CNS progenitor cells are vulnerable to clinically relevant levels of 5-FU exposure. (a) A summary of the putative relationships between the different cell types under study (for discussion of this and alternative views on lineage relationships in the CNS, see [199,200]). Pluripotent neuroepithelial stem cells (NSC) give rise to glial restricted precursor (GRP) cells and neuron restricted precursor (NRP) cells. GRP cells in turn give rise to astrocytes and oligodendrocyte-type-2 astrocyte progenitor/oligodendrocyte precursor cells (O-2A/OPCs), the ancestors of oligodendrocytes. (b,c) Primary CNS cells (b) or various cancer cell lines (c) were grown on coverslips and exposed to 5-FU for 24 h before analysis of cell viability as described in Materials and methods. 5-FU concentrations were chosen on the basis of drug concentrations reached in humans after conventional 5-FU treatment. None of the tumor lines tested were sensitive to 5-FU treatment in this dose range, whereas O-2A/OPCs, oligodendrocytes, GRP cells and human umbilical vein endothelial cells (HUVECs) were sensitive. (d,e) Exposure conditions designed to mimic the exposure levels associated with long-term infusion (d) or high-dose bolus administration (e) yielded similar results, with vulnerability of O-2A/OPCs and non-dividing oligodendrocytes to 5-FU exceeding the vulnerability of rapidly dividing cancer cells. As shown in (b,d), the vulnerability of HUVECs also exceeds the vulnerability of cancer cells. Each experiment was carried out in quadruplicate and was repeated at least twice in independent experiments. Data represent mean of survival ± s.e.m, normalized to control values.