Imaging of Prostate Cancer with Immuno-PET and Immuno-SPECT Using a Radiolabeled Anti-EGP-1 Monoclonal Antibody
Department of Nuclear Medicine, Radboud University Nijmegen Medical Center, Nijmegen, The Netherlands. Journal of Nuclear Medicine
(Impact Factor: 6.16).
08/2011; 52(10):1601-7. DOI: 10.2967/jnumed.110.086520
hRS7 is a humanized IgG1 monoclonal antibody directed against the epithelial glycoprotein-1 (EGP-1; also known as TROP2). This antigen is found in many epithelial cancers, including prostate cancer, and therefore this antibody could be suitable for targeting this cancer. In this study, the characteristics of hRS7 for targeting prostate cancer were examined. The potential for immuno-PET with (89)Zr-hRS7 and immuno-SPECT with (111)In-hRS7 was assessed using nude mice with human prostate cancer xenografts.
EGP-1 expression was assessed by immunohistology in human primary and metastatic prostate cancer samples and in PC3 xenografts. The optimal antibody protein dose for prostate cancer targeting was examined in nude mice with subcutaneous PC3 xenografts, and then the biodistribution of (111)In-, (125)I-, and (89)Zr-labeled hRS7 was determined in subcutaneous PC3 xenografts at 1, 3, and 7 d after injection. Immuno-PET and immuno-SPECT were performed with (89)Zr-hRS7 and (111)In-hRS7 in mice with subcutaneous and intraprostatic PC3 xenografts, respectively.
Immunohistochemical analysis showed abundant EGP-1 expression in human primary and metastatic prostate cancers and in PC3 xenografts. (111)In-hRS7 and (89)Zr-hRS7 preferentially and specifically accumulated in PC3 xenografts, with tumor uptake as high as 60% injected dose per gram at a protein dose of 0.1 μg per mouse. PC3 tumors in nude mice were clearly visualized with both tracers with immuno-PET and immuno-SPECT.
hRS7 shows excellent in vivo tumor targeting in human prostate cancer xenografts. Therefore, hRS7 is a potential vehicle for targeting prostate cancer.
Available from: Andreas Kjaer
- "Zr is a popular radionuclide for the radiolabeling of monoclonal antibodies (mAbs) and similar proteins for PET imaging     . However, several studies on 89 Zr-radiolabeled mAbs show significant bone uptake of 89 Zr   . This uptake is often attributed to chelate instability, with 89 Zr being released during the long circulation time of the mAb . "
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ABSTRACT: Preclinical studies involving 89Zr often report significant bone accumulation, which is associated with dissociation of the radiometal from the tracer. However, experiments determining the uptake of unbound 89Zr 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 89Zr 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 89Zr compound that would solubilize the 89Zr as a weak chelate, thus mimicking free or weakly bound 89Zr released in circulation. 89Zr 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 and PET/MR imaging followed to quantify the distribution of the radionuclide in the disease models.Results89Zr 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 89Zr on the order of 2-4% ID/cm3, which is comparable to EPR-mediated accumulation of certain species. In the aspergillosis model, the concentration of 89Zr 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 89Zr accumulation in the disease sites in the present study, we recommend control experiments mapping the biodistribution of free 89Zr in any preclinical study employing 89Zr where bone uptake is observed. Aqueous 89Zr 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.
Available from: Otto C Boerman
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ABSTRACT: RS7 is an internalizing anti-Trop-2 pancarcinoma antibody capable of targeting most epithelial cancers. Because pretargeting strategies could improve the tumor localization of radionuclides, a new anti-Trop-2 × antihapten bispecific antibody for pretargeting, based on humanized RS7, was prepared and evaluated with a radiolabeled hapten-peptide in vitro and in vivo to determine whether its internalization properties would interfere with pretargeting.
The anti-Trop-2 × antihapten bispecific antibody, TF12, was prepared using the modular dock-and-lock method. TF12 and humanized RS7 binding was assessed by cell binding assays and fluorescence-activated cell sorting analysis in a variety of human carcinoma cell lines. The internalization of TF12 was evaluated in vitro using a fluorescent TF12 conjugate or hapten-peptide and (111)In-labeled TF12 and RS7. The biodistribution of TF12 and its use as a pretargeting agent with an (111)In-labeled hapten-peptide were assessed in several human epithelial cancer xenografts. Dose optimization was examined in 2 tumor models.
TF12 internalizes, but a substantial fraction remained accessible on the tumor surface. Fluorescence-activated cell sorting analysis showed only a minor change in fluorescent signal when the tumor was probed with a fluorescent hapten-peptide over 4 h, and microscopy showed substantial membrane staining when reassessed at 24 h after TF12 exposure. Only 40.1% of (111)In-TF12 was internalized after 24 h. In vivo, excellent tumor localization of the (111)In-labeled peptide was observed in several tumor models.
TF12 was retained sufficiently on the cell surface in several epithelial cancers, thereby making it suitable for pretargeted imaging and therapy of various Trop-2-expressing carcinomas.
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ABSTRACT: 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.
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