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 F-18-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 IC50 assays. The presence of a pemetrexed-induced thymidine salvage pathway "flare" was then measured using H-3-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 IC50 for H460, H1299, H460R and H1299R measured as 0.141 mu M, 0.656 mu M, 22.842 mu M, 213.120 mu M, respectively. Thymidine salvage pathway H-3-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.