Abstract 2151: Development of small-molecule mTORC1 radiotracers for evaluation of targeted therapy resistance using PET
Authors: Parslow Adam C. , Wichmann Christian W. , Rigopoulos Angela , Ackermann Uwe , White Jonathan M. , Scott Andrew M. 
Published: August 2018

Abstract
Overcoming resistance is an area of unmet clinical need for patients undergoing trastuzumab-based HER2-directed therapy. PTEN loss or PI3K mutation has been shown to induce mTOR activation and mediate trastuzumab resistance. During the BOLERO-3 phase III clinical trial, the addition of everolimus to trastuzumab plus vinorelbine significantly prolonged progression-free survival in patients with trastuzumab-resistant and taxane-pretreated, HER2-positive advanced breast cancer. It is critically important to identify HER2-positive breast cancer patients who are sensitive to mTOR inhibition. Our aim is to develop noninvasive mTORC1 binding small-molecule imaging probes to predict breast cancer patient resistance to treatment utilizing positron emission tomography (PET). Utilizing the benzofuran-based compound ChemBridge 5219657, we have developed a panel of potent inhibitors specific for mTOR complex 1. Alkyne derivatives suitable for conjugation with 2-[18F]fluoroethylazide were developed via Cu(I)-catalyzed click chemistry. Radiolabeling was performed on the automated iPhase FlexLab module providing reliable amounts of radiotracer. Synthesis of click precursor was achieved over 5 linear steps with a 24% isolated yield. Quality control was characterized using NMR, MS, UV- and radio-HPLC analysis. 18F-labeling conjugation with precursor resulted in a 40 ± 5% isolated radiochemical yield with &gt;99% radiochemical purity and a molar activity of 12.5 ± 6.0 GBq/µmol.
Initial in vitro sensitivity to everolimus was performed via SRB proliferation analysis following cold-compound treatment on HER2-positive human breast cancer cell lines (BT-474 and HCC1419) and HER2 low expressing control cell lines (MDA-MB-468 and MDA-MB-231). In vitro 18F-labeled binding experiments demonstrated an increased uptake in BT-474 and HCC-1419 cells, 1.5 and 2.5-fold compared to the negative controls, respectively. NSG mice bearing BT-474, MDA-MB-231 and MDA-MB-468 xenografts were explored for in vivo dynamic PET imaging experiments confirming this selective uptake of radiotracer in everolimus-sensitive cells. Our synthesis strategies identified a reliable radiolabeling approach capable of producing sufficient amounts of radiochemically pure radiotracer for in vivo investigation. The increased tracer uptake in mTOR treatment-sensitive cell lines in vitro as well as in vivo provides an encouraging outcome warranting further preclinical studies for the investigation of HER2-positive therapy resistance in mTOR-sensitive models.
Citation Format: Adam C. Parslow, Christian W. Wichmann, Angela Rigopoulos, Uwe Ackermann, Jonathan M. White, Andrew M. Scott. Development of small-molecule mTORC1 radiotracers for evaluation of targeted therapy resistance using PET [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 2151.