CD19/CD3-scFv-Fc eliminates ibrutinib-resistant CLL cells, both in vitro and in patient-derived xenografts in vivo. PBMCs from 4 CLL patients with acquired ibrutinib-resistance were treated with CD19/CD3-scFv-Fc or HER2/CD3-scFv-Fc, either in culture (A-B) or after transfer into NSG mice (C-E). (A) CLL cell specific killing in culture over time. Bold lines indicate mean killing of 4 ibrutinib-resistant samples by treatment group; dashed lines indicate killing for each sample. (B) In vitro CLL cell specific killing for each ibrutinib-resistant patient at the end of culture. Medium-only control samples had average (range) CLL viability of 68% (52% to 78%). Asterisks indicate results of a paired Student t test; mean and 95% CIs are shown. (C) Once-weekly treatment with CD19/CD3-scFv-Fc or HER2/CD3-scFv-Fc was initiated 3 days after PBMC transfer into NSG mice. CLL counts in the peripheral blood are shown, where dashed lines indicate mean CLL counts by treatment of each xenograft source used and bold lines indicate the mean of all mice in the same treatment group (n = 4 patients; n = 37 mice). (D) CLL counts in the peripheral blood at experimental day 10. (E) CLL counts in the spleen at experimental day 17. Dots in panels D-E represent 1 animal, colors in panels B and D-E indicate patient source of xenografted cells and correspond to colors in supplemental Table 2. Mean and 95% CI are shown. Asterisks denote significance using unpaired Student t tests, taking into account the random batch effect from xenograft source. *P < .05; **P < .01; ***P < .001; ****P < .0001.