Abstract: Blastic plasmacytoid dendritic cell neoplasm is a rare and aggressive neoplasm for which there is still no current consensus on the best therapeutic approach. Most patients respond to intensive chemotherapy, but relapses are almost inevitable with median overall survival (OS) in the largest patient series ranging from 8 to 12 months except for patients who could benefit from allogenic hematopoietic stem cell transplantation (allo-HSCT). We present results of the first line treatments used in France between 2000 and 2013 for 86 patients recruited in the French network of BPDCN (abstract ASH 2015 N°78460). Seventeen patients were treated with acute lymphoid leukemia (ALL)-like therapy (median age : 63 yo) , 19 with acute myeloid leukemia (AML)-like therapy (median age : 40 yo), 16 patients with CHOP-like therapy (median age : 72 yo), 16 patients with NK/T-like therapy (based on high-dose methotrexate and L-asparaginase, ± dexamethasone, median age: 59 yo), and 12 patients received "other treatments" (OT, means variable drugs, median age : 82 yo). Thirty four patients obtained a complete remission (CR) and received HSCT (autologous n=4, or allogeneic n=30). The response rates for CHOP-like and OT groups were 31.3% and 25.0% respectively. For ALL-like, AML-like, and NK/T-like groups, response rates reached 70.6%, 78.9%, and 62.5% respectively (no statistic difference). Relapse rates among responders for CHOP-like and OT groups were 60% and 33.3% whereas there were only 25%, 26.7%, and 20% in ALL-like, AML-like, and NK/T-like groups respectively. For patients who obtained remission, the median of remission duration was 8.0 and 14.0 months for patients who received CHOP-like treatments (n=5) and OT (n=3) respectively and 10.0, 10.0, and 9.0 months for ALL-like (n=11), AML-like (n=14), and NK/T-like groups (n=9) respectively (p = 0.6339). In preclinical studies, we have shown that BPDCN cells are sensitive in vitro to idarubicine (Angelot Delettre F et al, 2015) so we studied patients receiving idarubicine in first line therapy in our series (n=9). From these 9 patients, 7 obtained CR and only one relapsed after 10 months. The 6 patients in continuous CR without any relapse have received HSCT (allo, n=5 or auto, n=1). Two out of those 6 patients are alive at the time of data collection with a follow-up of 40 and 87 months; the other 4 patients died after the graft, one relapsed after auto-HSCT, and 3 died of infectious complications after allo-HSCT. The median OS for patients who received HSCT, auto or allo (n=34) and other patients (n = 52) is respectively 49 and 8 months (p 〈 0.0001, Figure 1). The beneficial effect of HSCT persists independently of age in multivariate analysis. These results suggest that NK/T-like, AML-like, and ALL-like groups give better results than CHOP-like and OT groups. However, there is no significant statistical difference between AML-like, ALL-like, and NK/T-like groups. Thus it seems to be wise to combine "lymphoid" drugs like methotrexate, L-asparaginase and dexamethasone with "myeloid" drug such as idarubicine. The importance of allogenic stem cell transplantation to sustain remission is clear in this study and other one (Roos-Weil et al, 2013). We also observed a prolonged CR in one patient after auto-HSCT. Based on our results, we will propose the first prospective, multicentric, phase II trial in BPDCN, testing a combination of 3 cycles of methotrexate, L-asparaginase, idarubicine and dexamethasone followed by an allo-HSCT in first clinical remission for all eligible patients or repeated cycle of these drugs for unfit patients with auto-HSCT if possible. Kaplan-Meier overall survival curves compared by the Log-Rank test in the cohort of 34 HSCT patients (auto and allo, blue line) and 52 non HSCT patients (red line) (p 〈 0.0001). Censured patients are patient's alive or lost (+). OS of HSCT patients is still statistically significative with adjustment of age in multivariate analysis (Cox multivariate). Figure 1. Overall survival of HSCT patients and non HSCT patients. Figure 1. Overall survival of HSCT patients and non HSCT patients. Disclosures Recher: Celgene; Amgen; Chugai: Research Funding; Janssen; Novartis; Amgen: Other: Travel, accommodations, expenses; Sunesis; Celgene: Consultancy. Deconinck:CHUGAI: Other: Travel for international congress; NOVARTIS: Other: Travel for international congress; ALEXION: Other: Travel for international congress; LFB loboratory: Consultancy; JANSSEN: Other: Travel for international congress; PFIZER: Research Funding; ROCHE: Research Funding.

Abstract: Blastic plasmacytoid dendritic cell neoplasm is a clonal disease derived from precursors of plasmacytoid dendritic cells (pDC). It is a rare neoplasm involving the skin which may or may not be associated from the outset with a leukemic component. The disease invariably progresses to aggressive leukemic dissemination, leading to a differential diagnosis with acute leukemia. In 2004, we set up a French network to recruit biological data at diagnosis. Diagnosis was according to recommendations (Swerdlow et al, 2008), with, in addition, a mandatory panel of pDC markers (Garnache-Ottou et al, 2009) detected by flow cytometry or by immunohistochemistry on infiltrated blood, bone marrow or cutaneous lesions. In total, 109 cases of BPDCN were included in 35 hospitals (2000-2013). BPDCN is more prevalent in men (sex ratio 4.4/1) and in elderly subjects (median age: 63 years; 7 patients were 〈 20 yo).S kin lesions are very prevalent (85%) with variable lesion types. Blood cell counts show variable leukocytosis (figure 1A) with presence of blasts in 65% of cases. Anemia and thrombopenia were present in 59% and 76% of cases respectively. Bone marrow aspiration showed blastic infiltration in 94% of cases. Indeed, in 7 cases, there was isolated cutaneous involvement at diagnosis, with neither blood nor bone marrow infiltration. Morphologies of blast cells were heterogeneous. Typical morphologies were the most frequent, including medium-sized cells with a blastic round or irregular nucleus, cytoplasm displayed faint and irregular basophilia and no granulation. In a contingent of the blastic populations, we observed small vacuoles in a peculiar arrangement under the cytoplasmic membrane (42%) or the presence of large pseudopodia (28%) or both (17%) (Figure 1B, C, D). Some cases showed a more immature morphology with larger cells, higher nucleo-cytoplasmic ratio, very visible nucleoli and reinforced basophilia (28%) or a pseudomonoblastic morphology (5%) (Figure 1E). Rare cases presented a pseudolymphocytic form (7%, figure 1F) or large granulations in the cytoplasm (2%). Peroxidase and esterase were negative in all cases. Dysplasia of hematopoietic lineages was observed in 29% (figure 1G). For 8% of patients, myelodysplastic syndrome was diagnosed before the diagnosis of BPDCN. Immunophenotype showed that HLA-DR and CD4 were expressed in all cases, but 4 cases did not express CD56 (confirmed using 3 different antibodies). Expression of markers of others hematopoietic lineages was frequent. Among myeloid markers, the most frequent was CD33 (46%), followed by CD117 (23%), whereas CD13, CD11c, CD15 and CD65 were rarely expressed. Monocyte markers (CD14, CD64, CD11b) and myeloperoxidase were never expressed. For the T lineage, CD2 and CD7 were the most frequent (62% and 58% respectively) whereas CD5 was rare (7%). No cytoplasmic or surface CD3 were detected. For the B lineage, CD22 was expressed in 16%, and low levels of cCD79a in 5%. Both were never expressed together, and no CD19, CD20 and immunoglobulins were found. Generally, we observed one of these antigens (Ags) per case, but in 44% of cases, there was a combination of 2 or 3 Ags from 2 or 3 different lineages. Immature Ags such as CD34 and CD133 were never found, and Tdt was found in 14% of cases. Cytogenetic analysis revealed abnormal caryotype in 65% of the 78 caryotypes evaluated, with 20 cases having a complex caryotype. The frequency of the chromosomal abnormalities involved are shown in Figure 1H. In conclusion, we describe the largest series of BPDCN to date in the literature. Detailed clinical and biological data at presentation allow improved recognition of this rare form of acute and aggressive leukemia, enabling early initiation of appropriate management. Figure 1. A: Blood cell count in 109 BPDCN patients at diagnosis. Bars represent the median. B: Typical BPDCN morphology. C: In this case, the nuclei were peripheral, cytoplasm presented heterogenous basophilia, vacuoles were rare but large pseudopodia are frequent. D: Typical morphology with frequent microvacuoles under the cytoplasmic membrane. E: Immature morphology. F: Pseudolymphocytic morphology. G: Presence of dysplasia in myeloid cells with Auer Rods in the granulocytes. The morphology of the Blastic cells is typical. H. Chromosomal abnormalities in 78 caryotypes evaluated: The histogram represents the number of cases in which each chromosome was involved (deletion, gain, translocations). Figure 1. A: Blood cell count in 109 BPDCN patients at diagnosis. Bars represent the median. B: Typical BPDCN morphology. C: In this case, the nuclei were peripheral, cytoplasm presented heterogenous basophilia, vacuoles were rare but large pseudopodia are frequent. D: Typical morphology with frequent microvacuoles under the cytoplasmic membrane. E: Immature morphology. F: Pseudolymphocytic morphology. G: Presence of dysplasia in myeloid cells with Auer Rods in the granulocytes. The morphology of the Blastic cells is typical. H. Chromosomal abnormalities in 78 caryotypes evaluated: The histogram represents the number of cases in which each chromosome was involved (deletion, gain, translocations). Disclosures Bardet: Celgene: Research Funding. Deconinck:CHUGAI: Other: Travel for international congress; PFIZER: Research Funding; ROCHE: Research Funding; NOVARTIS: Other: Travel for international congress; ALEXION: Other: Travel for international congress; JANSSEN: Other: Travel for international congress; LFB loboratory: Consultancy.