ALBERT

Description: Advances in gene editing are leading to new medical interventions where patients’ own cells are used for stem cell therapies and immunotherapies. One of the key limitations to translating these treatments to the clinic is the need for scalable technologies for engineering cells efficiently and safely. Toward this goal, microfluidic strategies to induce membrane pores and permeability have emerged as promising techniques to deliver biomolecular cargo into cells. As these technologies continue to mature, there is a need to achieve efficient, safe, nontoxic, fast, and economical processing of clinically relevant cell types. We demonstrate an acoustofluidic sonoporation method to deliver plasmids to immortalized and primary human cell types, based on pore formation and permeabilization of cell membranes with acoustic waves. This acoustofluidic-mediated approach achieves fast and efficient intracellular delivery of an enhanced green fluorescent protein-expressing plasmid to cells at a scalable throughput of 200,000 cells/min in a single channel. Analyses of intracellular delivery and nuclear membrane rupture revealed mechanisms underlying acoustofluidic delivery and successful gene expression. Our studies show that acoustofluidic technologies are promising platforms for gene delivery and a useful tool for investigating membrane repair.

Description: Background: Severe combined immunodeficiency due to adenosine deaminase deficiency (ADA-SCID) is a rare disorder caused by ADA gene mutations, leading to lymphotoxic build-up of purine metabolites and profound immunodeficiency. Historically, enzyme replacement therapy (ERT) has been used as a bridge therapy until patients can receive an allogeneic hematopoietic stem cell transplantation (HSCT), ideally from a matched related donor (MRD) or, if none is identified, a non-matched and/or unrelated donor. We developed a self-inactivating lentiviral vector (LV), denoted EFS-ADA LV, encoding the human ADA cDNA sequence under the control of a shortened human elongation factor 1α gene promoter. A fresh or cryopreserved formulation of a drug product (OTL-101), composed of autologous hematopoietic stem and progenitor cells (HSPCs) transduced ex vivo with EFS-ADA LV, was evaluated in 2 prospective, non-randomized Phase I/II clinical trials at 2 USA centers. We report on safety and efficacy of OTL-101 in 30 ADA-SCID pediatric gene therapy (GT) subjects treated from 2013-2017 with a median follow up (FU) of 24 months (mo; range 12-26 mo), compared to a historical cohort of 26 ADA-SCID patients treated with HSCT. Methods: UCLA Fresh Study (NCT01852071): Autologous CD34+ HSPCs were isolated from bone marrow and pre-stimulated with cytokines before transduction with EFS-ADA LV to yield OTL-101, which was infused as a fresh formulation in 20 subjects (9 male, 11 female; aged 4 mo-4.3 yrs). Single dose busulfan (4 mg/kg) was administered prior to infusion of OTL-101. Subjects were followed for 24 mo. UCLA Cryo Study (NCT02999984): 10 subjects (4 male, 6 female; aged 5-15 mo) received a cryopreserved formulation of OTL-101, which allowed for an extended shelf-life and full quality control prior to infusion. Busulfan was administered in 2 doses, the first at 3 mg/kg and the second adjusted to target a total area under the curve of 4,900 µM*min (20 ng/mL*hr). At the time of analysis, all subjects reached 12 mo FU (except 1 subject who was withdrawn from the study due to lack of engraftment); 7 subjects reached 18 mo of FU. Historical Control Group: 26 patients (aged 0.2 mo-9.8 yrs) were treated with allogeneic HSCT (MRDs n=12, non-MRDs n=14) at Great Ormond Street Hospital, UK (n=16) or Duke University Children's Hospital, USA (n=10) from 2000-2016. Results: Sustained engraftment of genetically modified HSPCs was observed in 29/30 GT subjects by 6-8 mo and persisted through FU in both studies, based on vector gene marking in granulocytes and CD3+ T cell reconstitution (Figure). Subjects who engrafted maintained long-term metabolic detoxification from deoxyadenosine nucleotides after stopping ERT approximately 1 mo post-GT. At last FU (median 24 mo; range 12-24 mo) in the GT group, overall survival (OS) was 30/30 (100%) and event-free survival (survival in the absence of ERT reinstitution or rescue allogeneic HSCT; EvFS) was 29/30 (97%). OS and EvFS were higher in the GT group at last FU compared with HSCT controls (with or without an MRD) at 2 years (Table). One of 30 OTL-101 subjects (3%) did not engraft and was restarted on ERT; the subject was withdrawn from the study at 5.9 mo and subsequently received a rescue HSCT, whereas 42% of HSCT patients required rescue HSCT, PEG-ADA ERT or died. Among the 20 OTL-101 subjects in the UCLA Fresh Study who reached 2 years FU, 18 (90%) stopped immunoglobin replacement therapy (IgRT), compared to 52% of HSCT patients. Preliminary results were observed in 5/7 (71%) OTL-101 subjects in the UCLA Cryo Study with more limited (18 mo) FU. Twelve OTL-101 subjects experienced one or more serious adverse events, most frequently infections and gastrointestinal events; only 1 of which was considered treatment-related (bacteremia due to product contamination). In the GT group, there were no events of autoimmunity with ≤24 mo FU. Due to the autologous nature of OTL-101, there was no incidence of graft vs host disease (GvHD); in contrast, 8 HSCT patients experienced GvHD events (5 acute, 3 chronic events), 1 of which resulted in death. Conclusions: Based on sustained gene correction and restoration of immune function in all subjects who engrafted, treatment of ADA-SCID with OTL-101 has a favorable benefit-risk profile. Key correlates of engraftment were consistent across the expanded cohort. Importantly, higher rates of OS and EvFS compared with HSCT (with or without an MRD) were observed. Disclosures Kohn: Orchard Therapeutics: Consultancy, Membership on an entity's Board of Directors or advisory committees, Patents & Royalties: Inventor on IP licensed from UC Regents to Orchard Therapeutics. Future royalties may occur., Research Funding; NIH: Research Funding. Shaw:Orchard Therapeutics: Consultancy, Other: Personal fees and non-financial support; NIH: Research Funding. Carbonaro-Sarracino:NIH: Other: Salary while working on project at UCLA 2013-2016, Research Funding; Orchard Therapeutics: Consultancy, Employment. De Oliveira:National Institute for Health Research Biomedical Research Centre at Great Ormond Street Hospital for Children NHS Foundation Trust and University College London: Research Funding; CIRM: Research Funding; National Gene Vector Repository: Research Funding; NIAID, NHI: Research Funding; Medical Research Council: Research Funding. Terrazas:California Institute for Regenerative Medicine: Research Funding; Gene Therapy Resource Program, NHLBI/NIH: Research Funding. Hollis:Curative Therapeutics: Consultancy, Other: Personal fees. Trevisan:Orchard Therapeutics: Research Funding. Arduini:Orchard Therapeutics: Employment, Equity Ownership. Lynn:Orchard Therapeutics: Employment, Equity Ownership. Kudari:Orchard Therapeutics: Employment, Equity Ownership. Spezzi:Orchard Therapeutics: Employment, Equity Ownership. Buckley:Duke University: Research Funding. Booth:SOBI: Consultancy; GSK: Honoraria; NovImmune: Consultancy. Thrasher:Generation Bio: Consultancy, Equity Ownership, Membership on an entity's Board of Directors or advisory committees; Orchard Therapeutics: Consultancy, Equity Ownership, Membership on an entity's Board of Directors or advisory committees; 4BIOCapital: Membership on an entity's Board of Directors or advisory committees; Rocket Pharmaceuticals: Consultancy, Membership on an entity's Board of Directors or advisory committees. Gaspar:Orchard Therapeutics: Employment, Equity Ownership, Patents & Royalties: Lentiviral vector for gene therapy of ADA-SCID.

Description: Background: Patients with refractory or recurrent B-lineage hematological malignancies have less than 50% of chance of cure. Trials using autologous T-cells engineered with anti-CD19 chimeric antigen receptors (CAR) have demonstrated complete remissions even in chemotherapy-resistant malignancies, but the persistence of the cells is transient, limiting efficacy. Our hypothesis is modification of hematopoietic stem cells (HSC) with CAR will lead to persistent production of target-specific immune cells in multiple lineages, enhancing graft-versus-tumor activity and development of immunological memory. Design/Methods: Using CD19 as target, we generated second-generation CD28- and 4-1BB-costimulated CAR constructs for modification of human HSC for assessment in vitro and in vivo, using third-generation lentiviral vectors. Additionally, co-delivery of suicide gene systems was tested to allow ablation of gene-modified cells. Results: Gene modification of HSC with anti-CD19 CAR using lentiviral vectors did not impair differentiation or proliferation, and led to functional CAR-expressing cell progeny, at 40-50% transduction efficiency and engineered antigen-dependent cytotoxicity in myeloid, NK and T-cells. In vivo studies using humanized NSG engrafted with CAR-modified HSC demonstrated similar levels of humanization to non-modified HSC, with multilineage CAR-expressing cells present in bone marrow, spleen, blood and thymus in stable levels up to 44 weeks of life. No animals engrafted with CAR-modified HSC presented autoimmunity or inflammation. Ex vivo cells presented antigen-dependent cytotoxicity against targets. Mice engrafted with CAR-modified HSC had decreased CD19+ populations and successfully presented tumor growth inhibition and survival advantage at tumor challenge (55-60%). CAR-modified HSC led to development of T-cell effector memory and T-cell central memory phenotypes, confirming the development of long-lasting phenotypes due to directed antigen specificity. Mice humanized with gene-modified HSC presented significant ablation of gene-modified cells after treatment (p=0.002). Remaining gene-modified cells were close to background on flow cytometry and within two logs of decrease of vector copy numbers by ddPCR in mouse tissues. Conclusions: CAR modification of HSC for cancer immunotherapy is feasible. This approach can be applied to different cancers by adjusting target specificity and could be employed in the context of HSC transplantation to augment the anti-lymphoma activity. It also bears the possibility of decreased morbidity and mortality and offers alternative treatment for patients with no available sources for bone marrow transplantation, benefiting ethnic minorities. These results also give proof of principle for CAR-modified HSC regulated by suicide gene; further studies are needed to enable clinical translation. Disclosures No relevant conflicts of interest to declare.

Description: Patients with refractory or recurrent B-lineage hematological malignancies have less than 50% of chance of cure, despite intensive therapy. Chimeric Antigen Receptors (CARs) successfully engineer antigen specificity in immune cells, with clinical trials currently being conducted using ex vivo expanded gene-modified mature T cells. Results from preclinical studies and clinical trials show that effector cells usually have transient in vivo persistence that could significantly limit clinical efficacy and allow tumor recurrence. Our main hypothesis is that modification of hematopoietic stem cells (HSCs) with CARs will lead to persistent in vivo production of target-specific immune cells in multiple lineages, enhancing graft-versus-tumor activity and development of immunological memory. Using CD19 as target, we have generated first-generation and CD28- and 4-1BB-containing-second-generation CAR lentiviral constructs for modification of human HSCs, for assessment in vitro and in vivo. Gene modification with anti-CD19 CAR of CD34+cells isolated from human umbilical cord blood (UCB) did not impair normal differentiation and proliferation, with fully functional CAR-expressing cell progeny. Transduction with lentiviral vectors consistently achieved 40-50% efficiency at the clinically relevant vector copy number of 1-2 copies/cell. While first- and second-generation CARs triggered antigen-dependent cytotoxicity by myeloid and T cells in a similar fashion, only second-generation constructs successfully activated NK cells for antigen-dependent elimination of cell targets. In vivo studies using humanized NSG engrafted with CAR-modified human UCB CD34+ cells demonstrated similar levels of engraftment of human cells as compared to non-modified UCB CD34+ cells, with CAR-expressing cells in multiple lineages (myeloid, NK, T) successfully engrafted into bone marrow, spleen, peripheral blood and thymus detectable by flow cytometry and qPCR, in stable levels up to 35 weeks of life, with gene modification with first- or second-generation anti-CD19 CARs. No animals engrafted with CAR-modified HSCs presented signs of autoimmunity or chronic inflammation. Cells presented ex vivo antigen-dependent cytotoxicity against cell targets. Mice successfully engrafted with CAR-modified HSCs harbored decreased CD19+populations, and only HSCs modified with second-generation CARs successfully led to tumor growth inhibition and survival advantage at tumor challenge. CAR-modified HSCs led to development of T cell effector memory and T cell central memory subsets, confirming the expectation of development of long-lasting phenotypes due to directed antigen specificity. Longer survival of mice with developing tumors was also significantly correlated to higher number of CAR-expressing cells infiltrating subcutaneous tumors. Our results demonstrate feasibility of CAR modification of human HSCs for cancer immunotherapy. This approach can be applied to different cancers just by adjusting the target specificity. Furthermore, it could be easily employed in the context of HSC transplantation to augment the anti-leukemic activity, with CAR-expressing myeloid and NK cells to ensure tumor-specific immunity until de novo production of T cells from CAR-modified HSCs. It also bears the possibility of decreased morbidity and mortality, being desirable for vulnerable populations such as children and elderly patients, and offers alternative treatment for patients with no available HLA-matched sources for bone marrow transplantation, benefiting ethnic minorities. Disclosures Larson: Millenium Pharmaceuticals, Inc.: Speakers Bureau.

Description: Background: Patients with refractory or recurrent B-lineage hematological malignancies have less than 50% of chance of cure, despite intensive therapy. Innovative approaches are needed to decrease their morbidity and mortality. Chimeric Antigen Receptors (CAR) successfully engineer antigen specificity in immune cells, with clinical trials currently being conducted using ex vivo expanded gene-modified mature T cells. Results from preclinical studies and clinical trials show that effector cells usually have transient in vivo persistence that could significantly limit clinical efficacy and allow tumor recurrence. Building upon the standard of care and seeking an approach that could foster persistence of the CAR-modified cells, we have published studies using anti-CD19 CAR-modified human hematopoietic stem cells (HSC) engrafted in NSG immunodeficient mice. We hypothesize that modification of HSC with CD19-specific CAR will generate persistent multi-lineage anti-tumor activity for immunotherapy of CD19+ hematological malignancies. To increase the safety of the modification of HSC, a suicide gene can be inserted into the vector to eradicate the modified cells in the setting of toxicity. Thorough evaluation of this approach in relevant study models is required for advancement to clinical trials. Significance: This approach is untested in clinical translation to this date, and implies harnessing a patient's own HSC to create a whole self-renewing immune system directed to destroy cancer, a concept that can be applied to different cancers just by adjusting the target specificity. The prospect of modifying autologous cells to enhance graft-versus-cancer activity bears the possibility of decreased morbidity and mortality, being desirable for specifically vulnerable populations, as children and elderly patients, and offering alternative therapy for those without cell sources available for allogeneic HSC transplantation, benefiting patients from ethnic minorities. This approach can be easily adopted in the clinical setting for patients planned to receive autologous HSC transplant as their standard therapy, enhancing graft-versus-cancer activity with anti-CD19 specificity. Methods: High-titer third-generation lentiviral constructs were produced carrying second-generation CD19-specific CAR co-stimulated by CD28. These vectors also co-delivered HSV-sr39TK to provide a suicide gene to allow ablation of gene-modified cells if necessary. Human HSC isolated from umbilical cord blood and G-CSF-mobilized apheresed peripheral blood stem cells (PBSC) were transduced with such lentiviral vectors and injected into NSG pups after irradiation for in vivo evaluation of engraftment, function and suicide gene activation. Results: We have consistently achieved engraftment of human cells in about 95% of study mice, with engraftment of CAR+ cells in about 80% of the animals. Human HSC were successfully transduced with lentiviral vectors carrying anti-CD19 CAR with no impairment of differentiation or proliferation in vitro and in vivo. Immune cells differentiated in vivo from CAR+ HSC had antigen-specific cytotoxicity directed by CAR. CAR+ human cells were detected in BM, spleen, blood and thymus of injected mice. CAR+ T cells were stably detected in the blood of engrafted mice up to 40 weeks post-injection, demonstrating lymphopoiesis of CAR+ T cells successfully escaping thymic deletion and persisting throughout murine lifetime. As a surrogate of the antigen specificity and efficacy, CD19+ cells were significantly decreased in all mice engrafted with anti-CD19 CAR demonstrating that CAR+ immune cells were not inactivated or developed tolerance. Mice humanized with at least 4% of CAR+ cells in blood had significant protection against challenge with CD19+ tumor cell line, with inhibition or elimination of tumor development and consequent survival advantage. Activation of HSV-sr39TK suicide gene by ganciclovir treatment successfully led to ablation of gene-modified cells in vitro and in vivo. Conclusions: Our results demonstrate feasibility of CAR modification of human HSC for cancer immunotherapy. It could be easily employed in the context of HSC transplantation to augment the anti-cancer activity, with CAR-expressing myeloid and NK cells to ensure tumor-specific immunity until de novo production of T cells from CAR-modified HSC. Disclosures Larson: BMS: Consultancy.

Description: Abstract 4168 Chimeric Antigen Receptors (CAR) against CD19 have been shown to direct T cells to specifically target B-lineage malignant cells in animal models and clinical trials, with efficient tumor cell lysis. But, there has been insufficient persistence of effector cells, limiting the clinical efficacy. We propose gene transfer to hematopoietic stem/progenitor cells (HSPC) as a novel approach to ensure persistent production of effector cells targeting B-lineage malignant cells, exponentially increasing the number of effectors that may be generated against tumor cells. Experiments were performed using NOD-SCID-IL2 receptor gamma chain null (NSG) mice engrafted with human CD34+ HSPCs transduced with lentiviral vectors carrying first and second generations of CD19-specific CAR. There was efficient and stable transduction with 1–2 copies of CAR/cell as determined by qPCR. Differentiation of modified HSPC in vivo was not impaired by gene transfer, as observed in vitro. Results of in vivo studies showed that CAR-transduced human HSPC successfully differentiated into all lineages, with CAR-expressing T, NK and myeloid cells populating bone marrow, spleen and peripheral blood. The human CD19+ B cell populations normally formed in the xenografted NSG mice were significantly reduced when the transplanted HSPC were transduced with the anti-CD19 CAR, demonstrating in vivo biological activity. Cells harvested from bone marrow and spleen of mice engrafted with modified HSPC lysed CD19-positive cell targets ex vivo. Leukemic challenges of engrafted mice are in progress. Our results provide evidence for the feasibility and efficacy of the modification of HSPC with CAR as a protocol for generation of effector cells for immunotherapy against B-lineage malignancies. Disclosures: No relevant conflicts of interest to declare.

Description: Background: Chemotherapy-refractory or recurrent B-lineage leukemias and lymphomas yield less than 50% of chance of cure. Therapy with autologous T-cells expressing chimeric antigen receptors (CAR) have led to complete remissions, but the effector cells may not persist, limiting clinical efficacy. Our hypothesis is the modification of hematopoietic stem cells (HSC) with anti-CD19 CAR will lead to persistent generation of multilineage target-specific immune cells, enhancing graft-versus-cancer activity and leading to development of immunological memory. Design/Methods: We generated second-generation CD28- and 4-1BB-costimulated CD19-specific CAR constructs using third-generation lentiviral vectors for modification of human HSC for assessment in vivo in NSG mice engrafted neonatally with human CD34-positive cells. Cells were harvested from bone marrows, spleens, thymus and peripheral blood at different time points for evaluation by flow cytometry and ddPCR for vector copy numbers. Cohorts of mice received tumor challenge with subcutaneous injection of lymphoma cell lines. Results: Gene modification of HSC with CD19-specific CAR did not impair differentiation or proliferation in humanized mice, leading to CAR-expressing cell progeny in myeloid, NK and T-cells. Humanized NSG engrafted with CAR-modified HSC presented similar humanization rates to non-modified HSC, with multilineage CAR-expressing cells present in all tissues with stable levels up to 44 weeks post-transplant. No animals engrafted with CAR-modified HSC presented autoimmunity or inflammation. T-cell populations were identified at higher rates in humanized mice with CAR-modified HSC in comparison to mice engrafted with non-modified HSC. CAR-modified HSC led to development of T-cell effector memory and T-cell central memory phenotypes, confirming the development of long-lasting phenotypes due to directed antigen specificity. Mice engrafted with CAR-modified HSC successfully presented tumor growth inhibition and survival advantage at tumor challenge with lymphoma cell lines, with no difference between both constructs (62.5% survival for CD28-costimulated CAR and 66.6% for 41BB-costimulated CAR). In mice sacrificed due to tumor development, survival post-tumor injection was directly correlated with tumor infiltration by CAR T-cells. Conclusions: CAR modification of human HSC for cancer immunotherapy is feasible and continuously generates CAR-bearing cells in multiple lineages of immune cells. Targeting of different malignancies can be achieved by adjusting target specificity, and this approach can augment the anti-lymphoma activity in autologous HSC recipients. It bears decreased morbidity and mortality and offers alternative therapeutic approach for patients with no available sources for allogeneic transplantation, benefiting ethnic minorities. Disclosures De Oliveira: National Institute for Health Research Biomedical Research Centre at Great Ormond Street Hospital for Children NHS Foundation Trust and University College London: Research Funding; NIAID, NHI: Research Funding; Medical Research Council: Research Funding; CIRM: Research Funding; National Gene Vector Repository: Research Funding.

Description: Optimization of transgene expression is paramount for successful gene modification of primary cells for clinical applications, and careful selection of the viral vector construct is a critical part of this process. Viral promoters based on the U3 region of the Moloney murine leukemia virus (such as MNDU3 and MSCV) are currently the most commonly used for gene transfer in human primary cells. These viral promoter-containing vectors, however, can activate nearby genes, potentially causing toxicity and/or neoplastic transformation. EF1alpha (or its short, intron-less form, EFS) is a promoter that has been recently used in many clinical trials. It is a cellular-derived enhancer/promoter with decreased cross-activation of nearby promoters, therefore hypothetically decreasing the risk of genotoxicity. We have produced vector constructs carrying the internal enhancer/promoters MNDU3, MSCV, or EFS driving clinically relevant transgenes for modification of primary human T lymphocytes and hematopoietic stem cells. Lentiviral vectors containing either the MNDU3 or EFS promoters driving the EGFP reporter gene were used to transduce Jurkat cells and primary human T cells. In Jurkat cells, MNDU3-driven vectors provided 2-3 times higher vector copy integrations with a corresponding higher percentage of EGFP expression, across a wide range of multiplicity of infection (MOI). In primary T cells, however, there was no significant increase in vector copy numbers per cell, but a significant increase in transduction efficiency and geometric mean fluorescence intensity of EGFP expression in cells transduced with MNDU3-driven vectors at all MOI studied, even when corrected for vector copy number. Lentiviral vectors containing either a MNDU3 or EFS promoter driving a first-generation anti-CD19 chimeric antigen receptor (CAR) were used to transduce primary human T cells. We found that integrated vector copy numbers per cell were 0.8 with MNDU3 and 0.5 with EFS, and resultant transgene expression in the transduced populations was 45% with MNDU3 and 22% with EFS. Primary human T cells were also transduced with a lentivirus carrying MSCV or EFS driving a codon-optimized MART-1-specific T cell receptor (TCR) and then analyzed by tetramer staining. MSCV promoter-driven vectors resulted in 33.76%, 33.1%, and 29% higher transgene expression at 5 ng, 10 ng, and 25 ng p24 equivalents compared with T cells transduced with vectors driven by the EFS promoter using the same amount of p24. After correction for integrated vector copy numbers, T cells had more than 2-fold increase in transgene expression when using the MSCV promoter. CD34+ hematopoietic stem cells isolated from human cord blood were transduced using the same high-titer MSCV- or EFS-driven MART-1-specific TCR expression vectors; MSCV-driven lentiviral vectors provided an average vector copy number of 0.5 copies per cell compared to 0.7 copies per cell with the similar EFS-containing vectors. These gene-modified cells were then injected into NOD-scid-IL2rγnull mice, with peripheral blood analyzed by flow cytometry after 8 weeks. HuCD45+/huCD3+/huCD4+ and huCD45+/huCD3+/huCD8+cells had mean transgene expression of 18% and 16% in the MSCV group, compared to 0% and 0% in the EFS group. Together, these results demonstrate more efficient transgene expression is conveyed by the virally-derived MSCV and MNDU3 promoters versus the cellular EFS promoter in gene-modified primary human hematopoietic cells. Higher transgene expression relative to integrated vector copies is consistent with higher promoter function, and transgene expression may be significantly decreased when using the EFS promoter in lentiviral vectors for clinical applications. Further studies are needed to carefully evaluate genotoxic effects of the MNDU3 and MSCV promoters in comparison to the EFS promoter for safe and efficient clinical translation. Disclosures Larson: Millenium Pharmaceuticals, Inc.: Speakers Bureau.

Description: The Blood and Marrow Transplant Clinical Trials Network (BMT CTN 0501; NCT00412360) randomized children with hematologic malignancy to one or two cord blood unit transplantation between December 2006 and February 2012. While the trial concluded that survival was similar regardless of number of units infused, results were generally better than those previously reported after single UCB transplant. The apparently improved survival in recipients of BMT CTN 0501 compared to prior studies in children transplanted with a single UCB unit prompted a comparison of trial versus non-trial treatment outcomes to determine 1) the generalizability of trial results and 2) whether survival was better for patients treated with the trial regimen. Using data reported to the Center for International Blood and Marrow Transplant Research during the trial period, 396 recipients of one UCB unit transplant met the broad eligibility criteria for BMT CTN 0501 (i.e. aged 1- 21 years, high-risk acute leukemia, performance score ≥70). Trial and non-trial patients were comparable in their HCT-CI score. Trial patients (n=100) received total body irradiation (TBI) 1320 cGy, cyclophosphamide 120 mg/kg, and fludarabine 75 mg/m2 (TCF). Non-trial patients either received the same regimen as in the trial (N=62; non-trial TCF) or an alternative regimen (N=334; non-trial non-TCF regimen). Excluded were 13 patients on BMT CTN 0501 randomized to receive a single UCB unit for malignant diseases other than acute leukemia. Patient and disease characteristics of those treated on BMT CTN 0501 and others were similar except that those who received non-trial TCF regimen were more likely to report performance scores of 80 or 70 (24% versus 11%, p=0.04) and transplanted in relapse (18% vs. 5%, p=0.02) and those that received non-TCF regimens were more likely to 1-10 years of age (76% versus 53%, p

Description: Background: Patients with refractory or recurrent B-lineage hematologic malignancies have less than 50% of chance of cure despite intensive therapy and innovative approaches are needed to decrease their morbidity and mortality. Our previous published results demonstrate feasibility of stable gene modification of hematopoietic cells (HSC) with chimeric antigen receptor (CAR) directing antigen specificity of T-, NK- and myeloid cells that are continuously produced with effective anti-tumor activity in humanized mouse model. We hypothesize that gene modification of HSC with an anti-CD19 CAR will produce a multi-lineage, persistent immunotherapy against B-lineage hematological malignancies that can be controlled by the HSV-sr39TK suicide gene. Hematopoietic stem cell transplantation is frequently part of the standard of care for patients with relapsed and refractory B cell malignancies. Following HSC collection, a portion of these cells could be modified to express the CD19-specific CAR and give rise to a persistent, multi-cell lineage, HLA-independent immunotherapy, enhancing the graft-versus-malignancy activity. Based on current clinical trials of gene therapy, gene modification of a partial portion of HSC would be sufficient to give rise to a large number of antigen-specific immune cells, and ablation of such gene-modified cells would not cause myeloablation. The generation of NK and myeloid cells bearing CAR would allow earlier directed anti-tumor activity until post-transplant thymopoiesis can take place, enhancing the graft-versus-cancer activity. Methods: High-titer third-generation self-inactivating lentiviral constructs were developed to deliver second-generation CD19-specific CAR co-stimulated by CD28. These vectors also co-delivered the hyper-active herpes simplex virus thymidine kinase HSV-sr39TK to provide a suicide gene to allow ablation of gene-modified cells if necessary. Such vector constructs were thoroughly evaluated using Jurkat cell line and primary human T-cells. Human HSC isolated from umbilical cord blood were transduced with such lentiviral vectors and evaluated for function of both CAR and HSV-sr39TK. Gene-modified human HSC were also injected into NSG pups after irradiation for in vivo evaluation of engraftment, anti-tumor activity and suicide gene activation. In vivo ablation of gene-modified cells was performed by treatment with intraperitoneal ganciclovir 50mg/kg/day over 5 days. Persistence of gene-modified cells was assessed by flow cytometry and ddPCR of animal tissues. Results: Two promoters were compared in such vector constructs, the human elongating factor-1 short (EFS) and the retrovirus-derived MNDU3, regarding transduction efficiency of primary cells. Those vector constructs with MNDU3 as the promoter showed improved CAR expression (p=0.003) and antigen-specific cytotoxicity, despite similar integrated vector copy numbers (VCN) per cell. The addition of HSV-sr39TK to the construct did not impair CAR expression or antigen-specific cytotoxicity, and determined marked cytotoxicity to ganciclovir. Transduction of human HSC with high-titer lentiviral vectors led to satisfactory transduction efficiency with second-generation CAR and HSV-sr39TK. When tested in vivo, mice transplanted with gene-modified human HSC showed similar humanization in bone marrow and spleen (as defined by CD45+ cells) to non-modified HSC. CAR expression by flow cytometry and VCN were not significantly different between transduced cells with and without HSV-sr39TK, and expression of anti-CD19 CAR conferred anti-tumor survival advantage, with 100% of survival of mice engrafted with HSC modified with CAR and HSV-sr39TK for up to 120 days post-tumor challenge. When treated with ganciclovir, the flow cytometry detection of gene-modified cells was decreased in bone marrows (p=0.07) and spleens (p=0.05) to levels similar to background, and with significant decrease of ddPCR detection of gene-modified cells in both bone marrows (p=0.05) and spleens (p=0.01). Conclusions: The modification of HSC with second generation anti-CD19 CAR has provided specific and persistent anti-lymphoma protection. The addition of the suicide gene system HSV-sr39TK to the construct did not affect transduction efficiency, and has determined significant ablation of gene-modified cells in mouse tissues. Disclosures No relevant conflicts of interest to declare.