ALBERT

Description: Background: Patients with acute myeloid leukemia (AML) presenting with hyperleukocytosis have frequent complications and early mortality. Pulmonary, central nervous system, and cardiovascular complications are common. Attempts to improve outcomes in the 10% of AML patients arriving with hyperleukocytosis have included leukapheresis. No prospective randomized study has supported the use of leukapheresis, and retrospective reports have revealed persistently poor outcomes as well as emerging concerns of acquired coagulopathy and worsening hypoxemia after leukapheresis. While many centers use a leukapheresis protocol processing two blood volumes, Johns Hopkins protocol routinely processes three-five blood volumes. This study aimed to assess coagulopathy, hypoxemia, and mortality with large volume leukapheresis. Methods: 32 patients with newly diagnosed AML treated with large volume leukapheresis for WBC depletion are included in this report. Demographic, clinical, laboratory, and apheresis-related data were collected. Coagulopathy and hypoxemia-related metrics were evaluated within 6 hours before leukapheresis and within 6 hours of completion of leukapheresis. Descriptive and inferential statistics (chi square and Mann-Whitney U test) were used to compare pre and post-leukapheresis findings and assess clinical outcomes. Results: Twenty-nine of 32 (93.8%) patients presented with symptomatic leukostasis (with pulmonary and/or CNS symptoms in 26/29). Median blood volume processed was 14.8 liters (range 4-23.4L). Mean platelet count decreased from 60x109/L to 37x109/L after leukapheresis (p

Description: Background: Despite recent advances in the therapeutic armamentarium for AML, outcomes remain dismal for patients (pts) with relapsed/refractory (R/R) AML. Response rates with high dose cytarabine (HiDAC) salvage chemotherapy are approximately 20%. Multiple immune aberrations in AML lead to immune suppression, exhaustion, and senescence. Programmed Death-1 (PD-1), a co-inhibitory receptor (IR) on immune cells, suppresses immune activation and is exploited by leukemic cells to evade immune surveillance. PD-1 and other IRs are up-regulated during disease progression. We hypothesized that pembrolizumab, a monoclonal antibody targeting PD-1, after HiDAC would stimulate a T-cell mediated anti-leukemic immune response. Methods: Eligibility for this study included R/R AML 18-70 years, ECOG PS 0-1 and adequate organ function. Treatment consisted of HiDAC (60 years: 1.5 gm/m2 IV Q12hours days 1-5) followed by pembrolizumab 200 mg IV on day 14. The primary objective of this study was to estimate the overall complete remission (CR + CRi) rate. Secondary objectives included assessment of safety, durability of CR, overall survival (OS) and biomarker correlates of response. Overall responders were eligible to receive maintenance phase pembrolizumab 200 mg IV Q3weeks for up to 2 years until progression. Allogeneic stem cell transplant (alloSCT) was permissible before or after maintenance phase. Results: Thirty-seven pts were enrolled and evaluable (Table 1). Sixteen (43%) pts had refractory disease and 16 (43%) pts had relapsed AML with CR1 duration 3: n=1), AST elevation (32%; Grade 〉3: n=1), fatigue (27%), alkaline phosphatase elevation (24%), and maculopapular rash (19%; Grade 〉3: n=2). Grade 〉3 immune-related adverse events (iRAE) were rare (maculopapular rash: n=2, AST/ALT increase: n=2, right upper quadrant pain with lymphocytic infiltrate in liver: n=1) and self-limiting. Five (14%) pts required steroid administration for grade 2 hyperbilirubinemia (n=1), grade 3 ALT elevation (n=1), grade 3 AST elevation with liver biopsy revealing no evidence of iRAE (n=1), grade 3 bilirubin subsequently deemed to be a delayed hemolytic transfusion reaction (n=1), and grade 3 systolic dysfunction without evidence of myocarditis by endomyocardial biopsy or cardiac MRI (n=1). Sixty-day mortality was 3% (1/37) due to progressive AML. Median time to full neutrophil (〉1x109/L) and platelet (〉100x109/L) recovery was 32 and 31 days, respectively. The overall response (ORR: CR+CRi+PR+MLFS) and composite CR (CR+CRi) rates were 46% [29%,63%] and 38% [22%,55%], respectively, meeting the primary endpoint of the study. Notably, 13/28 (46%) pts receiving HiDAC + pembrolizumab as their first salvage regimen achieved CR/CRi. Two pts refractory to HiDAC (administered within past 6 months) achieved CR including one pt who was refractory to HiDAC salvage 1 month prior to enrollment and ultimately achieved CR without evidence of minimal residual disease. Nine (24%) pts received an alloSCT. There were no instances of Grade 〉3 acute GVHD or veno-occlusive disease post-alloSCT. Nine (24%) pts received maintenance phase pembrolizumab (median # of cycles = 3; range: 1-12) for CR (n=8) or PR (n=1). Seven out of 9 pts relapsed/progressed after maintenance phase. Median follow-up among survivors, and median OS, event-free survival and disease-free survival was 7.8 months, 8.9 months [6.0,13.1], 6.9 months [4.2,11.5], and 5.7 months [1.9,7.3], respectively. Conclusions: Pembrolizumab can be safely administered after HiDAC salvage in R/R AML. Severe iRAE's were uncommon despite administration after cytotoxic chemotherapy. The addition of pembrolizumab to HiDAC led to an encouraging overall CR rate meeting the primary endpoint of the study. Immunogenomic biomarker analyses consisting of B cell receptor amplicon sequencing, RNA-seq of blasts and CD8+ T cells, CD8+ T cell receptor repertoire, whole exome sequencing and flow cytometry analyses are ongoing to determine predictors of response. These results warrant further investigation of IR blockade and other immunomodulatory therapeutic strategies after intensive cytotoxic chemotherapy in AML. Disclosures Zeidner: Takeda: Research Funding; Merck: Research Funding; AsystBio Laboratories: Consultancy; Pfizer: Honoraria; Tolero: Honoraria, Research Funding; Daiichi Sankyo: Honoraria; Celgene: Consultancy, Honoraria, Research Funding; Agios: Honoraria; AbbVie: Honoraria. Vincent:Pharmacyclics: Research Funding; Merck: Research Funding. Foster:Bellicum Pharmaceuticals: Research Funding; Macrogenics: Research Funding; Celgene: Research Funding; Daiichi Sankyo: Consultancy. Coombs:Dedham Group: Consultancy; Covance: Consultancy; Cowen & Co.: Consultancy; Octopharma: Honoraria; H3 Biomedicine: Honoraria; Loxo: Honoraria; Pharmacyclics: Honoraria; Medscape: Honoraria. Webster:Pfizer: Consultancy; Amgen: Consultancy; Genentech: Research Funding. DeZern:Astex Pharmaceuticals, Inc.: Consultancy; Celgene: Consultancy. Smith:Jazz: Consultancy; Pfizer: Consultancy; Novartis: Consultancy; Agios: Consultancy. Levis:Amgen: Consultancy, Honoraria; Astellas: Consultancy, Research Funding; FUJIFILM: Consultancy, Research Funding; Menarini: Consultancy, Honoraria; Novartis: Consultancy, Research Funding; Daiichi Sankyo Inc: Consultancy, Honoraria; Agios: Consultancy, Honoraria. Luznik:Merck: Research Funding, Speakers Bureau; Genentech: Research Funding; AbbVie: Consultancy; WindMiL Therapeutics: Patents & Royalties: Patent holder. Serody:Merck: Research Funding; GlaxoSmithKline: Research Funding. Gojo:Amphivena: Research Funding; Amgen Inc: Consultancy, Honoraria, Research Funding; Juno: Research Funding; Merck: Research Funding; Jazz: Consultancy, Honoraria; Novartis: Consultancy, Honoraria; Abbvie: Consultancy, Honoraria. OffLabel Disclosure: Pembrolizumab is investigational for AML.

Description: Background: CD8+ T-cells in AML pts co-express multiple inhibitory receptors (IRs), including PD1, and IR expression increases with disease progression (Knaus et al, JCI Insight 2018). AZA upregulates pathways related to immunity and immune evasion in tumor cells, including PD-L1, (Wrangle et al, Oncotarget 2013) providing rationale for exploring AZA/Pembro combination in AML. Aims: To assess safety and response to AZA/Pembro after minimum 2 cycles of therapy in relapsed/refractory (R/R) (Cohort 1) and newly diagnosed (dx) older AML (Cohort 2). Methods: Cohort 1: Pts must have failed prior AML therapy. The first 6 pts (run in phase) received AZA 75 mg/m2 Days (D) 1-7 with Pembro 200 mg beginning on D8 and every (q)3 weeks (wks) thereafter. AZA cycles were repeated q4wks. No pts experienced dose limiting toxicity after minimum 3 cycles observation. After safety was established with the dosing schedule, patients with prior allogeneic stem cell transplant (alloSCT) were included and Cohort 2 started enrollment. Cohort 2: Pts ≥65 years (yrs) with newly dx AML and not candidates, or unwilling to receive, intensive chemotherapy. Other eligibility criteria (Cohort 1 and 2): ECOG PS 0-2 (changed to PS 0-1), adequate organ function, and no autoimmune processes requiring systemic immunosuppression. Results: Efficacy: Cohort 1 : 37 R/R pts have been enrolled. Baseline characteristics are summarized in Table 1A. 29 (78%) pts completed at least 2 cycles and are evaluable for response: 4 achieved complete remission (CR)/CR with incomplete hematologic recovery (CRi) (2/2) (14%) (Table 1B), 1 partial remission (PR) (4%), 4 hematologic improvement (HI) (14%), and 7 stable disease (SD) for at least 6 cycles (24%). The median # of cycles to response was 4 (range, 2-6). The 4- and 8-week mortality were 8% [all with rapidly progressive disease (PD): 2 received AZA for 3 and 5 days only] and 13%, respectively. With a median follow-up of 14.9 months (mos), the median overall survival (OS) for the whole cohort, responders + SD, and CR/CRi/PR is 10.8 mos (40% 1-yr), 13.9 mos (51% 1-yr), and 17.2 mos (75% 1-yr). The median event-free survival (EFS) is 6 mos, for all responders + SD 8.7 mos versus 2.6 mos for others (P

Description: Background: Patients (pts) with higher-risk (HR)-MDS in whom HMAs fail have a dismal prognosis with a median overall survival (OS) of 6 months; one had a prolonged SD after an IRAE, and one experienced an ongoing SD for 〉16 months. Three pts underwent alloSCT post ipilimumab without any additional toxicities and remain in complete remission at 2, 12 and 18 months post alloSCT, respectively. Median and mean OS for entire cohort (censoring at time of alloSCT) was 368 and 352 days, respectively (95%CI for mean OS, 264-440 days) [Figure 2]. Correlative studies evaluating dynamic changes in T-cell subsets, myeloid derived suppressor cells, cytokine levels, and T-cell receptor repertoire are in progress. Conclusions: Monotherapy withanti-CTLA-4 antibody ipilimumabat 3mg/kg is tolerable and can lead to prolonged disease stabilization. This dose is currently being evaluated further in a dose-expansion multi-center study. AlloSCT appears feasible post ipilimumab use but further data are needed. These results provide rationale for further exploration of immune checkpoint inhibition therapy in pts with MDS. Figure 1. Administration schedule Figure 1. Administration schedule Figure 2. Overall survival from first dose (censored at alloSCT or 6/30/2015). Figure 2. Overall survival from first dose (censored at alloSCT or 6/30/2015). Disclosures Off Label Use: Ipilimumab for MDS. Gore:Celgene: Consultancy, Honoraria, Research Funding.

Description: Background: Outcomes of patients (pts) with refractory and relapsed higher-risk myelodysplastic syndromes (HR-MDS) and acute myeloid leukemia with trilineage dysplasia (AML-TD) are dismal and therapeutic options are limited. Two phase 2 clinical trials reported clinical benefit using single agent high dose (50 mg) lenalidomide in elderly pts with untreated AML with and without deletion 5q (del5q), but this approach has not been formally studied in pts with AML-TD or HR-MDS who were refractory or relapsed following frontline therapy. Methods: This single-arm phase II study conducted at Johns Hopkins University enrolled pts with HR-MDS defined as IPSS high and INT-2 and pts with AML-TD regardless of karyotype. Pts received 2 cycles of lenalidomide at 15 or 50 mg daily on days 1-28 out of 42-day cycles; the protocol was originally designed at 15 mg/day, but dose was increased to 50 mg/day upon publication of safety/efficacy data in AML. Up to 3 weeks off treatment were allowed for count recovery but dose reductions for hematologic toxicity were not allowed. Responses were evaluated using the International Working Group 2006 criteria (IWG-2006). Pts with stable disease were allowed to receive 2 additional cycles. Responding pts were allowed to continue to receive drug until progression. The primary endpoint was overall hematologic response rate (ORR) following administration of 2-4 cycles of lenalidomide. Toxicities were graded according to CTCAE 3.0 criteria. Overall survival (OS) was calculated from first dose of lenalidomide and estimated by Kaplan-Meier (KM) methods. The study protocol was approved by Johns Hopkins Institutional Review Board. The study was sponsored by Celgene Corporation. Results: Twenty seven pts were enrolled between July 2009 and March 2014, of whom 9 received the 15 mg dose and 18 received the 50 mg dose. Median age was 72 years (range, 47-88 years); 76% were ≥65 years; 16 pts had HR-MDS and 11 had AML-TD. No pt had isolated del5q but 2 pts had del5q as part of complex karyotype; 41% had poor-risk karyotypes. Median bone marrow blast percentage was 15% for the entire cohort (range, 2-90%) and 40% for AML-TD pts. Median platelet count was 37×10⁹/L (range, 3-293×10⁹/L), and 63% of pts had platelet count of 50 or lower. Median white blood cell count was 2.4×10⁹/L (range, 0.5-6.4×10⁹/L) while median hemoglobin was 9.2 gm/dL (range, 6.7-14). Median number of prior regimens was 1 (range, 1-4). Median follow-up on study was 67 days (range, 9-245). Among the 9 pts who received 15 mg dose, 5 (55.6%) completed ≥2 cycles, none achieved an objective response, and none had grade 3 or 4 non-hematologic toxicity that was likely related to lenalidomide. Among the 18 pts treated at 50 mg dose, all had grade 3 or 4 non-hematologic toxicity, most commonly neutropenic infections, and only 7 pts (38.9%) completed ≥2 cycles of therapy. Among 18 pts treated on the 50 mg dose, ORR was 11% (both partial responses) and therefore the study was stopped due to futility. Both responders had AML-TD; one had normal karyotype and the other had monosomy 7. Overall, 30- and 60-day mortality rates were 14.8% and 29.6%, respectively. Median OS for the entire cohort [Figure 1] was 114 days (range, 15-841). Five pts (18.5%) survived ≥1 year; of those 1 pt got salvage low dose cytarabine; another underwent bone marrow transplantation, whereas the others chose supportive care alone. Conclusions: High dose lenalidomide(50 mg daily) administered on a 28/42-day cycles in pts with relapsed/refractory HR-MDS or AML-TD is poorly tolerated and associated with signficant toxicity and minimal activity. Further exploration of high dose lenalidomide in pts with AML-TD and HR-MDS should probably employ an intermittent dosing schedule and focus on intensive chemotherapy-ineligible untreated pts with AML. Disclosures Off Label Use: Use of high dose lenalidomide for higher risk MDS and AML. Gore:Celgene: Consultancy, Honoraria, Research Funding.

Description: All-trans retinoic acid (ATRA) causes terminal differentiation and apoptosis of non-APL AML cells in vitro, but has not proven clinically effective. We have recently shown that BM stroma expressing CYP26 inactivates ATRA and protects leukemia cells from differentiation (Su M et al 2015). ATRA signals through retinoic acid receptors (RARs) α, β and γ and induces not only terminal differentiation of AML, but also upregulation of CYP26, potentially forming an even more protective niche for leukemia. In an effort to decouple these two biological effects and improve differentiation therapy for AML, we investigated the relative contribution of RARα and RARγ to differentiation and CYP26 upregulation. ATRA (pan-RAR agonist), AM80 (strong RARα agonist, weak RARγ), and IRX5183 (RARα-specific agonist; previously known as NRX 195183, Io Therapeutics, Inc.) all induced significant differentiation of NB4 APL cells (i.e., upregulation of myeloid differentiation antigen CD11b and decreased clonogenic activity) at 0.1 μM. In contrast, the RARγ-specific agonist CD437 (0.1 μM) produced no evidence of differentiation. To determine the impact of RARα and RARγ signaling on stromal CYP26B1 levels, we treated murine OP9 BM stroma cells in low serum conditions with the aforementioned agents. ATRA induced a 40.5±30.1 fold upregulation of CYP26B1 at 24h (p=0.02 compared to control) and 24.7±20.4 fold upregulation at 48 hours (p=0.03). CD437 showed progressive upregulation of CYP26B1 (14.4±5.2 fold at 24h and 27.7±8.8 fold at 48h, p

Description: Mutations in the nucleophosmingene (NPM1) are common in patients with cytogenetically-normal acute myeloid leukemia (AML) and have been associated with positive outcomes. Although most leukemic stem cells (LSCs) arise from CD34+ primitive progenitors, our recent data suggest that a fraction arise from less primitive CD34- progenitors and seem to have a better prognosis (Gerber et al. Haematologica 2016). Conversely, poor-risk AMLs such as those harboring internal tandem duplications of the FMS-like tyrosine kinase 3 (FLT3-ITD) or with poor-risk cytogenetics appear to arise from LSCs with a primitive hematopoietic stem cell (HSC) phenotype. NPM1-mutated AMLs can be either CD34- or CD34+, and have been associated with variable prognoses. We hypothesize that the cell of origin, CD34- or CD34+, of NPM1-mutated AML has prognostic implications. A retrospective cohort analysis was conducted using electronic medical record data from patients aged 18 or older with newly-diagnosed NPM1-mutated AML treated with intensive induction chemotherapy at Johns Hopkins Hospital from 2008-2015. NPM1 and FLT3-ITD mutations were determined from PCR assays on diagnostic samples. We (Gerber et al. Haematologica2016) and others have shown that CD34+ cells represent normal, non-leukemia progenitors when they comprise /= 1% MNCs was the cut-off used to distinguish CD34+ from CD34- AMLs. Clinical outcomes including complete remission (CR) rate were compared using Chi-squared tests and event-free survival (EFS) and overall survival (OS) were estimated per the Kaplan-Meier method, using Wilcoxon tests to determine the impact of CD34 status. Of 46 consecutive, newly diagnosed NPM1-positive patients treated between 2008-15, 36 (78%) had normal cytogenetics and 37 (80%) had de novo AML. 26/46 (57%) had CD34+ disease; of these, 18/26 (69%) had FLT3-ITD mutations, compared to 9/20 (45%) with CD34- AML (p=0.10). In the CD34- patients with FLT3-ITD, the mean allelic ratio (mutant to wild type allele) ±SEM was 0.26±.08 compared to 2.3±1.0 in the CD34+ patients (p=0.07). Comparing CD34+ and CD34- patients, there were no significant differences in mean age at diagnosis (52 vs 59), sex, initial hemoglobin, platelet count, or lactate dehydrogenase, but CD34+ patients had significantly higher baseline white blood cell count (mean 78 vs 38 x 103 cells/mm3; p=0.04). There was a trend toward lower CR rates following induction for CD34+ patients (69 vs 89%; p=0.11), and similar proportions of patients underwent bone marrow transplantation (BMT) (50% vs 37%, p=0.38). However, CD34+ patients had significantly shorter EFS (figure 1; median 8.0 vs 34.3 months; p=0.04), and a trend toward inferior OS (15.4 vs 35.4 months; p=0.36). The lack of statistical significance in OS may reflect effective salvage by allogeneic transplantation. In conclusion, in this small retrospective cohort analysis of NPM1-mutated AML patients, CD34 expression was associated with significantly reduced EFS and, among FLT3-ITD mutated patients, a trend toward higher allelic ratios. Hence, CD34 positivity may be an early indicator of poor prognosis in NPM1+ AML and requires further characterization to determine its full prognostic and predictive value in guiding AML therapy. These data support evidence that the biology of AML is not only a function of genetic mutations, but also in part a functionof the stage of hematopoietic differentiation at which the leukemogenic mutations develop. Figure 1 EFS of NPM1-mutated AML patients by CD34 expression. Median follow-up of 23 months. EFS was significantly shorter for CD34+ patients (median 8.0 vs 34.3 months; p=0.01). Figure 1. EFS of NPM1-mutated AML patients by CD34 expression. Median follow-up of 23 months. EFS was significantly shorter for CD34+ patients (median 8.0 vs 34.3 months; p=0.01). Disclosures No relevant conflicts of interest to declare.

Description: Background: We recently showed that early lymphocyte recovery (ELR) following intensive induction chemotherapy in AML patients (pts) is characterized by complex immune system aberrations. (Blood 117(2):608, 2011) Full characterization of lymphoid T cell dynamics during this period can provide critical insights into their dysfunction and rationale for targeted therapeutic intervention to augment anti-leukemia immunity. Pomalidomide (POM), a small molecule immunomodulatory agent (IMiD), has direct effects on T cell co-stimulation by promoting the ubitiquitination of IL-2 transcriptional repressor Aiolos (IKZF3). (Br J Haematol 164(6):811, 2014) We hypothesized that administration of POM at the time of ELR may influence T cell differentiation and function in vivo. Methods: We serially collected peripheral blood (PB) samples at the time of ELR from 31 AML pts (median age 57, range 29-76 years; 26 de novo and 5 secondary AML) treated with intensive induction chemotherapy (18 with timed-sequential therapy (TST) and 13 with standard 7+3: cytarabine and idarubicin), 11 AML pts (median age 52, range 31-66 years; 6 de novo, and 5 secondary AML) treated on NCI/CTEP#9524 study of TST induction (AcDVP16: cytarabine, daunorubicin, and etoposide) followed by POM given daily for 10 days at ELR at escalating doses: 2mg (3pts)-4mg (3pts)-8mg (5pts) (Figure 1) and 17 healthy controls (HC) (median age 40, range 25-71 years). ELR was defined as absolute white blood cell (lymphocyte) count 〉200/mm3 above nadir. Using multi-parameter flow cytometry we performed extensive phenotypic characterization of lymphocyte populations, focusing on T cell differentiation status (CD45RA, CCR7), activation/proliferation (Ki-67), expression of POM-target gene Aiolos (IKZF3), and cytokine secretion. Statistical significance was determined using multiple t-tests using GraphPad Prism software. All protocols were IRB approved. Results: Administration of POM at ELR did not affect absolute lymphocyte count (ALC) compared to control pts not receiving POM (P=〉0.5). ALC values ranges (both groups): pre-treatment 2538-3645/mm3, ELR 428-525/mm3, and 931-1360/mm3 at full count recovery (FR1), respectively. The percentages and total numbers of CD4+ and CD8+ T cells behaved similarly; however, POM pts had increase in the frequency of CD4+ central memory subset (CD45RA- CCR7+) (P=0.001) at FR1 and decrease in the terminally differentiated effector memory subset (CD45RA+ CCR7-) of CD8+ T cells only for the duration of POM administration at ELR (P=0.04). Tregs (CD4+ FoxP3+ Tcells) increased in control pts during ELR (p=0.004; n=26) but not in POM-treated group (n=5). A dramatic but dose-dependent decrease of Aiolos expression in T cell subsets in vivo (P

Description: Background: Concerns have been raised whether immune checkpoint inhibitor therapy in the alloBMT setting will result in graft versus host disease (GvHD) and transplant related mortality (TRM). We report our experience with a variety of checkpoint inhibitors used before or after allogeneic bone marrow transplantation (alloBMT). Our series comprises patients who received T cell-replete hematopoietic stem cells from HLA-haploidentical or -matched donors and is limited to those treated with post-transplant cyclophosphamide (PTCy) as primary GvHD prophylaxis. Patient selection: We retrospectively reviewed the records of alloBMT recipients who received PTCy and received checkpoint inhibitor therapy before or after alloBMT. GvHD was assessed using the CIBMTR GVHD index. Results: Eleven patients received checkpoint inhibitor therapy prior to alloBMT: anti-PD-1: Nivolumab n=6, anti-CTLA4: Ipilimumab n=8 (3 patients received both nivolumab and ipilimumab). These patients received a median of 4 (range 1 - 18) cycles of therapy. The median interval from last checkpoint inhibitor treatment to day of transplant was 43 (range 18-302) days. All patients received nonmyeloablative conditioning; 6 received partially mismatched allografts (5 were HLA haploidentical). Four patients developed Grade II aGvHD: Three patients who had received partially mismatched allografts (haplo-2, 9/10 unrelated-1) experienced stage 3 cutaneous GvHD only; one patient who received a 10/10 unrelated donor allograft developed stage 3 cutaneous GvHD with stage 1 liver involvement. Three patients were on immunosuppression when GvHD developed, the fourth patient with cutaneous and liver GvHD had been taken off tacrolimus on day 68 due to concerns of graft failure. GvHD resolved with treatment in each case. None of these patients developed chronic GvHD and none have died [median follow-up of 0.66 (range 0.91 - 2.0) years post alloBMT]. Nine patients received checkpoint therapy following alloBMT: anti-PD-1: Pembrolizumab n = 1, Nivolumab n= 6, anti-CTLA4: Ipilimumab n= 3 (one patient received nivolumab and ipilimumab). Eight patients had received nonmyeloablative conditioning; 5 received haploidentical allografts. Six received treatment for relapse of their hematologic malignancy, 1 for relapsed pediatric sarcoma, and 2 for newly diagnosed lung cancer. The median time to initiation of checkpoint inhibitor therapy was 1.2 (range: 0.8 - 5.8) years post alloBMT. Patients received a median of 5 (range 1 - 24) cycles of therapy. There was 1 case of Grade II aGvHD; stage 3 cutaneous GvHD when DLI from a 10/10 matched unrelated donor was given for relapsed disease after ipilimumab. This resulted in GvHD which was not accompanied by the desired graft-vs-leukemia effect. There were no other cases of acute or chronic GvHD in this group. There were 4 tumor-related deaths: pediatric sarcoma (1), lung cancer (1), and AML (2). The median follow-up for this group is 2 years (range 0.85 - 8.0) post alloBMT. Conclusions: In this small series, the incidence and severity of GvHD seen in patients who received checkpoint inhibitors was similar to that seen in patients treated with PTCy as GvHD prophylaxis without checkpoint inhibitors. GvHD was seen in patients treated with checkpoint inhibitors prior to alloBMT, but was generally mild and readily controlled and there were no associated deaths. In patients treated with checkpoint inhibitors after alloBMT, the only case of GvHD occurred after the patient received DLI. We caution that use of checkpoint inhibitors in closer temporal proximity to transplant might well be associated with increased risk of GvHD or severity of GvHD. Disclosures Borrello: WindMIL Therapeutics: Equity Ownership, Patents & Royalties, Research Funding; Celgene: Honoraria, Research Funding, Speakers Bureau; BMS: Honoraria, Research Funding. Wagner-Johnston:Seattle Genetics: Research Funding. Smith:Celgene: Consultancy, Other: member of DSMB.

Description: BACKGROUND: Acute myeloid leukemia (AML) is an aggressive leukemia with 5-year overall survival of 20-25%. The major reason for treatment failure in AML is resistance to chemotherapy. Thus, there is an urgent need for identification of novel therapeutic agents for AML. Neoplastic cells, including AML, have dysfunctional redox regulation that results in increased reactive oxygen species (ROS). Accumulation of ROS leads to oxidation of free and incorporated nucleotides, leading to DNA damage and cell death. MTH1 is a nudix family hydrolase that sanitizes the oxidized nucleotide pool to prevent incorporation of these damaged bases in the DNA. MTH1 is thought to be non-essential for normal cells but crucial for neoplastic cells in order to avoid incorporation of oxidized dNTPs into DNA, thereby evading DNA damage and cell death. Whether MTH1 inhibitors have any activity against AML is not known. METHODS: Neoplastic myeloid cell lines HL-60, HEL, K562, KG1A, ML1, MV-4-11, SET2, and U937 were treated with varying concentrations of TH588 for a total of 48 hours. In experiments using the pan-caspase inhibitor Q-VD-OPh (Qvd), cells were pre-treated with 5µM Qvd for 1 hour followed by TH588. Cells were washed and stained with annexin, propidium iodide (PI), or MitoTracker (Life Technologies, Carlsbad, CA) for flow cytometry. To evaluate the potential impact of MTH1 inhibition on chemorefractory AML, HL-60/VCR cells were treated with vehicle control or TH588 in culture medium with or without vincristine (1µg/ml). Percentage apoptosis was calculated by normalizing to vehicle only control. With IRB approval, bone marrow aspirate samples were obtained from patients with untreated AML or healthy controls. Mononuclear cells were analyzed using colony-forming unit (CFU) assays. The total number of erythroid (CFU-E) and myeloid (CFU-G, CFU-GM) colonies containing ≥50 cells were read on day 14 and reported as percentage colonies compared to vehicle control. RESULTS: TH588 induced dose-dependent cell death in each of the neoplastic cell lines tested except HEL. In particular, treatment with TH588 resulted in a dose-dependent increase in the number of cells undergoing apoptosis as indicated by annexin V and/or PI staining (IC50 3.1-21.3µM, Figure 1). Pre-treatment with Qvd significantly inhibited TH588-induced cell death in all the cell lines studied except KG1A and SET2, suggesting a caspase-dependent mechanism of cell death. In further studies, cells treated with TH588 exhibited decreased MitoTracker staining; and Qvd pretreatment increased the number of MitoTrackerLow cells at the same time apoptotic cells decreased, suggesting that mitochondrial damage is upstream of caspase activation in TH588-induced apoptosis. Treatment with TH588 not only induced apoptosis in HL-60/VCR cells, but also facilitated further apoptosis in cells co-treated with vincristine and TH588 (Figure 2). Treatment with TH588 also diminished colony formation in a primary AML sample (IC50 6µM, Figure 3). Analysis of additional primary AML samples is ongoing. DISCUSSION: Our results show that the MTH1 inhibitor TH588 induces apoptosis in most neoplastic myeloid cells. MTH1 causes mitochondrial damage that, in turn, leads to caspase-dependent apoptosis in these cells. In HL-60/VCR cells representing chemorefractory phenotype, TH588 induces apoptosis as a single agent and resensitizes cells to vincristine. Moreover, TH588 significantly diminished colony formation in primary AML ex vivo. Further preclinical and possible clinical study of this class of agent appears warranted. Figure 1. Induction of cell death by MTH1 inhibitor TH588 in neoplastic myeloid cell lines. Figure 1. Induction of cell death by MTH1 inhibitor TH588 in neoplastic myeloid cell lines. Figure 2. TH588 induces apoptosis in HL-60/VCR cells and resensitizes cells to vincristine. Figure 2. TH588 induces apoptosis in HL-60/VCR cells and resensitizes cells to vincristine. Figure 3. TH588 significantly diminished colony formation in primary AML ex vivo indose-dependent manner. Figure 3. TH588 significantly diminished colony formation in primary AML ex vivo indose-dependent manner. Disclosures No relevant conflicts of interest to declare.