The STAT3 inhibitor, CPA-7, can synergize with Dox to kill ABC-DLBCL cells both in vitro and in vivo. (A) Toxicity of CPA-7 in 7 GCB and 7 ABC cell lines was measured by Resazurin-based viability assays. Although most of the ABC lines showed sensitivity to CPA-7 (IC₅₀ ≤ 3 μM), the majority of the GCB lines fell into the resistant group (IC50 > 7 μM). The 3 outliers are individually labeled and discussed in the text. (B) CPA-7 treatment reduced PY-STAT3, c-Myc, and Cyclin D2 in a dose- and time-dependent manner in Ly3 cells. (C) The effect of CPA-7 on basal and Dox-induced ROS and cell viability change. Ly3 and HBL1 cells were pretreated with 0.3 and 1 μM of CPA-7, respectively, for 1 h before exposing to IC₅₀ concentrations of Dox for 24 hours. ROS and cell viability measurements were made as in Figure 3. Results shown are mean ± SD and were representative of 3 independent experiments. *P < .05; **P < .01, based on 2-tailed Student t test was used. ns, not significant. nil, vehicle control. (D) Isobolograms for the combination of Dox with CPA-7 that were isoeffective (IC₅₀) for inhibition of proliferation of Ly3 and Ly10 cells. The diagonal line indicates the zero interaction isobole. Cell viability was measured after drug treatment of 48 hours, using Resazurin-based assays. Results are representative of 3 independent experiments. CPA-7 (3.5 mg/kg) synergized with CHOP to kill established Ly3 (E) and Ly10 (F) tumors in xenograft mouse models. Results presented are mean tumor volumes with error bars indicating standard error of the mean. Compared with CHOP or CPA-7 monotherapy, CHOP/CPA-7 combination showed significant activity (P <. 05, 2-tailed Student t test) at all points after day 6 post the single treatment on day 0, marked by the thick arrow. (G) Model depicting Dox-triggered mechanism of cytotoxicity in ABC-DLBCLs. Specifically, Dox is often inefficient in activating the DDR-p53-cell death axis; instead, it relies on ROS accumulation to cause oxidative cell death, a process that is countered by activated STAT3 and the endogenous antioxidant program. PARP, poly(ADP-ribose) polymerase 1.