Development of a PET probe detecting tumors, which are responsive to gefitinib treatment

Objectives Gefinitib is a molecularly target drug approved by FDA for the treatment of patients with metastatic non-small cell lung cancer (NSCLC), whose tumors have epidermal growth factor receptor (EGFR) exon 19 deletions or L858R mutations. However, within one year after the onset of the treatment, more than 90% of the NSCLC patients show drug resistance caused by second EGFR mutations (For example, T790M). Therefore, to detect the second EGFR mutation is very important in the therapeutic process of NSCLC. However, the only way to detect the mutation directly is invasive biopsy test and the development of noninvasively detected methodology has been desired. Our research target is to develop a PET tracer, which can detect the second EGFR mutations.

Methods Five thienopyrimidine derivatives (FTPs1~5) were designed and synthesized as candidate compounds. Utilizing Promega ADP-Glo kinase assay kit, inhibitory activities of these compounds against wild type EGFR(WT), single mutated EGFR(L858R), EGFR(T790M), and double mutated EGFR(DM L858R/T790M) were evaluated. Next, using compounds radiolabeled by fluorine-18, in vivo imaging study was performed using mice bearing human NSCLC of NCI-H3255 and NCI-H1975.

Results FTPs1~5 were synthesized by conventional organic chemical method. All synthesized compounds were confirmed based on the 1H NMR measurements. IC50 value (EGFR tyrosine kinase inhibition) of Gefitinib against EGFR(WT), EGFR(L858R), EGFR(T790M) and EGFR(L858R/T790M) was 0.020, 0.021, 0.868 and 7.11 μM, respectively. FTPs1~5 showed weak inhibitory activity against EGFR(T790M) and EGFR(L858R/T790M), and the IC50 values were over 10 μM, indicating FTPs1~5 could not bind to the EGFR with T790M mutation. FTPs2 and 4 kept inhibitory activity against EGFR(WT) and EGFR(L858R), and those IC50 (0.009 - 0.04 μM) were slightly lower or comparable to that of Gefitinib. Then, FTP2 showing the lowest IC50 value of 0.009 μM against EGFR(L858R) among FTPs1~5 was radiolabeled by fluorine-18, and i.v. injected to mice bearing human NSCLC of NCI-H3255 (EGFR(L858R)), NCI-H1975 (EGFR(L858R/T790M)), and PET images were acquired at 3 h post-injection. The following ex vivo study indicated accumulation of [18F]FTP2 at NCI-H3255 and NCI-H1975 were 2.4 and 1.0 %ID/g, respectively, and those signal intensity ratio against muscle was 6.0 and 2.0.

Conclusions The results in the present study suggest that [18F]FTP2 can detect NSCLC, which can be subjected to the EGFR targeted molecular therapy.