Using fluorodeoxythymidine to monitor anti-egfr inhibitor therapy in squamous cell carcinoma xenografts

Background. 3′-18F-fluoro-3′-deoxy-fluorothymidine ( ¹⁸F-FLT), a nucleoside analog, could monitor effects of molecularly targeted therapeutics on tumor proliferation. Methods. We tested whether ¹⁸F-FLT positron emission tomography (PET) uptake changes are associated with antitumor effects of erlotinib in A431 xenografts or cetuximab in SCC1 xenografts. Results. Compared with pretreatment FLT PET scans, 3 days of erlotinib in A431 reduced the standardized uptake value (SUV) by 18%, whereas placebo increased SUV by 1% (p = .005). One week of cetuximab in SCC1 reduced SUV by 62%, whereas placebo reduced SUV by 16% (p = .005). FLT uptake suppression following anti-epidermal growth factor receptor (EGFR) treatment was associated with reduced tumor thymidine kinase-1 (TK1) activity. In vitro TK1 knockdown studies confirmed the importance of TK1 activity on intracellular FLT accumulation suppression. Conclusions. ¹⁸F-FLT PET imaging detects tumor responses to EGFR-inhibitors within days of starting therapy. This technique may identify patients likely to benefit from EGFR-inhibitors early in their treatment course.