Intensity of Cyclophosphamide-Based Bridging Therapy Before Chimeric Antigen Receptor T Cell Therapy in Myeloma

Patients receiving autologous chimeric antigen receptor T cell (CAR-T) therapy for multiple myeloma (MM) may require bridging therapy (BT) before CAR-T infusion to maintain some level of disease control. Alkylators, such as cyclophosphamide (Cy), are often used in regimens, either in high-intensity regimens, such as modified hyperCVAD (cyclophosphamide, vincristine, doxorubicin, and dexamethasone), or once-weekly regimens, such as KCd (carfilzomib, cyclophosphamide, and dexamethasone). However, there is no consensus regarding the optimal BT alkylator dose intensity in MM. We performed a single-center analysis of all instances of BT before planned autologous CAR-T for MM during a 5-year period ending in April 2022. We classified bridging regimens into 3 cohorts: (1) hyperfractionated Cy (HyperCy) with inpatient Cy every 12 to 24 hours or as a continuous i.v. infusion; (2) less intensive Cy dosing (WeeklyCy), such as KCd; and (3) NonCy, in which no alkylators were used in BT. Demographic, disease-related, and treatment-related characteristics were collected for all patients. The 3 BT cohorts were compared using the Fisher exact test, Kruskal-Wallis test, and log-rank test, as appropriate. We identified 70 discrete BT instances among 64 unique patients, including 29 (41%) with HyperCy, 23 (33%) with WeeklyCy, and 18 (26%) with NonCy. The median total Cy dosing during BT in the 3 groups were 2100 mg/m², 615 mg/m², and 0 mg/m², respectively. Age, number of prior lines of therapy, triple-class refractory status, presence of high-risk cytogenetics, extramedullary disease, bone marrow plasma cell burden, involved free light chain (iFLC) kinetics before collection, and other measures of disease aggressiveness were comparable across the 3 cohorts. iFLC levels rose ≥25% and ≥100 mg/L during BT (approximating progressive disease) in comparable proportions (P = .25) among the cohorts: 52% for HyperCy, 39% for WeeklyCy, and 28% for NonCy. All BT instances without subsequent CAR-T were due to manufacturing failures. Among 61 instances of BT followed by CAR-T, vein-to-vein times were slightly longer (P = .03) with HyperCy (45 days) compared with WeeklyCy (39 days) and NonCy (46.5 days). Neutrophil recovery times were similar in the 3 cohorts, but platelet recovery took longer with HyperCy (64 days) compared with WeeklyCy (42 days) and NonCy (12 days). Progression-free survival was comparable among the cohorts, but median overall survival (OS) was not: 15.3 months with HyperCy, versus 30.0 months with WeeklyCy and not reached with NonCy. In our retrospective analysis of BT before CAR-T therapy in MM, HyperCy did not result in superior disease control than WeeklyCy despite a 3-fold higher dose of Cy. In contrast, HyperCy was associated with longer post-CAR-T platelet recovery and worse OS despite comparable measurements of disease aggressiveness and tumor burden. Study limitations include our small sample size, as well as confounding from gestalt markers of MM aggressiveness that might have led to poorer outcomes as well as physicians' decision to prescribe HyperCy. Given the rarity of objective disease responses to chemotherapy in relapsed/refractory MM, our analysis suggests that hyperfractionated Cy regimens do not outperform once-weekly Cy regimens for most patients who require BT before CAR-T therapy.