Background: Imaging of tumor proliferation has been studied with FLT-PET in various tumor types including NSCLC. Pemetrexed inhibits thymidylate synthase(TS), dihydrofolate reductase (DHFR), and glycinamide ribonucleotide formyltransferase (GARFT). TS inhibition upregulates the thymidine salvage pathway including relocalisation of ENT1 to membrane and TK1 activation as a transient “flare” response. We hypothesise that this can be detected as an increase in FLT tumoral uptake that subsequently decreases with reduced proliferation. This study was conducted to assess FLT uptake as an early pharmacodynamics(PD) marker of pemetrexed response. Methods: This was an open-label imaging study in 21 patients with Stage 3/4 NSCLC treated with pemetrexed and platinum-based chemotherapy. Patients underwent FLT PET/CT scan at baseline and 4h after administration of pemetrexed. Platinum component of treatment was administered on the day after second FLT scan for cycle 1. Plasma for TK1 activity expression were collected before each scan time point and analysed by ELISA. Percentage change in standardized uptake value (%ΔSUV) was calculated as [SUV(PET2) – SUV(PET1)]/ SUV(PET1)*100. Treatment response calculated by RECIST 1. 1 and survival data were collected. Results: 17 patients had evaluable PET/CT scans for pemetrexed response. Median percentage difference for SUVmean and SUVmax in tumour lesions increased by 3% and 10.3% respectively. 5 patients showed homogeneous FLT flare at 4h after pemetrexed, 2 patients had decrease, 10 patients had heterogeneous FLT response (regardless of platinum doublet). There was no significant correlation between plasma TK1 activity and FLT flare. At 9 weeks, 4 patients had partial response, 9 stable disease and 4 progressive disease. Baseline and weighted average ΔSUVmax were not associated with survival. The 5 patients with FLT flare in all lesions showed a median OS of 31 months, unlike the group with heterogenous or decrease uptake(15 months). FLT uptake in bone marrow and small bowel significantly increased at 4h (t test p = 0.004, p = 0.004, respectively) indicating increased thymidine salvage activity. Early FLT uptake was not predictive for tumour RECIST response or OS. In multivariable cox regression analysis, pre-treatment TK1 activity, adjusted for performance status, smoking history and age, independently affected survival in this group (p = 0.011). Conclusions: Early FLT flare at 4h was seen in NSCLC post pemetrexed administration indicating activation of thymidine salvage pathway. Median overall survival of patients with an FLT flare response was more than twice longer than patients with mixed or no response. However, the small sample size lacked power to show statistical significance in the OS comparison. Further studies should evaluate this and the relationship to other prognostic variables in a larger cohort of patients. Clinical trial information: NCRI UK badge 9249.