The epidermal growth factor receptor (EGFR) is overexpressed in many cancers including lung, breast, head and neck and brain. Furthermore, mutations of this receptor have been shown to play crucial roles in response to EGFR-targeted therapies in non small cell lung carcinoma (NSCLC) patients. Imaging EGFR or its function using positron emission tomography (PET) could aid in selection and monitoring patient’s therapeutic response to small molecule tyrosine kinase inhibitors (TKIs) including gefitinib.
The aims of this project are first to further investigate the use of a series of cyanoquinoline tracers for imaging the EGFR and second to assess the role of PET imaging to predict early response to EGFR directed therapy such as gefitinib.
First, the uptake of a representative cyanoquinoline radiotracer, [18F](2E)-N-{4-[(3-chloro-4-fluorophenyl)amino]-3-cyano-7-ethoxyquinolin-6-yl}}4-({[1-(2-fluoroethyl)-1H-1,2,3-triazol-4-yl]methyl*amino)but-2-enamide ([18F]FED6), was examined in cell lines harbouring mutant forms of EGFR. In vitro assays, evaluating the affinity of cyanoquinoline compounds for different mutant EGFR and in vivo PET imaging in NSCLC xenograft models expressing mutant or wild type (WT) EGFR were carried out. The lack of specificity for mutant vs WT EGFR and overall low tumour uptake led us to investigate the potential interaction of the radiotracer with members of the ABC transporters. In vitro experiments of radiolabelled [18F] showed substrate specificity of the cyanoquinoline tracer for at least two ABC transporters, ABCG2 and ABCB1. This was confirmed by inhibiting the activity of the transporters through drug and siRNA treatment. To overcome the transporter substrate specificity of FED6, investigations into a novel radiotracer, (2E)-N-{4-[(3-chloro-4-fluorophenyl)amino]-3-cyano-7-ethoxyquinolin-6-yl}-4-[({1-[(2R,5S)-3-fluoro-4,5-dihydroxy-6-(hydroxymethyl)oxan-2-yl]-1H-1,2,3-triazol-4-yl}methyl)amino]but-2-enamide (FED20), designed so as to limit the affinity for the ABC transporters were carried out. These studies showed that FED20 lacked the substrate specificity for both ABCB1 and ABCG2, and maintained a strong affinity for EGFR.
Second, the role of choline kinase as a biomarker of response to gefitinib treatment in sensitive vs resistant NSCLC cell lines was investigated. In vitro western blots, Q-PCR, cell viability and cell cycle analysis assays were undertaken. In vitro cell uptake experiments using tritiated choline were compared with uptake of [18F]3’ deoxy-3’ Fluorothymidine (FLT) and [18F]Fluoro-D glucose (FDG). Higher levels of choline uptake were found in the sensitive compared to the resistant cell line whereas both [18F]FLT and [18F]FDG showed higher accumulation in the resistant versus sensitive cell line. These experiments confirmed that differential responses of the sensitive and the resistant cell lines to gefitinib treatment were detectable by tracer pulse-chase. In vivo PET imaging of NSCLC xenografts using [18F]FLT, [18F]FDG and [18F]D4Choline showed that gefitinib treatment was associated with a decrease in the fractional retention of all three tracers in the sensitive but not resistant NSCLC xenografts.