Aim: 5-Iodo-(2-deoxy-2-fluoro-beta-D-arabinofuranosyl) uracil (FIAU) has been used for non-invasive monitoring of gene therapy and as an antiviral agent experimentally and in patients. However, FIAU metabolism in tumor cells is largely unknown. Here, the biological characteristics of FIAU in human leukemia and lymphoma cells in vitro and in a xenotransplant severe combined immunodeficient (SCID)-mouse model were investigated.
Methods: The susceptibility of FIAU to glycosidic bond cleavage by thymidine phosphorylase (TP) and its phosphorylation by human thymidine kinase 1 (hTK1) were examined. Cellular uptake and DNA-incorporation were determined in the leukemia cell line HL60 and the lymphoma cell line DoHH2. Biodistribution, in vivo stability of FIAU and expression of proliferation marker(67)Ki and thymidylate synthase were assessed in SCID-mice bearing HL60 xenotransplants. Cellular distribution of FIAU was imaged by microautoradiography.
Results: FIAU proved to be stable against degradation by TP and was phosphorylated by hTK1. Significant cellular uptake in DoHH2 and in HL60 cells was observed. The majority of intracellular [(131)I]FIAU was DNA incorporated. In vivo, moderate dehalogenation of [(131)I]FIAU was observed. Biodistribution studies showed a tumor uptake of 1.8+/-0.4% ID/g after 30 min. The half-life of [(131)I]FIAU in blood was 43+/-2 min. Microautoradiography showed a modest accumulation of [(125)I]FIAU in proliferating cells of small intestine, spleen and tumor.
Conclusion: Despite phosphorylation by the hTK, efficient incorporation into the DNA and high in vivo stability, FIAU accumulates only moderately and transiently in proliferating cells, suggesting that FIAU is probably not appropriate for imaging of proliferation.