1–2.3 μM on HL-60 cells. Regarding to normal cells (PBMC), IC50 values ranged from 3.2 to 13.4 μM and were less pronounced than those found in cancer cells. As shown in Fig. 2A, compounds 2, 3 and 4 caused reduction in HL-60 cell number in the concentration of 2 μM after 24 h treatment and evaluation by trypan blue exclusion test (46.7 ± 2.0, 43.0 ± 2.1 and 48.5 ± 3.3 × 104 cells/mL, respectively) when compared to control cells (65 ± 5.5 × 104 cells/mL) (p < 0.05), while no differences between the compounds were noticed (p > 0.05). The positive control Dox also caused a significant reduction on viable cell population

(42.2 ± 1.0 × 104 cells/mL, p < 0.05). Interestingly, though all compounds has decreased cell number after 24 h exposure,

none of them altered viability of the check details remaining cells, since it was not noticed statistically significant differences in viable and non-viable cells in comparison to control ( Fig. 2B). The cytotoxicity is not related to the membrane lysis of leukemia cells, since compounds 3 and 4 did not led to membrane disruption or increased DAPT mouse fluorescence after ethidium bromide incorporation. The exception was the compound 2 (2 μM), which induced a slight but significant decreasing in cells with intact membranes (93.0 ± 1.6%, p < 0.05) ( Fig. 2C). Since sesquiterpene lactones are known inhibitors of enzymes and cellular processes, we investigated whether the inhibition of cell proliferation is related to DNA synthesis inhibition using the BrdU assay. This method revealed that

all compounds were able to reduce the BrdU incorporation, presenting the compound 4 the highest potential to diminish BrdU-positive cells in both dose tested (1 μM and 2 μM, 28.5 ± 2.2% and 28 ± 1.9%, respectively) PD-1 antibody inhibitor in comparison to negative control (51.4 ± 3.15%). To define the mechanism responsible for the action of santonin derivatives involved on HL-60 cell death, cell-cycle distribution was assessed after 24 h and 48 h of treatment (Fig. 3A and B). A significant inhibition on HL-60 cell-cycle progression was observed within 24 h, where Dox (37 ± 3.4%), compound 3 (7.6 ± 0.5% and 9.0 ± 0.9%) and 4 (9.0 ± 0.9% and 8.6 ± 9.6%) (1 and 2 μM, respectively) caused an increasing of cells in G2/M phase when compared to untreated cells (3.4 ± 0.5%). On the other hand, 48 h exposure provoked G2/M reduction [(2.6 ± 0.7% and 1.5 ± 1.0%), (1.7 ± 0.3% and 1.5 ±.0.5%) and (0.6 ± 0.2% and 1.0 ± 0.8%), for compounds 2, 3 and 4, respectively] when compared to negative control (5 ± 0.8%) (Fig. 3C, p < 0.05), findings indicating time and concentration dependent activity of the molecules. Interestingly, only compound 2 at highest concentration was able to increase sub-G0/G1 DNA content after 24 h (34 ± 4.8%, indicated in pink part) in comparison with control (13 ± 1.3%) ( Fig. 3D). However, after 48 h exposure, α-santonin derivatives 3 and 4 also caused increasing on DNA fragmentation [(45.3 ± 1.2% and 91.0 ± 2.0%) and (64.4 ± 1.