Development of a Phase 1 Study Evaluating the Activity of Modular CAR T for Multiple Myeloma (MCARTY) Targeting BCMA and CD19 for Improved Persistence

INTRODUCTION: BCMA directed CAR T cell therapy for multiple myeloma (MM) shows promise, but very long-term survival may be limited. Notably, BCMA CAR T persistence in MM is shorter than that seen in with CD19 CAR T cells in B cell malignancies where long-term CD19 CAR T persistence is associated with durable responses. Shorter BCMA CAR T persistence may be explained by the lower surface density of BCMA on tumour compared with that of CD19. Alternatively, CD19 CAR T persistence may be driven by recognition of continuous emergence of newly differentiated B cells.

We sought to improve BCMA CAR T persistence by (a) developing a BCMA CAR with high sensitivity to BCMA, and (b) co-expression with a CD19 CAR. Notably, while CD19 is absent or expressed at low level on myeloma cells, CD19 may be expressed on a subpopulation of myeloma stem cells, providing an additional rationale for co-targeting CD19 in MM.

METHODS: We performed functional screening of several mid to high affinity BCMA binders in CAR format. We additionally explored whether a more stable CAR format would improve sensitivity. We explored co-expression of the new BCMA CAR with the CD19 CAR from obecabatagene autoleucel (also known as CAT19), which is known to drive long-term persistence. We also designed and started a clinical study (MCARTY, NCT04795882) testing the BCMA CAR alone or co-expressed with a CD19 CAR in patients with refractory Myeloma.

RESULTS:The binding affinity for BCMA was determined for 13 antibody clones from a rat library, before being assessed functionally in single chain variable fragment (scFv) and fragment antigen-binding (Fab) formats in a second generation (41bbζ) CAR backbone. Noting superior target binding, functional efficacy and minimal basal activation of effector cells expressing the D8 binder in Fab format we then assessed this construct against standard BCMA targeting CARs based on the bb2121 and LCAR-B38M binders used in Ide-Cel and Cilta-Cel respectively. Compared to these competitor CARs, D8 demonstrated improved sensitivity to BCMA with improved activation, target kill, and cytokine release in a series of cocultures with plate bound antigen and cell targets expressing low level BCMA.

Successful manufacture in an academic setting of dual targeting CAR T cells was achieved by double transduction of autologous CD4/CD8 selected T cells with separate lentiviral vectors titrated for equal transduction. Manufacture by CliniMACS Prodigy included AKT inhibitor VIII to reduce T cell differentiation and LentiBoost, a polaxamer-based transduction enhancer, in a protocol lasting 6-8 days. This resulted in 3 populations of CAR T cells (D8+CAT19-, D8-CAT19+, D8+CAT19+) which can be individually tracked in treated patients.

The MCARTY trial is a single centre, Phase 1 dose escalation trial uniquely designed with two cohorts to evaluate the safety of D8 CAR T cells (Cohort 1) and D8/CAT19 CAR T cells (Cohort 2) in patients with relapsed/refractory MM at a dose of 50 or 150x10^6 CARs. Inclusion criteria are: progression or stable disease to last treatment, 3 or more previous lines of therapy, exposure to proteasome inhibitors (PI), immunomodulatory drugs (IMID) and CD38 antibodies, ECOG 0/1. There was no selection based on previous exposure to BCMA targeting agents or tumour BCMA expression.

As of 11 July, 8 patients have been treated, 6 patients with D8 CAR alone (n=3 at each dose level) and 2 patients with the lower dose of dual targeting D8/CAT19 CAR T cells. Median age was 57 (range 33-66), high risk cytogenetics in 4 of 7 patients (57%), previous lines of therapy 4 (range 3-8).

Doses were well tolerated with no cases of ICANS or CRS≥ grade 3. At a median follow up of 4.5 months (range 1-12), clinical responses ≥PR were seen in all patients (ORR: 100%). Of 6 patients, 5 (83%) CR/sCR, 1 (17%) PR at 3 months. We observed robust expansion of D8 CAR in Cohort 1, and of dual and single targeting CARs in Cohort 2.

CONCLUSION: We report a unique approach to prolong CAR persistence in MM by developing a CAR T cell co-expressing a sensitive BCMA CAR construct and CD19 CAR with proven durability. We report feasibility of double transduction, the first clinical use of a FabCAR and a unique trial design allowing comparison of treatment with single vs dual targeting CAR constructs.