ABCB1 and glioblastoma: Since (i) serum concentrations of 17-AAG achieve micromolar levels [39], (ii) IC50 values for 17-AAG do not exceed 50nM concentration in studied cell lines (Table 2), and (iii) ABCB1- and ABCG2-mediated efflux efficacy of 17-AAG is relatively low (Figure 4 and Figure 5), we hypothesize that 17-AAG can cross the BBB at concentrations that may be cytotoxic to GBM.