Liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis of 3′-deoxy-3′-fluorothymidine (FLT) and 3′-deoxy-3′-fluorothymidine monophosphate (FLT-MP) accumulation in xenograft tumors and liver extracts from HCT116 tumor-bearing mice. FLT is distributed equally well in tumors and non-proliferative liver control tissues from all treatment groups (ng FLT/g tissue, panel A). FLT-MP is detectable in tumor tissues, but not in liver controls (panel B, FLT-MP peak area). Compared to vehicle-treated tumors (gray bars, n = 5) FLT-MP is significantly reduced 24 h (white bars; n = 4; P = 0.009, Student’s t- test) and 48 h (hashed bars, n = 4; P = 0.165) after docetaxel treatment. FLT-MP concentrations were below the validated lower limit of quantitation of the assay in matched liver extracts (dotted lines) from all treatment groups. Thymidine and thymidine monophosphate (TMP) were detectable in all tissues (panels C and D). Results represent means +/− SD for each treatment group.