Abstract: 7552 Background: Axicabtagene Ciloleucel (Axi-cel) is an autologous anti-CD19 CAR T-cell therapy approved for the treatment of relapsed/refractory diffuse large B-cell lymphoma (r/r DLBCL). Long‐term analysis of the Zuma‐1 clinical trial showed ~40% of patients remained progression-free at 2 years (Locke, Lancet Oncology 2018). Early identification of patients who will later progress after CAR T cell therapy may improve clinical care and patient outcomes. Methods: As of 2/1/2019, we enrolled 50 patients on a multi-institutional, prospective study measuring circulating tumor DNA(ctDNA) minimal residual disease (MRD) in r/r DLBCL patients undergoing treatment with Axi-cel. Using an next generation sequencing-MRD assay (Adaptive Biotechnologies; Seattle WA), ctDNA levels were measured pre, 0, 7, 14, 21, 28, 56, 90, 180, 270, and 365 days following Axi-cel infusion. A pre-planned comparison between EDTA, Streck, and CFD tubes for the initial 10 enrolled patients determined the CFD tube provided optimal analyte stability over 144 hours following sample collection. CFD tubes are being used to collect all study samples. Results: 24 subjects have 3 or more months clinical follow up and their treatment ctDNA MRD significantly associated with clinical outcomes. A day 28 landmark analysis shows 12 patients were MRD negative and 12 patients were MRD positive as defined by detection of none or any tumor-associated ctDNA, respectively. 10 of 12 MRD+ patients subsequently developed progressive disease. In contrast, only 2 MRD- patients subsequently progressed and the 10 other patients remain in a CR. (p = 0.0033, Fisher's exact test). With a median follow up of 237 days, median PFS after Axi-cel infusion for day 28 MRD+ vs. MRD- patients is 93 days vs. not reach, p = 0.0010 by Log-rank test. Median OS for day 28 MRD+ vs. MRD- patients is 281 days after Axi-cel infusion vs. not reach, p = 0.0399 by Log-rank test. Conclusions: After Axi-cel infusion, day 28 ctDNA-based MRD significantly associates with PFS and OS and identified early at-risk patients prior to progression. These results provide a rationale for designing MRD-based risk-adaptive CAR T-cell clinical trials.

Abstract: CD19 CAR T cells have revolutionized the treatment of relapsed and refractory (R/R) large B cell lymphomas (LBCL), mediating durable complete responses in approximately 40-50% of patients. Besides a loss or decrease in CD19 expression, no studies have identified tumor specific factors driving inherent or acquired resistance to CAR T cells in LBCL. Mutations in and loss of expression of LFA-3 (CD58) have been described in approximately 20% of cases of LBCL. As the ligand for CD2 on T cells, CD58 provides costimulation to T cells and CD58 loss or mutation has been linked to immune resistance in LBCL. We evaluated CD58 status in fifty-one R/R LBCL patients treated at Stanford with commercial axicabtagene ciloleucel (axi-cel) through immunohistochemistry (IHC) on tumor biopsy samples and/or deep sequencing of circulating tumor DNA by CAPP-Seq. We identified 12/51 (24%) patients with a CD58 aberration (lack of expression by IHC or mutation by CAPP-Seq). Progression-free survival (PFS) was significantly decreased in patients with a CD58 aberration (median PFS for CD58 aberration 3 months vs. not reached for CD58 intact, p & lt;0.0001). In fact, only 1/12 patients with a CD58 alteration achieved a durable, complete response to axi-cel, while the remaining 11 patients progressed, most commonly after a period of initial response. Partial responses were more common among patients with CD58 aberrations (58% for CD58 aberration vs 10% for CD58 intact, p & lt;0.001), and complete responses were less common (25% for CD58 aberration vs 82% for CD58 intact, p & lt;0.0001). To probe the biology of CAR T cell responses towards tumors lacking functional CD58, we generated a CD58 knockout Nalm6 model. CD19.CD28.ζ, CD19.4-1BB.ζ, and CD22.4-1BB.ζ CAR T cells demonstrated significantly reduced cytokine production and cytolytic activity in response to CD58 KO vs wildtype (WT) tumor cells. Additionally, while mice inoculated with WT Nalm6 and treated with any of the three CARs demonstrate complete responses and prolonged leukemia-free survival, mice inoculated with CD58KO Nalm6 demonstrated only partial responses, eventual tumor progression, and death from leukemia. CD2, the T cell ligand for CD58, plays both an adhesive role and a costimulatory role in T cells. CD2 knockout resulted in significantly reduced cytokine production after CAR stimulation. Re-expression of only the CD2 extracellular domain did not rescue CAR function, indicating that CD2 signaling is essential for full CAR activation. Additionally, when we stimulated CD19 CAR T cells with anti-idiotype antibody (CAR stimulation), soluble CD58 (CD2 stimulation), or both, we observed significantly enhanced phosphorylation of both CD3ζ and ERK by western blot in CAR T cells stimulated through both the CAR and CD2. Phosphorylation analysis by mass spectrometry revealed that CD2 stimulation enhances phosphorylation of proximal signaling molecules in the TCR pathway (LCK, LAT, CD3ε among others) and also mediators of actin-cytoskeletal rearrangement in CAR T cells, consistent with effects in natural T cell responses. To overcome CD58 loss in LBCL, we generated second- and third-generation CAR T cell constructs integrating CD2 costimulatory domains within the CAR molecule. While these cis constructs demonstrated increased potency against CD58KO cells in vitro, they were unable to ultimately overcome CD58 loss in vivo. However, when CARs were co-expressed with an additional CD2 receptor in trans, they mediated significant anti-tumor activity in vivo, overcoming CD58 knockout in tumor cells. In conclusion, we have identified that CD58 status is an important biomarker for durable response to CAR T cells in LBCL. We modeled the biologic basis for this finding and generated CAR T cells capable of overcoming CD58 loss in B cell malignancies. CD58 mutations have been reported in many cancers, including multiple myeloma and colon cancer, and are likely to play a role in immune evasion for CAR T cells as they are developed for additional histologies. These data provide rationale for investigating CD58 status for patients receiving CAR based therapeutics and devising next generation CARs capable of overcoming this newly discovered mechanism of resistance. Disclosures Majzner: Xyphos Biopharma: Consultancy; Zai Lab: Consultancy; Lyell Immunopharma: Consultancy; GammaDelta Therapeutics: Membership on an entity's Board of Directors or advisory committees; Aprotum Group: Consultancy; Illumina Radiopharmaceuticals: Consultancy. Kurtz:Roche: Consultancy; Genentech: Consultancy; Foresight Diagnostics: Other: Ownership. Sotillo:Lyell Immunopharma: Consultancy, Other: Consultancy. Alizadeh:Janssen: Consultancy; Genentech: Consultancy; Pharmacyclics: Consultancy; Chugai: Consultancy; Celgene: Consultancy; Gilead: Consultancy; Roche: Consultancy; Pfizer: Research Funding. Miklos:Miltenyi Biotec: Research Funding; Janssen: Consultancy, Other: Travel support; Pharmacyclics: Consultancy, Other: Travel support, Patents & Royalties, Research Funding; Novartis: Consultancy, Other: Travel support, Research Funding; Allogene Therapeutics Inc.: Research Funding; Juno-Celgene-Bristol-Myers Squibb: Consultancy, Other: Travel support, Research Funding; Kite-Gilead: Consultancy, Membership on an entity's Board of Directors or advisory committees, Other: Travel support, Research Funding; Adaptive Biotech: Consultancy, Other: Travel support, Research Funding. Mackall:BMS: Consultancy; Allogene: Current equity holder in publicly-traded company; Apricity Health: Consultancy, Current equity holder in private company; Nektar Therapeutics: Consultancy; NeoImmune Tech: Consultancy; Lyell Immunopharma: Consultancy, Current equity holder in private company.

Abstract: Introduction Axi-cel is an autologous anti-CD19 CAR T-cell therapy approved by the US FDA 10/18/2017, for the treatment of adults with relapsed or refractory (r/r) large B-cell lymphoma (DLBCL), primary mediastinal large B-cell lymphoma (PMBCL), transformed lymphoma (tFL), and high-grade B-cell lymphoma (HGBCL) who have failed at least two prior systemic lines of therapy. In the pivotal ZUMA-1 trial, 108 patients (pts) with r/r DLBCL were treated with axi-cel: the best overall response rate (ORR) was 82% and complete response (CR) rate was 58%. At a median follow-up of 15.4 months, 42% of the pts had ongoing remission (Neelapu and Locke et al. NEJM 2017). Grade 3 or higher cytokine release syndrome (CRS) by Lee criteria and neurologic events (NEs) occurred in 13% and 31% of the pts, respectively. Here, we evaluated the real world outcomes of pts treated with standard of care axi-cel under the commercial FDA label. Methods and Results Seventeen US academic centers contributed data to this effort independently of the manufacturer. As of 6/30/2018, 211 pts were leukapheresed with intention to manufacture commercial axi-cel. Of these, 165 (78%) pts completed axi-cel infusion as of 6/30/18 and a further 23 (11%) pts are scheduled for axi-cel infusion in July 2018. Of the 23 remaining pts, 7 (3%) received axi-cel therapy on ZUMA-9 expanded access trial (NCT03153462) due to non-conforming cell therapy product, 15 (7%) pts died before axi-cel infusion (14 from lymphoma progression and 1 from sepsis) and 1 (1%) patient attained CR from bridging therapy and was not infused. Safety was evaluable in 163 pts receiving axi-cel. Grade ³3 CRS and NEs occurred in 7% and 31% of pts. Tocilizumab was administered in 62% of pts and 57% received corticosteroids. Outside of lymphoma progression, 3 deaths occurred post-axi-cel; 1 due to HLH, 1 due to systemic candidiasis, and a third due to septic shock. There were no grade 5 NEs observed. Response assessment was done for pts infused with axi-cel and who were re-staged at day 30 and/or day 100, or were deemed to have clinical progression. Of 112 pts evaluable at day 30, ORR was 79% with 50% CR, 29% PR, 6% SD and 14% with PD. Of 39 pts evaluable at day 100, 59% of pts had ongoing response (CR 49%, PR 10%). At the time of abstract submission, more detailed patient characteristics and treatment course data were available in 134/165 pts infused. Median age was 59 (range 21-82) with 57% male. Performance status (PS) was ECOG 0-1 (81%), ECOG 2 (16%) and ECOG 3 (3%). By histology, 61% of pts had DLBCL including HGBCL, 31% had tFL and 8% PMBCL. Thirty-one percent had a prior autologous stem cell transplant. Bridging therapy between apheresis and infusion was given in 56% of patients, the majority of which consisted of chemotherapy. Sixty-six of 134 (49%) would not have met eligibility criteria for ZUMA-1 at the time of leukapheresis. Common criteria that would have made these patients ineligible for ZUMA-1 included ECOG PS 〉 1 (n = 22), platelets 〈 75k (n = 17), GFR 〈 60 (n =12), active DVT/PE (n = 13), liver enzyme abnormalities (n = 10), a history of CNS lymphoma (n = 8), recent checkpoint inhibitor therapy (n = 7), ejection fraction 〈 50% (n = 4), prior CD19 CAR T therapy (n = 4), and prior allogeneic transplant (n=2). Median time from leukapheresis to start of conditioning chemotherapy was 21 days and median time from leukapheresis to axi-cel infusion was 26 days. Median hospitalization period was 14 days. Conclusions This multicenter retrospective study delineates the real world outcomes of axi-cel CAR T-cell therapy for r/r aggressive B-cell lymphoma when used as a standard of care. Though limited by relatively short follow up, 30 day responses in the real world setting are comparable to the best responses observed on the pivotal ZUMA-1 clinical trial (Table 1). Importantly, safety appears comparable to the ZUMA-1 trial despite nearly half the pts failing to meet ZUMA-1 eligibility criteria. Updated results including PFS and OS will be presented at the meeting. L.J.N. and M.D.J. contributed equally; D.B.M., S.S.N. and F.L.L. contributed equally. Disclosures Nastoupil: Merck: Honoraria, Research Funding; Celgene: Honoraria, Research Funding; TG Therappeutics: Research Funding; Karus: Research Funding; Genentech: Honoraria, Research Funding; Janssen: Research Funding; Gilead: Honoraria; Juno: Honoraria; Novartis: Honoraria; Spectrum: Honoraria. Lunning:Celgene: Consultancy; Spectrum: Consultancy; Portola: Consultancy; Genzyme: Consultancy; Janssen: Consultancy; Seattle Genetics: Consultancy; Kite: Consultancy; Juno: Consultancy; AbbVie: Consultancy; Astra-Zeneca: Consultancy; Genentech: Consultancy; Gilead: Consultancy; Genentech: Consultancy; Bayer: Consultancy; TG Therapeutics: Consultancy; Verastem: Consultancy. Reagan:Seattle Genetics: Research Funding. McGuirk:Gamida Cell: Research Funding; Astellas Pharma: Research Funding; Fresenius Biotech: Research Funding; Bellicum Pharmaceuticals: Research Funding; Novartis Pharmaceuticals Corporation: Honoraria, Other: speaker, Research Funding; Kite Pharma: Honoraria, Other: travel accommodations, expenses, speaker ; Pluristem Ltd: Research Funding. Deol:Novartis: Consultancy; Kite Pharmaceuticals: Consultancy. Hill:Pharmacyclics: Honoraria, Membership on an entity's Board of Directors or advisory committees; Genentech: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Seattle Genetics: Honoraria, Membership on an entity's Board of Directors or advisory committees; Amgen: Research Funding; Seattle Genetics: Honoraria, Membership on an entity's Board of Directors or advisory committees; Abbvie: Honoraria, Membership on an entity's Board of Directors or advisory committees; Pfizer: Honoraria, Membership on an entity's Board of Directors or advisory committees; Novartis: Honoraria, Membership on an entity's Board of Directors or advisory committees; Novartis: Honoraria, Membership on an entity's Board of Directors or advisory committees; Pfizer: Honoraria, Membership on an entity's Board of Directors or advisory committees; Pharmacyclics: Honoraria, Membership on an entity's Board of Directors or advisory committees; Abbvie: Honoraria, Membership on an entity's Board of Directors or advisory committees. Andreadis:Gilead: Consultancy; Genentech: Consultancy, Employment; Astellas: Consultancy; Bayer: Consultancy; Celgene: Consultancy; Seattle Genetics: Consultancy; Pharmacyclics: Consultancy; Juno: Research Funding; Kite: Consultancy; Novartis: Consultancy, Research Funding. Munoz:Pfizer: Consultancy; Bayer: Consultancy, Speakers Bureau; Juno: Consultancy, Honoraria; Janssen: Consultancy; Genentech: Consultancy, Honoraria; Bristol-Myers Squibb: Consultancy; Pharmacyclics: Consultancy, Honoraria; Kite: Consultancy, Honoraria, Speakers Bureau; Alexion: Consultancy; Gilead: Speakers Bureau. Westin:Novartis Pharmaceuticals Corporation: Membership on an entity's Board of Directors or advisory committees; Celgen: Membership on an entity's Board of Directors or advisory committees; Kite Pharma: Membership on an entity's Board of Directors or advisory committees; Apotex: Membership on an entity's Board of Directors or advisory committees. Vose:Kite Pharma: Research Funding; Acerta Pharma: Research Funding; Celgene: Research Funding; Novartis: Honoraria, Research Funding; Roche: Honoraria; Seattle Genetics, Inc.: Research Funding; Legend Pharmaceuticals: Honoraria; Merck Sharp & Dohme Corp.: Research Funding; Abbvie: Honoraria; Epizyme: Honoraria; Bristol Myers Squibb: Research Funding; Incyte Corp.: Research Funding. Miklos:Genentech: Research Funding; Kite - Gilead: Consultancy, Research Funding; Novartis: Consultancy, Research Funding; Pharmacyclics - Abbot: Consultancy, Research Funding; Janssen: Consultancy, Research Funding; Adaptive Biotechnologies: Consultancy, Research Funding. Locke:Kite Pharma: Other: Scientific Advisor; Cellular BioMedicine Group Inc.: Consultancy; Novartis Pharmaceuticals: Other: Scientific Advisor.

Abstract: The prognosis of patients with large B-cell lymphoma (LBCL) that progresses after treatment with chimeric antigen receptor (CAR) T-cell therapy targeting CD19 (CAR19) is poor. We report on the first 3 consecutive patients with autologous CAR19-refractory LBCL who were treated with a single infusion of autologous 1 × 106 CAR+ T cells per kilogram targeting CD22 (CAR22) as part of a phase 1 dose-escalation study. CAR22 therapy was relatively well tolerated, without any observed nonhematologic adverse events higher than grade 2. After infusion, all 3 patients achieved complete remission, with all responses continuing at the time of last follow-up (mean, 7.8 months; range, 6-9.3). Circulating CAR22 cells demonstrated robust expansion (peak range, 85.4-350 cells per microliter), and persisted beyond 3 months in all patients with continued radiographic responses and corresponding decreases in circulating tumor DNA beyond 6 months after infusion. Further accrual at a higher dose level in this phase 1 dose-escalation study is ongoing and will explore the role of this therapy in patients in whom prior CAR T-cell therapies have failed. This trial is registered on clinicaltrials.gov as #NCT04088890.

Abstract: Introduction: Autologous anti-CD19 chimeric antigen receptor T cells (CAR19) have shown dramatic clinical responses in relapsed-refractory large B cell lymphomas, but more than 50% of patients will have disease progression. Here we characterize the observed mechanisms of treatment failure following Axicabtagene Ciloleucel (Axi-cel) therapy. Methods: Sixty-nine patients with refractory large B cell lymphoma were referred for CAR19 therapy from October 2017 to June 2018. The WHO diagnosis and B cell antigen expression on lymphoma cells were assessed by immunohistochemistry and/or flow cytometry before and at the time of progression. We assessed peripheral blood CAR-T cell numbers at Days 7, 14, 21, and 28 by flow cytometry immunophenotyping and monitored disease response with PET-CT at Day 28, 3 months, and 6 months post-infusion. Results: Twenty-two patients who received CAR19 therapy, including patients with transformed large B cell lymphoma (N =5), diffuse large B cell lymphoma, not otherwise specified (N =11), high grade B cell lymphoma, not otherwise specified (N = 2), primary mediastinal large B cell lymphoma (N = 2), and high grade B cell lymphoma with rearrangement of MYC and BCL2 or BCL6 (N =2). The Day 28 ORR was 86%: 10 patients had complete response, 9 had a partial response, 1 had stable disease, and 2 had progressive disease. There was no statistically significant difference in age, gender, underlying disease, or number of prior treatment regimens between patients who achieved a clinical response versus those who failed therapy. Both patients (2 of 2) with progressive disease at Day 28 showed dim or partial CD19 expression prior to CAR-T infusion but nonetheless demonstrated robust Axi-cel expansion. In contrast, one patient with Day 28 stable disease showed no CAR-T cell expansion despite intact CD19 expression. Overall, there was no statistical difference in relative or absolute CAR+ T cells in patients who responded versus those who did not at Day 28 (Figure 1). Day 90 response was available for 12 patients with either CR or PR at Day 28. Five patients (26%) developed progressive disease, and 4 of 5 underwent repeat biopsy. Of these patients, 2 had complete loss of tumor CD19 (Figure 2) and another had downregulation of CD19 with variable expression of other B cell antigens. Conclusion: Eight of 22 (36%) of patients who underwent CAR19 infusion did not respond or relapsed after Day 28 response. Five patients (62%) who failed therapy had loss or downregulation of CD19, which emphasizes that single target antigen loss is a common mechanism of CAR-T failure. However, lack of CAR-T cell expansion was noted in multiple patients, suggesting that there may be T cell intrinsic causes of treatment failure. Further studies are necessary to help identify and predict which patients will benefit from targeted immunotherapy. Disclosures No relevant conflicts of interest to declare.

Abstract: Introduction: Chimeric antigen receptor (CAR) T cells targeting either CD19 or CD22 have yielded striking complete remission (CR) rates of 70%-90% in patients with relapsed/refractory B-cell acute lymphoblastic leukemia (ALL), but CD19 negative and CD22 low relapse limits the curative potential of these single-antigen CAR T cell approaches. We hypothesized that a bivalent CAR-T construct that can target CD19 and/or CD22 would prevent antigen negative/low relapse. Here we present the combined single institution experience to date of pediatric and adult patients with R/R ALL treated with this novel bispecific CAR. Methods: We conducted parallel Phase I clinical trials of CD19/CD22 bispecific CAR T cells in pediatric and adult patients with relapsed/refractory ALL. We utilized lentiviral transduction of a bivalent CAR construct incorporating the fmc63 CD19 and m971 CD22 single chain variable fragments (scFvs) and a 41BB costimulatory endodomain. After lymphodepletion with fludarabine and cyclophosphamide, patients were infused with fresh or cryopreserved CAR T cells manufactured using a 7-11 day process. Two dose levels were tested during dose escalation: Dose level 1 was 1x106 CAR T cells/kg and dose level 2 was 3x106 cells/kg. Primary objectives assessed the ability to successfully manufacture CAR19/22 CAR T cells and safety while response at Day 28 post-infusion was a secondary objective. Blood, bone marrow and cerebrospinal fluid samples were obtained at protocol defined intervals for correlative biology studies. Results: Nineteen patients have been enrolled (10 pediatric; 9 adult) with a median age of 23 years (range, 2-68) and median of 4 (range, 2-11) prior lines of leukemia-directed therapy. Ten patients received prior HCT, 9 were treated with prior Blinatumomab, 3 with prior CD19 directed CAR T cells and 4 with prior Inotuzumab. Fourteen patients (8 pediatric, 6 adult) have been infused to date with CD19/CD22 bispecific CAR T cells; 7 were treated at dose level 1 (DL1) and 7 at dose level 2 (DL2). Successful manufacturing of cells at target dose levels was achieved in all patients. Twelve patients have reached day 28 and are included in the safety and response analysis presented here. Nine of 12 (75%) experienced cytokine release syndrome (CRS) and 2/12 (17%) developed immune-effector cell neurotoxicity syndrome (ICANS). The CRS and ICANS were all grade 1 or 2 across both dose levels and across pediatric and adult patients except for one adult with high disease burden who experienced grade 4 CRS and grade 4 ICANS, both of which were reversible. No differences in toxicities were seen across the patient age spectrum and there were no cases of treatment-related mortality within 28 days following CAR T infusion. Eleven of 12 (92%) patients achieved a CR, 10 of whom achieved CR at day 28 and one with a PR of extramedullary disease at day 28 which improved to CR by day 180 without further leukemia-directed intervention. One patient had primary progressive disease prior to day 28. Peak CAR expansion as detected by peripheral blood flow cytometry reached a median level of 11.13% (DL1) and 29.1% (DL2) CAR T of CD3+ cells with a range of 0.7-22.54% and 3.8-86.96%, respectively. To date, 3 patients (1 pediatric and 2 adult patients) have relapsed, all with retention of CD19. Post-remission practice differed across pediatric and adult patients; Six pediatric patients reaching day 28 underwent consolidative hematopoietic cell transplantation (HCT) whereas no adult patients received subsequent HCT. One patient died from complications post HCT while in remission. Therefore, the overall survival for all infused patients was 92% with a median follow-up of 9.5 months from time of infusion (range, 1-20). Conclusion: The combined pediatric and adult phase I trials of bispecific CD19/CD22 targeting CAR T cells in relapsed/refractory ALL demonstrates safety and tolerability at two dose levels. Expanded accrual at dose level 2 is ongoing and clinical outcomes will be updated. This work additionally demonstrates feasibility of delivering unified B-ALL CAR T cell therapy across age boundaries. Multi-parametric CyTOF studies permitting CAR T cell phenotyping in conjunction with single cell TCR tracking, proteomics, epigenomics and cytokine profiling are ongoing and will be used to further characterize persisting CAR T cells and define inter-product and inter-patient variability. Disclosures Muffly: Pfizer: Consultancy; KITE: Consultancy; Adaptive: Research Funding. Majzner:Xyphos Inc.: Consultancy; Lyell Immunopharma: Consultancy. Feldman:Octane Biotech, Inc.: Employment; Personalized Medicine Initiative Science: Membership on an entity's Board of Directors or advisory committees. Miklos:Adaptive Biotechnologies: Membership on an entity's Board of Directors or advisory committees; Kite-Gilead: Membership on an entity's Board of Directors or advisory committees, Research Funding; BMS: Membership on an entity's Board of Directors or advisory committees; Celgene: Membership on an entity's Board of Directors or advisory committees; Juno: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees, Research Funding; Becton Dickinson: Research Funding; Miltenyi Biotech: Membership on an entity's Board of Directors or advisory committees; Precision Bioscience: Membership on an entity's Board of Directors or advisory committees; AlloGene: Membership on an entity's Board of Directors or advisory committees. Mackall:Obsidian: Research Funding; Lyell: Consultancy, Equity Ownership, Other: Founder, Research Funding; Nektar: Other: Scientific Advisory Board; PACT: Other: Scientific Advisory Board; Bryologyx: Other: Scientific Advisory Board; Vor: Other: Scientific Advisory Board; Roche: Other: Scientific Advisory Board; Adaptimmune LLC: Other: Scientific Advisory Board; Glaxo-Smith-Kline: Other: Scientific Advisory Board; Allogene: Equity Ownership, Membership on an entity's Board of Directors or advisory committees; Apricity Health: Equity Ownership, Membership on an entity's Board of Directors or advisory committees; Unum Therapeutics: Equity Ownership, Membership on an entity's Board of Directors or advisory committees.

Abstract: Axicabtagene ciloleucel (Axi-cel) is an autologous anti-CD19 chimeric antigen receptor (CAR) T-cell therapy approved for the treatment of relapsed or refractory diffuse large B-cell lymphoma (r/r DLBCL). Long-term analysis of the ZUMA-1 phase 1-2 clinical trial showed that ~40% of Axi-cel patients remained progression-free at 2 years (Locke et al., Lancet Oncology 2019). Those patients who achieved a complete response (CR) at 6 months generally remained progression-free long-term. The biological basis for achieving a durable CR in patients receiving Axi-cel remains poorly understood. Here, we sought to identify CAR T-cell intrinsic features associated with CR at 6 months in DLBCL patients receiving commercial Axi-cel at our institution. Using mass cytometry, we assessed expression of 33 surface or intracellular proteins relevant to T-cell function on blood collected before CAR T cell infusion, on day 7 (peak expansion), and on day 21 (late expansion) post-infusion. To identify cell features that distinguish patients with durable CR (n = 11) from those who developed progressive disease (PD, n = 14) by 6 months following Axi-cel infusion, we performed differential abundance analysis of multiparametric protein expression on CAR T cells. This unsupervised analysis identified populations on day 7 associated with persistent CR or PD at 6 months. Using 10-fold cross-validation, we next fitted a least absolute shrinkage and selection operator (lasso) model that identified two clusters of CD4+ CAR T cells on day 7 as potentially predictive of clinical outcome. The first cluster identified by our model was associated with CR at 6 months and had high expression of CD45RO, CD57, PD1, and T-bet transcription factor. Analysis of protein co-expression in this cluster enabled us to define a simple gating scheme based on high expression of CD57 and T-bet, which captured a population of CD4+ CAR T cells on day 7 with greater expansion in patients experiencing a durable CR (mean±s.e.m. CR: 26.13%±2.59%, PD: 10.99%±2.53%, P = 0.0014). In contrast, the second cluster was associated with PD at 6 months and had high expression of CD25, TIGIT, and Helios transcription factor with no CD57. A CD57-negative Helios-positive gate captured a population of CD4+ CAR T cells was enriched on day 7 in patients who experienced progression (CR: 9.75%±2.70%, PD: 20.93%±3.70%, P = 0.016). Co-expression of CD4, CD25, and Helios on these CAR T cells highlights their similarity to regulatory T cells, which could provide a basis for their detrimental effects. In this exploratory analysis of 25 patients treated with Axi-cel, we identified two populations of CD4+ CAR T cells on day 7 that were highly associated with clinical outcome at 6 months. Ongoing analyses are underway to fully characterize this dataset, to explore the biological activity of the populations identified, and to assess the presence of other populations that may be associated with CAR-T expansion or neurotoxicity. This work demonstrates how multidimensional correlative studies can enhance our understanding of CAR T-cell biology and uncover populations associated with clinical outcome in CAR T cell therapies. This work was supported by the Parker Institute for Cancer Immunotherapy. Figure Disclosures Muffly: Pfizer: Consultancy; Adaptive: Research Funding; KITE: Consultancy. Miklos:Celgene: Membership on an entity's Board of Directors or advisory committees; BMS: Membership on an entity's Board of Directors or advisory committees; Kite-Gilead: Membership on an entity's Board of Directors or advisory committees, Research Funding; AlloGene: Membership on an entity's Board of Directors or advisory committees; Precision Bioscience: Membership on an entity's Board of Directors or advisory committees; Miltenyi Biotech: Membership on an entity's Board of Directors or advisory committees; Becton Dickinson: Research Funding; Adaptive Biotechnologies: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees, Research Funding; Juno: Membership on an entity's Board of Directors or advisory committees. Mackall:Vor: Other: Scientific Advisory Board; Roche: Other: Scientific Advisory Board; Adaptimmune LLC: Other: Scientific Advisory Board; Glaxo-Smith-Kline: Other: Scientific Advisory Board; Allogene: Equity Ownership, Membership on an entity's Board of Directors or advisory committees; Apricity Health: Equity Ownership, Membership on an entity's Board of Directors or advisory committees; Unum Therapeutics: Equity Ownership, Membership on an entity's Board of Directors or advisory committees; Obsidian: Research Funding; Lyell: Consultancy, Equity Ownership, Other: Founder, Research Funding; Nektar: Other: Scientific Advisory Board; PACT: Other: Scientific Advisory Board; Bryologyx: Other: Scientific Advisory Board.