18F-Labeled Pyrido[3,4-d]pyrimidine as an Effective Probe for Imaging of L858R Mutant Epidermal Growth Factor Receptor

The Development of Positron Emission Tomography Tracers for In Vivo Targeting the Kinase Domain of the Epidermal Growth Factor Receptor

Multiple small molecule PET tracers have been developed for the imaging of the epidermal growth factor receptor (EGFR). These tracers target the tyrosine kinase (TK) domain of the receptor and have been used for both quantifying EGFR expression and to differentiate between EGFR mutational statuses. However, the approaches for in vivo evaluation of these tracers are diverse and have resulted in data that are hard to compare. In this review, we analyze the historical development of the in vivo evaluation approaches, starting from the first EGFR TK PET tracer [11C]PD153035 to tracers developed based on TK inhibitors used for the clinical treatment of mutated EGFR expressing non-small cell lung cancer like [11C]erlotinib and [18F]afatinib. The evaluation of each tracer has been compiled to allow for a comparison between studies and ultimately between tracers. The main challenges for each group of tracers are thereafter discussed. Finally, this review addresses the challenges that need to be overcome to be able to efficiently drive EGFR PET imaging forward.

[11C]Erlotinib PET cannot detect acquired erlotinib resistance in NSCLC tumor xenografts in mice

INTRODUCTION

[¹¹C]Erlotinib PET has shown promise to distinguish non-small cell lung cancer (NSCLC) tumors harboring the activating epidermal growth factor receptor (EGFR) mutation delE746-A750 from tumors with wild-type EGFR. To assess the suitability of [¹¹C]erlotinib PET to detect the emergence of acquired erlotinib resistance in initially erlotinib-responsive tumors, we performed in vitro binding and PET experiments in mice bearing tumor xenografts using a range of different cancer cells, which were erlotinib-sensitive or exhibited clinically relevant resistance mechanisms to erlotinib.

METHODS

The following cell lines were used for in vitro binding and PET experiments: the epidermoid carcinoma cell line A-431 (erlotinib-sensitive, wild-type EGFR) and the three NSCLC cell lines HCC827 (erlotinib-sensitive, delE746-A750), HCC827_EPR (erlotinib-resistant, delE746-A750 and T790M) and HCC827_ERLO (erlotinib-resistant, delE746-A750 and MET amplification). BALB/c nude mice with subcutaneous tumor xenografts underwent two consecutive [¹¹C]erlotinib PET scans, a baseline scan and a second scan in which unlabeled erlotinib (10mg/kg) was co-injected. Logan graphical analysis was used to estimate total distribution volume (V_T) of [¹¹C]erlotinib in tumors.

RESULTS

In vitro experiments revealed significantly higher uptake of [¹¹C]erlotinib (5.2-fold) in the three NSCLC cell lines as compared to A-431 cells. In all four cell lines co-incubation with unlabeled erlotinib (1μM) led to significant reductions in [¹¹C]erlotinib uptake (-19% to -66%). In both PET scans and for all four studied cell lines there were no significant differences in tumoral [¹¹C]erlotinib V_T values. For all three NSCLC cell lines, but not for the A-431 cell line, tumoral V_T was significantly reduced following co-injection of unlabeled erlotinib (-20% to -35%).

CONCLUSIONS

We found no significant differences in the in vitro and in vivo binding of [¹¹C]erlotinib between erlotinib-sensitive and erlotinib-resistant NSCLC cells. Our findings suggest that [¹¹C]erlotinib PET will not be suitable to distinguish erlotinib-sensitive NSCLC tumors from tumors with acquired resistance to erlotinib.