GSK3B and neoplasm: The addition of a suboptimal concentration of metformin did reduce the IC50 concentration of the GSK-3 inhibitor tideglusib in both MCF-7 + pLXSN and MCF-7 + KD-GSK-3β cells, 5.1- and 10-fold, respectively, but increased the IC50 of tideglusib in MCF-7 + WT-GSK-3β cells 5.6-fold, indicating that GSK-3 was playing a tumor suppressor role in these cells, and suppression of its activity increased therapeutic resistance (Figure 21, Table 3).