Abstract: Introduction: Selinexor is a first-in-class Selective Inhibitor of Nuclear Export (SINE) compound that selectively binds and inactivates XPO1, therefore forcing the nuclear retention and re-activation of cell cycle regulators such as p53, FOXO, IkB, and Rb. Selinexor in combination with low dose dexamethasone (Sel-dex) was recently approved based on data from the STORM study, which induced an overall response rate (ORR) of 26.2% in patients with penta-exposed, triple-class refractory multiple myeloma (TCR-MM). Patients with TCR-MM often present with plasmacytomas along with serological markers of MM. Methods: Here we analyzed the effects of Sel-dex in patients from the STORM study who had baseline plasmacytomas. Results: 122 patients were in enrolled in the STORM study including 27 with a baseline plasmacytoma. The majority of plasmacytomas were soft tissue (22 patients) and 5 patients had soft tissue disease extension from a bone (rib (2), iliac (2), sacral vertebral). The median age of patients with plasmacytomas was 64 years, the median prior therapies were 7 (range 4 - 15), and 8/27 patients with a plasmacytoma had high risk cytogenetics. Of the 27 patients, 11 patients did not have a follow up plasmacytoma assessment: 6 were not evaluable for response as they came off therapy due to clinical progression and/or adverse events, 4 had stable disease (SD) with no evidence of plasmacytoma change, and 1 had progressive disease (PD) on serum M-protein with no evidence of plasmacytoma change. Sixteen of the 27 patients did have follow-up plasmacytoma assessments (methods of measurements included PET, CT, MRI or Clinical). The median days from baseline plasmacytoma evaluation to follow up was 41 days (range 22 - 119). Five patients had objective responses, based upon para-protein and plasmacytoma reductions according to IMWG criteria (1 very good partial response [VGPR], 4 partial responses [PR] ) for an ORR of 18.5%. In addition, 2 patients had a minimal response (MR), 4 had SD and 5 had objective PD. Among the 5 patients with ≥PR, 3 plasmacytomas completely resolved, 1 showed near complete resolution, and another showed size reduction with no metabolic activity on PET. Of the 2 patients with a MR, 1 plasmacytoma completely resolved and 1 showed reduced PET uptake. Among the 4 patients with SD, 1 plasmacytomas completely resolved, 1 increased in size and 2 had unknown outcomes as they were assessed clinically. Among the 5 patients with PD, 1 plasmacytomas decreased in size, 1 increased in size, and 3 had unknown outcomes as they were assessed clinically. Conclusions: Of the 16 patients with TCR-MM and a follow up plasmacytoma assessment enrolled on STORM, 9/16 of the plasmacytomas either completely resolved or decreased in size and/or metabolic activity. Effects on plasmacytomas occurred in patients with objective responses (≥PR), as well as in patients with MR, SD and PD. These observations support the finding that Sel-dex is active in patients with plasmacytomas and heavily pretreated TCR-MM. Disclosures Yee: Amgen: Consultancy, Honoraria; Celgene: Consultancy, Honoraria, Research Funding; Takeda: Consultancy; Karyopharm: Consultancy; Adaptive: Consultancy; Bristol-Myers Squibb: Consultancy, Research Funding. Huff:Member of Safety Monitoring Board for Johnson and Johnson: Membership on an entity's Board of Directors or advisory committees; Karyopharm, Sanofi, MiDiagnostics: Consultancy. Chari:Seattle Genetics: Membership on an entity's Board of Directors or advisory committees, Research Funding; Sanofi: Membership on an entity's Board of Directors or advisory committees; Pharmacyclics: Research Funding; Oncoceutics: Research Funding; Amgen: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Bristol-Myers Squibb: Consultancy; Celgene: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Millennium/Takeda: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Janssen: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Karyopharm: Consultancy, Membership on an entity's Board of Directors or advisory committees; Array Biopharma: Research Funding; GlaxoSmithKline: Research Funding; Novartis Pharmaceuticals: Research Funding. Vogl:Active Biotech: Consultancy; Janssen: Consultancy; Karyopharm Therapeutics: Consultancy; Takeda: Consultancy; Celgene: Consultancy; Amgen: Consultancy. Gavriatopoulou:Genesis: Honoraria, Other: Travel expenses; Amgen: Honoraria; Janssen: Honoraria, Other: Travel expenses; Takeda: Honoraria, Other: Travel expenses. Nooka:Adaptive technologies: Honoraria, Other: advisory board participation; Spectrum pharmaceuticals: Honoraria, Other: advisory board participation; GSK: Honoraria, Other: advisory board participation; Janssen: Honoraria, Other: advisory board participation; Amgen: Honoraria, Other: advisory board participation; Takeda: Honoraria, Other: advisory board participation; Celgene: Honoraria, Other: advisory board participation; BMS: Honoraria, Other: advisory board participation. Moreau:AbbVie: Consultancy, Honoraria; Celgene: Consultancy, Honoraria; Janssen: Consultancy, Honoraria; Amgen: Consultancy, Honoraria; Takeda: Consultancy, Honoraria. Dingli:Karyopharm: Research Funding; Rigel: Consultancy; Millenium: Consultancy; Janssen: Consultancy; alexion: Consultancy. Lonial:Janssen: Consultancy, Research Funding; GSK: Consultancy; Karyopharm: Consultancy; BMS: Consultancy; Celgene Corporation: Consultancy, Research Funding; Takeda: Consultancy, Research Funding; Amgen: Consultancy; Genentech: Consultancy. Dimopoulos:Sanofi Oncology: Research Funding. Vij:Takeda: Honoraria, Research Funding; Sanofi: Honoraria; Karyopharm: Honoraria; Janssen: Honoraria; Genentech: Honoraria; Celgene: Honoraria, Research Funding; Bristol-Myers Squibb: Honoraria, Research Funding. Tuchman:Prothena: Research Funding; Amgen: Research Funding; Karyopharm: Honoraria; Alnylam: Honoraria, Research Funding; Sanofi: Research Funding; Merck: Research Funding; Roche: Research Funding; Celgene: Honoraria, Research Funding, Speakers Bureau. Hoffman:Celgene: Speakers Bureau. Costa:Abbvie: Consultancy; Celgene: Consultancy, Honoraria, Research Funding; Sanofi: Consultancy, Honoraria, Speakers Bureau; GSK: Consultancy, Honoraria, Research Funding; Karyopharm: Consultancy; Amgen: Consultancy, Honoraria, Research Funding, Speakers Bureau; Janssen: Research Funding, Speakers Bureau; Fujimoto Pharmaceutical Corporation Japan: Other: Advisor. Biran:Janssen: Consultancy, Honoraria, Research Funding; Merck: Research Funding; Amgen: Consultancy, Honoraria, Research Funding; Celgene: Consultancy, Honoraria; Bristol Meyers Squibb: Research Funding; Takeda: Consultancy, Honoraria. Siegel:Takeda: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau; Amgen: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau; Celgene: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau; Bristol-Myers Squibb Company: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau; Janssen: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau. Shah:Karyopharm Therapeutics: Employment, Equity Ownership. Picklesimer:Karyopharm Therapeutics: Employment, Equity Ownership. Saint-Martin:Karyopharm Therapeutics: Employment, Equity Ownership. Li:Karyopharm Therapeutics: Employment, Equity Ownership. Kauffman:Karyopharm Therapeutics Inc: Employment, Equity Ownership, Membership on an entity's Board of Directors or advisory committees. Shacham:Karyopharm Therapeutics Inc: Employment, Equity Ownership, Membership on an entity's Board of Directors or advisory committees, Patents & Royalties. Richardson:Takeda: Membership on an entity's Board of Directors or advisory committees, Research Funding; Celgene: Membership on an entity's Board of Directors or advisory committees, Research Funding; Oncopeptides: Membership on an entity's Board of Directors or advisory committees, Research Funding; Bristol-Myers Squibb: Research Funding; Karyopharm: Membership on an entity's Board of Directors or advisory committees; Sanofi: 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. Jagannath:Multiple Myeloma Research Foundation: Speakers Bureau; Medicom: Speakers Bureau; Merck: Consultancy; Celgene: Consultancy; Novartis: Consultancy; BMS: Consultancy.

Abstract: Introduction: Selinexor is a novel, oral selective inhibitor of nuclear export (SINE) which forces nuclear retention and activation of tumor suppressor proteins. Selinexor plus low dose dexamethasone (Sel-dex) was recently approved in the United States based on data from the STORM study wherein, Sel-dex induced an overall response rate (ORR) of 26.2% in patients with penta-exposed, triple-class refractory multiple myeloma. The presence of high-risk cytogenetic abnormalities is known to have a poor prognosis in multiple myeloma, with transient responses. We performed post-hoc analyses to determine the outcomes in patients with relapsed/refractory myeloma treated with Sel-dex based on the baseline cytogenetic abnormalities. Methods: STORM was a phase 2b, open-label study which enrolled patients with relapsed and refractory myeloma. Selinexor 80 mg in combination with dexamethasone 20 mg was administered orally, twice weekly. The primary endpoint was ORR. For this analysis, we pooled 78 patients (48 quad-refractory and 30 penta-refractory) from STORM, part 1 and 122 patients (penta-exposed and triple class refractory) from STORM, part 2 to compare outcomes between high-risk and standard risk cytogenetics groups. High-risk cytogenetics was defined as having at least 1 of the following abnormalities by fluorescence in-situ hybridization (FISH) at baseline: del(17p), t(4;14), t(14;16), and gain(1q) in 〉 5% of screened plasma cells. The FISH analyses were performed at a central laboratory and used to assess cytogenetic risk status. Results: Of the 200 patients, 122 (61%) had high-risk disease (del(17p): 36%, t(4;14): 18%, t(14;16): 5%, and gain(1q): 36%). The ORR in high-risk patients was 20.5% (very good partial response [VGPR)]: 5.7% and partial response [PR] : 14.8%) and the ORR was 29.5 % (complete response [CR]: 2.6%, VGPR: 3.8%, and PR: 23.1%) in standard-risk patients. The clinical benefit rate (CBR) was 35.2% in high-risk patients compared with 38.5% in standard-risk patients. Median duration of clinical benefit (DOCB) was 4.4 and 6.2 months in the high-risk and standard-risk patients respectively. Median progression-free survival (PFS) was 3.8 and 4.2 months and overall survival (OS) was 8.6 and 9.4 months in the high-risk and standard-risk patients, respectively. Efficacy by specific cytogenetic abnormality is presented in Table 1 below (Due to the small sample size (n=11), data for the t(14;16) subgroup are not presented separately). Conclusions: Sel-dex demonstrated a similar CBR in patients with high risk and standard risk disease and preserved clinical benefit in heavily pre-treated patients who had rapidly proliferative disease and high-risk cytogenetics at baseline. The benefit was maintained across cytogenetic risk subgroups with a higher ORR in the t(4;14) and gain(1q) subgroups. The DOR and OS was similar across all the subgroups. These analyses support the use of Sel-dex in patients with high-risk cytogenetics and warrant further evaluation of selinexor in combination with other anti-myeloma therapies in high-risk disease. Disclosures Nooka: GSK: Honoraria, Other: advisory board participation; Amgen: Honoraria, Other: advisory board participation; BMS: Honoraria, Other: advisory board participation; Janssen: Honoraria, Other: advisory board participation; Spectrum pharmaceuticals: Honoraria, Other: advisory board participation; Adaptive technologies: Honoraria, Other: advisory board participation; Celgene: Honoraria, Other: advisory board participation; Takeda: Honoraria, Other: advisory board participation. Yee:Adaptive: Consultancy; Karyopharm: Consultancy; Takeda: Consultancy; Bristol-Myers Squibb: Consultancy, Research Funding; Amgen: Consultancy, Honoraria; Celgene: Consultancy, Honoraria, Research Funding. Huff:Karyopharm, Sanofi, MiDiagnostics: Consultancy; Member of Safety Monitoring Board for Johnson and Johnson: Membership on an entity's Board of Directors or advisory committees. Vogl:Karyopharm Therapeutics: Consultancy; Takeda: Consultancy; Celgene: Consultancy; Amgen: Consultancy; Janssen: Consultancy; Active Biotech: Consultancy. Chari:Novartis Pharmaceuticals: Research Funding; GlaxoSmithKline: Research Funding; Array Biopharma: Research Funding; Karyopharm: Consultancy, Membership on an entity's Board of Directors or advisory committees; Janssen: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Millennium/Takeda: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Celgene: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Bristol-Myers Squibb: Consultancy; Amgen: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Sanofi: Membership on an entity's Board of Directors or advisory committees; Seattle Genetics: Membership on an entity's Board of Directors or advisory committees, Research Funding; Pharmacyclics: Research Funding; Oncoceutics: Research Funding. Moreau:Celgene: Consultancy, Honoraria; Janssen: Consultancy, Honoraria; Amgen: Consultancy, Honoraria; Takeda: Consultancy, Honoraria; AbbVie: Consultancy, Honoraria. Dingli:Karyopharm: Research Funding; Rigel: Consultancy; Millenium: Consultancy; alexion: Consultancy; Janssen: Consultancy. Lonial:Genentech: Consultancy; Karyopharm: Consultancy; Takeda: Consultancy, Research Funding; Amgen: Consultancy; BMS: Consultancy; Janssen: Consultancy, Research Funding; GSK: Consultancy; Celgene Corporation: Consultancy, Research Funding. Dimopoulos:Sanofi Oncology: Research Funding. Vij:Takeda: Honoraria, Research Funding; Sanofi: Honoraria; Karyopharm: Honoraria; Janssen: Honoraria; Genentech: Honoraria; Celgene: Honoraria, Research Funding; Bristol-Myers Squibb: Honoraria, Research Funding. Tuchman:Karyopharm: Honoraria; Celgene: Honoraria, Research Funding, Speakers Bureau; Amgen: Research Funding; Merck: Research Funding; Prothena: Research Funding; Roche: Research Funding; Sanofi: Research Funding; Alnylam: Honoraria, Research Funding. Biran:Bristol Meyers Squibb: Research Funding; Amgen: Consultancy, Honoraria, Research Funding; Merck: Research Funding; Janssen: Consultancy, Honoraria, Research Funding; Celgene: Consultancy, Honoraria; Takeda: Consultancy, Honoraria. Siegel:Amgen: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau; Celgene: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau; Bristol-Myers Squibb Company: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau; Janssen: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau; Takeda: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau. Richardson:Karyopharm: Membership on an entity's Board of Directors or advisory committees; Janssen: Membership on an entity's Board of Directors or advisory committees; Sanofi: Membership on an entity's Board of Directors or advisory committees; Bristol-Myers Squibb: Research Funding; Oncopeptides: Membership on an entity's Board of Directors or advisory committees, Research Funding; Amgen: Membership on an entity's Board of Directors or advisory committees; Celgene: Membership on an entity's Board of Directors or advisory committees, Research Funding; Takeda: Membership on an entity's Board of Directors or advisory committees, Research Funding. Liu:Karyopharm Therapeutics: Employment, Equity Ownership. Joshi:Karyopharm Therapeutics: Employment, Equity Ownership. Shah:Karyopharm Therapeutics: Employment, Equity Ownership. Shacham:Karyopharm Therapeutics Inc: Employment, Equity Ownership, Membership on an entity's Board of Directors or advisory committees, Patents & Royalties. Kauffman:Karyopharm Therapeutics Inc: Employment, Equity Ownership, Membership on an entity's Board of Directors or advisory committees. Jagannath:Medicom: Speakers Bureau; Multiple Myeloma Research Foundation: Speakers Bureau; BMS: Consultancy; Celgene: Consultancy; Novartis: Consultancy; Merck: Consultancy.

Abstract: Abstract 837 Background: Combination therapy with cytotoxic chemotherapy and tyrosine kinase inhibitors has improved the outcome for patients with Ph+ ALL with durable remissions in some patients even without an allogeneic stem cell transplant. The dual Src and Abl inhibitor dasatinib has ∼325 times more potent in vitro kinase inhibition than imatinib against BCR-ABL with significant clinical activity in patients with imatinib-resistant lymphoid blast phase CML (CML-LB) and Ph+ ALL. Aim: To determine the efficacy and safety of combining chemotherapy with dasatinib for treating patients with Ph+ ALL. Methods: In this phase II trial, patients with newly diagnosed Ph+ ALL receive dasatinib 50 mg po bid (or 100 mg daily) for the first 14 days of each of 8 cycles of alternating hyperCVAD and high dose cytarabine and methotrexate. Patients in CR continue to receive maintenance dasatinib 50 mg po bid (or 100 mg daily) and vincristine and prednisone monthly for 2 years followed by dasatinib indefinitely. Results: We have enrolled in the study 34 patients with untreated Ph+ ALL and 7 patients with 1 prior cycle of chemotherapy (before Ph+/BCR-ABL+ status was known). Patients younger than 50 years old have received a median of 6 cycles (range 2-8) and patients 50 years and older have received a median of 6 cycles (range 1-8). 20 patients are receiving maintenance in CR and two have completed the entire treatment regimen. Median age is 51 years (range 21 – 79); 22 patients were older than 50 years, Median WBC at diagnosis was 13.6 × 109/L (range, 1-276 × 109/L). 12 patients had CNS involvement at presentation. All patients are evaluable for assessment of response to induction; 39 (95%) achieved CR after first cycle or were CR at start. Two patients died before response assessment from infections. Thirty-one of 39 (79%) evaluable patients achieved cytogenetic (CG) CR after 1 cycle; 4 had a major CG response (3 had 5% and one had 15% Ph+), 2 had insufficient metaphases, and 2 are unknown (no CG exam on day 21 marrow). To date, 22 patients (56%) have achieved complete molecular remission (CMR) and another 8 (21%) have achieved a major (but not complete) molecular response (MMR) at a median of 14 weeks from initiation of treatment (range 2 – 59 weeks). Minimal residual disease assessment by flow cytometry is negative in 35 (90%) patients at a median of 3 weeks (range, 2-18 weeks). The median time to neutrophil and platelet recovery for cycle 1 is 18 and 23 days and for subsequent cycles is 15 and 20 days. Grade 3 and 4 adverse events have included bleeding (GI, GU, soft tissue and subdural hematomas)(18), pleural effusions (9), pericardial effusion (1), reversible rise in creatinine (10), deep vein thromboses (6), pulmonary emboli (3), as well as diarrhea, infections, hypophosphatemia, hypokalemia, hypocalcemia, hyperglycemia, and elevated transaminases. With a median follow up of 13 months (range 1-33), 29 patients (71%) are alive and 27 (66%) are in CR; 4 patients died in CR; 1 from an unrelated cardiac event and 3 from infections. Three patients have undergone an allogeneic stem cell transplant. The median disease free survival is 48+ weeks (range,1 to 140+) and the median overall survival is 52+ weeks (range, 3 to 143+). Eight patients have relapsed with a median response duration of 51 weeks (range 23-73) and 6 of them have died. In 5 patients morphological relapse was preceded by flow and molecular relapse. Five relapsed patients had ABL mutations (3 T315I, 1 F359V, and 1 V299L). Conclusion: Combination of chemotherapy with dasatinib is effective in achieving long term remissions in patients with newly diagnosed Ph+ ALL. Disclosures: Ravandi: Bristol Myers Squibb: Honoraria, Research Funding. Kantarjian:Bristol Myers Squibb: Research Funding. Wierda:Genzyme: Research Funding; Genentech: Consultancy, Honoraria. Cortes:Bristol Myers Squibb: Research Funding. O'Brien:Bristol Myers Squibb: Research Funding.

Abstract: CLL cells proliferate in secondary lymphatic tissues where the CLL cells interact with T cells, monocyte-derived nurselike cells and mesenchymal stromal cells, collectively referred to as the tissue microenvironment. The CLL lymph node morphology with proliferation centers resembles normal B cell follicles, suggesting that mechanisms regulating the expansion of antigen-selected normal B cells during the germinal center (GC) reaction also regulate CLL cell expansion. Normal GC reactions require specialized T follicular helper (Tfh) cells, which provide T cell help to antigen-stimulated B cells. Another important T cell subset are Tregs which regulate immune tolerance and homeostasis. NLC co-culture have been used to dissect cellular and molecular interactions between CLL cells and the lymph node microenvironment in vitro. To gain insight into the role of T cell subsets in CLL, we utilized the NLC model to characterize T cell subsets and their dynamics in NLC co-cultures. We quantified CD4 + and CD8 + T cells in co-cultures from 18 different patients, observing that CD4 + T cell counts remained stable over time (449±62 at baseline to 443±80 cells/µl after 14 days). In contrast, numbers of CD8 + T cells declined from 180±42 cells/µl to 98±16 cells/µl on day 14 (p & lt;0.05). T helper cell subset analyses in 28 different samples revealed an increase in Tfh cells (from 1.7±0.3% to 6.0±0.9%, p & lt;0.001) and Tregs (from 3.3±0.5% to 9.6±1.7%, p & lt;0.001) after 14 days. In contrast, Th17 cell frequency remained unchanged. PD-1 expression levels on Tfh cells were tested to distinguish memory (PD-1 +) from germinal (PD-1 +++) Tfh cells. NLC co-culture resulted in an increase of germinal (0.2±0.1% to 2.4±0.3%, p & lt;0.0001) and memory Tfh cells (10.2±1.5% to 18.8±1.7%, p & lt;0.0001). In addition, germinal Tfh cells expressed high levels of BCL6 (39.4±4.4 to 179.0±5.9, n=8, p & lt;0.001), ICOS (119.0±68.0 to 574.0±154.0, n=6, p & lt;0.05), and IL-21 after co-culture (10.3±2.6% to 29.5±6.5%, p & lt;0.01, n=12). In contrast, CD40L was downregulated on memory Tfh cells from 24.4±8.6% to 8.7±2.4% (p=0.05, n=9). Tregs showed an increase of TGFß levels (4.3±1.4% to 25.3±8.5%, p & lt;0.05), and IL-10 expression (2.2±0.4% to 7.4±2.0%, p & lt;0.05, n=12), indicating activation during co-culture. Interestingly, NLC co-cultures also induced Ki-67 in germinal (13.0±4.9% to 39.5±5.7%, p & lt;0.001) and memory Tfh cells (1.7±0.3% to 13.0±4.9%, p & lt;0.05), and Tregs (2.5±0.6% to 18.0±3.3%, p & lt;0.001, n=13). To examine whether the expansion of Tfh and Treg cells in NLC co-cultures correlated with changes in T cell receptor (TR) gene repertoire we performed TR beta chain immunosequencing. Using the beta-binomial method, we analyzed the abundance of TR gene rearrangements, identifying clonotypes that were differentially abundant at 14 days. Next, to assess the similarity/overlap between the CD4 + TR gene repertoires at baseline and after co-culture, we utilized the Morisita overlap index (MOI). We found that the 6 samples with increased clonality had MOIs ranging from 0.12 to 0.53. Then, we compared the top 20 most expanded CDR3 sequences from each sample with low MOI with CDR3 sequences obtained in previous TR immunoprofiling studies from untreated CLL patients. Interestingly, we found numerous shared TR beta CDR3 sequences between NLC co-cultures and unrelated CLL patient samples. Furthermore, we correlated the Tfh expansion at 14 days normalized to baseline with MOI, a significant indirect correlation was noted (R 2=0.5, p=0.02, n=10). Interactions between CLL cells and Tfh or Tregs were evaluated in six LN CLL samples using multispectral microscopy. High magnification imaging and 3D Z-stack reconstructions revealed that there was a close spatial relationship between clusters of Ki-67 +CLL and Tfh cells within CLL LNs. In contrast, FOXP3 +CD4 + cells were generally found in cell groups in close proximity to other CD4 + cells. We next assessed whether the ICOS +CD4 + cell expansion was correlated with CLL proliferation and found a correlation between the percentage of ICOS +CD4 + cells with proliferating CLL cells in situ (R 2=0.75, p=0.03, n=6), whereas, no correlation was found with FOXP3 +CD4 + cell frequency. Collectively, these data provide new insight into the cellular and molecular cross-talk between CLL and T cell subsets, resulting in clonal expansion of T helper cells and interaction of Tfh cells with proliferating CLL cells opening new avenues for therapeutic targeting. Disclosures Ferrajoli: Janssen: Other: Advisory Board ; AstraZeneca: Other: Advisory Board, Research Funding; BeiGene: Other: Advisory Board, Research Funding. Wierda: Oncternal Therapeutics, Inc.: Research Funding; Sunesis: Research Funding; Gilead Sciences: Research Funding; GSK/Novartis: Research Funding; Acerta Pharma Inc.: Research Funding; Genentech: Research Funding; Karyopharm: Research Funding; KITE Pharma: Research Funding; Juno Therapeutics: Research Funding; Pharmacyclics LLC, an AbbVie Company: Research Funding; Cyclacel: Research Funding; Miragen: Research Funding; Loxo Oncology, Inc.: Research Funding; Janssen: Research Funding; Xencor: Research Funding; AstraZeneca: Research Funding; Genzyme Corporation: Consultancy; AbbVie: Research Funding. Patten: ABBVIE: Honoraria; ASTRA ZENECA: Honoraria; GILEAD SCIENCES: Honoraria, Research Funding; JANSSEN: Honoraria; NOVARTIS: Honoraria; ROCHE: Research Funding. Burger: Novartis: Other: Travel/Accommodations/Expenses, Speakers Bureau; Beigene: Research Funding, Speakers Bureau; Pharmacyclics LLC: Consultancy, Other: Travel/Accommodations/Expenses, Research Funding, Speakers Bureau; Gilead: Consultancy, Other: Travel/Accommodations/Expenses, Research Funding, Speakers Bureau; TG Therapeutics: Other: Travel/Accommodations/Expenses, Research Funding, Speakers Bureau; AstraZeneca: Consultancy; Janssen: Consultancy, Other: Travel/Accommodations/Expenses, Speakers Bureau.

Abstract: Leukemia stem cells (LSCs) are believed to be a prominent source of relapse in acute myeloid leukemia (AML). Human AML LSCs are classically studied via xenotransplantation and defined as cells with (a) self-renewal ability, giving rise to leukemic engraftment that can be maintained over serial transplantation; and (b) ability to give rise to more differentiated progeny that are unable to engraft. LSCs are chemoresistant presumably because they have properties distinct from those of the bulk AML cells. A better understanding of the properties of LSCs can ultimately allow us to develop new therapies specifically targeting these cells to achieve lasting responses or even cures. We recently reported that induced pluripotent stem cells (iPSCs) generated from AML patients (AML-iPSCs) exhibit leukemic features upon hematopoietic differentiation, including extensive proliferation, maintenance of hematopoietic stem cell (HSC) markers, and serial engraftment of a lethal leukemia in immunodeficient mice (Kotini et al. Cell Stem Cell 2017). More recently, we found that the hematopoietic stem/progenitor cells (HSPCs) derived from AML-iPSCs exhibit phenotypic and functional heterogeneity. We first observed the presence of two cell populations with distinct growth characteristics and morphology: a population exhibiting adherent growth (adherent, A) and a population growing in suspension (S). The A cell population contained cells with immature morphology and HSC immunophenotype (CD34+/CD38-/CD90+/CD45RA-/ CD49f+), while the S fraction contained more differentiated cells (CD34-/low/CD38+/CD90-/CD45RA+). Experiments utilizing serial replating, single-cell plating of GFP-labeled cells and mathematical modeling revealed a hierarchical organization, whereby the A cells continuously give rise to the S cell fraction. Time-lapse imaging showed that A cells divide both symmetrically and asymmetrically with the majority (~60%) of cell divisions giving rise to two A cells and less frequently generating either one A and one S (~15%) or two S cells (~15%). Serial transplantation experiments in NSG mice revealed that the engraftment potential was largely contained within the A cell fraction. Cell cycle analysis showed that adherent cells contained a lower S and higher G0/G1 phase fraction than suspension cells. Thus, these AML-iPSC-derived hematopoietic cells exhibit hallmarks of an LSC model, namely phenotypic and functional heterogeneity and hierarchical organization, with the A fraction containing LSCs that serially transplant leukemia and give rise to more differentiated cells (S fraction) without engraftment potential. Transcriptome analyses showed that the A to S transition was predominantly mediated by gene upregulation with only 2 out of 418 differentially expressed genes being downregulated and revealed enrichment for HSC and LSC gene sets in the A fraction. Consistent with a general transcriptional upregulation characterizing the A to S transition, differentially accessible regions by ATAC-seq analysis were found predominantly in intragenic regions and enhancers in A cells, while they mostly localized in gene promoters in S cells. Time-course single cell-RNA-seq analyses and their integration with bulk RNA-seq and ATAC-seq data revealed cell clusters specific to the A cell fraction, which were enriched in HSC, LSC and hESC genes. These integrated analyses revealed candidate genes with a role in maintaining LSC properties. The transcription factor RUNX1 was particularly prominent, with RUNX1 motifs found highly accessible in A compared to S cells. Overexpression of RUNX1 in S cells conferred a more LSC-like immunophenotype. RUNX1 is a key transcriptional regulator of hematopoiesis. It has a well-characterized role during development of the hematopoietic system and is often mutated or translocated in AML. A role of RUNX1 in LSC maintenance was unanticipated in view of its known tumor-suppressor function. In further support for a role for RUNX1 in LSCs, we found RUNX1 expression to significantly correlate with survival in AML patient cohorts. In summary, we developed a new model that enables us to prospectively isolate large numbers of genetically clonal human AML LSCs and perform genome-wide integrative molecular studies, with which we obtained new insights into the biology of AML LSCs. Disclosures No relevant conflicts of interest to declare.

Abstract: A heteroclitic WT1 peptide vaccine is well tolerated and induces immunologic responses in most acute myeloid leukemia patients post-CR1. Median overall survival for the group of patients vaccinated was not reached but is poised to reach or exceed 67.6 months.

Abstract: Introduction: Selinexor is a novel, oral Selective Inhibitor of Nuclear Export (SINE) compound that blocks exportin 1 (XPO1). Selinexor treatment results in nuclear accumulation and activation of tumor suppressor proteins, inhibition of NF-kB, and translational suppression of several oncoprotein mRNAs (e.g., c-myc, cyclin D).Multiple myeloma (MM) remains incurable, and most patients (pts) eventually progress through standard drug classes of proteasome inhibitors (PIs), immunomodulatory drugs (IMiDs), anti-CD38 mAbs and others. The increased use of combinations in MM treatment, (PIs/IMiDs/mAbs), has led to a growing number of pts with penta-refractory MM (pts that have been treated with bortezomib (bort), carfilzomib (carfil), lenalidomide (len), pomalidomide (pom) and daratumumab (dara)). Active novel therapies with different mechanisms of actions are needed to address this unmet medical need. Part 1 of STORM enrolled pts with both quad- (bort, carfil, len, pom, treated MM) or penta-refractory MM and demonstrated an overall response rate (ORR) of 21% (Vogl et al, JCO 2018). Based on these findings, the Pivotal Part 2 of STORM was initiated, enrolling an additional cohort of 122 patients with penta-refractory MM. Methods: Pts with penta-refractory MM were treated with 80 mg selinexor plus 20 mg dexamethasone (Sd) twice weekly. Pts must have received an alkylator, bort, carfil, len, pom and dara, and had MM refractory to ≥1 PI, ≥1 IMiD, dara, a glucocorticoid, and their last therapy. Pts must have a total ANC ≥1000 mm3,platelets ≥50k/mm3 (or ≥75k if marrow plasma cells 〈 50%), and creatinine clearance ≥20 mL/min. The primary endpoint was ORR. Secondary endpoints: duration of response (DOR), clinical benefit rate (CBR), progression free survival (PFS), and overall survival (OS). Efficacy was assessed by an Independent Review Committee (IRC) based on IMWG criteria. OS was also compared to a cohort of pts with PI, IMiD, dara refractory MM from the Flatiron Health Analytic Database (FHAD), (ref: ASH 2018 abs ID: 116493),who met all the inclusion criteria for STORM. Results: As of 1-Jun-2018, 122 pts (71 M/ 51 F) were enrolled in 38 sites (US and EU). Pt characteristics were [medians (range)]: age 65 yrs (40-85); 7 (3 - 18) prior treatment regimens, 6.6 yrs ( 〈 1 - 23.4) from initial MM diagnosis.65 pts (53%) had high risk cytogenetics, 86 pts (70%) had prior dara in combination, 102 pts (84%) had prior stem cell transplantation, 2 pts had prior CAR-T therapy. All pts enrolled with progressive disease (PD), 72% of pts had increases (3% - 792%) in MM markers from screening to C1D1 (median 11 days). Frequently reported Sd treatment related adverse events (AEs) included (all grades, grades 3/4): thrombocytopenia (67%, 53%), nausea (67%, 10%), fatigue (68%, 21%), anorexia (50%, 2%), anemia (46%, 28%), and weight loss (46%, 0%). Eight pts remain on study and 114 pts discontinued treatment (most commonly for PD). There were 4 deaths on treatment: sepsis, respiratory failure, pulmonary embolism, and an unrelated, unspecified cardiac event. IRC determined ORR (≥PR) was 26.2%, with 6.5% ≥ very good partial response, including 2 stringent complete responses (sCRs; MRD negative at 1:10-6 and at 1:10-4sensitivity). Both pts who relapsed after CAR-T achieved PRs. The CBR (≥minimal response, [MR]) was 39.3%, and 79% of pts achieved ≥stable disease (SD). Responses typically occurred within the first month. Medians: DOR 4.4 months (mo) (range 〈 1 - 10 mo), PFS 3.7 mo, and OS 8.0 mo. Pts with ≥MR had significantly longer OS than pts with PD/NE (median not reached vs 1.9 mo, p= 〈 0.0001). Compared to the FHAD cohort, STORM cohort had longer OS (Figure 1, HR 0.41, p=0.0001). Conclusions: Results of the pivotal STORM Part 2 in penta (PI, IMiD, dara)-refractory MM demonstrated that oral selinexor plus low-dose dexamethasone (Sd) was highly active with an ORR of 26.2%. Importantly, responses were rapid and deep with 2 patients achieving sCRs (both MRD negative) in these heavily pre-treated penta-refractory MM pts (median 7 prior regimens, 53% high risk). AEs are a function of dose/schedule/disease severity and can be managed with dose modifications and supportive care. No major organ toxicity was observed and AEs were typically transient and reversible. Sd is an all-oral, first in class mechanism with novel MOA and represents a potential therapeutic option to the growing number of pts with penta-refractory MM who have exhausted approved therapies. Disclosures Chari: Novartis: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Adaptive Biotechnology: Membership on an entity's Board of Directors or advisory committees; The Binding Site: Consultancy; Seattle Genetics: Membership on an entity's Board of Directors or advisory committees; Array Biopharma: Research Funding; Celgene: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Amgen: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Pharmacyclics: Research Funding; Takeda: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Janssen: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Bristol Myers Squibb: Consultancy. Vogl:Karyopharm Therapeutics: Consultancy. Dimopoulos:Takeda: Honoraria; Bristol-Myers Squibb: Honoraria; Janssen: Honoraria; Amgen: Honoraria; Celgene: Honoraria. Nooka:GSK: Consultancy, Membership on an entity's Board of Directors or advisory committees; Janssen pharmaceuticals: Consultancy, Membership on an entity's Board of Directors or advisory committees; Adaptive technologies: Consultancy, Membership on an entity's Board of Directors or advisory committees; Takeda: Consultancy, Membership on an entity's Board of Directors or advisory committees; Celgene: Consultancy, Membership on an entity's Board of Directors or advisory committees; Amgen: Consultancy, Membership on an entity's Board of Directors or advisory committees; BMS: Consultancy, Membership on an entity's Board of Directors or advisory committees; Spectrum Pharmaceuticals: Consultancy, Membership on an entity's Board of Directors or advisory committees. Moreau: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; Celgene: 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; Takeda: Honoraria, Membership on an entity's Board of Directors or advisory committees. Cole:University of Michigan: Employment; Cancer Support Community myeloma advisory board: Membership on an entity's Board of Directors or advisory committees. Dingli:Millennium Takeda: Research Funding; Alexion Pharmaceuticals, Inc.: Other: Participates in the International PNH Registry (for Mayo Clinic, Rochester) for Alexion Pharmaceuticals, Inc.; Alexion Pharmaceuticals, Inc.: Other: Participates in the International PNH Registry (for Mayo Clinic, Rochester) for Alexion Pharmaceuticals, Inc.; Millennium Takeda: Research Funding. Vij:Jazz Pharmaceuticals: Honoraria; Karyopharm: Honoraria; Janssen: Honoraria; Amgen: Honoraria; Takeda: Honoraria, Research Funding; Bristol Myer Squibb: Honoraria, Research Funding; Celgene: Honoraria, Research Funding. Raab:BMS: Consultancy, Honoraria, Research Funding; Novartis: Consultancy, Honoraria, Research Funding; Celgene: Consultancy, Honoraria; Amgen: Consultancy, Honoraria, Research Funding. Weisel:Amgen, BMS, Celgene, Janssen, and Takeda: Honoraria; Amgen, Celgene, Janssen, and Sanofi: Research Funding; Amgen, BMS, Celgene, Janssen, Juno, Sanofi, and Takeda: Consultancy, Membership on an entity's Board of Directors or advisory committees. Delforge:Celgene and Janssen: Research Funding; Amgen, Celgene, Janssen and Takeda: Consultancy. Stewart:Amgen Inc., BMS, Celgene, Takeda, Roche, Seattle Genetics, Janssen, Ono: Consultancy; Amgen Inc., Celgene, Roche, Seattle Genetics: Research Funding. Mohty:Amgen: Consultancy, Honoraria; Molmed: Consultancy; Sanofi: Consultancy, Honoraria, Research Funding, Speakers Bureau; Servier: Consultancy; Jazz Pharmaceuticals: Honoraria, Research Funding, Speakers Bureau; Janssen: Honoraria, Research Funding, Speakers Bureau; MaaT Pharma: Consultancy, Membership on an entity's Board of Directors or advisory committees; Celgene: Consultancy, Honoraria; Takeda: Honoraria, Speakers Bureau; Bristol Myers: Consultancy, Research Funding. Sylvain:Gilead: Other: scientific advisor board. Costa:Celgene: Honoraria, Research Funding; Abbvie: Research Funding; Karyopharm: Research Funding; Janssen: Research Funding; Amgen: Honoraria, Research Funding; Sanofi: Honoraria; BMS: Research Funding. Shah:Karyopharm Therapeutics: Employment. Picklesimer:Karyopharm Therapeutics: Employment. Saint-Martin:Karyopharm Therapeutics: Employment. Li:Karyopharm Therapeutics: Employment. Kauffman:Karyopharm Therapeutics: Employment, Equity Ownership, Membership on an entity's Board of Directors or advisory committees. Shacham:Karyopharm Therapeutics: Employment, Equity Ownership, Membership on an entity's Board of Directors or advisory committees. Richardson:Celgene: Membership on an entity's Board of Directors or advisory committees, Research Funding; Oncopeptides: Membership on an entity's Board of Directors or advisory committees; Takeda: Membership on an entity's Board of Directors or advisory committees, Research Funding; Jazz Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees, Research Funding; Karyopharm: Membership on an entity's Board of Directors or advisory committees; BMS: Research Funding; Amgen: Membership on an entity's Board of Directors or advisory committees; Janssen: Membership on an entity's Board of Directors or advisory committees. Jagannath:Celgene: Consultancy; Bristol-Myers Squibb: Consultancy; Multiple Myeloma Research Foundation: Speakers Bureau; Medicom: Speakers Bureau; Novartis: Consultancy; Merck: Consultancy.

Abstract: Background: Activating mutations in Notch receptors are found in multiple hematopoietic malignancies, including 〉 50% of patients with T-ALL, making it the most common genetic abnormality in this disease. GSIs block activation of Notch receptors and limit growth and survival in pre-clinical T-ALL models. However, various GSIs evaluated in clinical trials have had on target toxicities and have not been reported to show significant responses. CA216002 is a multicenter phase 1 trial designed to assess the safety of a novel intravenous GSI BMS-906024 in patients with relapsed or refractory T-ALL and T-cell lymphoblastic lymphoma (T-LL). We are presenting the initial toxicity profile and response data on this trial. Methods: Adults with relapsed/refractory T-ALL or T-LL were enrolled and received BMS-906024 intravenously weekly at doses of 0.6 mg, 4 mg, and 6 mg. Due to the rapid progression of T-ALL in many cases, administration of glucocorticoids or other agents was permitted and dosing guidelines for dexamethasone were provided in the protocol. Results: As of July 2, 2014, safety and response data are available on 25 patients (age 18-74 yrs) with relapsed/refractory T-ALL/T-LL that received at least one dose of BMS-906024, at doses of 0.6 mg (n=1), 4 mg (n=10), and 6 mg (n=14). Seven patients did not complete the first 4-week cycle due to rapid disease progression or disease-related death (n=5), infusion reaction (n=1), or an unrelated adverse event (n=1). Safety: The drug-related grade 3-5 adverse events included grade 4 events of anemia, hypophosphatemia, and thrombocytopenia, and grade 3 events of diarrhea, febrile bone marrow aplasia, hepatotoxicity, hypophosphatemia, pancytopenia, and tumor lysis syndrome (n=1 each). Drug-related diarrhea was common (n=11, 44%), consistent with expectations for Notch inhibition, but was generally grade 1-2 with only one grade 3 event. One dose-limiting toxicity involving grade 3 elevations of ALT and AST without bilirubin elevation (reported as grade 3 hepatotoxicity) occurred at the 4 mg dose level. One death not related to disease progression occurred, due to GI and post-surgical hemorrhage associated with pancytopenia; hemorrhage was considered not related, but pancytopenia was considered related to study drug. Responses: Eight patients (32%; 4 at 4 mg and 4 at 6 mg) had at least 50% reduction in bone marrow (BM) blasts, including one formal CR and one PR (both at 6 mg), and three of these eight had 98-100% clearance of BM blasts. (One patient, marked “*” below, began the study with 0.1% BM blasts and is not included in the eight.) The patient who achieved a CR began the study with 85% BM blasts and an absolute peripheral blood (PB) blast count of 38 k/mcL. By the end of the first cycle the BM and PB were cleared of blasts, and by the end of the second cycle there was count recovery. This patient received dexamethasone during the first cycle only, and left the study after 3 cycles in CR for an allogeneic transplant. The patient who achieved a PR began with 32% BM blasts, and by the end of the first cycle the BM blasts had decreased to 7% with improvement in ANC. The additional six patients with 50-100% decreases in BM blasts had residual lymphadenopathy, had incomplete count recovery or failed to meet other criteria which prevented them from being considered CR or PR based on the protocol definitions. One of these six patients also received hydroxyurea beginning on study day 16. Biomarkers: The figure shows change in BM blasts in 12 patients with paired BM assessments and Notch mutation status available. BM responses occurred in both Notch mutant and wildtype patients. Conclusions: Overall 8 of the 25 patients (32%) showed at least 50% reduction in BM blasts including one CR and one PR. Although the contribution of concurrent glucocorticoid therapy to the improvement in some patients is not clearly defined, the multiple responses on this trial suggest anti-leukemia activity of BMS-906024. This represents the first Notch targeting trial leading to multiple responses in relapsed/refractory T-ALL. BMS-906024 was relatively well tolerated, with minimal diarrhea in this population. The weekly dosing of this long-acting GSI shows promise for targeting Notch in T-ALL. Pharmacokinetic and additional biomarker data will be presented. Figure 1 Figure 1. Maximum Percent Reduction from Baseline of BM Blasts in Patients with Paired BM Assessments and Known Mutation Status Disclosures Zweidler-McKay: BMS: Research Funding. Off Label Use: BMS-906024 is in early Phase I clinical trials, and does not yet have an FDA-approved indication.. Douer:BMS: Research Funding. Ottmann:BMS: Consultancy, Honoraria, Research Funding. Vey:BMS: Honoraria. Thomas:BMS: Research Funding. Zhu:BMS: Employment. Huang:BMS: Employment, Equity Ownership. Bajaj:BMS: Employment. Fischer:BMS: Employment, Equity Ownership.

Abstract: Abstract 526 The prognosis of patients with high-risk myeloma (HR MM) continues to be dismal, despite the early incorporation of novel agents. Early phase trials of allogeneic hematopoietic stem cell transplant (alloHCT) suggest the possibility of an immunologic graft-versus-myeloma effect that might favorably affect survival. Less toxic reduced-intensity HCT preparative regimens now allow more widespread use of alloHCT in the MM population. BMT CTN 0102 is a phase III multicenter clinical trial that biologically assigned patients to either melphalan 200mg/m2 (MEL 200) auto-auto without (obs) or with 1 year of thalidomide and dexamethosone (ThalDex), or an auto-allo approach using MEL 200 followed by alloHCT using 2 Gy total body irradiation. Graft-versus-host disease (GVHD) prophylaxis was cyclosporine and mycophenolate mofetil. Patients were stratified by biological prognostic factors that were considered to be high risk at the time of the trial design: chromosome 13 deletions by metaphase karyotype and beta-2 microglobulin ≥4 mg/dl. The primary endpoint was 3-year progression free survival (PFS). Between December 2003 and March 2007, 710 patients from 43 US centers were enrolled, and 85 fulfilled the criteria of HR MM. Among them, 48 were assigned to auto-auto (24 Thal-Dex and 24 obs) and 37 to auto-allo. Groups differed in age (median 57 y and 51y, p=0.02) but were otherwise balanced. Compliance with second transplant was 65% for auto-auto and 78% for auto-allo. Compliance with ThalDex was poor, so the two auto-auto arms were pooled for the primary analysis. Three-year PFS was 33% (95% Confidence Interval (CI), 22–50%) and 40% (95% CI, 27–60%, p=0.74) and 3-year OS was 67% (95% CI, 54–82%) and 59% (95% CI, 49–78%, p=0.46) for auto-auto and auto-allo, respectively. Corresponding probabilities for 3-year progression/relapse was 53% and 33% (p=0.09), and 3 year treatment-related mortality was 8% and 20% (p=0.3). Among auto-allo patients, probabilities of grade 3–4 acute and chronic GVHD were 9% and 48%, respectively. Among the 59 (31 auto-auto, 28 auto-allo) patients who received second transplant, 3 year PFS was 35% and 46% (p=0.6). Disease response at day 56 after second transplant was 57% for very good partial response (VGPR) or better and 37% for complete response (CR) and near CR (nCR) in the auto-auto group; and 48% (VGPR or better) and 41% (CR+nCR) in the auto-allo group. In conclusion, this planned secondary analysis of a cohort of HR MM patients demonstrated equivalent 3-year PFS and OS for auto-auto and auto-allo in both intention-to-treat and as-treated analyses. However, trends in late PFS and time to progression/relapse suggest further follow-up is needed before final conclusions regarding the utility of auto-allo in this HR cohort can be made. Finally, this study shows the feasibility of an alloHCT approach for HR MM patients and may serve as a platform for future studies seeking to enhance graft-versus-myeloma effects. Disclosures: Stadtmauer: Celgene: Speakers Bureau. Krishnan:Celgene: Speakers Bureau. Qazilbash:Celgene: Speakers Bureau. Vesole:Celgene: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau. Giralt:Celgene: Honoraria, Speakers Bureau; Millenium: Honoraria, Speakers Bureau.

Abstract: Although the nucleoside analogs cladribine and pentostatin produce high response rates in patients with hairy cell leukemia (HCL), a significant number of patients eventually relapse. Several studies have demonstrated that patients with complete remission (CR) have a longer disease-free survival. Therefore, strategies to improve on the initial response to nucleoside analog therapy are likely to be beneficial, at least for a proportion of patients. We have treated 13 patients with newly diagnosed HCL (n = 11) or after failure of one prior chemotherapy (n = 2) with cladribine (5.6 mg/m2 given intravenously over 2 hours daily for 5 days) followed by 8 weekly doses of rituximab (375 mg/m2). All patients achieved a CR and minimal residual disease (MRD) assessed by consensus primer polymerase chain reaction (PCR) or flow cytometry was eradicated in 11 (92%) of 12 and in 12 (92%) of 13 of patients, respectively. There was no decline in the absolute CD4 and CD8 lymphocyte number after rituximab. We conclude that eradication of MRD in HCL is possible. Whether this leads to a reduced risk of relapse would need to be evaluated in a larger number of patients and with longer follow-up. Disease characteristics may potentially be used to identify patients that are more likely to benefit from such additional therapy.