Γ-Secretase Inhibitors Enhance the Potency of BCMA-Targeting T Cell Engagers Against Multiple Myeloma Cells without Adverse Impact on T-Cell Activation and Differentiation

BCMA-directed therapy is the first efficacious mono-immunotherapy to treat patients with relapsed and refractory multiple myeloma (MM). However, membrane BCMA (mBCMA) receptor molecule is constantly cleaved by γ-secretase (GS) and shed/soluble BCMA (sBCMA) is released to the circulation. This could interfere BCMA targeting thereby reduce killing of patient MM cells. We here defined the impact of γ-secretase inhibitors (GSIs) on T-cell-dependent MM cell lysis and immunomodulatory effects induced by BCMAxCD3 bispecific antibodies (BisAbs). Therapeutic significance of combination GSI with BCMAxCD3 BisAb were further determined in a MM xenograft adoptive transfer model. First, ED₅₀ values of 9 tested GSIs are comparable for mBCMA accumulation and sBCMA reduction, ranging from 0.07 to 777 nM. LY411575 is most effective, showing ~2-log greater potency than DAPT in all tested MM cell lines and patient MM cells regardless of baseline BCMA expression and statuses of disease and drug resistance. GSI-induced mBCMA upregulation reached near maximum within 4h and sustained over 42h-study period on MM cell lines and patient MM cells. GSIs, i.e., 2 nM LY411575 or 1 μM DAPT, robustly increased mBCMA densities on CD138+ but not CD3+ patient cells, associated with rarely detectable sBCMA in supernatants of patient bone marrow stromal cell cultures for 1d. In patient serum samples (n =52), sBCMA levels were significantly elevated in the cohort with active disease (n = 35, from 10.25 to 150.2 ng/mL) vs maintenance therapy (n = 17, from 3.69 to 21.69 ng/mL) (P < .0001). In the MM-patient T cell co-cultures at the low E:T ratio of 1:1 which mimicked to the immune compromised conditions in patients, the addition of sBCMA (12.5-200 ng/mL) decreased BCMAxCD3 BisAb-induced % CD107a+ patient T cells and MM cell lysis, starting from 12.5 ng/mL (P < .01). Significantly, GSIs abrogated sBCMA-diminished MM cell lysis and further enhanced autologous patient MM cell killing induced by all 4 tested BCMAxCD3 BisAbs, accompanied by elevated cytolytic markers (CD107a, IFNg, IL2, TNFα) and memory effector differentiation in patient T cells. GSI (LY411575) did not further extend BCMAxCD3 (PL33)-induced T cell activation and checkpoint marker expression, indicating minimal effects on T-cell exhaustion. At later time points in ex vivo co-cultures, PL33-induced % T-cell subsets with inhibitory characteristics (Treg, IL10+, TGFβ+) were decreased by GSI treatment vs control media. In longer 7d-co-cultures, LY411575 minimally affected PL33-induced transient expression of checkpoint (PD1, TIGIT, TIM3, LAG3) and co-stimulatory (41BB, CD28) proteins, as well as time-dependent increases in % subsets with effector memory (CD45RA⁻CD62L⁻) plus central memory (45RA^-CD62L⁺) phenotypes and CD8/CD4 ratios in patient T cells (n = 8-11). These data suggest that combination therapy may trigger persistent anti-MM activity in the clinic. Importantly, a single low LY41157 treatment (3 mg/kg) rapidly cleared human sBCMA from the serum of KMS11-luc MM-bearing NSG mice reconstituted with human T cells and further augmented anti-MM activity of a single sub-curative PL33 administration with prolonged host survival (P < .002). Taken together, GSI potently prevented mBCMA loss from MM cells and abolished sBCMA decoy, thereby enhancing MM cell targeting and efficacy of BCMAxCD3 BisAbs without any adverse impacts on T effector cells. Moreover, a single treatment with GSI at ~ 2-log lower drug concentration than used to induce anti-cancer activity maximizes mBCMA density without harming normal cells. Rationally incorporating GSI into all BCMA-targeting immunotherapy therefore represents a promising novel combination approach to further improve patient outcome in MM.