Early detection of thymidylate synthase resistance in non-small cell lung cancer with FLT-PET imaging

INTRODUCTION: Inhibition of thymidylate synthase (TS) results in a transient compensatory “flare” in thymidine salvage pathway activity measureable with (18)F-thymidine (FLT)- positron emission tomography (PET) at 2hrs. of therapy which may predict non-small cell lung cancer (NSCLC) sensitivity to TS inhibition. MATERIALS AND METHODS: Resistance to TS inhibition by pemetrexed was induced in NSCLC cell lines H460 and H1299 through TS overexpression. TS overexpression was confirmed with RT-PCR and Western blotting and pemetrexed resistance confirmed with IC(50) assays. The presence of a pemetrexed-induced thymidine salvage pathway “flare” was then measured using (3)H-thymidine in both pemetrexed sensitive (H460 and H1299) and resistant (H460R, H1299R, CALU-6, H522, H650, H661, H820, H1838) lines in vitro, and validated with FLT-PET in vivo using H460 and H460R xenografts. RESULTS: Overexpression of TS induced pemetrexed resistance with IC(50) for H460, H1299, H460R and H1299R measured as 0.141 μM, 0.656 μM, 22.842 μM, 213.120 μM, respectively. Thymidine salvage pathway (3)H-thymidine “flare” was observed following pemetrexed in H460 and H1299 but not H460R, H1299R, CALU-6, H522, H650, H661, H820 or H1838 in vitro. Similarly, a FLT “flare” was observed in vivo following pemetrexed therapy in H460 but not H460R tumor-bearing xenografts. CONCLUSIONS: Imaging of TS inhibition is predictive of NSCLC sensitivity to pemetrexed.