In vivo generation of resistance to PI3K-δ inhibitor GS-649443. (A) Scheme for generation of resistance to PI3K-δ inhibitor in vivo. TCL1-192 cells were transferred into immunodeficient recipients, and starting from day 5 after transfer, the mice were treated with vehicle or GS-649443. Tumors from critically sick mice treated continuously with PI3K-δ inhibitor or vehicle were retransferred into recipient mice, and the treatments were repeated. A total of 4 transfer and treatment rounds was performed. (B) Simplified treatment cohort scheme with the 4 rounds of transfer and treatment to generate resistance and their corresponding survival curves. Red: GS-649443–treated recipient mice that were transferred with cells treated continuously with PI3K-δ inhibitor in all previous transfers. Green: GS-649443–treated recipient mice transferred with cells treated with vehicle in all previous transfers. Blue: vehicle-treated recipient mice transferred with cells that were treated continuously with vehicle in all previous transfers. Purple: vehicle-treated recipient mice transferred with cells treated with GS-649443 in all previous transfers. Red vs blue first round: P = .0047; red vs green third round: P = .0001; red vs green fourth round: P = .0004. All P values were calculated using the Mantel Cox log-rank test. (C) Spleen and liver weights and WBC counts as a representation of the tumor burden in mice from the fourth round of transfer and treatment. Data were collected following 5 days of treatment. Spleen, P = .0286; liver, P = .0294; WBC, P = .0571, Mann-Whitney U test (n = 6). ns, not statistically significant; p.o., by oral gavage *P ≤ .05; **P ≤ .01; ***P ≤ .001; ns, P > .05.