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TPS1659 Background: Financial toxicity, the material and psychological burden of treatment cost, affects up to half of people with cancer and can affect adherence and survival. Financial toxicity is a health equity issue, disproportionately affecting Black patients. Patient education about cost and patient-provider cost discussions are recommended to mitigate financial toxicity but occur infrequently. Our goal is to mitigate financial toxicity through a tailorable education and communication intervention, the DISCO App. The DISCO App was shown to be feasible, acceptable, and preliminarily effective at prompting cost discussions and improving related outcomes in a pilot trial. The aim of the ongoing trial is to test the effectiveness of the DISCO App on short- and longer-term outcomes for Black and White patients with cancer. Methods: This study is a longitudinal RCT. Oncologists are eligible if they treat patients with solid tumors at the trial site. Patients of participating oncologists are eligible if they are ≥ 18 years of age; identify as Black or White; can read and write in English; have an email address; and were diagnosed with a solid tumor for which systemic therapy is a likely recommended treatment. Strata were created to balance arms by patient race, income, age, and sex. Upon consent, patients are randomized to one usual care arm (1) or one of two intervention arms (2 and 3). All patients are asked to allow one treatment discussion with their oncologist to be video recorded for analysis. Prior to the recording, intervention patients utilize the DISCO App on an iPad. The DISCO App includes a video about treatment costs, ways to manage costs, and the importance of discussing costs with oncologists. Once patients enter their socio-demographic information (e.g., employment, insurance) and any financial concerns, they receive a tailored list of questions to ask their oncologist. Arm 3 patients receive an intervention booster via email two months after the recording. Patients complete measures at baseline, right after the recording, and at 1, 3, 6 and 12 months after the recording. Measures assess outcomes including cost discussions, communication quality, cost knowledge, self-efficacy for cost management, referrals for support, short- and longer-term financial toxicity, and treatment adherence. The patient-oncologist interaction is the unit of analysis and we will use multi-linear models to compare outcomes by arm. We anticipate recruiting up to 15 oncologists and 240 patients. Data collection began in March 2021 and will continue until July 2025. Participants to date include 13 oncologists and 192 patients (116 Black, 76 White). Most patients completed the baseline assessment (n=164), the post-interaction assessment (n=137), and at least 1 follow-up assessment (n=132); 125 treatment discussions have been recorded. The IRB reviewed the trial in December 2024 and approved continuation. Clinical trial information: NCT04766190 .

TPS1657 Background: Cancer treatments are traditionally administered in clinical settings, which can isolate patients from their familiar environments and exacerbate physical, psychosocial, and financial burdens. Travel requirements further amplify these challenges, particularly for underserved populations. Studies indicate that patients prefer receiving care at home, and international models have demonstrated the safety of home-delivered chemotherapy since the 1990s; however, no U.S. clinical trial data exists. In response, Mayo Clinic has developed the Cancer CARE (Connected Access and Remote Expertise) Beyond Walls (CCBW) program, a distributed cancer care delivery model that expands access to quality cancer care by bringing it to the home environment, providing in-home cancer treatment, lab testing, telemedicine, and community paramedic support. This trial evaluates the safety, acceptability, and impact of home-based subcutaneous (SC) nivolumab (Nivo) administration compared to in-clinic treatment within the CCBW initiative. Methods: This open-label, single-arm trial evaluates the impact of SC Nivo administration location—home versus infusion center—on patient reported cancer care experience, patient-preferred treatment location, acceptability, safety, and patient-reported outcomes. Eligible adult patients (ECOG 0-1) receiving IV Nivo for an FDA-approved indication and residing within 75 miles of Mayo Clinic Florida will transition to SC Nivo, receiving two initial in-clinic cycles. If tolerated without injection reactions, four cycles will be administered at home before resuming in-clinic treatment. Exclusion criteria include concurrent investigational/standard treatments or contraindications to immunotherapy. Fifty patients will be enrolled, with an estimated 75% (n=38) providing cancer care ratings after 8 weeks in-clinic and 8 weeks at-home, offering 85% power to detect a mean difference in ratings of 0.50 standard deviations. The primary endpoint is within-patient change in cancer care rating (0–10 scale, CAHPS Cancer Care Survey) comparing 8 weeks of in-clinic vs. at-home care. Secondary endpoints include patient-preferred treatment location, comfort with home injections, safety (grade 3+ adverse events), function (EORTC QLQ-F17), symptoms (PRO-CTCAE), side effect impact (GP5), and healthcare utilization (ER visits, hospitalizations). Cost will be assessed as a tertiary endpoint. The trial has FDA approval (IND #170079), Mayo Clinic IRB approval (#23009663), and ClinicalTrials.gov registration (NCT06265285). Enrollment began in April 2024, with an expected accrual period of 24 months. To date, 10 patients have been enrolled, and final analysis is expected 2.5 years after trial activation. Clinical trial information: NCT06265285 .

e13575 Background: Cellular therapies continue to expand in diversity, complexity, and financial impact leading clinicians and administrators to seek innovative care options. The paradigm of late has been to move to an outpatient treatment model. However, centers may not find this model to be the safest option for their patients. This pilot project aims to use established outpatient resources to reduce hospital length of stay (LOS) for patients receiving ciltacabtagene autoleucel in an inpatient setting. Methods: Chart review was conducted for 42 patients treated for incidence of CRS, days to onset, LOS, readmissions, and hospital expenses and charges. The cohort showed an 83% incidence of CRS with a mean onset of 7 days. 71% of CRS was limited to Grade 1. The mean LOS was 10.8 days and the readmission rate was 21%. Financial modeling estimated overall cost reduction to range from $13,000 to $20,000 per patient for a LOS of 3 days. A pilot project to reduce LOS was developed with inclusion/exclusion criteria prioritizing patient safety and likelihood of remaining in the outpatient setting. A workflow for administering one dose of tocilizumab for Grade 1 CRS in the outpatient setting was also established to counterbalance readmissions. The goal of the pilot is to reduce the overall LOS from 10 days to 3 days while maintaining readmission rates and comparable charge to reimbursement ratios on net operating income. Results: In the first five months, 25 patients were screened for eligibility. 60% were deemed eligible and 66% of those were discharged after three days. No patients discharged after a minimum admission were readmitted within three days. Current data suggests the readmission rate within 30 days is higher than baseline at 40%. The overall LOS of patients enrolled in the pilot upon infusion was six days. Patients successfully remaining on the pilot have a 60% reduction in LOS. There have been no major safety events. Conclusions: We identified that select patients could be managed in the outpatient setting after a minimum three-midnight hospital admission. A pilot project was developed using our comprehensive care model coupled with a minimum LOS of 3 days. After discharge, patients are followed daily in our outpatient clinic using in-person visits and remote patient monitoring. To counterbalance readmissions, Grade 1 CRS has been treated in the outpatient clinic including administration of tocilizumab. Early data indicates that the workflow is reducing LOS by 60% under patient selection criteria capturing approximately 60% of patients. While readmissions in the first three days post discharge have been parried, there is opportunity to further reduce readmissions in the first 30 days following CAR-T infusion. We anticipate the pilot will reduce costs without negatively impacting reimbursement or readmissions.

e13574 Background: AL (amyloid light chain) amyloidosis is associated with poor prognosis especially if diagnosis occurs with cardiac involvement. Current AL treatments target the plasma cell clone producing the light chains that misfold, aggregate and deposit in various tissues. The multi-organ manifestations of AL amyloidosis require a multidisciplinary care team of hematologist-oncologists, cardiologists, and other healthcare providers (HCPs) for comprehensive, patient-centric management. Since 2022, Amyloidosis Stakeholder Partnerships for Impact, Reach & Equity (ASPIRE), facilitated by Amyloidosis Research Consortium (ARC) has been bringing together biotech and pharmaceutical companies with the shared goal of optimizing amyloidosis patient care. Through this study, ARC-ASPIRE sought to document and share the current best practices and key barriers to patient-centric amyloidosis care at US specialized amyloidosis centers (SACs). A broad range of stakeholders in amyloidosis care – healthcare providers at SACs, patients, referring physicians, and patient advocacy group representatives – were interviewed for this study. This abstract presents findings relevant to the management of patients with AL amyloidosis. Methods: Structured interviews were conducted between December 2023 and February 2024 with 77 amyloidosis stakeholders (physicians, advanced practice providers, registered nurses from 17 SACs, patients, referring physicians, and patient advocacy group representatives), including 12 hematology-oncologists and 8 AL patients. SACs were selected to be representative of current US amyloidosis care, based on geography, amyloidosis patient volume, years since establishment, and types of amyloidosis treated. Results: Three-quarters of participating SACs treat AL patients under a single multidisciplinary amyloidosis program. Multidisciplinary care is facilitated by regular team meetings, dedicated clinic days, and informal communications across specialties. At diagnosis, collaboration between cardiology and hematology-oncology ensures AL patients are accurately diagnosed and promptly treated. Sixty percent of SACs have a centralized intake system, and patients suspected with AL are prioritized to ensure an appointment within 1 week of referral. Key barriers were delayed or missed diagnosis due to low awareness of AL amyloidosis within community oncologists, travel burden to SACs, lack of seamless medical records sharing and telehealth regulations. Conclusions: The best practices outlined in this study serve to educate AL amyloidosis and non-AL amyloidosis HCPs for better multidisciplinary collaboration and early diagnosis. The key barriers identified should be addressed to improve patient care.

6579 Background: Treatment strategies in chronic myelomonocytic leukemia (CMML) are not standardized and include hydroxyurea (HU) and hypomethylating agents (HMA). In a phase 3 study comparing HU and decitabine, in proliferative CMML, response rates were higher for decitabine (56% vs 31%; p<0.01) but overall survival was similar (p=0.67) between the two groups (Itzykson, R. JCO, 2023. 41:1888). In the current retrospective study, we examined the survival impact of different treatment strategies among 457 Mayo Clinic patients with CMML. Methods: The current study was conducted under institutional review board approved minimum risk protocols allowing retrospective patient data collection and analysis. Diagnostic criteria were according to the International Consensus Classification (Arber et al. Blood 2022. 140:1200). All statistical analyses were conducted using JMP 17 software. For survival analysis, patients were censored at time of allogeneic stem cell transplant (ASCT). Results: A total of 457 patients were considered (median age 72 years; 68% males). 209 patients received CMML-directed therapy: HU (N=102), HMA (N=78; azacitidine 32 and decitabine 46). Responses were adjudicated separately for leukocytosis and anemia; normalization of leukocyte count was achieved in 21% vs. 16 % (p<0.01) for HU vs. HMA and overall response in anemia in 1% vs. 9% (p=0.02), respectively. Overall survival did not appear to be impacted by different treatment strategies at any stage of CMML; (i) treated vs. untreated (p=0.3), (ii) HU vs. HMA for first-line therapy (p=0.3), and (iii) HU/HMAs vs. other drugs as first-line therapy (p=0.1). Blast transformation-free survival (BTFS) was also similar between HU vs. HMA as well as HU/HMA vs. other drugs. In univariate analysis, BTFS was inferior in patients receiving CMML-directed therapy (p<0.01); the particular association (HR 2; 95% CI 1.2-3.3) retained its significance in multivariable analysis that also included other risk factors for BTFS: bone marrow blast ≥10% (HR 4.3), circulating blast ≥2% (HR 2.7), WBC ≥13 x 10 9 /L (HR 1.8), and ASXL1 mutation (HR 1.7). Conclusions: In the current retrospective study that included a large number of patients with CMML, chemotherapy with HU or HMA did not appear to affect overall survival but might have increased the risk of blast transformation. The study also suggests superiority of HU for the treatment of leukocytosis and HMA for anemia. Predictors of blast transformation-free survival (BTFS) in 457 patients diagnosed with chronic myelomonocytic leukemia (CMML). BTFS Univariable analysis p-value (HR) Multivariable analysis p-value (HR) CMML-directed therapy <0.01 (2.9; 1.8-4.7) <0.01 (2; 1.2-3.3) Bone marrow blasts ≥10% <0.01 (12.4) 0.01 (4.3) Circulating blasts ≥2% <0.01 (5.2) <0.01 (2.7) WBC ≥13 x 10 9 /L <0.01 (2.9) 0.02 (1.8) ASXL1 mut <0.01 (2.1) 0.02 (1.7)

569 Background: Several studies established the prognostic role of both the amount and locations of tumor-infiltrating lymphocytes (TILs) in TNBC. Three distinct immunotypes were described based on the amount and locations of TILs: immune enriched (IN), immune excluded, and immune desert. Using single-cell spatial transcriptomic analysis in the Mayo Clinic TNBC cohort, our previous studies showed the central role of interferon (IFN) signaling in IN phenotype. Herein, we evaluated the association between IFN and outcomes in TNBC in 3 independent datasets. Methods: NanoString IO360 was performed in 114 samples from FinXX (NCT00114816) to generate 22-gene IFNα and 33-gene IFNγ signatures. RNA sequencing was performed in 388 samples from CALGB 40603 (NCT00861705). 3038 TNBC samples were tested by WTS (NovaSeq; Caris Life Sciences, Phoenix, AZ). Median values were used as cut-offs for high vs low IFNγ RNA expression and 18-gene IFNγ signature scores. Caris Life Science CODEai was used to evaluate real-world overall survival (OS) obtained from insurance claims and calculated from tissue collection to last contact using Kaplan-Meier estimates. Chi-square, Mann-Whitney U, ANOVA, and Cox regression were used. Results: A high 22-gene IFNα signature score was associated with significantly improved recurrence-free survival (RFS) in FinXX (hazard ratio [HR] 0.32, 95% confidence interval [CI] 0.14-0.74, p 0.007) and OS (HR 0.28, 95%CI 0.12-0.66, p 0.003). Similar findings were observed with 33-gene IFNγ signature with significant improvement in RFS (HR 0.21, 95%CI 0.09-0.51, p < 0.001) and OS (HR 0.18, 95%CI 0.08-0.44, p < 0.001). Furthermore, in CALGB 40603, both IFNα and IFNγ scores were positively associated with pathologic complete response (pCR: IFNα p 0.019 and IFNγ p 0.007) and residual cancer burden (RCB: IFNα p 0.044 and IFNγ p 0.013). Using the Caris data platform to further validate, we identified 2899 TNBC patients (pts) with genomic and clinical outcome data. High IFNγ expression was associated with significant improvement in OS (25.95 vs 17.43 months; HR 0.65, 95% CI 0.59 – 0.72, p < 0.001). Similarly, pts with high IFNγ signature scores had significant improvement in median OS (25.79 vs 16.22 months; HR 0.66, 95% CI 0.6 – 0.73, p < 0.001). Conclusions: This study underscores the pivotal role of IFN signaling in TNBC. High IFNα and IFNγ signatures were consistently associated with improved RFS, OS, higher pCR rates, and lower RCB across clinical trial cohorts and real-world data. These findings signify IFN signaling as a potential key biomarker and therapeutic target in TNBC. Support: U10CA180821, U10CA180882, U24CA196171; Breast Cancer Research Foundation, Mayo Clinic Breast Cancer SPORE (P50CA116201-17), Bankhead Coley, W81XWH-15-1-0292, P50CA015083, R35CA253187; https://acknowledgments.alliancefound.org . Genentech. Clinical trial information: NCT00114816 and NCT00861705 .

561 Background: Obesity and other lifestyle factors are associated with breast cancer (BC) risk and outcomes. These relationships appear to vary in pre- vs postmenopausal women. Here we explore associations between pathological features, diet quality and physical activity (PA) in patients (pts) with BC enrolled in the BWEL (Alliance for Clinical Trials in Oncology A011401) trial, stratifying for menopausal status. Methods: BWEL is a phase III trial evaluating the impact of a weight loss intervention on disease outcomes in 3180 pts with stage II-III HER2– BC and body mass index ≥27 kg/m 2 . The first 542 BWEL pts underwent assessment of PA (7-Day PA Recall) and diet (24-hour Diet Recall) at enrollment. Healthy Eating Index 2020 (HEI) score (0-100, higher value = healthier diet) and minutes/week (min/wk) of moderate/vigorous PA (MVPA) were calculated. Estrogen receptor (ER) and progesterone receptor (PR) expression level (%) were abstracted from pathology reports. Analyses of links between HEI score, MVPA min/wk, % ER/PR, stratified by menopausal status, were conducted using t-tests; differential associations by menopausal status used linear regression models with interaction term. Results: In 523 pts with available pathology data, 76.5% had ER/PR+ BC, 56.2% were postmenopausal. Median HEI score was 50.1 (range 18.1-96.4), median MVPA min/wk was 0 (range 0-630), median time from diagnosis to enrollment was 10.0 months (range 2.4-13.1). In postmenopausal pts, higher diet quality was linked to increased % ER and PR (HEI above vs below median, mean % ER 78.0 vs 66.1, p=0.012; mean % PR 57.7 vs 43.1, p=0.004). The relationship between HEI score and % ER/PR differed significantly by menopausal status (interaction term of menopausal status and % ER: p=0.006; and % PR: p=0.012); there were no significant associations between HEI scores and % ER/PR in premenopausal pts. There were no significant associations between PA and % ER/PR. Conclusions: Healthier diet was associated with higher % ER and PR in postmenopausal pts with BC, but no relationship was seen in premenopausal pts. These data suggest that lifestyle factors may influence BC pathologic features related to outcomes in older women. Support: U10CA180821, U10CA180882, UG1CA189823, U24CA196171, https://acknowledgments.alliancefound.org . Clinical trial information: NCT02750826 . % ER mean (SD) p value % PR mean (SD) p value Post-Menopausal HEI* score≤50.1 [n=134]>50.1 [n=156] 66.1 (43.3)78.0 (36.8) 0.012 43.1 (41.8)57.7 (41.0) 0.004 MVPA † min/wk0 [n=147]≥1 [n=147] 74.5 (39.3)70.3 (41.2) 0.37 50.0 (41.8)53.0 (42.0) 0.41 Pre-Menopausal HEI* score≤50.1 [n=124]>50.1 [n=103] 70.5 (38.5)65.5 (43.0) 0.15 53.5 (41.1)49.1 (43.0) 0.43 MVPA † min/wk0 [n=112]≥1 [n=113] 66.3 (41.0)66.3 (41.2) 0.99 49.5 (41.4)52.6 (42.7) 0.59 *Healthy Eating Index. † Moderate/Vigorous Physical Activity.

548 Background: TNBC is associated with higher risk of recurrence and poorer survival rates than other breast cancer subtypes. For node-positive TNBC or tumors larger than 0.5 cm, chemotherapy is generally recommended. For stage I TNBC, the benefit from chemotherapy, the optimal regimen, and whether to administer it in the neoadjuvant versus adjuvant setting remain controversial. Here, we report the treatment patterns and clinical outcomes of patients with stage I TNBC treated at Mayo Clinic. Methods: Using the Mayo Clinic Tumor Registry data, we identified patients with stage I TNBC treated between 2010 and 2021. We used the pathologic tumor (T) category for patients treated with upfront surgery and the clinical T category for patients receiving neoadjuvant chemotherapy. We used the Kaplan-Meier method and log-rank test to compare recurrence-free survival (RFS) according to treatment groups, measured from the day of surgery. RFS was reported at a median follow-up of 3.9, 6.2, and 7.3 years for patients who received chemotherapy neoadjuvantly, adjuvantly, and surgery alone with no chemotherapy, respectively. Results: A total of 602 patients with Stage I TNBC were included, with a median age of 62 years. 290 (48%) underwent upfront surgery followed by adjuvant chemotherapy (pT1a: 2%, pT1b: 25%, pT1c: 74%), 127 (21%) received neoadjuvant chemotherapy followed by surgery (cT1a: 2%, cT1b: 11%, cT1c: 87%), and 185 (31%) underwent primary surgery without any chemotherapy (pT1mi/T1a: 33%, pT1b: 32%, pT1c: 35%). Most patients treated with neoadjuvant therapy received anthracycline/cyclophosphamide + taxane (70%), while most patients treated in the adjuvant setting received a non-anthracycline containing regimen (60%). The 5-year RFS was 96% (95% CI: 93-100%) for patients who received chemotherapy neoadjuvantly, 95% (95% CI: 92-98%) for those who received chemotherapy adjuvantly, and 85% (95% CI: 80-91%) for those treated with surgery alone and no chemotherapy ( P < 0.0001). 71 (56%) of the 127 patients who received neoadjuvant therapy achieved a pCR, with only 1 RFS event (at 14 months) in that group, compared to 4 RFS events among those not achieving pCR (at 12, 20, 28 and 75 months, respectively). Among patients not receiving chemotherapy (either adjuvantly or neoadjuvantly), the 5-year RFS rates were 94% for pT1mi/T1a, 92% for pT1b and 72% for pT1c (T1mi/a/b vs T1c, P = 0.009). Conclusions: In this large cohort of stage I TNBC, patients who received chemotherapy (adjuvantly or neoadjuvantly) had better 5-year RFS compared with those treated with locoregional therapy only. Among patients receiving chemotherapy, 5-year RFS was nearly identical regardless of whether chemotherapy was administered before or after surgery. Interestingly, patients undergoing upfront surgery were more likely to receive an anthracycline-sparing chemotherapy regimen.

6558 Background: Allogeneic stem cell transplantation (ASCT) is currently the only curative therapy in chronic myelomonocytic leukemia (CMML). Methods: A Mayo Clinic enterprise-wide database search identified 138 CMML cases who underwent ASCT. Conventional statistical methods were used for analyses. Results: 138 CMML patients (transplanted between 1995-2024) were included (median age 63 years; males 62%);104 (group A) received ASCT before and 34 (group B) after blast transformation (BT). At initial diagnosis, CMML-1/CMML-2 and dysplastic/proliferative representations were 78%/22% and 54%/46%; 31% displayed abnormal karyotype and most frequent mutations were ASXL1 (56%), TET2 (44%) and SRSF2 (38%). At time of initial diagnosis, CPSS-Mol risk categories were low (17%), intermediate-1 (11%), intermediate-2 (40%), and high (32%). Median time from diagnosis to ASCT was 11 months (range 0-201). Median overall survival (OS) from the time of initial diagnosis was 67 months (range 4-239) and from the time of ASCT 54 (range 0-212) months. Occurrence of BT before ASCT was associated lower post-transplant survival (PTS; 16 vs 95 months, P=0.01, HR 1.9, 95%CI 1.2-3.2). Bone marrow (BM) blasts, at the time of ASCT, <5%, 5-9%, and 10-19% correlated with median OS of 171, 81, and 18 months in group A (p=0.01) and 50, 25, and 13 months in group B (P=0.07), respectively. Pre-ASCT hypomethylating agent exposure was associated with lower PTS in group A (P=0.02). PTS was also adversely affected by DNMT3A and SETBP1 mutations. In group A, PTS was the longest with myeloablative busulfan-based (median not reached) and the shortest (median 22 months) with Cy-TBI-based conditioning (p=0.1). Donor type did not impact PTS: matched unrelated (63%), matched sibling (23%), mismatched unrelated (7%), or haplo-identical (7%). Post-transplant cyclophosphamide was associated with a numerically lower median PTS (22 months), compared to other forms of GVHD prophylaxis (107 months; p=0.1) and significantly higher non-relapse mortality (p=0.02). Documentation of morphologic CR at day 100 was associated with significantly longer PTS in both groups A (92%,164 vs. 18 months; p=0.01) and B (84%, 42 vs. 11 months; p=0.01). The presence of abnormal karyotype at day 100 was associated with shorter PTS in group A (p=0.01). Grade ≥3 acute and moderate to severe chronic GVHD occurred in 20% and 34%, respectively, in group A patients and in 26% and 44% in group B. GRFS at 1/3 years were 42/21% in group A and 31/16% in group B. At last follow up, 44% in group A and 65% in group B were dead. Causes of death were relapse/non-relapse related in 36/64% in group A and 39/61% in group B. Conclusions: ASCT is effective in securing long-term survival in CMML, especially when the procedure is performed prior to BT and in the setting of <5% BM blasts/promonocytes. Response assessment at day 100 was highly informative of outcome.

6554 Background: SER-155 is an investigational, oral, live biotherapeutic product (LBP) comprised of 16 bacterial strains designed to decolonize gastrointestinal (GI) pathogens, improve epithelial barrier integrity, and modulate immune responses to prevent bloodstream infections (BSI). In the placebo-controlled cohort 2 of the phase 1b study, SER-155-001 (NCT04995653), SER-155 was generally well tolerated with a safety profile similar to placebo and the incidence of BSIs, a secondary endpoint, was significantly lower after treatment with SER-155 when compared to placebo. We report exploratory analyses of biomarkers of T cell expansion relevant for immune reconstitution after HCT. Methods: Participants were randomized 1:1 to receive 4 days of oral vancomycin (for microbiome conditioning) and 10 days of SER-155 or placebo/placebo administered pre-HCT (course 1) and post-neutrophil engraftment (course 2). Primary endpoints were safety and SER-155 strain engraftment (PK). Exploratory endpoints included plasma cytokine concentrations measured by ELISA and analysis of peripheral blood mononuclear cells (PBMCs) by flow cytometry. Results: Demographics were comparable across treatment arms. 34 of 45 randomized participants were treated and received allo-HCT (SER-155, 20; placebo, 14); 28 received course 2 (SER-155, 19; placebo, 9). SER-155 strain engraftment was observed in the peri-transplant period (median 11.5 strains after course 1) and post-HCT (median 11 strains after course 2 and HCT Day 100). Significant differences and trends in cytokines of systemic inflammation and immune homeostasis were observed relative to placebo prior to HCT Day 0 and post-HCT. On HCT Day 0, both arms had similar concentrations of IL7 and IL15 (Table 1). However, after course 2, and at HCT Day 100, significantly higher concentrations of IL7 were observed in the SER-155 arm relative to placebo (p=0.02, and p=0.003, respectively). In a preliminary analysis of PBMCs from a subset of participants, a high frequency of CD4+ T cells were observed in the SER-155 arm at the same timepoints. For homeostatic cytokine IL15, no significant differences were observed between arms. Conclusions: The significantly higher concentrations of IL7 with SER-155 treatment and observed frequency of CD4+ T cells support the potential role of the GI microbiome in promoting homeostatic expansion of peripheral T cells and immune reconstitution important for successful HCT. Clinical trial information: NCT04995653 . Median plasma IL7 and IL15 (pg/mL), SER-155 vs. placebo (generalized linear model). HCT Day or event SER-155 IL7 Placebo IL7 SER-155 IL15 Placebo IL15 D0 13.8 13.0 31.9 30.4 D7 15.1 14.6 50.9 62.6 D14 12.8 15.5 56.6 69.8 Post-neutrophil engraftment 13.4 9.0 16.3 15.4 Post-course 2 7.4* 3.9 6.2 9.1 D100 7.1** 3.5 6.2 6.2 *p<0.05, **p<0.01.