Abstract: Chronic lymphocytic leukemia (CLL) patients harboring 11q22.3 deletion, del(11q), are characterized by a rapid disease progression. One of the suggested genes to be involved in the pathogenesis of this deletion is BIRC3, a negative regulator of NF-κB, which is monoallelically deleted in ~80% of del(11q) CLL cases. In addition, truncating mutations in the remaining allele of this gene can lead to BIRC3 biallelic inactivation, which accounts for marked reduced survival in CLL. Nevertheless, the biological mechanisms by which monoallelic or biallelic BIRC3 lesions could contribute to del(11q) CLL pathogenesis, progression and therapy response are partially unexplored. We used the CRISPR/Cas9 system to model monoallelic and biallelic BIRC3 loss in vitro. First, we generated an isogenic HG3 CLL cell line harboring monoallelic del(11q) - HG3-del(11q) - by the introduction of 2 guide RNAs targeting 11q22.1 and 11q23.3 (~17 Mb). Loss-of-function BIRC3 mutations (MUT) were introduced in the remaining allele, generating 3 HG3-del(11q) BIRC3MUT clones. In addition, single BIRC3MUT were introduced in HG3 and MEC1 CLL-derived cells for experimental validation (n = 3 clones/cell line). We first questioned whether monoallelic and biallelic BIRC3 loss had an impact in the DNA-binding activity of NF-κB transcription factors. Interestingly, HG3-del(11q) had higher p52 and RelB (non-canonical NF-κB signaling) activity than HG3WT cells (P = 0.005; P = 0.007), being this activity further increased in HG3-del(11q) BIRC3MUT cells (P & lt; 0.001; P & lt; 0.001). In depth analysis of the non-canonical signaling components by immunoblot revealed that HG3-del(11q) and, to a greater extent, HG3-del(11q) BIRC3MUT cells presented NF-κB-inducing kinase (NIK) cytoplasmic stabilization, high p-IKKα levels and p52-RelB nuclear translocation. Besides, HG3-del(11q) BIRC3MUT cells showed increased levels of the anti-apoptotic proteins BCL2 and BCL-xL. We next assessed this pathway ex vivo in stroma and CpG-stimulated primary CLL cells with or without BIRC3 deletion (n = 22; 11 each group). Remarkably, stimulated BIRC3-deleted primary cells showed higher p52 and RelB activity than BIRC3WT cases (P = 0.01; P = 0.07), and the percentage of BIRC3-deleted cells correlated with p52 activity in del(11q) cases (P = 0.04). We further performed western blot analyses in a homogenous cohort of del(11q) cases including (n = 4) or not including (n = 3) BIRC3 within the deleted region. Interestingly, del(11q)/BIRC3 deleted cases presented high levels of stabilized NIK, which correlated with higher p52 processing (P = 0.003). These patients also showed higher BCL2 levels than those del(11q)/BIRC3 undeleted, and we could further observe a correlation between p52 and BCL2 levels (P = 0.01). Given this p52-dependent BCL2 upregulation, we treated the CRISPR/Cas9 edited clones with venetoclax, demonstrating that HG3-del(11q) BIRC3MUT cells were more sensitive upon BCL2 inhibition than HG3WT clones (mean IC50 3.5 vs. 5.75 μM; P = 0.005). In vitro proliferation assays were performed to interrogate the impact of BIRC3 loss in CLL cell growth, revealing that HG3 BIRC3MUT cell lines had higher growth rates than BIRC3WT cells (P = 0.001). HG3-del(11q) BIRC3MUT cells also showed enhanced proliferation in comparison to HG3-del(11q) clones (P = 0.009). We further determined the clonal dynamics of del(11q) and/or BIRC3MUT cell lines in clonal competition experiments, showing that HG3 BIRC3MUT and HG3-del(11q) BIRC3MUT cells progressively outgrew HG3WT and HG3-del(11q) cells, respectively, overtime (P = 0.02; P = 0.006). Furthermore, we injected these edited cell lines into NSG mice (n = 20) in vivo, showing that mice xenografted with HG3 BIRC3MUT and HG3-del(11q) BIRC3MUT cells presented, by flow cytometry, an increase of human CD45+ cells in spleen 14 days after injection, compared to HG3WT and HG3-del(11q) cells (P = 0.02; P = 0.015). In summary, this work demonstrates that biallelic BIRC3 deletion through del(11q) and mutation triggers non-canonical NF-κB signaling, driving BCL2 overexpression and conferring clonal advantage, which could account for the negative predictive impact of BIRC3 biallelic inactivation in CLL. Taken together, our results suggest that del(11q) CLL patients harboring BIRC3 mutations should be considered as a CLL subgroup at a high risk of progression that might benefit from venetoclax-based therapies. Funding: PI18/01500 Disclosures No relevant conflicts of interest to declare.

Abstract: An autosomal dominant form of congenital dyserythropoietic anemia type III (CDA III) is caused by a missense mutation in the KIF23 gene whose protein product, mitotic kinesin-like protein (MKLP1), is part of the centralspindlin complex involved in cytokinesis. Several case reports suggested the existence of an autosomal recessive inheritance form of CDA III so far not genetically characterized. By means of whole exome sequencing in a Spanish CDA III family with healthy parents, we identified in the male proband a novel homozygous missense mutation p.Pro432Ser in the RACGAP1 gene, which encodes for the RACGAP1 protein (Rac GTPase-activating protein 1, also known as MgcRacGAP or CYK-4), the partner of MKLP1 in the centralspindlin complex. A second CDA III Spanish patient has a different rare and novel homozygous missense mutation, p.Thr220Ala, in the RACGAP1 gene. Both patients presented with macrocytic anemia, aberrant multinucleated erythroblasts in the bone marrow typically seen in CDA III cases, no iron overload and skull defects secondary to severe anemia. Silencing of RACGAP1 using siRNA in HeLa cells mimics the cytokinesis defect observed in the bone marrow of our patients. Both mutations disrupt normal cytokinesis and alter the GTPase balance in patients' cells. We conclude that the autosomal recessive form of CDA type III is caused by mutations in the RACGAP1 gene, encoding for RACGAP1 protein, which is the partner of MKLP1 in the centralspindlin complex critical for cytokinesis and now both proteins are associated with CDA type III. Figure 1 Figure 1. Disclosures No relevant conflicts of interest to declare.

Abstract: Continuous treatment with lenalidomide (R) and dexamethasone (d) is a standard of care for multiple myeloma (MM) patients (pts) not candidates for autologous stem cell transplantation (ASCT). As previously reported, the addition of Clarithromycin (C) to Rd has proven to be safe and effective, and case-control analyses suggested a significant additive value with the combination. C optimizes the therapeutic effect of glucocorticoids by increasing the area under the curve, has immunomodulatory effects and may have direct antineoplastic properties. However, there are not randomized phase III trials confirming these results. GEM-Claridex in an open, randomized, phase III trial for untreated newly diagnosed MM pts ineligible for ASCT. Enrolled pts were randomly assigned 1:1 to receive 28-day cycles of R (25mg po qd days 1-21), d (40mg po [20mg in pts & gt;75 years], days 1, 8, 15 and 22) plus or minus C (500mg po bid) until disease progression or unacceptable toxicity. The primary endpoint was progression-free survival (PFS). Secondary endpoints included overall response rate (ORR), overall survival (OS) and minimal residual disease (MRD) negativity rate and safety. MRD was evaluated in 99 pts using Euroflow NGF (limit of detection, 2x10-6). As expected, most pts in CR were tested for MRD whereas the majority of pts with missing MRD data achieved VGPR or less and were thus considered as MRD-positive for intent to treat analyses. Two hundred and eighty-eight pts were included (144 to C-Rd and 144 to Rd). Median age was 76 (range: 65-93), 36.8% of pts had ISS 3 and 15.6% presented with high-risk cytogenetic abnormalities. Key baseline characteristics were well balanced between the two arms. The addition of C to Rd resulted in deeper responses with a ≥ complete response (CR) rate of 20.1% in the C-Rd arm compared to 11.2% in the Rd arm (p = 0.037). Also, the ≥ very good partial response (VGPR) rate was 52.8% in the C-Rd arm as compared to the 37.1% in the Rd arm (p = 0.007). MRD analysis was performed at suspected CR and yearly afterwards. On intent-to-treat, 5/144 (3,5%) and 9/143 (6,2%) of pts achieved undetectable MRD with C-Rd and Rd, respectively (p = 0,7). With a median follow-up of 16 months (range, 1-47), no significant differences were observed in PFS: in the C-Rd arm the median was 23 months and has not been reached in the Rd arm (p = 0.09); furthermore, although disease progression and/or death rate was comparable in both arms (C-Rd: 57/144 [39.6%] vs Rd: 45/144 [31.2%] ), a trend towards shorter PFS was observed in the C-Rd group (Figure 1). This effect was less evident in younger ( & lt;75) pts (median PFS, C-Rd: 24 months vs Rd NR, p = 0,588) but, in older pts (≥ 75), the addition of C to Rd resulted into a significant deleterious effect on PFS (median PFS, C-Rd: 19 vs Rd 28 months, p = 0.03) (Figure 2a and 2b). Irrespectively of treatment arm, pts with MRD negative had significantly longer PFS (NR vs 26 months, p = 0,03). Concerning OS, no differences have been identified (p = 0.41), although median has not been reached yet in any arm. Out of the 33 and 28 deaths documented in the C-Rd and Rd arms respectively, the percentage of pts dying w/o documented PD was significantly higher in the C-Rd group (27/33 [82%] vs 13/27 [48%] , p = 0.004). Furthermore, in the C-Rd arm, the most frequent causes of death were severe infections (14/27 [52%] and cardiovascular events 6/27 [22%] ) the majority of them occurring in older (≥75) pts (20/27, 74%). The most common G3-4 adverse events (AE) in the C-Rd and Rd arms were hematologic (neutropenia: 10,4% vs 16,7% [p = ns] and anemia: 2,1% vs 6,9% [p = 0,04] , respectively). G3-4 infections occurred in 16% of cases in both arms and were the most frequent non-hematological AE. 7% of pts in both arms developed G3-4 GI toxicity and there were no differences between the two arms in G3-4 skin-related AEs (2,8% vs 3,5%). Only one case of invasive SPM (colon cancer) in the C-Rd arm was reported. In conclusion, the addition of C to Rd in transplant ineligible newly diagnosed MM pts significantly increases the rate and depth of responses but it is not associated with an improved PFS and OS due to a higher proportion of deaths in the C-Rd arm, mostly infectious, in pts & gt; 75 years and being early deaths. Overexposure to steroids due to the delayed clearance induced by C in this elderly population could explain our results. Figure Disclosures Puig: The Binding Site: Honoraria; Celgene: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau; Takeda: Consultancy, Honoraria; Amgen: Consultancy, Honoraria; Janssen: Consultancy, Honoraria, Research Funding. Rosinol Dachs:Janssen, Celgene, Amgen and Takeda: Honoraria. De Arriba:Celgene: Consultancy, Honoraria; Janssen: Consultancy, Honoraria; Amgen: Consultancy, Honoraria; Takeda: Honoraria. Oriol:Celgene Corporation: Consultancy, Speakers Bureau; Takeda: Consultancy, Speakers Bureau; Janssen: Consultancy; Amgen: Consultancy, Speakers Bureau. De La Rubia:AbbVie: Consultancy; AMGEN: Consultancy; Celgene Corporation: Consultancy; Takeda: Consultancy; Janssen: Consultancy. Amor:Celgene: Membership on an entity's Board of Directors or advisory committees; Amgen: Membership on an entity's Board of Directors or advisory committees; Janssen: Membership on an entity's Board of Directors or advisory committees; Takeda: Membership on an entity's Board of Directors or advisory committees. Martín Sánchez:GILEAD SCIENCES: Research Funding. Rossi:BMS: Research Funding; Janssen, Celgene, Amgen: Consultancy. Coleman:Merck: Research Funding; Pharmacyclics: Speakers Bureau; Kite Pharmaceuticals: Equity Ownership; Gilead, Bayer, Celgene: Consultancy, Research Funding, Speakers Bureau. Paiva:Amgen, Bristol-Myers Squibb, Celgene, Janssen, Merck, Novartis, Roche, and Sanofi; unrestricted grants from Celgene, EngMab, Sanofi, and Takeda; and consultancy for Celgene, Janssen, and Sanofi: Consultancy, Honoraria, Research Funding, Speakers Bureau. San-Miguel:Amgen, Bristol-Myers Squibb, Celgene, Janssen, MSD, Novartis, Roche, Sanofi, and Takeda: Consultancy, Honoraria. Bladé:Jansen, Celgene, Takeda, Amgen and Oncopeptides: Honoraria. Niesvizky:Takeda, Amgen, BMS, Janssen, Celgene: Consultancy, Research Funding. Mateos:EDO: Membership on an entity's Board of Directors or advisory committees; Pharmamar: Membership on an entity's Board of Directors or advisory committees; Abbvie: Membership on an entity's Board of Directors or advisory committees; Takeda: 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; Amgen: Honoraria, Membership on an entity's Board of Directors or advisory committees; Celgene: Honoraria, Membership on an entity's Board of Directors or advisory committees; GSK: Membership on an entity's Board of Directors or advisory committees; Adaptive: Honoraria.

Abstract: Background: Within the spectrum of monoclonal gammopathies, there are various subgroups with unique biological and clinical profiles. Namely, the presence of multiple myeloma (MM) and light-chain amyloidosis (AL) pts with MGUS-like phenotype has been hypothesized, but the criteria to identify this subgroup are poorly defined and lack clinical validation. Aim: Develop an algorithm based on a large flow cytometry dataset across the spectrum of monoclonal gammopathies, for automated identification of MM and AL pts with MGUS-like phenotype. Methods: This study included 5,114 pts with monoclonal gammopathies and available flow cytometry data on the frequency of bone marrow (BM) plasma cells (PC) and the percentages of normal and clonal PC within the BM PC compartment, at diagnosis. An algorithm to classify pts with MGUS-like phenotype was developed based on these three parameters, obtained from 548 MGUS, 393 smoldering MM (SMM) and 2,011 MM pts. Newly diagnosed MM pts were homogeneously treated according to the GEM2000 (n = 486), GEM2005MENOS65 (n = 330), GEM2005MAS65 (n = 239), GEM2010MAS65 (n = 230), GEM2012MENOS65 (n = 450) and CLARIDEX (n = 276) protocols. The prognostic value of the MGUS-like phenotype was validated in 96 SMM pts studied in Arkansas and 1,859 MM pts treated outside clinical trials in Czech Republic. The clinical significance of the algorithm was investigated in two independent series of Spanish (n = 102) and Italian (n = 105) AL pts. Results: The frequency of BM PC and of normal and clonal PC within the BM PC compartment were used to plot MGUS, SMM and MM pts in a principal component analysis (PCA). Lines defining 1.5 standard deviations of MGUS and MM pts were used as reference to classify each of the 5,114 cases. Once plotted against the dataset, individual pts were classified as MGUS-, intermediate- or MM-like, if their location in the PCA fell inside the MGUS, the overlapping or the MM reference lines, respectively. In the training SMM series, patient classification into MGUS-, intermediate- and MM-like phenotype resulted in significantly different rates of disease progression (0%, 54% and 66% at 5y, respectively; P & lt; .001). These results were validated in the Arkansas series (8%, 27% and 71% at 5y, respectively; P & lt; .001). Only 5% of SMM pts with high-risk disease according to Mayo or PETHEMA criteria had an MGUS-like phenotype, and these had virtually no risk of progression at 5y. In the training MM series, pts with MGUS-like phenotype showed significantly longer progression free (PFS) and overall survival (OS) vs the remaining pts. Median PFS was 10y vs 3y (hazard ratio [HR]: 0.46, P & lt; .001) and median OS was not reached (NR) vs 6.5y (HR: 0.48, P & lt; .001), respectively. These results were validated in the Czech Republic series with significant differences in PFS (HR: 0.45, P & lt; .001) and OS (HR: 0.38, P & lt; .001) between MGUS-like vs other MM pts. MGUS-like classification in the training MM series retained independent prognostic value in multivariate analyses of PFS (HR: 0.48, P & lt; .001) and OS (HR: 0.54, P = .033), together with ISS, LDH, cytogenetics, induction regimen, transplant-eligibility and complete remission (CR). MGUS-like pts showed similar PFS (P = .932) and OS (P = .285) regardless of having standard vs high risk cytogenetics. Notably, MGUS-like transplant-eligible MM pts treated with proteasome inhibitors, immunomodulatory drugs and corticoids during induction showed PFS and OS rates at 5y of 86% and 96%, respectively. Differences in PFS among MGUS-like MM pts achieving ≥CR vs & lt;CR were not significant (median of 13y vs 9y, respectively; P = .122), which suggests that attaining CR is not mandatory to reach long-term survival in this subgroup of pts, treated with fixed-duration regimens. Classification of AL pts into the MGUS-, intermediate- and MM-like phenotype resulted in significantly different PFS in the Spanish (median of 28, 20 and 1 months, respectively; P = .001) and Italian (median 32, 11 and 3 months, respectively; P & lt; .001) cohorts. Conclusions: We developed an algorithm that can be readily installed in clinical flow cytometry software, and requires three parameters that are routinely assessed at screening. Patient' automated classification using the algorithm was validated in large series across the spectrum of monoclonal gammopathies. Because pts with MGUS-like phenotype have a distinct clinical behavior, their identification could become part of the diagnostic workup in SMM, MM and AL. Disclosures Cedena: Janssen, Celgene and Abbvie: Honoraria. Milani: Celgene: Other: Travel support; Janssen-Cilag: Honoraria. Cordon: Cytognos SL: Research Funding. Oriol: Takeda: Consultancy, Speakers Bureau; Celgene: Consultancy, Speakers Bureau; Amgen: Consultancy, Speakers Bureau; Janssen: Consultancy. de la Rubia: Amgen, Bristol Myers Squibb,: Honoraria, Speakers Bureau; Celgene, Takeda, Janssen, Sanofi: Honoraria; Ablynx/Sanofi: Consultancy; Amgen: Consultancy, Membership on an entity's Board of Directors or advisory committees; Celgene: Consultancy; Janssen: Consultancy, Membership on an entity's Board of Directors or advisory committees, Other: TRAVEL, ACCOMMODATIONS, EXPENSES; AbbVie: Consultancy; Bristol Myers Squibb: Consultancy, Membership on an entity's Board of Directors or advisory committees, Other: Travel Accommodations; GSK: Consultancy; Takeda: Consultancy; Sanofi: Membership on an entity's Board of Directors or advisory committees. De Arriba: Amgen: Consultancy, Honoraria; Glaxo Smith Kline: Consultancy, Honoraria; BMS-Celgene: Consultancy, Honoraria, Speakers Bureau; Janssen: Consultancy, Honoraria, Speakers Bureau. Cabañas: Janssen: Consultancy, Honoraria; BMS: Consultancy, Honoraria; Sanofi: Honoraria. Gonzalez De La Calle: Celgene-BMS, Janssen, Amgen: Honoraria. Rodríguez-Otero: 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; 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; 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. Hajek: Pharma MAR: Consultancy, Honoraria; Takeda: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Novartis: Consultancy, Research Funding; Amgen: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; AbbVie: Consultancy, Honoraria; Janssen: Consultancy, Honoraria, Research Funding; BMS: Consultancy, Honoraria, Research Funding; Celgene: Consultancy, Honoraria, Research Funding. Jimenez-Zepeda: BMS, Amgen, Takeda, Janssen: Honoraria. Palladini: Janssen Global Services: Honoraria, Other: advisory board fees; Pfizer: Honoraria; Siemens: Honoraria. Rosinol: Janssen, Celgene, Amgen and Takeda: Honoraria. Bladé Creixenti: Janssen, Celgene, Takeda, Amgen and Oncopeptides: Honoraria. Martínez-López: Janssen, BMS, Novartis, Incyte, Roche, GSK, Pfizer: Consultancy; Roche, Novartis, Incyte, Astellas, BMS: Research Funding. Mateos: Roche: 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; Regeneron: 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; Adaptive Biotechnologies: Honoraria, Membership on an entity's Board of Directors or advisory committees; Celgene - Bristol Myers Squibb: 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; Sea-Gen: Honoraria, Membership on an entity's Board of Directors or advisory committees; Takeda: Honoraria, Membership on an entity's Board of Directors or advisory committees; AbbVie: Honoraria; Oncopeptides: Honoraria, Membership on an entity's Board of Directors or advisory committees; Bluebird bio: Honoraria; Janssen: Honoraria, Membership on an entity's Board of Directors or advisory committees; GSK: Honoraria; Oncopeptides: Honoraria. San-Miguel: AbbVie, Amgen, Bristol-Myers Squibb, Celgene, GlaxoSmithKline, Janssen, Karyopharm, Merck Sharpe & Dohme, Novartis, Regeneron, Roche, Sanofi, SecuraBio, Takeda: Consultancy, Other: Advisory board. 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.

Abstract: Abstract 4832 Background Myeloproliferatives neoplasms (MPN) are clonal stem cell disorders characterized by proliferation of one or more of the myeloid lineages, associated with genetic abnormalities that include translocations or point mutations of genes that encode cytoplasmic or tyrosin quinase (TK) receptor proteins. This produces an abnormal constitutively activation of signal transduction pathways, leading to an unregulated proliferation. According to the WHO criteria, MPN are classified into BCR-ABL+/Philadelfia Ph+: chronic myeloid leukemia (CML) and MPN BCR-ABL fusion-/Ph-. A single acquired point mutation, JAK2V617F, has been described in 95% of Polycytemia vera (PV), in 50 % of essencial thrombocythemia (ET) and idiopathic myelofivrosis (IMF), and generally absent in MPN Ph+. In the last years, it has been described the co-ocurrence of both BCR-ABL and V617F mutation in few cases of CML patients. We report here the rare and concomitant ocurrence of JAK2V617F mutation with BCR-ABL translocation at presentation in atypical CML. Methods Blood samples from six patients with clinical suspicion of MPN diagnosis, were referred to our laboratory to cytogenetic studies and molecular analysis of BCR-ABL fusion gene expression by conventional RT-PCR and JAK2V617F status mutation by ASO-PCR in three of them. All patients showed a slightly elevated white blood cells level (7200-25900), trombocythosis (700 -1036 platelets) and small splenomegaly. Three patients, after CML diagnosis, recieved Imatinib therapy and were monitored by quantitative BCR-ABL real time PCR. Due to persistent thrombocytosis, slightly elevated white blood cells level and small splenomegaly; JAK2 status was analized in these blood specimens, and later retrospectively in the stored initial diagnosis samples. Results BCR-ABL rearrangement (b3a2 isoform) and JAK2V617F mutation were identified in all 6 patients at diagnosis. Three cases showed lack of Ph chromosome, 1 patient showed 15 % Ph+ metaphases and in the remained two patients no data was available. Quantitative PCR for BCR-ABL expression performed in 3 patients during follow-up (8-12 months) showed BCR-ABL/BCR ratio 〈 = 0.0018 % (scored according to the International Scale) and the presence of JAK2V617F mutation. Retrospective assessment of stored bood samples showed that JaK mutation was already present at the time of the diagnosis of CML. Conclusions The coexistence of both genetic defects, BCR-ABL fusion gene and JAK2V617F mutation in NMP patients is a rare and uncommon feature. We found 6 MPN patients hourboring both genetics features in blood dignosis samples. In three cases JAK2V617F mutation was detected in MPN patients BCR-ABL positive after the remission induction with Imatinib. The rapid remission of BCR-ABL transcript, after a short period of Imatinib treatment, led us to think that BCR-ABL fusion gene expression was present in a low burden at diagnosis. The complete reduction of BCR-ABL rearrangement, after the imatinib therapy, and the persistence of JAK 2 mutation suggests two possible mechanisms for this double genetic alteration: 1) a haematopoyetic cell subclone with a pre-existing JAK2V617F acquires the BCR-ABL fusion gene, which confers a selective advantage to double mutant progenitor; 2) two clons, one of them having BCR-ABL rearrangement, and the other one the JAK2V617F mutation (biclonal origin). These cases intend to contribute to the discussion about the onset of the molecular alterations, and their correlation with the differente phenotypes and clinical management. Disclosures: No relevant conflicts of interest to declare.

Abstract: B ackground: ARI0002h is an academic autologous CAR T-cell product with a 4-1BB co-stimulatory domain and a humanized single chain variable fragment targeting BCMA, which is being tested in the CARTBCMA-HCB-01 clinical trial for patients (pts) with RRMM (NCT04309981). A fractionated infusion of 3x10 6 CAR+ cells (ARI0002h)/kg (10/30/60%) was administered after lymphodepletion regimen (LR). A second dose of 3x10 6 CAR+ cells/kg was planned starting at 4 months after the first dose, with or without LR according to persistence of CAR-T in peripheral blood (PB). Clinical results have been submitted in a separate abstract (#147188); here we present the correlative biological studies performed and related to disease response. Methods: ARI0002h production data were obtained and analyzed, including % CAR transduction in the final product (FP) by flow cytometry (FC). PB samples were obtained at inclusion and sequentially after infusion. Bone marrow (BM) samples were obtained at inclusion, at d28, d100 and at 6 months (m), to evaluate BCMA expression on plasma cells (PCs) and minimal residual disease (MRD) by next generation flow (NGF) with 10 -6 sensitivity. Apheresis, FP, and PB/BM samples on d28 were analyzed for T-cell subpopulations and surface markers. Persistence of ARI0002h was evaluated in PB after infusion by PCR. If a second infusion was administered, CART detection after 3 and 7d was also checked. Plasmatic soluble BCMA (sBCMA) was determined by ELISA prior to infusion, at d28 and if a relapse was observed. Human anti-human antibodies (HAHAs) were evaluated at d100 (using FC on transduced HEK cells). Clinical and response data were collected for analysis. Results: Mean BCMA molecules/cell on malignant BM PC at inclusion were 1306 (SD ±889) with non-significant (NS) correlation with response at d100. Thirty-three apheresis were obtained and transduced, with a median manufacturing time of 11d (range 9-14). All FP were successfully obtained at first attempt but one, which could be produced with a second apheresis. The mean % transduction of ARI0002h on autologous T cells was 56% (SD ±25). A higher CD4/CD8 ratio in the apheresis was correlated with achievement of stringent complete response (sCR) (p=0.003); this association was not observed in the FP. Moreover, T-cell subpopulations in the FP did not correlate with response (Figure 1A). A significant increase of PD-1 (p=0.0001), TIGIT (p=0.037) and LAG3 (p=0.003) expression was observed in CD4 CAR-T cells, as well as PD-1 in CD8 (p=0.0001), between the FP and d28 PB paired samples; similar results were observed in BM CAR-T cells at d28, except for LAG3 (Figure 1B). ARI0002h CAR-T cells showed maximum PB expansion at 14d (range 7d - 5m). Among pts with 3 and 6 m follow-up, 54% and 24% had evidence of CAR+ T cells in PB, with a median persistence of 4m (range 2-not reached). Expansion levels by PCR and PB persistence did not correlate with MM response. 22 out of 27 eligible pts (81%) received a 2 nd dose, with a subsequent CART expansion in 10 of 20 (50%) evaluable pts; LR was readministered in only 30% of these 10 pts. B-cell aplasia was observed in all pts concurring with CART expansion (Figure 1C). Also, a decline in B-cell counts was observed in some pts after reinfusion, with no association with response. Patients with detectable sBCMA at inclusion became sBCMA negative in all the subsequent tested samples at d28 and d100. None of these pts have relapsed. 27 pts had MRD assessed at day +100. In 3, the BM was severely hemodiluted and inadequate for MRD testing. Of 24 MRD-evaluable pts, 22 (92%) were MRD-negative (59% were negative at a sensitivity of 1x10 -6, 36% at 1x10 -5 and 5% at 1x10 -4), and 2 were MRD positive (one BCMA positive and another with BCMA undetermined result). Only 1 of 27 (4%) evaluable pts had positive HAHAs at day 100. CAR-T cells were undetectable at day 70 in this patient, but after receiving a 2 nd dose at m4 a short-lasted expansion was observed. Despite that, the patient deepened the response from partial response (PR) (d100) to very good PR (m6). Conclusion: ARI0002h production is fast and feasible. The peak of CART expansion occurs after 14d of infusion and also in some pts after reinfusion, regardless of LR, which suggests that LR may not be mandatory to allow further expansion after booster dose. Responses obtained are deep and construct immunogenicity seems to be low. Figure 1 Figure 1. Disclosures Gonzalez-Calle: BMS, Janssen, Amgen: Honoraria. Cabañas: Sanofi: Honoraria; BMS: Consultancy, Honoraria; Janssen: Consultancy, Honoraria. Reguera: Janssen, Kite/Gilead, Novartis: Speakers Bureau; BMS-Celgene, Novartis: Membership on an entity's Board of Directors or advisory committees. Rodríguez-Otero: Regeneron: Honoraria; Celgene-BMS, Janssen, Amgen, Sanofi, GSK, Oncopeptides: Honoraria, Membership on an entity's Board of Directors or advisory committees. Rosinol: Janssen, Celgene, Amgen and Takeda: Honoraria. Moraleda: Pfizer: Other: Educational Grants, Research Funding; Sanofi: Other: Educational Grants, Research Funding; MSD: Other: Educational Grants, Research Funding; ROCHE: Consultancy, Honoraria, Other: Educational Grants, Research Funding; Takeda: Consultancy, Honoraria, Other: Educational Grants, Research Funding; Sandoz: Consultancy, Honoraria; Novartis: Consultancy, Honoraria, Other: Educational Grants, Research Funding; Gilead: Consultancy, Honoraria, Other: Educational Grants, Research Funding; Jazz Pharmaceuticals: Consultancy, Honoraria, Other: Educational Grants, Research Funding; NovoNordisk: Other: Educational Grants, Research Funding; Janssen: Other: Educational Grants, Research Funding; Celgene: Other: Educational Grants, Research Funding; Amgen: Other: Educational Grants, Research Funding. Prósper: Oryzon: Honoraria; Janssen: Honoraria; BMS-Celgene: Honoraria, Research Funding. Mateos: Roche: 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; Celgene - Bristol Myers Squibb: Honoraria, Membership on an entity's Board of Directors or advisory committees; Sea-Gen: Honoraria, Membership on an entity's Board of Directors or advisory committees; Takeda: 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; Oncopeptides: Honoraria; GSK: Honoraria; Adaptive Biotechnologies: Honoraria, Membership on an entity's Board of Directors or advisory committees; Bluebird bio: Honoraria; Sanofi: Honoraria, Membership on an entity's Board of Directors or advisory committees; Regeneron: 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; AbbVie: Honoraria; Oncopeptides: Honoraria, Membership on an entity's Board of Directors or advisory committees. 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. Fernandez de Larrea: GSK: Honoraria; Takeda: Honoraria, Research Funding; Amgen: Consultancy, Honoraria, Research Funding; BMS: Consultancy, Honoraria, Research Funding; Sanofi: Consultancy; Janssen: Consultancy, Honoraria, Research Funding.

Abstract: Introduction: Carfilzomib dosed at 56 mg/m2 twice a week in combination with dexamethasone (Kd) is a standard of care for RRMM after 1-3 prior lines (PL) based on the ENDEAVOR study. Later, the ARROW study showed Kd dosed at 70 mg/m2 weekly to be superior to Kd dosed at 27 mg/m2 twice a week on RRMM patients (pts) after 2-3 PL. On the other side, Cyclophosphamide is an alkylating agent that has been widely combined with proteasome inhibitors and immunomodulatory drugs in MM, improving their efficacy with a good safety profile. In this phase 2 randomized study, we have compared Kd plus cyclophosphamide (KCyd) with Kd in RRMM after 1-3PL, both with K dosed weekly at 70 mg/m2. Patients and methods: RRMM after 1-3 PL of therapy were included in the trial. Consistently with the ENDEAVOR population, previous therapy with proteasome inhibitors was allowed but refractory patients were excluded. Pts were randomized 1:1 to receive K at a dose of 70 mg/m2 iv on days 1, 8 and 15 plus dexamethasone at a dose of 20 mg PO the day on and the day after K plus/minus KCyd at a dose of 300 mg/m2 IV on days 1, 8 and 15 of each 28 days-cycle, as continuous treatment until progressive disease or unacceptable toxicity. The primary endpoint was PFS and key secondary endpoints included response rates, safety profile, and OS. Results: Between January 2018 and February 2020, 198 RRMM pts were included. 97 pts were randomized to KCyd and 101 to Kd. The baseline characteristics of the patients were well balanced between both groups. The median age was 70 years, and 70% and 28% of pts were older than 65 and 75. The median number of PL was one; 61% of pts had received 1 prior line. 94% and 92% of patients had been exposed to bortezomib in the KCyd and Kd and all of them were sensitive. 72% and 67% of patients had been exposed to IMiD's and 51% and 55% of them were IMiD's-refractory in the KCyd and Kd. Only 4 and 6 patients in KCyd and Kd, had received anti-CD38 antibodies being all refractory. After a median f/u of 15.6 months, median PFS was 20.7 m and 15.2 m in KCyd and Kd (p=0.2). In pts after 1PL, median PFS has not been reached in any arm (p=0.4) and in patients after 2-3PL, KCyd resulted in a median PFS of 20.7 vs 11m for Kd (p=0.4). Of note, in the IMiD-refractory population, the addition of Cy to Kd resulted in a significant benefit in terms of PFS: 26.2 months vs 7.7 months in the Kd arm (p=0.01). OS is immature with 23 and 25 events so far in KCyd and Kd, respectively. The ORR was 78% for KCyd and 73% for Kd: 20% of patients in both arms achieved at least complete response, 33% and 28% very good partial response, respectively, and 25% partial response in both arms. The MRD-ve rate was 4% and 5%. As far as toxicity is concerned, neutropenia was the only hematological adverse event more frequently reported in KCyd compared with Kd, of any grade (24% vs 11%) and grade 3-4 (13% vs 7%). This did not translate into more infections and the rate was comparable in both arms (5% G3-4 in both arms). Thrombocytopenia of any grade and grade 3-4 occurred in 14%/1% and 18%/10% in KCyd/Kd. Cardiovascular events of any grade occurred in 22% and 30% of patients in KCyd and Kd. Nine pts in KCyd developed G3-4 cardiovascular events, these included atrial fibrillation (1pt), cardiac failure (2 pts), myocardial infarct (2 pts), and hypertension (4 pts). In the Kd arm, 11 patients developed G3-4 cardiovascular events and consisted of hypertension in most of them (9 pts). Conclusion: Cyclophosphamide added to Kd 70 mg/m2 weekly in RRMM pts after 1-3 PL prolonged the PFS as compared to Kd particularly in the lenalidomide-refractory population. The administration of K at a dose of 70 mg/m2 weekly was safe and more convenient and overall, the toxicity profile was manageable in both arms. Disclosures Mateos: Abbvie/Genentech: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; Pfizer: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; Regeneron: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; Janssen-Cilag: Consultancy, Honoraria; PharmaMar-Zeltia: Consultancy; Sanofi: Honoraria, Membership on an entity's Board of Directors or advisory committees; Oncopeptides: Honoraria, Membership on an entity's Board of Directors or advisory committees; Roche: Honoraria, Membership on an entity's Board of Directors or advisory committees; Seattle Genetics: Honoraria, Membership on an entity's Board of Directors or advisory committees; GlaxoSmithKline: Consultancy; Takeda: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; Amgen: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; Janssen: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; Celgene: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; Adaptive Biotechnologies: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees. Ocio:Janssen: Consultancy, Honoraria, Speakers Bureau; Celgene: Consultancy, Honoraria; Asofarma: Honoraria; Sanofi: Consultancy, Honoraria; Amgen: Consultancy, Honoraria; Takeda: Honoraria; GSK: Consultancy; MDS: Honoraria; Secura-Bio: Consultancy; Oncopeptides: Consultancy. Sureda Balari:Novartis: Consultancy, Honoraria; Janssen: Consultancy, Honoraria; Roche: Honoraria; Takeda: Consultancy, Honoraria, Speakers Bureau; Sanofi: Consultancy, Honoraria; Merck Sharpe and Dohme: Consultancy, Honoraria, Speakers Bureau; Celgene/Bristol-Myers Squibb: Consultancy, Honoraria; BMS: Speakers Bureau; Incyte: Consultancy; Celgene: Consultancy, Honoraria; Gilead/Kite: Consultancy, Honoraria. Oriol:Celgene/Bristol-Myers Squibb: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Amgen: Consultancy, Speakers Bureau; Janssen: Consultancy; Sanofi: Consultancy, Membership on an entity's Board of Directors or advisory committees; GlaxoSmithKline: Membership on an entity's Board of Directors or advisory committees. Rosinol Dachs:Janssen: Honoraria; Celgene: Honoraria; Amgen: Honoraria; Takeda: Honoraria; Sanofi: Honoraria. Blade Creixenti:Takeda: Membership on an entity's Board of Directors or advisory committees; Oncopeptides: Membership on an entity's Board of Directors or advisory committees; Janssen: Membership on an entity's Board of Directors or advisory committees; Celgene: Membership on an entity's Board of Directors or advisory committees; Amgen: Membership on an entity's Board of Directors or advisory committees. San-Miguel:Amgen, BMS, Celgene, Janssen, MSD, Novartis, Takeda, Sanofi, Roche, Abbvie, GlaxoSmithKline and Karyopharm: Consultancy, Membership on an entity's Board of Directors or advisory committees.

Abstract: Background. Point mutations in the kinase domain (KD) of the BCR-ABL are the most frequent mechanism of drug resistance in CML patients treated with kinase inhibitors (TKI). More than 80 mutations with different frequency and clinical significance have been reported. One of them, the T315I confers resistance to all TKIs available. The detection of mutations in KD allows early identification of high-risk patients and therefore guides clinical therapy decisions. Aim. To assess the mutation status of a group of CML pts resistant to TKI from Uruguay (n=35) and Brazil (n=30). Methods. KD mutation screening was performed by RT-PCR and direct sequencing according to Branford et al. (2002). Additionally, we developed a rapid, specific, sensitive and low cost allele specific (AS)-RT-PCR assay to identify T315I, using Branford’s KD amplification primers in combination with an allele specific primer for the T315I point mutation detection. BCR-ABL transcript levels were also measured by RQ-PCR according to international recommendations. Results and Discussion. RT-PCR and direct sequencing analyses performed in all pts showed the presence of T315l mutation in 3/65 cases. Other 11 showed the alternative mutations Y253H (n=2), E450A, G250E (n=2), E459K (n=2), E450G, F317L (n=2) and E255K; and the remaining 55 showed no mutations in the ABL KD. All 65 samples together with cDNA from 15 non-resistant CML pts and 10 cDNA from non-CML were analyzed by AS-RT-PCR assay for T315l mutation in order to validate the method. T315l was identified in the 3 samples in which the mutation was previously detected by direct sequencing and in 1 pt that had been classified as KD mutation negative. This result was then confirmed by direct sequencing of the AS-PCR product. T315 was neither detected in samples positive for other mutations nor in samples of non-resistant CML and non-CML patients, supporting the specificity of the method. Assessment of the sensitivity of the AS-RT-PCR was performed on serial dilutions experiments using RNA from T315 positive pt into RNA from CML-T315l negative pt, showing that the T315I mutation was detectable to a level of 0.01 % by AS-PCR, while through direct sequencing method the sensitivity was 10–20%. The prevalence of mutations in our study was 15/65 (23%). Conclusions. Our results showed that the AS-RT-PCR described here is a convenient and easy tool to be used in a clinical routine laboratory for rapid screening for BCR-ABL T315. This, together with direct sequencing, constitutes a suitable approach for CML resistance monitoring and therapeutic choice.

Abstract: Acute myeloblastic leukemias (AML) are the most frequent acute leukemias in adults. Recent genomic and candidate gene studies identified novel recurrent somatic mutations in patients with AML with biological, clinical and therapeutic significance. In Uruguay, the characterization of AML is dependent on the integration of cytomorphology, immunophenotype, cytogenetics and molecular biology (mutations in FLT3, NPM1, CEBPA and cKIT). This allows stratification in prognostic risk groups and rationalization of therapeutic resources. However, with this molecular markers, complete stratification is not always succesful in patients with normal karyotype ( NK) . In order to expand the AML genetic markers analysis in Uruguayan patients ( 3.000.0000 persons country ) ; DNA samples from 49 adult patients at the onset of the disease and 3 healthy controls were analyzed by next generation sequencing (NGS, Illumina) using a customized panel of 30 cancer associated genes. After excluding synonymous variants and those with a population frequency 〉 1%, we ended with 493 variants in promoters and intron ends, and 173 in the 28 genes coding regions. Notably, all patients had at least one variant. While none of the non-coding region variants were reported as pathogenic in COSMIC and/or ClinVar databases, 45 of those falling in coding regions were reported as pathogenic. The pathogenic variants were detected in 36/49 patients and 1 healthy control distributed in 16 genes. The most affected genes were TET2, NRAS, DNMT3, FLT3 and cKIT with 7, 6, 5, 4, 4 variants respectively. This preliminar findings in this study, highlight the relevance of detecting gene variability underlying prognostic risk subgroups to deliver patient tailored clinical decision support. The correlation between the genetic abnormalities, associated with the clinicopahtological features and the epigenetic studies have prognostic significance, and are continued to be analyzed. Disclosures No relevant conflicts of interest to declare.