Bernadette V. Marquez

Evaluation of 89Zr-pertuzumab in Breast Cancer Xenografts

Pertuzumab is a monoclonal antibody that binds to HER2 and is used in combination with another HER2–specific monoclonal antibody, trastuzumab, for the treatment of HER2+ metastatic breast cancer. Pertuzumab binds to an HER2 binding site distinct from that of trastuzumab, and its affinity is enhanced when trastuzumab is present. We aim to exploit this enhanced affinity of pertuzumab for its HER2 binding epitope and adapt this antibody as a PET imaging agent by radiolabeling with 89Zr to increase the sensitivity of HER2 detection in vivo. Here, we investigate the biodistribution of 89Zr-pertuzumab in HER2–expressing BT-474 and HER2–nonexpressing MDA-MB-231 xenografts to quantitatively assess HER2 expression in vivo. In vitro cell binding studies were performed resulting in retained immunoreactivity and specificity for HER2–expressing cells. In vivo evaluation of 89Zr-pertuzumab was conducted in severely combined immunodeficient mice, subcutaneously inoculated with BT-474 and MDA-MB-231 cells. 89Zr-pertuzumab was systemically administered and imaged at 7 days postinjection (p.i.) followed by terminal biodistribution studies. Higher tumor uptake was observed in BT-474 compared to MDA-MB-231 xenografts with 47.5 ± 32.9 and 9.5 ± 1.7% ID/g, respectively at 7 days p.i (P = 0.0009) and blocking studies with excess unlabeled pertuzumab showed a 5-fold decrease in BT-474 tumor uptake (P = 0.0006), confirming the in vivo specificity of this radiotracer. Importantly, we observed that the tumor accumulation of 89Zr-pertuzumab was increased in the presence of unlabeled trastuzumab, at 173 ± 74.5% ID/g (P = 0.01). Biodistribution studies correlate with PET imaging quantification using max SUV (r = 0.98, P = 0.01). Collectively, these results illustrate that 89Zr-pertuzumab as a PET imaging agent may be beneficial for the quantitative and noninvasive assessment of HER2 expression in vivo especially for patients undergoing trastuzumab therapy.

Imaging the L-Type Amino Acid Transporter-1 (LAT1) with Zr-89 ImmunoPET

The L-type amino acid transporter-1 (LAT1, SLC7A5) is upregulated in a wide range of human cancers, positively correlated with the biological aggressiveness of tumors, and a promising target for both imaging and therapy. Radiolabeled amino acids such as O-(2-[18F]fluoroethyl)-L-tyrosine (FET) that are transport substrates for system L amino acid transporters including LAT1 have met limited success for oncologic imaging outside of the brain, and thus new strategies are needed for imaging LAT1 in systemic cancers. Here, we describe the development and biological evaluation of a novel zirconium-89 labeled antibody, [89Zr]DFO-Ab2, targeting the extracellular domain of LAT1 in a preclinical model of colorectal cancer. This tracer demonstrated specificity for LAT1 in vitro and in vivo with excellent tumor imaging properties in mice with xenograft tumors. PET imaging studies showed high tumor uptake, with optimal tumor-to-non target contrast achieved at 7 days post administration. Biodistribution studies demonstrated tumor uptake of 10.5 ± 1.8 percent injected dose per gram (%ID/g) at 7 days with a tumor to muscle ratio of 13 to 1. In contrast, the peak tumor uptake of the radiolabeled amino acid [18F]FET was 4.4 ± 0.5 %ID/g at 30 min after injection with a tumor to muscle ratio of 1.4 to 1. Blocking studies with unlabeled anti-LAT1 antibody demonstrated a 55% reduction of [89Zr]DFO-Ab2 accumulation in the tumor at 7 days. These results are the first report of direct PET imaging of LAT1 and demonstrate the potential of immunoPET agents for imaging specific amino acid transporters.

Investigation of a vitamin B12 conjugate as a PET imaging probe.

Nutrient demand is a fundamental characteristic of rapidly proliferating cells. Vitamin B12 is vital for cell proliferation; thus neoplastic cells have an increased demand for this essential nutrient. In this study we exploited the vitamin B12 uptake pathway to probe the nutritional demand of proliferating cells with a radiolabeled B12 derivative in various preclinical tumor models. We describe the synthesis and biological evaluations of copper‐64‐labeled B12–ethylenediamine–benzyl‐1,4,7‐triazacyclononane‐N,N′,N′′‐triacetic acid (B12‐en‐Bn‐NOTA‐64Cu), the first example of a B12 derivative for positron emission tomography (PET) imaging. Small‐animal imaging and pharmacological evaluation show high tumor uptake ranging from 2.20 to 4.84 % ID g−1 at 6 h post‐administration. Competition studies with excess native B12 resulted in a 95 % decrease in tumor accumulation, indicating the specificity of this radiopharmaceutical for B12 endocytotic transport proteins. These results show that a vitamin B12 PET radiopharmaceutical has potential utility for non‐invasive imaging of enhanced nutrient demand in proliferating cells.

Cyclotron Production of High–Specific Activity 55Co and In Vivo Evaluation of the Stability of 55Co Metal-Chelate-Peptide Complexes

This work describes the production of high–specific activity 55Co and the evaluation of the stability of 55Co-metal-chelate-peptide complexes in vivo. 55Co was produced via the 58Ni(p,α)55Co reaction and purified using anion exchange chromatography with an average recovery of 92% and an average specific activity of 1.96 GBq/μmol. 55Co-DO3A and 55Co-NO2A peptide complexes were radiolabeled at 3.7 MBq/μg and injected into HCT-116 tumor xenografted mice. Positron emission tomography (PET) and biodistribution studies were performed at 24 and 48 hours postinjection and compared to those of 55CoCl2. Both 55Co-metal-chelate complexes demonstrated good in vivo stability by reducing the radiotracers’ uptake in the liver by sixfold at 24 hours with ˜ 1% ID/g and at 48 hours with ˜ 0.5% ID/g and reducing uptake in the heart by fourfold at 24 hours with ˜ 0.7% ID/g and sevenfold at 48 hours with ˜ 0.35% ID/g. These results support the use of 55Co as a promising new radiotracer for PET imaging of cancer and other diseases.

Glypican-3–Targeted 89Zr PET Imaging of Hepatocellular Carcinoma: Where Antibody Imaging Dares to Tread

Recently we have seen an increase in PET imaging of 89Zr-radiolabeled antibodies in preclinical and clinical investigations, particularly in oncology. 89Zr-antibodies are examples of the immuno-PET imaging class, in which antibodies equipped with a PET isotope are detected and quantified in vivo. 89Zr, with a half-life of 78.4 h, is an excellent PET isotope to pair with antibodies to address the limitations of antibodies’ long blood circulation, requiring several days to clear from the bloodstream to achieve optimal imaging. The comprehensive groundwork conducted to make 89Zr more widely available, along with published procedures for radiolabeling and

Harvesting 67Cu from the Collection of a Secondary Beam Cocktail at the National Superconducting Cyclotron Laboratory

Isotope harvesting is a promising new method to obtain isotopes for which there is no reliable continuous supply at present. To determine the possibility of obtaining radiochemically pure radioisotopes from an aqueous beam dump at a heavy-ion fragmentation facility, preliminary experiments were performed to chemically extract a copper isotope from a large mixture of projectile fragmentation products in an aqueous medium. In this work a 93 MeV/u secondary beam cocktail was collected in an aqueous beam stop at the National Superconducting Cyclotron Laboratory (NSCL) located on the Michigan State University (MSU) campus. The beam cocktail consisted of ∼2.9% (67)Cu in a large mixture of co-produced isotopes ranging in atomic number from ∼19 to 34. The chemical extraction of (67)Cu was achieved via a two-step process: primary extraction using a divalent metal chelation disk followed by anion-exchange chromatography. A significant fraction (74 ± 4%) of the (67)Cu collected in the aqueous beam stop was recovered with >99% radiochemical purity. To illustrate the utility of this product, the purified (67)Cu material was then used to radiolabel an anti-EGFR antibody, Panitumumab, and injected into mice bearing colon cancer xenografts. The tumor uptake at 5 days postinjection was found to be 12.5 ± 0.7% which was in very good agreement with previously reported studies with this radiolabeled antibody. The present results demonstrate that harvesting isotopes from a heavy-ion fragmentation facility could be a promising new method for obtaining high-quality isotopes that are not currently available by traditional methods.

Development of a Radiolabeled Irreversible Peptide Ligand for PET Imaging of Vascular Endothelial Growth Factor

Imaging agents based on peptide probes have desirable pharmacokinetic properties provided that they have high affinities for their target in vivo. An approach to improve a peptide ligand’s affinity for its target is to make this interaction covalent and irreversible. For this purpose, we evaluated a 64Cu-labeled affinity peptide tag, 64Cu-L19K-(5-fluoro-2,4-dinitrobenzene) (64Cu-L19K-FDNB), which binds covalently and irreversibly to vascular endothelial growth factor (VEGF) as a PET imaging agent. We compared the in vivo properties of 64Cu-L19K-FDNB in VEGF-expressing tumor xenografts with its noncovalent binding analogs, 64Cu-L19K-(2,4-dinitrophenyl) (64Cu-L19K-DNP) and 64Cu-L19K. Methods: The L19K peptide (GGNECDIARMWEWECFERK-CONH2) was constructed with 1,4,7-triazacyclononane-1,4,7-triacetic acid at the N terminus for radiolabeling with 64Cu with a polyethylene glycol spacer between peptide and chelate. 1,5-difluoro-2,4-dinitrobenzene was conjugated at the C-terminal lysine for cross-linking to VEGF, resulting in L19K-FDNB. 64Cu-L19K-FDNB was assayed for covalent binding to VEGF in vitro. As a control, L19K was conjugated to 1-fluoro-2,4-dinitrobenzene, resulting in L19K-DNP. PET imaging and biodistribution studies of 64Cu-L19K-FDNB, 64Cu-L19K-DNP, and the native 64Cu-L19K were compared in HCT-116 xenografts. Blocking studies of 64Cu-L19K-FDNB was performed with a coinjection of excess unlabeled L19K-FDNB. Results: In vitro binding studies confirmed the covalent and irreversible binding of 64Cu-L19K-FDNB to VEGF, whereas 64Cu-L19K-DNP and 64Cu-L19K did not bind covalently. PET imaging showed higher tumor uptake with 64Cu-L19K-FDNB than with 64Cu-L19K-DNP and 64Cu-L19K, with mean standardized uptake values of 0.62 ± 0.05, 0.18 ± 0.06, and 0.34 ± 0.14, respectively, at 24 h after injection (P < 0.05), and 0.53 ± 0.05, 0.32 ± 0.14, and 0.30 ± 0.09, respectively, at 48 h after injection (P < 0.05). Blocking studies with 64Cu-L19K-FDNB in the presence of excess unlabeled peptide showed a 53% reduction in tumor uptake at 48 h after injection. Conclusion: In this proof-of-concept study, the use of a covalent binding peptide ligand against VEGF improves tracer accumulation at the tumor site in vivo, compared with its noncovalent binding peptide analogs. This technique is a promising tool to enhance the potency of peptide probes as imaging agents.

Abstract 109: PET imaging of 89Zr-labeled Pertuzumab in HER2-positive breast cancer xenografts

The sensitivity and quantitative properties of Positron Emission Tomography (PET) imaging combined with the selectivity and high affinity of radiolabeled monoclonal antibodies (mAbs) make a powerful tool in molecular imaging of diseases including breast cancer. Radiolabeling mAbs with the positron-emitting 89Zr has recently been useful in clinical trials due to its relatively long half-life (t1/2 = 3.3 days), which matches the pharmacokinetic properties of mAbs. Pertuzumab (Perjeta™) is an FDA-approved mAb for the treatment of human epidermal growth factor receptor 2 (HER2)-positive metastatic breast cancer, in combination with Trastuzumab (Herceptin™) and chemotherapy. Herein, we evaluated 89Zr-labeled Pertuzumab as a PET imaging agent in transgenic mice-bearing human breast cancer. The 89Zr chelator, p-isothiocyanatobenzyl-desferrioxamine, was conjugated to Pertuzumab and subsequently radiolabeled with 89Zr. The immunoreactivity and specificity of 89Zr-Pertuzumab for HER2-expressing cells were determined in vitro and showed retention of 89Zr-Pertuzumab immunoreactivity and specific binding to HER2-expressing cells. Human breast cancer cells were inoculated subcutaneously in severely immunocompromised NOG mice, a model which allows for the growth of HER2-positive tumors without exogenous stimuli. 89Zr-Pertuzumab was injected via tail vein and imaged and 5 and 7 days post-injection (p.i.) followed by biodistribution studies. Optimal contrast was achieved at 5 days p.i., with higher tumor uptake in the HER2-positive xenograft than the HER2-negative model. Immunohistochemical analysis of these tumors is in agreement with PET imaging analysis. These results illustrate that 89Zr-Pertuzumab may be beneficial for the noninvasive assessment of HER2 expression. Citation Format: Bernadette V. Marquez, Oluwatayo F. Ikotun, Brian Wright, Alexander Zheleznyak, Pierce Richard, Suzanne E. Lapi. PET imaging of 89Zr-labeled Pertuzumab in HER2-positive breast cancer xenografts. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 109. doi:10.1158/1538-7445.AM2014-109

Abstract 4209: Targeting GPNMB with 89Zr-CR011 for PET imaging of triple negative breast cancer

Glycoprotein non-metastatic melanoma B (GPNMB) is a transmembrane protein overexpressed in 30 - 40% of triple negative breast cancer (TNBC) and has shown to be associated with metastasis and disease recurrence. An anti-GPNMB antibody drug conjugate, Glembatumumab Vedotin (CDX-011), is currently in Phase II clinical trials for the treatment of metastatic TNBC patients, with promising outcomes. Positron Emission Tomography using radiolabeled antibodies could be advantageous in stratifying patients who may benefit from CDX-011, tracking the biodistribution of CDX-011, and assessing GPNMB expression in vivo. To this end, we radiolabeled the naked antibody, Glembatumumab (CR011), with the positron-emitting 89Zr (half life = 3.3 days). We characterized the stability, affinity, rate of cellular internalization, and specificity of 89Zr-CR011 using various cell-binding assays in human TNBC cell lines. We determined that 89Zr-CR011 is stable in serum solution for up to 5 days, binds specifically to GPNMB+ TNBC cells with high affinity (KD = 16 nM), and internalizes rapidly (50% within 30 - 60 min). We conducted a biodistribution study from 1 - 12 days post administration via tail vein in GPNMB+ MDA-MB-468 xenografts to determine the time point at which we achieve the optimal tumor-to-nontarget ratios. A subset of mice was administered a blocking dose of unlabeled CR011 (100-fold excess, 1 mg/mouse), where we observed a 2.5-fold reduction in 89Zr-CR011 tumor uptake, confirming the specificity of 89Zr-CR011 for GPNMB+ TNBC tumors. PET imaging studies and dosimetry calculations are currently in progress. This preliminary study demonstrates that 89Zr-CR011 may be an excellent companion diagnostic agent for CDX-011 therapy and an essential tool to assess the function of GPNMB in vivo. Citation Format: Bernadette V. Marquez, Supum Lee, Tibor Keler, Thomas Hawthorne, Jeremy Hoog, Shunqiang Li, Cynthia Ma, Suzanne E. Lapi. Targeting GPNMB with 89Zr-CR011 for PET imaging of triple negative breast cancer. [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 4209.