Combination of forimtamig with BCMA TCB. (A-B) BCMA and GPRC5D expression was analyzed in ∼1000 mm³ NCI-H929 tumors that relapsed upon forimtamig treatment at 0.1 mg/kg in experiment 5F by immunohistochemistry and quantified using visiopharm (n = 5 animals per group); control tumors were matched in tumor size but were removed from the study at earlier time points; data are shown as mean ± SEM expression and statistical differences between multiple groups were determined using Student t tests. ∗P < .05; ∗∗P < .005; ∗∗∗P < .0005. (C) huNSG mice were subcutaneously implanted with NCI-H929 tumor cells and when tumors reached 200 to 250 mm³ injected once weekly with forimtamig at 0.1 mg/kg or 1+1 BCMA TCB at 1 mg/kg or the combination of both TCBs using a concomitant schedule starting at C1, C2, or C3; an alternating schedule; or different switch schedules in which 1 TCB was dosed after stopping the second TCB; treatment was stopped after 8 injections and tumor escape was monitored for 10 additional days; tumors growth was monitored twice weekly using caliper measurements. (D) Serum was harvested from n = 5 animals 48 hours after C1, C2, C3, and C4 dosing and cytokine levels were measured by bioplex analysis; and graphs summarizing the mean ± SEM expression and statistical differences between multiple groups were determined by 1-way ANOVA with Tukey posttest analysis. ∗P < .05; ∗∗P < .005; ∗∗∗P < .0005. (E) Blood was collected 48 hours after C1, C2, C3, and C4 TCB dosing and human CD3⁺ T cells were quantified and characterized using spectral flow cytometry; and graphs summarizing the mean ± SEM expression and statistical differences between multiple groups were determined by 1-way ANOVA with Tukey posttest analysis. ∗P < .05; ∗∗P < .005; ∗∗∗P < .0005; ∗∗∗∗P < .0001. TNF-α, tumor necrosis factor α.