Background: Axitinib is an oral, potent, and selective inhibitor of VEGF receptors (VEGFR) 1, 2, and 3. Clinical trials of VEGFR inhibitors in combination with cytotoxic chemotherapies have yet to yield positive results. Inhibition of cancer cell proliferation by VEGFR inhibitors may be antagonizing cytotoxic chemotherapy. This serial FLT-PET imaging study was performed to determine whether continuous dosing of axitinib inhibits FLT uptake and whether a 3- or 7-day drug holiday could reverse the FLT uptake inhibition. Methods: Pts with CRC or other GI cancers with suitable lesions were enrolled. Axitinib 7 mg (cohort 1) or 10 mg (cohort 2) twice daily (BID) starting dose was administered continuously for 7 days, then interrupted for ≥7 days. Dynamic and whole-body FLT-PET/CT scans were obtained at baseline, at the end of 7 days of continuous axitinib BID dosing, and then 3 and 7 days after axitinib interruption. Kinetic parameters calculated included the Patlak influx constant (K_patlak) and changes in standardized uptake values (SUV_max, SUV_mean). For liver lesions, SUV_mean was used. Results: A total of 19 pts were enrolled, 3 and 16 into Cohorts 1 and 2, respectively. 15 had CRC and 4 had other GI cancers; 14 were male, and median age was 56 yrs. Serial FLT-PET data were available for 18 pts: 6 with lung, 5 with liver, 3 with lymph node, 2 with abdominal wall, and 2 with pelvic lesions. Following an initial 7 days of continuous axitinib, tumor FLT uptake (mean DSUV) changed by –46.0% (95% CI: –31.8%, –60.2%) from baseline. Then, after 3 days of axitinib holiday, tumor FLT uptake increased from nadir by +54.5% (95% CI: +17.5%, +91.5%), which further increased by +96.6% (95% CI: +36.4%, +156.8%) after 7 days of axitinib holiday. Conclusions: Continuous axitinib therapy for 7 days markedly inhibited fluorothymidine uptake. Since inhibition of cancer cell proliferation can protect against the cytotoxic effects of chemotherapeutics, this could explain the failure of VEGFR inhibitor + chemotherapy regimens to improve chemotherapy. Inclusion of a short drug holiday in axitinib dosing schedule may allow resensitization of cancer cells.