Abstract: Remission durability following single-antigen targeted chimeric antigen receptor (CAR) T-cells is limited by antigen modulation, which may be overcome with combinatorial targeting. Building upon our experiences targeting CD19 and CD22 in B-cell acute lymphoblastic leukemia (B-ALL), we report on our phase 1 dose-escalation study of a novel murine stem cell virus (MSCV)-CD19/CD22-4-1BB bivalent CAR T-cell (CD19.22.BBζ) for children and young adults (CAYA) with B-cell malignancies. Primary objectives included toxicity and dose finding. Secondary objectives included response rates and relapse-free survival (RFS). Biologic correlatives included laboratory investigations, CAR T-cell expansion and cytokine profiling. Twenty patients, ages 5.4 to 34.6 years, with B-ALL received CD19.22.BBζ. The complete response (CR) rate was 60% (12 of 20) in the full cohort and 71.4% (10 of 14) in CAR-naïve patients. Ten (50%) developed cytokine release syndrome (CRS), with 3 (15%) having ≥ grade 3 CRS and only 1 experiencing neurotoxicity (grade 3). The 6- and 12-month RFS in those achieving CR was 80.8% (95% confidence interval [CI]: 42.4%-94.9%) and 57.7% (95% CI: 22.1%-81.9%), respectively. Limited CAR T-cell expansion and persistence of MSCV-CD19.22.BBζ compared with EF1α-CD22.BBζ prompted laboratory investigations comparing EF1α vs MSCV promoters, which did not reveal major differences. Limited CD22 targeting with CD19.22.BBζ, as evaluated by ex vivo cytokine secretion and leukemia eradication in humanized mice, led to development of a novel bicistronic CD19.28ζ/CD22.BBζ construct with enhanced cytokine production against CD22. With demonstrated safety and efficacy of CD19.22.BBζ in a heavily pretreated CAYA B-ALL cohort, further optimization of combinatorial antigen targeting serves to overcome identified limitations (www.clinicaltrials.gov #NCT03448393).

Abstract: Introduction: HLH is a T cell mediated inflammatory syndrome associated with immune activation and NK cell dysfunction. In the context of CAR T cells, HLH occurs in a subset of those with cytokine release syndrome (CRS), yet predisposing factors are not yet well established. Utilizing our ongoing phase I study of CD22/4-1BB CAR T cells in children and young adults with CD22+ B cell malignancies (NCT02315612), we evaluated patient and product characteristics to elucidate factors associated with HLH. Methods: We retrospectively reviewed multiple variables to identify correlates of HLH in those infused. Definitions for CRS and HLH are in Table 1. Multiple univariate analyses were performed to identify risk factors for HLH; variables with significance were incorporated into a multiple logistic regression model to predict those at risk of HLH-like CRS. Table 1.Patient and Product Characteristics Amongst those with CRS, Distinguished by HLH versus non HLH-like toxicitiesNo HLH^HLH ^pBaseline DemographicsHLH manifestations amongst those with CRS, n, (%)31 (59.6%)21 (40.4%)Age (years), median (range)17 (4 - 30)13 (4 - 30)nsTime to CRS onset, day post infusion, median (range)8 (3-13)7.5 (3-13)nsTime to HLH onset, day post infusion, median (range)N/A14 (7-26)N/A# patients with prior HSCT, n (%)23/31 (74.2%)13/21 (61.9%)ns# patients with prior CAR, n (%)20/31 (64.5%)11/21 (52.4)nsPatient Factors; median (IQR)Baseline# PB12 (7.1-23.6)7.4 (4.4-14.6)0.04NK%Baseline# PB75 (58.5 - 88.5)83.9 (75.3-92.1)0.05CD3%Baseline# PB2.3 (1.2 - 4.3)6.1 (2.6 - 10.8)0.008CD8/NK ratioBaseline# BM3.6 (1.6 - 6.1)8.4 (4.9 - 15.7)0.0009CD3/NK ratioBaseline# Disease burden, % marrow blasts50 (18.9 - 90)71.4 (40.8 - 88.5)nsBaseline# Soluble IL2Ra (pg/mL)728 (460 - 1001)1176 (556.3 - 1550)0.07Product Factors; median (IQR)TCS of apheresis product, n, (%)13/31 (41.9%)18/21 (85.7%)0.002Product culture day 7, CD3%99.6 (99.3-99.8)99.8 (99.7 - 99.8)0.02Product culture day 7, NK%.1 (.01 - .23)0 (0 -.10)0.002Final product, CD3%99.7 (99.6 - 99.9)99.9 (99.8 - 99.9)0.01Final product, NK%.03 (0 - .12)0 (0 - .04)0.1Max Grade CRSGrade 121/31 (67.7%)5/21 (23.8%)0.004Grade 27/31 (22.3%)14/21 (66.7%)Grade 3 & 43/31 (9.6%)2/21 (9.5%)CAR T-cell Expansion (PB); median (IQR)Day 14, %CD871.6 (60.3 - 79.4)85.9 (61.1-92.9)0.02Day 14, %CD419.8 (11.9 - 27.2)7.8 (4.5 - 32.7)0.06Day 14, %NK4.0 (1.9 - 5.4)1.7 (.7 - 5.2)nsDay 14, CD8:NK ratio16.7 (10.5 - 41.1)42.1 (12.3 - 151.1).05Day 14, CD4:NK5.3 (1.8 - 8.1)5.5 (1.7 - 15)nsDay 28, %CD843.6 (37.8 - 55.7)69.8 (43.6 - 81.7)0.02Day 28, %CD435.8 (26 - 47.2)22.3 (10.7 - 44.9)0.07Day 28, %NK9 (4.5 - 15.5)1.9 (.7 - 11.7)0.02Day 28, CD8:NK ratio4.5 (2.6 - 12.8)21.5 (2.9 - 109.2)0.03Day 28, CD4:NK3.9 (1.8 - 7.6)7.7 (3.9 - 19.6)0.05Inflammatory markers and select cytokine profiling**; median (IQR)Ferritin (ng/mL)Baseline#1979 (992.5 - 3802)1942 (1397 - 4192)nsDay 51433 (888.5 - 2491)3621 (1587 - 5024)0.02Day 95920 (1097 - 18321)14063 (5367 - 61503)0.02IL18 (pg/mL)Day 0347.1 (251.8 - 544.4)645 (331.7 -863.3)0.02Day 5450 (232.5 - 589)578.6 (426.5 - 887.4)0.05Day 9835 (586.8 - 1035)894.4 (843.5 - 2971)nsDay 131494 (843.3 - 1970)2058 (1386 - 5001)0.04IL-1B (pg/mL)Day 0.29 (.28 - .33).28 (.28 - .29)nsDay 5.29 (.28 - .33).28 (.28 - .29)nsDay 9.33 (.28. - .82).53 (.28 - 3.36)nsDay 13.29 (.28 - .645).89 (.28 - 1.528)0.03IL-6** (pg/mL)Day 02.9 (1.4 - 8.9)4.1 (2.1 - 8.5)nsDay 53.3 (1.7 - 14.9)6.2 (2.7 - 9.3)nsDay 920.5 (5.0- 59.1)43.0 (15.5 - 983)0.04Day 139.025 (4.118 - 22.54)127.2 (20.92 - 597.4)0.0008IFNy (pg/mL)Day 011.1 (3.8 - 17.4)8.5 (3.2 - 15.4)nsDay 518.01 (5.0 - 46.9)13.6 (4.0 - 25.5)nsDay 9169.5 (93.3 - 665.4)485.2 (188.2 - 1887).06Day 1381.07 (21.1 - 132.8)287.3 (123 - 829.7)0.001Predictive model1.9882 x (TCS, 1=Yes/0=No) + 0.8013 x (max grade CRS) + 0.2368 x (bone marrow T/NK ratio); values & gt; 4.47 were predictive for HLH.(Model based on 59/60 infused patients, with full data available for 55 subjects. 1 subject excluded due to rapidly progressive disease requiring steroids prior to CAR expansion)*CRS (cytokine release syndrome) was defined by Lee et al. (Blood 2014).^HLH was defined by the following criteria, modified from Neelapu et al.: peak ferritin & gt;100,000 µg/L with at least two of the following criteria: a) hepatic transaminases or bilirubin & gt; grade 3, b) creatinine & gt; grade 3, c) pulmonary edema & gt; grade 3 and/or d) evidence of hemophagocytosis on bone marrow aspirate/biopsy.#Baseline: Timepoint prior to apheresis.**IL-6R antagonist (Tocilizumab) administration confounds results.Abbreviations: PB: peripheral blood; BM: bone marrow; TCS: T-cell selection of the apheresis product using CD4/CD8 beads; IQR: interquartile range; N/A: not applicable; ns: not significant.P values determined using Fisher's exact and Mann Whitney U tests for categorical and continuous variables, respectively, except Max Grade CRS which utilized a Chi-Squared test. Results: 52 of 60 (86.7%) treated subjects developed CRS; 21 (40.4%) of whom subsequently developed HLH. The following factors were associated with HLH: lower baseline blood and marrow NK% and higher CD3% cells; CD4/CD8 T cell selection (TCS) of the apheresis product (which increased CD3 and reduced NK cell populations); and higher IL18. Following infusion, distinct cytokine profiles and prolonged predominance of CD8 T cells distinguished those with HLH from those without it. Multiple logistic regression identified a) use of TCS, b) higher grade CRS and c) high baseline bone marrow T/NK ratio as most predictive of HLH; leading to a model with 75% sensitivity and 80% specificity. Conclusion: Our results provide novel insights into factors associated with CAR T cell related HLH and implicate preexisting T and NK cell populations, baseline cytokines, apheresis selection methodologies and CAR T cell expansion parameters in the pathophysiology. With a goal of earlier intervention to prevent HLH, the predictive model will be prospectively evaluated. Biologic correlative studies and assessment of generalizability in alternate CAR T cell constructs are planned. Citation Format: Daniel A. Lichtenstein, Seth M. Steinberg, Steven L. Highfill, Bonnie Yates, Ping Jin, Jianjian Jin, Sandhya Panch, Haneen Shalabi, David F. Stroncek, Terry J. Fry, Nirali N. Shah. Factors predictive of CAR T cell associated hemophagocytic lymphohistiocytosis (HLH) [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 4231.

Abstract: Introduction: Patients who relapse after or are resistant to CD19 targeting are in need of novel therapies. We previously reported on the initial experience with our highly-active, first-in-human, first-in-child, CD22 CAR trial in the first 21 subjects with ALL. We now report on the cumulative experience with 52 treated subjects. Methods: This phase I study of anti-CD22 CAR-T cells (Clinicaltrials.gov NCT02315612) enrolled subjects between the ages of 3-30 years with relapsed/refractory CD22+ disease. All subjects received fludarabine 25 mg/m2 x 3 days and cyclophosphamide 900 mg/m2 x 1 day for lymphodepletion. Disease assessments were performed prior to initiation of lymphodepletion and at day 28 (+/- 4 days) post CAR infusion. Three dose levels were explored; with an interim manufacturing modification incorporating CD4/CD8 bead T-cell selection (TCS); current dose is DL1-TCS. (Table 1) Results: The median age was 18.1 years (range, 4.4-30.6). 36 (69.2%) subjects had undergone HSCT; 30 (57.7%) had prior CD19 CAR; 22 (42.3%) had prior blinatumomab; 28 (53.8%) subjects had a CD19 negative population, including two who were inherently partial CD19 negative without prior targeted therapy. 46 (88.4%) experienced CRS, 5 (10.9%) had grade 3-4 CRS. Unique toxicities apparent with expanded experience included capillary leak syndrome (n=3), including one grade 5 event; atypical HUS (n=2), symptomatic coagulopathy (n=8) and HLH-like manifestations (n=18). The complete remission rate was 72.5% overall; 84% at the current dose level. This included complete remissions seen in subjects who were non-responders to CD19 CAR and/or blinatumomab. The longest remission is & gt; 3 years (n=1) post-CAR. Relapse occurred at a median of 6 months post CAR in 23 (64%) subjects primarily due to CD22 modulation. 12 proceeded to HSCT following CD22 CAR. Conclusion: In the largest study of CD22 CAR T-cell therapy to date, this extended experience confirms the initial efficacy, while highlighting novel aspects of the toxicity profile that warrant special attention. Results from our study support further testing of this CD22 CAR in a phase 2 clinical trial. Treated SubjectsDL1 (3 x 10e5)DL2 (1 x 10e6)DL3 (3 x 10e6)DL2 TCS (1 x 10e6)DL1 TCS & gt; (3 x 10e5)n5261827#19n, (% of all subjects)526 (11.5%)18 (34.6%)2 (3.8%)7 (13.4%)19 (36.5%)DemographicsMedian age, (range, years)18.1 (4.4-30.6)21.3 (7.3-22.7)16.7 (8.0-30.7)17.1 (7.9-26.4)12.8 (4.4-28.9)18.5 (4.9-30.4)Prior HSCT, n (%)36 (69.2%)6 (100%)13 (72.2%)2 (100%)6 (85.7%)9 (47.3%)Prior CD19 CAR, n (%)30 (57.7%)6 (100%)11 (61.1%)1 (50%)5 (71.4%)7 (36.8%)Prior Blinatumomab, n (%)22 (42.3%)1 (16.7%)4 (22.2%)2 (100%)2 (28.6%)13 (68.4%)Prior Inotuzumab, n (%)14 (26.9%)1 (16.7%)4 (22.2%)1 (50%)3 (42.9%)5 (26.3%)Prior CD22 CAR, n (%)3 (5.8%)0002 (28.6%)1 (5.3%)Any CD19 negative population ( & lt;90%+), n (%)28 (53.8%)4 (66.7%)9 (50%)05 (71.4%)10 (52.6%) & gt;M2 marrow, n (%)38 (73.0%)4 (66.7%)11 (61.1%)2 (100%)6 (85.7%)15 (78.9%)Toxicity ProfileTotal with CRS, n (%)46 (88.4%)3 (50%)16 (88.9%)2 (100%)6 (85.7%)19 (100%)Amongst all CRSCRS Grades 1-2, n (% of all with CRS)41 (89.1%)3 (100%)15 (93.8%)2 (100%)6 (100%)15 (78.9%)CRS Grades & gt; 3, n (% of all with CRS)5 (10.9%)01 (5.6%)004 (21.1%)Received Tocilizumab, n (%)19 (36.5%)03 (16.7%)04 (57.1%)12 (63.2%)Received Steroids, n (%)17 (32.7%)02 (11.1%)1 (50%)4 (57.1%)10 (52.6%)Developed symptomatic coagulopathy, n (%)8 (15.4%)03 (16.7%)04 (57.1%)1 (5.3%)Developed HLH, n (%)18 (34.6%)03 (16.7%)05 (71.4%)10 (52.6%)Developed CLS, n (%)3 (5.8%)01 (5.6%)^002Developed aHUS, n (%)2 (3.8%)0001 (14.2%)2Grade 5 events, n (%)2 (3.8%)02 (11.1%)000Response RateComplete Remissions, n (%)@37 (72.5%)*1 (16.7%)13 (76.5)**1 (50%)6 (85.7%)16 (84.2%)MRD negative CR, n(%)@32 (62.7)*1 (16.7%)10 (58.8%)**0 (0%)6 (85.7%)15 (78.9%)CRS: cytokine release syndrome, as graded per Lee et al. HLH: hemophagocytic lymphohistiocytosis, retrospectively identified and defined as present if the following criteria were met: peak ferritin & gt;100,000 with at least one of the following criteria: a) liver function tests & gt; grade 3, b) creatinine & gt; grade 3, c) pulmonary edema & gt;grade 3 or d) evidence of hemophagocytosis on the bone marrow. *51 subjects evaluable for response. One subject had a grade 5 toxicity prior to disease restaging; **17 subjects evaluable for response. One subject had a grade 5 toxicity prior to disease restaging; ^CLS developed into fatal adult respiratory distress syndrome; #Subject 27 had stable disease with the first infusion with grade 1 CRS not requiring steroids or tocilizumab and limited CAR expansion. Notably he had received a CD22 CAR construct at an outside hospital prior to treatment on this protocol. Data presented in this table reflect the response and toxicity profile following the second infusion as it informed the toxicity and response profile at this dose. @Reflects the best response at any time point without any interval therapy; & gt;Implementation of pre-emptive tocilizumab dosing initiated in this cohort. Citation Format: Nirali N. Shah, Haneen Shalabi, Bonnie Yates, Constance Yuan, Haiying Qin, Amanda Ombrello, Hao-Wei Wang, Leah Hoffman, Minh Tran, Sandhya Panch, Maryalice Stetler-Stevenson, Jianjian Jin, Crystal Mackall, Steve Highfill, David Stroncek, Terry J. Fry. Phase I CD22 CAR T-cell trial updates [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr LB-146.

Abstract: Chimeric antigen receptor T-cells (CART) are active in relapsed/refractory (r/r) B-cell acute lymphoblastic leukemia (B-ALL), but relapse remains a substantial challenge. Reinfusion with the same CART product (CART2) in patients with suboptimal response or antigen positive relapse following first infusion (CART1) represents a potential treatment strategy, though early experiences suggest limited efficacy of CART2 with CD19 targeting. We report on our experience with CART2 across a host of novel CAR T-cell trials. This was a retrospective review of children and young adults with B-ALL who received reinfusion with an anti-CD19, anti-CD22, or anti-CD19/22 CART construct on one of 3 CAR T-cells trials at the National Cancer Institute ( NCT01593696 , NCT02315612 , NCT0344839) between July 2012 and January 2021. All patients received lymphodepletion (LD) pre-CART (standard LD: 75 mg/m 2 fludarabine, 900 mg/m 2 cyclophosphamide; or intensified LD: 120 mg/m 2 fludarabine, 1200 mg/m 2 cyclophosphamide). Primary objectives were to describe response to and toxicity of CART2. Indication for CART2, impact of LD intensity, and CAR T-cell expansion and leukemia antigen expression between CART infusions was additionally evaluated. Eighteen patients proceeded to CART2 due to persistent (n=7) or relapsed antigen positive disease (n=11) following CART1. Seven of 18 (38.9%) demonstrated objective response (responders) to CART2: 5 achieved a minimal residual disease (MRD) negative CR, 1 had persistent MRD level disease, and 1 showed a partial remission, the latter with eradication of antigen positive disease and emergence of antigen negative B-ALL. Responders included four patients who had not achieved a CR with CART1. Limited cytokine release syndrome was seen following CART2. Peripheral blood CART1 expansion was higher than CART2 expansion (p=0.03). Emergence of antigen negative/dim B-ALL in 6 (33.3%) patients following CART2 contributed to lack of CR. Five of seven (71.4%) responders received intensified LD pre-CART2, which corresponded with higher CART2 expansion than in those receiving standard LD (p=0.029). Diminished CAR T-cell expansion and antigen downregulation/loss impeded robust responses to CART2. A subset of patients, however, may derive benefit from CART2 despite suboptimal response to CART1. Intensified LD may be one strategy to augment CART2 responses, though further study of factors associated with CART2 response, including serial monitoring of antigen expression, is warranted.

Abstract: Patients with B-cell acute lymphoblastic leukemia who experience relapse after or are resistant to CD19-targeted immunotherapies have limited treatment options. Targeting CD22, an alternative B-cell antigen, represents an alternate strategy. We report outcomes on the largest patient cohort treated with CD22 chimeric antigen receptor (CAR) T cells. PATIENTS AND METHODS We conducted a single-center, phase I, 3 + 3 dose-escalation trial with a large expansion cohort that tested CD22-targeted CAR T cells for children and young adults with relapsed/refractory CD22 + malignancies. Primary objectives were to assess the safety, toxicity, and feasibility. Secondary objectives included efficacy, CD22 CAR T-cell persistence, and cytokine profiling. RESULTS Fifty-eight participants were infused; 51 (87.9%) after prior CD19-targeted therapy. Cytokine release syndrome occurred in 50 participants (86.2%) and was grade 1-2 in 45 (90%). Symptoms of neurotoxicity were minimal and transient. Hemophagocytic lymphohistiocytosis–like manifestations were seen in 19/58 (32.8%) of subjects, prompting utilization of anakinra. CD4/CD8 T-cell selection of the apheresis product improved CAR T-cell manufacturing feasibility as well as heightened inflammatory toxicities, leading to dose de-escalation. The complete remission rate was 70%. The median overall survival was 13.4 months (95% CI, 7.7 to 20.3 months). Among those who achieved a complete response, the median relapse-free survival was 6.0 months (95% CI, 4.1 to 6.5 months). Thirteen participants proceeded to stem-cell transplantation. CONCLUSION In the largest experience of CD22 CAR T-cells to our knowledge, we provide novel information on the impact of manufacturing changes on clinical outcomes and report on unique CD22 CAR T-cell toxicities and toxicity mitigation strategies. The remission induction rate supports further development of CD22 CAR T cells as a therapeutic option in patients resistant to CD19-targeted immunotherapy.

Abstract: Background: With the hypothesis that dual antigen targeting strategies may prevent antigen negative escape, we tested a novel humanized bispecific CD19/CD22 CAR T cell construct in patients with relapsed/refractory B ALL. Building upon our experience with effective CD19 (NCT01593696) and CD22 (NCT02315612) CAR T cell constructs, we report our initial findings. Design: This was a phase 1 dose escalation study which started at 3 x 105 transduced CAR T-cell/kg. The CD19/CD22 construct was comprised of FMC63 (CD19 scFv) and m971 (CD22 scFv) with a 4-1BB costimulatory domain (NCT: 03448393). The primary objective was safety and toxicity; secondary objectives included efficacy, CAR persistence and cytokine profiling. CAR T cells were manufactured onsite utilizing a closed system device (CliniMACS Prodigy®). ASTCT consensus guidelines were used for cytokine release syndrome (CRS) grading. Prior CAR T cells were not exclusionary. Results: Eleven subjects (median age 21) were infused at 3 dose levels. Four experienced mild, reversible CRS without neurotoxicity (Table). Eight subjects had an objective response (MRD negative CR, n=4; partial response, n=4), and 2 proceeded to transplant. Complete responders were CAR naïve and received & gt; 1 x 106 CAR T-cells/kg. CAR expansion peaked at a median of 13 days (range, 6-20) and were detectable by flow cytometry to a median of 54 days (range: 25-110 days) post infusion. Patients with higher disease burden trended towards having higher CRS, CAR expansion, and cytokine elevation. Two patients have relapsed (1 post BMT) with CD19+/CD22+ disease. Table 1:Subject Characteristics, Toxicity, Response, and CAR PersistenceSubjectAge (years)Dose Level(DL)CD19/22 CAR T cell Dose(x 106)Disease StatusExtramedullary DiseasePrior CARGrade CRSResponseD28Peak absolute circulating CD19 CAR T cells (cells/mL)Peak BM CAR% at 1-monthPersistence of Circulating CAR-T(Day %CAR=0)Consolidative HSCTFollow up##118115.9M2NoYes, CD19 4-1BB; CD19/22 4-1BB0SD00%N/ANo1-month2516.3M3NoYes, CD19-28z1PR44.6815%56No1-month319117.1M1YesYes, CD22 4-1BB0SD3.974.8%61*No1-month419116.8M1YesYes,CD19-28z0PR1.210.1%26No1-month528275M3YesNo3^MRD negative CR84.2222%110Yes8.5-months**628268M3NoNo3^MRD negative CR33.832.1%54No4-months**723272M2NoYes,CD19 4-1BB0SD00%N/ANo1-month818248M1YesNo0PR$72.535.2%87No2-months9213183M1NoNo1MRD negative CR6.160%27Yes5-months@10283264M1NoNo0 & MRDNegative CR76.641.3%25No4-months@12283258M1YesNo0 & PR#11.410.3%27No1-monthDL1: 3×105 transduced CAR T-cells/kg, DL2: 1×106 transduced CAR T-cells/kg, and DL3: 3×106 transduced CAR T-cells/kg; M1 & lt; 5% blasts, M2 5-25% blasts, M3 & gt;25% blasts; SD: stable disease, PR: partial response defined by decrease in marrow blasts by at least 50% with absence of peripheral blasts, MRD negative CR: minimal residual disease negative complete remission; CRS cytokine release syndrome; ;^ received tocilizumab & Patient had CAR expansion and malaise without fever $ Patient had complete response in bone marrow however had extramedullary disease that was progressive; #Patient had complete response in CSF, partial response in extramedullary breast mass, and residual low-level bone marrow disease. N/A: not applicable; * CAR=0.2%, ##Time to event (relapse, progression, or remission) **at the time of relapse, @remain in remission Conclusion: In our preliminary experience, CD19/22 CAR was well tolerated and effective in CAR naïve patients, with 4/6 patients achieving MRD negative CR. Relapses were antigen positive likely due to limited CAR persistence. Future plans include exploring an additional dose level, intensifying lymphodepletion for prior CAR patients, and evaluating CAR T-cell product characteristics with outcomes. Citation Format: Haneen Shalabi, Bonnie Yates, Shilpa Shahani, Haiying Qin, Steven L. HIghfill, Sandhya Panch, Minh Tran, David Stroncek, Leah Hoffman, Lauren Little, Katherine Graap, Maryalice Stetler-Stevenson, Constance Yuan, Hao-Wei Wang, Terry J. Fry, Nirali N. Shah. Safety and efficacy of CD19/CD22 CAR T cells in children and young adults with relapsed/refractory ALL [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr CT051.