In vivo administration of GSI prevents BCMA shedding and increases BCMA CAR efficacy. (A) Concentration of LY3039478 in peripheral blood of mice at various time points after administration of 2 doses of 1 or 3 mg/kg LY3039478 36 hours apart. (B-C) Mice were engrafted with MM.1R for 27 days, treated with 1 mg/kg LY3039478 (filled circles) or vehicle (open squares) twice 18 hours apart and euthanized at various time points after the second dose. (B) Fold change in BCMA expression on CD138⁺GFP⁺ cells from bone marrow was determined, as well as sBCMA levels (C) in peripheral blood serum of mice. (D) Quantified bioluminescence imaging and (E) survival of mice engrafted with MM.1R cells and treated with 7.5 × 10⁵ cells BCMA-CAR or CD19 CAR T cells 20 days after tumor injection; 1 mg/kg LY3039478 or vehicle was administered 3 times a week (TIW) starting at day −1 before T-cell treatment. (F) Staining of BCMA (red histograms) on 3 primary MM samples at screening for enrollment and 4 to 6 hours after 3 doses of 25 mg of LY3039478. LY3039478 was dosed orally on days 1, 3, and 5 and flow cytometry was performed on screen and day 5 samples. Time between the screening and day 5 samples is at most 39 days. Blue histograms depict staining with fluorescence minus 1 (FMO) control. Shaded area indicates positive above background, with BCMA⁺ percentages in top right corner. BCMA ABC is depicted in the right panel. Plots are gated on abnormal plasma cells of freshly collected BMAs. Significance was tested by the 2-tailed paired Student t test. (G) Percentage of abnormal plasma cells from samples shown in panel F. (H) Ki67 stain on samples from patients 2 and 3. Percentage of Ki67⁺ cells is depicted. *P < .05 as tested by the Mann-Whitney test. ****P ≤ .0001 as tested by 1-way ANOVA with the Dunnett posttest. **P ≤ 0.01 as tested by the log-rank (Mantel-Cox) test.