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7507 Background: CARTITUDE-1 evaluated cilta-cel in pts with heavily pretreated RRMM who historically have an expected median progression-free survival (PFS) of <6 months (mo) and median overall survival (OS) of ~1 year (y). At 33.4 mo median follow-up, median PFS was 34.9 mo, and median OS was not reached (36-mo OS rate, 62.9%; Lin et al, ASCO 2023). We report OS, ≥5 y progression-free outcomes, and safety with a median study follow-up of 60.3 mo. Methods: Pts in CARTITUDE-1 received a single cilta-cel infusion. Correlative analyses were performed utilizing drug product, baseline, and postinfusion samples. Pts are followed for progression, survival, and safety in a 15-y follow-up study, CARTinue (NCT05201781), with pt evaluations per local standard of care (reported annually at a minimum). Results: Of 97 pts treated, 32 (33.0%) remain alive and progression free for ≥5 y after cilta-cel, without further MM treatment. For these 32 pts, prior to enrollment in CARTITUDE-1, median time from start of last line of therapy (LOT) to progression was 4.0 mo (range, 0.7–48.6). Among the 32, median age was 60 y (range, 43–78), median number of prior LOT was 6.5 (range, 3–14), 23.3% had high risk cytogenetics, 12.5% had extramedullary disease (EMD), 90.6% were triple-class refractory, and 46.9% were penta-drug refractory. Baseline characteristics of pts who were progression free for ≥5 y, including those with high-risk cytogenetics and EMD, were comparable to pts with progressive disease (PD) within 5 y. Compared with pts who had PD within 5 y, biomarkers significantly associated with ≥5 y progression free status included a higher fraction of naïve T cells in the drug product, lower neutrophil to T cell ratio, higher hemoglobin and platelets at baseline, and higher effector-to-target ratio (Cmax to sBCMA at baseline). At Cmax, these pts also had significantly higher CD4 central memory CAR+ T cell subsets and CAR+ T cells that were positive for the activation markers CD38, CD25, and PD-1. Data were collected on a subset of pts from a single center where local serial MRD assessments were performed. All 12 pts at this center who were progression free for ≥5 y were MRD negative at 10 –6 and imaging negative by PET/CT yearly for 5 y. Overall, at 60.3 mo median follow-up in CARTITUDE-1 (N=97), median OS was 60.6 mo (95% CI, 41.9–NE). With continued follow-up, 3 additional pts reported a second primary malignancy (1 of which was acute myeloid leukemia; onset, 2.8 y after infusion). No new cases of movement and neurocognitive disorders were reported. Conclusions: The median OS for pts enrolled in CARTITUDE-1 was 5 y, and 33% of pts remain progression free for ≥5 y following a single cilta-cel infusion. These data provide the first evidence that cilta-cel is potentially curative in pts with RRMM. Clinical trial information: NCT03548207 , NCT05201781 .

5064 Background: FoundationOneCDx (F1CDx) supports two FDA-approved biomarkers to guide treatment decisions for ICI for patients with mCRPC: MSI status and TMB. MSI-H and TMB-H (10+ mut/MB) have strongly overlapping prevalence. We sought to better characterize ICI outcome associations of TMB-H / non-MSI-H population and relative effectiveness of taxanes & ICI among patients who received these agents in sequence. Methods: Following a prespecified analysis plan, this study used the nationwide (US-based) de-identified Flatiron Health-Foundation Medicine mCRPC clinico-genomic database (FH-FMI CGDB), with data originating from ~280 US cancer clinics (~800 sites of care). Inclusion criteria included patients with mCRPC treated with single-agent anti-PD(L)1 therapy in the FH network between 1/1/2011 - 3/30/2024. This study used the MSI and TMB algorithms from the tissue based F1CDx. Time to next treatment (TTNT) and OS were assessed with Kaplan-Meier plots and in multivariable Cox models adjusted for ECOG performance score, socioeconomic status, prior treatment history, and baseline PSA. Among patients who received taxanes in a prior line of therapy, the effectiveness (TTNT1 vs. TTNT2) of taxane and ICI were compared. Results: Among 2995 prostate cancer tissue specimens in the database, 95 (3.1%) were MSI-H and 142 (4.7%) were TMB-H. 94 (3.1%) were MSI-H & TMB-H, 1 was MSI-H & TMB-L, and 48 (1.6%) were TMB-H & not MSI-H. Among these, 84 patients with mCRPC were treated with ICI and met inclusion criteria, including MSI-H & TMB-H (n = 30), non-MSI-H & TMB-H (n = 8), and non-MSI-H and TMB-L (n = 46). The respective median TTNT on ICI was 8.0 vs. 9.6 vs. 3 months. The respective median OS from initiation of ICI was 10.9 vs. not reached vs. 4.4 months. In multivariable models evaluating ICI only, compared to non-MSI-H & TMB-L, the MSI-H & TMB-H group had more favorable TTNT (HR: 0.20, 95%CI: 0.10 – 0.41, p < 0.001) and OS (HR: 0.33, 95%CI: 0.16 – 0.70, p = 0.004), and the non-MSI-H & TMB-H group also had more favorable TTNT (HR: 0.13, 95%CI: 0.04 – 0.45, p = 0.002) and OS (HR: 0.20, 95%CI: 0.05 – 0.75, p = 0.017). 50 of the 84 (60%) patients treated with ICI had prior mCRPC taxane treatment. Better TTNT2 on subsequent ICI vs. prior taxane was observed for MSI-H (HR: 0.49, 95%CI: 0.23 – 1.01, p = 0.051) and TMB-H (0.54, 95%CI: 0.30 – 0.98, p = 0.044), but the opposite was true for non-MSI-H and TMB-L subgroups, with significant treatment interactions for each (p = 0.0018, p = 0.00052). Conclusions: TMB-H (4.7%) is more prevalent than MSI-H (3.1%) by F1CDx in mCRPC. Non-MSI-H / TMB-H (1.6%) in the routine practice cohort have similar outcome associations on ICI to MSI-H. Both MSI-H and TMB-H by F1CDx are predictive of differential benefit for ICI vs. taxanes in later mCRPC treatment lines.

e15003 Background: Antibody-drug conjugates (ADCs) have shown substantial promise as therapies for metastatic breast cancer (mBC). However, direct comparisons of different ADC-based regimens are limited. We assessed survival outcomes in mBC patients treated with ADCs. Methods: Randomized controlled trials (RCTs) published before October 2024 were searched in Medline, Embase, ClinicalTrials.gov, and Cochrane Library. Progression-free survival (PFS) and overall surcvival (OS) outcomes were extracted from the most updated publications. Participants were stratified based on molecular subtypes, hormone receptor (HR) status, duration of cyclin-dependent kinase 4/6 inhibitor (CDK4/6i) treatment, and sensitivity to endocrine therapy (ET). Bayesian frameworks were deployed, presenting the synthesized outcomes of indirect comparisons using the surface under the cumulative ranking curve (SUCRA), hazard ratio (HR), and 95% credible interval (Crl). This study was prospectively registered on the International Platform of Registered Systemic Reviews and Meta-analysis Protocols (INPLASY) (INPLASY2024110021). Results: A total of 6449 patients from 12 RCTs of ADCs ado-trastuzumab emtansine (T-DM1), sacituzumab govitecan (SG), trastuzumab deruxtecan (T-DXd), and datopotamab deruxtecan (Dato-DXd) were included in the analysis. Compared to chemotherapy, T-DXd significantly improved PFS (HR 0.35, 95% Crl 0.15-0.84) and OS (HR 0.56, 95% Crl 0.32-1.0). SG also significantly extended OS (HR 0.65, 95% Crl 0.43-0.97) but not PFS. In HER2+ mBC, pooled analyses revealed T-DXd was the most effective treatment in terms of PFS, with a SUCRA of 94.2%, followed by T-DM1 (58.9%), and chemotherapy with HER2-directed therapy (34.2%). Synthesized outcomes of OS mirrored the PFS outcomes. In HR+/HER2- mBC (some of which was HER2-low), Dato-DXd (76.1%) showed slightly superior efficacy over SG (67.4%) for PFS, while SG (85.1%) was advantageous over Dato-DXd (49.1%) in terms of OS. Among patients with HR+/HER2+ mBC, T-DXd (80.0%), SG (55.5%), and chemotherapy with HER2-directed therapy (53.1%) were the most effective therapies for prolonging PFS. Regarding OS, SG (70.3%) and T-DM1 (60.3%) were the most effective, with comparable efficacy observed between chemotherapy with HER2-directed therapy (52.1%) and T-DXd (52.0%). In HR-/HER2+ mBC, T-DXd (78.1%), SG (56.1%), and T-DM1 (51.5%) significantly prolonged PFS, while SG (69.4%) had the most substantial benefit for OS, followed by T-DXd (62.8%) and chemotherapy with HER2-directed therapy (57.6%). Dato-DXd showed an advantage over SG in mBC patients received CDK4/6i for > 12 months. Conclusions: This network meta-analysis showed prominent survival benefits with ADC-containing regimens for mBC. Our results provide valuable insights into personalized treatment strategies for mBC patients.

9002 Background: The field of AI is rapidly evolving. With recent development of more user-friendly AI software, its integration into healthcare across different medical specialties, including HemOnc has emerged. Data detailing the perception and concerns of the HemOnc workforce about AI’s roles is lacking and is needed if AI is to be integrated in HemOnc CliPr and MedEd. Methods: Questionnaires about the perception and concerns regarding AI in CliPr and MedEd were created on REDCap and approved by IRB. We surveyed the entire HemOnc workforce at the 3 Mayo Clinic major sites between 11/07/24 and 01/20/25, including physicians, both faculty (FAC) and fellows (FEL), advanced practice providers (APP), and nurses (RN). Participation was voluntary. Simple statistical analyses were employed for the results. Results: 344 participants (PTP) responded to the survey, 118 physicians (41 FEL and 77 FAC), 49 APP, and 177 RN. 64% of PTP report having used AI but only 31% used it in HemOnc MedEd, and 28% in HemOnc CliPr, with 67% considering themselves to have little to no knowledge about AI. 94% of PTP believe AI will be integrated in HemOnc MedEd, with such integration being seen as beneficial by 90%. Among physicians, 85% of FEL and 92% of FAC report that fellowship programs should incorporate AI training into curricula. 95% of PTP believe AI will be incorporated into HemOnc CliPr. Meanwhile, 62% of PTP (50.8% of physicians, and 68% of APP and RN) are concerned about risks it may pose to CliPr. Nevertheless, 90% of PTP would embrace AI’s use in HemOnc. Table 1 details the perceptions and concerns about AI in HemOnc MedEd and CliPr. The main perceived risks are decreased time spent with patients (pts), and inaccuracies or worse pt care. Meanwhile, 33% believe AI would increase efficiency and quality of care, while 30% believe it would increase time spent with pts. 14% of PTP worry their role could be replaced by AI. Conclusions: To the best of our knowledge, this is the largest assessment of HemOnc workforce’s perception and concerns about AI. Based on our survey, the HemOnc workforce, in general, endorses AI use, but with some concerns raised. While incorporation in MedEd is perceived with excitement, many envision the use in CliPr involves risks and challenges. Proper systematic education of the workforce about AI, with well-designed CliPr integration methods are needed to mitigate the legitimate existing concerns. Perceptions & concerns about AI in HemOnc MedEd & CliPr. FEL % APP % RN % FAC % AI will assist documentation in CliPr 98 82 79 92 AI will be used in pt communication 73 80 78 87 AI will screen pts for clinical trial enrollment 85 86 76 96 AI will increase the efficiency and quality of CliPr 43 37 24 46 AI will increase time spent with pts 39 27 25 38 AI will decrease time spent with pts 29 41 48 23 AI will lead to inaccuracies or worse CliPr 12 20 25 20 AI may replace the HemOnc workforce 11 16 19 5

4119 Background: TP53 mutations are frequent genetic alterations in cancers and also prevalent in biliary tract cancer (BTC). In non-small cell lung cancer, TP53 mutations have been reported to enhance immune checkpoint inhibitor (ICI) efficacy and improve outcomes. However, their impact on BTC remains unclear. This study investigated the clinico-molecular characteristics, prognosis, and ICI efficacy in TP53 -mutated BTC. Methods: Patient data were collected from the SCRUM-Japan GOZILA and MONSTAR-SCREEN-1/2 databases (Japan) and the Duke university and Mayo clinic databases (U.S.), which included patients receiving first-line ICI therapy. Comparisons between TP53 wild-type (WT) and mutation groups were performed in the Japanese cohort. Subsequently, analyses were expanded to include U.S. patients, focusing on comparisons between non-ICI and ICI groups. Tumor Immune Dysfunction and Exclusion (TIDE) scores were calculated using MONSTAR-SCREEN-2 whole transcriptome sequencing (WTS) data to evaluate ICI responsiveness, and gene set enrichment analysis (GSEA) explored pathways associated with TP53 mutations. Results: In the Japanese cohort (n=594), TP53 mutations were identified in 311 patients (52.4%). KRAS , ERBB2 , CDKN2A , and SMAD4 alterations were more frequent in the TP53 mutation group, while IDH1, BAP1 , and FGFR2 fusions were more common in the WT group. Multivariable analysis showed TP53 mutations were independent prognostic factors for poor outcomes (progression-free survival [PFS]: HR 1.37, 95% CI: 1.11–1.69, P=0.003; overall survival [OS]: HR 1.36, 95% CI: 1.06–1.75, P=0.017). Incorporating the U.S. cohort (n=625; non-ICI: 548; ICI: 77), no significant differences in overall response rate (ORR) or disease control rate (DCR) were observed in the non-ICI group between the groups. However, in the ICI group, TP53 mutations tended to show higher ORR (21.1% vs. 38.5%, P=0.16) and DCR (47.4% vs. 66.7%, P=0.14). In the non-ICI group, TP53 mutations were associated with worse PFS (HR 1.46, median PFS: 7.4 vs. 5.3 months, P<0.001), whereas, in the ICI group, no significant difference was observed; however, a trend toward better PFS was seen in the TP53 mutation group (median PFS: 5.0 vs. 6.4 months, P=0.163). TIDE scores were significantly lower in the TP53 mutation group, indicating higher ICI responsiveness. GSEA revealed enrichment of oxidative phosphorylation and hypoxia pathways. Conclusions: TP53 mutations are independent prognostic factors for poor outcomes in BTC. However, patients with TP53 mutations may derive greater benefits from ICIs, underscoring the importance of tailored therapeutic strategies in TP53- mutated BTC.

7032 Background: The impact of immune cell counts pre LD and during immune reconstitution on CAR T therapy outcomes is poorly understood. We investigated the association between CD4, CD8 and NK cell counts and CAR T therapy outcomes in patients with LBCL. Methods: Retrospective study of R/R LBCL patients who received CAR T cells between 2016-24 at Mayo Clinic, Rochester, were included in this analysis. Peripheral blood CD4, CD8 and NK counts were measured pre LD and at M1 post CAR T infusion. The receiver operating characteristic (ROC) curve was used to determine the optimal cutoff for pre LD and M1 immune cells to predict patients who were alive and in remission at 6 months (M6) post CAR T infusion. Patients who progressed or were lost to follow up prior to day 30 were excluded from M1 ROC analysis. Results: Of 140 patients, 81 were alive and in remission at M6 (Group A), while 59 had relapse or death (Group B). Axicabtagene ciloleucel was the CAR T product given in 81% of patients (114/140). Median pre LD CD4 counts were significantly lower in Group B compared to group A (143 vs 280 cells/µL, p = 0.001). No significant difference was observed in median pre LD CD8 counts (205 vs 221 cells/µL, p = 0.8). ROC analysis identified an optimal pre LD CD4 count of 124.5 cells/µL for M6 alive+remission. Lower pre LD CD4 counts (<124.5) predicted worse progression free survival (PFS) in univariate (HR = 3.02, 95% confidence interval [CI]: 1.73–5.27, p< 0.01) and multivariable analysis (MVA) adjusted for IPI and the number of prior lines (aHR = 2.54, 95% CI: 1.39–4.62, p< 0.01).Lower pre LD CD4 counts also predicted inferior overall survival (OS) in MVA (aHR = 2.27, 95% CI: 1.08–4.77, p = 0.03). (Table 1) On day 30 landmark analysis, ROC identified M1 optimal CD4 count of ≥ 99.5 cells/µL to be associated with a trend toward superior PFS (P=0.09), but not OS (p=0.90) in MVA. Median pre LD NK cell counts were significantly lower in Group B (73 vs 98 cells/µL, p = 0.04). ROC analysis identified an optimal pre LD NK count of 151 cells/µL for M6 alive + remission. Lower pre LD NK counts (<151) predicted worse PFS, both on univariate (HR = 4.17, 95% CI: 1.29–13.47, p = 0.02) and MVA (aHR = 4.64, p< 0.01). The lower pre LD NK cell count group had worse OS in MVA (aHR = 5.69, 95% CI: 1.16–28.1, p = 0.01). (Table 1) On D30 landmark analysis, M1 NK cell count of ≥ 128.5 cells/µL was associated with a trend toward superior PFS (P=0.08), but not OS (p=0.30) in MVA. Conclusions: Pre LD CD4 and NK cell counts are significantly associated with PFS and OS post CAR T therapy in patients with R/R LBCL. Pre-treatment immune subset levels may identify patients at higher risk of relapse or death after CAR-T therapy. Groups Median PFS 2 Year PFS Median OS 2 Year OS CD4 ≥124.5 cells/µL (N=75) NR 58% 3.31 years 67% CD4 <124.5 cells/µL (N=30) 2.2 months 22% 1.59 years 45% NK ≥151 cells/µL (N=17) NR 77% NR 84% NK <151 cells/µL (N=82) 10.6 months 44% 3.31 years 62%

7023 Background: Axi-cel is an autologous chimeric antigen receptor (CAR) T-cell therapy approved for adults with R/R LBCL after ≥ 1 prior line of therapy (LoT). Adverse events, such as cytokine release syndrome (CRS) and neurologic events (NEs), may deter centers from using axi-cel in an outpatient (OPT) setting, though individual centers have observed comparable safety and effectiveness in OPT and inpatient (IPT) care (Furqan et al. Blood Adv . 2024). Here, we present safety and effectiveness outcomes of axi-cel by intention to treat in OPT and IPT settings in a multicenter real-world dataset. Methods: Pts receiving axi-cel for R/R LBCL in the US from 01/2021-07/2023 with data in the Center for International Blood and Marrow Transplant Research (CIBMTR) registry were eligible for analysis. Pts with prior allogeneic transplant or unknown intended care setting were excluded. Of potential pts, 119 OPT pts were identified from 29 centers where an increasing trend was seen (9.6% of pts in 2021 were OPT, 13.5% in 2022, 22.8% in 2023). Pts were matched to 119 IPT pts by propensity score matching on age, sex, comorbidities, lactate dehydrogenase (LDH), bulky disease, prior LoT, chemosensitivity, and infusion year (see table). Results: OPT pts had median age of 63 y (25% ≥ 70), 66% were male, and 67% had ≥ 1 comorbidity. Half (50%) had elevated LDH and 73% had 1 prior LoT. Bulky disease was reported in 3%, and 60% had chemo-resistant disease. Outcomes were analyzed at median follow-up of 12 mo. Safety and effectiveness outcomes were similar between OPT and IPT pts (see table). In multivariate analyses, no differences were found between intended care setting and CRS (odds ratio [OR] 1.09 [95% CI 0.51-2.35]), CRS Gr ≥ 3 (OR 0.57 [0.12-2.60]), NEs (OR 1.14 [0.65-2.00]), or NEs Gr ≥ 3 (OR 0.98 [0.48-2.00]). Among OPT pts, 24% and 50% did not require hospital admission within 30 d and 3 d, respectively. In pts aged ≥ 70 y, only any Gr NEs were higher in the OPT group. Conclusions: After matching on key factors that may be used to select pts for OPT infusion, outcomes were comparable between intended care settings. These findings corroborate prior results and support the consideration of axi-cel in appropriate OPT care settings. Outcomes between matched pts with R/R LBCL receiving axi-cel intended for OPT or IPT. OPT (n=119) a IPT (n=119) a CRS any Gr / Gr ≥ 3 83 (75−89) / 3 (<1−7) 83 (74−89) / 4 (1−10) NE any Gr / Gr ≥ 3 47 (38−57) / 19 (12−27) 46 (37−56) / 21 (14−30) Overall / complete response rates 78 (69−85) / 68 (59−76) 76 (67−83) / 62 (52-70) Duration of response @ 12 mo 64 (52−74) 69 (58−78) Progression-free survival @ 12 mo 53 (43−62) 53 (43−61) Overall survival @ 12 mo 71 (61−78) 72 (62−79) Non-relapse mortality @ 12 mo 6 (2−11) 4 (1−8) Hospital admission within 30 days of infusion / median duration (range), d 76 (67−83) / 9 (2−53) Not Applicable a Percent (95% CI) unless otherwise specified.

7014 Background: Mosun is a CD20xCD3 bispecific antibody that can be administered in the outpatient setting for a fixed duration. Intravenous Mosun is approved for the treatment of relapsed/refractory FL after ≥2 prior lines of therapy. Mosun SC achieved high response rates with manageable safety in patients with 3L+ FL in a pivotal Phase II study (Bartlett et al. ASH 2024). We report the efficacy and safety of Mosun SC in patients with previously untreated high-tumor burden FL in the Phase II MorningSun study (NCT05207670). Methods: Patients with previously untreated high-tumor burden FL, per GELF criteria, were enrolled into this cohort. Mosun SC was administered with step-up dosing in Cycle (C)1 (Day [D]1, 5mg; D8, 45mg; D15, 45mg) then 45mg on D1 for up to 17 cycles (1 year; 21-day cycles). Patients with a partial or complete metabolic response (CMR) after C17 could receive additional Mosun maintenance therapy (45mg every 8 weeks for up to 1 year). Corticosteroid prophylaxis to reduce the risk of cytokine release syndrome (CRS) was mandatory in C1–2 and optional thereafter. The primary endpoint was progression-free survival (PFS) rate at 24 months. Key secondary endpoints included objective response rate (ORR), time to response (TTR), and safety. Results: As of May 29, 2024, 102 patients were enrolled; 55 patients had completed initial treatment (17 cycles), 31 had discontinued (most commonly due to progressive disease [n=19] and adverse events [AEs; n=4]), and 16 were ongoing initial treatment. Forty-two patients received maintenance treatment, and this was ongoing in 38 patients. Median age was 65 years (range: 24–86); 52.0% of patients were female. Most patients had Ann Arbor stage III/IV (91.2%) and a FLIPI score ≥2 (78.4%). Median duration of follow-up was 13.9 months. The 12-month PFS rate was 82.8% (95% confidence interval [CI]: 73.0–89.3). ORR was 87.3%; 60.8% of patients had a CMR. Among the 89 patients with a response, the median TTR was 2.7 months (range: 1.2–5.8). The most common AEs (≥30%) were injection-site reaction (57.8%), fatigue (42.2%), CRS (34.3%), headache (31.4%), and nausea (30.4%). Grade ≥3 AEs and serious AEs (SAEs) were reported in 44.1% and 29.4% of patients, respectively. CRS events were all Grade 1/2 and SAE of CRS occurred in 10.8% of patients (Table); all events resolved. Conclusions: Mosun SC demonstrated promising efficacy in patients with previously untreated high-tumor burden FL. The manageable safety profile, including rate of CRS, supports the administration of fixed duration Mosun SC in an outpatient setting. Additional data, including cytokine profiling and T/B/NK dynamics, will be presented. Clinical trial information: NCT05207670 . CRS events, n (%) PatientsN=102 Any grade 35 (34.3) Grade12 30 (29.4)5 (4.9) SAE 11 (10.8) ManagementSteroidsTocilizumabSteroids + tocilizumabFluidsICU admission 8 (7.8)6 (5.9)3 (2.9)3 (2.9)2 (2.0)

e20120 Background: In May 2024, the FDA approved tarlatamab, a Delta-like ligand (DLL3)/CD3-targeted bispecific T-cell engager, for patients with extensive-stage small cell lung cancer (SCLC) with disease progression following platinum-based chemotherapy and at least one other prior line of therapy. We aim to evaluate the real-world outcomes of agent. Methods: We conducted a retrospective multi-site single center analysis of adult patients who received tarlatamab at Mayo clinic sites including Arizona, Florida, Iowa, Minnesota, and Wisconsin. Data analysis was performed with Blue Sky statistics version 10.4. Results: Twenty patients with a median age 67 years (range 43-76) were included. Patients were predominantly women (55%) and all had a history of tobacco use. Metastatic disease commonly involved liver (55%) and CNS (60%). Majority (90%) had received prior PDL-1 therapy. 25% (5/20) of patients received prior gamma knife and in addition 10% (2/20) received whole brain radiation treatment in our cohort. Median time from CNS radiation and tarlatamab initiation was 4 months (range 3-28). Other characteristics, outcomes, and toxicity are presented in Table 1. Cytokine release syndrome (CRS) Grade (G) 1 was observed in 45% patients. G 2, 3 and 4 CRS were observed in 5% patients respectively. G 1 CRS was managed with acetaminophen initially followed dexamethasone 10mg or methylprednisone 1g if no improvement in 24 hours. Tocilizumab was used 20% patients for G 1,2,3 and 4 CRS depending on clinical course. Immune effector cell-associated neurotoxicity (ICANS) G 1 occurred in 10% patients, followed by G 2 in 15% and G 3 in 5% patients. ICANS was managed in a multidisciplinary fashion. All patients had resolution of CRS and ICANS. Conclusions: In this study investigating the real-world outcomes and toxicity of tarlatamab, a higher incidence of ICANS was observed than that reported in the DeLLphi-301 trial. The association between presence of CNS metastatic disease, prior CNS radiation and incidence of ICANS needs to be explored further. Mayo Clinic report of tarlatamab real world outcomes and toxicity. Patient Characteristics, Tarlatamab Outcomes and Toxicity Mayo Clinic Report DeLLphi-301 trial- Part 1 & 2 DeLLphi-301 trial- Part 3 ECOG no. (%) 2-4 4 (20) 0 (0) 0 (0) Metastatic Disease no.(%) 19 (95) 98 (98) 32 (94) Median no. of previous lines of therapy (range) 3 (2-4) 2 (1-6) 2 (2-6) Objective Response (%) 30% 40% CRS no (%) 12 (60) 49 (49) 19 (56) ICANS no (%) 6 (30) 7 (7) 4 (12) Neutropenia Grade 3 no (%) 2 (10) 6 (6) 2 (6) Fatal CRS, ICANS or Neutropenia 0 (0) 0 (0) 0 (0)

e19008 Background: Cytokine release syndrome (CRS) is typically reversible in CAR-T therapy, while hyperinflammatory syndromes like macrophage activation-like syndrome (MAS-L) and high-grade immune effector cell-associated hemophagocytic syndrome (IEC-HS) are associated with significant morbidity and mortality. Early interventions are critical for improving outcomes. We aim to analyze clinical cytokine assays and cellular phenotypes early in hyperinflammatory syndromes to identify profiles for severe cases to inform management decisions. Methods: This study included 113 CAR-T patients (pt) (September 2019-March 2024) with cytokine profiles (CP) performed at the second tocilizumab dose or with suspected hyperinflammatory states (CP1), followed by assessments at 24 hours (CP24) and 48 hours (CP48). 52 pts consented to blood (PB) and bone marrow (BM) cell immune phenotyping at pre-treatment (pre-LD) and day 1 for PB. Results: Among the 113 pts, CRS pts (N=74), when compared to MAS-L(N=19) & IEC-HS (N=20) at CP1, had lower levels of TNF (median in pg/mL; 31, 62.9, 47.7) , IL-18 (794, 1353, 2011), and MCP-1 (689, 2722, 2485.5). While MAS-L and IEC-HS had similar CP1 profiles, CP24 and CP48 differed between them: TNF & MCP-1 levels decreased in MAS-L but remained elevated or increased in IEC-HS. IL-18 was stable in MAS-L but increased steadily daily in IEC-HS. Infections were more frequent in IEC-HS (CRS: 43%; MAS-L: 58%; IEC-HS: 85%; p=0.002). Notably, patients who died with persistent E. faecium bacteremia had higher IL-18 in CP48. In the 52 pts with cell phenotypes available by flow, 33 had CRS, 18 MAS-L and 8 IEC-HS. The correlation between cell phenotypes and cytokine levels showed that elevated TNF at CP1 correlated with increased non-classical monocytes in pre-LD BM and higher NK and NK-T cells in pre-LD PB. Similarly, elevated IL-18 CP48 levels correlated with increased CD8 T-cells, NK-T cells, and intermediate monocytes in pre-LD PB, as well as increased NK cells at both pre-LD and day 1 (Table). Conclusions: We report distinct cytokine profiles in IEC-HS early in the inflammatory course post-CAR-T therapy. These elevated cytokines correlate with specific immune cell subsets, including BM-resident monocytes, intermediate monocytes with heightened cytokine trafficking and inflammatory capacity. This study demonstrates the feasibility for identifying potential biomarkers for early risk stratification and targeted interventions. Cytokine Sample source Immune cells R P value TNF CP1 BM pre-LD Non-classical monocytes (CD14loCD16+) 0.38 0.025 PB pre-LD Monocytes (All) 0.47 0.022 NK cells (CD56+CD16+) 0.34 0.024 NK-T cells (CD3+CD56+CD16-) 0.29 0.04 IL-18 CP48 PB pre-LD CD8+ T-cells 0.59 0.0093 NK-T cells (CD3+CD56+CD16-) 0.53 0.023 Intermediate monocytes (CD14+CD16+) 0.58 0.012 PB pre-LD NK cells (CD56+CD16+) 0.56 0.49 0.015 0.024