ALBERT

Description: Introduction: Current therapy for acute myeloid leukemia (AML) is inadequate. Treatment of older patients considered unfit for standard induction therapy is particularly challenging (Juliusson et al. Blood 2009). Hypomethylating agents (HMA) are commonly used alternatives for these patients (Fenaux et al. JCO 2010). In the case of azacitidine, complete and partial response rates range between 15-30% (Maurillo et al. Cancer 2012) and the addition of other drugs (e.g. lenalidomide and vorinostat) has been limited by toxicity. In this study, we sought to improve the activity of azacitidine by adding the non-steroidal anti-inflammatory drug (NSAID), sodium salicylate (NSal). NSAIDs exert anti-cancer effects through impaired signal transduction (inhibition of NF-kB and Wnt/B catenin pathways) (Kop et al. Science 1994; Reya et al. Nature 2003), epigenetic modulation and disruption of cellular metabolism (activation of AMPK) (Wang et al. CMLS 2013). In addition, previous reports have demonstrated that NSal combines synergistically with established anti-leukemic agents (daunorubicin) (Klampfer et al. Blood 1999), through down-regulation of the anti-apoptotic protein Mcl-1. Importantly, NSal has no effect on platelet function and a small pilot trial (N=11) confirmed the clinical feasibility of this agent in patients with refractory myeloid neoplasms (Klimek et al. Leukemia Research 2012). In this study, therapeutic plasma concentrations of NSal were safely achieved without excessive NSAID class effects (GI and renal toxicity, bleeding). Here, we report the in vitro effects of NSal combined with azacitidine and suggest a rationale for exploring this combination further in the clinic. Methods: AML cell lines (HL-60, KG-1a and THP-1) were treated with increasing concentrations of NSal (0.2–20mM), azacitidine (0.02-10µM) and both agents combined. Consequent effects on cell viability were measured by trypan blue exclusion and cell proliferation assay (MTT based). Apoptosis was measured by FACS analysis for propidium iodide (PI) and annexin V binding. Synergistic lethality was calculated using the Chou-Talalay method (Calcusyn software). Critical mediators underlying mechanism were measured by immunoblotting (Mcl-1). Results: The degree of apoptosis in AML cells treated with sub-lethal concentrations of NSal (0.2-20mM) and azacitidine (0.02-10µM) was compared to the degree of apoptosis induced by the combination. Little effect on the viability of HL-60, KG-1a and THP-1 cell lines was observed after 48 hours exposure to either agent alone, however, in the presence of 2 µM azacitidine and 2mM NSal, 39.2% of THP-1 cells were dead at 48 hours (annexin V binding). The combination of 0.2 µM azacitidine and 2mM NSal had significantly inhibited cell proliferation at 72 hours. Cell proliferation decreased by 47.4%, 25%, and 66.1% in HL-60, KG-1a, and THP-1, cell lines, respectively. Combination index (CI) values (Calcusyn software) were less than 1 indicating synergistic activity. The expression of Mcl-1 decreased in a dose- and time-dependent manner in salicylate treated cells which might account for its ability to enhance cell killing by azacitidine. Conclusions: Sodium salicylate has a priming effect when combined with azacitidine in pre-clinical models of AML. Despite the high pre-clinical concentrations of NSal evaluated, previous studies have confirmed the feasibility of achieving therapeutically active plasma concentrations in patients (Klimek et al. Leukemia Research 2012). This study forms a rationale for a clinical investigation of this approach. Additional in vitro mechanistic studies elucidating the combined effects of NSal and azacitidine will be presented. Disclosures No relevant conflicts of interest to declare.

Description: Microarray - assisted gene - expression screens of human macrophages revealed WNT5A, a homolog of Wingless, a key regulator of Drosophila melanogaster embryonic segmentation and patterning, to be consistently up-regulated following stimulation with different mycobacterial species and conserved bacterial structures. The expression of WNT5A required Toll-like receptor signaling and NF-κB activation, which identifies a novel induction pathway for a Wingless homolog. We show that human peripheral-blood mononuclear cells express the WNT5A receptor Frizzled-5 (FZD5). Both WNT5A and FZD5 also were detected in granulomatous lesions in the lungs of Mycobacterium tuberculosis–infected patients. Functional studies showed that WNT5A and FZD5 regulate the microbially induced interleukin-12 response of antigen-presenting cells and interferon-γ production by mycobacterial antigenstimulated T cells. Our findings implicate the evolutionarily conserved WNT/Frizzled signaling system in bridging innate and adaptive immunity to infections.

Description: Background: Several novel therapeutics are being developed for AML with demonstrable effects on response rates (e.g. complete remission [CR], CR with incomplete hematologic recovery [CRi], CR with incomplete platelet recovery [CRp], overall response rate [ORR = CR+CRi/CRp]), and EFS. Improvement in EFS recently served as the basis of approval of gemtuzumab ozogamicin (GO) for the treatment of newly-diagnosed CD33-positive AML in adults (Jen et al. Clin Cancer Res 2018). The relationship between response rate, EFS, and OS in newly-diagnosed AML has not been conclusively established. Therefore, we conducted trial-level and patient-level meta-analyses of newly-diagnosed AML trials submitted to the FDA. Methods: We searched for trials submitted with Biologics License or New Drug Applications for treatment of newly-diagnosed AML between 2007 and 2017. Criteria for inclusion were randomized, active-controlled, multicenter trials of intensive AML induction and consolidation chemotherapy. The estimated odd ratios (OR - ratio of odds of response in treatment to odds of response in controls) of CR and ORR and hazard ratios (HR - ratio of hazard of treatment versus control) for EFS and OS were calculated for each study. EFS was defined as time from randomization to treatment failure (defined as date of randomization for patients who failed to achieve CR following induction), disease relapse following CR, or death. Associations between treatment effects at the trial-level were evaluated using weighted least square (WLS) regression analyses on the log-scale (weighted by sample size of each randomized comparison). Coefficient of determination (R2) and 95% confidence interval (CI) were calculated to measure the strength of associations. Individual patient data for OS were plotted against EFS to explore their relationship. An exploratory patient-level responder analysis was performed to compare OS between responders and non-responders, regardless of treatment assignment in the pooled dataset. We estimated HRs of OS from Cox proportional hazards models. OS estimates by response were obtained based on the Kaplan-Meier method. Results: We identified 8 trials with a total of 4482 patients and 3 experimental agents (GO, n=5; [daunorubicin and cytarabine] liposome injection, n=2; midostaurin, n=1) for treatment of newly-diagnosed AML. Two trials tested therapy in defined patient populations (FLT3 mutant = 1 and secondary AML = 1). The association between OS HR and CR OR at the trial-level was moderate (R2 = 0.67, 95% CI: 0.16 - 0.94; Figure 1), whereas the association between OS HR and ORR OR was weak (R2 = 0.43, 95% CI: 0.03 - 0.98). For the OS vs. EFS HR analysis, a weaker than expected association was observed (data to be presented at the meeting). Individual patient level data scatter plots suggested that one possible reason for the lack of a strong association between EFS and OS was that early failures and relapses did not always result in worse OS (Figure 2). In the patient-level responder analysis, patients who achieved a CR response had better OS compared with CRi+CRp response and no response (Figure 3). Conclusions: On a trial level, the meta-analysis of randomized, active-controlled trials in newly-diagnosed AML suggests a moderate association between OS and CR rate, but not ORR. A strong association between EFS and OS is not established and may be confounded by improvements in salvage therapy, supportive care, hematopoietic stem cell transplantation, and/or differing censoring across trials. A patient-level responder analysis showed that CR responders have better OS compared with CRi+CRp responders and nonresponders. While our results are limited by the number of trials, they suggest that CR remains the response endpoint associated with greatest long-term benefit and that EFS, while clinically meaningful, is not a surrogate for OS. Disclosures No relevant conflicts of interest to declare.

Description: Background: The PD-1 inhibitors release T cells from PD-L1 and PD-L2 mediated blockade, resulting in enhancement of immune responses. These immune responses include not only antitumor activity but also autoimmune reactions. The most serious toxicities of PD-1 inhibitors are immune-mediated, consistent with their mechanism of action. The purpose of this retrospective analysis was to characterize the immune-related adverse reactions (IMARs) of PD-1 inhibitor therapy in patients with cHL. Methods: Data were pooled from three single-arm trials in adults with multiply-relapsed or refractory cHL treated with nivolumab or pembrolizumab at their approved doses. The pool included 243 patients from Study CA209-205 (NCT02181738), 23 patients from Study CA209-039 (NCT01592370), and 210 patients from Study MK-3475-087 (NCT02453594). IMARs were ascertained using grouped MedDRA terms for the following categories: adrenal insufficiency, cardiac disorder, diabetes, diarrhea, encephalitis, hepatitis, hypophysitis, infusion reaction, kidney injury, myositis, pancreatitis, pneumonitis, rash, thyroid disorder, and uveitis. The pooled data set was queried for the incidence, timing, and outcome of IMARs in the population overall. Key patient characteristics assessed as risk factors for IMARs included age, sex, race, ethnicity, weight, body mass index (BMI), B symptoms, bulky disease, prior irradiation, prior autologous stem cell transplantation (aHSCT), prior brentuximab use, and prior lines of therapy. To identify risk factors, multivariable models were developed using a backward-stepping elimination procedure after the initial selection using p-value 〈 0.2 in the univariable analysis. In the multivariable analysis, p-values 〈 0.05 were considered statistically significant. Results: The study cohort included 476 patients of median age 34 years (range, 18-76 years); 56% were male, and 87% were Caucasian. Treatment consisted of a median of 13 doses over a median of 9.3 months. An IMAR was reported in 301 (63.2%) patients, and the IMAR was grade 3-4 in 52 (10.9%) patients. No fatal IMARs were reported. There were 51 (11%) patients treated with corticosteroids, and 48 (10%) had study drug interrupted or withdrawn. The most common (〉10%) IMARs were rash (28.6%), diarrhea (26.5%), thyroid disorder (14.3%), infusion reaction (13.2%), and hepatitis (11.1%). The most common (〉1%) grade 3-4 IMARs were pancreatitis (3.6%), hepatitis (2.9%), and diarrhea (2.3%). The table below shows the IMARs listed in order of time to onset. Risk factors for the occurrence of any IMAR were prior aHSCT (p=0.004) and elevated BMI (p=0.049). When assessed by individual IMAR, the risk factors identified were age for cardiac disorder; prior aHSCT for diarrhea; race for hepatitis; race and weight for infusion reaction; number of prior lines of therapy and BMI for kidney injury; weight and B symptoms for pneumonitis; prior aHSCT and BMI for rash; and sex and prior radiation for thyroid disorders. No risk factors were identified for adrenal insufficiency, diabetes, hypophysitis, myositis, pancreatitis, or uveitis. Conclusions: The majority of patients with cHL treated with a PD-1 inhibitor experienced an adverse event consistent with an IMAR, although few events were grade 3-4. Most of these events started after 2-3 months of treatment, but overall, they occurred at any time during treatment. Prior aHSCT was identified as a significant risk factor for IMAR, although this was driven largely by the categories rash and diarrhea. Acknowledgment: Data were provided for nivolumab by Bristol-Myers Squibb Company and for pembrolizumab by Merck Sharp & Dohme Corp. Table. Table. Disclosures No relevant conflicts of interest to declare.

Description: 1. Five patients who died as a result of a variety of diseases, all characterized by severe anemia for which numerous transfusions had been given, and all of whom developed features of hemochromatosis are presented. 2. Eight similar cases found in the literature are summarized. 3. It is postulated that the hemochromatosis developing in these patients is the end result of the deposition and subsequently irritating action of the excess amounts of iron in the parenchymatous tissues. 4. The underlying anemia and the not infrequent transfusion reactions are thought to act as predisposing factors for the development of exogenous hemochromatosis. 5. The name Exogenous Hemochromatosis is proposed for this syndrome. 6. The clinical similarities and dissimilarities and the differences in pathogenesis between exogenous and endogenous hemochromatosis are discussed.

Description: There is a continued reliance on antibody therapies for treating chronic lymphocytic leukaemia (CLL). Whilst the inclusion of antibody therapies in many standard treatment regimes results in good outcomes, acquired resistance remains a significant clinical challenge for many CLL patients resulting in insensitivity to the antibody treatment. Thus, understanding the mechanisms driving treatment resistance is likely to lead to therapies to reverse resistance and improve patient outcomes. Earlier studies from our laboratory have shown that resistance to therapeutic antibodies, in CLL, is due to a reduced ability of monocyte derived macrophages (MDMs) to participate in FcγR-dependent antitumour responses (e.g. ADCC and antibody-dependent phagocytosis (ADP). In this regard, we recently showed that SYK and BTK activation are downstream of the FcγRs (Oncogene, 36(17):2366-2376, 2017). Moreover, we showed that signalling through the FcγR pathway was reduced in antibody-resistant MDMs and could be reversed using inhibitors of SHIP1. These studies indicated that knowledge of FcγR signalling could exploited to reverse resistance. Unfortunately, knowledge of the exact signalling events controlling FcγR activity in MDMs from CLL patients is unclear. In this study we investigated the involvement of PI3K isoforms in FcγR-dependent ADCC and ADP in MDMs from CLL patients. PI3K isoforms have been shown to be important pathway regulators for immune-receptor function in various immune cells such as T cells, B cells and NK cells as well as in cancerous cells. In the first instance, I examined the expression of PI3K isoforms in MDMs from CLL patients. This showed that PI3Kα, β, and δ are expressed in MDMs whereas PI3Kγ is below the limit of detection. Next, I examined the involvement of the different PI3K isoforms to contribute to FcγR-dependent ADCC by MDMs. For this we used a suite of isoform-selective inhibitors to target each PI3K isoform and examined their effect on ADCC responses by MDMs. The PI3Kδ-selective inhibitor, idelalisib and the pan PI3K inhibitor BKM120 (Buparlisib) were able to inhibit ADCC responses to the CD20-targeting therapeutic antibody, obinutuzumab. Similarly, both buparlisib and idelalisib were able to inhibit AKT phosphorylation at concentrations that also inhibited ADCC. In contrast. None of the other isoform-selective inhibitors were able to suppress ADCC responses to obinutuzumab. These studies have been repeated using isoform-specific siRNAs. This is the first report to show that PI3Kδ is involved in FcγR signalling in MDMs from CLL patients or in MDMs from any tumour type. Based on these findings we conclude that PI3Kδ is a critical effector molecule for antitumour responses to therapeutic antibodies in CLL. Disclosures Mollee: Amgen: Membership on an entity's Board of Directors or advisory committees; Celgene: 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. Gill:Pharma: Membership on an entity's Board of Directors or advisory committees.

Description: Abstract 4074 Aberrant protein folding results in the accumulation of misfolded/unfolded proteins in the endoplasmic reticulum (ER), which in turn triggers ER stress followed by unfolded protein response (UPR), an adaptive response against ER stress. Since multiple myeloma (MM) cells have high protein synthesis, they are sensitive to ER stress and require strict ER quality control for cell survival. Upon UPR, IRE1α is activated by auto-phosphorylation resulting in activation of its endoribonuclease domain to splice XBP1 mRNA from XBP1 unspliced form (XBP1u: inactive) to XBP1 spliced form (XBP1s: active). Since XBP1 is a transcription factor regulating genes which are responsible for protein folding and ER associated degradation (ERAD), IRE1α-XBP1 pathway acts as a pro-survival signaling pathway under the UPR condition. In this study, we examined whether IRE1α-XBP1 pathway is a potential novel therapeutic option in MM. We first examined IRE1α expression and confirmed its expression in all MM cell lines. In contrast, XBP1s was not detected by RT-PCR in most cell lines except in for RPMI8226 cells. To assess biologic significance of IRE1α in MM cell, we knock-downed its expression using shRNA and found that downregulation of IRE1α inhibited MM cell growth, indicating that IRE1α has a crucial role in MM cell survival. We next examined the impact of inhibition of XBP1 splicing by a small molecule IRE1α endoribonuclease inhibitor MKC-3946 (Mannkind, Valencia CA) in MM cells in vitro. As expected, MKC-3946 significantly inhibited tunicamycin-induced XBP1s without affecting phosphorylation of IRE1α. MKC-3946 induced only modest cytotoxicity in MM cell lines without toxicity in normal mononuclear cells from healthy donors; however, it significantly enhanced cytotoxicity in combination with bortezomib or 17-AAG. Both bortezomib and 17-AAG induced ER stress evidenced by induction of XBP1s; conversely, MKC-3946 blocked XBP1s triggered by these agents. Furthermore, apoptosis induced by these agents was enhanced with MKC-3946 associated with increased CHOP, which is a known pro-apoptotic protein induced in uncompensated ER stress condition. Importantly, MKC-3946 enhanced the cytotoxicity of bortezomib or 17-AAG in INA6 cells, even in the presence of increased IL-6 or bone marrow stromal cells. Finally, MKC-3946 was active inhibiting XBP1 splicing in a model of ER stress and significantly inhibited growth of RPMI8226 plasmacytoma in a xenograft murine model when used in combination with a low dose of bortezomib. Taken together, our results demonstrate that inhibition of XBP1 splicing by blockade of IRE1α is a promising therapeutic option in MM. Disclosures: Blumenthal: Mannkind Corporation: Employment, Equity Ownership. Tam:Mannkind Corporation: Employment, Equity Ownership. Kertesz:Mannkind Corporation: Employment, Equity Ownership. Zeng:Mannkind Corporation: Employment, Equity Ownership. Patterson:Mannkind Corporation: Employment, Equity Ownership. Munshi:Celgene: Membership on an entity's Board of Directors or advisory committees; Millennium: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees. Richardson:Celgene: Membership on an entity's Board of Directors or advisory committees; Millennium: Membership on an entity's Board of Directors or advisory committees; Johnson & Johnson: Membership on an entity's Board of Directors or advisory committees. Anderson:Celgene: Membership on an entity's Board of Directors or advisory committees; Millennium: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees.

Description: Background: Philadelphia chromosome-like acute lymphoblastic leukemia (Ph-like ALL) is a high-risk subtype occurring in 15-40% of older children, adolescents, and adults with B-ALL and is associated with high relapse rates and poor survival. Approximately 50% of Ph-like ALL cases have CRLF2 (cytokine receptor-like factor 2 rearrangements (CRLF2-R), often with concomitant JAK2 point mutations. We and others have shown that targeted inhibition of JAK/STAT and PI3K/AKT signaling in CRLF2-rearranged Ph-like ALL only partially inhibits leukemia proliferation. Our preliminary data indicates that modified B cell receptor signaling results in ERK activation and Ph-like ALL cell growth that appears independent of CRLF2-activated signaling. This study aimed to define the extent to which BCR signaling mediates drug resistance in Ph-like ALL after cytokine receptor signaling inhibition and to develop novel combinatorial treatment strategies. Methods and Results: We first validated high basal levels of JAK/STAT and PI3K pathway phosphoproteins via immunoblotting of cell lysates from 〉20 primary childhood and adult Ph-like ALL samples and patient-derived xenograft (PDX) models. Despite lack of B-cell receptor (BCR) μ heavy chain on Ph-like ALL cells by flow cytometry analysis, we detected high expression levels of the BCR-associated proteins CD79A, CD79B, and BLNK. We further identified constitutive phosphorylation of downstream BCR-associated signaling molecules, including SRC family kinases (SFKs), BTK, and ERK. Given these new observations, we then sought to delineate more precisely alternative pathways in Ph-like ALL (beyond JAK/STAT and PI3K) that may contribute to adaptive signaling regulation and confer resistance to kinase inhibitor monotherapy. We hypothesized that JAK inhibitor (JAKi) resistance is mediated by compensatory activation of PI3K/AKT/mTOR through 'BCR-like' signaling via CD79/CD19/SRC/BTK/ERK. To test this hypothesis, we treated CRLF2-R Ph-like ALL cells in vitro with selective inhibitors of JAK1/2 (ruxolitinib) or PI3Kd (INCB050465) and assessed effects upon signaling via immunoblotting. We observed i) effective dephosphorylation of STAT5 and moderate dephosphorylation of AKT with full reactivation within 72h following JAKi or PI3Kdi monotherapy, respectively, ii) combined JAKi and PI3Kdi prevented PI3K pathway, reactivation, and iii) JAKi and PI3Kdi had no effects upon ERK signaling. These results suggested that oncogene-independent activation of BCR and PI3K pathway signaling occurs in Ph-like ALL. We then performed immunofluorescence analysis and detected distinct clustering and homodimerization of CD79B at the cell membrane of Ph-like ALL cells, as well as physical proximity of CD79B and CD19, suggesting that CD19 could be phosphorylated by SRC/SFKs independently of an activated BCR. Interestingly however, CRISPR-mediated deletion of CD19 or CD79B resulted in hyperactivation of ERK signaling. Finally, we tested if triple inhibition of JAK/STAT, PI3K, and SRC/SFKs signaling is sufficient to eradicate Ph-like ALL. Strikingly, combined JAKi (ruxolitinib), PI3Kdi (INCB050465), and SRCi (dasatinib) silenced STAT5, PI3K, and ERK signaling and induced near-complete cytotoxicity in vitro and in vivo in Ph-like ALL cell lines and patient-derived xenograft (PDX) models. While triple inhibitor treatment appeared well-tolerated in mice, we also assessed dual kinase inhibition of JAK2 and PI3K with dexamethasone (a known repressor of BCR signaling) as a more clinically relevant combinatorial strategy. We observed similarly robust inhibition of signaling pathways and leukemia eradication in vitro and in vivo in our Ph-like ALL cell lines and PDX models. Conclusions: Children and adults with Ph-like ALL have a 〉60% risk of relapse despite maximally intensive chemotherapy, demonstrating need for alternative therapeutic approaches. While earlier studies by our group and others have reported partial anti-leukemia activity of JAKi and PI3Ki monotherapies, our new data provide compelling evidence that three essential cooperating signaling pathways are required for Ph-like leukemogenesis. We postulate that triple JAK, PI3K, and SRC inhibition or dual kinase inhibition with chemotherapy can overcome innate compensatory mechanisms of signaling resistance and may in fact be necessary to eradicate Ph-like ALL in patients. Disclosures Perl: Daiichi Sankyo: Consultancy, Honoraria, Other, Research Funding; Arog: Consultancy, Other: Non-financial support included travel costs for advisory board meetings.; AbbVie: Consultancy, Honoraria, Other: Non-financial support included travel costs for advisory board meetings.; Actinium Pharmaceuticals: Consultancy, Honoraria, Other: Clinical Advisory Board member, Research Funding; Agios: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: Non-financial support included travel costs for advisory board meetings.; Jazz: Consultancy, Honoraria, Other: Non-financial support included travel costs for advisory board meetings.; NewLink Genetics: Consultancy, Honoraria, Other: Non-financial support included travel costs for advisory board meetings.; Astellas: Consultancy, Honoraria, Other: Non-financial support included travel costs for advisory board meetings as well as a medical writing company that assisted with manuscript preparation/submission and slide deck assembly for academic meeting presentations of trial data., Research Funding; Takeda: Consultancy, Honoraria, Other: Non-financial support included travel costs for advisory board meetings.; Bayer: Research Funding; BioMed Valley Discoveries: Research Funding; FujiFilm: Research Funding; Novartis: Honoraria, Other: Advisory board, Non-financial support included travel costs for advisory board meetings as well as a medical writing company that assisted with manuscript preparation/submission and slide deck assembly for academic meeting presentations of the data., Research Funding. Carroll:Astellas Pharmaceuticals: Research Funding; Incyte: Research Funding; Janssen Pharmaceuticals: Consultancy. Tasian:Gilead Sciences: Research Funding; Incyte Corportation: Research Funding; Aleta Biotherapeutics: Membership on an entity's Board of Directors or advisory committees.

Description: Background: ATO was approved by the U.S. Food and Drug Administration in 2000 for treatment of patients with relapsed/refractory APL and in 2018 in combination with all-trans retinoic acid (ATRA) for adults with newly-diagnosed low-risk APL. ATRA and ATO combinations result in long-term remissions in most patients with APL (Lo-Coco et al. NEJM 2013, Burnett et al. Lancet Oncol 2015). However, epidemiologic studies have shown associations between exposure to inorganic arsenic and development of skin, lung, bladder, and potentially liver, kidney, and prostate cancers (IARC 2004). The prescribing information for ATO includes a warning for carcinogenesis, with advice to monitor patients for development of second primary malignancies. Retrospective cohort analyses of second cancers in APL patients treated with ATO have been performed, finding incidence rates of 1-5% (Eghtedar et al. Leuk Lymphoma 2015, Au et al. Leuk Res 2007, Zhu et al. Blood 2016). We sought to perform an exploratory population-based analysis of second cancers in adults with APL treated with ATO compared to those treated with other systemic APL therapies without ATO using the linked Surveillance, Epidemiology, and End Results (SEER)-Medicare database. Methods: Using SEER-Medicare linked data, we identified APL patients diagnosed from January 1, 2006 to December 31, 2015. Patients whose first SEER cancer diagnosis was APL, who were continuously enrolled in Medicare Parts A, B, & D from the month of their APL diagnosis, and who received treatment with ≥ 1 systemic APL therapy within 1 year of diagnosis were included. Patients were followed from their first month of treatment group-defining APL therapy (ATO or non-ATO containing) until disenrollment from Medicare Parts A, B, or D, end of study period, or death. We determined the cumulative incidence of SEER-confirmed second cancers and overall survival (OS) according to whether patients were treated with or without ATO using the Kaplan-Meier method. We used a multivariate Cox proportional hazards model to estimate the hazard ratios (HR) and 95% confidence intervals (CI) of cumulative incidence of second cancers and OS adjusted for relevant covariates of age, sex, and year of diagnosis. Results: We identified 1,442 APL cases, with 1,179 having APL as their first cancer diagnosis, of which 246 were enrolled in Medicare Parts A, B, and D. Of these, 64 received ATO and 60 received systemic APL therapy without ATO within 1 year of diagnosis. Characteristics, follow-up, and second malignancies for these cohorts are presented in the Table. Absolute incidence rates of second cancer were 3.4 per 1000 person-months compared to 1.4 per 1000 person-months in patients treated with and without ATO, respectively, and cumulative incidence rates at 24 months were 9.9% and 6.0%, respectively (p=0.20) (Figure 1). Mortality rates were 1.9 per 1000 person-months compared to 5.1 per 1000 person-months in patients treated with and without ATO, respectively, and OS rates at 24 months were 90.8% and 81.5%, respectively (p=0.10) (Figure 2). After adjusting for relevant covariates, HR for cumulative incidence of second malignancies in patients treated with ATO was 1.27 (95% CI 0.29-5.49; p=0.75) and for OS was 0.46 (95% CI 0.16-1.29; p=0.14) compared to APL therapy without ATO. Conclusions: This exploratory analysis revealed a high incidence of second malignancies in APL patients treated with ATO, although the risk was not significantly increased compared to patients who received other APL therapies. Most second malignancies following ATO were solid tumors, in line with prior epidemiologic studies of inorganic arsenic exposures. Despite the occurrence of second malignancies, there was a tendency towards longer OS in patients treated with ATO. Given the small sample size, short follow-up, potential selection and immortal time bias, and unaccounted for differences between comparator groups, firm conclusions cannot be inferred. However, the nearly 10% cumulative incidence of second malignancies at 24 months follow-up in Medicare patients with APL treated with ATO suggests the need for close monitoring for second malignancies following ATO therapy. Further prospective research into second malignancies following ATO is needed. Disclosures No relevant conflicts of interest to declare.

Description: Abstract 133 Multiple myeloma (MM) cells are characterized by high protein synthesis resulting in chronic endoplasmic reticulum (ER) stress, which is adaptively managed by the unfolded protein response (UPR). Therefore blockade of UPR could provide a novel therapeutic option in MM. Upon UPR, inositol-requiring enzyme 1α (IRE1α) is activated by auto-phosphorylation, resulting in activation of its endoribonuclease domain to cleave XBP1 mRNA from XBP1 unspliced form (XBP1u: inactive) to generate the XBP1 spliced form (XBP1s: active). XBP1s protein in turn regulates genes responsible for protein folding and degradation, playing a pro-survival signaling role in the UPR. In this study, we specifically examined whether IRE1α-XBP1 pathway is a potential therapeutic target in MM. We first examined the biologic significance of IRE1α by knockdown using lentiviral shRNA and observed significant growth inhibition in IRE1α knockdown cells. We next examined the impact of inhibition of XBP1 splicing using a novel small molecule IRE1α endoribonuclease domain inhibitor MKC-3946 (MannKind, Valencia CA). MKC-3946 blocked not only the basal level, but also inducible (by tunicamycin) XBP1s, evidenced by RT-PCR analysis in RPMI8226 cells, without affecting phosphorylation of IRE1α. Importantly, MKC-3946 also inhibited XBP1s in primary tumor cells from MM patients. We also confirmed functional inhibition of XBP1s, with target genes including SEC61A1, p58IPK, and ERdj4 downregulated by MKC-3946 treatment. Importantly, MKC-3946 triggered growth inhibition in MM cell lines, without toxicity in normal mononuclear cells. Furthermore, it significantly enhanced cytotoxicity induced by bortezomib or 17-AAG in RPMI8226 and INA6 cells, as well as primary tumor cells from MM patients. Both bortezomib and 17-AAG induced ER stress with XBP1s, which was markedly blocked by MKC-3946. Moreover, apoptosis induced by bortezomib or 17-AAG was enhanced by MKC-3946, associated with increased CHOP mRNA and protein, a proapoptotic factor triggered by ER stress. We next demonstrated that XBP1s was induced by bortezomib in INA6 cells co-cultured with bone marrow (BM) stromal cells, which was inhibited by MKC-3946, associated with enhanced cytotoxicity induced by the combination. Finally, MKC-3946 inhibited XBP1s in a model of in vivo ER stress induced by tunicamycin. To evaluate the anti-MM effect of MKC-3946, we used the subcutaneous RPMI8226 xenograft model in mice. MKC-3946 significantly reduced MM tumor growth in the treatment versus control group, associated with prolonged overall survival. We also confirmed that MKC-3946 treatment significantly inhibited XBP1s in excised tumors, assessed by RT-PCR. In order to examine the activity of MKC-3946 on MM cell growth in the context of the human BM microenvironment in vivo, we used the SCID-hu model, in which INA6 cells are directly injected into a human bone chip implanted subcutaneously in SCID-mice. MKC-3946 treatment significantly inhibited tumor growth compared with vehicle control. Moreover, XBP1s in excised tumor cells was inhibited, evidenced by RT-PCR. In conclusion, these data demonstrate that blockade of XBP1s by MKC-3946 triggers MM cell growth inhibition in vivo and prolongs host survival. Taken together, our results demonstrate that blockade of XBP1 splicing by inhibition of IRE1α endoribonuclease domain is a potential novel therapeutic option in MM. Disclosures: Tam: MannKind Corporation: Employment, Equity Ownership. Zeng:MannKind Corporation: Employment, Equity Ownership. Patterson:MannKind Corporation: Employment, Equity Ownership. Richardson:Bristol Myers Squibb: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees; Johnson & Johnson: Membership on an entity's Board of Directors or advisory committees; Celgene: Membership on an entity's Board of Directors or advisory committees; Millennium: Membership on an entity's Board of Directors or advisory committees. Munshi:Millennium: Membership on an entity's Board of Directors or advisory committees; Celgene: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees. Anderson:Millennium: Membership on an entity's Board of Directors or advisory committees; Onyx: Membership on an entity's Board of Directors or advisory committees; MannKind: Membership on an entity's Board of Directors or advisory committees.