ALBERT

Description: Proteasome inhibitors (PIs) capitalize on the constitutive activation of NF-KB in AML cells and increase chemosensitivity to anthracyclines and cytarabine. We combined the second generation PI, ixazomib, with the standard AML salvage regimen of MEC (mitoxantrone, etoposide, cytarabine). The primary objectives of this study were to determine the dose limiting toxicity (DLT), maximum tolerated dose (MTD), and phase 2 dose of ixazomib in combination with MEC in relapsed/ refractory (R/R) AML. Secondary objectives included evaluating the efficacy of this combination and correlating response to the gene expression profile and CD74 expression, which may identify a subset of leukemias in which NF-KB is operative with increased sensitivity to PI (Attar et al. CCR 2008; 14: 1446-54). Methods: Patients (pts) were treated at Cleveland Clinic and University Hospitals of Cleveland from Oct 2014 to present. An IND was approved by the FDA, and the protocol was approved by each institutional review board. Eligibility: age 18-70 yrs, R/R AML, and cardiac ejection fraction ≥ 45%. The fraction of blasts positive for CD74 was assessed by flow cytometry. Samples were stored for gene expression profiling pre- and post-treatment (at the time of response assessment). Pts received MEC: mitoxantrone (8 mg/ m2), etoposide (80 mg/m2), and cytarabine (1000 mg/m2) intravenous (IV) Days 1-6. Ixazomib, provided by Takeda, was given orally on Days 1, 4, 8, and 11 and was dose escalated using a standard 3x3 design. Dose levels (DLs): 1 (1.0 mg), 2 (2.0 mg), 3 (3.0 mg), 4 (3.7 mg). An additional 18 pts were to be treated at the MTD. One cycle of treatment was administered. Response was assessed by bone marrow aspirate/ biopsy by Day 45 and complete remission (CR) was defined by IWG criteria (Cheson 2006). Toxicities were graded according to NCI CTCAE v 4.03. Toxicities secondary to neutropenia or sepsis were not considered DLTs. DLTs included: (1) ≥ Grade 4 non-hematologic toxicity (NHT) with the exception of nausea, vomiting/ alopecia and drug-related fevers; (2) any ≥ Grade 3 neurologic toxicity; (3) grade 4 platelet or neutrophil count 50 days beyond the start of chemotherapy and not related to leukemia; (4) any Grade 4 NHT 〉 grade 2 by 45 days beyond the start of chemotherapy. Grade 2, 3, and 4 hyperbilirubinemia were redefined as 1.5-〈 10x upper limits of normal (ULN), 10-20 x ULN, and 〉 20 x ULN. Results: Of 23 pts enrolled, 22 are evaluable. The median age was 58 yrs (range 31-70), 12 (52%) were male and the median baseline WBC was 2.56 K/ uL (range 0.1-62.9). The median time from initial diagnosis to registration was 7.1 months (range 1.4-36.8) and 7 pts (30%) had a history of an antecedent hematologic disorder. Thirteen pts were in 1st relapse and 10 pts were refractory to their last therapy. One pt had received a prior allogeneic hematopoietic cell transplant (AHCT), 7 pts had FLT3 ITD mutations and 7/ 21 pts (33%) had adverse cytogenetics per CALGB 8461 criteria at the time of relapse. At DL1, 1 DLT occurred (grade 4 thrombocytopenia), so this DL was expanded to 6 pts. At DL2, 2 pts developed Grade 4 thrombocytopenia; therefore, the MTD of ixazomib was 1.0 mg. The most common grade 3-5 NHTs in the dose escalation phase were febrile neutropenia (100%), hypoalbuminemia (25%), hypokalemia (42%), hypotension (33%), and respiratory failure (33%). No adverse events in the dose escalation phase were attributed to ixazomib alone. The overall response rate was 55% [CR/ CR with incomplete count recovery (CRi)], and 9 pts proceeded to AHCT. Five of these 9 pts remain alive with a median follow-up of 12.8 months. Five pts had CD74 expression performed. Two pts had high levels of CD74 expression (〉 80%); and both achieved CRi. Myeloid mutation panel data was available in 14 pts. Previous data has demonstrated the number of mutations in DNTMT3A, TP53, ASXL1, and NRAS (0, 1, 〉1) is associated with a worse response to salvage therapy (Advani et al, abstract 3825, ASH 2015). Seven pts had at least one of these mutations and 6 of the 7 achieved CR/ CRi. Conclusions: The combination of MEC and ixazomib was well-tolerated and produced an overall response rate of 55% in patients with relapsed/ refractory AML irrespective of molecular mutation status. The combination is safe with a similar toxicity profile to MEC alone. CD74 expression may represent a biomarker for response to this therapy. Results from gene expression profiling will be complete by the time of the meeting and will be presented. Disclosures Mukherjee: Novartis: Consultancy, Honoraria, Research Funding; Ariad: Consultancy, Honoraria, Research Funding; Celgene: Consultancy, Honoraria, Research Funding. Caimi:Genentech: Speakers Bureau; Gilead: Consultancy; Roche: Research Funding; Novartis: Consultancy. Maciejewski:Alexion Pharmaceuticals Inc: Consultancy, Honoraria, Speakers Bureau; Apellis Pharmaceuticals Inc: Membership on an entity's Board of Directors or advisory committees; Celgene: Consultancy, Honoraria, Speakers Bureau. Sekeres:Millenium/Takeda: Membership on an entity's Board of Directors or advisory committees; Celgene: Membership on an entity's Board of Directors or advisory committees.

Description: Abstract 3597 The c-kit (CD117) receptor is expressed on 〉 10% blasts in 64% of de novo AMLs and mediates proliferation and anti-apoptotic effects. High c-kit levels correlate with a shorter time to relapse and decreased overall survival (OS). Imatinib mesylate (IM), a c-kit inhibitor, has activity against relapsed/refractory AML. The primary objective of this study was to determine whether adding maintenance IM for 1 yr after completion of standard induction (IT) and post-remission therapy (PRT) in patients (pts) with newly diagnosed c-kit+ AML improves relapse-free survival (RFS) compared to historical controls. Secondary objectives included: (1) assessing the feasibility of this approach; (2) evaluating outcomes based on c-kit expression (c-kit mean fluorescent intensity [MFI]); (3) determining whether c-kit expression correlates with AF1q gene and/or multi-drug resistance (MDR) gene expression. Methods: Pts were treated at Cleveland Clinic, Duke, Roswell Park, and University Hospitals of Cleveland from 2008 to 2012. IM was supplied by Novartis. Eligibility criteria included: pts age ≥ 18 yrs, AML in first complete remission (CR1), ≥ 20% c-kit+ blasts at diagnosis (dx), ECOG performance status 0–2. Cytogenetics (CG) were classified by CALGB 8461. Pts must have received IT (7+3 [continuous infusion cytarabine (C) and an anthracycline] or ADE [C, daunorubicin, etoposide]) and PRT (≥ 1 course for pts 〉 60 yrs; ≥ 2 courses for pts 〈 60 yrs). CR status was confirmed by bone marrow analysis prior to study enrollment. MDR expression was analyzed by immunohistochemistry on diagnostic samples (n=19); AF1q gene expression was analyzed by RT-PCR on RNA from available diagnostic pt samples (n=9) as previously described (Tse et al. Blood 2004; 104: 3058–63). C-kit MFI was calculated as the mean channel number (MCN) of the blasts/MCN autofluorescence using a CD45/orthogonal light scatter gate to isolate blasts. All pts received IM 600 mg/day for 12 months (mos) unless they experienced toxicity or disease progression. Dose modifications were made for Grades 2–4 non-hematologic toxicity and Grades 3–4 neutropenia and thrombocytopenia. Pts remaining off IM for 〉 4 wks were removed from treatment. Cumulative dose intensity was defined as the proportion of the total optimum dose administered over time. Results: Thirty-three pts were enrolled, with 32 pts having complete data. The median age was 54 yrs (range 19–81), median WBC at dx 22.13 K/μL (1.55–98.44), median peripheral blood blasts at dx 23.6% (range 0–85), and 44% were male. CG risk included: 47% (15) good, 31% (10) intermediate, and 22% (7) poor. The median c-kit % was 79.9, and median c-kit MFI 39.8 (range 6.5–120.1). Median AF1q expression was 9.59 (range 1.83–161.8.5). Eighty-four percent of pts had moderate or high levels of MDR expression (GSTP1, MDR1, LRP1, and/or MRP1); almost half (47%) had high expression. The majority of pts (74%, n=20) received PRT with high dose C (3 g/m2/dose × 6 doses/cycle). Pts received IM for a median of 4.0 mos (range 0.1–12.2), and the median daily dose was 600 mg. Twelve pts (38%) were dose reduced to 400 mg. Forty-five percent (13/29) of pts experienced grade 3 reactions possibly related to treatment, with the majority (31%) being myelosuppression. The most commonly reported adverse events were Grade 1/2 nausea and vomiting (72%), edema (59%), and fatigue (41%). Twelve pts (38%) discontinued treatment for adverse events. The median RFS survival is 18.8 mos, with a median follow-up of 19.1 mos (range 6.4–37.2). Estimated 2-yr OS is 62% ± 10%. Predictors of RFS included: age, WBC at dx, % peripheral blasts at dx, and CG risk. Dose intensity of IM did not correlate with outcome. AF1q and MDR expression did not correlate with c-kit MFI; although the number of pts with AF1q data was small. Of note, neither c-kit MFI nor AF1q expression were prognostic in this subset of pts treated with IM. With the exception of LRP1 expression (p=0.03), there was no correlation of MDR expression with RFS. Conclusions: Previous studies have demonstrated that c-kit MFI 〉 20.3 is an independent adverse prognostic factor for RFS and OS (median RFS 10.7 months). Considering the high c-kit MFI of pts in this study, the outcomes using IM maintenance are encouraging, and suggest that further study of this approach is warranted. Given the toxicities observed, reducing the dose of IM to 400 mg in the maintenance setting may be better tolerated. Disclosures: Advani: Novartis: Research Funding. Rizzieri:Novartis: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau. Kalaycio:Novartis: Research Funding, Speakers Bureau. Maciejewski:Novartis: Research Funding.

Description: Abstract 3595 The overexpression of proteasomes and constitutive activation of NF-KB in acute myeloid leukemia (AML) cells suggest that proteasome inhibitors (PI) such as Bortezomib (Bz) may be an effective therapy. PI or a decoy NF-KB oligonucleotide increases chemosensitivity to both anthracyclines and cytarabine. Thus, PI may improve the effectiveness of MEC (mitoxantrone, etoposide, cytarabine), a standard regimen for relapsed/refractory (R/R) AML. The primary objectives of this study were to determine the dose-limiting toxicity (DLT), maximum tolerated dose (MTD), and recommended Phase 2 dose of Bz in combination with MEC in patients (pts) with R/R AML. Secondary objectives included evaluating the preliminary activity of this combination and correlating CD74 antigen expression with response. CD74 may identify a subset of leukemias in which NF-KB is operative, with increased sensitivity to PI (Attar et al. CCR 2008; 14: 1446–54). Methods: All pts were treated at the Cleveland Clinic from August 2010-July 2012. This protocol was reviewed and approved by the institution's review board. Eligibility included: age 18–70 yrs, R/R AML, cardiac ejection fraction ≥ 45%. CD74 was assessed by flow cytometry using CD45 PE (BD Biosciences San Jose, CA) and CD74-Alexa 488 (AbD Serotec Raleigh, NC). All pts received MEC: mitoxantrone (6 mg/m2/d), etoposide (80 mg/m2), and cytarabine (1000 mg/m2) Days 1–6. Bz was administered IV on Days 1, 4, 8, and 11 and was dose escalated using a standard 3 × 3 design. Dose levels (DL) were: −1 (0.40 mg/m2), 1 (0.70 mg/m2), 2 (1.00 mg/m2), 3 (1.30 mg/m2). One cycle of treatment was administered. Response was assessed by bone marrow aspirate/biopsy by Day 45, and CR was defined by IWG criteria (Cheson, 2006). Toxicities secondary to neutropenia or sepsis were not considered DLTs. DLT included: (1) ≥ Grade 4 non-hematologic toxicity (NHT) with the exception of nausea, vomiting, alopecia, and drug-related fevers; (2) any ≥ Grade 3 neurologic toxicity; (3) grade 4 platelet or neutrophil count 50 days beyond the start of chemotherapy (not related to leukemia); (4) Any grade 3 NHT 〉 grade 2 by 45 days beyond the start of chemotherapy. The following were redefined as not being DLT: (1) anorexia requiring TPN; (2) fatigue requiring bed rest; (3) grade 2, 3, and 4 hyperbilirubinemia were redefined as 1.5- 20 × ULN respectively. Results: Seventeen pts have enrolled; and 15 are evaluable for response. The median age was 54 years (range 33–69), 7 (47%) were male, and median baseline WBC 3.58 K/μL (range 0.96–76.53). The median time from initial diagnosis to enrollment was 7.1 months (range 1.4–84.9) and 2 pts had a history of an antecedent hematologic disorder. Nine pts (60%) were in first relapse, 2 (13%) in second relapse, and 4 (27%) refractory. One pt had received a prior allogeneic hematopoietic stem cell transplant; and 4 out of 15 pts (27%) had adverse cytogenetics at the time of relapse based on CALGB 8461 criteria. At DL 1, 1 DLT occurred (Grade 4 thrombocytopenia). No DLTs occurred at DL 2. Three pts were enrolled on DL 3, with one DLT occurring thus far (Grade 4 transaminases: likely related to leukemia). Only one pt had their Day 11 Bz held secondary to Grade 2 ileus (DL 1). Overall, 3 pts (all on DL 1) have died from blood stream infections. In addition to Grade 4 hematologic toxicity, the most commonly reported adverse events (AEs) have been gastrointestinal (GI). GI toxicities were the mostly commonly reported AEs attributable to Bz. Nine pts reported constipation and/or abdominal pain (7: Grade 1 or 2; 2: Grade 3). Six of the 15 evaluable pts (40%) have achieved a complete remission (CR) or CRp (complete remission without platelet recovery). Eight of the fifteen pts had CD74 expression testing, but only 6 of 8 were evaluable for response. The median CD74 expression was higher in refractory pts (35.9%) (range 14–87) than in responders (3.2%) (range 0.9–16). Conclusions: The MTD of MEC in combination with Bz will be reported at the meeting; but currently we are in the last DL (3) and the MTD has not been reached. The toxicities of the combination are similar to that of MEC, except potentially an increase in GI toxicities. We await the results of preliminary response in the expanded cohort of pts, once the MTD is achieved. Higher CD74 expression appears to correlate with refractory disease rather than response in this R/R AML cohort, but larger pt numbers are needed to confirm this. Disclosures: Advani: Millenium: Research Funding. Hsi:Millenium: Research Funding.

Description: The c-kit (CD117) receptor is expressed on 〉 10% blasts in 64% of de novo AMLs and mediates proliferation and anti-apoptotic effects. High c-kit levels [defined as mean fluorescent intensity (MFI) 〉 20] correlate with a shorter time to relapse and decreased overall survival (OS). Imatinib mesylate (IM), a c-kit inhibitor, has activity against relapsed/ refractory AML. The primary objective of this study was to determine whether adding maintenance IM for 1 yr after completion of standard induction (IT) and post-remission therapy (PRT) in pts with newly diagnosed c-kit + AML improves progression-free survival (PFS) compared to historical controls. We previously presented our toxicity and correlative data at ASH 2012 (Abstract 3597). Here, we present our long term follow-up results. Methods: Pts were treated at Cleveland Clinic, Duke, Roswell Park, and University Hospitals of Cleveland from 2008 to 2012. IM was supplied by Novartis. Eligibility criteria: pts age ≥ 18 yrs, AML in first complete remission (CR1), ≥ 20% c-kit+ blasts at diagnosis (dx), ECOG performance status 0-2. Cytogenetics (CG) were classified per CALGB 8461. Pts must have received IT (7+3 [continuous infusion cytarabine and an anthracycline] or ADE [cytarabine, daunorubicin, etoposide]) and PRT (≥ 1 course for pts ≥ 60 yrs; ≥ 2 courses for pts 〈 60 yrs). CR was confirmed by bone marrow analysis prior to study enrollment. MDR expression was analyzed by IHC on diagnostic samples (n=19); AF1q gene expression was analyzed by RT-PCR on RNA from available diagnostic pt samples (n=9). C-kit MFI was calculated as the mean channel number (MCN) of the blasts/ MCN auto fluorescence using a CD45/orthogonal light scatter gate to isolate blasts and lymphocytes. All pts received IM 600 mg/day for 12 months (mos) unless they experienced toxicity or disease progression. Dose modifications were made for Grade 2-4 non-hematologic toxicity and Grades 3-4 neutropenia and thrombocytopenia. PFS was measured from the CR date to the time of relapse or death. Primary endpoints: Based on historical data from the Cleveland Clinic and SWOG, the median PFS for all AML pts undergoing IT 〈 60 yrs of age is 13 mos and for pts ≥ 60 yrs of age is 8 mos. The goal of this study was to see a 30% improvement in PFS at these time points in the respective age groups (i.e. 65% PFS at 13 mos for pts 〈 60 yrs; 65% PFS at 8 mos for pts ≥ 60 yrs). Results: Of 32 pts enrolled, the median age was 54 yrs (range 19-81), median WBC at dx 22.13 K/ uL (1.55-98.44), median peripheral blood blasts at dx 23.6% (range 0-85), and 44% were male. CG risk included: 16% (5) good, 66% (21) intermediate, 16% (5) poor, 3% (1) miscellaneous. Of the pts with normal CG, 10 were NPM1+, FLT3 ITD negative; and 1 pt was FLT3 ITD+. The median c-kit+ blast % was 79.9, and median c-kit MFI 39.8 (range 6.5-120.1). Median AF1q expression was 9.59 (range 1.83-161.85) (〉 9 is considered high and is associated with a poor prognosis; high AF1q is also associated with high c-kit expression). Eight-four percent of pts had moderate or high levels of drug resistance factors (GST1, MDR1, LRP1, and/or MRP1); almost half (47%) had high expression. There was no correlation between MDR and c-kit MFI. Pts received IM for a median of 4.0 mos (range 0.1-12.2) and the median daily dose was 600 mg. Twelve pts (38%) were dose reduced to 400 mg. Forty-five percent of pts experienced Grade 3 reactions possibly related to treatment, with the majority (31%) being myelosuppression. With a median follow-up time of 56.3 mos, the estimated median OS was 51.3 mos and estimated median relapse-free survival (RFS) 18.9 mos. The estimated PFS at 13 mos for pts 〈 60 yrs of age was 71 ± 10% (p=0.017, compared to the null hypothesis); and the estimated PFS at 8 mos for pts ≥ 60 yrs of age was 64 ± 15% (p=0.166, compared to the null hypothesis). Predictors of worse RFS included: age, WBC at dx, % peripheral blasts at dx, CG risk, and MDR expression. C-kit MFI and Af1q were not associated with RFS or OS. Conclusions: Use of IM maintenance therapy appeared to be associated with improved PFS compared to historical controls in pts 〈 60 yrs of age. In addition to a high c-kit MFI, these pts had other adverse characteristics (moderate to high levels of MDR. high AF1q). Though previous studies have demonstrated that c-kit MFI 〉 20.3 was an independent adverse prognostic factor for RFS and OS (median RFS 10.7 months) in AML, use of IM maintenance therapy in this study appeared to mitigate this, supporting further investigation. Disclosures Off Label Use: imatinib in the treatment of AML. Rao:Boehringer-Ingelheim: Other: Advisory Board; amgen: Other: ad board; novartis: Other: ad board. Rizzieri:Teva: Other: ad board, Speakers Bureau; Celgene: Other: ad board, Speakers Bureau. Wang:Immunogen: Research Funding. Griffiths:Alexion Pharmaceuticals: Honoraria; Astex: Research Funding; Celgene: Honoraria. Sekeres:TetraLogic: Membership on an entity's Board of Directors or advisory committees; Amgen: Membership on an entity's Board of Directors or advisory committees; Celgene Corporation: Membership on an entity's Board of Directors or advisory committees.

Description: C-kit is a tyrosine kinase receptor expressed on more than 10% of blasts in 95% of relapsed AMLs. Signaling pathways, such as STAT3 and STAT5, may be activated downstream of c-kit. Imatinib mesylate (IM) is a potent c-kit inhibitor and has activity in relapsed/ refractory AML. We conducted a Phase 1 trial of IM in combination with cytarabine (A) and daunorubicin (D) in pts with c-kit+ relapsed AML. Methods: All pts were treated at the Cleveland Clinic from 2003–2008. Cytogenetic (CG) risk was defined by CALGB criteria. Eligibility criteria included: age ³18 yrs, AML in first relapse (excluding APL), relapse more than 6 mos from induction therapy, ECOG performance status 0–2, ³20% blasts c-kit+ (CD117+ by flow cytometry). Phosphorylated (P) (activated) STAT3 (nuclear and cytoplasmic) was assessed by immunhistochemistry (IHC) on pre-tx samples. IHC studies for P-STAT5 are ongoing. Cases were defined as positive if any staining was present in blasts. All pts received A 100 mg/m2/d by continuous infusion for 7 days and D 45 mg/m2 IV for 3 days (7+3). IM dose was escalated using a standard 3 + 3 design. Six additional pts were then treated at the MTD. Planned dose levels were: −1 (300 mg), 1 (400 mg), 2 (600 mg), 3 (800 mg). IM was administered with the first dose of chemotherapy and continued daily until disease progression, intolerance, removal from study for another treatment (including alloBMT), or dose-limiting toxicity (DLT). A Day 14 BM was performed; pts with persistent leukemia but ≥ 50% reduction in BM blasts continued IM and received 5 days of A (100 mg/m2/d) and 2 days of D (45 mg/m2/d). Results: Twenty-one pts have been enrolled: median age was 47 yrs (range 24–75) and 48% were male. The median time from CR to relapse was 439 d (range 216–1100). CG risk: poor 14%, intermediate 71%, and good 14%. The median percentage of c-kit+ blasts was 89% in the bone marrow (range 52–98). In patients expressing nuclear P-STAT3, 2–6% of the blasts stained positive. Eleven pts had no nuclear P-STAT3. No pts had cytoplasmic P-STAT3. At 400 mg IM, 2 of 6 pts had a DLT; both were ³ Grade 3 hyperbilirubinemia. Pharmacodynamic/ pharmacokinetic studies to better understand the etiology of the hyperbilirubinemia are ongoing. Subsequent pts were treated with 300 mg IM. One pt died during induction and was not evaluable for response. Eleven of the 19 evaluable pts (58%) achieved CR or CRp . Pts received IM for a median of 23 d (range 9–574+). Three discontinued IM to proceed to alloBMT. Two pts continue on IM and remain in a remission at 52+ and 574+ days. C-kit expression did not correlate with CR. However, patients with no nuclear P-STAT3 expression had a higher CR rate (82%) than patients with P-STAT3 nuclear expression (33%) (p=0.18). Conclusions: The MTD of IM in combination with 7+3 was 300 mg. Since the hyperbilirubinemia appeared to be reversible with discontinuation of IM, it may be possible to escalate IM doses further and re-challenge. The CR rate is encouraging, and merits evaluation in a Phase 2 study. In particular, pts with no nuclear P-STAT3 had a CR rate of 82%; whereas, pts with nuclear P-STAT3 expression had a CR rate of 33%. STAT3 may be activated independently of c-kit; therefore, directly targeting STAT3 in this latter group of patients may potentially improve prognosis. However, these are preliminary data, and will need to be evaluated in a larger study.

Description: MEC (mitoxantrone, etoposide, cytarabine) is a standard regimen for relapsed/ refractory (R/R) acute myeloid leukemia (AML), but outcomes remain poor. The overexpression of proteasomes and constitutive activation of NF-KB in AML cells suggest that proteasome inhibitors (PI) such as bortezomib (Bz) may be effective anti-leukemia therapy. PI or a decoy NF-KB oligonucleotide increase chemosensitivity to both anthracyclines and cytarabine. To test the hypothesis that PI may improve the efficacy of MEC, we conducted a phase 1 trial of Bz in combination with MEC. Here, we present final results of this trial: response rate, toxicity, and correlation of outcomes with mutation analysis. As CD74 expression may identify a subset NF-KB-dependent AML with predicted increased sensitivity to PI (Clin Can Res 2008; 14: 1446-54), we also explored this correlation. Methods: All pts were treated at the Cleveland Clinic from Aug 2010-Mar 2014. This protocol was approved by the institution’s review board. Eligibility included: age 18-70 yrs, R/R AML, cardiac ejection fraction ≥ 45%. CD74 was assessed by flow cytometry using CD45 PE (BD Biosciences San Jose, CA) and CD74-Alexa 488 (AbD Serotec Raleigh, NC). A myeloid panel mutational analysis was performed on extracted DNA in pts with banked samples (n=26). All pts received 1 cycle of MEC: mitoxantrone (6 mg/m2/d), etoposide (80 mg/ m2), and cytarabine (1000 mg/ m2) Days 1-6. Bz was administered IV on Days 1, 4, 8, and 11. Dose was escalated using a standard 3 x 3 design. Dose levels (DL) were: -1 (0.40 mg/ m2), 1 (0.70 mg/ m2), 2 (1.0 mg/ m2), and 3 (1.3 mg/m2). Response was defined by IWG criteria (Cheson, 2006). The maximum tolerated dose (MTD) of Bz with MEC was 1.0 mg/m2 (Advani et al, ASH 2012, Abstract 3595). Results: Of 35 pts enrolled, the median age was 55 yrs (range 33-69), 13 (38%) were male, and median baseline WBC was 4.0 K/ µL (range 0.82-84.7). The median time from initial diagnosis of AML to enrollment was 8.4 months (range 1.1-88.2) and 6 pts (17%) had an antecedent hematologic disorder. Salvage status (S) at enrollment: S1 (24 pts, 69%), S2 (7 pts, 20%), S4 (4 pts, 11%). Nine pts (26%) were refractory to all prior therapies, and 3 pts (9%) had received prior allogeneic hematopoietic cell transplant (AHCT). Adverse cytogenetics per CALGB/ Alliance 8461 criteria occurred in 19% of pts at study entry and 15 of 26 pts (58%) had poor-risk molecular mutations (RUNX1, ASXL1, TET2, p53, IDH1, MECOM, FLT3 ITD). Ten pts were enrolled on DL1, 13 pts on DL2, 11 pts on DL3, and 1 pt died prior to treatment. Overall, 3 pts (9%) died during induction. In addition to febrile neutropenia and Gr 4 hematologic toxicity, the most commonly reported adverse events (AEs) were metabolic, constitutional, gastrointestinal (GI), and dermatologic, with the majority of these being Gr 1 or 2. GI toxicity was the only reported AE attributable to Bz: 12 pts had constipation or ileus (10: Gr 1 or 2; 2: Gr 3 or 4). Seventeen of the 33 evaluable pts (52%) have achieved a complete remission (CR) or complete remission with incomplete count recovery (CRi); with 1 pt inevaluable due to donor lymphocyte infusion. The estimated median overall survival was 7.2 months; median duration of response was 10.3 months. DL did not correlate with response. Eleven pts (32%) went on to receive AHCT. Among pts with poor-risk molecular mutations, 64% achieved CR/ CRi. Inhibition of NF-KB signaling in leukemia cells with mutated RUNX1 efficiently blocks growth and development of leukemia (Blood 2011; 118: 6626-37). Of the 5 pts with RUNX1 mutations, 3 (60%) achieved CR/ CRi, suggesting that Bz may have promising clinical benefit in this difficult subset of pts. Among the 17 pts with CD74 expression testing who were evaluable for response, the mean CD74 expression trended higher in non-responding pts (32.6%) than in responders (11.1%) (p=0.14). Conclusions: The combination of MEC/Bz was well-tolerated and resulted in high response rates, even within a molecularly-defined poor risk population of pts with R/R AML. Our data do not confirm the expectation that higher CD74 expression would correlate with response in this R/ R AML cohort, but larger pt numbers are needed. These results, especially in pts with poor-risk mutations, support development of a randomized study to address the benefit of adding Bz to MEC in the treatment of R/R AML. Disclosures Advani: Takeda: Research Funding. Carew:Takeda: Research Funding. Sekeres:Celgene Corp.: Membership on an entity's Board of Directors or advisory committees; Amgen: Membership on an entity's Board of Directors or advisory committees; Boehringer Ingelheim: Membership on an entity's Board of Directors or advisory committees.