SRC and glioblastoma: Next, we determined the concentration of c-Src inhibitors necessary to inhibit P-gp activity by 50% (IC50 of P-gp inhibition, Table 2) We examined the accumulation of Rho 123 in sensitive -U87 and MDR-U87-TxR GBM cells after 30 min and compared this accumulation with that obtained after treatment with increasing concentrations of Si306 and pro-Si306 (Figure 3).