Pirtobrutinib inhibits cellular proliferation, B-cell activation, and calcium mobilization. (A) Activity of pirtobrutinib, ibrutinib, acalabrutinib, and zanubrutinib on cellular proliferation of TMD8 cells and (B) TMD8 cells expressing BTK C481S (mean and SD are graphed). (C) Pirtobrutinib inhibited anti-IgM–stimulated calcium flux in TMD8 and (D) REC-1 cells. Representative traces of 3 independent experiments are shown. (E) Human PBMCs from healthy donors were treated with pirtobrutinib, and B-cell activation was measured by upregulation of the CD69 activation marker after IgM stimulation. Representative flow cytometry density plots showing a reduction in the percentage of CD19⁺ B cells expressing CD69 with increasing doses of pirtobrutinib. (F) Dose-response curve of pirtobrutinib inhibition of B-cell activation in PBMCs from 4 separate healthy donors. Data are mean ± SD.