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Alterations in KRAS have been identified as the most recurring somatic variants in the multiple myeloma (MM) mutational landscape. Combining DNA and RNA sequencing, we studied 756 patients and observed KRAS as the most frequently mutated gene in patients at diagnosis; in addition, we demonstrated the persistence or de novo occurrence of the KRAS aberration at disease relapse. Small-molecule inhibitors targeting KRAS have been developed; however, they are selective for tumors carrying the KRASG12C mutation. Therefore, there is still a need to develop novel therapeutic approaches to target the KRAS mutational events found in other tumor types, including MM. We used AZD4785, a potent and selective antisense oligonucleotide that selectively targets and downregulates all KRAS isoforms, as a tool to dissect the functional sequelae secondary to KRAS silencing in MM within the context of the bone marrow niche and demonstrated its ability to significantly silence KRAS, leading to inhibition of MM tumor growth, both in vitro and in vivo, and confirming KRAS as a driver and therapeutic target in MM.

Introduction: The role of minimal residual disease (MRD) in Multiple Myeloma (MM) as a surrogate biomarker of patients' outcome, as well as the prognostic information of functional imaging response after treatment have been established in recent years. Furthermore, the predictive relevance of sustained MRD negativity assessed by marrow and imaging techniques and its association with an excellent outcome is emerging in clinical trials. Diffusion-weighted whole-body MRI (DW-MRI) is increasingly used in the management of MM patients, but data regarding the predictive role of sustained DW-MRI response after treatment are lacking. The Myeloma Response Assessment and Diagnosis System (MY-RADS) recommendations have established criteria for Response Assessment Category (RAC) (Messiou C et al, Radiology 2019) with a 5 point scale defining the probability of complete imaging response (i.e. RAC 1) or progressive disease after treatment (i.e. RAC 5). We implemented the RAC criteria in our clinical practice and DW-MRI at 1-year in MM patients treated with autologous stem cell transplantation (ASCT) followed by maintenance therapy. Patients and methods: We retrospectively analyzed the outcome of 57 newly diagnosed MM patients (median age 61 years) diagnosed at our institution from January 2015 to December 2019 receiving maintenance therapy after ASCT. Patients underwent DW-MRI evaluation according to MY-RADS criteria at day +100 after ASCT, before maintenance, and after 1 year with the aim of monitoring imaging residual disease. Bone marrow samples were collected for MRD assessment by 8-color FCM (sensitivity 10-5) at day +100 after ASCT, before maintenance, and after 1 year. We focused on sustained 1-year DW-MRI negativity evaluated according to RAC response, and its potential predictive role at that timepoint on progression free survival (PFS) and overall survival (OS). In patients with available 1-year MRD evaluations, concordance between 1-year MRD and DW-MRI results was calculated and the level of agreement was expressed by Cohen's kappa statistics. Results: Out of 57 patients, 23 (40%) were ISS stage 3 and 14 (25%) showed high risk cytogenetics. Patients were treated with the following induction regimens: VTD 42, VRD 5, Dara-VRD 6, KRD 3, KCD 1. Single ASCT with MEL200 conditioning was performed in 33 patients (58%), whereas 24 patients (42%) received double ASCT. Subsequent maintenance was performed with lenalidomide (49 patients, 86%) or daratumumab-lenalidomide (8 patients, 14%). Response rates after ASCT were PR 9%, VGPR 23%, CR 51% and sCR 17%. According to MY-RADS, a complete imaging response (RAC1) at day +100 after ASCT was observed in 34 patients (60%). Sustained 1 year complete imaging response during maintenance therapy was observed in 43 patients (75%). Some residual disease was identified at that timepoint in 14 patients (25%) [RAC 2: 6 (11%), RAC > 2: 8 (14%)]. After a median follow up of 36 months, PFS and OS were significantly longer in patients with sustained 1-year imaging negativity, compared to patients with imaging residual disease (RAC 1 vs RAC ≥2): median PFS 56 vs 24,1 months, p Conclusion: Our real-life data analysis confirms the predictive value of imaging residual disease assessment with DW-MRI after ASCT and highlights the importance of achieving sustained imaging MRD negativity during maintenance therapy regardless of different treatment strategies. Moreover, given the high rates of CR seen in patients with MM with novel effective treatment combinations, the detection of residual disease with the combined evaluation of marrow and functional imaging techniques during maintenance therapy can help the physician to identify patients with increased risk of early relapse and particularly poor prognosis. Our preliminary data regarding the high concordance observed between DW-MRI and MRD results during maintenance therapy suggest that DW-MRI could represent a reliable non-invasive technique to monitor residual disease. Disclosures Belotti: 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; Takeda: Consultancy; GSK: Membership on an entity's Board of Directors or advisory committees. Ribolla: Janssen: Membership on an entity's Board of Directors or advisory committees. Cancelli: Amgen: Membership on an entity's Board of Directors or advisory committees. Roccaro: Amgen, Celgene, Janssen, Takeda: Membership on an entity's Board of Directors or advisory committees; AstraZeneca,: Research Funding; Associazione Italiana per la Ricerca sul Cancro (AIRC): Research Funding; European Hematology Association: Research Funding; Fondazione Regionale per la Ricerca Biomedica (FRRB), Transcan-2 ERA-NET: Research Funding. Rossi: Sanofi: Honoraria; Jazz: Membership on an entity's Board of Directors or advisory committees; Pfizer: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees; Abbvie: Membership on an entity's Board of Directors or advisory committees; Celgene: Membership on an entity's Board of Directors or advisory committees; Janssen: Membership on an entity's Board of Directors or advisory committees; Daiichi Sankyo: Consultancy, Honoraria; Astellas: Membership on an entity's Board of Directors or advisory committees; Amgen: Honoraria, Membership on an entity's Board of Directors or advisory committees; Alexion: Membership on an entity's Board of Directors or advisory committees; Takeda: Membership on an entity's Board of Directors or advisory committees. Tucci: janssen: Membership on an entity's Board of Directors or advisory committees; Gentili: Membership on an entity's Board of Directors or advisory committees; Takeda: Membership on an entity's Board of Directors or advisory committees.

Background: 18F-FDG-PET/CT is currently the standard technique to define imaging-minimal residual disease (MRD) in multiple myeloma (MM) patients. A joint analysis of 2 prospective randomized trials in newly diagnosed transplant-eligible MM (NDTEMM) patients applied for the first time the Deauville Scores (DS) to focal lesions (FS) and bone marrow uptake (BMS) showing the liver background (DS < 4) as the best cut-off to define PET/CT negativity after therapy and complete metabolic response (CMR) (Zamagni et al, ASH 2018). Validation of this new standardized criteria in an independent prospective series of NDTEMM patients is highly required. Imaging-PET/CT MRD also showed to be complementary to Multiparameter Flow Cytometry (MFC) in predicting patient's outcomes (Moreau P, JCO 2017). In this analysis, we aimed at confirming the applicability and validity of DS criteria to define PET/CT CMR and showing their impact on patient's outcomes in the multicenter phase II randomized FORTE trial for NDTEMM patients. We also looked at the complementarity with BM techniques, especially MFC. Methods: NDTEMM patients ≤ 65 years were randomized to receive carfilzomib, lenalidomide, dexamethasone (KRD) induction - autologous stem cell transplantation (ASCT) intensification-KRd consolidation (arm A); KRd12 (arm B) and carfilzomib, cyclophosphamide, dexamethasone (KCd) induction-ASCT intensification-KCd consolidation (arm C). Thereafter, patients were randomized to maintenance with lenalidomide alone or plus carfilzomib. PET/CT scans were performed locally at baseline (B) and prior to the start of maintenance (PM). DS were applied both in the BM and FLs, as previously described (Zamagni E, EHA 2020). CMR was defined as DS < 4 both in the FLs and BM. MRD evaluation was performed by 8-color second-generation flow cytometry (sensitivity 10-5) in patients who achieved at least VGPR before maintenance (Gay F, ASCO 2019). The impact of each parameter on outcomes was evaluated by landmark analyses at PM. A multivariable Cox regression analysis was adopted to identify independent predictors for PFS and OS. Results: 182 out of the 474 global patients enrolled in the trial had a pre- and post-treatment PET/CT evaluation available and were included in this analysis. Patients' baseline characteristics were: median age 57 years, ISS and R-ISS stage III 18% and 10%, respectively, high-risk cytogenetics (t(4;14) ± del(17p) ± t(14;16), detected by FISH) 26%, reflecting baseline clinical features of the entire FORTE population. At B, 93% of patients had FLs, with a median maximum standardized uptake value (SUVmax) of 5.7 [IQR: 4.1-8.1], 7% presented extra-medullary lesions and nearly all patients had increased BM uptake, with a median SUVmax of 3.4 [IQR: 2.8-4.3]. FS and BMS ≥ 4 were present in 87% and 57% of patients, respectively. At PM, PET/CT negativity according to DS < 4, was present in 80% in the FLs and 85% in the BM, respectively. 63% showed CMR. 92% and 61% of patients achieved ≥ VGPR and CR as best response, respectively, while 73% of them achieved MFC MRD negativity. The achievement of a best CR significantly correlated with BMS < 4 at PM (P= 0.003). In univariate analysis, at Landmark time PM, FS Conclusion: In conclusion, the present analysis confirms the applicability and validity of DS criteria to define PET/CT CMR in an independent prospective series of NDTEMM patients. CMR significantly and independently correlated in uni- and multivariable analysis with patient's outcomes in terms of PFS and OS and was complementary to the MFC MRD negativity. Figure Disclosures Zamagni: Takeda: Honoraria, Other: Travel, Accommodations, Expenses, Speakers Bureau; Celgene Corporation: Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; BMS: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: Travel, Accommodations, Expenses, Speakers Bureau; Amgen: Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Janssen: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: Travel, Accommodations, Expenses, Speakers Bureau; Sanofi: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: Travel, Accommodations, Expenses, Speakers Bureau. Gay:Bristol-Myers Squibb: Honoraria, Membership on an entity's Board of Directors or advisory committees; Roche: Membership on an entity's Board of Directors or advisory committees; AbbVie: Membership on an entity's Board of Directors or advisory committees; Adaptive: Membership on an entity's Board of Directors or advisory committees; Seattle Genetics: Membership on an entity's Board of Directors or advisory committees; Oncopeptides: Membership on an entity's Board of Directors or advisory committees; GSK: Membership on an entity's Board of Directors or advisory committees; Celgene: 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; Janssen: Honoraria, Membership on an entity's Board of Directors or advisory committees. D'Agostino:GSK: Membership on an entity's Board of Directors or advisory committees. Galli:Takeda: Honoraria; Janssen: Honoraria; Celgene: Honoraria; BMS: Honoraria. Zambello:Janssen: Membership on an entity's Board of Directors or advisory committees; Celgene: Membership on an entity's Board of Directors or advisory committees. Rivolti:Celgene: Membership on an entity's Board of Directors or advisory committees. Tacchetti:Bristol-Myers Squibb: Honoraria; AbbVie: Honoraria; Takeda: Honoraria; Celgene: Honoraria; Oncopeptides: Honoraria; Janssen: Honoraria; Amgen: Honoraria. Fanti:Bayer, Astellas, GE Healthcare, ANMI, Janssen: Membership on an entity's Board of Directors or advisory committees; Bayer, Astellas, GE Healthcare, Sanofi, AAA: Honoraria. Boccadoro:Bristol-Myers Squibb: Honoraria, Research Funding; Celgene: Honoraria, Research Funding; Janssen: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; AbbVie: Honoraria; Mundipharma: Research Funding; GlaxoSmithKline: Membership on an entity's Board of Directors or advisory committees; Sanofi: Honoraria, Research Funding; Novartis: Honoraria, Research Funding; Amgen: Honoraria, Research Funding. Cavo:AbbVie: 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, Speakers Bureau; Novartis: Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Sanofi: Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; BMS: Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Janssen: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: Travel accomodations, Speakers Bureau; Celgene: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: Travel accomodations, Speakers Bureau; GlaxoSmithKline: Honoraria, Speakers Bureau; Karyopharm: Honoraria. Musto:Amgen: Honoraria; Celgene: Honoraria. Oliva:Amgen: Honoraria, Membership on an entity's Board of Directors or advisory committees; Takeda: Membership on an entity's Board of Directors or advisory committees; Janssen: Honoraria, Membership on an entity's Board of Directors or advisory committees; Adaptive Biotechnologies: Membership on an entity's Board of Directors or advisory committees; Celgene: Honoraria.

INTRODUCTION. The multiple myeloma (MM) mutational landscape has identified KRAS as the most recurring somatic variant, observed in around 26% of cases, therefore KRAS may represent an important therapeutic target. Despite several attempts to develop a targeted therapeutic for KRAS mutant cancers, either direct KRAS enzymatic inhibition, or inhibition of MAPK- and PI3K- downstream effector cascades have not been successful. Therefore, there is a need to develop novel therapeutic approaches that may target the KRAS mutational event in MM. We have studied AZD4785, a novel, potent and selective high affinity 2'-4' constrained ethyl residues containing therapeutic antisense oligonucleotide (ASO) targeting KRAS, both in vitro and in vivo. METHODS. AZD4785 productive uptake was assessed by measuring KRAS knockdown at both the mRNA and protein level. Molecular mechanisms underlying AZD4785-dependent anti-MM activity were studied, interrogating the transcriptome profiling of AZD4785-treated MM cells. Anti-MM activity of AZD4785 was assessed in vitro in the context of primary MM patients' derived bone marrow stromal cells (BMSCs). Endpoints included evaluation of cell proliferation, cytotoxicity, cell cycle modulation, apoptosis, MM cell migration and adhesion; modulation of MAPK-, PI3K-, apoptotic-signaling. KRAS-mutated (MM1S; KMS20); -wild type (U266; KMS11) MM cell lines; BM MM patients' and peripheral blood healthy donor derived cells were tested. A non-targeting ASO (ASO-ctrl) was used as control. Synergism between AZD4785 and bortezomib, in modulating MM growth was tested. AZD4785-dependent modulation of tumor growth was studied in vivo in a subcutaneous MM.1S.-Luc model and a disseminated GFP/Luc-MM.1S model (BLI); MM cell dissemination to distant BM niches was studied ex vivo, using confocal laser scanning microscopy. RESULTS. AZD4785 led to specific dose-dependent inhibition of KRAS mRNA and protein expression, in KRAS-mutant, -wild-type cell lines and MM patient-derived CD138+ cells; without affecting NRAS and HRAS content. Wide mRNA transcriptome was performed using AZD4785 treated MM.1S cells vs control: GSEA showed down-regulation of MAPK, cell cycle, TP53 signaling pathways (FDR CONCLUSION. Taken together, these data suggest that AZD4785 may represent a novel therapeutic approach for targeting mutant KRAS in MM, either alone or in combination with proteasome inhibitors; and warrant further development. Disclosures Giacomini: Fondazione Cariplo: Research Funding. Belotti:Amgen: 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. Revenko:Ionis Pharmaceuticals: Employment. MacLeod:Ionis Pharmaceuticals: Employment. Willis:AstraZeneca: Employment. Cai:AstraZeneca: Employment. Hauser:AstraZeneca: Employment. Rooney:AstraZeneca: Employment. Ambrose:AstraZeneca: Employment. Staniszewska:AstraZeneca: Employment. Hanson:AstraZeneca: Employment. Rossi: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; Gilead: Membership on an entity's Board of Directors or advisory committees; Sanofi: Membership on an entity's Board of Directors or advisory committees; Abbvie: Membership on an entity's Board of Directors or advisory committees; Pfizer: Membership on an entity's Board of Directors or advisory committees; Jazz: Membership on an entity's Board of Directors or advisory committees; Astellas: Membership on an entity's Board of Directors or advisory committees; Novartis: Honoraria; Mundipharma: Honoraria; BMS: Honoraria; Sandoz: Honoraria; Daiichi-Sankyo: Consultancy; Roche: Membership on an entity's Board of Directors or advisory committees. Ronca:Associazione Italiana per la Ricerca sul Canctro (AIRC): Research Funding. Bolli:GILEAD: Other: Travel expenses; JANSSEN: Honoraria; CELGENE: Honoraria. Moschetta:AstraZeneca: Employment. Ross:AstraZeneca: Employment. Roccaro:Celgene: Membership on an entity's Board of Directors or advisory committees; European Hematology Association: Research Funding; Amgen: Membership on an entity's Board of Directors or advisory committees; Transcan2-ERANET: Research Funding; AstraZeneca: Research Funding; Janssen: Membership on an entity's Board of Directors or advisory committees; Associazione Italiana per al Ricerca sul Cancro (AIRC): Research Funding.

Background: 18F-FDG-PET/CT is currently the standard technique to define minimal residual disease (MRD) status outside the bone marrow (BM) in multiple myeloma (MM) patients. A joined analysis of two prospective randomized trials in newly diagnosed transplant-eligible MM (NDTEMM) patients applied for the first time the Deauville Scores (DS) to focal lesions (FS) and bone marrow uptake (BMS) and showed the liver background (DS < 4) to be the best cut-off to define PET negativity after therapy (Zamagni et al, ASH 2018). Multiparameter Flow cytometry (MFC) at the sensitivity level of 10-5 is one of the standardized methods to assess MRD in the BM (Kumar SK, Lancet Oncol 2016). In this analysis, we aimed at comparing MRD data by PET/TC assessment and MFC in the multicenter phase II randomized FORTE trial for NDTEMM patients. Methods: NDTEMM patients ≤65 years were randomized to receive carfilzomib, lenalidomide, dexamethasone (KRD) induction - autologous stem cell transplantation (ASCT) intensification-KRd consolidation (arm A); KRd12 (arm B) and carfilzomib, cyclophosphamide, dexamethasone (KCd) induction-ASCT intensification-KCd consolidation (arm C). Thereafter, patients were randomized to maintenance with lenalidomide alone or lenalidomide plus carfilzomib. MRD evaluation was performed by 8-color second-generation flow cytometry (sensitivity 10-5) in patients who achieved at least VGPR before maintenance (Gay F, ASCO 2019). PET/CT scans were performed locally at baseline (B) and prior to the start of maintenance (PM) and the DS were applied both in the BM and FLs as previously described. Cramér's V coefficient was used to measure the concordance between PET/TC and MFC; Fisher or X2 tests were adopted, where appropriate, to evaluate the statistical significance, at the level of 0.05. Results: 182 out of the 474 global patients enrolled in the trial had a matched PET/CT and MFC evaluation available and were included in the present analysis. Baseline characteristics of the patients were as follows: median age 57 years, ISS and R-ISS stage III 18% and 10%, respectively, high-risk cytogenetics (t(4;14) ± del(17p) ± t(14;16), detected by FISH) 26%, reflecting baseline clinical features of the entire FORTE population. At B, 92% of the patients had FLs, with a median maximum standardized uptake value (SUVmax) of 5.7 [IQR: 4.1-8.1], 11% presented extra-medullary lesions and nearly all the patients had increased BM uptake, with a median SUVmax of 3.3 [IQR: 2.8-4.3]. FS and BMS ≥4 were present in 87% and 59% of the patients, respectively. A higher FS at B was significantly correlated with ISS stage III (P= 0.04), while higher BMS with lower hemoglobin level (P= 0.002) and higher free light chain ratio (P= 0.004). At PM, PET/TC negativity according to DS < 4, was present in 78% in the FLs and 85% in the BM, respectively. No significant correlations between PET/CT negativity after therapy and baseline prognostic factors or PET/CT characteristics were found. 95% and 67.5% of the patients achieved ≥ VGPR and CR as best response, respectively, while 75% of them achieved MFC MRD negativity. The achievement of a best CR significantly correlated with BMS < 4 at PM (P= 0.013). We analyzed the concordance between MRD results by the two techniques and Cramér's V coefficient measured a strong association, with a value of 0.76 (p Conclusion: In conclusion, the present analysis confirms the applicability and validity of DS criteria for the definition of PET/CT MRD outside the BM in an independent prospective series of NDTEMM patients. PET/CT negativity significantly correlated with the achievement of best CR. PET/CT and MFC at the sensitivity level of 10-5 showed a good concordance in the BM, while were also confirmed to be complementary outside (FLs). Future analyses will show the impact of PET/CT in comparison with BM MRD techniques on patient's outcomes. Disclosures Zamagni: Celgene Corporation: Honoraria, Other: Advisory board, Speakers Bureau; Janssen: Honoraria, Other: Advisory board, Speakers Bureau; Amgen: Honoraria, Other: Advisory board, Speakers Bureau; BMS: Honoraria, Other: Advisory Board, Speakers Bureau; Takeda: Honoraria, Speakers Bureau; Sanofi: Honoraria, Other: Advisory Board, Speakers Bureau. Gay:Celgene: 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; Janssen: 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; Takeda: Honoraria, 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; Amgen: Honoraria, Membership on an entity's Board of Directors or advisory committees; Roche: Membership on an entity's Board of Directors or advisory committees; Amgen: Honoraria, Membership on an entity's Board of Directors or advisory committees; Bristol-Myers Squibb: Honoraria, Membership on an entity's Board of Directors or advisory committees; Roche: Membership on an entity's Board of Directors or advisory committees; AbbVie: Membership on an entity's Board of Directors or advisory committees; Bristol-Myers Squibb: Honoraria, Membership on an entity's Board of Directors or advisory committees. Omedé:Janssen: Membership on an entity's Board of Directors or advisory committees. Galli:Janssen: Honoraria; Leadiant (Sigma-Tau): Honoraria; Takeda: Honoraria; Bristol-Myers Squibb: Honoraria; Celgene: Honoraria. Zambello:Janssen: Membership on an entity's Board of Directors or advisory committees; Celgene: Membership on an entity's Board of Directors or advisory committees. Gamberi:Amgen: Honoraria, Membership on an entity's Board of Directors or advisory committees; BMS: 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; Janssen: Honoraria, Membership on an entity's Board of Directors or advisory committees. Patriarca:Janssen: Membership on an entity's Board of Directors or advisory committees; Celgene: Membership on an entity's Board of Directors or advisory committees; Takeda: Membership on an entity's Board of Directors or advisory committees. Tacchetti:Celgene: Honoraria, Speakers Bureau; Amgen: Honoraria, Speakers Bureau; Takeda: Honoraria, Speakers Bureau; Janssen: Honoraria; Oncopeptides: Honoraria, Speakers Bureau; BMS: Honoraria. Boccadoro:Sanofi: Honoraria, Research Funding; Celgene: Honoraria, Research Funding; Amgen: Honoraria, Research Funding; Janssen: Honoraria, Research Funding; Novartis: Honoraria, Research Funding; Bristol-Myers Squibb: Honoraria, Research Funding; AbbVie: Honoraria; Mundipharma: Research Funding. Cavo:bms: Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; sanofi: Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; celgene: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: travel accommodations, Speakers Bureau; novartis: Honoraria; takeda: Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; amgen: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; AbbVie: 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, Other: travel accommodations, Speakers Bureau. Oliva:Adaptive Biotechnologies: Membership on an entity's Board of Directors or advisory committees; Celgene: Honoraria; 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; Takeda: Membership on an entity's Board of Directors or advisory committees.

Introduction: Autologous stem cell transplantation (ASCT) has proven effective in Multiple Myeloma (MM) patients (pts) aged 65. With the increase of treatment options, several geriatric assessment tools have been proposed to help physicians in selecting treatment of appropriate intensity. However specific criteria for evaluating ASCT eligibility in elderly MM pts have been seldom investigated.Patients and Methods: From January 2013 to December 2017 131 consecutive newly diagnosed symptomatic MM pts aged 65-75 (M/F: 66/65) were considered for ASCT at our center according to physician's clinical judgement. The variables included in Charlson Comorbidity Index (CCI), Hematopoietic cell transplantation comorbidity index (HCT-CI), and International Myeloma Working Group (IMWG) frailty score were not considered for selection but were retrieved for this study and pts were classified accordingly. Of note, ADL and IADL were referred to pts status prior the occurrence of MM related symptoms. The impact of age and the predictive role of the above mentioned scores on progression-free survival (PFS) of pts selected or not for ASCT was analyzed. Pts characteristics are shown in Table 1. Results: Of 131 pts, 85 (65%) were judged transplant eligible (ASCT ITT arm) by the clinician and received bortezomib-based induction (VTD 94%, VD 5%, VCD 1%); 72 of them (85%) actually underwent ASCT (70% single, 30% double) with melphalan conditioning 200 mg/sqm in 68% of cases. The 46 pts considered ineligible to ASCT received a less intensive first line treatment (89% VMP, 4% Daratumumab-VMP, 2% MP, 5% steroids/palliation). Complete remission (CR) after the first ASCT was higher in the ASCT (ITT) vs the NO ASCT group (43,5% vs 26%, respectively; p 0.048), whereas ORR and ³VGPR rates were comparable (83% vs 74% and 76,4% vs 61%, respectively). Transplant related mortality (TRM) was 0%. 2 year death rate was 13% in the ASCT (ITT) arm and was due to PD in 91% of cases (the latter 9% concerns a cardiac arrest secondary to aspiration pneumonia in a pt in VGPR 3 months after first ASCT). After a median follow-up of 27 months, PFS was 35,6 in the ASCT (ITT) group vs 19,9 months in NO ASCT pts, respectively; p 0,013, HR 0,42 (95% CI: 0,25-0,71). HCT-CI was ≥2 in 87% of pts overall. PFS was better in pts with HCT-CI 0-1 compared to HCT≥2 (NR vs 27,3 months; p 0,025, HR 0,45, 95% CI 0,23-0,91) and also in pts with HCT-CI ³2 undergoing ASCT (ITT) than in NO ASCT pts: median 34 vs 19,9 months, p 0,008 (HR 0,5, 95%CI 0,30-0,84). CCI was 0-3 in 78% of pts. Their outcome according was similar to pts with CCI >3 but ASCT performed better than NO ASCT also in the few (median PFS 51,4 vs 16,9 months; p 0,04, HR 0,37, 95%CI 0,15-0,95). IMWG frailty score was more useful. No pts classified as FRAIL had been considered eligible to ASCT by clinical decision with a significantly worse outcome compared to FIT and UNFIT pts (median PFS: 7,9 months vs 32,9 and 29,6; FIT vs FRAIL: p < 0.0001, HR 0,02, 95%CI 0,004-0,008; UNFIT vs FRAIL: p < 0,001, HR 0,03 95%CI 0,007-0,12). Unlike HCT-CI and CCI, the distribution of patients according to IMWG score was more balanced both overall and between the ASCT (ITT) and NO ASCT groups, particularly for UNFIT pts. However on the whole no outcome differences were observed between FIT and UNFIT pts. PFS was better in FIT&UNFIT pts in the ASCT (ITT) group than in the NO ASCT group: median 35,6 months vs 25,8 months; p 0,04, HR 0,54, 95%CI 0,31-0,97. However, in the ASCT (ITT) group, the age group 65-69 years fared better than pts ³70 years (51,5 vs 27,7 months, p 0,0037; HR 0.34, 95% CI 0.17-0.7). Indeed, in IMWG UNFIT patients aged ³70, the PFS of the ASCT (ITT) group was comparable to NO ASCT group (18 vs 27 months, p 0,33) whereas in UNFIT pts aged 65-69, PFS was superior in pts treated more intensively by physician choice: 43,3 months in ASCT (ITT) arm vs 18,4 months in NO ASCT (p 0.01, HR 0.03; 95%CI 0.003-0.24). Conclusion: In an unselected series of elderly MM pts aged 65-75 and undergoing ASCT according to clinical judgement the outcome of ASCT pts was better than those of NO ASCT. CCI and HCT-CI score proved of little help for better identifying the best candidates to ASCT. Conversely the IMWG frailty score would be of help in identifying the category of UNFIT pts aged 70-75 whose outcome with ASCT selected by clinical judgement was no better than with less intensive treatments. Table 1: Disclosures Belotti: Amgen: Other: Advisory Board; Celgene: Other: Advisory Board. Cattaneo:GILEAD: Other: Advisory Board. Rossi:TEVA: Other: ADVISORY BOARD; CELGENE: Other: ADVISORY BOARD; SANOFI: Other: ADVISORY BOARD; MUNDIPHARMA: Honoraria; ABBVIE: Other: ADVISORY BOARD; JANNSEN: Other; PFIZER: Other: ADVISORY BOARD; ROCHE: Other: Advisory Board; NOVARTIS: Honoraria; SANDOZ: Honoraria; GILEAD: Other: ADVISORY BOARD; BMS: Honoraria; JAZZ: Other: ADVISORY BOARD; AMGEN: Other: ADVISORY BOARD.

Introduction Multiple myeloma (MM) can be associated with paraskeletal (PP) or extramedullary (EMP) plasmocytomas (PL). Although PLs are relatively frequent, even at diagnosis, our knowledge on the subject mainly relies on small case series or single center experiences. Remarkably, little is known regarding the role of new drugs on MM with PL. Aim To perform a meta-analysis of 8 EMN-GIMEMA studies focused on the description of PLs characteristics, clinical outcome, and response to new drugs. The trials involved in this meta-analysis have the following identification codes: EUDRACT 2005-00473041, NCT01093196, NCT01063179, NCT01091831, NCT01857115, NCT01190787, NCT00551928, NCT01346787. Patients A total of 2332 newly diagnosed MM patients have been included in the analysis, and 267 (11.4%) had a PL, defined as PP in 243 (10.4%), EMP in 12 (0.5%), and not classified in 12 (0.5%) patients. PLs were evaluated with different techniques as PET, CT, NMR or physical examination accordingly to their site and local guidelines. PP and EMP were single in 195, and multiple in 60 cases, and their median size was 4 cm (range 1-26 cm). Median age at diagnosis was 68 years (range 18-87) in PL, and 69 years (range 25-91) in non-PL patients; male patients were 149 (55.8%) in PL and 1007 (48.8) in non-PL patients. ISS stage in PL and non-PL patients was as follows: stage I in 119 (44.6%) and 682 (33.0%), stage II in 89 (33.3%) and 792 (38.3%), stage III in 38 (14.2%) and 508 (24.6%), unknown in 21 (7.8%) and 83 (4.0%) patients, respectively. Cytogenetics was available in 166 PL, and 1528 non-PL patients, and high risk status, defined as having t(4;14) or t(14;16) or del(17p), was observed in 51 (30.7%) PL, and 446 (21.6%) non-PL patients. PL and non-PL patients received treatment with the following drugs: bortezomib in 65 (24.3%) and 598 (29.0%), lenalidomide in 182 (68.1%) and 1366 (66.1%), and carfilzomib in 20 (7.5%) and 101 (4.9%), respectively. Autologous stem cell transplantation (ASCT) was performed in 112 (41.9%) PL and 782 (37.9) non-PL patients. PL and non-PL patients received maintenance in 108 (40.4%) and 815 (39.5%), continuous treatment in 128 (47.9%) and 1007 (48.7%), and fixed-duration treatment in 31 (11.6%) and 243 (11.7%) cases, respectively. Results With a median follow-up of 62 months (IQ range 34-75) in PL and 65 months (IQ range 40-77) in non-PL patients, 4-years overall survival (OS) was 56% in PL and 67% in non-PL patients (p3 cm PL, 53% ≤3 cm PL vs 67% in non-PL), and a 5 cm threshold (55% >5 cm PL, 57% ≤5 cm PL vs 67% in non-PL), neither by the number of PLs (58% single PL, 50% multiple PLs vs 67% in non-PL), or the onset site (57% PP, 55% EMP vs 67% in non-PL). Four-years OS in PL patients treated with either bortezomib or carfilzomib was 44%; it was 56% in patients treated with lenalidomide; in non-PL patients was 62% when treated with bortezomib or carfilzomib, or 65% when treated with lenalidomide. Four-year progression-free survival (PFS) was 26% in PL and 27% in non-PL patients. In patients that received maintenance, the 4-years PFS from maintenance start was 24% in PL and 30% in non-PL patients. When considering the PFS-2, i.e. the PFS calculated from the treatment start to the time of second relapse/progression, the median time was 42.3 months (95% CI, 36.3 - 51.5) in PL and 46.4 months (95% CI, 44.1 - 48.9) in non-PL patients. Conclusions Our meta-analysis studied the largest number of newly diagnosed MM patients with PL so far reported. We observed that in patients treated with new generation therapies incorporating new drugs, the detrimental effect of PL at diagnosis is limited. The 4-years OS is modestly reduced for PL patients, while we observed that PSF and PFS-2 were similar in PL and non-PL patients, suggesting that effective treatment can overcome the unfavorable prognostic significance of PLs. We also observed that lenalidomide is effective in patients with MM and PL, as are bortezomib and carfilzomib. Disclosures Montefusco: Janssen: Other: Advisory Board; Amgen: Other: Advisory Board; Celgene: Other: Advisory Board. Gay:Roche: Other: Advisory Board; Seattle Genetics: Other: Advisory Board; BMS: Honoraria; Janssen: Honoraria; Celgene: Honoraria, Other: Advisory Board; Amgen: Honoraria; Takeda: Honoraria, Other: Advisory Board. De Paoli:Gilead: Other: Advisory Board; Celgene: Other: Advisory Board; Amgen: Other: Advisory Board; Janssen: Other: Advisory Board. Di Raimondo:Celgene: Honoraria; Takeda: Honoraria, Research Funding. Patriarca:Janssen: Other: advisory role; Celgene: Other: advisory role & travel, accommodations, expenses; MSD Italy: Other: advisory role; Medac: Other: travel, accommodations, expenses; Jazz: Other: travel, accommodations, expenses. Musto:Amgen: Honoraria; BMS: Honoraria; Takeda: Honoraria; Janssen: Honoraria; Celgene: Honoraria. Cavo:GlaxoSmithKline: Honoraria, Membership on an entity's Board of Directors or advisory committees; AbbVie: Honoraria, Membership on an entity's Board of Directors or advisory committees; Janssen: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau; Takeda: Honoraria, Membership on an entity's Board of Directors or advisory committees; Bristol-Myers Squibb: 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; 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, Research Funding, Speakers Bureau. Ballanti:BMS: Honoraria; Amgen: Honoraria; Janssen: Honoraria; Celgene: Honoraria. Hajek:Takeda: Consultancy, Honoraria, Research Funding; Bristol Myers Squibb: Consultancy, Honoraria; Amgen: Consultancy, Honoraria, Research Funding; Janssen: Consultancy, Honoraria, Research Funding; Celgene: Consultancy, Honoraria, Research Funding; Novartis: Research Funding. Offidani:Amgen: Honoraria, Other: Advisory Board; Takeda: Honoraria, Other: Advisory Board; Janssen: Honoraria, Other: Advisory Board; Celgene: Honoraria, Other: Advisory Board; Bristol-Myers Squibb: Honoraria, Other: Advisory Board. Corradini:Gilead: Honoraria, Other: Advisory Board & Lecturer; Amgen: Honoraria, Other: Advisory Board & Lecturer; Takeda: Honoraria, Other: Advisory Board & Lecturer; Celgene: Honoraria, Other: Advisory Board & Lecturer; Roche: Honoraria, Other: Advisory Board & Lecturer; Sanofi: Honoraria, Other: Advisory Board & Lecturer; Sandoz: Other: Advisory Board; Novartis: Honoraria, Other: Advisory Board & Lecturer; Abbvie: Honoraria, Other: Advisory Board & Lecturer; Janssen: Honoraria, Other: Lecturer. Boccadoro:Janssen: Honoraria, Research Funding; Novartis: Honoraria, Research Funding; AbbVie: Honoraria; Bristol-Myers Squibb: Honoraria, Research Funding; Mundipharma: Research Funding; Amgen: Honoraria, Research Funding; Sanofi: Honoraria, Research Funding; Celgene: Honoraria, Research Funding.

Introduction: Immunoglobulin light chain amyloidosis (AL) may occur in association with a recognizable plasma cell (PC) clone diagnostic for multiple myeloma (MM) both with CRAB symptoms (AL-CRAB) and without them but with more than 10% marrow PCs (AL-PCMM). Prognosis of AL-CRAB and AL-PCMM was significantly poorer than AL in a large retrospective study by the Mayo Clinic (Kourelis et al, JCO 2013). Treatment with autologous stem cell transplantation (ASCT) improved the 5-y overall survival (OS) rates of all the three entities by approximately 40%. Therefore it is debated if patients with AL should receive more MM-oriented treatment including ASCT and if patients with AL-PCMM or AL-CRAB should receive the same treatment as MM patients. Aims: to contribute to this debate we analysed the outcome of patients with amyloidosis treated at our center according to their underlying disease: AL, AL-PMCC and AL-CRAB. Furthermore, we analysed the outcome according to the use of ASCT as part of first line treatment strategy. Methods: Of 53 patients newly diagnosed between Jan 1999 to June 2016 (median age 62 years), 19 (36%) were affected by AL, whereas 23 (43%) presented with AL-PCMM and 11 (21%) with AL-CRAB. Of 53 patients 18 (34%) underwent first line ASCT (21% of AL patients, 35% of AL-PCMM and 55% of AL-CRAB patients), upfront (6) or following induction chemotherapy (12). The 35 patients (66%) who didn't receive ASCT were treated with CyBorD regimen (21), melphalan-dexamethasone (8), VMP (4) or lenalidomide-dexamethasone (2). Risk groups were identified as follows: cTnI >0.1ng/ml and/or ECOG PS ³3: high risk; age ²65 years with normal cTnI levels, ECOG PS 50ml/min: low risk. Intermediate risk patients were defined if not meeting criteria for high or low risk. Mayo Stage classification (Dispenzieri et al, JCO 2004) was also applied to stratify risk categories. MM diagnosis was made according to IMWG criteria. Haematological and organ response (OR), OS and event free survival (EFS: time to 2nd line therapy or death) were analysed. Results: the proportion of high risk patients was 37%, 44% and 18% in AL, AL-PCMM and AL-CRAB, respectively, whereas the proportion of Mayo stage III patients was 47%, 30% and 9% for each subgroup. Median dFLC was similar (150, 150 and 146, respectively), such as baseline NT-proBNP (1318, 1301 and 1396). No difference was observed in terms of haematologic and organ response between patients with AL only or with concomitant MM: overall response rate (ORR) and complete remission (CR) were 63% and 37% in AL only patients and 88% and 32% if concomitant MM was present, whereas organ response was 47% and 56%, respectively. Similar results were seen among high risk or Mayo Stage III patients between the two subgroups. Haematologic response was similar between patients receiving ASCT in first line treatment and patients who did not: ORR and CR rates were 89% and 44% in the ASCT group and 71% and 29% in the no ASCT group, respectively. However, OR was significantly higher in the ASCT group (83% vs 37%, p 0.0014). Toxicity was manageable in both groups. After a median follow up of 32 months no significant difference was seen overall between AL only and AL with concomitant MM in terms of EFS (65% vs 85% at 2 years, respectively; HR 1,46, 95% CI 0,64-3,35) and OS (71% vs 87% at 2 years, respectively; HR 2,28, 95% CI 0,75-6,95). ASCT significantly improved both EFS and OS, comparing patients receiving or not transplantation (EFS 87% vs 64% at 2 years, p 0,013; HR 0,41, 95% CI 0,21-0,83; OS 100% vs 71% at 2 years, p 0,005; HR 0,23, 95% CI 0,08-0,64, see Figure 1). Of note, in patients achieving OR, similar OS was observed regardless of having received ASCT or not. Our data confirm the survival advantage with ASCT reported by Kourelis et al. The better outcome observed in AL with concomitant MM may be due to the more frequent use of a transplant strategy in first line treatment for AL-MM at our institution. Conclusion: We confirm that selected patients with AL AL-PCMM and AL-CRAB may have a survival advantage when receiving ASCT. These results may be related to the higher OR rates obtained with ASCT and to the improvement in OS in patients achieving OR. As no difference in terms of treatment response were observed between different underlying diseases, our study also supports the use of conventional MM treatment therapies incorporating high doses of melphalan followed by ASCT also for patients with AL Figure 1 Figure 1. Disclosures No relevant conflicts of interest to declare.

Introduction and aim of the study: monoclonal gammopathy of undetermined significance (MGUS) and smoldering multiple myeloma (SMM) have a distinct risk of evolving towards symptomatic multiple myeloma (MM) but it is widely accepted that they should not be treated until clinical evolution. The clinical presentation of symptomatic MM includes severe complications like acute renal failure, spinal cord compression by vertebral collapse, pathologic bone fractures which cannot be completely avoided even by strict follow-up of MGUS/SMM patients. The recent availability of effective and less toxic therapies has fueled renewed interest in the identification of prognostic models able to predict the risk of impending progression of MGUS/SMM patients to symptomatic MM and criteria for MM defining events have been recently broadened to allow an earlier start of treatment even in asymptomatic patients (Rajkumar SV et al, Lancet Oncol 2014). However, from a clinical perspective it would be more important to prevent the development of severe MM related complications by treating those MGUS/SMM considered at high risk of developing them. To this end, we have analyzed known risk factors for MGUS/SMM progression and other disease biomarkers in order to identify those able to consistently predict an high risk of "severe MM" development. Methods: we retrospectively analyzed 67 patients with symptomatic MM with a known previous history of MGUS/SMM, referred to our division from 1985 to December 2014 (23 males, 44 females; median age at MGUS/MM diagnosis 60 years). Focusing on the first assessment of their asymptomatic disease, we compared patients who subsequently developed severe CRAB criteria (Bladé J et al, Hematol Oncol Clin North Am. 2007), defined as having at least one of the following: hypercalcemia > 11.5mg/dL, creatinine > 4mg/dL, Hb < 8 mg/dL, severe bone lesions characterized by pathological fractures or causing spinal nerve compression, and called "severe MM" with patients who lacked severe CRAB symptoms at MM diagnosis, in order to identify risk factors for "severe MM" during its asymptomatic phase. Variables analyzed for comparison were: involved/uninvolved free light chains (FLC) ratio, monoclonal component (MC) quantification, presence of evolving pattern of MC (defined as an increase in the level of MC of at least 10% in 6 months), presence of immunoparesis, LDH levels, 24h-urinary protein levels, Ig isotype and bone marrow plasma cells percentage. Frequency of follow up was also considered (every 6 months or more). Time to treatment start (TTTS) from MGUS/SMM diagnosis, progression free survival (PFS) from first line treatment and overall survival (OS) were also calculated. Results: median 24-hour urine protein levels were significantly higher in MGUS/SMM patients who subsequently developed "severe MM", compared with patients without severe CRAB symptoms at MM diagnosis (140 vs 90mg/24h, p 0.022, see Figure 1), with an increased risk of severe myeloma related complications for MGUS/SMM patients with more than 200mg/24h urine proteins (Odds Ratio 2.65, p 0.037). Risk was highest in 4 MGUS/SMM patients with 24h-urinary protein levels higher than 1500mg (Odds Ratio 8.43, p 0.030) who invariably developed "severe MM". No difference were found comparing other variables. Of note, there was no difference between the two subgroups in terms of median TTTS (5.3 vs 8.3 years for MGUS patients and 1.7 vs 1.6 years for SMM, respectively), PFS ( 24.4 vs 32.2 months, respectively) and OS (nr vs 4.36), although a higher early mortality was observed in patients with "severe MM" (2 y OS 69% vs 93%). Conclusions: increased 24h-urinary protein levels (> 200mg) in MGUS/SMM patients are associated with a higher risk of the occurrence of severe CRAB symptoms at MM progression. Further data with larger series of patients are needed to better define the best predictive threshold for "severe MM". However, patients with more than 1500mg/24h urinary protein loss represent a small subgroup of MGUS/SMM patients who should better start treatment while still asymptomatic. Figure 1. 24h-urinary protein (mg) levels in the asymptomatic phase Figure 1. 24h-urinary protein (mg) levels in the asymptomatic phase Disclosures No relevant conflicts of interest to declare.

Abstract 4676 Introduction Myeloproliferative diseases (MPDs) are a group of haematological disorders characterized by the hyper proliferation of different blood cells in peripheral blood and other hematopoietic organs. The clinical heterogenity of these neoplasms reflects the different gene pathways involved, most of which are only partially known. Given the genetic homology and the physiological similarity to mammals, zebrafish has emerged as an ideal model to study human normal and malignant haematopoiesis. In the last decade several oncogenes involved in the development of hematopoietic neoplasms have been used to model leukemia in zebrafish with the aim to discovery new molecular pathways involved in malignant transformation. Despite the first encouraging results these experimental models failed to fully recapitulate human myeloproliferative disorders. Methods We took advantage of the Gal4/UAS binary system to induce the expression of human oncogenic HRASV12G in the zebrafish hematopoietic compartment. We used a specific transgenic line that drives oncogene expression in zebrafish early hematopoietic progenitors under control of the FLI.1 (Friend Leukemia virus Integration 1) promoter. Results We observed the development of a myelo-erythroid proliferative disease in few days in zebrafish transgenic larva. The pathological phenotype is characterized by the expansion of the hematopoietic tissue, an increased expression of myelo-erythroid specific genes (PU.1, gata1, mpx, c-mpl) associated with a slight increase of staminality markers (lmo2, scl, c-myb, runx.1), and a higher number of l-plastin expressing cells. Moreover blood smear of pathological larva displayed leukemic blasts and the arrest of erythrocyte differentiation whereas kidney marrow of juvenile fish displayed abnormal myelopoiesis characterized by the increase of erythro-myeloid progenitors. We found that the pathological phenotype is associated with a down regulation of the Notch pathway as shown by the decreased gene expression of notch pathways target genes (notch1, notch3, her6). Furthermore we discovered a novel set of genes involved in neoplastic transformation induced by HRASV12 expression through RNA-Seq analysis of pathological larva. Conclusions The expansion of the zebrafish hematopoietic compartment characterized by the hyper-proliferation of the myelo-erythroid progenitors that we found in this model reproduces some of the pathological features of human myeloproliferative disorders. This study showed that forcing oncogene expression in the hemogenic endothelial cells induces the transdifferentiation of the early hemogenic pluripotent stem cells into abnormal myeloerythoid progenitors by repressing the Notch pathways. Transcriptome analysis identified a number of potential effectors of this transformation. Disclosures: No relevant conflicts of interest to declare.

Abstract 4711 Background: Myeloproliferative diseases (MPDs) are a group of hematological malignancies characterized by the abnormal increase of different blood cells in peripheral blood and in other hematological organs. The molecular pathways involved in disease pathogenesis and progression as well as in drug sensitivity and resistance are not fully elucidated yet. Given the physiological similarity to mammals, the zebrafish (ZF) could accelerate the study of the molecular basis of these diseases. In the last decade several human leukaemia associated oncogenes have been transiently over-expressed in ZF embryos in order to perturb the myelo-erythroid compartment, with the aim to create experimental models to discover new molecular pathways that can became potential targets for new chemical compounds. Methodogy: We took advantage of the binary Gal4/UAS system to express oncogenic human HRAS in the ZF hematopoietic compartment. We used the tg(MAZe) driver line, a specific transgenic line that allows random mosaic expression of Gal4-VP16 after heat shock(hs) treatment(1). This driver line was mated with a tg(UAS:eGFP-HRASV12)(2) responder line in order to mimic somatic events leading to human oncogene expression. Results: Surprisingly a single heat shock at 30hpf, (referred here as HRASV12hs) induces human HRASV12 expression in hematopoietic progenitors as judged by the expansion of the hematopoietic tissue. Histological analysis showed an increased number of monocytes/macrophages. Blood smear of HRASV12hs larvae showed the expansion of blasts and myeloid precursors. Quantitative analysis of gene expression highlights a remarkable increase of myelo-erythroid restricted genes associated with a slight increase of staminality markers (pu1, mpx, lmo2, mpl, CD41 and c-myb). Moreover fish raised to adulthood developed hyper-plastic kidney marrow, the site of definitive haematopoiesis, the equivalent of mammalian bone marrow. Conclusions and Future Perspectives: The combination of hyper-proliferation of blood cells associated with the expansion of the hematopoietic compartment that we found in this model reproduces the pathological features of human myeloproliferative disorders. This study shows that it is possible to drive the expression of leukemia associated oncogenes to the hematopoietic compartment in developing ZF thus reproducing some of the features of human blood malignancies in an in vivo model. Our Group is now testing different driver lines in order to induce oncogene expression in earliest hematopoietic progenitors and is focusing on different responder lines expressing human oncogenes involved in hematopoietic malignancies. Disclosures: No relevant conflicts of interest to declare.