Abstract 4208: Development of 18F-IL2: a PET radiotracer for imaging activated T-cells

Introduction: Activation of T-cells is accompanied by a strong up-regulation of interleukin-2 (IL2) receptor (CD25). Therefore PET imaging of IL2 receptors might be a suitable imaging biomarker for T-cell activation. 18F-IL2 PET could detect CD25-positive T-cells and the migration of these T-cells to distant sites of inflammation in SCID mice subcutaneously injected with human peripheral blood mononuclear cells1 and NOD mice with insulitis. Also a strong correlation was found between the accumulation of 18F-IL2 and the number of injected activated T-cells in immune-competent rats.2 In tumor bearing mice, 18F-IL2 PET could detect treatment-induced accumulation of activated T-cells in the tumor following local radiotherapy and/or vaccination. Cancer immunotherapy is increasingly obtaining a place in clinical practice. However not all patients benefit. A biomarker for upfront or early response prediction for these immunotherapies might support patient selection before and during therapy. Potentially 18F-IL2 PET might serve this purpose. Therefore we aimed to accommodate the production of 18F-IL2 for use in clinical imaging studies. Material and methods: In order to produce a GMP-compliant tracer the production is being implemented on the Eckert & Ziegler PharmTracer synthesis module. In this synthesis module, disposable cassettes, reactors and vials are used to avoid cross-contamination between productions. First the prosthetic group N-succinimidyl 4-fluorobenzoate (18F-SFB) is produced in 3 steps from cyclotron-produced 18F-fluoride. Subsequently, 18F-SFB is conjugated to human recombinant IL2 (Proleukin®). Various methods for synthesis and purification of 18F-SFB have been evaluated. Also purification of 18F-IL2 has been optimized. Quality control has been performed using ultra performance liquid chromatography (UPLC) and Thin Layer Chromatography (TLC). Results: 18F-SFB was successfully synthesized with the Eckert & Ziegler PharmTracer synthesis module with decay-corrected radiochemical yields comparable to literature (range 28-64%). Major challenges have been encountered, most importantly regarding the purification of the 18F-SFB and 18F-IL2, stability of the IL2 and specific activity. The activated ester 18F-SFB was purified by high performance liquid chromatography (HPLC) to remove any impurities that could interfere with the conjugation. 18F-IL2 has been purified using PD-10 columns with PBS containing 0.05% SDS as mobile phase. Conclusions: Several ch