Effects of Prior Alkylating Therapies on Preinfusion Patient Characteristics and Starting Material for CAR T Cell Product Manufacturing in Late-Line Multiple Myeloma

Introduction: Identifying prior therapy exposures that affect the patient or their peripheral blood mononuclear cell (PBMC) material is one strategy to optimize outcomes to CAR T cell therapy. Alkylating agents commonly used in multiple myeloma management, such as cyclophosphamide, have been reported to impair the proliferative capacity of T lymphocytes and to blunt their functional activity (Ercolini et al. J Exp Med. 2005;201:1591; Banissi et al. Cancer Immunol Immunother. 2009;58:1627; Litterman et al. J Immunol. 2013;190:6259). In the pivotal phase 2 KarMMa trial (NCT03361748) investigating the BCMA-directed CAR T cell therapy idecabtagene vicleucel (ide-cel, bb2121) in triple-class exposed patients with RRMM, 80% of patients had a history of prior anticancer treatment with ≥1 alkylating agents. In this retrospective analysis, patient and PBMC characteristics associated with time from last dose of alkylating agent(s) until apheresis of PBMCs for CAR T cell manufacture were identified.

Methods: PBMCs isolated from patient apheresis material, which serves as starting material for CAR T cell manufacturing, were immunophenotyped by polychromatic flow cytometry for markers associated with T cell differentiation, memory, senescence, and exhaustion. Data from relevant prespecified clinical and exploratory endpoints were collected, and a novel implementation of left-censored time-to-event analysis (Ware et al. Biometrics. 1976;32:459) was used to identify statistically significant relationships between washout time after prior alkylator exposure (encompassing 14 individual drugs) and patient and PBMC variables. Dose intensity of prior alkylators was not considered due to sparse annotations in the patient histories. Optimal cutpoints were identified for each variable that maximized the proportional hazard of receiving an alkylator between patients above and below the cutpoint, and P values were adjusted for testing multiple cutpoints. Relationships were verified by nonparametric correlation, in which alkylator washout was encoded as 1/log(−washout).

Results: More recent exposure to an alkylating agent (after diagnosis but before apheresis) was associated with patients receiving more prior therapies per year to manage their disease (hazard ratio [HR]=2.63, ρ=−0.54, P<0.0001), having a lower body mass index (HR=0.93, ρ=0.27, P=0.0021), and having higher ferritin levels at baseline (log-scaled HR=1.33, ρ=−0.31, P=0.0004). Patients with more recent alkylator exposure also had fewer T cells in their PBMC material (HR=2.28, ρ=0.24, P=0.0068; Figure) along with more CD8⁺ T_EM (CCR7⁻/CD45RA⁻) and fewer CD8⁺ T_EMRA (CCR7⁻/CD45RA⁺) T cells (HR=1.02 and 0.98, ρ=−0.2 and 0.21, P=0.023 and 0.016, respectively). A 50% reduction in the median CD3⁺ T cell composition of patient PBMCs was detectable up to 9 months after the last dose of alkylator, relative to patients who never received this drug class. In a multivariate model evaluating the correlation between the T cell fraction in PBMCs and number of therapies per year and alkylator washout period, number of therapies per year did not significantly improve model performance compared with the null model including alkylator washout alone.

Conclusions: Associations between patient characteristics and alkylator washout suggest that patients who more recently received alkylating agents to manage their myeloma had a more aggressive disease course, having progressed more quickly through prior regimens, and had lower weight and elevated systemic inflammation. Although these factors suggest a suboptimal patient profile, the depletion of T cells by alkylator therapy may be especially disadvantageous for autologous CAR T cell therapies (Wang et al. Mol Ther Oncolytics. 2016;3:16015; Perica et al. Biol Blood Marrow Transplant. 2018;24:1135). Our analysis found that the use of alkylators prior to CAR T cell therapy exhibits a detrimental effect on the apheresis PBMC material up to 6-9 months after the last dose.