Prolonged Persistence and Functional Activity of Anti-BCMA CAR-T Durcabtagene Autoleucel (PHE885) Manufactured Using the T-Charge™ Platform

T-Charge^TM is a novel platform for manufacturing CAR-T cells that reduces ex-vivo processing time to <2 days. A phase 1 clinical trial (NCT04318327) of durcabtagene autoleucel (PHE885) manufactured using the T-Charge^TM platform in relapsed/refractory multiple myeloma (R/RMM) has shown promising clinical activity. Previous correlative studies showed that T-Charge^TM manufacturing preserved T cell stemness in the CAR-T product and PHE885 expanded robustly with high TCR clonal diversity (Ikegawa et al., ASH 2023). CAR-T cells persisted for prolonged periods in some patients, allowing the detailed analysis of long-term persisting CAR-T cells.

Among 32 patients with R/RMM treated at our center, we analyzed samples from 25 patients who had been followed for at least six months. At six months, CAR-T cells remained detectable in peripheral blood in 12 of 25 patients (median 38.3%; range, 2.1-70.8% of CD3+) by flow cytometry. CAR-T cells persisted for 12 months in 7 of 14 patients (median 15.6%; range, 1.04-75.9% of CD3+). At 24 months after CAR-T infusion, two of four patients continued to have detectable CAR-T by flow cytometry (61% and 0.8% of CD3+, respectively).

CAR-T cells were predominantly CD4+ with effector memory phenotype at peak expansion, whereas the persisting CAR-T cells at later time points contained more less-differentiated T cells, including naïve and stem-like memory T cells. Mass cytometry revealed that the expression of activation, proliferation, and exhaustion markers on CAR-T cells declined after peak expansion. CAR-T TCR repertoire continued to be highly diverse 6-18 months after infusion and CAR-T cells present at later time points shared TCR clonotypes with those at peak expansion.

Next, we evaluated the anti-BCMA function of post-infusion CAR-T cells in 12 patients who had persistent CAR-T cells in peripheral blood for >6 months. CAR-T cells were isolated by cell sorting from cryopreserved PBMC obtained during early expansion and at later time points (>3 months) and assayed their ability to directly kill reporter cells expressing BCMA. Isolated CAR-T cells continued to have anti-BMCA-specific cytotoxic activity that was not impaired at late time points. Notably, anti-BCMA-specific cytotoxicity in vitro was comparable in CAR T cells obtained from patients with ongoing clinical response and those with disease relapse. sBCMA levels at disease relapse were very low in 6 of 7 patients with CAR-T persistence. In contrast, sBCMA levels increased in 4 of 8 patients who relapsed after loss of CAR-T cells. These results indicate that T-charge^TM manufacturing produced CAR-T cells capable of long-term persistence with sustained anti-BCMA specific cytotoxic activity and BCMA loss likely contributes to relapse in these patients.

We also explored various factors that could affect CAR-T persistence. The number of CAR-T cells at peak expansion and the number of Tscm/central memory T cells in the CAR-T product/leukapheresis were not associated with long-term CAR-T persistence. CD127 (IL-7Ra) expression on CAR-T cells was significantly higher in patients with long-term CAR-T persistence in CD4 T cells (day14; p = 0.04, day21; p = 0.01, and day28; p = 0.02) and CD8 T cells (day14; p = 0.04 and day21; p = 0.005), and this difference was not observed for non-CAR-T cells. Mass cytometry data confirmed that the CAR-T subset with high expression of CD127 at peak expansion was significantly increased in patients with long-term CAR-T persistence (p = 0.03).

Because the T-charge^TM platform reduces ex-vivo CAR-T expansion, PHE885 expands predominantly in vivo. In conventional CAR-T manufacturing, common g-chain receptor cytokines, especially IL-7 and IL-15, promote ex-vivo expansion and long-term CAR-T persistence. Similarly, baseline cytokine levels may have an important role in the memory formation of post-infusion PHE885. Among common g-chain receptor cytokines, IL-7 levels at CAR-T infusion were significantly higher in patients with long-term CAR-T persistence (p = 0.03).

In summary, BCMA CAR-T cells manufactured using the T-Charge process exhibit robust expansion in vivo and long-term polyclonal persistence for >6 months in 48% of patients. Long-term persisting CAR-T cells remain functionally active against BCMA expressing target cells in vivo and in vitro, and the IL-7/IL-7R pathway appears to have an important role in promoting long-term persistence of memory CD4+ CAR-T cells.