Sandahl, Tyler B.

Abstract Background The advent of T-cell redirecting bispecific antibodies (BsAb) therapies has led to deeper and durable responses in patients with RRMM. We sought to identify adverse prognostic factors for patients treated with BsAbs. We also explored whether the extent of the non-involved free light chain (niFLC) suppression could serve as a potential marker of T-cell engager activity, given the binding of BsAbs to both clonal and non-clonal plasma cells and the short half-life of serum FLCs. Methods We conducted a retrospective analysis of RRMM patients treated with BsAbs between 2022 and 2025 at Mayo Clinic. Baseline lymphocyte counts and ferritin levels were collected and analyzed both as continuous and dichotomized variables. niFLC was considered undetectable if it was below the quantification limit, while the magnitude of niFLC suppression was defined as the absolute difference between niFLC levels and the lower normal range threshold at the time of best response. In addition to standard statistical methods, interaction models were used to explore the potential effect modification of niFLC dynamics. Results A total of 158 patients were included (teclistamab, 73%; talquetamab, 15%; elranatamab, 12%). The median follow-up is 12 months (range 0 to 28 months). Median age is 71(range 65 to 77); 52% (n=85) were triple-class refractory (refractory to an IMiD, a PI, and CD38-antibody) and 37% (n=60) were penta-refractory. CAR-T therapy was previously administered in 28% (n = 46) patients, and 70% had undergone an autologous stem cell transplant. One or more high risk cytogenetic features including del(17p), gain(1q), amp(1q), del(1p), t(4;14), and t(14;16) were present in 57% (n = 90) of patients. Extramedullary disease was noted at the time of BsAb therapy in 44% of patients. Utilizing IMWG response criteria, 68% (n=107) achieved ≥partial response, and 64% (n=101) had ≥ very good partial response (VGPR) as best response following BsAb treatment. The median time to best response was 1.6 months (range 0.7 to 4.4). On multivariable analyses, baseline immunoparesis (HR 1.96; 95% CI, 1.08–3.58; p = 0.0274) and elevated ferritin levels (≥ 337 ng/mL) (HR 1.96; 95% CI, 1.24–3.09; p = 0.0041) were independent predictors of inferior progression-free survival (PFS). Lower baseline absolute lymphocyte count, analyzed as a continuous variable, was significantly associated with shorter PFS in the univariable analysis (HR 0.66; 95% CI, 0.44–0.98; p = 0.04), but not in the multivariable model (HR 0.75; 95% CI, 0.5–1.13; p = 0.166). Detectable niFLC at the time of best response was also significantly associated with inferior PFS (HR 1.80; 95% CI, 1.14–2.84; p = 0.0114). However, the magnitude of niFLC suppression did not show a statistically significant impact on PFS. The median PFS was 5 months in patients with high ferritin levels (≥ 337 ng/mL) and 21 months in those with lower ferritin (p = < 0.005). Similarly, PFS was 8 months in those with immunoparesis and not reached in those without baseline immunoparesis (p = 0.008). At the time of best response, detectable niFLC was associated with a trend toward shorter PFS (5 vs. 13 months), although the difference was not significant (p = 0.06). Logistic regression analysis supported these findings, with higher ferritin levels (OR 2.28; 95% CI: 1.09–4.96; p = 0.032) and the presence of baseline immunoparesis (OR 2.60; 95% CI: 1.18–6; p = 0.02) being significantly associated with higher likelihood of progression or death. Patients with deep reduction in niFLC achieved a VGPR or better. In contrast, those with detectable or higher-than-normal niFLC levels did worse in terms of response. Notably, niFLC became undetectable in 72% of patients with ≥VGPR (73/101), compared to 52% of those who did not achieve this level of response (30/57). Conclusion Among RRMM patients treated with BsAbs, baseline immunoparesis and elevated ferritin levels emerged as independent predictors of worse PFS. While failure to induce a deep reduction in niFLC at the time of best response suggests a tumor extrinsic mechanism of resistance, and it is also linked to inferior outcomes.

1652 Background: Tarlatamab is a bispecific T-cell engager targeting DLL3 that has shown promise as a therapy in small cell lung cancer. Administering tarlatamab requires careful consideration of immune-related adverse events (AE), including cytokine release syndrome (CRS) and immune effector cell-associated neurotoxicity syndrome (ICANS). Prescribing information recommends patients to be monitored for 22-24 hours in an appropriate healthcare setting for cycle 1 (C1), days 1 and 8. At Mayo Clinic in Rochester MN (MCR), tarlatamab is initially administered in a hospital-based outpatient (HBO) setting utilizing an innovative remote patient monitoring system (RPM). In this study, we describe our RPM process and patient outcomes thus far. Methods: We retrospectively reviewed records of patients treated with tarlatamab from August through December 2024. All patients received tarlatamab in the HBO setting and were enrolled into RPM for C1, days 1–10. Patients attended daily HBO visits on days 1–3 and 8–10 of C1 and were required to stay within 30 minutes of the hospital with a 24-hour caregiver for 48 hours following infusion days in C1-2. Data on patient characteristics, frequency of RPM alerts, escalations, rate of CRS & ICANS, and need for hospitalizations were reported using descriptive metrics. Results: As part of RPM, patients are provided with a kit containing preconnected Bluetooth-enabled devices to measure vital signs (VS), including blood pressure (BP), heart rate (HR), temperature (T), and pulse oximetry (SpO2). The kit also includes a cellular-enabled tablet for electronic ICANS questionnaires, which upload directly into our electronic medical record system. Patients are required to log VS four times daily, monitored in real-time by a centralized virtual RPM nursing (RN) team. Embedded decision trees trigger alerts, prompting the RN team to contact patients and follow care pathways for escalations as needed. Among the 16 patients treated, the median age was 68 years, and the median ECOG PS was 1. During the 10-day RPM period, a total of 2,233 VS entries were recorded. Of these, 88% (14/16) of patients had alerts triggered, with a median of 2 alerts per patient. Alerts were mostly for out-of-range systolic BP (56%), HR (56%), and T (25%), with none for SpO2. The RN team escalated alerts in 50% (8/16) of the patients. Overall, 38% (6/16) of patients were managed entirely in the HBO setting. Among the 63% hospitalized, 56% had CRS limited to grade (G) 2, and 31% had ICANS limited to G3. Two patients were hospitalized for other indications. The mean length of stay was 2.6 days, with no ICU admissions. Conclusions: Our experience shows that frequent monitoring required with tarlatamab can be safely and effectively executed in an outpatient setting with the utilization of an RPM system. It can potentially minimize the burden of hospitalizations for patients and the healthcare system.

PURPOSE Cytokine release syndrome (CRS) of any grade occurs in 56%-80% of bispecific antibodies (BsAbs) used in multiple myeloma (MM). Risk mitigation strategies are required to expedite escalation of care if toxicities develop. The rarity of grade 3 or 4 CRS compels protocolized outpatient management of BsAbs as outpatient practice may avoid hospital admissions and costly resource utilization without compromising safety. MATERIALS AND METHODS Patients with MM who received BsAb step-up dosing (SUD) from August 23, 2023, to March 29, 2024, were enrolled. Baseline demographics, patient outcomes, and CRS management were reviewed to assess the safety of outpatient practice. Resource utilization was also analyzed. RESULTS In this study, 34 patients received outpatient SUD of BsAbs (teclistamab n = 17, talequetamab n = 17) with 16 remaining outpatient throughout the entire SUD period. CRS occurred in seven patients who did not require hospitalization, demonstrating the safety of outpatient management of BsAb toxicity, regardless of low-grade CRS. CRS was observed in 24 patients (maximum grade 2) and immune effector cell–associated neurotoxicity syndrome in four patients (maximum grade 3). All patients with CRS received steroids, including 12 patients at home when instructed. Tocilizumab was given to 13 patients, accounting for 18 doses. If all doses of BsAbs and tocilizumab were given outpatient, the medication margin would be $115,004 in US dollars (USD). CONCLUSION An outpatient-based practice for BsAb administration demonstrated safety and cost savings. The description of our practice and results of this study provide valuable insights into the safety, feasibility, and resource stewardship of outpatient management of BsAbs. Future research should attempt to predict and stratify CRS risk to deliver a tailored supportive strategy and continued increase of outpatient management.

ABSTRACT Background and Hypothesis Teclistamab, a novel bispecific monoclonal antibody targeting CD3 and B cell maturation antigen (BCMA), and chimeric antigen receptor T cell (CAR-T) therapy are promising options for treating relapsed/refractory multiple myeloma (MM). However, the rates of acute kidney injury (AKI) associated with teclistamab remain inadequately characterized. This study aims to compare the incidence, severity, and outcomes of AKI between patients receiving teclistamab and CAR-T therapy. Methods This was a retrospective study involving 64 patients with relapsed/refractory MM treated with either teclistamab or CAR-T therapy. All patients had previously received at least four lines of chemotherapy before being treated with either teclistamab or CAR-T. The primary outcome was the incidence of AKI, and secondary outcomes included AKI severity, kidney recovery rates, and mortality. Kaplan–Meier estimates for AKI-free survival were calculated, and hazard ratios (HRs) for AKI risk were determined using Cox proportional hazards models. Results Sixty-four patients met inclusion criteria for this study (30 received CAR-T and 34 received teclistamab therapy). Among these patients, 14 AKI events occurred in total (22%), with 10 events (29%) in the teclistamab group and four events (13%) in the CAR-T group. AKI-free survival estimates at 180 days after treatment initiation were 68% [95% confidence interval (CI): 53%–87%] for teclistamab patients and 90% (95% CI: 79%–100%) for CAR-T patients. While patients receiving teclistamab were found to have an increased risk of an AKI event compared to those receiving CAR-T therapy, the results were not statistically significant [HR (95% CI): 3.38 (0.93–12.31), P = .065]. Conclusions This study suggests that patients treated with teclistamab may experience a higher incidence of AKI compared to those receiving CAR-T therapy. However, further research is required to determine whether this increased risk is attributable to disease progression or teclistamab itself. These results highlight the need for close kidney function monitoring in patients receiving teclistamab.

PURPOSE Teclistamab is initiated with a step-up dosing (SUD) schedule to mitigate the risk of cytokine release syndrome (CRS) and immune effector cell–associated neurotoxicity syndrome (ICANS). Early teclistamab users commonly received SUD in a hospital setting. This study aimed to evaluate safety and health care resource utilization (HRU) in real-world patients with multiple myeloma who initiated teclistamab SUD in an outpatient setting. METHODS This was a retrospective study using Mayo Clinic's electronic medical records from October 26, 2022, to October 31, 2023. Patient characteristics were summarized for all patients treated with teclistamab and separately for patients who started SUD outpatient. SUD pattern, safety, HRU, and post-SUD dosing schedule were described in patients with complete SUD. RESULTS At data cutoff, 65 patients received ≥1 teclistamab dose, including 58 patients who initiated SUD outpatient (median age, 69.2 years; male, 63.8%; White, 89.7%). Among 57 patients who completed SUD in an outpatient setting, all received premedications on the days of teclistamab administrations per label recommendation; 18 (31.6%) developed CRS (13 grade 1, four grade 2, and one grade 4) and two developed ICANS (one each with grade 2 and 4). All CRS and ICANS resolved with supportive care and all patients continued treatment. Eighteen patients were admitted to the hospital for CRS treatment, with a median CRS-related hospital stay of 2 days per admission. Most (60%) doses during SUD required <1 hour clinic time between administration and checkout. Post-SUD, clinic time for treatment doses decreased to <30 minutes for most doses (82%). CONCLUSION Outcomes of this study support outpatient administration as a safe and feasible option for teclistamab SUD to potentially reduce HRU and improve patient experiences.

Cytokine release syndrome (CRS) commonly occurs during initiation of bispecific T-cell engaging therapy (BsAbs) for multiple myeloma (MM) (NEJM 2022;387:495, NEJM 2022;387:2232, Nat Med 2023;29:2259). Pre-medications, step-up dosing (SUD), and monitoring minimize high-grade toxicity risk, while protocolized outpatient (OP) management of BsAbs and toxicities could reduce resource utilization and improve patient satisfaction. At Mayo Clinic, SUD, clinical assessment and grade 1 CRS management occurs primarily as an OP for patients with MM receiving BsAbs. This study aims to assess the current OP management practices of BsAbs and related toxicities including patient outcomes, admission rates, and resource utilization. Patients with MM who received SUD from August 23, 2023, to March 29, 2024, were included. Baseline demographics, patient outcomes, and CRS management details are summarized. Resource utilization outcomes included level of care, length of hospital stay (LOS), and high-cost drug administration. 38 patients received MM BsAbs during the study timeframe. 34 patients initiated OP SUD (teclistamab n=17 and talquetamab n= 17). The median age was 71 years, 59% were male, 82% were white, and 88% had Medicare coverage. Median number of prior lines of therapy was 6. There were 317 OP encounters with a median of 10 encounters per patient. Of these, 72% were planned in-person assessments, 5% were unplanned clinic visits for potential toxicity-related symptom assessment, and 20% were phone visits that obviated in-person assessment. 66 of 68 doses of talquetamab (total of 1,628 mg) and 48 of 50 doses of teclistamab (total of 2,048 mg) were administered and billed in the OP setting. 24 patients (71%) that initiated OP SUD developed CRS (grade 1 n=16, grade 2 n=7). Management of the initial CRS event included steroids and acetaminophen, with or without tocilizumab. 2,600 mg of tocilizumab administered in the OP setting. Recurrent CRS was seen in 5 patients and also treated entirely OP. 4 patients developed ICANS (grade 1 n=3, grade 3 n=1). Hospitalization following OP SUD initiation occurred in 18 (53%) patients at some point during treatment. Indications included grade 2 CRS (n=6), ICANS (grade 1 n=3), refractory CRS (n=7), infection (n=1), GI bleed (n=1), planned observation for comorbidities or prior CRS (n=2), loss of caregiver (n=1), atrial fibrillation (n=1), heart failure (n=1), and pain crisis (n=1). Median LOS was 2 days. Recurrent admission occurred in 41% and median LOS was also 2 days for subsequent admissions. Two patients were admitted to the intensive care unit for hypotension or arrhythmia. 1 patient required dialysis. In total for SUD during the study time frame, 51 mg of teclistamab, 176 mg of talquetamab, and 8,240 mg of tocilizumab were billed under an IP encounter, which likely was due to inpatient administration or hospital admission within 72 hours of receiving the OP SUD. This descriptive study of our OP BsAb management practice demonstrated that 47% of patients remained OP, even with a CRS event. The CRS rate did not surpass those in landmark clinical trials and aligns with real-world data previously reported (Blood Cancer J 2024;14:35.). Availability and use of patient monitoring capabilities along with lack of sentinel outcomes emphasizes the safety of an OP BsAb practice. Drivers of financial burdens and savings include location and reimbursement of high-cost drugs and admission rates. Median LOS was less than half of what's anticipated with inpatient SUD. Inpatient administration of high-cost drugs can be more costly compared to OP due to associated medical costs and insufficient reimbursement in a population primarily insured by Medicare which reimburses inpatient mediations based on the Diagnostic Related Group (DRG) unless the medication has New Technology Add on Payment (NTAP). Using expected Medicare reimbursement rates and wholesale acquisition costs while excluding variable DRG reimbursement and NTAP impacts for teclistamab, the total net revenue improvement for shifting use of all teclistamab, talquetamab, and tocilizumab OP is $115,004 for the period analyzed (CMS.gov 2024). In conclusion, OP management of patients undergoing SUD for MM BsAbs is safe and has substantial reductions in resource utilization and costs.

Background and Methods: IEC-HS is a hyperinflammatory state with high morbidity and mortality. Effective interventions to reverse the process early in the clinical course may be critical to improve outcomes. We previously examined cytokine differences between IEC-HS and other secondary hemophagocytic lymphohistiocytosis, and found significant differences in the patterns and outcomes.(1) This study examines cytokine profiles (CP) of patients treated with CAR-T from 9/2019 to 3/2024 who developed CRS, MAS-L and IEC-HS. Demographics and lab results were compared among the three groups and correlated with outcomes. Results: CP was tested in 113 of 366 CAR-T patients: 74 with CRS, 19 with MAS-L, and 20 with IEC-HS. The racial distribution was 93% White/Caucasian, 2% Black or African American, 1% Asian, 1% Native American and 3% other or unavailable. No differences were seen in demographics between groups except a higher percentage of patients with ECOG performance status ≥2 in IEC-HS (p=0.037). Incidence of IEC-HS by disease: 3.6% (7/193) in lymphoma, 5% (8/159) myeloma and 28.5% (4/14) B-acute lymphoblastic leukemia (p=0.0002). Incidence of IEC-HS by CAR-T product: 3% (4/150) in Axi-cel, 23% (6/26) in Brexu-cel, 8% (1/13) in Tisa-cel, 5% each (3/60) in Cilta-cel and Ide-cel, and 0/17 in Liso-cel (p=0.001). Prior to lymphodepletion, labs were already different among the CRS, MAS-L, and IEC-HS groups: hemoglobin (g/dL: 11.1, 9.6, 8.45); platelets (x109/L: 168, 116, 52.5); ferritin (µg/L: 115, 946, 1944). CP was started when patients needed a second dose of tocilizumab or concern for developing MAS-L/IEC-HS, and continued daily until clinical improvement or resolution of inflammatory state. Tocilizumab and dexamethasone use were similar across groups. Other therapies used were vasopressors (3%, 10.5%, 45%; p&lt;0.0001), Methylprednisolone (20%, 31.6%, 50%; p=0.027), Anakinra (12%, 63%, 85%; p&lt;0.0001), Siltuximab (9.5%, 16%, 30%; p=0.06), Basiliximab (30% of IEC-HS), and continuous veno-venous hemofiltration (30% of IEC-HS). Among the first CP, the CRS group had lower levels of cytokines (in pg/mL) compared to MAS-L and IEC-HS groups: TNF (31, 62.9, 47.7), IL-10 (29.3, 97.7, 169), IL-18 (794, 1353, 2011), IFN-gamma (74, 281, 250), MCP-1 (689, 2722, 2485.5), MIP-1 alpha (220, 234, 247.5), IL-6 (&gt;315: 45%, 79%, 70%), soluble IL-2R alpha (&gt;4000: 45%, 84%, 70%). The percentage of patients who received anakinra before CP1 were 4% (CRS), 21% (MAS-L) and 25% (IEC-HS) (p=0.007). IL-1 beta was below threshold among all patients who received anakinra in CRS and MAS-L and 75% in IEC-HS, suggesting rapid and effective IL-1 suppression by anakinra except in IEC-HS. Cytokines trends 24 hours and 48 hours after the first CP differed between MAS-L and IEC-HS. MAS-L had decreased TNF (62.9, 40.4, 18.5) and MCP-1 (2722, 355, 104) while these cytokines remained elevated or increased in IEC-HS (TNF: 47.7, 105.5, 62.6; MCP-1: 2486, 2276, 1585). IL-18 was minimally elevated in MAS-L (1353, 1955, 3068) and significantly increased in IEC-HS (2011, 2420, 4048). Notably, CP2 and CP3 levels of these cytokines in IEC-HS were statistically significantly higher than corresponding CPs among CRS and MAS-L. In addition, patients who died with persistent E. faecium bacteremia had higher IL-18 on CP3. Infections occurred in 43%, 58%, and 85% respectively (p=0.002), including E. faecium bacteremia (3%, 5%, 35%; p=0.0004), CMV viremia (30% of IEC-HS patients vs. 11% of non-IEC-HS; p=0.025), and invasive fungal infections (15% of IEC-HS patients; p=0.0001). Between the IEC-HS and non-IEC-HS groups, day 100 non-relapse mortality was 35% vs. 0 (p&lt;0.0001), day 100 all-cause mortality was 45% vs. 3% (p&lt;0.0001), and E. faecium bacteremia-related deaths were 50% vs. 1% (p&lt;0.0001). Conclusions: CRS, MAS-L andIEC-HS are known complications of CAR-T. Cytokine-directed therapy can modify the cytokine milieu and control the inflammatory process. However, excess immunosuppression may result in life-threatening infections. Further studies are needed to identify optimal biomarker-guided immunosuppressive strategies to control the inflammatory cascade, while avoiding excess immunosuppression. References: 1. Leung, Nelson, et al. “Outcomes of patients who received CAR T cell therapy and developed IEC-HS treated with cytokine directed therapy.” (2024): 7516-7516.

7542 Background: Teclistamab, a novel bispecific monoclonal antibody targeting CD3 and BCMA, exhibits promising effects for patients with relapsed and refractory multiple myeloma (MM). However, rates of acute kidney injury (AKI) associated with this therapy remains inadequately characterized. Methods: This was a retrospective observational cohort study of patients with relapsed refractory MM who received teclistamab or Chimeric Antigen Receptor T-cell Therapy (CAR-T) therapy at Mayo Clinic between 12/1/2022-5/15/2023 and who were not in kidney failure at the time of treatment. The primary endpoint was incidence of AKI during follow-up; secondary endpoints included treatment and recovery from AKI. AKI was defined based on Kidney Disease Improving Global Outcomes criteria as an increase of serum creatinine levels > 1.5 times their baseline value after starting therapy. Cumulative incidences and Cox proportional hazard regression models were used to evaluate time to event data. Results: A total of 64 patients met inclusion criteria for the study (30 received CAR-T and 34 received teclistamab therapy). There were 14 AKI events occurring during follow-up (10 teclistamab [n = 6 stage 1, n = 3 stage 2 and n = 1 stage 3] and 4 CAR-T [n = 3 stage 1 and n = 1 stage 2]). Cumulative incidence estimates of AKI at 120 days after treatment were 32% (95% CI: 14%-47%) for teclistamab patients and 10% (95% CI: 0%-21%) for CAR-T patients. While patients receiving teclistamab were found to be at increased risk of an incident AKI event compared to patients receiving CAR-T therapy, results were not statistically significant (HR (95% CI): 3.38 (0.93-12.31), p = 0.065). Median time to development of AKI was 30 days (IQR 16, 73) in teclistamab and 59 days (IQR 9, 146) in CAR-T patients. Among the 10 patients in the teclistamab group with an AKI event, 4 (40%) had complete renal recovery and 4 (40%) had partial recovery, while among the 4 patients with CAR-T with an AKI event, 2 (50%) experienced complete renal recovery and 2 (50%) experienced partial recovery. Conclusions: Our study showed that almost one third of patients receiving teclistamab therapy for relapsed and refractory multiple myeloma experienced an incident AKI event during follow-up. This finding underscores the importance of monitoring for kidney function in these patients.

7520 Background: There are 2 CAR-T products and 3 bispecific antibodies (BsAbs) that are FDA-approved for patients (pts) with relapsed, refractory multiple myeloma (MM). Clinical trials experience to date suggests that clinical response of BsAb is less impacted after CAR-T therapy. We examined the outcome of pts who received BsAb in standard-of-care (SOC) practice at Mayo Clinic Rochester. Methods: Retrospective analysis of MM patients who received FDA-approved BsAb at Mayo Clinic, Rochester between 12/2022- 1/2024. IMWG criteria was used for clinical response. Results: Sixty-two patients received BsAbs: 77% (48/62) received teclistamab, and 23% (14/62) received talquetamab. Median age was 62 years (range: 33-81), 60% (37/62) were males, and 45% (28/62) received prior CAR-T. Patients who had prior CAR-T therapy had more prior lines of therapy compared to those who did not. Overall, ORR to teclistamab was 61%, which is comparable to previous studies and other real-world reports. ORR and ≥CR rates were comparable between patients with and without prior CAR-T (Table). Of the 28 pts who received CAR-T before BsAbs, 61% (17/28) received idecabtagene vicleucel, 10% (3/28) were ciltacabtagene autoleucel, and 29% (8/28) on clinical trial. We then compared outcome of BsAb between pts who had relapsed from CAR-T within 1 year (n=21, median PFS: 8.84 months (range, 1.12, 10.84)) or after 1 year (n=7, median PFS: 16.62 months (range, 13.30, 36.20)). At the time of BsAb infusion, pts with who relapsed < 1 year vs ≥1 year post CAR-T had comparable blood counts, ALC, M-protein and involved FLC levels, CRP, ferritin and LDH. BsAb CR/sCR rate was significantly higher in pts with disease relapse ≥1 year compared to <1 year post CAR-T (57% vs 14%, p=0.02). Conclusions: Irrespective of prior CAR-T exposure, BsAb have clinical activity in SOC practice in pts with MM. Our preliminary data suggest that responses to BsAbs are better in pts with late compared to early relapse post CAR-T. Longer follow-up is needed to understand the duration of response. Larger, multi-center studies with longer follow-up will help identify factors that impact BsAb response post CAR-T.[Table: see text]

7516 Background: Immune effector cell hemophagocytic lymphohistiocytosis IEC-HS is a complication of CAR T cell therapy (CART). This study aims to look at the outcomes of CART patients who develop IEC-HS. Methods: Patients who developed IEC-HS after receiving CART and patients who developed IEC-HS without (nonCART) between December 2020 and March 2022 were included. Results: The 20 CART patients were older than the 25 nonCART patients. Peak ferritin, fibrinogen, CRP, serum creatinine and transaminases were similar between the 2 group. CART patients had lower white blood cell count and platelets and bilirubin than the nonCART patients. CART patients were more like to have markedly elevated IL-6 levels (> 315 mg/ml), higher IL-18, higher MCP-1, and suppressed IL-1β and GMCSF than nonCART patients (Table). Treatment of IEC-HS included tocilizumab (n = 19), anakinra (n = 18) siltuximab (n = 4) and basiliximab (n = 4), dexamethasone (n = 18) methylprednisolone (n = 6), etoposide (n = 1) and ruxolitinib (n = 1). A significant response defined by a 90% reduction or normalization of the cytokine was observed with IL-10 and IFN-ϒ in 92.3% and 84.6% of the CART patients respectively. An intermediate response was noted with TNF, IL-6, MCP-1 and MIP-1 in 38.4%, 30.8% and 61.6% and 38.5% of the patients respectively. No patient had a significant response in IL-2Rα or IL-1β and only 7.7% had a significant response in IL-18. Of the 5 patients who had a MIP-1 response, 4 were on basiliximab +/- high flow continuous venovenous hemofiltration and the only patient that had an IL-18 response was on both modalities. The 100-day mortality after IEC-HS in the CART patients was 40% and was associated with higher peak and lower best ferritin but not the percentage reduction (Table). Higher TNF (90.3 pg/ml vs 37.2 pg/ml, p = 0.05), markedly elevated levels of IL-6 (92% vs 50%, p = 0.03) and IL-10 (33% vs 0%) and an IL-6 response (44.4% vs 0%) were associated with survival at day 100. IL-6 responders had lower peak ferritin vs IL-6 nonresponders (14066 mcg/L vs 45051 mcg/L, p = 0.08). Enterococcus faecium bacteremia developed in 75% of the nonsurvivors vs 16.6% of the 100-day survivors (p = 0.009). Survivors cleared the enterococcus bacteremia within days while nonsurvivors had protracted bacteremia. Conclusions: IEC-HS is devastating complication of CART. Anti-cytokine therapy can reduce some cytokines and ferritin but overimmunosuppression may lead infectious complication, in particular with E. faecium. Better biomarkers are needed to finetune immunosuppressive therapy in order to avoid over-immunosuppression. [Table: see text]