Second Primary Malignancies in Chronic Lymphocytic Leukaemia; Skin, Solid Organ, Haematological, and Richter's Syndrome

Chronic lymphocytic leukaemia (CLL) is invariably accompanied by some degree of immune failure. CLL patients have a high rate of second primary malignancy (SPM) compared to the general population. We comprehensively documented the incidence of all forms of SPM including skin cancer (SC), solid organ malignancy (SOM), second haematological malignancy (SHM), and separately Richter's Syndrome (RS) across all therapy eras. Among the 517 CLL/SLL patients, the overall incidence of SPMs with competing risks were SC 31.07%, SOM 25.99%, SHM 5.19% and RS 7.55%. Melanoma accounted for 30.3% of SC. Squamous cell carcinoma (SCC), including 8 metastatic SCCs, was 1.8 times more than basal cell carcinoma (BCC), a reversal of the typical BCC:SCC ratio. The most common SOM were prostate (6.4%) and breast (4.5%). SHM included 7 acute myeloid leukaemia and 5 myelodysplasia of which 8 were therapy-related. SPMs are a major health burden with 44.9% of CLL patients with at least one, and apart from SC, associated with significantly reduced overall survival. Dramatic improvements in CLL treatment and survival have occurred with immunochemotherapy and targeted therapies but mitigating SPM burden will be important to sustain further progress.

Venetoclax plus Dose-Adjusted R-EPOCH (VR-EPOCH) for Richter's Syndrome

Richter's Syndrome (RS) of chronic lymphocytic leukemia (CLL) is typically chemoresistant, with a poor prognosis. We hypothesized that the oral Bcl-2 inhibitor venetoclax could sensitize RS to chemoimmunotherapy and improve outcomes. We conducted a single-arm, investigator-sponsored, phase 2 trial of venetoclax plus dose-adjusted rituximab, etoposide, prednisone, vincristine, cyclophosphamide, and doxorubicin (VR-EPOCH) to determine the rate of complete response (CR). Patients received R-EPOCH for 1 cycle, then after count recovery, accelerated daily venetoclax ramp-up to 400 mg, then VR-EPOCH for up to 5 more 21-day cycles. Responders received venetoclax maintenance or cellular therapy off-study. Twenty-six patients were treated, and 13 of 26 (50%) achieved CR, with 11 achieving undetectable bone marrow minimal residual disease for CLL. Three additional patients achieved partial response (overall response rate 62%). Median progression-free survival was 10.1 months, and median overall survival was 19.6 months. Hematologic toxicity included grade ≥3 neutropenia (65%) and thrombocytopenia (50%), with febrile neutropenia in 38%. No patients experienced tumor lysis syndrome with daily venetoclax ramp-up. VR-EPOCH is active in RS, with deeper, more durable responses than historical regimens. Toxicities from intensive chemoimmunotherapy and venetoclax were observed. Our data suggest that studies comparing venetoclax with chemoimmunotherapy to chemoimmunotherapy alone are warranted. (Funded by Genentech/AbbVie; ClinicalTrials.gov number, NCT03054896).

Evidence of a Synergistic Cross-Talk between the B Cell Receptor (BCR) and Nicotinamide Phosphoribosyl Transferase (NAMPT) in Richter's Syndrome Patient-Derived Xenograft Models: Therapeutic Implications

Background: A rare complication of Chronic Lymphocytic Leukemia (CLL), Richter's Syndrome (RS) represents the transformation of a pre-existing CLL into a Diffuse Large B-cell Lymphoma (DLBCL), generally associated with poor prognosis. Current therapeutic approaches are limited and do not significantly reduce disease progression. For these reasons there is intense investigation to identify potentially druggable molecular circuits, opening the way to innovative combination therapies. Among the several oncogenic signalling pathways that may contribute to disease progression, the B Cell Receptor (BCR) is a main driver and an actionable target. We previously showed that BCR ligation in CLL cells increases expression of nicotinamide phosphoribosyltransferase (NAMPT), the rate-limiting enzyme in the salvage NAD pathway starting from nicotinamide, a finding in line with the notion of oncogenic-driven metabolic reprogramming. In this context, increased NAMPT expression leading to heightened NAD+ levels could sustain proliferation through the modulation of the activity of several intracellular enzymes, including sirtuins (NAD-dependent deacetylases). Aim: This work explores the connections between BCR signalling and the NAMPT/NAD/sirtuin axis in RS cells, asking the question of whether BCR and NAMPT can be simultaneously targeted. Methods: RS-PDX cells were freshly purified from tumor masses grown in mice and immediately used for short ex-vivo experiments. For in vivo experiments, NSG mice were injected subcutaneously with RS-PDX cells, which were left to engraft until a palpable mass was evident. Mice were then randomized to receive duvelisib, OT82 or a combination of the two for two consecutive weeks. Control mice were similarly treated with vehicle only. Mice were under a niacin-free diet. Results: By using our 4 RS patient-derived xenografts (PDX) models, we invariably observed high levels of NAMPT expression. High levels of NAMPT expression were also observed in 15 primary RS lymph node biopsies analyzed by RNA sequencing. BCR engagement through αIgM polyclonal antibodies significantly up-regulated NAMPT expression, as determined by qRT-PCR and protein analysis, with a concomitant increase in intracellular NAD+ levels. We then asked whether RS cells are sensitive to NAMPT inhibition, alone or in combination with drugs that target the BCR pathway. As most RS patients would likely have been treated during the preceding CLL phase with a BTK inhibitor, possibly developing resistance, we turned to PI3K inhibitors, which are less commonly used for CLL therapy. As NAMPT inhibitors (NAMPTi) we used both FK866 and OT-82, which are validated small molecules. Results indicated that the combination of the dual PI3K-δ/γ inhibitor duvelisib, with either FK866 or OT-82 induces dramatic apoptosis in all 4 models tested, as confirmed by annexinV/PI staining, by caspase 3 activation and by a significant drop in ATP and NAD+ levels. Importantly, two RS-PDX models (RS9737 and RS1316) were fully resistant to NAMPTi used alone, likely due to high levels of nicotinate phosphoribosyltransferase (NAPRT), which is the rate-limiting enzyme in the NAD salvage pathway that starts from nicotinic acid. However, addition of duvelisib, which was mildly effective when used alone, was followed by marked apoptosis even in these two models. Molecular dissection of the pathway showed that the combination of duvelisib and NAMPTi was followed by complete inhibition of the PI3K pathway, which was only partially blocked by duvelisib alone, even at high doses. The connection between NAMPT and PI3K is represented by cytoplasmic sirtuins, particularly SIRT2, which activate AKT through de-acetylation. Immunoprecipitation and two-dimensional gel electrophoresis showed that in the presence of NAMPTi, the amount of acetylated, i.e., inactive, AKT increased considerably. Consistently, treatment of RS-PDX mice with a combination of duvelisib and OT-82 was followed by significantly higher responses and longer animal survival. Conclusions: These results highlight a crosstalk between BCR signalling and NAMPT/sirtuin axis in RS models, showing the increased efficacy of the dual targeting (i.e., PI3K-δ/γ and NAMPT), and supporting this novel and promising therapeutic strategy for the treatment of RS patients. Disclosures Deaglio: Heidelberg Pharma: Research Funding; Astra Zeneca: Research Funding.

Dissecting Richter's Syndrome in a Multiplexed CRISPR-Based Mouse Model Reveals Key Changes in MYC, Interferon and BCR Signaling Underlying Transformation

Richter's syndrome (RS) represents one of the foremost challenges in CLL management, and its pathogenesis remains largely undefined. We recently leveraged CRISPR-Cas9 in vivo gene editing to develop mouse models of RS by engineering multiplexed loss-of-function lesions typical of CLL (Atm, Trp53, Chd2, Birc3, Mga, Samhd1) in early stem and progenitor cells [Lineage - Sca-1 + c-kit + (LSK)] from MDR-Cd19Cas9 donor mice. These animals express Cas9-GFP in a B-cell restricted fashion and the leukemogenic MDR lesion, which mimics del(13q) when the sgRNA-transduced LSK cells are transplanted in CD45.1 immunocompetent recipients. Through these methods, we observed not only development of CLL, but also transformation into RS, and even captured a stage where CLL and RS were co-existing in the same animal (CLL/RS). We hypothesized that the molecular events underlying RS development would be markedly distinct from those of CLL and performed transcriptome analysis of FACS-sorted CLL and/or RS cells (5 CLL, 4 CLL/RS, 10 RS) and normal B cell controls from 4 age-matched wild type MDR-Cd19Cas9 mice. We identified a unique transcriptional profile of RS (ANOVA, FDR<0.1), characterized by upregulation of pathways involved in cell survival and proliferation (E2F/MYC targets, G2-M checkpoint, mitotic spindle). In contrast, genes involved in interferon gamma response, JAK-STAT and BCR signaling were predominantly downregulated. We asked whether these oncogenic circuitries would be recapitulated in human RS. By correlating the differentially expressed genes in murine RS with those of 7 human RS cases (compared to matched CLL), we identified similar pathway dysregulations with >100 commonly altered genes including upregulated cell cycle regulators (CDK1, CCNA2) and downregulated signaling adapters (ITPKB, MAP3K9). To further dissect gene regulatory networks driving transformation in the mouse, we profiled one CLL and one RS case by single cell ATAC sequencing (scATAC-seq). Consistent with the RNA-seq profiles, we detected increased chromatin accessibility of MYC-family associated transcription factor motifs (MAX, MYCN), and reduced accessibility of the pro-inflammatory STAT2 motif in RS (-log10adjP>50). Functionally, decreased interferon gamma responses were confirmed by the reduced ability of RS cells to phosphorylate STAT1 and STAT3 at 5' and 15' after IFN-gamma stimulation, compared to CLL and normal B cells (Western Blot). To define the genetic landscape underlying these changes, we performed whole genome sequencing analysis, and identified loss of chr12 and chr16 as recurrent events in RS (6/8 cases) and CLL/RS (2/2), but not in CLL cases (0/5). Among the genes encoded by these chromosomes, we identified several epigenetic drivers (Dnmt3a, Crebbp, Setd3/4), MAP kinase family members (Map4k5, Mapk1), cytoskeletal regulators (Hcls1, Rhoj), and interferon family receptors (Ifnar1/2, Ifngr2), suggesting that broad epigenetic modifications together with loss of BCR and interferon signaling molecules represent key events of transforming disease. RS cases were also characterized by a significantly higher number of full chromosome amplifications or deletions (median=6; range: 2-9), as compared to CLL or CLL/RS (1; 0-5, P=0.0008), consistent with the high degree of genomic instability observed in human disease. Finally, we asked whether the observed changes would impact RS therapeutic vulnerabilities, and exposed 15 primary murine RS splenocyte samples to 20 drugs in vitro for 24 hours, followed by CellTiter-Glo assessment of cellular viability. We observed strong sensitivity to the BRD4 inhibitor JQ1 and the mTOR inhibitor everolimus (both reported to interfere with MYC signaling, P<0.0001), and to CDK inhibitors (e.g. the CDK4/6 inhibitor palbociclib, P=0.0007), modest activity of the JAK1/2 inhibitor ruxolitinib (P=0.05), and minimal, if any, response to ibrutinib, venetoclax and fludarabine. In conclusion, we define the evolutionary trajectories and therapeutic vulnerabilities of RS in a mouse model, with unique transcriptional, genetic, and epigenetic features, indicative of broad changes in MYC, IFN and BCR signaling pathways and remarkable similarities with human disease. In-depth analyses of BCR signaling and in vivo treatment studies are underway and will refine mechanistic insights into the biology of RS. Disclosures Davids: Surface Oncology: Research Funding; Eli Lilly and Company: Consultancy; Genentech: Consultancy, Research Funding; Takeda: Consultancy; MEI Pharma: Consultancy; Janssen: Consultancy; Verastem: Consultancy, Research Funding; Ascentage Pharma: Consultancy, Research Funding; Pharmacyclics: Consultancy, Research Funding; TG Therapeutics: Consultancy, Research Funding; Astra-Zeneca: Consultancy, Research Funding; Merck: Consultancy; Adaptive Biotechnologies: Consultancy; Research to Practice: Consultancy; AbbVie: Consultancy; MEI Pharma: Consultancy, Research Funding; Novartis: Consultancy, Research Funding; BMS: Consultancy, Research Funding; Celgene: Consultancy; BeiGene: Consultancy. Letai: Dialectic Therapeutics: Other: equity holding member of the scientific advisory board; Flash Therapeutics: Other: equity holding member of the scientific advisory board; Zentalis Pharmaceuticals: Other: equity holding member of the scientific advisory board. Neuberg: Madrigal Pharmaceuticals: Other: Stock ownership; Pharmacyclics: Research Funding. Wu: Pharmacyclics: Research Funding; BioNTech: Current equity holder in publicly-traded company.

Venetoclax Combined with Dose-Adjusted R-EPOCH (VR-DA-EPOCH) As Treatment of Richter's Syndrome: A Real-World Study

Background: To evaluate the efficacy and safety of venetoclax, rituximab plus dose-adjusted etoposide, prednisone, vincristine, cyclophosphamide, and doxorubicin (VR-DA-EPOCH) in Richter's syndrome (RS), we conducted a single-arm, retrospective, observational, real-world study in our center. Methods: Patients who had history of CLL/SLL were diagnosed as RS by biopsy during treatment or watch and wait strategy. VR-DA-EPOCH was given as follow, venetoclax was administered with accelerated ramp-up from 20 mg per day to 400 mg per day, d1-10 during cycle 1, 400 mg daily on day1-10 of cycle 2-6, rituximab 375 mg/m 2 on day 0 of cycle 1 and 500 mg/m 2 on day 0 of cycle 2-6, plus etoposide (50 mg/m 2,day1-4), vincristine (0.4 mg/m 2 day 1-4) or vindesine 3 mg/m 2 day 1 , doxorubicin (10 mg/m 2 day 1-4), prednisone (60 mg/m 2, day 1-5), cyclophosphamide (750 mg/m 2 day 5), 21 days per cycle，dose adjustment on the basis of nadir ANC and platelet count are as previously reported by Wison WH. Response assessment was conducted after 2 or 3 cycles by enhanced CT or PET/CT and after 6 cycles (EOT) by PET/CT according to 2014 Lugano criteria. Minimal residual disease (MRD) of CLL cell in peripheral blood (PB) and bone marrow (BM) was detected after 2 or 3 and 6 cycles by flow cytometry. uMRD was defined as less than 1 CLL cell per 10 4 leukocytes. Results: 7 RS patients were enrolled in Pukou CLL Center from 10/2019 to 7/2021 and the last follow up was 07/25/2021. The median age was 52 years old. Unmutated IGHV, complex karyotype (CK) and TP53 deletion and/or mutation was detected in 100% (6/6), 20% (1/5) and 40% (2/5) patients, respectively. 5 patients received at least one prior line (range: 1-5) treatment for CLL/SLL, with 4 patients received ibrutinib as last prior therapy and one patient previously exposed to venetoclax. 2 patients were diagnosed as RS during watch and wait. The median duration from diagnoses or previous treatment for CLL/SLL to RS was 12 months (range: 3-14). All patients underwent lymph node (n=6) or bone biopsy(n=1) at the site of SUVmax or secondary SUV uptake (unaccessible for SUVmax) by PET/CT and was confirmed as transformed to non-GCB type of diffuse large B-cell lymphoma (DLBCL). Furthermore, 4 of 4 (100%) available patients were confirmed as clonal-related RS by detecting IGHV gene usage. 3 patients acquired CK, and 2 patients appeared BTK C481S mutation. 7 patients completed at least 2 cycles and were available for efficacy and safety assessment. Overall response rate (ORR) was 100% after 2 or 3 cycles, and CR rate (CRR) was 60% after 6 cycles in 5 patients who completed 6 cycles. 2 patients experience disease progression (PD) after cycle 2 and cycle 4 respectively, with one ceased after the addition of brentuximab and the other received CD20 UCAR-T and progressed 3 months later, transit to allo-hemapoietic stem cell transplant (allo-HCT). One patient received auto-hemapoietic stem cell transplant (auto-HCT) and CD19-CAR-T as consolidation and remains in CR, one patient experience PD after 6 cycles and attained CR with chidamide , programmed death-1 (PD-1) inhibitor Sintilimab and XPO1 inhibitor Selinexor, bridging to allo-HCT and remains in CR. Another patient who achieved CR after 6 cycles progressed 6 months later and ceased within one month. 2 patients transformed without previous treatment wait for final evaluation. 60% (3/5) patients attained MRD negativity both in the PB and BM after 6 cycles. The median tolerable dose was 60% (50%-70%) of standard EPOCH and the median dose of venetoclax was 400mg daily for 7 days each cycle. No tumor lysis syndrome (TLS) happened during venetoclax ramp-up. The most common grade 3 or 4 adverse effects (AEs) was agranulocytic fever (6/7, 85.7%), thrombocytopenia (3/7, 42.9%) and sepsis (2/7, 28.6%). 3 (42.9%) patients discontinued venetoclax due to severe AEs and the median duration of discontinuation was 3 days. 85.7% (6/7) had venetoclax dose reduction or interruption due to grade 3 or 4 neutropenia. Conclusion: VR-DA-EPOCH showed high response rate, impressive CR with uMRD and manageable toxicity in patients with RS, even with previous exposure to new target drugs. VR-DA-EPOCH could be recommended as an effective treatment choice for RS, CAR-T or allo-HCT should be considered as subsequent strategy for long term disease control. Figure 1 Figure 1. Disclosures No relevant conflicts of interest to declare.

Blinatumomab for Patients with Richter's Syndrome: A Multicenter Phase 2 Trial from the Filo Group

Background Richter syndrome (RS) refers to the onset of aggressive lymphoma, mostly diffuse large B cell lymphoma (DLBCL), in patients with chronic lymphocytic leukemia (CLL). The outcome of RS patients is usually very poor with both low response rates to chemoimmunotherapy and short survival. While BCR and BCL2 inhibitors have transformed the management of CLL patients, these drugs do not prevent the onset of RS. Modulating anti-tumor immunity has recently been suggested as a promising approach in RS (Ding, 2017). Blinatumomab is a bi-specific T-cell engaging antibody construct that transiently links CD3-positive T cells to CD19-positive B-cells, inducing T-cell activation and subsequent lysis of tumor cells. It has been approved for the treatment of patients with relapsed or refractory B-ALL and has also been evaluated in the setting of persisting minimal residual disease. Recently, blinatumomab (stepwise dosing 9-28-112 μg/d) has been evaluated in patients with relapsed or refractory DLBCL and demonstrated promising results (ORR 43%) with acceptable safety (Viardot, 2016). We hypothesized that blinatumomab would improve response in RS patients failing to achieve CR after initial debulking with R-CHOP. Methods We report here the first results of a phase 2 multicenter study investigating the efficacy and safety of blinatumomab after R-CHOP debulking therapy for patients with untreated RS of DLBCL histology (NCT03931642). The patients with persisting (PR, SD) or progressive disease (PD) after 2 cycles of R-CHOP were eligible to receive an 8-week course of blinatumomab induction. Blinatumomab was administered at a stepwise dose of 9 μg/d in the first week, 28 μg/d in the second week, and 112 μg/d thereafter. The primary endpoint was CR rate according to the revised Lugano criteria after the 8-week induction course of blinatumomab. An additional 4-week consolidation cycle was optional. Allo-HSCT was further allowed for eligible patients. Results A total of 34 patients out of 41 has already been enrolled in the trial to date. Median age was 66 years (range, 38-82) and sex ratio M/F was 23/12. CLL features at baseline were as follows: 57% had 17p deletion and 67% TP53 mutations. Sixty-five percent had complex karyotype and 79% unmutated IGHV status. Median number of prior therapeutic lines for CLL was 2 (range, 0-7): 19 (54%) patients previously received chemo-immunotherapy, 23 (66%) patients were exposed to ibrutinib and 11 (31%) to venetoclax. As of the data cut-off of June 1st, 2021, the blinatumomab induction course has been completed for 18 patients. Ten patients did not receive blinatumomab for the following reasons: 7 patients achieved CR after R-CHOP, 2 patients died because of febrile neutropenia after R-CHOP and 1 patient presented severe pneumonia after R-CHOP. Three patients are still on R-CHOP and 3 others on blinatumomab to date. Regarding toxicity during blinatumomab, data are available for the 18 patients having completed the blinatumomab induction to date. All patients had at least one grade 1 adverse event (AE), 10 had grade ≥3 AE. The most common AE (> 1 case), regardless of relationship to blinatumomab, were fever (4 patients), CRS (2 patients), sepsis (2 patients), vein thrombosis (2 patients), anemia (4 patients), neutropenia (3 patients), lymphopenia (5 patients), thrombocytopenia (3 patients) and hyperglycemia (5 patients). In terms of neurologic events, 5 (28%) experienced neurotoxicity (all recovered) including grade 3 encephalopathy, grade 4 confusion, grade 3 anxiety, grade 1 myoclonus, grade 2 ataxia, grade 1 sleep disorder and grade 1 ICANS (each in 1 pt). Blinatumomab was temporarly stopped in 3 patients and permanently in 2. In terms of efficacy, after R-CHOP debulking therapy (n=31 evaluable patients), 7 patients achieved CR, 6 patients were in PR, 7 patients were stable and 11 patients were progressive. At evaluation after the blinatumomab induction (n=18 evaluable patients), 4 (22.2%) patients achieved CR, 4 (22.2%) patients PR, 2 (11.1%) patients were stable and the remaining 8 (44.5%) were progressive. Considering the whole strategy (including R-CHOP debulking) (n=28), 15 (54%) patients achieved overall response including 11 (39%) CR. Conclusions Our preliminary data suggest that blinatumomab suggests encouraging anti-tumor activity and acceptable toxicity in patients with RS. Disclosures Ysebaert: Abbvie, AstraZeneca, Janssen, Roche: Other: Advisory Board, Research Funding. Ferrant: AstraZeneca: Honoraria; Janssen: Other: Travel, Accommodations, Expenses; AbbVie: Honoraria, Other: Travel, Accommodations, Expenses. de Guibert: Janssen: Consultancy, Honoraria; AbbVie: Consultancy, Honoraria; Gilead: Consultancy, Honoraria. Laribi: Astellas Phama, Inc.: Other: Personal Fees; AstraZeneca: Other: Personal Fees; Novartis: Other: Personal Fees, Research Funding; Le Mans Hospital: Research Funding; IQONE: Other: Personal Fees; Jansen: Research Funding; BeiGene: Other: Personal Fees; Takeda: Other: Personal Fees, Research Funding; AbbVie: Other: Personal Fees, Research Funding. Feugier: Abbvie: Consultancy, Honoraria; Gilead: Consultancy, Honoraria; Amgen: Honoraria; Astrazeneca: Consultancy, Honoraria; Janssen: Consultancy, Honoraria. OffLabel Disclosure: blinatumomab is approved for acute lymphoblastic leukemia. The aim of this phase 2 study is to evaluated it in patients with Richter's syndrome.

Anti-CD37 Alpha-Amanitin Conjugated Antibodies As Therapeutic Weapons for Richter's Syndrome

The leukocyte surface antigen CD37 (TSPAN26), a member of the tetraspanin superfamily, is widely expressed on most malignant B cells, making it an actionable target for treatment of patients with chronic lymphocytic leukemia (CLL) and other B-cell non-Hodgkin lymphoma (NHL) indications. Accordingly, αCD37 antibodies have shown promising results in phase 1/2 clinical trials for CLL and NHL. Richter's syndrome (RS) is the transformation of CLL into an aggressive and rapidly fatal lymphoma, typically a diffuse large B cell lymphoma (DLBCL). RS is a challenging disease since very few effective treatment options exist for these patients and the available regimens, mainly based on R-CHOP scheme, show limited efficacy. We recently established patient-derived xenograft (PDX) models from RS patients and have shown that they can be used to test the efficacy of novel drugs and drug combinations 1,2. All available RS-PDX models were characterized by high-levels of CD37 expression, when assessed by RNA sequencing, reverse-transcriptase-polymerase chain reaction (RT-PCR), flow cytometry (FACS), western blot (WB) and immunohistochemistry (IHC). More precisely, two models (RS1316 and IP867/17) showed slightly higher CD37 levels compared to the others (RS9737 and RS1050). These models were used to test three different αCD37-ATACs®, ADCs which comprise amanitin-derivatives as payload. Amanitin (the main poison in the green deathcap mushroom) belongs to the well-known amatoxin family. Amanitin is taken up by OATP1B3 transporter, solely expressed on hepatocytes. Upon mushroom intoxication, it can lead to severe liver toxicity by inhibiting the RNA polymerase II. Upon conjugation to target-specific antibodies, the maximal tolerated dose is significantly increased by reducing the non-specific liver uptake. By binding to its antigen, ATACs deliver amanitin only into target-positive cancer cells while target negative cells show no off-target toxicity. Consistent with CD37 expression on the cell surface, ex-vivo treatment of freshly purified cells from RS-PDX tumor masses to αCD37-ATACs® resulted in increased apoptosis after 72 hours of treatment, with only minor differences among the 3 ATACs® and the models used. Since alpha-amanitin is a deadly toxin known to target human RNA polymerase II and, at high doses, also RNA polymerase III, we checked messenger RNA levels in basal conditions and after CD37-ATAC® treatment by looking at different housekeeping genes, and confirmed a reduction in global mRNA levels. αCD37-ATAC® efficacy was then assessed in vivo in systemic RS-PDX models where RS cells are intra-venously (i.v.) injected in the tail vein and cells distribute to different tissues (blood, spleen and bone marrow), resembling the human disease. Cells from RS1316, RS1050 and RS9737 models were injected into the tail vein and left to engraft 14 days, before mice were randomly assigned to vehicle or ATAC® groups. A single i.v. treatment for each αCD37-ATAC® was administered, testing two different doses for each compound, and mice were then monitored for survival. Overall, the single administration of all three ATACs® caused highly significant disease regression. In the RS1316 model, independently of the dose or tested ATAC®s, all treated mice, except one, were alive and disease-free until the end of the experiment, 140 days post cells injection, while survival of vehicle-treated mice was 65 days. FACS analysis to trace neoplastic cells in parenchymatous organs and bone marrow confirmed the absence of neoplastic cells. In the other 2 models, RS9737 and RS1050, even though ATAC® treatment did not result in complete disease eradication, a single administration of αCD37-ATAC® resulted in a dramatically increased survival (approximately 35-60 days, depending on the model and ATAC® used). Finally, CD37 expression was confirmed by RNA sequencing on a cohort of 15 primary RS samples, even though with variable levels. Compared to CLL cells, RS samples showed CD37 expression levels comparable to those reported for DLBCL cells. Overall, these data indicate CD37 as a potential target to treat RS patients with selective targeting αCD37-ATACs®. ATACs® is a registered trade mark of Heidelberg Pharma Research GmbH, Germany References Vaisitti T et al. Blood. 2021;137(24):3365-3377. Iannello A, et al. Blood. 2021;137(24):3378-3389. Disclosures Orlik: Heidelberg Pharma: Current Employment. Kulke: Heidelberg Pharma: Current Employment. Pahl: Heidelberg Pharma: Current Employment. Deaglio: Heidelberg Pharma: Research Funding; Astra Zeneca: Research Funding.

Utilizing Clinical Features of Progression to Predict Richter's Syndrome in Patients with CLL Progressing after Ibrutinib

Background: Aggressive lymphoma arising in the setting of chronic lymphocytic leukemia (CLL), known as Richter's syndrome (RS), is associated with poor outcomes with standard of care therapies. There is limited capacity for PET-CT to distinguish patients (pts) who develop RS after ibrutinib (Mato Haematologica 2019). Data on outcomes of pts with RS having previously received ibrutinib is limited. Here we sought to determine if clinical characteristics associated with iwCLL criteria for progression (Hallek Blood 2018) predict the risk of RS and overall survival (OS) in pts treated with ibrutinib. Methods: We conducted a retrospective analysis of pts with CLL treated with ibrutinib from 2010-2019 at The Ohio State University. We identified pts that progressed after ibrutinib and classified the progression by 2018 iwCLL criteria. We then identified who developed RS on or after progression. Risk of developing RS was assessed through Fine and Gray model treating death as the competing risk. OS was measured from time of progression and estimated using Cox model. Results: We analyzed 559 pts who had received ibrutinib for CLL, and identified 179 pts who progressed per iwCLL criteria. 94% of the pts who progressed were relapsed/refractory prior to ibrutinib. 116 pts progressed on ibrutinib, and the median time to progression from ibrutinib start was 40.8 months (mos) (range: 0.2-103.9). 63 pts progressed after stopping ibrutinib due to an adverse event or development of a resistance mutation, and the median time to progression from ibrutinib start for these pts was 28.5 mos (range: 0.7-92.9). Of the 179 pts who progressed, 54 developed RS. Of these 54 pts; 83% had enlarging lymphadenopathy, 9% had an enlarging liver or spleen, 17% had constitutional symptoms, 31% had increasing lymphocytosis, 15%, 15%, and 2% had worsening thrombocytopenia, anemia, or neutropenia respectively. No pts had worsening of CLL in the bone marrow (BM) and 2% had new appearance of other organ involvement. As lymphadenopathy and lymphocytosis were the most common clinical features identified we analyzed them jointly; 61% had lymphadenopathy without lymphocytosis, 9% had lymphocytosis without lymphadenopathy, 22% had both, and 7% had neither. Among pts with RS, median time from progression to RS was 0.4 mos (range: 0-49.3). Nine pts had biopsy confirmed RS on the date of progression. Median time from ibrutinib start to RS was 27.8 mos (range: 0.7-92.9). We performed a univariable analysis to determine whether clinical signs of relapse were associated with subsequent risk of RS, and found that presence of lymphadenopathy without lymphocytosis at progression was significantly associated with risk of RS (HR 3.58, 95% CI 1.44-8.88, p=0.006) (Table 1, Figure 1A). To determine if there was an association between clinical features of progression and OS we evaluated all 179 pts that progressed; 72% had enlarging lymphadenopathy, 10% had an enlarging liver or spleen, 15% had constitutional symptoms, 46% had increasing lymphocytosis, 15%, 17%, and 2% had worsening thrombocytopenia, anemia, or neutropenia respectively, only 2% had worsening of CLL in the BM, and 1% had new appearance of other organ involvement. When analyzed jointly; 45% had lymphadenopathy without lymphocytosis, 20% had lymphocytosis without lymphadenopathy, 26% had both, and 9% had neither. Median OS from progression was 24.4 mos (95% CI: 18.6-45.5), while median OS from RS diagnosis was 4.0 mos (95% CI: 2.1-7.1). Median OS from progression was 15.2 mos (95% CI: 7.8-24.6), and 49.9 mos (95% CI: 20.0-NR) for the lymphadenopathy without lymphocytosis and the lymphocytosis without lymphadenopathy groups respectively (Figure 1B). On univariable analysis lymphadenopathy without lymphocytosis was associated with a shorter OS (Table 1). These findings were maintained on multivariable analysis, with lymphadenopathy without lymphocytosis remaining an independent predictor of OS (HR 2.12, CI 1.17-3.87, p=0.01). Conclusions: Here we show that pts who have received prior ibrutinib for CLL who progress with lymphadenopathy have a higher likelihood of having RS than those pts who progress without lymphadenopathy. Furthermore, pts who progressed with lymphadenopathy without lymphocytosis have a shorter OS. Our data suggests that consideration should be given to perform a biopsy to rule out RS in any pts progressing with lymphadenopathy after receiving ibrutinib therapy for CLL. Figure 1 Figure 1. Disclosures Kittai: Abbvie: Consultancy; Janssen: Consultancy; Bristol-Meyers Squibb: Consultancy. Bhat: Beigene: Consultancy; Onclive: Honoraria; AstraZeneca: Consultancy; Aptitude Health: Honoraria. Bond: Kite/Gilead: Honoraria. Byrd: Pharmacyclics LLC: Research Funding; Acerta Pharma: Research Funding; Genentech, Inc.: Research Funding; Janssen Pharmaceuticals, Inc.: Research Funding. Rogers: Genentech: Consultancy, Research Funding; Janssen: Research Funding; Abbvie: Consultancy, Research Funding; Novartis: Research Funding. Woyach: AbbVie Inc, ArQule Inc, Janssen Biotech Inc, AstraZeneca, Beigene: Other: Advisory Committee; AbbVie Inc, ArQule Inc, AstraZeneca Pharmaceuticals LP, Janssen Biotech Inc, Pharmacyclics LLC, an AbbVie Company,: Consultancy; Gilead Sciences Inc: Other: Data & Safety; AbbVie Inc, Loxo Oncology Inc, a wholly owned subsidiary of Eli Lilly & Company: Research Funding.