B Cell Maturation Antigen-Specific CAR T Cells for Relapsed or Refractory Multiple Myeloma: A Meta-Analysis

Background

Cellular immunotherapies represent an enormously promising strategy for relapsed/refractory multiple myeloma (RRMM). Chimeric antigen receptor (CAR) T cells targeting B cell maturation antigen (BCMA) have shown impressive results in early-phase clinical studies. Here, we summarize the current body of evidence on the role of anti-BCMA CAR T cell therapy for RRMM.

Methods

We performed a systematic literature review to identify all publicly available prospective studies. We searched Medline, Cochrane trials registry, and www.clinicaltrials.gov. To include the most recent evidence, meeting abstracts from international hematology congresses were added. A conventional meta-analysis was conducted using meta and metafor packages in R statistical software. Pooled event rates and 95% confidence intervals (CIs) were calculated using the inverse variance method within a random-effects framework. Main efficacy outcomes were overall response, complete response (CR), and minimal residual disease (MRD). Furthermore, relapse rates, progression-free survival, and overall survival were evaluated. In terms of safety, outcomes were cytokine release syndrome (CRS), neurotoxicity, and hematologic toxic effects.

Results

Fifteen studies comprising a total of 285 patients with heavily pretreated RRMM were included in quantitative analyses. Patients received a median of seven prior treatment lines (such as proteasome inhibitors, immunomodulatory drugs, monoclonal antibodies, stem cell transplantation) which included autologous stem cell transplantation in 90% of patients. The median age of patients was 59 years and median follow-up duration ranged from 1.1 to 11.3 months. Most studies used 4-1BB (or CD137), a member of the TNF receptor superfamily, as an activation-induced T-cell costimulatory molecule. Most studies used fludarabine and cyclophosphamide for lymphodepletion while one study used busulfan and cyclophosphamide and one study used cyclophosphamide only. Most studies used the former Lee criteria for CRS grading.

Anti-BCMA CAR T cells resulted in a pooled overall response of 82% (95% CI, 74-88%). The pooled proportion of CR in all evaluable patients was 36% (95% CI, 24-50%). Within responders, the pooled proportion of MRD negativity was 77% (95% CI, 67-85%). Higher dose levels of infused CAR⁺ cells were associated with higher overall response rates resulting in a pooled proportion of 88% (95% CI, 78-94%). In addition, peak CAR T cell expansion appeared to be associated with responses.The presence of high-risk cytogenetics appeared to be associated with lower overall response rates resulting in a pooled proportion of 68% (95% CI, 47-83%). The presence of extramedullary disease at time of infusion did not influence outcome and was associated with similar response rates compared with RRMM patients who did not have extramedullary disease, resulting in a pooled proportion of 78% (95% CI, 47-93%).

The pooled relapse rate of all responders was 45% (95% CI, 27-64%) and the median progression-free survival was 10 months. In terms of overall survival, pooled survival rates were 84% (95% CI, 60-95%) at last follow-up (median, 11 months).

In terms of safety, the pooled proportion of CRS of any grade was 69% (95% CI, 51-83%). Notably, the pooled proportions of CRS grades 3-4 and neurotoxicity were 15% (95% CI, 10-23%) and 18% (95% CI, 10-31%). Peak CAR T cell expansion appeared to be more likely in the setting of more severe CRS in three studies. Most hematologic toxic effects of grade 3 or higher were neutropenia (85%), thrombocytopenia (70%), and leukopenia (60%).

Conclusion

Anti-BCMA CAR T cells showed high response rates, even in high-risk features such as high-risk cytogenetics and extramedullary disease at time of CAR T cell infusion. Toxicity was manageable across all early-phase studies. However, almost half of the patients who achieved a response eventually relapsed. Larger studies with longer follow-up evaluating the association of response and survival are needed.