Abstract: Relapse following chimeric antigen receptor (CAR) T-cell therapy directed against CD19 for relapsed/refractory B-acute lymphoblastic leukemia (r/r B-ALL) remains a significant challenge. Three main patterns of relapse predominate: CD19 positive (CD19pos) relapse, CD19 negative (CD19neg) relapse, and lineage switch (LS). Development and validation of risk factors that predict relapse phenotype could help define potential pre- or post-CAR T-cell infusion interventions aimed at decreasing relapse. Our group sought to extensively characterize preinfusion risk factors associated with the development of each relapse pattern via a multicenter, retrospective review of children and young adults with r/r B-ALL treated with a murine-based CD19-CAR construct. Of 420 patients treated with CAR, 166 (39.5%) relapsed, including 83 (50%) CD19pos, 68 (41%) CD19neg, and 12 (7.2%) LS relapses. A greater cumulative number of prior complete remissions was associated with CD19pos relapses, whereas high preinfusion disease burden, prior blinatumomab nonresponse, older age, and 4-1BB CAR construct were associated with CD19neg relapses. The presence of a KMT2A rearrangement was the only preinfusion risk factor associated with LS. The median overall survival following a post-CAR relapse was 11.9 months (95% CI, 9-17) and was particularly dismal in patients experiencing an LS, with no long-term survivors following this pattern of relapse. Given the poor outcomes for those with post-CAR relapse, study of relapse prevention strategies, such as consolidative hematopoietic stem cell transplantation, is critical and warrants further investigation on prospective clinical trials.

Abstract: Abstract 837 Based upon demonstrated activity of mTOR inhibitors in preclinical models of ALL and a promising pilot study, we hypothesized that the use of sirolimus added to a standard GVHD prophylaxis regimen would decrease relapse and improve survival after HSCT for children with high-risk CR1 and CR2 ALL. The COG performed a randomized phase III trial comparing tacrolimus/methotrexate (tac/mtx) with tac/mtx plus sirolimus. Sirolimus was initiated on day 0 of transplant and continued at therapeutic levels for 6 months. All patients received a preparative regimen consisting of fractionated TBI (1200–1320 cGy), and cyclophosphamide (60mg/kg × 2) +/− thiotepa (5mg/kg × 2) or VP-16 (1500mg/m2). Patients with primary induction failure who attained CR, hypodipoid ALL ( 〈 44 chromosomes), and Ph+ ALL were eligible for HCT in CR1 (intermediate risk); eligible CR2 patients included B-lineage with early bone marrow (BM) relapse ( 〈 36m from diagnosis) or T-lineage with BM relapse at any time (high risk, HR) and B-lineage with late BM relapse or isolated extramedullary relapse occurring 〈 18m from diagnosis (intermediate risk, IR). HLA-matched sibling donors, 7–8/8 HLA matched related or unrelated donors, and 4–6/6 HLA matched cord blood stem cell sources were allowed. Randomization was stratified by donor type and relapse risk group. Events included relapse and treatment related mortality (TRM); the study was designed to enroll 259 patients to detect a two-year event free survival difference of 16% with 80% power. Results: A total of 146 patients enrolled, with 142 providing data to the analysis. As of July 1, 2011, there were 27 events in the 69 control arm patients and 31 events in the 73 experimental arm patients. The study was closed on May 10, 2011 when a stopping rule for futility of the primary endpoint (improved EFS) was met. The rates of grade III–IV acute GVHD, relapse, and TRM were 22%, 28% and 12% in control and 15%, 29%, and 14% in experimental arm patients (p=NS). Rates of CMV reactivation, venoocclusive Disease (VOD), and transplant associated microangiopathy (TAM) were 19%, 9%, and 1% in control arm patients and 12%, 17%, and 8% in experimental arm patients (p=0.26, 0.13, 0.10, respectively). Multistate modeling results indicate that the treatment group experienced less grade 2 to 4 aGVHD, but relapse rates in aGVHD patients were a quarter of those in non-aGVHD patients (p= 〈 .001) while TRM rates were 1.5 times that of patients with no or grade I aGVHD (p=NS, see table). Survival after sibling donor transplantation was identical to URD transplantation. There was a trend toward increased risk of relapse in recipients of cord blood (p=0.14); further analysis of HLA type, pre-HSCT minimal residual disease (MRD), and patient risk is underway to clarify this finding.NaGVHD (III–IV)RelapseTRM1 year EFSaGVHD (II–IV) not present9333 (31%)10 (11%).45a (.32–.61)aGVHD (II–IV) present497 (15%)8 (16%).79a (.60–.89)P value 〈 0.0010.38Matched Sib7513 (17%)20 (27%)5 (7%).58 (.42–.71)URD/Other RD387 (18%)7 (18%)7 (18%).54 (.32–.71)Cord296 (21%)13 (45%)6 (21%).33 (.16–.51)P value0.720.140.30 Conclusions: The addition of sirolimus to tac/mtx resulted in a trend toward higher rates of non-fatal TAM and VOD, but identical rates of severe VOD and TRM. There was a lower rate of grade II–IV aGVHD in the experimental arm, but no decrease in relapse. For the combined cohort, grade II–IV acute GVHD was associated with significantly less relapse and improved survival, suggesting that GVL is important in decreasing relapse after HSCT for pediatric ALL. The cytotoxic effect of sirolimus against ALL was insufficient to decrease relapse when given to very high risk ALL patients after HSCT as part of GVHD prophylaxis, possibly because of the offsetting effect of decreasing GVL. Disclosures: Off Label Use: The trial used sirolimus for graft vs. host disease prophylaxis in children. Sirolimus is approved for kidney transplantation in children. Borowitz:BD Biosciences: Research Funding. Grupp:Children's Hospital of Philadelphia: Patents & Royalties.

Abstract: Abstract 470 We previously reported lower rates of grade II-IV aGVHD, but no improvement in survival with the addition of sirolimus to tacrolimus/methotrexate GVHD prophylaxis in a randomized phase III Children's Oncology Group/Pediatric Blood and Marrow Transplant Consortium trial, ASCT 0431. We performed further analysis of this cohort to test whether the presence or absence of aGVHD combined with detection of pre- and/or post-HCT minimal residual disease (MRD) would allow definition of high-risk populations amenable to early intervention to prevent relapse. MRD was measured on patient bone marrow samples with multi-channel flow cytometry at a single reference laboratory (detection threshold 0.01%). The trial included children ages 1–21yrs with high-risk CR1 and CR2 ALL receiving related and unrelated donor TBI-based allogeneic HCT. The analysis included 144 eligible patients with an estimated median follow up of 757d (interquartile range 558–1022d). Both the absence of grade II-IV aGVHD at day 100 and positive MRD pre-and/or post-HCT were correlated with relapse. To illustrate the effect of aGVHD on relapse, Table 1 shows the cumulative incidence of TRM, relapse, and EFS at 100 days, 1 yr, and 2 yrs after HCT in patients with or without aGVHD. Table 1. Effect of grade II-IV aGVHD by day 100 on TRM, Relapse, and EFS Time post-HCT # of Pts at Risk TRM Relapse EFS yes aGVHD no aGVHD yes aGVHD no aGVHD yes aGVHD no aGVHD yes aGVHD no aGVHD Day 100 69 44 3.0% 6.6% 0.8% 3.7% 35% 51% 1 year 32 55 4.6% 7.4% 5.0% 24% 31% 28% 2 years 17 19 7.0% 7.4% 6.4% 30% 27% 22% Thirty eight percent of patients developed grade II-IV aGVHD (most (89%) by day +50). Analysis of patients who did or did not experience aGVHD by day 100 showed that patients who developed aGVHD rarely experienced TRM or relapse from that time forward (7.0 and 6.4% with TRM and relapse at 2 yrs post-HCT, respectively). Conversely, patients who did not develop aGVHD had a small risk of early relapse (3.7% at d100) but a steadily increasing rate of relapse starting day 200 post-HCT. Patients who did not develop grade II-IV aGVHD relapsed nearly 5 times more than those with aGVHD (30% vs. 6.4 at 2 yrs), with the result that EFS at 2 years was 19% (17/89) in those with no aGVHD vs. 35% (19/55) in those with aGVHD (p=0.001). It is important to note that the occurrence of aGVHD did not impact the rate of TRM. To assess the effect of the MRD status on relapse, we measured MRD by flow cytometry of BM pre-HCT, within 2 wks of engraftment, and at 3, 6, 9, or 12m after HCT and correlated the presence of MRD in these samples with relapse rates. More than 400 samples were analyzed. For patients positive for pre-HCT MRD, the rate ratio (RR) for relapse was 3.7 (1.6–8.2) with an estimated two-year relapse risk of 27.4% (17.4–43.1) and 70.8% (50.4–99.4) in pre-HCT MRD negative and positive patients, respectively. For patients positive for post-HCT MRD, we constructed a Cox regression model to assess the effect of positive post-HCT MRD in the context of other transplant-related variables. In the post-HCT period, an MRD positive result was associated with a 14-fold increase in relapse rate over an MRD negative result (RR=14.3, 95% CI:6.1–33.6). These findings did not change after controlling for risk group, immunophenotype, donor type and pre-HCT MRD status. Finally, we did an analysis of the combined effect of aGVHD and MRD status. Patients who developed grade II-IV aGVHD had low rates of post-HCT MRD and relapse, and a combined predictive effect of MRD status in patients with aGVHD could not be assessed. However, among patients who did not develop aGVHD, pre- and post-HCT MRD statuses were found to be independent risk factors for relapse (Table 2). Table 2. Effect of Pre- and Post-HCT MRD on Relapse risk of Patients without aGHVD Pre-HCT MRD Post HCT MRD RR for Relaspe (CI) - - 1 (reference) + - 2.5 (1.0-5.8) - + 12.7 (4.2-38.2) + + 31.2 (7.5-129.3) In summary, measurement of flow MRD on BM samples pre- and early post-HCT is feasible and positive results along with absence of grade II-IV aGVHD is highly predictive of relapse. There is a brief “window of opportunity” after day 50 and before day 200 post-HCT when most patients who will get aGVHD have experienced it, and the large majority of high-risk patients identified by MRD and aGHVD status have yet to relapse. It is imperative that novel post-HCT interventions be developed and given during this window in order to decrease relapse in the very high-risk patients we have defined. Disclosures: No relevant conflicts of interest to declare.

Abstract: Background: The Therapeutic Advances in Childhood Leukemia & Lymphoma (TACL) Consortium and Children's Oncology Group (COG) have recently adopted a modified UK ALLR3 (R3) re-induction regimen as a chemotherapy backbone to test novel agents in multiply relapsed pediatric acute lymphoblastic leukemia (ALL). A detailed toxicity profile of this platform has not previously been published, nor have data using this regimen beyond 1st relapse. Such baseline data with this backbone would be helpful in the design, conduct and assessment of future clinical trials of R3 in combination with novel agents. Methods: This was a multi-institutional, retrospective study to investigate adverse events (AEs) and outcome following R3 re-induction chemotherapy (vincristine, mitoxantrone, dexamethasone, and asparaginase) in pediatric relapsed/refractory (R/R) ALL. The objectives of the study were to: 1) establish baseline treatment related AEs during re-induction block 1; and 2) analyze the complete remission (CR) rate and minimal residual disease (MRD) status at the end of re-induction. Patients were identified using medical records from 5 pediatric oncology centers. The study was approved by IRBs at all sites. Data about diagnosis, relapse, prior therapy, AEs, and response to R3 block 1 were extracted from medical records and de-identified. AEs were graded according to the NCI CTCAE v4.0. Results: 59 patients were included in the study (Table 1). The median number of prior treatment attempts in multiply relapsed patients was 2.5 (range 2-4). Grade ≥ 3 infection was reported in 54/59 patients (92%) and 41 (76%) of these had an infectious source identified by culture, biopsy, and/or imaging. Poly-microbial infection was detected in 18 patients. Bacterial infections occurred most frequently, followed by viral and fungal infections (Table 2). The only grade 5 event in re-induction was due to sepsis. 9 patients (15%) were admitted to the intensive care unit (ICU) due to infection. The most common site of infection was blood, followed by gastrointestinal (GI) and pulmonary. Other grade ≥ 3 non-hematologic toxicities were metabolic derangements (39%), GI (22%), and pain (8%). There was no difference in the grade 〉 3 toxicities between 1st relapsed vs. multiply relapsed patients. 36 patients were discharged home prior to neutrophil count recovery. Among these patients, 92% (33/36) had hospital readmission, 7 (19%) required ICU care, and 10 (28%) required chemotherapy modification. In contrast, only 9% (2/23) of patients who remained hospitalized until count recovery received ICU care or had chemotherapy modifications. The median duration from the start of block 1 to block 2 was 39 days (range 28-115 days). 53 patients achieved CR after block 1; 5 had refractory disease; and 1 died of septic shock prior to disease evaluation. Conclusion: Our study confirmed that R3 block 1 is a highly active re-induction chemotherapy regimen for children with R/R ALL. However, it was associated with very high risk of life-threatening infections. Toxicity reported from this study should be used as a reference for future combination clinical trials using R3 as a backbone in pediatric R/R ALL. Hospitalization until blood count recovery may reduce the need for ICU care and chemotherapy modification. Table 1. Patient Characteristics and Response All Patients 1st Relapse 〉 2nd Relapse N 59 43 16 Median Age (yrs) 7 (0.4 – 19) 7 (0.4 – 19) 6.5 (2 – 15) Gender Male 41 (69%) 30 (70%) 11 (69%) Female 18 (31%) 13 (30%) 5 (31%) Site of Relapse Isolated BM 32 (54%) 22 (51%) 10 (63%) Isolated EM 15 (25%) 11 (26%) 4 (25%) Combined 12 (20%) 10 (23%) 2 (13%) Induction Death 1 (2%) 1 (2%) 0 Response Data CR/CRi 53 (90%) 41 (95%) 12 (75%) MRD - 30 (59%)* 23 (59%)* 7 (58%) MRD + 21 (41%) 16 (41%) 5 (42%) Refractory Disease 5 (8%) 1 (2%) 4 (25%) Subsequent HSCT Yes 24 (41%) 14 (33%) 10 (63%) No 35 (59%) 29 (67%) 6 (38%) BM = bone marrow; EM = extra medullary; CRi = CR with incomplete blood count recovery; HSCT = hematopoietic stem cell transplant *MRD was not performed in 2 patients. Table 2. Organism identified Biopsy/Culture confirmed or Suspected Infection by Imaging* 65 Bacterial Infection 44 (68%) Gram + 18 Gram - 19 Other 7 Fungal Infection 11 (17%) Biopsy/Culture confirmed 7 Suspected by CT/MRI 4 Viral Infection 10 (15%) Parasite 1 *Eighteen patients had 〉 1 organism identified Disclosures No relevant conflicts of interest to declare.

Abstract: Abstract 3697 Background: Iboctadekin (recombinant human interleukin-18; rhIL-18) is an immunostimulatory cytokine with anti-tumor activity in preclinical animal models. Phase I clinical trials have demonstrated that rhIL-18 is well-tolerated as monotherapy for patients with advanced solid tumors. Preclinical and clinical data indicate that rhIL-18 acts a costimulatory cytokine that may be optimally used in combination with other agents such as monoclonal antibodies. Methods: Patients with CD20+ B-cell non-Hodgkin's lymphoma (NHL) were given rituximab 375 mg/m2 IV weekly for 4 consecutive weeks in combination with rhIL-18 (in six dose cohorts of 1, 3, 10, 20, 30, and 100 mcg/kg) IV weekly for 12 weeks. Eligible patients had disease which progressed after standard therapy or for which there was no effective standard treatment. Assessments included safety/tolerability, pharmacokinetics, pharmacodynamics (serum cytokines, peripheral blood phenotypic markers and tumor biomarkers), immunogenicity, and anti-tumor activity. Results: Nineteen patients were enrolled on study (10 follicular NHL, 5 mantle cell lymphoma, 2 diffuse large B-NHL, 2 other subtypes). The combination was well-tolerated with a safety profile similar to that observed with rituximab or rhIL-18 given as monotherapy. The pharmacodynamic response is as expected based on results of prior clinical trials of rhIL-18 as monotherapy for cancer. Using the IWC response criteria, two (11%) of 18 evaluable patients had complete responses (CR) and 3 (16%) had partial responses (PR). Six (33%) patients had stable disease. Overall response rate for follicular lymphoma was 33% (1 CR, 2 PR). Conclusions: The combination of rhIL-18 and rituximab is safe, well-tolerated, and induces potent biological activity. A maximum tolerated dose of rhIL-18 was not identified. Further study of rhIL-18 plus rituximab in patients with CD20+ B cell malignancies is warranted. Disclosures: Bauman: GlaxoSmithKline: Employment, Equity Ownership. Gardner:GlaxoSmithKline: Employment, Equity Ownership. Jonak:GlaxoSmithKline: Employment, Equity Ownership. Struemper:GlaxoSmithKline: Employment, Equity Ownership. Germaschewski:GlaxoSmithKline: Employment, Equity Ownership. Koch:GlaxoSmithKline: Employment, Equity Ownership. Murray:GlaxoSmithKline: Employment, Equity Ownership. Toso:GlaxoSmithKline: Employment, Equity Ownership.

Abstract: Introduction: Chimeric antigen receptor (CAR) T-cells redirected against CD19 have demonstrated remarkable clinical activity in children and adults with relapsed/refractory (r/r) B-cell malignancies. The risk of lineage switch (LS) following CD19-directed therapies has been well documented but has been primarily limited to case reports. Additionally, the risk of subsequent malignant neoplasms (SMN) following CAR T-cells has not yet been described. Distinguishing LS (B-ALL to myeloid malignancy) from a therapy-related myeloid neoplasm is both clinically and biologically relevant. The former emerges from a highly refractory leukemic clone, likely resistant to salvage therapy, whereas the latter represents a new malignancy that can be associated with long-term survival. Methods: We conducted a multicenter, retrospective review of children and young adults with r/r B-acute lymphoblastic leukemia (B-ALL) who received either commercial tisagenlecleucel or 1 of 3 investigational murine-based CD19-CAR constructs on clinical trials at 7 US centers between 2012-2019. Patients diagnosed with B-ALL before age 25 years were included and patients who had received any prior CAR product were excluded. Results: Of 420 CAR-treated patients, with a median follow-up of 30.1 months, 12 (2.9%) experienced LS and 6 (1.4%) developed a SMN (Table). The median time to diagnosis of LS following CAR T-cell infusion was significantly shorter compared to diagnosis of SMN (65.5 days vs. 883.5 days; p=0.005). Eleven of 12 patients (91.7%) with LS converted to acute myeloid leukemia (AML). One patient converted to mixed phenotype acute leukemia, B/myeloid type. The leukemia of 10 of 12 patients with LS harbored cytogenetics similar to those at initial diagnosis. For the remaining 2 patients with LS, cytogenetics were unavailable, but the leukemias were considered LS by the treating institution. KMT2Ar rearrangement (KMT2Ar) was a predominant cytogenetic abnormality seen in patients with LS. Overall, 38 of 420 patients (9%) had a KMT2Ar. KMT2Ar was present in 9 of 12 (75%) patients with LS compared to 20 of 408 (7.1%) non-LS patients (p & lt;0.001). Patients with LS were younger at initial diagnosis compared to the remaining cohort (median age, 1.6 years vs. 7.7 years; p=0.001), reflecting the inherent association between KMT2Ar and infant ALL. Otherwise, there were no significant differences in gender, prior hematopoietic stem cell transplant (HSCT), prior blinatumomab exposure, or CAR response. Within the KMT2Ar cohort, 31 (81.6%) patients achieved a complete remission post-CAR. Eight of these patients received a consolidative HSCT (representing 4 first and 4 second HSCTs). No KMT2Ar patient experienced a post-HSCT LS and 3 are alive with a median follow-up of 1164 days post-CAR. In contrast, of the 23 KTM2Ar patients who did not receive HSCT post-CAR, 7 developed LS and 14 are alive with a median follow-up of 864 days post-CAR. Relative contraindications to post-CAR HSCT included a prior HSCT (n=11) or early LS (n=5). Of the 7 CAR non-responding patients with KMT2Ar, 2 (28.6%) had rapid emergence of LS by the first restaging timepoint. There are no long-term survivors following LS, regardless of KMT2A status, dying a median of 123 days (range, 36-594 days) after diagnosis of LS. The 6 SMNs were cholangiocarcinoma, synovial sarcoma, malignant melanoma and 3 therapy-related myeloid neoplasms (MDS/AML), distinguished from LS based on loss of original cytogenetics. Notably, 4/6 (67%) patients that developed a SMN had received an allogeneic HSCT prior to development of SMN. Four patients (67%) remain alive and in remission with a median follow-up of 304 days after diagnosis of SMN, including 2 patients with MDS/AML. Conclusions: In the largest series of pediatric patients treated with CAR T-cell therapy, we show that LS occurs in 2.9% of children. The presence of a KMT2Ar was the biggest risk factor, with 23.7% of these patients experiencing LS. We found that LS can occur very early in a patient's post CAR T-cell course, and despite a variety of treatment approaches, the outcomes for these patients are dismal. Given the predisposition to LS, the role for consolidative HSCT in KMT2Ar patients warrants further study. Limited by a short follow-up period, we saw SMNs in only 1.4% of our patients. Causality is unknown and likely unrelated to CAR-T, but this further supports the long-term safety of CAR T-cells in children with B-ALL. Figure 1 Figure 1. Disclosures Borowitz: Amgen, Blueprint Medicines: Honoraria. Lee: Harpoon Therapeutics: Consultancy; Amgen: Membership on an entity's Board of Directors or advisory committees; BMS: Membership on an entity's Board of Directors or advisory committees; Kite Pharma: Other: trial funding; Gilead: Other: trial funding. Grupp: Novartis, Adaptimmune, TCR2, Cellectis, Juno, Vertex, Allogene and Cabaletta: Other: Study steering committees or scientific advisory boards; Novartis, Roche, GSK, Humanigen, CBMG, Eureka, and Janssen/JnJ: Consultancy; Novartis, Kite, Vertex, and Servier: Research Funding; Jazz Pharmaceuticals: Consultancy, Other: Steering committee, Research Funding. Verneris: jazz: Other: advisory board; Novartis: Other: advisory board; Fate Therapeutics: Consultancy. Gore: Mirati: Current equity holder in publicly-traded company; Novartis: Consultancy, Membership on an entity's Board of Directors or advisory committees; Amgen: Consultancy, Current equity holder in publicly-traded company, Honoraria, Membership on an entity's Board of Directors or advisory committees; Roche/Genentech: Consultancy, Honoraria; Clovis: Current equity holder in publicly-traded company; Celgene: Current equity holder in publicly-traded company, Membership on an entity's Board of Directors or advisory committees; Sanofi Paris: Current equity holder in publicly-traded company; Anchiano: Current equity holder in publicly-traded company; Blueprint Medicines: Current equity holder in publicly-traded company. Brown: Novartis: Membership on an entity's Board of Directors or advisory committees; Takeda: Membership on an entity's Board of Directors or advisory committees; Amgen: Membership on an entity's Board of Directors or advisory committees; Kura: Membership on an entity's Board of Directors or advisory committees; KIte: Membership on an entity's Board of Directors or advisory committees. Pulsipher: Equillium: Membership on an entity's Board of Directors or advisory committees; Adaptive: Research Funding; Jasper Therapeutics: Honoraria. Rheingold: Pfizer: Research Funding; Optinose: Other: Spouse's current employment. Gardner: BMS: Patents & Royalties; Novartis: Consultancy.

Abstract: Target identification for chimeric antigen receptor (CAR) T-cell therapies remains challenging due to the limited repertoire of tumor-specific surface proteins. Intracellular proteins presented in the context of cell surface HLA provide a wide pool of potential antigens targetable through T-cell receptor mimic antibodies. Mass spectrometry (MS) of HLA ligands from 8 hematologic and nonhematologic cancer cell lines identified a shared, non-immunogenic, HLA-A*02–restricted ligand (ALNEQIARL) derived from the kinetochore-associated NDC80 gene. CAR T cells directed against the ALNEQIARL:HLA-A*02 complex exhibited high sensitivity and specificity for recognition and killing of multiple cancer types, especially those of hematologic origin, and were efficacious in mouse models against a human leukemia and a solid tumor. In contrast, no toxicities toward resting or activated healthy leukocytes as well as hematopoietic stem cells were observed. This shows how MS can inform the design of broadly reactive therapeutic T-cell receptor mimic CAR T-cell therapies that can target multiple cancer types currently not druggable by small molecules, conventional CAR T cells, T cells, or antibodies.

Abstract: CAR T-cell targeting of leukemic infiltrates in the optic nerve and retina caused retinal detachment as a presentation of pseudoprogression. Treatment of this intraocular inflammation with intravitreal triamcinolone and orbital radiation led to marked improvement in visual acuity.

Abstract: Background: The youngest patients referred for CAR T cell therapy are those with relapsed or refractory (R/R) KMT2A-rearranged infant B-ALL. Infants with relapsed ALL following Interfant-99 therapy have a dismal reported 3-yr OS of 20.9%, indicating the need for novel therapies. Smaller patient size, heavily pre-treated disease and high leukemia burden are often characteristics of this subgroup of patients that pose unique challenges to apheresis and manufacture of a T cell product. Additionally, reports of KMT2A-rearranged leukemia undergoing lineage switch following CD19-targeting pressure raises concern for an increased risk of myeloid leukemia relapses after B-lineage targeted CAR T cell therapy in this population. Here we report our experience using CAR T cell immunotherapy for patients with R/R infant ALL enrolled on clinical trials PLAT-02 (NCT02028455) and PLAT-05 (NCT03330691). Methods: PLAT-02 is a phase 1/2 trial of CD19-specific (FMC63scFv:IgG4hinge:CD28tm:4-1BB:ζ) CAR T cells. PLAT-05 is a phase 1 trial of CD19xCD22 dual specific CAR T cells, transduced with two separate lentiviral vectors to direct the co-expression of the CD19-specific CAR above and a CD22-specific CAR (m971scFv:IgG4hinge-CH2(L235D)-CH3-CD28tm:4-1BB:ζ). Eligible subjects on both studies have R/R B-ALL, an absolute lymphocyte count ≥100 cells/µL, and were at least 1 year of age. In addition, subjects on PLAT-02 were ≥ 10kg, and ≥ 8kg on PLAT-05. For cell manufacture, apheresis products were immuno-magnetically selected for CD4 and CD8 cells. Selected T cells were activated with anti-CD3/CD28 beads, transduced, and grown in culture with homeostatic cytokines to numbers suitable for clinical use. Infant ALL subjects received a range of 5x105 to 10x106 CAR+ T cells/kg following lymphodepleting chemotherapy. Disease response assessments were required at Day 21 and Day 63 following CAR T cell infusion. Adverse events were graded according to CTCAEv4, except CRS which was graded according to 2014 Lee criteria. Results: Eighteen subjects with R/R infant ALL have enrolled on PLAT-02 (n=14) or PLAT-05 (n=4), with a median age of 22.5 months at enrollment (range: 14.5 - 40.1 months). Of these, 2 (11.1%) had primary refractory disease, 8 (44.4%) were in 1st relapse, 7 (38.9%) were in 2nd relapse and 1 (5.6%) was in 3rd or greater relapse. Ten subjects (55.6%) had an M2 marrow or greater at enrollment prior to apheresis, and 9/18 had a history of hematopoietic cell transplant (HCT). The mean ALC was 1309 cells/µL (range 253-6944). Successful CAR T cell products were manufactured in 17/18 subjects, including in 9/9 subjects with no prior history of HCT. Of these, 16/17 subjects with available products were infused, with a median follow up of 26.9 months. One subject died of disease complications prior to CAR T cell infusion. Of the 16 treated subjects, 1 is pending disease and toxicity assessments. The maximum grade of CRS was 3 and occurred in two of 15 evaluable subjects (13%) and neurotoxicity was limited to a maximum grade of 2. Fourteen of 15 (93.3%) achieved an MRD negative complete remission (MRD-CR) by Day 21. Of the 14 subjects with an MRD-CR, 6 went on to HCT with 1 subsequent CD19 negative relapse. Of the 8 subjects who did not proceed to HCT, 1 developed lineage switch at one month following CAR T cells, and 1 died of infectious complications with aplasia. A "wait and watch" approach was taken for the remaining 6 subjects, and 2 developed CD19+ relapse. The incidence of lineage switch among the infant ALL group was 1/15 (6.7%). The estimated 1-year LFS was 66.7% and 1-year OS was 71.4%. Conclusion: This is the largest reported cohort to date of R/R infant B-ALL subjects treated with CAR T cell therapy. We report successful manufacture and administration of a CAR T cell product in the significant majority of infant subjects. Toxicity and MRD-CR rates are comparable to that of non-infant ALL subjects. In our experience, subjects with R/R infant ALL are not at increased risk for lineage switch relapse compared with the entire study populations following B-antigen targeting CAR T cell immunotherapy. Numbers in this report are too small to make definitive conclusions about the value of consolidative HCT. However, the LFS of this cohort is remarkably higher when compared with historical controls. Future work is focused on overcoming feasibility issues for the smallest of subjects, to enable a larger number of these cases to access CAR T cell therapy. Disclosures Pulsipher: Amgen: Other: Lecture; Bellicum: Consultancy; Miltenyi: Research Funding; Medac: Honoraria; Novartis: Consultancy, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Jazz: Other: Education for employees; Adaptive: Membership on an entity's Board of Directors or advisory committees, Research Funding; CSL Behring: Membership on an entity's Board of Directors or advisory committees. Wayne:AbbVie: Consultancy; Spectrum Pharmaceuticals: Consultancy, Research Funding; Servier: Consultancy; Kite, a Gilead Company: Consultancy, Research Funding. Jensen:Bluebird Bio: Research Funding; Juno Therapeutics, a Celgene Company: Research Funding. Gardner:Novartis: Honoraria.