Figure 1. Evolutional landmarks in FTO inhibitor discovery. Rhein, the first FTO inhibitor identified in 2012, did not impair viability in BE(2)-C neuroblastoma cells at a dose of 20 μM. MO-I-500, an inhibitor discovered in 2014, was found to inhibit survival of SUM149 breast cancer cells in glutamine (Gln)-free medium with an IC50 of 20 μM, but had little effect on cells cultured in complete medium. Meclofenamic acid (MA) and its analogs were reported to be highly selective inhibitors of FTO. However, more than 90% of the Hela cells treated with 120 μM MA2 (an MA analog) remained viable. Fluorescein and its derivatives could simultaneously inhibit and label FTO and were therefore considered "bifunctional". At a concentration of up to 150 μM, fluorescein derivatives FL6 and FL8 did not display inhibitory effects on Hela cells with > 95% viable cells. In 2019, guided by the structural complex of FTO/MA, FB23 and FB23-2 were designed and optimized as two more potent FTO inhibitors. But the IC50 for both inhibitors in acute myeloid leukemia (AML) were still in the micromolar range (23.6 – 44.8 μM for FB23 and 0.8 – 16 μM for the optimized FB23-2). In contrast, the most recently discovered small molecule FTO inhibitors, CS1 and CS2, have much lower IC50 values in AML and solid tumors, especially in cancer cells that highly express FTO (in the low nanomolar range). CS1 and CS2 exert their anti-tumor activity by suppressing the FTO-mediated upregulation of MYC/CEBPA as well as LILRB4, thereby attenuating cancer stem cell (CSC) self-renewal and overcoming tumor immune evasion.