chelate. For comparison of the in vitro stability of (89)Zr-Zevalin and (88)Y-Zevalin (as a substitute for (90)Y), samples were incubated in human serum at 37 degrees C up to 6 days. Biodistribution of (89)Zr-Zevalin and (88)Y-Zevalin was assessed at 24, 48, 72 and 144 h p.i. by co-injection in nude mice bearing the non-Hodgkin's lymphoma (NHL) xenograft line Ramos. The clinical performance of (89)Zr-Zevalin-PET was evaluated via a pilot imaging study in a patient with NHL, who had undergone [(18)F]FDG-PET 2 weeks previously.

RESULTS

Modification of Zevalin with N-sucDf resulted in an N-sucDf-to-antibody molar ratio of 0.83+/-0.04. After radiolabelling and purification, the radiochemical purity and immunoreactivity of (89)Zr-Zevalin always exceeded 95% and 80%, respectively. (89)Zr-Zevalin showed the same stability in serum as (88)Y-Zevalin, with a radiochemical purity >95% during a period of 6 days. The co-injected (89)Zr-Zevalin and (88)Y-Zevalin conjugates showed a very similar biodistribution, except for liver and bone accumulation at 72 and 144 h p.i., which was significantly higher for (89)Zr than for (88)Y. PET images obtained after injection of (89)Zr-Zevalin showed clear targeting of all known tumour lesions.

CONCLUSION

(89)Zr-Zevalin and (88)Y-Zevalin showed a very similar biodistribution in mice, implying that (89)Zr-Zevalin-PET might be well suited for prediction of (90)Y-Zevalin biodistribution in a myeloablative setting.

(89)Zr as a PET surrogate radioisotope for scouting biodistribution of the therapeutic radiometals (90)Y and (177)Lu in tumor-bearing nude mice after coupling to the internalizing antibody cetuximab

Immuno-PET as a scouting procedure before radioimmunotherapy (RIT) aims at confirming tumor targeting and accurately estimating radiation dose delivery to both tumor and normal tissues and might therefore be of value for selection of patient candidates for RIT. A prerequisite for this approach is that PET radioimmunoconjugates and RIT radioimmunoconjugates must show a similar biodistribution. In the present study, we evaluated the potential of the long-lived positron emitter (89)Zr to predict biodistribution of the residualizing therapeutic radiometals (88)Y (as a substitute for (90)Y) and (177)Lu when labeled to the monoclonal antibody (mAb) cetuximab via different types of chelates. Cetuximab was selected as a model mAb because it abundantly internalizes after binding to the epidermal growth factor receptor.

METHODS

Cetuximab was labeled with (89)Zr using succinylated desferrioxamine B (N-sucDf). The chelates p-benzyl isothiocyanate-1,4,7,10-tetraazacyclododecane-1,4,7, 10-tetraacetic acid (p-SCN-Bz-DOTA) and p-isothiocyanatobenzyl diethylenetriaminepentaacetic acid (p-SCN-Bz-DTPA) were both used for radiolabeling with (88)Y and (177)Lu. For measurement of the in vitro stability of each of the 5 radioimmunoconjugates, samples were incubated in freshly prepared human serum at 37 degrees C up to 16 d. Biodistribution was assessed at 24, 48, 72, and 144 h after intraperitoneal coinjection of the PET and RIT conjugates in nude mice bearing the squamous cell carcinoma xenograft line A431.

RESULTS

Cetuximab premodification with N-sucDf, p-SCN-Bz-DOTA, or p-SCN-Bz-DTPA resulted in chelate-to-mAb molar ratios of about 1. After radiolabeling and purification, the radiochemical purity and immunoreactive fraction of the conjugates always exceeded 97% and 93%, respectively. All conjugates were stable in serum, showing a radioactivity release of less than 5% until day 7. From day 7 until day 16, an enhanced release was observed for the (89)Zr-N-sucDf, (88)Y-p-SCN-Bz-DTPA, and (177)Lu-p-SCN-Bz-DTPA conjugates. The coinjected PET and RIT conjugates showed similar biodistributions, except for the thighbone and sternum. For example, the (89)Zr-N-sucDf conjugate showed a 2.0-2.5 times higher radioactivity accretion in the thighbone than did the RIT conjugates at 72 h after injection.

CONCLUSION

In view of the advantages of PET over SPECT, (89)Zr-immuno-PET is a promising modality for in vivo scouting of (90)Y- and (177)Lu-labeled mAbs, although care should be taken when estimating bone marrow doses.

Adsorption of Co(II), Ni(II), Cu(II), and Zn(II) on hexagonal templated zirconia obtained thorough a sol-gel process: the effects of nanostructure on adsorption features

Using zirconium tetrabutoxide, diaminedecane, and diamineoctane as precursors, a templated hexagonal zirconia matrix is synthesized and characterized by X-ray diffractometry and scanning electron microscopy. The adsorption capacity of such a matrix toward Co(II), Ni(II), Cu(II), and Zn(II) from aqueous solutions is studied. The adsorption affinity of the synthesized hexagonal templated zirconia toward the cations is Cu(II)>Zn(II) >>Ni(II)>Co(II). It is also verified that the adsorption of the cations follows a Langmuir and not a Freundlich isotherm. All obtained isotherms are of type I, according to the IUPAC classification. The observed adsorption affinity sequence can be explained by taking into account the velocity constant for the substitution of water molecules into the cation coordination spheres, as well as the Irving-Williams series.

PET Radioimmunoscintigraphy of Renal Cell Cancer Using 89Zr-Labeled cG250 Monoclonal Antibody in Nude Rats

INTRODUCTION

With the introduction of positron-emitting radionuclides with half-lifes in days, such as 89Zr and 124I, radioimmunoscintigraphy (RIS) with positron-emitter-labeled monoclonal antibodies (moAbs) becomes feasible. RIS, using positron emission tomography (immuno-PET), combines the specific localization of an antibody with the high resolution of a PET camera. In the present study, scintigraphic tumor imaging using chimeric moAb G250 labeled with 89Zr (immuno-PET) or 111In (RIS), and [18F]FDG-(PET) was explored in rats with s.c. renal cell carcinoma (RCC) tumors.

METHODS

Nude rats (6-8 rats per group) with s.c. SK-RC-52 tumors were i.v. injected with 4 MBq 111InDTPA-cG250, 20 MBq 89Zr-Df-cG250 or 4 MBq [18F]FDG. Planar 111In-DTPA-cG250 images were obtained 5 minutes, and 24, 48, and 72 hours postinjection (p.i.). 3D PET imaging was performed 5 minutes, and 24, 48, and 72 hours after a 89Zr-Df-cG250 injection and 1 hour after a [18F]FDG injection using a Siemens ECAT EXACT PET camera. Rats were killed after the last imaging session, and the uptake of the radiolabel in the dissected tissues was determined.

RESULTS

Both radiolabeled antibody preparations were stable during 4 days of incubation in serum at 37 degrees C, and the immunoreactivity was preserved. Two (2) days after injection, s.c. tumors (100 mg) were clearly visualized, both with 89Zr-Df-cG250 and 111In-DTPA-cG250. Tumors were not visualized with [18F]FDG (uptake in tumor of 0.5 +/- 0.1 %ID/g, 1 hour p.i.). The biodistribution experiments showed an identical uptake in the tumor for both 89Zr-Df-cG250 and 111In-DTPA-cG250 at 3 days p.i. (5.0 +/- 2.4 and 4.9 +/- 2.9 %ID/g, respectively). Blood levels at 3 days p.i. were also identical (1.4 +/- 0.4 versus 1.7 +/- 0.7 %ID/g), and no significant differences were found in the biodistribution of normal tissues between the two radiolabeled cG250 preparations.

CONCLUSION

The cG250 antibody can be stably labeled with the positron-emitter 89Zr, while preserving the immunoreactivity of the moAb. In this rat model, the in vivo biodistribution of 89Zr-Df-cG250 was identical to that of 111In-DTPA-cG250. Immuno-PET of RCC is feasible with 89Zr-cG250, and relatively small tumors could be visualized, even without a dedicated PET camera for small animals.

Quantitative 89Zr immuno-PET for in vivo scouting of 90Y-labeled monoclonal antibodies in xenograft-bearing nude mice

Immuno-PET as a scouting procedure before radioimmunotherapy (RIT) aims at the confirmation of tumor targeting and the accurate estimation of radiation dose delivery to both tumor and normal tissues. Immuno-PET with (89)Zr-labeled monoclonal antibodies (mAbs) and (90)Y-mAb RIT might form such a valuable combination. In this study, the biodistribution of (89)Zr-labeled and (88)Y-labeled mAb ((88)Y as substitute for (90)Y) was compared and the quantitative imaging performance of (89)Zr immuno-PET was evaluated.

METHODS

Chimeric mAb (cmAb) U36, directed against an antigen preferentially expressed in head and neck cancer, was labeled with (89)Zr using the bifunctional chelate N-succinyldesferrioxamine B (N-sucDf) and with (88)Y using the bifunctional chelate p-isothiocyanatobenzyl-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (p-SCN-Bz-DOTA). The radioimmunoconjugates were coinjected in xenograft-bearing nude mice, and biodistribution was determined at 3, 24, 48, 72, and 144 h after injection. (89)Zr was evaluated and compared with (18)F in phantom studies to determine linearity, resolution, and recovery coefficients, using a high-resolution research tomograph PET scanner. The potential of PET to quantify cmAb U36-N-sucDf-(89)Zr was evaluated by relating image-derived tumor uptake data (noninvasive method) to (89)Zr uptake data derived from excised tumors (invasive method).

RESULTS

(89)Zr-N-sucDf-labeled and (88)Y-p-SCN-Bz-DOTA-labeled cmAb U36 showed a highly similar biodistribution, except for sternum and thigh bone at later time points (72 and 144 h after injection). Small differences were found in kidney and liver. Imaging performance of (89)Zr approximates that of (18)F, whereas millimeter-sized (19-154 mg) tumors were visualized in xenograft-bearing mice after injection of cmAb U36-N-sucDf-(89)Zr. After correction for partial-volume effects, an excellent correlation was found between image-derived (89)Zr tumor radioactivity and gamma-counter (89)Zr values of excised tumors (R(2) = 0.79).

CONCLUSION

The similar biodistribution and the favorable imaging characteristics make (89)Zr a promising candidate for use as a positron-emitting surrogate for (90)Y.

89Zr immuno-PET: comprehensive procedures for the production of 89Zr-labeled monoclonal antibodies

The use of immuno-PET, the combination of PET with monoclonal antibodies (mAbs), is an attractive option to improve tumor detection and mAb quantification. The long-lived positron emitter (89)Zr has ideal physical characteristics for immuno-PET, such as a half-life of 3.27 d, which is compatible with the time needed for intact mAbs to achieve optimal tumor-to-nontumor ratios. Thus far, a major limitation in the use of (89)Zr has been the lack of suitable methods for its stable coupling to mAbs. In this article, practical protocols for reproducible isolation of highly pure (89)Zr and the production of optimal-quality mAb-(89)Zr conjugates are provided.

METHODS

(89)Zr was produced by a (p,n) reaction on natural yttrium ((89)Y), isolated with a hydroxamate column, and used for labeling of premodified mAbs. mAbs were premodified with a novel bifunctional derivative of the chelate desferrioxamine B (Df) via a new linker chemistry. To this end, Df was initially succinylated (N-sucDf), temporarily filled with Fe(III), esterified by use of tetrafluorophenol, and then directly coupled to mAbs. Chimeric mAb (cmAb) U36, directed against head and neck cancer, was used for in vitro and in vivo evaluation. The in vitro stability of cmAb U36-N-sucDf-(89)Zr was assessed in human serum, and its in vivo behavior was evaluated by biodistribution and PET imaging studies in tumor-bearing nude mice. A cmAb U36-Df-(89)Zr conjugate containing a previously described succinimide ring-thioether unit in the linker was used as a reference.

RESULTS

(89)Zr was produced in large batches (6.5-13.5 GBq) and isolated with improved radionuclidic purity (>99.99%) and high yield (>94%). The Df-premodified mAbs gave (89)Zr-labeling efficiencies of 80% within 30 min, resulting in conjugates with preserved integrity and immunoreactivity. With respect to stability, the novel cmAb U36-N-sucDf-(89)Zr conjugate appeared to be superior to the reference conjugate. In vivo, the novel conjugate demonstrated selective tumor targeting, and on PET images obtained at 24, 48, and 72 h after injection of this conjugate, small tumors in the range of 19-154 mg were readily visualized.

CONCLUSION

Methods were developed for improved purification of the long-lived positron emitter (89)Zr. Moreover, a novel bifunctional Df chelate was synthesized for the reproducible coupling of (89)Zr to mAbs. The suitability of such conjugates to detect millimeter-sized tumors in xenograft-bearing nude mice was demonstrated.

[Dynamics of 95Zr in simulated marine water-sediment-organisms system]

To provide scientific evidence to evaluate the behavior of 95Zr in ocean ecosystem, the dynamic model of the transference, accumulation and disappearance of 95Zr among the simulated marine water, sediment and organisms was investigated using Nassarius semiplicatus and Boleophthalums pectinirostris as experimental stuffs. The result showed that 95Zr(Bq.g-1 or Bq.ml-1) in the marine water was decreased more than 90% in the first 4 h, and then descended gradually. 95Zr in sediment was increased in the peak in 48 h and then declined. The radioactivity percent of 95Zr in the shell and muslce of Nassarius semiplicatus was 68.7% and 31.30% respectively, while the radioactivity percent was 22.80%, 12.64%, 34.82%, 10.31%, 4.48%, 11.55% and 3.71%, respectively in the fill, fin, viscera, skull, skin, vertebra and muscle of Boleophthalmus pectinirostris. Nassarius semiplicatus had a greater concentrating capability of 95Zr than Boleophthalmus pectinirostris. The order of the 95Zr concentration was found to be sediment > Nassarius semiplicatus > water > Boleophthalmus pectinirostris. A dynamic model of closed four-compartment was constructed with exponent function.

A re-evaluation of the biokinetics of zirconium in humans

There is much interest in understanding the biokinetics of zirconium in humans due to the potential radiological risk represented by the radionuclide 95Zr and by its daughter 95Nb. Despite the significance of zirconium, few data are available on the actual biokinetics of zirconium in humans. Accordingly the biokinetic model currently recommended by ICRP for this element is based mainly on data from animal experiments. In this study, the use of the stable isotopes 90Zr and 96Zr as tracers has enabled the conduct of 6 biokinetic investigations in 3 healthy volunteers. These studies have provided new valuable information about intestinal absorption and kinetics in blood plasma of zirconium and have been used for the set-up of a more realistic compartmental model with possible applications for dosimetric purposes.

Stable tracer investigations in humans for assessing the biokinetics of ruthenium and zirconium radionuclides

The interest in the biokinetics of ruthenium and zirconium in humans is justified by the potential radiological risk represented by their radionuclides. Only a few data related to the biokinetics of ruthenium and zirconium in humans are available and, accordingly, the biokinetic models currently recommended by the ICRP for these elements are mainly based on data from animal experiments. The use of stable isotopes as tracers, coupled with a proper analytical technique (nuclear activation analysis with protons) for their determination in biological samples, represents an ethically acceptable methodology for biokinetic investigations, being free from any radiation risk for the volunteer subjects. In this work, the results obtained in eight biokinetic investigations for ruthenium, conducted on a total of three healthy volunteers, and six for zirconium, performed on a total of three subjects, are presented and compared to the predictions of the ICRP models.

Determination of biokinetic parameters for ingestion of radionuclides of zirconium in animals using stable tracers

Exposure to the radioactive isotope 95Zr, as in nuclear accidents, and to stable zirconium, due to its use in industry, has increased the interest in the biokinetics of this element. Information has been derived mainly from tests performed on animals by means of radioactive tracers. Due to the fact that extrapolation from animals to humans is always open to question, there is an increasing need of a methodology which allows data to be obtained directly from humans. The use of stable tracers, being ethically justifiable, is a powerful tool for providing this information. As two tracers of the same element must be utilized in order to evaluate gut absorption, an analytical technique which is capable of distinguishing and measuring simultaneously different isotopes of zirconium in biological samples is required. Preliminary tests on laboratory animals were performed in order to assess the feasibility of the double tracer technique combined with proton activation analysis.

Zirconium-labeled monoclonal antibodies and their distribution in tumor-bearing nude mice

A method to label monoclonal antibodies (MAbs) with 88Zr and 89Zr has been developed and tested on the MAbs 323/A3 and E48.

METHODS

The bifunctional chelating agent desferal (Df) was linked through a thioether bond to the MAbs. Labeling was accomplished by addition of the premodified antibodies to isolated Zr. The retention of the in vivo behavior of the MAbs was determined by comparing the biodistribution of 88Zr-labeled MAbs with those of 123I and 99mTc in mice bearing tumor xenografts.

RESULTS

The labeling was simple and the yields were high (above 90%). The obtained conjugates retained their immunoreactivity (> 80%). The blood clearance and biodistribution of Zr-labeled MAbs resembled those of the reference conjugates. The Zr-Df-MAb conjugates showed a specific tumor accumulation. Zirconium-89-labeled 323/A3 could be visualized with a PET camera. The absence of large amounts of Zr present in the bone pointed to a good in vivo stability of the Zr-Df-MAb conjugates.

CONCLUSION

This method is well suited for labeling MAbs with Zr isotopes. Using 89Zr, the biodistribution of the radioimmunoconjugate can easily be visualized with a PET camera.

[The effect of algisorb on the level of the accumulation of zirconium, ruthenium, iodine and cesium radioactive isotopes in the body of rats]

The sorption effect of Algisorbum has been studied in rats following single and multiple intragastric administration. Algisorbum doses of 250-2000 mg/kg decrease the absorption of 106Ru and 95Zr by 50%, that of 137Cs by 15% and have no effect on 131I absorption. Application of a complex of agents to protect the body from nuclear fission products is discussed.

A facile method for the labeling of proteins with zirconium isotopes

To label proteins with positron emitters with a half-life in the order of days, a method has been developed to label proteins with zirconium (Zr)-isotopes. Therefore, the bifunctional chelating agent desferal (Df) was coupled to albumins via a thioether bond. Labeling of the premodified proteins was easily performed by addition of these proteins to freeze-dried Zr-oxalate. This labeling was efficient (> 90%) and accomplished in several minutes. The conjugates showed a high in vitro stability. Biodistribution studies were performed with 88Zr-citrate, 88Zr-Df, and 88Zr-labeled mouse serum albumin (88Zr-Df-MSA), modified with different amounts of chelating groups. Whereas Zr-citrate was found to accumulate in bone, Zr-Df was cleared very fast by glomerular filtration. The 88Zr-Df-MSA showed similar blood clearance as did 123I-labeled MSA. The biodistribution pattern of 88Zr-Df-MSA differed only from 123I-MSA in that a higher accumulation of Zr in liver, kidney, and spleen was found. The absence of large amounts of 88Zr in bone indicated that in vivo the conjugates are also reasonably stable.

Retention and excretion of 95Zr-95Nb in humans

This note describes the retention and excretion of 95Zr-95Nb in humans based on a recent CANDU experience and a literature survey of reported cases. Two data bases, QUEST and INIS were used for the survey. Three reported cases were discovered: two for occupational exposures and one for public exposure from nuclear weapons fallout. Human lung retention from these three cases, plus whole body retention and some limited fecal excretion data from a recently occurred exposure at a CANDU station, were reviewed and tested against predictions based on ICRP Publication 30 model. Based on the fits of this model to the reported data it seems that the three occupational exposures exhibit class Y behavior while the public exposure exhibits class W behavior. For only one case is the chemical compound known with certainty: ZrO2. Zirconium oxides are currently classified as class W in ICRP Publications 30 and 54. This work confirms a suggestion that oxides of zirconium be classified as class Y and should be taken into account by the ICRP in its future publications.

Effect of hydrostatic pressure on the diffusion of monomers through dentin in vitro

In previous work, the diffusion of monomers from composite and bonding resins through dentin was demonstrated in vitro. The monomers triethylene glycol dimethacrylate (TEGDMA) and 2-hydroxyethyl methacrylate (HEMA) were identified in samples from the pulp space. In the current study, we examined the effects of two levels of positive hydrostatic pressure on the passage of resin monomers through dentin in vitro from a composite-resin/bonding-resin combination to test the hypothesis that monomer diffusion is prevented by such pressure. An occlusal cavity prepared in the tooth crown was restored with the resins. Distilled water samples from the pulpal space were removed over time and analyzed for monomer content by high-performance liquid chromatography and mass spectrometry. Positive pulpal pressure reduced but did not prevent pulpward movement of diluent monomers that leach from bonding agents and from resin composites through dentin in vitro. The degree of reduction of diffusion was greater with TEGDMA than with the lower-molecular-weight monomer HEMA.

Behavior of neutron-activated uranium dioxide dust particles in the gastrointestinal tract of the rat

The behavior of neutron-irradiated, simulated Chernobyl UO2 particles containing 141Ce, 144Ce, 95Zr, 95Nb, and 103Ru in the gastrointestinal tract was investigated to obtain basic information for dosimetric and risk analyses of nuclear accidents. After the UO2 particles were administered to rats intragastrically, the distribution and retention of specific radionuclides were studied by using whole-body autoradiography and gamma-spectrometric analysis of tissues. None of the radionuclides were detected in liver, kidney, muscle, bone, brain, blood, and urine. Approximately 98% of the total administered radioactivity was excreted in feces within 3 days. A two times greater intestinal retention (about 6%) of 95Nb than for the other radionuclides was observed 1 day after administration. The results indicate that this kind of relatively insoluble particulate material is not absorbed or retained significantly in the epithelial cells of the intestinal wall. Fallout particles containing high-energy beta sources, 106Ru and 144Ce, result in a very high radiation dose (up to several Gy/day) in the vicinity of a hot particle. Niobium-95 with low average beta energy (0.043 MeV (100%)) does not increase the total dose to the GI tract significantly despite its longer retention in the intestine. Evaluation of the biological effects of these particles in the GI tract by using a dosimetric model based on uniform distribution of activity may be misleading.