PET imaging of c-Met activity in NSCLC using ¹⁸F Labelled c-MET inhibitor

Objectives: c-Met is a receptor tyrosine kinase, implicating cellular processes including proliferation and apoptosis [1]. Abnormal activation of c-Met pathway plays an important role in tumorigenesis and progression in NSCLC (non-small cell lung cancer) and mediates acquired resistance to targeted therapy [2]. Nevertheless, lack of accurate detection methods for c-Met in NSCLC challenges the overall efficacy of c-Met targeted therapy. Over past decades, important progresses have been made in field of c-Met imaging [3]. However, they were mostly associated with c-Met antibody-based tracers (150 kDa), which have long biological half-life, slow clearance and potential immunogenicity challenging the clinical translation. Recently, small molecule tracers have been brought into focus because of superior properties, including their small size and lower toxicity. In this study, we developed a novel small molecule PET agent based on c-Met inhibitor PF-04217903 to image c-Met in vivo in NSCLC.

Methods: We first modified oxhydryl of PF-04217903 with leaving group -OTs to obtain the precursor (TSPF), which was then radiofluorinated with 18F-tetrabutyl ammonium fluoride (TBAF). After HPLC purification, 18F-TSPF was obtained and evaluated in series of in vitro experiments to verify its specificity to c-Met. In vivo evaluation of 18F-TSPF is performed in NSCLC subcutaneous and orthotopic tumors bearing mice through PET imaging and ex-vivo biodistribution study. The results are further confirmed with immunohistochemical analysis or western-blot.

Results: 18F-TSPF was prepared with more than 60% radiochemical yield. In vitro evaluation demonstated 18F-TSPF has high specificity towards c-Met positive NSCLC cell line (Hcc827) instead of c-MET negative cell line (H1299) as confirmed with western-blot analysis. In PET imaging study, Hcc827 subcutaneous tumor show significantly high uptake (more than 6 %ID/g) of 18F-TSPF than H1299 tumors ( about 2 %ID/g). In addition, Hcc827 orthotopic tumors in lung showed higher uptake (more than 8 %ID/g) of 18F-TSPF than subcutaneous tumors (Figure 1). As expected, the tracer uptake of 18F-TSPF could be specifically blocked by the added inhibitor, PF-04217903, indicating specificity of 18F-TSPF to c-Met. Moreover, 18F-TSPF could also be used to monitor c-Met targeted treatment by dynamic PET imaging followed by immunohistochemistry confirmation. Conclusion: We developed a novel small molecule probe, 18F-TSPF, with high radiochemical yield and verified its ability to noninvasively image c-Met and monitor c-Met targeted therapy in NSCLC subcutaneous and orthotopic tumor models. The PET agent 18F-TSPF is a great candidate for PET imaging c-MET in future clinical trials. References 1 Trusolino L, Bertotti A, Comoglio PM. MET signalling: principles and functions in development, organ regeneration and cancer. Nat Rev Mol Cell Biol. 2010;11:834-848. 2 Engelman JA, Zejnullahu K, Mitsudomi T, et al. MET amplification leads to gefitinib resistance in lung cancer by activating ERBB3 signaling. Science. 2007;316:1039-1043. 3 Han ZG, Wu YY, Wang K, et al. Analysis of progress and challenges for various patterns of c-MET-targeted molecular imaging: a systematic review. EJNMMI Res. 2017;7:41.

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