Improved non-invasive positron emission tomographic imaging of chemotherapy-induced tumor cell death using Zirconium-89-labeled APOMAB®

Diagnostic Positron Emission Tomography Imaging with Zirconium-89 Desferrioxamine B Squaramide: From Bench to Bedside

Molecular imaging with antibodies radiolabeled with positron-emitting radionuclides combines the affinity and selectivity of antibodies with the sensitivity of Positron Emission Tomography (PET). PET imaging allows the visualization and quantification of the biodistribution of the injected radiolabeled antibody, which can be used to characterize specific biological interactions in individual patients. This characterization can provide information about the engagement of the antibody with a molecular target such as receptors present in elevated levels in tumors as well as providing insight into the distribution and clearance of the antibody. Potential applications of clinical PET with radiolabeled antibodies include identifying patients for targeted therapies, characterization of heterogeneous disease, and monitoring treatment response.Antibodies often take several days to clear from the blood pool and localize in tumors, so PET imaging with radiolabeled antibodies requires the use of a radionuclide with a similar radioactive half-life. Zirconium-89 is a positron-emitting radionuclide that has a radioactive half-life of 78 h and relatively low positron emission energy that is well suited to radiolabeling antibodies. It is essential that the zirconium-89 radionuclide be attached to the antibody through chemistry that provides an agent that is stable in vivo with respect to the dissociation of the radionuclide without compromising the biological activity of the antibody.This Account focuses on our research using a simple derivative of the bacterial siderophore desferrioxamine (DFO) with a squaramide ester functional group, DFO-squaramide (DFOSq), to link the chelator to antibodies. In our work, we produce conjugates with an average ∼4 chelators per antibody, and this does not compromise the binding of the antibody to the target. The resulting antibody conjugates of DFOSq are stable and can be easily radiolabeled with zirconium-89 in high radiochemical yields and purity. Automated methods for the radiolabeling of DFOSq-antibody conjugates have been developed to support multicenter clinical trials. Evaluation of several DFOSq conjugates with antibodies and low molecular weight targeting agents in tumor mouse models gave PET images with high tumor uptake and low background. The promising preclinical results supported the translation of this chemistry to human clinical trials using two different radiolabeled antibodies. The potential clinical impact of these ongoing clinical trials is discussed.The use of DFOSq to radiolabel relatively low molecular weight targeting molecules, peptides, and peptide mimetics is also presented. Low molecular weight molecules typically clear the blood pool and accumulate in target tissue more rapidly than antibodies, so they are usually radiolabeled with positron-emitting radionuclides with shorter radioactive half-lives such as fluorine-18 (t1/2 ∼ 110 min) or gallium-68 (t1/2 ∼ 68 min). Radiolabeling peptides and peptide mimetics with zirconium-89, with its longer radioactive half-life (t1/2 = 78 h), could facilitate the centralized manufacture and distribution of radiolabeled tracers. In addition, the ability to image patients at later time points with zirconium-89 based agents (e.g. 4-24 h after injection) may also allow the delineation of small or low-uptake disease sites as the delayed imaging results in increased clearance of the tracer from nontarget tissue and lower background signal.

Imaging of cell death in malignancy: Targeting pathways or phenotypes?

Cell death is fundamental in health and disease and resisting cell death is a hallmark of cancer. Treatment of malignancy aims to cause cancer cell death, however current clinical imaging of treatment response does not specifically image cancer cell death but assesses this indirectly either by changes in tumor size (using x-ray computed tomography) or metabolic activity (using 2-[18F]fluoro-2-deoxy-glucose positron emission tomography). The ability to directly image tumor cell death soon after commencement of therapy would enable personalised response adapted approaches to cancer treatment that is presently not possible with current imaging, which is in many circumstances neither sufficiently accurate nor timely. Several cell death pathways have now been identified and characterised that present multiple potential targets for imaging cell death including externalisation of phosphatidylserine and phosphatidylethanolamine, caspase activation and La autoantigen redistribution. However, targeting one specific cell death pathway carries the risk of not detecting cell death by other pathways and it is now understood that cancer treatment induces cell death by different and sometimes multiple pathways. An alternative approach is targeting the cell death phenotype that is "agnostic" of the death pathway. Cell death phenotypes that have been targeted for cell death imaging include loss of plasma membrane integrity and dissipation of the mitochondrial membrane potential. Targeting the cell death phenotype may have the advantage of being a more sensitive and generalisable approach to cancer cell death imaging. This review describes and summarises the approaches and radiopharmaceuticals investigated for imaging cell death by targeting cell death pathways or cell death phenotype.

Evaluation of coumarin-tagged deferoxamine as a Zr(IV)-based PET/fluorescence dual imaging probe

Desferoxamine (DFO) is currently the golden standard chelator for 89Zr4+, a promising nuclide for positron emission tomography imaging (PET). The natural siderophore DFO had previously been conjugated with fluorophores to obtain Fe(III) sensing molecules. In this study, a fluorescent coumarin derivative of DFO (DFOC) has been prepared and characterized (potentiometry, UV-Vis spectroscopy) for what concerns its protonation and metal coordination properties towards PET-relevant ions (Cu(II), Zr(IV)), evidencing strong similarity with pristine DFO. Retention of DFOC fluorescence emission upon metal binding has been checked (fluorescence spectrophotometry), as it would - and does - allow for optical (fluorescent) imaging, thus unlocking bimodal (PET/fluorescence) imaging for 89Zr(IV) tracers. Crystal violet and MTT assays on NIH-3 T3 fibroblasts and MDA-MB 231 mammary adenocarcinoma cell lines demonstrated, respectively, no cytotoxicity nor metabolic impairment at usual radiodiagnostic concentrations of ZrDFOC. Clonogenic colony-forming assay performed on X-irradiated MDA-MB 231 cells showed no interference of ZrDFOC with radiosensitivity. Morphological biodistribution (confocal fluorescence, transmission electron microscopy) assays on the same cells suggested internalization of the complex through endocytosis. Overall, these results support fluorophore-tagged DFO as a suitable option to achieve dual imaging (PET/fluorescence) probes based on 89Zr.

The RNA-binding protein La/SSB associates with radiation-induced DNA double-strand breaks in lung cancer cell lines

BACKGROUND

Platinum-based chemotherapy and radiotherapy are standard treatments for non-small cell lung cancer, which is the commonest, most lethal cancer worldwide. As a marker of treatment-induced cancer cell death, we have developed a radiodiagnostic imaging antibody, which binds to La/SSB. La/SSB is an essential, ubiquitous ribonuclear protein, which is over expressed in cancer and plays a role in resistance to cancer therapies.

AIM

In this study, we examined radiation-induced DNA double strand breaks (DSB) in lung cancer cell lines and examined whether La/SSB associated with these DSB.

METHOD

Three lung cancer lines (A549, H460 and LL2) were irradiated with different X-ray doses or X-radiated with a 5 Gy dose and examined at different time-points post-irradiation for DNA DSB in the form of γ-H2AX and Rad51 foci. Using fluorescence microscopy, we examined whether La/SSB and γ-H2AX co-localise and performed proximity ligation assay (PLA) and co-immunoprecipitation to confirm the interaction of these proteins.

RESULTS

We found that the radio-resistant A549 cell line compared to the radio-sensitive H460 cell line showed faster resolution of radiation-induced γ-H2AX foci over time. Conversely, we found more co-localised γ-H2AX and La/SSB foci by PLA in irradiated A549 cells.

CONCLUSION

The co-localisation of La/SSB with radiation-induced DNA breaks suggests a role of La/SSB in DNA repair, however further experimentation is required to validate this.

Fc gamma receptor is not required for in vivo processing of radio- and drug-conjugates of the dead tumor cell-targeting monoclonal antibody, APOMAB®

The Fc region of a monoclonal antibody (mAb) can play a crucial role in its biodistribution and therapeutic activity. The chimeric mAb, chDAB4 (APOMAB®), which binds to dead tumor cells after DNA-damaging anticancer treatment, has been studied pre-clinically in both diagnostic and therapeutic applications in cancer. Given that macrophages contribute to the tumor accumulation of chDAB4 and its potency as an antibody drug conjugate in vivo, we next wanted to determine whether the Fc region of the chDAB4 mAb also contributed. We found that, regardless of prior labeling with chDAB4, dead EL4 lymphoma or Lewis Lung (LL2) tumor cells were phagocytosed equally by wild-type or Fcγ knock-down macrophage cell lines. A similar result was seen with bone marrow-derived macrophages from wild-type, Fcγ knock-out (KO) and NOTAM mice that express Fcγ but lack immunoreceptor tyrosine-based activation motif (ITAM) signaling. Among EL4 tumor-bearing wild-type, Fcγ KO or NOTAM mice, no differences were observed in post-chemotherapy uptake of ⁸⁹Zr-labeled chDAB4. Similarly, no differences were observed between LL2 tumor-bearing wild-type and Fcγ KO mice in post-chemotherapy uptake of ⁸⁹Zr-chDAB4. Also, the post-chemotherapy activity of a chDAB4-antibody drug conjugate (ADC) directed against LL2 tumors did not differ among tumor-bearing wild-type, Fcγ KO and NOTAM mice, nor did the proportions and characteristics of the LL2 tumor immune cell infiltrates differ significantly among these mice. In conclusion, Fc-FcγR interactions are not essential for the diagnostic or therapeutic applications of chDAB4 conjugates because the tumor-associated macrophages, which engulf the chDAB4-labelled dead cells, respond to endogenous 'eat me' signals rather than depend on functional FcγR expression for phagocytosis.

Heptadentate chelates for 89Zr-radiolabelling of monoclonal antibodies

Zirconium complexation chemistry is an important area of research in the context of developing radiolabelled proteins for applications in diagnostic positron emission tomography (PET) imaging. Herein, we report the synthesis...

ImmunoPET imaging of hematological malignancies: From preclinical promise to clinical reality

Immuno-positron emission tomography (immunoPET) imaging is a paradigm-shifting imaging technique for whole-body and all-lesion tumor detection, based on the combined specificity of tumor-targeting vectors [e.g., monoclonal antibodies (mAbs), nanobodies, and bispecific antibodies] and the sensitivity of PET imaging. By noninvasively, comprehensively, and serially revealing heterogeneous tumor antigen expression, immunoPET imaging is gradually improving the theranostic prospects for hematological malignancies. In this review, we summarize the available literature regarding immunoPET in imaging hematological malignancies. We also highlight the pros and cons of current conjugation strategies, and modular chemistry that can be leveraged to develop novel immunoPET probes for hematological malignancies. Lastly, we discuss the use of immunoPET imaging in guiding antibody drug development.

Positron Emission Tomographic Imaging of Tumor Cell Death Using Zirconium-89-Labeled APOMAB® Following Cisplatin Chemotherapy in Lung and Ovarian Cancer Xenograft Models

PURPOSE

Early detection of tumor treatment responses represents an unmet clinical need with no approved noninvasive methods. DAB4, or its chimeric derivative, chDAB4 (APOMAB®) is an antibody that targets the Lupus associated antigen (La/SSB). La/SSB is over-expressed in malignancy and selectively targeted by chDAB4 in cancer cells dying from DNA-damaging treatment. Therefore, chDAB4 is a unique diagnostic tool that detects dead cancer cells and thus could distinguish between treatment responsive and nonresponsive patients.

PROCEDURES

In clinically relevant tumor models, mice bearing subcutaneous xenografts of human ovarian or lung cancer cell lines or intraperitoneal ovarian cancer xenografts were untreated or given chemotherapy followed 24h later by chDAB4 radiolabeled with [⁸⁹Zr]Zr^IV. Tumor responses were monitored using bioluminescence imaging and caliper measurements. [⁸⁹Zr]Zr-chDAB4 uptake in tumor and normal tissues was measured using an Albira SI Positron-Emission Tomography (PET) imager and its biodistribution was measured using a Hidex gamma-counter.

RESULTS

Tumor uptake of [⁸⁹Zr]Zr-chDAB4 was detected in untreated mice, and uptake significantly increased in both human lung and ovarian tumors after chemotherapy, but not in normal tissues.

CONCLUSION

Given that tumors, rather than normal tissues, were targeted after chemotherapy, these results support the clinical development of chDAB4 as a radiodiagnostic imaging agent and as a potential predictive marker of treatment response.

Preclinical evaluation of anti-VEGFR2 monoclonal antibody ramucirumab labelled with zirconium-89 for tumour imaging

The key factors participating in angiogenesis include vascular endothelial growth factor (VEGF) and its receptors (VEGFRs), particularly VEGFR2. Angiogenesis suppression comprises the blocking of the VEGFR2 binding site by the monoclonal antibody ramucirumab (RAM). Our study focused on RAM radiolabelling with zirconium-89 along with subsequent in vitro and in vivo biological evaluation. RAM was conjugated with the bifunctional chelator p-SCN-Bn-deferoxamine (DFO) and subsequently radiolabelled with [89 Zr]Zr-oxalate. The binding affinity of [89 Zr]Zr-DFO-RAM to VEGFR2 was tested in vitro on prostate (PC-3) and ovary adenocarcinoma (SK-OV-3) cell lines. The positron emission tomography/computed tomography (PET/CT) imaging and ex vivo biodistribution experiments were performed in PC-3 and SK-OV-3 xenografted mice. The in vitro experiments revealed the preserved binding affinity of [89 Zr]Zr-DFO-RAM to VEGFR2. The obtained ex vivo biodistribution data showed the uptake in PC-3 and SK-OV-3 tumours at about 8.7 ± 0.2 and 12.1 ± 1.6%ID/g, respectively. The tumour-to-muscle ratio for 1, 3 and 6 days post injection was 3.9, 5.5 and 5.12 for PC-3 and 6.0, 8.0 and 8.82 for SK-OV-3 tumours, respectively. PET/CT images showed high radioactivity accumulation in the tumours starting already on the first day after tracer administration. The obtained results proved the potency of [89 Zr]Zr-DFO-RAM to target and image VEGFR2-positive tumours in vivo.

State of the Art in Radiolabeling of Antibodies with Common and Uncommon Radiometals for Preclinical and Clinical Immuno-PET

Inert and stable radiolabeling of monoclonal antibodies (mAb), antibody fragments, or antibody mimetics with radiometals is a prerequisite for immuno-PET. While radiolabeling is preferably fast, mild, efficient, and reproducible, especially when applied for human use in a current Good Manufacturing Practice compliant way, it is crucial that the obtained radioimmunoconjugate is stable and shows preserved immunoreactivity and in vivo behavior. Radiometals and chelators have extensively been evaluated to come to the most ideal radiometal–chelator pair for each type of antibody derivative. Although PET imaging of antibodies is a relatively recent tool, applications with ⁸⁹Zr, ⁶⁴Cu, and ⁶⁸Ga have greatly increased in recent years, especially in the clinical setting, while other less common radionuclides such as ⁵²Mn, ⁸⁶Y, ⁶⁶Ga, and ⁴⁴Sc, but also ¹⁸F as in [¹⁸F]AlF are emerging promising candidates for the radiolabeling of antibodies. This review presents a state of the art overview of the practical aspects of radiolabeling of antibodies, ranging from fast kinetic affibodies and nanobodies to slow kinetic intact mAbs. Herein, we focus on the most common approach which consists of first modification of the antibody with a chelator, and after eventual storage of the premodified molecule, radiolabeling as a second step. Other approaches are possible but have been excluded from this review. The review includes recent and representative examples from the literature highlighting which radiometal–chelator–antibody combinations are the most successful for in vivo application.