PCI-32765 induces cytotoxicity against Btk-expressing MM cells stimulated by IL-6 or coculture with BMSCs or OCs. (A) Immunoblotting analysis showed Btk expression in CD138⁺ MM cell lines and patient MM cells (n = 5). (B) INA6 cells, with or without IL-6 (B) or BMSCs (C) or OC (D), prepared from MM patients were cultured with PCI-32765. (E) INA6-GFP cells were cocultured with (+) or without (−) BMSCs overnight, sorted by FACS, and subjected to mRNA and protein extraction for Btk levels by quantitative RT-PCR and immunoblotting. (F) PCI-32765 was added for 3 days to CD138⁺ cells from 2 representative MM patients, alone or with BMSCs, followed by a luminescent cell viability assay.