18F-fluorothymidine PET/CT as an early predictor of tumor response to treatment with cetuximab in human lung cancer xenografts.

We investigated whether 18F-fluorothymidine-positron-emission tomography/computed tomography (18F-FLT-PET/CT) is useful for the evaluation of the very early response to anti-epidermal growth factor receptor (EGFR) antibody cetuximab therapy in human lung cancer xenografts. A human tumor xenograft model was established with a human non-small cell lung cancer cell line. The mice were randomly assigned to four groups: tumor growth follow-up, ex vivo study, PET/CT imaging and non-treated control. Mice were administered saline as control or cetuximab on day 1. An immunohistochemical study with Ki-67 was performed. Tumor volume treated with cetuximab was kept significantly smaller than control after day 8, although there was no difference on day 3. On day 3, 18F-FLT distribution was higher in the tumor than in other tissues, and was significantly decreased by treatment with cetuximab. On PET/CT imaging, 18F-FLT distribution in the tumor was clearly visualized, and the maximum standardized uptake value (SUV) was significantly decreased after treatment with cetuximab (p<0.01). Ki-67 expression was also significantly decreased on day 3 (p=0.01). These results suggest that 18F-FLT-PET/CT can be a useful predictor to determine the response to molecular targeted drugs such as cetuximab at an earlier time point than the change of tumor size.