Kimberly J. Edwards

Selective Imaging of VEGFR-1 and VEGFR-2 Using 89Zr-Labeled Single-Chain VEGF Mutants

Vascular endothelial growth factor-A (VEGF-A) acts via 2 vascular endothelial growth factor receptors, VEGFR-1 and VEGFR-2, that play important and distinct roles in tumor biology. We reasoned that selective imaging of these receptors could provide unique information for diagnostics and for monitoring and optimizing responses to anticancer therapy, including antiangiogenic therapy. Herein, we report the development of 2 first-in-class 89Zr-labeled PET tracers that enable the selective imaging of VEGFR-1 and VEGFR-2. Methods: Functionally active mutants of scVEGF (an engineered single-chain version of pan-receptor VEGF-A with an N-terminal cysteine-containing tag for site-specific conjugation), named scVR1 and scVR2 with enhanced affinity to, respectively, VEGFR-1 and VEGFR-2, were constructed. Parental scVEGF and its receptor-specific mutants were site-specifically derivatized with the 89Zr chelator desferroxamine B via a 3.4-kDa PEG linker. 89Zr labeling of the desferroxamine B conjugates furnished scV/Zr, scVR1/Zr, and scVR2/Zr tracers with high radiochemical yield (>87%), high specific activity (≥9.8 MBq/nmol), and purity (>99%). Tracers were tested in an orthotopic breast cancer model using 4T1luc-bearing syngeneic BALB/c mice. For testing tracer specificity, tracers were coinjected with an excess of cold proteins of the same or opposite receptor specificity or pan-receptor scVEGF. PET imaging, biodistribution, and dosimetry studies in mice, as well as immunohistochemical analysis of harvested tumors, were performed. Results: All tracers rapidly accumulated in orthotopic 4T1luc tumors, allowing for the successful PET imaging of the tumors as early as 2 h after injection. Blocking experiments with an excess of pan-receptor or receptor-specific cold proteins indicated that more than 80% of tracer tumor uptake is VEGFR-mediated, whereas uptake in all major organs is not affected by blocking within the margin of error. Critically, blocking experiments indicated that VEGFR-mediated tumor uptake of scVR1/Zr and scVR2/Zr was mediated exclusively by the corresponding receptor, VEGFR-1 or VEGFR-2, respectively. In contrast, uptake of pan-receptor scV/Zr was mediated by both VEGFR-1 and VEGFR-2 at an approximately 2:1 ratio. Conclusion: First-in-class selective PET tracers for imaging VEGFR-1 and VEGFR-2 were constructed and successfully validated in an orthotopic murine tumor model.

PET Imaging of Extracellular pH in Tumors with 64Cu- and 18F-Labeled pHLIP Peptides: A Structure–Activity Optimization Study

pH (low) insertion peptides (pHLIP peptides) target acidic extracellular environments in vivo due to pH-dependent cellular membrane insertion. Two variants (Var3 and Var7) and wild-type (WT) pHLIP peptides have shown promise for in vivo imaging of breast cancer. Two positron emitting radionuclides (64Cu and 18F) were used to label the NOTA- and NO2A-derivatized Var3, Var7, and WT peptides for in vivo biodistribution studies in 4T1 orthotopic tumor-bearing BALB/c mice. All of the constructs were radiolabeled with 64Cu or [18F]-AlF in good yield. The in vivo biodistribution of the 12 constructs in 4T1 orthotopic allografted female BALB/c mice indicated that NO2A-cysVar3, radiolabeled with either 18F (4T1 uptake; 8.9 ± 1.7%ID/g at 4 h p.i.) or 64Cu (4T1 uptake; 8.2 ± 0.9%ID/g at 4 h p.i. and 19.2 ± 1.8% ID/g at 24 h p.i.), shows the most promise for clinical translation. Additional studies to investigate other tumor models (melanoma, prostate, and brain tumor models) indicated the universality of tumor targeting of these tracers. From this study, future clinical translation will focus on 18F- or 64Cu-labeled NO2A-cysVar3.

Non-invasive 89Zr-Transferrin PET Shows Improved Tumor Targeting Compared to 18F-FDG PET in MYC-overexpressing Human Triple Negative Breast Cancer

The current standard for breast PET imaging is 18F-FDG. The heterogeneity of 18F-FDG uptake in breast cancer limits its utility, varying greatly among receptor status, histopathologic subtypes, and proliferation markers. 18F-FDG PET often exhibits nonspecific internalization and low specificity and sensitivity, especially with tumors smaller than 1 cm3. MYC is a protein involved in oncogenesis and is overexpressed in triple-negative breast cancer (TNBC). Increased surface expression of transferrin receptor (TfR) is a downstream event of MYC upregulation and has been validated as a clinically relevant target for molecular imaging. Transferrin labeled with 89Zr has successfully identified MYC status in many cancer subtypes preclinically and been shown to predict response and changes in oncogene status via treatment with small-molecule inhibitors that target MYC and PI3K signaling pathways. We hypothesized that 89Zr-transferrin PET will noninvasively detect MYC and TfR and improve upon the current standard of 18F-FDG PET for MYC-overexpressing TNBC. Methods: In this study, 89Zr-transferrin and 18F-FDG imaging were compared in preclinical models of TNBC. TNBC cells (MDA-MB-157, MDA-MB-231, and Hs578T) were treated with bromodomain-containing protein 4 (BRD4) inhibitors JQ1 and OTX015 (0.5–1 μM). Cell proliferation, gene expression, and protein expression were assayed to explore the effects of these inhibitors on MYC and TfR. Results: Head-to-head comparison showed that 89Zr-transferrin targets TNBC tumors significantly better (P < 0.05–0.001) than 18F-FDG through PET imaging and biodistribution studies in MDA-MB-231 and MDA-MB-157 xenografts and a patient-derived xenograft model of TNBC. c-Myc and TfR gene expression was decreased upon treatment with BRD4 inhibitors and c-MYC small interfering RNA (P < 0.01–0.001 for responding cell lines), compared with vehicle treatment. MYC and TfR protein expression, along with receptor-mediated internalization of transferrin, was also significantly decreased upon drug treatment in MDA-MB-231 and MDA-MB-157 cells (P < 0.01–0.001). Conclusion: 89Zr-transferrin targets human TNBC primary tumors significantly better than 18F-FDG, as shown through PET imaging and biodistribution studies. 89Zr-transferrin is a useful tool to interrogate MYC via TfR-targeted PET imaging in TNBC.

89Zr-DFO-AMG102 Immuno-PET to Determine Local HGF Protein Levels in Tumors for Enhanced Patient Selection

The hepatocyte growth factor (HGF) binding antibody rilotumumab (AMG102) was modified for use as a 89Zr-based immuno-PET imaging agent to noninvasively determine the local levels of HGF protein in tumors. Because recent clinical trials of HGF-targeting therapies have been largely unsuccessful in several different cancers (e.g., gastric, brain, lung), we have synthesized and validated 89Zr-DFO-AMG102 as a companion diagnostic for improved identification and selection of patients having high local levels of HGF in tumors. To date, patient selection has not been performed using the local levels of HGF protein in tumors. Methods: The chelator p-SCN-Bn-DFO was conjugated to AMG102, radiolabeling with 89Zr was performed in high radiochemical yields and purity (>99%), and binding affinity of the modified antibody was confirmed using an enzyme-linked immunosorbent assay (ELISA)–type binding assay. PET imaging, biodistribution, autoradiography and immunohistochemistry, and ex vivo HGF ELISA experiments were performed on murine xenografts of U87MG (HGF-positive, MET-positive) and MKN45 (HGF-negative, MET-positive) and 4 patient-derived xenografts (MET-positive, HGF unknown). Results: Tumor uptake of 89Zr-DFO-AMG102 at 120 h after injection in U87MG xenografts (HGF-positive) was high (36.8 ± 7.8 percentage injected dose per gram [%ID/g]), whereas uptake in MKN45 xenografts (HGF-negative) was 5.0 ± 1.3 %ID/g and a control of nonspecific human IgG 89Zr-DFO-IgG in U87MG tumors was 11.5 ± 3.3 %ID/g, demonstrating selective uptake in HGF-positive tumors. Similar experiments performed in 4 different gastric cancer patient-derived xenograft models showed low uptake of 89Zr-DFO-AMG102 (∼4–7 %ID/g), which corresponded with low HGF levels in these tumors (ex vivo ELISA). Autoradiography, immunohistochemical staining, and HGF ELISA assays confirmed that elevated levels of HGF protein were present only in U87MG tumors and that 89Zr-DFO-AMG102 uptake was closely correlated with HGF protein levels in tumors. Conclusion: The new immuno-PET imaging agent 89Zr-DFO-AMG102 was successfully synthesized, radiolabeled, and validated in vitro and in vivo to selectively accumulate in tumors with high local levels of HGF protein. These results suggest that 89Zr-DFO-AMG102 would be a valuable companion diagnostic tool for the noninvasive selection of patients with elevated local concentrations of HGF in tumors for planning any HGF-targeted therapy, with the potential to improve clinical outcomes.

Fc-Mediated Anomalous Biodistribution of Therapeutic Antibodies in Immunodeficient Mouse Models

A critical benchmark in the development of antibody-based therapeutics is demonstration of efficacy in preclinical mouse models of human disease, many of which rely on immunodeficient mice. However, relatively little is known about how the biology of various immunodeficient strains impacts the in vivo fate of these drugs. Here we used immunoPET radiotracers prepared from humanized, chimeric, and murine mAbs against four therapeutic oncologic targets to interrogate their biodistribution in four different strains of immunodeficient mice bearing lung, prostate, and ovarian cancer xenografts. The immunodeficiency status of the mouse host as well as both the biological origin and glycosylation of the antibody contributed significantly to the anomalous biodistribution of therapeutic monoclonal antibodies in an Fc receptor-dependent manner. These findings may have important implications for the preclinical evaluation of Fc-containing therapeutics and highlight a clear need for biodistribution studies in the early stages of antibody drug development.Significance: Fc/FcγR-mediated immunobiology of the experimental host is a key determinant to preclinical in vivo tumor targeting and efficacy of therapeutic antibodies. Cancer Res; 78(7); 1820-32. ©2018 AACR.

A comparative evaluation of the chelators H4octapa and CHX-A″-DTPA with the therapeutic radiometal 90Y

OBJECTIVES To compare the radiolabeling performance, stability, and practical efficacy of the chelators CHX-A″-DTPA and H4octapa with the therapeutic radiometal (90)Y. METHODS The bifunctional chelators p-SCN-Bn-H4octapa and p-SCN-Bn-CHX-A″-DTPA were conjugated to the HER2-targeting antibody trastuzumab. The resulting immunoconjugates were radiolabeled with (90)Y to compare radiolabeling efficiency, in vitro and in vivo stability, and in vivo performance in a murine model of ovarian cancer. RESULTS High radiochemical yields (>95%) were obtained with (90)Y-CHX-A″-DTPA-trastuzumab and (90)Y-octapa-trastuzumab after 15min at room temperature. Both (90)Y-CHX-A″-DTPA-trastuzumab and (90)Y-octapa-trastuzumab exhibited excellent in vitro and in vivo stability. Furthermore, the radioimmunoconjugates displayed high tumoral uptake values (42.3±4.0%ID/g for (90)Y-CHX-A″-DTPA-trastuzumab and 30.1±7.4%ID/g for (90)Y-octapa-trastuzumab at 72h post-injection) in mice bearing HER2-expressing SKOV3 ovarian cancer xenografts. Finally, (90)Y radioimmunotherapy studies performed in tumor-bearing mice demonstrated that (90)Y-CHX-A″-DTPA-trastuzumab and (90)Y-octapa-trastuzumab are equally effective therapeutic agents, as treatment with both radioimmunoconjugates yielded substantially decreased tumor growth compared to controls. CONCLUSIONS Ultimately, this work demonstrates that the acyclic chelators CHX-A″-DTPA and H4octapa have comparable radiolabeling, stability, and in vivo performance, making them both suitable choices for applications requiring (90)Y.

Synthesis and evaluation of an 18F-labeled pyrimidine-pyridine amine for targeting CXCR4 receptors in gliomas

INTRODUCTION Chemokine receptor-4 (CXCR4, fusin, CD184) is expressed on several tissues involved in immune regulation and is upregulated in many diseases including malignant gliomas. A radiolabeled small molecule that readily crosses the blood-brain barrier can aid in identifying CXCR4-expressing gliomas and monitoring CXCR4-targeted therapy. In the current work, we have synthesized and evaluated an [(18)F]-labeled small molecule based on a pyrimidine-pyridine amine for its ability to target CXCR4. EXPERIMENTAL The nonradioactive standards and the nitro precursor used in this study were prepared using established methods. An HPLC method was developed to separate the nitro-precursor from the nonradioactive standard and radioactive product. The nitro-precursor was radiolabeled with (18)F under inert, anhydrous conditions using the [(18)F]-kryptofix 2.2.2 complex to form the desired N-(4-(((6-[(18)F]fluoropyridin-2-yl)amino)methyl)benzyl)pyrimidin-2-amine ([(18)F]-3). The purified radiolabeled compound was used in serum stability, partition coefficient, cellular uptake, and in vivo cancer targeting studies. RESULTS [(18)F]-3 was synthesized in 4-10% decay-corrected yield (to start of synthesis). [(18)F]-3 (tR ≈ 27 min) was separated from the precursor (tR ≈ 30 min) using a pentafluorophenyl column with an isocratic solvent system. [(18)F]-3 displayed acceptable serum stability over 2 h. The amount of [(18)F]-3 bound to the plasma proteins was determined to be > 97%. The partition coefficient (LogD7.4) is 1.4 ± 0.5. Competitive in vitro inhibition indicated 3 does not inhibit uptake of (67)Ga-pentixafor. Cell culture media incubation and ex vivo urine analysis indicate rapid metabolism of [(18)F]-3 into hydrophilic metabolites. Thus, in vitro uptake of [(18)F]-3 in CXCR4 overexpressing U87 cells (U87 CXCR4) and U87 WT indicated no specific binding. In vivo studies in mice bearing U87 CXCR4 and U87 WT tumors on the left and right shoulders were carried out using [(18)F]-3 and (68)Ga-pentixafor on consecutive days. The CXCR4 positive tumor was clearly visualized in the PET study using (68)Ga-pentixafor, but not with [(18)F]-3. CONCLUSIONS We have successfully synthesized both a radiolabeled analog to previously reported CXCR4-targeting molecules and a nitro precursor. Our in vitro and in vivo studies indicate that [(18)F]-3 is rapidly metabolized and, therefore, does not target CXCR4-expressing tumors. Optimization of the structure to improve the in vivo (and in vitro) stability, binding, and solubility could lead to an appropriate CXCR4-targeted radiodiagnositic molecule.

Abstract LB-182: Optimized antibody modification and radiolabeling conditions for zirconium-89 immuno-PET

OBJECTIVES: The PET imaging isotope 89Zr (t1/2 = ∼3.3 d) is the most commonly used isotope with antibodies (Abs) (immuno-PET). The chelator desferrioxamine (DFO) has been conjugated to many Abs, radiolabeled with 89Zr, and imaged in many animals and humans. It is generally known that conjugating too many chelators onto Abs will decrease binding affinity and immunoreactivity. We have conjugated DFO to the HER2/neu targeting Abs trastuzumab in varying quantities, and then radiolabeled with 89Zr under a comprehensive set of conditions and evaluated their effects on radiochemical yields (RCY), specific activity (SA), immunoreactivity, and ultimately in vivo tumor targeting. We believe a comprehensive study on these variables has not been sufficiently performed and would be of substantial value to anyone attaching isotopes to antibodies. METHODS: The chelator p-SCN-Bn-DFO was conjugated to trastuzumab (5, 10, 20, 40, 60, 100, 150, 200 equivalents), purified, and the number of attached chelators determined by MALDI-TOF mass spectrometry. These different modifications of DFO-trastuzumab were then radiolabeled and RCY were monitored from 15-60 min by iTLC (25 °C and 37 °C, n = 3, 50 mM EDTA mobile phase). The immunoreactivity of these different conjugates was determined by a cell binding assay with SKOV3 cells, and binding affinity by surface plasmon resonance. This was followed by 89Zr radiolabeling with varying quantities of Abs (5, 25, 50 μg), different buffers (PBS, chelexed PBS, TRIS/HCl, HEPES), different volumes (100, 300, 1000 μL), and activities (0.2, 1.0 mCi). To test radiolysis effects, 89Zr-DFO-trastuzumab (∼200 μCi) was radiolabeled and then placed in 100 μL of different buffers (PBS, TRIS/HCl, saline, HEPES, PBS + ascorbic acid, and PBS + gentisic acid) for 48 h before determining immunoreactivity. Representative samples (5, 10, 20, and 200 equiv. DFO conjugations) were radiolabeled and injected in mice bearing subcutaneous SKOV3 xenografts (n = 4 per group), and imaged by PET at 24, 72, and 120 h p.i. with biodistributions done at 120 h. RESULTS: Regardless of the equiv. chelator used during conjugations, a maximum average number of chelates/Abs of only ∼10-12 could be attached. To achieve maximum SA, a small quantity of Abs was used (5 μg), achieving RCY of ∼50-90% and SA of ∼15-35 mCi/mg (depending on chelates/Abs). The optimal conjugation level and radiolabeling conditions were 1-3 DFO per Abs (5-10 equiv. conjugation), chelexed PBS as buffer, minimum buffer volume, and no radioprotectant (gentisic acid or ascorbic acid). The in vivo effects of these chelate conjugations were dramatic, revealing a trend where higher amounts of attached DFO correlated to lower tumor uptake and higher liver and spleen uptake. The 89Zr-DFO-Trastuzumab that was prepared by reacting 5 equiv. of chelator (∼1.4 ± 0.5 chelates/Abs) had tumor and liver uptake at 120 h p.i. of 38.7 ± 3.8 and 6.3 ± 4.1%ID/g, respectively, where the 200 equiv. conjugation (∼10.9 ± 0.7 chelates/Abs) had values of 16.2 ± 3.2 and 27.5 ± 4.1%ID/g, respectively (p CONCLUSIONS: Increasing the degree of DFO attachment to trastuzumab resulted in a concomitant increase in maximum SA values (up to ∼35 mCi/mg), and a decrease in immunoreactivity/binding affinity, which translated to inferior in vivo performance. Optimized radiolabeling conditions provided very high SA of 17.5 ± 2.2 mCi/mg with only ∼1.4 ± 0.5 chelates/Abs. Citation Format: Eric W. Price, Jonathan M. Glaser, Kimberly J. Edwards, Jason S. Lewis. Optimized antibody modification and radiolabeling conditions for zirconium-89 immuno-PET. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr LB-182.

Abstract 4205: Targeting a variety of cancers with NOTA-derivatized pH (low) insertion peptide (pHLIP) complexes with 64Cu and 18F: What cancers are targetable

Introduction: Although heterogeneous, the extracellular tumor environment is generally marked by decreased pH relative to nontumor tissues due to cancer cell metabolism. Using a pH-sensitive peptide to deliver diagnostic PET isotopes can provide clinicians with improved diagnostic tools for cancer-specific imaging and an alternative to receptor-based cancer targeting. pH (low) Insertion Peptides (pHLIPs) have previously been shown to target cancerous tissues and are able to deliver PET metals to the cancer site utilizing a NOTA-based chelator. Methods: The thiol-containing cysteine moiety on the pHLIP peptide sequence was conjugated to the NOTA chelator. The NOTA-derivatized peptides were labeled with 64 CuCl 2 in 100 mM NH 4 OAc (pH 5) solution; the [ 18 F]-fluoride was reacted with AlCl 3 before forming the [ 18 F]-AlF-NOTA-pHLIP complexes in NH 4 OAc buffered reaction mixtures (pH ∼ 4.1). After purification and formulation, the radiolabeled peptides were injected intravenously into 4T1 breast, PC-3 prostate, LNCaP prostate, or B16-F10 melanoma tumor-bearing mice. Results: The 4T1 tumor-bearing mice showed significant uptake > 10%ID/g by 4 h p.i. and continued to show increased uptake of the 64 Cu-NOTA-cysVar3 over 20 h. Tumors were visible in the 4T1 tumor-bearing mice as early as 2 h, but had the greatest tumor-to-background ratios at 24 h. Initial studies with B16-F10 tumor-bearing mice showed > 9%ID/g uptake in the tumor by 4 h p.i. with slight increase of 64 Cu-NOTA-cysVar3 uptake over 24 h. Due to the placement of the melanoma tumors away from internal organs, tumors were visible by 1 h p.i. Additionally, the size of the melanoma tumors did not affect the%ID/g of radiotracer uptake. Conclusions: Our studies indicate that the NOTA-cys(pHLIP), specifically NOTA-cysVar3, radiolabeled with 64 Cu or 18 F is a viable imaging tool for detecting highly metabolic tumors in preclinical mouse models. Funding: NIH F32 CA186721 (D.W.D.), NIH R01 CA138468 (J.S.L.), NIH MSKCC Center Grant (P30-CA08748). Citation Format: Dustin W. Demoin, Kimberly J. Edwards, Linden C. Wyatt, Mirkka Sarparanta, Jacob Pourat, Oleg A. Andreev, Yana K. Reshetnyak, Nerissa Viola-Villegas, Jason S. Lewis. Targeting a variety of cancers with NOTA-derivatized pH (low) insertion peptide (pHLIP) complexes with 64 Cu and 18 F: What cancers are targetable. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4205.