GDC-0853 inhibits both wild-type and C481S-mutated BTK variants. (A) Inhibition of wild-type and C481S recombinant BTK protein by 1 µM GDC-0853 as measured by a biochemical kinase activity assay (N = 3). (B) The HEK 293T cell line was stably transfected with either wild-type or C481S-mutated BTK. Following BTK inhibitor treatment, these cells were immunoblotted to determine the phosphorylation status of BTK (Y223). (C) Primary CLL cells from patients expressing the C481S BTK mutation were treated with DMSO, 1 µM GDC-0853, or 1 µM ibrutinib for 48 hours and assessed for viability (N = 9). (D) CLL cells from patients acquiring C481S BTK mutations during the course of ibrutinib therapy were treated with GDC-0853 to determine its ability to inhibit BCR-mediated production of CCL3 (N = 8). *P ≤ .05; **P ≤ .01.