The uptake, Btz intracellular accumulation, and release of anti-BCMA-Exo-Btz in vitro. U266 cells and LP-1 cells were treated with anti-BCMA-Exo-Btz. Internalization was measured by flow cytometry (A) and fluorescence microscopy (B). Scale bar, 20 μm. (C) Intracellular Btz accumulation of U266 cells and LP-1 cells treated with free Btz and anti-BCMA-Exo-Btz. (D) In vitro release of Btz from anti-BCMA-Exo-Btz in pH 7.45 and pH 7.35 by HPLC. ∗P < .05; ∗∗P < .01; ∗∗∗P < .001; ∗∗∗∗P < .0001. DAPI, 4',6-diamidino-2-phenylindole.