Various cancer cell lines undergo cell death under the treatment of cally2R. (A) A structure of callyspongiolide R and S form at C-21. (B) IC50 of cally2R after 72 h of cally2R treatment in various cell lines (n = 3). (C) CTG assay in SKBR3 cells were treated of cally2R, ferrostatin-1, z-VAD-fmk or GSK963 for 48 h (n = 3). Each bar represents the mean + s.d. n.s. no significance; *P < 0.05, **P < 0.01, ***P < 0.001 compared to the cally2R-treated group.