Molecular and Phenotypic Profiling of Drug Product and Post-Infusion Samples from CRB-402, an Ongoing: Phase I Clinical Study of bb21217 a BCMA-Directed CAR T Cell Therapy

Introduction:

B-cell maturation antigen (BCMA) is primarily expressed by malignant and normal plasma cells, making it an attractive target for the treatment of multiple myeloma (MM). bb21217 is a BCMA-directed chimeric antigen receptor (CAR) T cell therapy that uses the same CAR molecule as idecabtagene vicleucel (ide-cel, bb2121), but adds the PI3K inhibitor bb007 during manufacturing to enrich the drug product (DP) for memory-like T cells, thereby reducing the proportion of highly differentiated or senescent T cells. We conducted correlative analyses to investigate the mechanistic hypothesis that CAR+ T cells with memory like phenotypes may persist and function longer, which may be one determinant of duration of response (DOR).

Methods:

An ongoing phase I clinical study (CRB-402; NCT03274219) is assessing safety and efficacy of bb21217 in relapsed/refractory MM patients. A total of 44 patients had PBMCs, collected from apheresis, and DP characterized by RNA sequencing (RNAseq) and mass Cytometry (CyTOF). The correlation of T cell phenotype with peak expansion, response and DOR per IMWG Uniform Response Criteria was explored. P-values were determined by Wilcoxon test, Spearman correlation, or Cox PH regression on DOR with categorical marker values (high/low).

Results:

In this patient population, substantial cross patient heterogeneity in T cell phenotypes was observed both in PBMCs and DP. Late differentiation/senescent markers in PBMCs were negatively correlated with clinical response. In particular, patients whose DP had higher expression of CD57 had lower peak expansion (p<0.0001), experienced more early relapse by month 6 (M6) (p<0.05) and had lower DOR (p<0.05) compared to those with lower CD57 expression. Paired analysis of PBMCs and DP demonstrated bb21217 DP is enriched for memory-like T cells (LEF-1+ (median increase 310%, p<0.0001), CD27+ (median increase 84.7%, p<0.0001), CCR7+ (median increase 188.3%, p<0.0001) and depleted of highly differentiated or senescent CD57+ T cells (median decrease 87.3%, p<0.0001), relative to PBMCs. As CAR+ T cell peak expansion is associated with initial clinical response, we investigated the relationship between peak expansion and DP phenotype. Expression of early memory T cell markers (eg, LEF1, CD27, CCR7) in DP were positively correlated with peak expansion, while markers of terminally differentiated effector cell markers in DP (eg, CD57, GZMA, GZMB) were negatively correlated with peak expansion. RNAseq also showed a significant enrichment in naïve/early memory gene signatures and a decrease in late differentiation gene signatures in DP from patients with high peak expansion (>2x10⁵copies/ug), consistent with the CYTOF findings. Early activation markers (CD38 p<0.001, IRF4 p<0.01) were expressed in the DP from patients with high peak expansion, suggesting that an early activation phenotype may contribute to robust expansion. We assessed the relationship between T cell memory markers in DP and sustained response by comparing DP from patients with or without progressive disease (PD) by M6. RNAseq analysis showed a significant enrichment for early memory (CCR7, p<0.05, SELL, p<0.05) and early activation (CD38 p<0.05, IRF4 p<0.05) T cell phenotypes and a significant reduction in late differentiation/senescence (CD57 p<0.05, GZMB p<0.01. KLRG1 p<0.01) T cell phenotypes in DP from patients without PD at M6. Gene set enrichment analysis showed enrichment of naïve/memory gene signatures in patients without PD at M6. ScRNAseq analyses from a subset of PBMC/DP as well as immunophenotyping data from clinical samples, including memory phenotypes, and their relationship to clinical outcome, will also be presented.

Conclusion:

As seen with other CAR T cell studies, the quality of incoming PBMCs, in particular the fraction of T cells with a late differentiation/senescent phenotype, influences initial and sustained clinical response. The analyses reported here support the mechanistic hypothesis of bb21217, suggesting the presence of early memory like T cells in PBMC and/or DP may contribute to high peak expansion and prolonged DOR, while presence of highly differentiated or senescent T cells may negatively impact these measures. Further clinical evaluation of bb21217 and robust correlative analyses will be important to help contextualize the influence of patient and product characteristics on clinical outcomes.