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Introduction: High-dose post transplant cyclophosphamide (PTCY) effectively prevents graft-versus-host disease (GVHD) after unmanipulated HLA-haploidentical hematopoietic stem cell transplant (HSCT) and offer low rates of GVHD in the setting of HLA identical transplant. The objective of our study was to compare the outcomes of haplo vs HLA identical HSCT in patients undergoing HSCT for acute myeloid leukemia (AML) using PTCY. Patients and methods: We conducted a retrospective study of 229 patients undergoing a first HSCT for AML using PTCY, 130 from an haploidentical donor between 2013 and 2018 (median follow up 62.5 months) and 99 from a matched sibling (MSD) (n=38) or unrelated donor (MUD) (n=61) (median follow up 27 months) between 2013 and 2019, in 20 centers in Spain. Last update of the cohort was performed in March 2021. Results: Baseline characteristics are summarized in Table 1. There were more patients with active disease at transplant (5% MSD/MUD vs. 20% haplo, p=0.001), high/very high DRI (32% vs. 67%, p=0.000) and prior autologous HSCT (2% vs. 11%, p=0.010) in the haplo group. Mobilized peripheral blood stem cells was the most frequent stem cell source in both groups. Most patients received myeloablative conditioning (55% vs. 64%, p=0.170). All Patients in the haplo group received PTCY days +3+4 followed by a calcineurin inhibitor (CNI) and MMF from +5. In the MSD/MUD group, 37% received both CNI+MMF, 33% only CNI and 30% PTCY with sirolimus+MMF (this group included only MUD donors). None of the patients received ATG. Cumulative incidence of neutrophil recovery at day 28 was 97% in both groups, with a median of 16 and 17 days respectively (p=0.948). Both 2-year overall survival (OS) (72% vs. 62%, p=0.07) and event-free survival (EFS) (70% vs. 54%, p=0.055) were higher in the MSD/MUD group, but the difference was not statistically significant (Figure 1). Multivariate analysis only identified age and pre-transplant status as independent risk factors for OS, and pre-transplant status for EFS. No differences were found in the cumulative incidence of relapse at 2 years (19% vs. 25%, p=0.13) and non-relapse mortality (14% vs. 19%, p=0.145). Cumulative incidence of grade II-IV acute GVHD was lower in MSD/MUD (14% vs. 47%, p=0.000, Figure 2), while III-IV aGVHD was similar (4% vs. 9%, p=0.14). Cumulative incidence of chronic GVHD and moderate-severe cGVHD at 2 years was similar for both groups (42% vs. 33% (p=0.051); 22% vs. 19% (p=0.28)). No differences were found in GRFS (48% vs. 46% (p=0.506)). Most frequent cause of death in the early post-transplant period was non-GVHD related infection in both groups. Conclusions: in our experience, PTCY as GVHD prophylaxis in both MSD/MUD and Haplo transplant in AML using mostly PBSC effectively prevents GVHD and offers similar NRM, relapse and survival rates. Poor control of the disease before transplant was the only factor affecting OS and EFS in this setting. Prospective studies are needed to confirm our results. Figure 1 Figure 1. Disclosures Bailen: Pfizer, Kite-Gilead, Gilead: Honoraria. Guerreiro: Novartis, Gilead: Consultancy, Honoraria. Oarbeascoa: Gilead: Honoraria, Speakers Bureau. Kwon: Novartis, Celgene, Gilead, Pfizer: Consultancy, Honoraria.

Background: Chimeric Antigen Receptor-modified T cell (CAR-T) therapies have revolutionized cancer immunotherapy, especially in hematological malignancies. Although great results have been achieved during the last years, long-term efficacy is still compromised in some cases and factors behind CAR-T cell disfunction are not fully understood. Recent studies have shown that the control of CAR expression influences CAR-T fitness and antitumoral efficacy 1. Therefore, we hypothesized that CAR density on the membrane of CAR-T cells could directly affect CAR-T cell function. In this study we perform a functional and genomic analysis of FACS-isolated subpopulations of CAR-T cells with different CAR densities (CAR High and CAR Low). Methodology: Second generation CAR-T cells with 4-1BB costimulatory domain targeting BCMA were generated by lentiviral transduction of αCD3/αCD28 activated T cells that were expanded for 12-14 days in the presence of IL-7/IL-15. Phenotypic analyses were performed by flow cytometry before and after coculture with MM cells. Cytotoxic activity and cytokine production were measured by standard procedures. In vivo antitumoral efficacy was evaluated in xenogeneic tumor models in NSG mice. Transcriptomic (RNA-seq) and epigenetic (ATAC-seq) analysis were performed following stablished protocols 2. Single cell analysis was performed using the Chromium Single Cell Immune Profiling solution from 10x Genomic that allows simultaneous analysis of gene expression and paired T-cell receptors from a single cell. Gene Regulatory Network (GRN) analysis was performed using SimiC, a novel computational method that infers regulatory dissimilarities 3. Results: RNA-seq and ATAC-seq analysis revealed completely different profiles between CAR High- and CAR Low-T cells in both CD4 +and CD8 + cell subsets, with >3500 differentially expressed genes (2086 for CD4 + and 1553 for CD8 +) that were related with increased tonic signaling, T cell activation and proliferation in CAR High-T cells. Functional studies at resting state (before antigen encounter) corroborated that CAR High-T cells presented increased tonic signaling, that lead to a higher basal activation and a more differentiated phenotype with skewed presence of CCR7 +/CD45RA +/CXCR3 + T SCM cells. After antigen-driven activation, increased cytotoxicity and cytokine production was observed in CAR High-T cells, that also presented higher percentage of terminally differentiated effector cells (CCR7 -/CD45RA +), along with increased exhaustion (PD1 +/LAG3 +/TIGIT +). This effect was also observed in the infusion products of CARTBCMA-HCB-01 clinical trial for patients with R/R MM (NCT04309981), where products enriched in CAR High-T cells presented increased cytotoxic activity. Although no significant differences were observed in the antitumoral efficacy in vivo, CAR Low-T cells presented increased persistence, suggesting that higher CAR levels could reduce long-term efficacy. Further characterization of CAR-T cells at single cell level (scRNA-seq) showed enrichment of CAR High-T cells in activated CD4 + and exhausted CD8 + cell clusters. The analysis of regulatory dissimilarities driven by different CAR densities with SimiC revealed an increased activity of the regulon associated to NR4A1 transcription factor (a well-known TF driving T cell exhaustion 4) in CAR High-T cells, providing mechanistic insights of the regulatory networks behind differential functionality of CAR High-T cells. Finally, to evaluate the impact of CAR density in the clinical outcome of CAR-T therapies, we developed a gene signature associated to increased CAR density, that was applied to transcriptomic data available from public studies 5. We score the infusion products of several clinical trials testing CTL019 (NCT01029366, NCT01747486 and NCT02640209) and we observed an enrichment on CAR High signature in the products from non-responder patients. Conclusions: Our data demonstrate that CAR density on the membrane of engineered T cells plays important roles in CAR-T activity with a significant impact on clinical outcome. Moreover, the comprehension of regulatory mechanisms driven by CAR densities at the single cell level offer an important tool for the identification of key regulatory factors that could be modulated for the development of improved therapies. Figure 1 Figure 1. Disclosures Rodríguez-Otero: Oncopeptides: Honoraria, Membership on an entity's Board of Directors or advisory committees; Kite: Honoraria, Membership on an entity's Board of Directors or advisory committees; Amgen: Honoraria, Membership on an entity's Board of Directors or advisory committees; Regeneron: Membership on an entity's Board of Directors or advisory committees; Abbvie: Honoraria, Membership on an entity's Board of Directors or advisory committees; Sanofi: Honoraria, Membership on an entity's Board of Directors or advisory committees; GlaxoSmithKline: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; Janssen: Honoraria, Membership on an entity's Board of Directors or advisory committees; BMS/Celgene: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: Travel and other expenses. Paiva: Bristol-Myers Squibb-Celgene, Janssen, and Sanofi: Consultancy; Adaptive, Amgen, Bristol-Myers Squibb-Celgene, Janssen, Kite Pharma, Sanofi and Takeda: Honoraria; Celgene, EngMab, Roche, Sanofi, Takeda: Research Funding. San-Miguel: AbbVie, Amgen, Bristol-Myers Squibb, Celgene, GlaxoSmithKline, Janssen, Karyopharm, Merck Sharpe & Dohme, Novartis, Regeneron, Roche, Sanofi, SecuraBio, Takeda: Consultancy, Other: Advisory board. Prósper: Oryzon: Honoraria; Janssen: Honoraria; BMS-Celgene: Honoraria, Research Funding.

Acute myeloid leukemia (AML) is a hematologic neoplasm characterized by a remarkable phenotypic and genomic heterogeneity. The recent characterization of genomic subtypes of AML based on large sequencing studies has provided the rationale for the development of targeted therapies based on the presence of specific genomic abnormalities. However, long term survival particularly in older patients remains a unmet medicalneed. Additionally, recent studies using RNA interference (RNAi) libraries have determined the existence of genes that are essential for the survival of multiple cancer cells. Understanding the effect of genomic alterations (mutations, deletions, translocations) on gene essentiality could favor the development of targeted therapies for specific subgroups of AML patients. However, current statistical methods such as the Benjamini-Hochberg (BH) procedure have shown limitations for controlling the false discovery rate (FDR) and have suboptimal sensitivity (recall of true positives) because the P-value correction does not include any prior information of individual tests. For this reason, in this study we developed a new large-scale statistical algorithm, which combine the RNAi libraries (more than 17.000 genes) data with mutational profiles, to identify gene essentialities associated with specific genomic mutations in order to explore this approach in AML. We adapted the Independent Hypothesis Weighting (IHW) procedure to the problem of identifying mutations as surrogate markers of gene essentiality, by using the gene mutation state in each cell line as prior information of a IHW problem. This approach was tested in 19 tumor subtypes, of the Cancer Cell Line Encyclopedia (CCLE) showing that it recalls new discoveries that cannot be identified with standard procedures in 17 out of 19 tumors, including the identification of up to 1,000 discoveries in tumor types in which BH recalls no discovery. These results demonstrated the accuracy of the IHW-based approach to identify gene mutations as surrogate markers of gene essentiality in the future. Once validated, we applied this computational model to the15 AMLcell lines of CCLE. The number of discoveries with an FDR of 20% increases from 2 (using the traditional BH correction), to 38 using our procedure, showing NRAS as the top mutation biomarker in the ranking. Interestingly, the algorithm identified one essential gene (NRAS) for NRAS mutated (NRAS-mut) and another essential gene (PTPN11) for NRAS wild type (NRAS-wt) AML cells, covering all samples of AMLs. To validate this hypothesis, we examined the effect of two different specific siRNAs for each gene (siPTPN11 and siNRAS) on cell proliferation of four AML cell lines: two lines with NRAS-mut (HL-60 and OCIAML3) and two with NRAS-wt (MV4-11 and HEL). Downregulation of NRAS expression significantly decreases the cell proliferation only in the 2 NRAS-mutated AML cell lines. Whereas the inhibition of PTPN11expression produced an equivalent effect, but specifically in the 2 NRAS-wt AML cell lines (Figure 1). These results confirmed our predictions and showed the essential role of NRAS or PTNPN11 in AML cell lines either with NRAS mutated or wild type, respectively. These results demonstrate that the application of our algorithm in the context of specific gene mutation not only may allow identification of directed therapies based on the mutation but can also define new gene essentialities amenable for targeted therapies providing new therapeutic strategies in patients with AML and potentially in other tumors. Disclosures Paiva: Amgen, Bristol-Myers Squibb, Celgene, Janssen, Merck, Novartis, Roche and Sanofi: Honoraria, Membership on an entity's Board of Directors or advisory committees; Celgene, Janssen, Sanofi and Takeda: Consultancy. San-Miguel:Amgen, Bristol-Myers Squibb, Celgene, Janssen, MSD, Novartis, Roche, Sanofi, and Takeda: Consultancy, Honoraria.

Acute myeloid leukemia (AML) is a malignant disease characterized by uncontrolled proliferation, differentiation arrest and accumulation of immature myeloid progenitors. Despite recent developments and the approval of new therapeutic agents in the last few years, long term survival of AML, particularly in elderly patients remains an unmet medical need.The use of all-trans retinoic acid (ATRA) in Acute Promyelocytic Leukemia has proven that differentiation therapy may significantly change the survival of AML patients, however the success in APL has not been translated to other groups of AML. Therefore, the identification of new therapeutic agents that may induce the differentiation of AML blasts represents an attractive new target. Furthermore, it is well known that epigenetic alterations have an important role in the development and maintenance of cancer and AML in particular. Thus, our aim was to develop new small molecules targeting epigenetic modifying enzymes like DNA methyltransferases (DNMT), histone methyltransferases or histone deacetylase (HDAC) with the aim of inducing differentiation in AML. We performed a screening of over 50 small molecules synthesized by our group. The design was performed in-house using a knowledge and structure based strategy and the read out of the screening was based on changes in expression of CD11b (a well described marker of myeloid differentiation) after in vitro treatment of AML cells lines. Interestingly, we found several compounds with high capacity to promote the differentiation of leukemic cells in AML cells lines at low non-cytotoxic doses, selecting CM-444 and CM-1758 as our lead compounds (Figure 1a).A complete biochemical characterization showed that both compounds are specific pan-HDACs inhibitors (HDACi). CM-444 and CM-1758 induced in vitro cell differentiation in all subtypes of AML, independently of the AML genetic subgroups or the presence of mutations, which was significantly more pronounced that differentiation induced by reference compounds such as Panobinostat, Vorinostat, Entinostat, Tubastatin or Quisinostat, previously described HDACi. CM-444 and CM-1758 also induced in vivo differentiation in xenogeneic models of AML. AML differentiation was associated with induction of CD11b, downregulation of c-MYC, overexpression of transcription factors that govern the myeloid differentiation and morphologic changes. In addition, these compounds promoted in vitro differentiation of patient-derived AML blasts. The complete transcriptome analysis by RNA-Seq carried out in AML cell lines after CM-444, CM-1758, Panobinostat or Vorinostat treatment showed changes in genes implicated in differentiation, but without explaining the differences among the different HDACi. Analysis of the complete acetylome and proteome before and after treatment with CM-444 and CM-1758 in comparison with other HDACi showed differential acetylation of non-histone proteins included in the GO categories of Zn binding proteins and nucleic acid binding proteins (Figure 1b). Most of these proteins are epigenetic enzymes and have been related to AML and myeloid differentiation, such as MLL2, EP300 or BRD4. In summary, we have developed and characterized novel epigenetic small molecules with a high in vitro and in vivo capacity of differentiating AML cells. These compounds might be an effective differentiation-based therapy to be tested in AML. Besides, the mechanism of differentiation of these compounds is due, at least in part, to the acetylation of non-histone epigenetic proteins, which are key in the myeloid differentiation. Disclosures Paiva: Celgene, Janssen, Sanofi and Takeda: Consultancy; Amgen, Bristol-Myers Squibb, Celgene, Janssen, Merck, Novartis, Roche and Sanofi: Honoraria, Membership on an entity's Board of Directors or advisory committees. San-Miguel:Amgen, Bristol-Myers Squibb, Celgene, Janssen, MSD, Novartis, Roche, Sanofi, and Takeda: Consultancy, Honoraria.

Background: Duvelisib (DUV) is a first-in class, oral dual inhibitor of PI3K-δ,-γ being developed for the treatment of advanced B- and T-cell malignancies. To date 304 patients (pts) with relapsed/refractory (RR) CLL/SLL have been treated with DUV monotherapy 25 mg BID in 4 studies: Study IPI-145-02 [NCT01476657], a Phase 1 study in advanced hematologic malignancies; DYNAMOTM [NCT01882803], a Phase 2 study in iNHL; DUOTM [NCT02004522], a Phase 3 study in CLL/SLL; and Study IPI-145-12 [NCT02049515], a Phase 3 crossover study from DUO. Here we present pooled efficacy and safety analyses from these 4 studies in RR CLL/SLL, examining the baseline characteristics, incidence and timing of AEs, and response in the subset of pts who received DUV monotherapy for >2 years in order to characterize the factors that contribute to long-term treatment with DUV. Methods: In this analysis we examined pts treated with DUV for >2 years (n=45) referred to as long-term (LT) pts. Efficacy and safety data from 4 studies of DUV monotherapy in pts with RR CLL/SLL treated at 25 mg BID were pooled. Response was based on investigator assessment per IWCLL/IWG criteria. Treatment-emergence (TE) was defined as those adverse events (AEs) that occurred from first dose to 30 days post last dose of DUV. All AEs were coded using MedDRA version 16.1; severity was assessed by investigators according to the National Cancer Institute-Common Terminology Criteria for Adverse Events version 4.03. Results: Baseline characteristics of LT pts were similar to pts who discontinued The median exposure for LT pts was 31 months, with 20 pts on DUV >3 years and 3 pts on DUV >4 years. The majority of LT pts (60%) remain on treatment as of June 2018. The median PFS in LT pts was 37 months, with an investigator-assessed ORR of 89%; best responses included 16% CR/CRi, 73% PR, and 11% SD. Among LT pts, 98% had an AE, 80% had a ≥ Gr 3 AE, 46% an SAE, and 9% an AE leading to discontinuation (after 2 years on DUV); 69% of pts had at least 1 dose modification due to an AE (69% hold, 40% reduction). Table 1 shows the incidence of AEs in LT pts by 6-month treatment intervals. Overall, ≥ Gr 3 AEs occurred less frequently over time but still occurred in 27% pts after 2 years. The rate of colitis was consistent over the 2 years of treatment (≤ 2.2% per 6-month interval). No ≥ Gr 3 AEs of diarrhea were observed within the first 6 months of treatment, while the overall rate of all grade diarrhea was consistent over time (16-27% per 6-month interval); dose modification for the management of diarrhea increased over time allowing pts to remain on therapy. Neutropenia, ALT increase, and lipase increase were more common in the first 6 months; most ≥ Gr 3 AEs of neutropenia did not require any dose modification. Compared to all pts with RR CLL/SLL treated with DUV (n=442), the incidences and types of AEs in LT pts were generally similar. Four (9%) LT pts discontinued DUV due to an AE, and included: colitis (n=2), diarrhea (n=1), Clostridium difficile colitis (n=1), and rectal adenocarcinoma (n=1). Pneumocystis jirovecii pneumonia occurred in 1 LT pt (not on prophylaxis at the time) and was the only opportunistic infection reported for this population. There have been no fatal events in the LT pts. Conclusions: 45 pts with RR CLL/SLL have been on DUV 25 mg BID for >2 years, with a median of 31 months on treatment. The investigator-assessed ORR for these long-term pts was 89% (16% CR/CRi, 78% PR) and the median PFS was 37 months. A majority (60%) of the long-term pts remain on treatment. Baseline characteristics were similar in these long-term pts compared to pts who discontinued < 2 years. Most of the commonly occurring ≥ Gr 3 AEs decreased over time, with the exception of diarrhea. The majority of ≥ Gr 3 AEs were managed through dose modification (dose interruption and or reduction). These data support DUV monotherapy as a long-term treatment option for pts with RR CLL/SLL with the potential for durable response and good tolerability over time. Disclosures Flinn: ArQule: Research Funding; Infinity: Research Funding; Verastem: Research Funding; BeiGene: Research Funding; Curis: Research Funding; Merck: Research Funding; Seattle Genetics: Research Funding; Agios: Research Funding; Forma: Research Funding; Portola: Research Funding; Kite: Research Funding; Forty Seven: Research Funding; Genentech: Research Funding; Gilead: Research Funding; Celgene: Research Funding; Verastem: Consultancy, Research Funding; Incyte: Research Funding; Janssen: Research Funding; Takeda: Research Funding; TG Therapeutics: Research Funding; Pharmacyclics: Research Funding; Pfizer: Research Funding; Trillium: Research Funding; Novartis: Research Funding; Calithera: Research Funding; Constellation: Research Funding. Montillo:Roche: Consultancy, Honoraria, Research Funding; Janssen: Consultancy, Honoraria; AbbVie: Consultancy, Honoraria, Speakers Bureau; Gilead: Consultancy, Honoraria, Speakers Bureau. Davids:BMS: Research Funding; TG Therapeutics: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Celgene: Consultancy; Merck: Consultancy; Surface Oncology: Research Funding; Janssen: Consultancy, Membership on an entity's Board of Directors or advisory committees; Gilead: Membership on an entity's Board of Directors or advisory committees; Verastem: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Pharmacyclics: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; MEI Pharma: Consultancy, Research Funding; Roche/Genentech: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Astra-Zeneca: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; AbbVie, Inc: Consultancy, Membership on an entity's Board of Directors or advisory committees. Le:Verastem Oncology: Employment. Lustgarten:Verastem Oncology: Employment.