Abstract B112: Personalizing anti-IL13R 2 therapy with real-time immunoPET visualization of biomarker expression.

Interleukin 13 Receptor 2 (IL13R 2) is a tumor-restricted plasma membrane receptor that is overexpressed on greater than 70% of Glioblastoma Multiformes (GBM) as well as in other malignancies, including melanoma, adenocarcinoma, ovarian cancer and renal cell carcinoma. We and others have therapeutically exploited this attractive tumor-restricted biomarker in a number of ways, including with bacterial toxins, chemotherapeutics, nanoparticles, viruses and immunotherapies. Our objective here is to for the first time develop a positron emission tomography (PET)-based molecular imaging probe that can non-invasively visualize IL13R 2 expression in order to determine if it is expressed on a particular patient9s tumor, which can then potentially be therapeutically targeted. To accomplish our goal, we created a novel IL13R 2-Targeted Quadruple Mutant of IL13 (TQM13; IL13.E13K/R66D/S69D/K105R) based on our prior work in which we identified functional “hotspot” amino acid mutations of IL13 that separately increased its affinity towards the tumor-restricted IL13R 2 but abrogated its binding to the physiologically abundant IL13R 1/IL4R heterodimer. In addition, we created a matching negative control, IL13R-Binding Deficient mutant (BDef13; IL13.E13K/R66D/S69D/K105A) with disrupted binding to both IL13R 1 and IL13R 2. In these experiments, our goals were to confirm the binding profile of this optimized ligand in vitro and its potential as a non-invasive PET probe to visualize the cancer-restricted IL13R 2 biomarker in vivo. We demonstrated that 125I-TQM13, but not 125I-BDef13, specifically bound to IL13R 2 via autoradiography on frozen GBM sections and electrophoretic mobility shift assay. An in vitro cell binding assay confirmed 125I-TQM13 had strong affinity to IL13R 2-expressing cells (Kd ∼ 5nM). This binding assay also established that G48a GBM cell line, which we utilized in our in vivo studies, highly expressed IL13R 2 with approximately three million binding sites per cell. Importantly, in an in vivo biodistribution study, 125I-TQM13 bound to IL13R 2-expressing G48a tumors at a 7-to-1 ratio compared to background muscles 24 hours after intravenous injection, in contrast to 125I-BDef13, which remained at background levels. We therefore radiolabeled TQM13 with the positron emitter 124I and intravenously delivered it to mice bearing subcutaneous G48a tumors. Subsequent microPET images at 24 and 48 hours post-injection demonstrated specific tumor localization with tumor-to-background ratio of 7:1 at 48 hours. In contrast, 124I-BDef13 only demonstrated background signal levels. In conclusion, we have successfully generated an optimized, biomarker-targeted IL13 derivative and demonstrated for the first time the potential to non-invasively image the attractive cancer-restricted biomarker, IL13R 2. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr B112.