Abeykoon, Jithma P.

Abstract Constitutive activation of B-cell receptor (BCR) signaling is a major driving mechanism for the proliferation and survival of various B-cell lineage non-Hodgkin lymphomas (NHL). Blocking BCR signaling using the first-in-class Bruton's tyrosine kinase (BTK) inhibitor, ibrutinib, has proved effective and is currently FDA approved for the treatment of several B-cell lymphomas. Unfortunately, significant subsets of patients possess either primary or acquired resistance to ibrutinib, often resulting in worse prognosis and poorer responses to subsequent therapies. Therefore, overcoming ibrutinib resistance is an urgent clinical need requiring new tools to both delineate the molecular mechanisms of resistance and develop viable therapeutics. To that end, we developed a mantle cell lymphoma (MCL) cell line, MCIR1, from a patient with clinically acquired ibrutinib resistance. MCIR1 is a bona fide MCL cell line featuring CD19+/CD5+/CD10-/CD23-/t(11;14)+/TP53+/-; and lacking the previously described mutations in BTK (C481S) and phospholipase C-gamma-2 (R665W, L845F). Importantly, MCIR1 cells possess hallmarks of ibrutinib resistance, including the insensitivity of ERK1/2 phosphorylation to ibrutinib in vitro, and the irresponsiveness of MCIR1 xenograft tumors to ibrutinib treatment in mice as compared with ibrutinib-sensitive Jeko-1 tumors. To delineate the molecular mechanism(s) of ibrutinib resistance, we identified by RNA-Seq analysis that MCIR1 cells possess robust activity of the non-canonical NFkB pathway and elevated expression of NFkB target genes, suggesting that this pathway likely drives their resistance to ibrutinib. Finally, we demonstrated the utility of MCIR1 as a tool for testing new drugs to combat ibrutinib resistance in vitro by showing its sensitivity to the anti-apoptotic B-cell lymphoma-2 (Bcl-2) protein inhibitor, venetoclax. To our knowledge, MCIR1 is the first patient-derived ibrutinib-resistant cell line to be established. Hence, MCIR1 is especially useful because uniquely possesses the relevant clinical and biological features of patients with ibrutinib-resistant MCL. We propose MCIR1 as a tool to further the understanding of molecular mechanisms conferring ibrutinib resistance by providing a clinically relevant system with the capacity for molecular and genetic manipulations. Further, these features make MCIR1 an ideal model to screen drugs for combating ibrutinib resistance through both in vivo and in vitro approaches. Citation Format: Kevin E. Nowakowski, Jithma P. Abeykoon, Mary J. Stenson, Michael M. Timm, Curtis A. Hanson, Daniel L. Van Dyke, Anne J. Novak, Xiaosheng Wu, Thomas E. Witzig. MCIR1: A patient-derived ibrutinib-resistant mantle cell lymphoma line for the study of ibrutinib resistance and drug discovery [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 650.

e20048 Background: No literature exists regarding whether hypomagnesemia at the time of diagnosis in Burkitt Lymphoma (BL) is associated with inferior survival. Methods: Patients with new diagnosis of BL who were seen at Mayo Clinic, MN from 2000-2019 with a serum magnesium level available prior to chemotherapy were included. Patients were allocated to two groups; Abnormal Magnesium Group (AMG), defined as a magnesium level < 1.7 mg/dL and Normal Magnesium Group (NMG), defined as ≥ 1.7 – 2.3mg/dL. Two-sided Wilcoxon rank sum test and Chi square/Fischer’s exact test were used to compare the continuous and categorical variables, respectively. Kaplan-Meier and log-rank tests were used to perform all time to event analysis which were done from time of treatment. Hazard ratios (HR) with confidence intervals (CI) were calculated using Cox-proportional hazards. Results: Of 90 patients with a diagnosis of BL, 42 patients had a magnesium level at or before time of diagnosis. The Table lists the baseline characteristics and significant findings of the AMG/NMG groups. The median follow-up was 30 months. Hypomagnesemia was predictive for inferior EFS at 30 months - 34.3% (95% CI: 8.7–74.0) for AMG and 79.3% (95% CI: 60.8-90.4) for NMG (p = 0.038). OS at 30 months for AMG was 42.9% (95% CI: 14.4-77.0) and for NMG was 93.7% (95% CI: 78.1-98.4%) (p = 0.002). Other parameters significant on univariate analysis included the Charlson Comorbidity Index (p = 0.034), creatinine (p = 0.009), and number of treatments (p = 0.048). In the multivariate analysis, creatinine (p = 0.005) and number of treatments (p = 0.004) were significant. Likelihood ratio testing of predictors associated with inferior OS were significant for hypomagnesemia (likelihood ratio 3.99, p = 0.046). Conclusions: Patients with hypomagnesemia at the time of BL diagnosis have inferior outcomes compared to BL patients with a normal magnesium level. Prospective studies are needed to confirm this finding and test Mg replacement strategies to mitigate the effects of hypomagnesemia. [Table: see text]

8550 Background: Waldenström Macroglobulinemia (WM) is a rare lymphoplasmacytic malignancy characterized by the presence of a recurrent point mutation in the MYD88 gene (MYD88L265P) in 80-95% of cases. Patients with MYD88WT genotype comprise a small subset that responds poorly to ibrutinib and other Bruton tyrosine kinase inhibitors. We examined the characteristics and outcome of WM patients with MYD88WT genotype predominantly treated with non-BTK inhibitor based therapies. Methods: Patients with a diagnosis of WM seen at Mayo Clinic, Rochester, between 1996 and 2018 were included. Their characteristics and outcomes were assessed from the time of active disease. Marrow MYD88 genotyping was assessed with an allele specific PCR assay (analytic sensitivity 1%). Categorical and continuous variables were compared using Chi square and Wilcoxon tests, respectively. Time-to-event analyses were performed using Kaplan Meier test. Results: Of 986 patients with active WM, MYD88 genotype data were available in 331 (34 %) patients; 72 (22%) and 260 (78%) patients harbored MYD88WT and MYD88L265P genotypes, respectively. The median follow-up was 5.8 years (95% CI: 5.0-6.5 years) from active WM; 6 years MYD88WT vs 5.4 years for MYD88L265P cohort. Median age was 63 years and 66 years in the MYD88WT and MYD88L265P cohorts, respectively (p = 0.07) with 46% and 53% patients being ≥65 years of age, respectively (p = 0.36). Pre-treatment marrow lymphoplasmacytic (LPL) infiltrate (median 40% for MYD88WT vs 60% for MYD88L265P; p = 0.001) and beta-2 microglobulin (median 3 µg/mL for MYD88WT vs 3.9 µg/mL for MYD88L265P; p = 0.02) were lower in the MYD88WT compared to the MYD88L265P cohort; other laboratory parameters at active disease were comparable. Per IPSSWM prognostic criteria, MYD88WT had fewer patients in the high risk group (18% for MYD88WT vs 42% MYD88L265P; p = 0.03). Patients with MYD88WT had higher likelihood of histological transformation [18% for MYD88WT vs 4% for MYD88L265P; odds ratio 5.8 (95% CI: 2.5-13.5; p < 0.0001)]. Among patients with treatment data available, only 35 (11%) patients received ibrutinib. The 5-year overall survival (OS) from active disease was comparable (85% in MYD88WT vs 82% in MYD88L265P cohort; p = 0.7). Conclusions: MYD88WT genotype in WM is associated with lower marrow LPL infiltration, lower likelihood of high-risk IPSSWM categorization and a higher likelihood of histological transformation in comparison to MYD88L265P mutant subpopulation. MYD88 genotype does not affect the OS from active disease in predominantly non-BTK inhibitor treated patients.

7559 Background: Waldenström macroglobulinemia (WM) is a rare indolent lymphoma commonly treated with rituximab (R)-based therapy. The use of rituximab maintenance (mR) in WM is controversial. We present a case-control study of patients (pts) with WM treated with mR. Methods: Pts evaluated at Mayo Clinic, Rochester with active WM that received mR between 1/2000 & 6/2018 were included. Cases comprised pts who received mR following R-based induction as primary therapy. Cases were matched based on the time of diagnosis in 1:2 ratio with a control group treated with R-based primary induction therapy without mR. Time to event analyses were performed from initiation of R-based induction. Results: Of 776 pts with active WM, 42 (5%) cases received mR and 84 pts were selected as controls. The median follow-up and the proportion of high risk pts were comparable between the two cohorts (Table). Pts in the mR cohort show a trend toward longer time to next therapy (TTNT) and a significantly longer overall survival (OS) compared to the control group (Table). The R-based induction therapies were comparable in the two cohorts (p = 0.6). Median duration of mR was 1.9 yrs (95% CI 1.6-2) and mR was used most frequently every (q) 2 (range 1-6) months. Of the 42 mR pts, 25 (60%) received an R-based combination for induction and 17 (40%) received R monotherapy as induction. Five (12%) pts discontinued mR due to toxicity, infections were reported in 13 pts (31%) during mR therapy and 3 pts (7%) received IVIg infusions for recurrent infections. Conclusions: R-based induction followed by mR demonstrates a longer OS in WM compared to R-treated control population not receiving mR, albeit at a high rate of infections. Despite limitations of a retrospective study, with a heterogeneously treated cohort, these data add to the body of literature supporting Rituximab maintenance. Results from an ongoing randomized controlled trial are awaited. [Table: see text]

7509 Background: Waldenstrom macroglobulinemia (WM) is a rare lymphoma for which scant comparative data exist to guide frontline therapy. Herein, we compare 3 commonly used regimens in WM: R-Benda, DRC, and BDR in frontline setting. Methods: Patients (Pts) with active WM seen at Mayo Clinic between 2000 & 2018 who received R-Benda, DRC or BDR as primary therapy were included in this retrospective study. Response rates were assessed by Consensus Criteria. All time to event analyses were performed from the frontline therapy, using Kaplan-Meier method. Results: The study included 172 pts with active WM (R-Benda, n=67, DRC, n=75, BDR, n=30).The median follow-up for the entire cohort was 3.7 years (y) (95% CI 3.7-3.0). Baseline characteristics, including IPSS, and time to frontline therapy from WM diagnosis were similar across the 3 cohorts. Clinically relevant endpoints are shown in the Table. Hematologic and non-hematologic toxicities were similar across the 3 groups. Grade 3 neuropathy requiring treatment discontinuation was encountered in 13% pts treated with BDR. 56 pts received subsequent salvage therapy [(10% in R-Benda arm, 44% in DRC arm, & 53% in BDR arm]; 29% pts in the R-Benda arm and 30% pts in DRC arm received a PI-based regimen while 69% pts in the BDR arm received alkylator-rituximab based therapy. Conclusions: Outcomes (MRR, TTNT and EFS) with frontline R-Benda are superior in comparison to frontline DRC or BDR in patients with WM. Clinically relevant endpoints are not significantly different with DRC vs. BDR. The toxicity profile across the 3 groups was comparable. [Table: see text]

AbstractHistological transformation in Waldenström macroglobulinemia (WM) is an uncommon complication, with limited data, particularly regarding the impact of MYD88 L265P mutation on transformation. We examined risk factors and outcomes associated with transformation in WM, highlighting the role of MYD88 L265P mutation. Patients with WM seen at Mayo Clinic, Rochester, USA and University Hospital of Reims, France, between 01/01/1996 and December 31, 2017 were included; 50 (4.3%) of 1147 patients transformed to a high‐grade lymphoma, with median time‐to‐transformation of 4.5 (range 0‐21) years in the transformed cohort. The MYD88 L265P mutation status was known in 435/1147 (38%) patients (406 with non‐transformed WM and 29 patients in transformed cohort). On multivariate analysis, MYD88 WT status alone was an independent predictor of transformation (odds ratio, 7[95%CI: 2.1‐23]; P = .003). Additionally, the MYD88 WT status was independently associated with shorter time‐to‐transformation (HR 7.9 [95%CI: 2.3‐27; P = .001]), with a 5‐year transformation rate of 16% for MYD88 WT vs 2.8% with MYD88 L265P mutated patients. Patients with transformation demonstrated a significant increase in risk of death compared to patients who did not transform (HR 5.075; 95%CI: 3.8‐6.8; P < .001). In conclusion, the MYD88 WT status is an independent predictor of transformation and associated with a shorter time‐to‐transformation. Additionally, transformation conferred an inferior overall survival in patients with WM.

Introduction: The landmark VE-BASKET trial demonstrated that Erdheim-Chester disease (ECD) patients with the BRAF-V600E mutation can be effectively treated with vemurafenib 1920 mg/day. However, all patients required dose reductions due to adverse effects. The efficacy of low dose BRAF-inhibitors is not well established in ECD. Further, as Langerhans cell histiocytosis (LCH) also harbors BRAF-V600E mutations in 50-60% of patients, BRAF-inhibitors may be effective in this disease as well. In this study, we evaluated the efficacy of low dose BRAF-inhibitors (vemurafenib and dabrafenib) in the treatment of ECD and LCH. Methods: We conducted a retrospective study of ECD and LCH patients who were seen at our institution from January 1998 to July 2018. All patients had a diagnosis of ECD and LCH determined by clinical criteria in conjunction with histopathologic findings. Based on the standard doses approved for malignant melanoma and ECD, patients were categorized into the low dose BRAF-inhibitor group if they were treated with vemurafenib < 960 mg/day or dabrafenib ≤ 150mg/day at any point in time. We used a simple response criteria that defined clinical progressive disease (PD) as worsening of symptoms attributed to ECD/LCH or radiologic PD as a progression or worsening of proven or suspected lesions due to ECD/LCH. The minimum duration for symptom improvement to be considered a response was set at 3 months. Results: A total of 89 ECD patients and 186 LCH patients were identified. Within the ECD cohort, 24 of 44 (55%) patients who were tested had the BRAF-V600E mutation. Eight patients were included in the low dose BRAF-inhibitor group. The median age at diagnosis was 57 years (range 37-74) and 5 (63%) were male. The most common areas of involvement included bone (88%), cardiovascular system (63%), kidneys (63%), and brain parenchyma (50%). The median time of follow up was 66 months (range 23-165) and the median time on low-dose BRAF-inhibitor was 10 months (range 1-27) [Table 1]. Three patients had a starting dose of vemurafenib 1920 mg/day, 4 had 960 mg/day, and 1 had 480 mg/day. All patients required dose reductions and 50% of the patients ultimately discontinued vemurafenib due to side effects. Side effects included fatigue, pruritus, nausea, facial swelling, blisters, papillomas, and/or subcutaneous nodules. Four patients were able to remain on low-dose vemurafenib for a median follow up time of 24.5 months (range 12-28) with ongoing response and no signs of clinical or radiologic PD. Within the LCH cohort, 18 of 31 tested (58%) patients had the BRAF-V600E mutation. Four patients were included in the low dose BRAF-inhibitor group. The median age at diagnosis of this cohort was 43 years (range 34-69) and 2 (50%) were male. Areas of involvement included bone (100%), brain parenchyma (hypothalamus/optic stalk and pons; 50%), and skin (25%). The median time of follow up was 31 months (range 21-46) and the median time on low-dose BRAF-inhibitor was 4 months (range 3-24) [Table 2]. Two patients had a starting dose of vemurafenib 960mg/day. The patients with brain parenchymal involvement had a starting dose of dabrafenib 150mg/day or 100mg/day. All patients taking vemurafenib 960mg/day required dose reductions and one patient discontinued treatment due to skin blistering in her feet. In the patients taking dabrafenib, no side effects have yet been reported. 3 patients had an ongoing response and did not have clinical or radiologic PD. Patient #10 however, had clinical and radiologic PD after being on vemurafenib 720mg/day for 22 months. Conclusion: Our study suggests a potential role for lower doses of BRAF-inhibitors in ECD and LCH patients harboring BRAF-V600E mutations. Dabrafenib was found to be particularly efficacious and well-tolerated in LCH involving the brain parenchyma. However, patients who undergo dose reductions should be closely monitored due to the risk of disease progression. Careful balance of toxicities and efficacy is needed for optimizing patient outcomes with targeted therapies. Disclosures Bennani: Adicet Bio: Other: Advisory board; Kite Pharma: Other: Advisory board; Purdue Pharma: Other: Advisory board; Purdue Pharma: Other: Advisory board; Seattle Genetics: Other: Advisory board; Purdue Pharma: Other: Advisory board; Adicet Bio: Other: Advisory board; Kite Pharma: Other: Advisory board; Bristol-Myers Squibb: Research Funding; Bristol-Myers Squibb: Research Funding; Adicet Bio: Other: Advisory board; Bristol-Myers Squibb: Research Funding; Kite Pharma: Other: Advisory board; Seattle Genetics: Other: Advisory board; Seattle Genetics: Other: Advisory board. OffLabel Disclosure: Vemurafenib and dabrafenib for Langerhans cell histiocytosis. Vemurafenib dosage for Erdheim-Chester disease is less than the approved dose of 960mg every 12 hours.