Thompson, Meghan C

Abstract Early-phase clinical trials of Bruton’s tyrosine kinase (BTK) degraders have demonstrated efficacy in patients with BTK inhibitor-resistant chronic lymphocytic leukemia (CLL). How clinical resistance to BTK degraders arises is unknown. Here we sequenced serial CLL samples from patients enrolled in the phase I trials of zelebrudomide and bexobrutideg and observed recurrent expansion of preexisting BTK A428D mutations at relapse. Unlike previously studied BTK inhibitor resistance mutations, BTK A428D conferred pan-resistance to BTK inhibitors and degraders. In the absence of BTK-directed therapies, however, cells bearing BTK A428D exhibited a competitive disadvantage. A crystal structure of BTK A428D revealed that the mutant aspartate clashes with the adenine ring of ATP and the adenine-mimetic moiety of BTK inhibitors and degraders. Combining BTK degraders with venetoclax mitigated the expansion of BTK A428D. These results provide the molecular basis for clinical resistance to BTK degraders and will inform the development of next-generation BTK degrader therapies.

Bruton tyrosine kinase inhibitors (BTKis) and BCL2 inhibitor (BCL2i)–containing regimens significantly improve survival outcomes in patients with chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL). Results from randomized clinical trials have demonstrated that time-limited treatment with BCL2i-containing regimens resulted in higher rates of undetectable measurable residual disease (uMRD) than BTKi monotherapy or chemoimmunotherapy (CIT). Pirtobrutinib (a noncovalent BTKi) and lisocabtagene maraleucel (CD19-directed CAR T-cell therapy) are newer options for relapsed or refractory disease after prior therapy with BTKi and BCL2i-contining regimens. Histologic transformation of CLL/SLL to diffuse large B-cell lymphoma (Richter transformation) is associated with a poor prognosis. Molecular analysis to determine whether there is clonal relationship between CLL/SLL and transformed diffuse large B-cell lymphoma is useful to select an appropriate treatment option. These NCCN Guideline Insights highlight significant updates to the NCCN Guidelines for the treatment of CLL/SLL and Richter transformation.

Abstract Introduction: Lisocabtagene maraleucel (liso-cel) was approved in the United States for the treatment of CLL in 3/2024 for patients who have received two prior lines of therapy including a covalent BTK inhibitor and a BCL-2 inhibitor. Data regarding outcomes of liso-cel is limited to reports from the registrational TRANSCEND-CLL 004 clinical trial which had strict inclusion and exclusion criteria. This work investigates the real-world efficacy and safety of liso-cel for patients with CLL and its efficacy in specific sub-populations. Methods: This is a multi-institutional retrospective study (Collaborative Assessment of Real-World Evidence of Lisocabtagene Maraleucel in Patients with Relapsed/Refractory CLL/SLL [CARE CAR-T CLL]) of patients with CLL who received commercial liso-cel in the United States. Patients with Richter transformation were excluded. Response was defined by iwCLL criteria. Patients who did not undergo a marrow biopsy but met other criteria for a complete response (CR) were categorized as unconfirmed CR (CRu). CRS and ICANS were graded per ASTCT criteria. Results: A total of 30 patients were identified with a median age of 67 years (range 44 – 80). Twenty of thirty patients (67%) had either del17p and/or mutated TP53, and 8/30 (27%) had complex karyotype (≥ 3 abnormalities) at time of liso-cel infusion. Patients received a median of 6 prior lines of therapy (range 1 - 12), and 24 (80%) were previously treated with chemotherapy. All patients had previous exposure to a covalent BTK inhibitor and a BCL-2 inhibitor. Twenty-seven (90%) of patients had prior exposure to pirtobrutinib. Twenty-nine patients (96.7%) received bridging treatment that was stopped after leukapheresis with 1 patient in CR and 25 patients in PR at time of liso-cel. Eighteen patients (60%) were treated with pirtobrutinib as the last line of treatment prior to liso-cel. Median length of time on pirtobrutinib prior to liso-cel infusion was 4 months (interquartile range [IQR], 2 – 8 months). Twelve (66.7%) patients stopped pirtobrutinib between leukapheresis and lymphodepletion, and 6 continued after liso-cel. Best CR/CRu rate was 60.0% (CR, n = 17/30; CRu, n = 1/30), and best overall response rate (ORR) was 83.3% (25/30) with median time to best response of 30 days (IQR, 28 – 30 days). Of the 18 patients who received pirtobrutinib as their last line of therapy, 13 (72%) achieved a CR/CRu. Meanwhile, of the 12 patients who did not receive pirtobrutinib as their last line of therapy, only 5 (28%) achieved a CR. Measurable residual disease testing (MRD) by flow cytometry was done in 14/30 patients, and 9 patients (64%) had achieved undetectable MRD (uMRD). MRD by next generation sequencing (NGS) was done in 17 patients, and 7 (41%) patients had uMRD. Twenty-seven of 30 patients (90%) were alive at time of data collection. Median follow-up was 3.3 months (IQR, 1.2 – 8.9 months). Twenty-six patients (87%) experienced any grade cytokine release syndrome (CRS), and 3 patients experienced grade 3 CRS (10%). Twelve patients (40%) experienced any grade immune effector cell-associated neurotoxicity syndrome (ICANS), and 4 patients (13%) experienced grade 3 ICANS. There were no grade 4 or 5 CRS or ICANS. One patient experienced grade 3 immune effector cell-associated hemophagocytic lymphohistiocytosis-like syndrome (IEC-HS). At day 30, patients had a median absolute neutrophil count of 1.7K/μL (IQR, 0.8 – 3.6K/μL), hemoglobin of 11.7 g/dL (IQR, 8.6 – 13.1 g/dL), and platelet count of 72K/μL (IQR, 29 – 119K/μL). Conclusion: Real-world analysis of liso-cel for patients with relapsed/refractory CLL showed CR and ORR of 60.0% and 83.3%, respectively. Despite having a heavily pre-treated and chemotherapy-exposed population and with over half the patients having high-risk features, these response rates are higher than what was described in the TRANSCEND-CLL 004 clinical trial. These results are limited by the short follow-up period and the variability in response assessment typical of a retrospective study. As we increase our sample size, we will also be interested if the use of the non-covalent BTK inhibitor pirtobrutinib prior to liso-cel is associated with a higher CR rate, mimicking the findings of the sub-cohort analysis of TRANSCEND-CLL 004 where the concurrent use of ibrutinib led to higher CR rates. Further updates and patients from additional collaborators will be presented at the meeting.

Abstract BTK degraders promote proteasomal degradation of BTK and can overcome resistance to covalent and noncovalent BTK inhibitors in patients with relapsed CLL. Currently the molecular basis for BTK degrader resistance is unknown. We therefore set out to understand BTK degrader resistance based on observations from the phase 1 trial of zelebrudomide (NX-2127) (NCT04830137), the first BTK degrader to enter clinical studies. Targeted 500-gene sequencing was performed on blood and/or tissue from four CLL patients treated with zelebrudomide. In one patient previously treated with chemoimmunotherapy, ibrutinib, venetoclax, and pirtobrutinib, BTK A428D was detected in the baseline blood sample with variant allele frequency (VAF) of <2%, along with BTK V416L (13%) and mutations in TP53 (60%) and SF3B1 (37%). Following daily treatment with 100mg zelebrudomide and subsequent dose escalation to 200mg, at 6 months the VAF of all four mutations increased, with BTK A428D VAF increasing to 6% in the blood and 29% in a lymph node. BTK A428D was detected at baseline but not at progression in a second patient. To evaluate the impact of BTK A428D on drug response, we transduced BTK-dependent TMD8 cells with BTK WT, C481S, L528W, and A428D, and found that BTK A428D uniquely conferred resistance to all FDA-approved BTK inhibitors (ibrutinib, acalabrutinib, zanubrutinib, pirtobrutinib) as well as three BTK degraders under clinical evaluation (zelebrudomide, bexobrutideg, BGB-16673). To model the subclonal nature of BTK A428D we conducted a competition assay in which TMD8 cells expressing GFP + BTK A428D (5%) were mixed with TMD8 cells expressing mCherry + BTK WT (95%); over 9 days the GFP + BTK A428D expanded from 5% to 88-97% with all four BTKi and all three BTK degraders. In vitro kinase assays revealed that the BTK A428D mutant protein lacks kinase activity yet facilitates calcium flux and ERK/AKT signaling upon B-cell receptor activation, suggesting a scaffolding effect similar to other kinase deficient BTK mutations. To determine whether BTK A428D affects BTK degradation, we developed a reporter that fuses the BTK kinase domain to GFP. BTK A428D showed minimal degradation following 20 hours of treatment with a titration of all three BTK degraders. Similarly, we knocked BTK A428D into the endogenous BTK locus of HBL1 cells and confirmed a lack of degradation at doses up to 1 µM of all 3 degraders. Consistent with these findings, surface plasmon resonance revealed that the FDA-approved BTK inhibitors as well as zelebrudomide, bexobrutideg, BGB-16673, and AbbVie-1 (Compound 1 from WO 2023183811) are unable to bind to BTK A428D. Additionally, an in-cell NanoBRET assay demonstrated that zelebrudomide was unable to mediate the interaction between cereblon and BTK A428D. Strikingly, NanoBRET also demonstrated that a panel of promiscuous kinase-binding compounds was unable to bind to BTK A428D. The lack of binding of both selective and promiscuous kinase binders to BTK A428D led us to speculate that the mutation disrupted the ATP binding pocket to an unusual degree. We therefore solved the crystal structure of apo BTK A428D and found that, in addition to its predicted steric clash with ligand binding in the hinge region, the mutation led the protein to crystallize with its activation loop occupying the catalytic cleft, rendering it inaccessible. This is the first apo BTK structure observed in this autoinhibited conformation, and explains the observed binding profile. We hypothesized that the addition of a second targeted agent such as venetoclax might mitigate expansion of the A428D subclone. To test this, we introduced the BTK A428D mutation into the endogenous BTK gene in 3% of TMD8 cells and then exposed them to 0.1µM of zelebrudomide, bexobrutideg, or BGB-16673 +/- venetoclax 0.1µM. Combining the BTK degraders with venetoclax slowed the emergence of the BTK A428D resistant population and, in the case of zelebrudomide, eradicated both the wild-type and A428D clones. In summary, we identified and validated BTK A428D as a clinically relevant acquired pan-resistance mutation that confers resistance by blocking access to the ATP binding pocket. This mechanism differs from other kinase dead BTK mutations in which the kinase's enzymatic pocket remained accessible.These data also provide rationale for combining BTK degraders with BCL2 inhibitors, particularly in patients with BTK A428D mutations, or high-risk mutations such as in TP53.

Abstract Background: Undetectable minimal residual disease (uMRD) at the end of first-line (1L) treatment (tx) with 12-cycles (C) of venetoclax (ven) + obinutuzumab (obin) is associated with prolonged survival for patients (pts) with chronic lymphocytic leukemia (CLL, Al Sawaf et al. Blood 2024). We hypothesized that MRD may be used to guide ven tx duration for CLL pts treated with 1L ven + obin. Methods In this ongoing phase II, multicenter investigator-initiated study, pts receive ven + obin for 1L tx of CLL/small lymphocytic lymphoma (NCT04447768). Eligible pts have a CIRS score of ≤ 6 and require 1L tx per iwCLL 2018 criteria. Pts receive 6 x 28-day C of obin and initiate ven on C1D22. Ven duration is dependent on peripheral blood (PB) MRD status measured by ClonoSeq next generation sequencing. At C7, pts undergo MRD testing. Pts uMRD6 (<10-6 sensitivity) at both C7 and C9 dc ven after 9C and enter treatment free observation (TFO). Pts with detectable MRD6 (dMRD6) at C7 undergo repeat testing at C12; if uMRD5 (<10-5) pts dc ven after 12C. Pts dMRD5 at C12 continue ven for an additional 12C (24C total) prior to TFO. The primary study endpoint is 36-month PFS; the unpromising rate is set at 82% and will be tested by a nonparametric survival estimate (Kaplan-Meier) along with Greenwood's formula. RNA sequencing (RNAseq) was performed pre-tx on patient bone marrow (or PB if inaspirable) samples. Unsupervised clustering using machine learning (ML) based on consensus nonnegative matrix factorization was applied to the most variable genes to identify patient clusters. Clusters were then biologically characterized by gene signature scoring using xCell algorithm and assessed for association with uMRD6 at C7 using a Fisher exact test. Results Baseline characteristics of the 100 pts who initiated tx include: median age 58 (range 34-81); 71% male, 90% White, 3% Black and 2% Asian, 5% unknown; 49% IGHV unmutated, 16% TP53 mutation, 8% del17p (previously reported, Roeker et al. ASH 2024). 3 pts stopped tx prior to C7. 51 pts were uMRD6 at C7 and C9 and completed 9C ven. 46 pts were dMRD6 at C7; 6 pts stopped tx outside of meeting protocol requirements and are included in intention-to-treat (IIT) analyses. 32 pts were uMRD5 at C12 and dc ven after 12C. 8 pts were dMRD5 at C12: 5 continued ven per protocol (3 completed 24C ven, 2 ongoing tx as of 15 May 2025 data cut-off), 2 stopped ven outside of protocol requirements (included in ITT analyses), and 1 pt withdrew consent. At a median follow-up of 35 months, the 36-month PFS rate is 90% (95% CI: 84%, 97%). 36-month OS is 94% (95% CI: 89%, 99%). Landmark PFS analyses from EOT were performed for pts who dc ven after 9C due to uMRD6 (24-month PFS 92%, 95% CI: 85, 100%, n=51) and pts who dc ven after 12C due to uMRD5 (24-month PFS 96%, 95% CI: 89%, 100%, n=32). For pts dc ven after 9C due to uMRD6 (n=51), PB MRD at 12C after EOT was: 59% uMRD6, 12% uMRD5, 4% uMRD4, 8% dMRD4 and 18% unknown. For pts dc ven after 12C due to uMRD5 status (n=32), PB MRD at 12C following EOT was: 9% uMRD6, 31% uMRD5, 9% uMRD4, 25% dMRD4 and 25% unknown. Of the 5 pts with PD, 3 pts have had Richter Transformation to diffuse large B-cell lymphoma (all uMRD6 at C7 / 9). Two pts with PD have not met iwCLL tx criteria and remain in TFO. Five pts have died (suicide, West Nile encephalitis, COVID-19, complications of glioblastoma and RT DLBCL). Five C12 dMRD5 pts who continued ven have reached C18, with only 1 of 5 converting to uMRD5 in PB. However, all 5 maintained uMRD4 (<10-4). Of 3 pts that completed 24C ven (2 ongoing tx), 1 pt had deepening of response to uMRD5 at C24, and 2 pts had rising MRD on ven (dMRD4 at C24). To investigate patient heterogeneity at baseline and its relationship to uMRD, we clustered patient RNAseq samples into two clusters using artificial intelligence/ML based unsupervised clustering. The clusters associated with uMRD6 at C7 (Fisher exact test p=0.038), and patients in the cluster with significantly higher immunosuppressive cells such as T-regulatory cells and M2 macrophages were less likely to be uMRD6 at C7. Conclusions At a median follow-up of 35 months, the estimated 36-month PFS rate is 90%. RNAseq transcriptomic analyses identified that patients with enriched immunosuppressive cells were less likely to be uMRD6 at C7. Additional follow-up is required to confirm the durability of this MRD-guided ven + obin tx strategy.

Abstract Background: Venetoclax (VEN)+obinutuzumab (VO) is an effective therapy for CLL/SLL requiring regular monitoring for tumor lysis syndrome (TLS) during VEN ramp-up. The CLL14 trial of VO reported no TLS during VEN ramp-up; however, there are limited real-world data on monitoring practices and TLS outcomes for different dose levels and monitoring timepoints. This study assessed VEN initiation prophylaxis and monitoring outcomes for patients (pts) with CLL/SLL starting VO in first line (1L) in an outpatient, real-world setting. Methods This interim analysis relied onretrospective observational data from 4 US sites of the CLL Collaborative Study of Real-World Evidence (CORE). Adult pts with CLL/SLL were eligible if they: (1) started VO per-label outside of a clinical trial in 1L, or immediately after exposure in 1L to agents with different mechanisms of action (i.e., BTKi, anti-CD20 only) of <30 days; (2) prior to VEN start, had either (i) low physician-determined tumor burden (TB) (i.e., all lymph nodes [LN] <5 cm and ALC <25 x 109/L), or (ii) medium physician-determined TB (i.e., any LN ≥5-<10 cm or ALC ≥25 x 109/L) with creatinine clearance ≥80 mL/min; (3) had no strong CYP3A inhibitors or prophylactic rasburicase from 3 days prior to VEN start until end of VEN ramp-up; and (4) had VEN ramp-up in an outpatient setting. TB-related labs/imaging was assessed from 2 months prior to obinutuzumab initiation (i.e., index date) until VEN start. VEN characteristics (i.e., dosing, duration), prophylaxis measures (i.e., antihyperuricemics and IV hydration), and TLS-related monitoring and management (i.e., lab results and interventions received) were collected per ramp-up week and TLS monitoring timepoint. Results Of 128 pts in CORE who initiated VO in any line, 72 pts met inclusion criteria (low TB: 57 [79.2%]; medium TB: 15 [20.8%]). Median (IQR) age was 62 (56.0, 68.5) years, 58.3% were male, and 81.9% had ECOG grade ≤1. Among pts tested, 7/68 (10.3%) had del(17p)/TP53 mutation and 43/66 (65.2%) had unmutated IGHV. Median (IQR) time from index to VEN start was 23.0 (22.0, 30.0) days. Most pts completed ramp-up (98.6% [71/72]) and followed a per-label regular dosing schedule (93.1% [67/72]). Prior to VEN ramp-up, all pts received prophylactic antihyperuricemics (allopurinol: 71 [98.6%]; febuxostat: 2 [2.8%]) and prophylactic hydration. In the 2 days prior to a dose escalation for at least 1 week of ramp-up, prophylactic IV hydration was reported for 27 (37.5%) pts based on physician determination. One pt stopped VEN after week 2 due to neutropenia (grade 3+). During ramp-up (median [IQR] of 9 [8, 12] blood tests per pt), no lab or clinical TLS was reported. Of 21 (29.2%) pts who received an intervention following a lab assessment, most had low TB at VEN start (16 [76.2%]). All interventions were either prophylactic or non-TLS-related. Following a lab assessment for at least 1 week of ramp-up, prophylactic IV or escalated oral hydration intervention was reported in 17/21 pts (81.0%) based on physician determination (e.g., to manage impaired baseline renal function in those with low TB, patient request, or institutional practice) (IV hydration: 15 pts; escalated oral hydration: 2 pts). By per-label monitoring timepoints, hydration intervention was given prophylactically following pre-dose lab tests for 7 pts (across weeks 1-5), 6-8 hours post-dose for 5 pts (weeks 1-2), and 24 hours post-dose for 10 pts (weeks 1-2). By per-label monitoring timepoints, other non-TLS-related interventions (i.e., blood transfusion for anemia, insulin for hyperglycemia, and potassium for non-TLS-related hypokalemia) were given following pre-dose lab tests for 2 pts (weeks 1-5) and 24 hours post-dose for 4 pts (weeks 1-2). Conclusion Consistent with the CLL14 trial, no TLS was reported in this real-world, interim analysis of pts with mostly low TB and good renal function at 1L VO initation. All interventions during VEN ramp-up were prophylactic or non-TLS-related, with prophylactic hydration commonly given based on physician preference. These results suggest that VO initiation is manageable in an outpatient setting. Consideration should be given for simplified monitoring requirements including potential removal of 6-8 hour post-dose lab in this population and be further explored in prospective studies. Data collection for this study is ongoing to further contextualize real-world VEN initiation prophylaxis and monitoring outcomes in a larger cohort.

Introduction: Complex karyotype (CK, ≥3 chromosome (chr) abnormalities (abn)) is an important adverse prognostic indicator in chronic lymphocytic leukemia (CLL). CK at diagnosis (dx) of CLL, particularly with major structural chr abn (i.e. unbalanced translocation, add, marker, ring chr), called type-2 CK (CK2), has been associated with an increased risk of Richter transformation (RT) (Visentin Haematologica 2022). RT is defined as development of aggressive lymphoma, most often large B-cell lymphoma, in the setting of CLL. In patients (pts) with relapsed/refractory CLL treated with ibrutinib, CK as a continuous variable and the presence of polyploidy were independent predictors for RT. CK is a common finding at the time of RT, and high-CK (≥5) has been associated with short survival in RT, although previous studies predominantly included pts with prior chemoimmunotherapy (CIT) administration for their CLL. The prognostic significance of CK in pts with RT whose prior CLL-directed therapies include only non-CIT, as is the current standard of care for CLL, has not been evaluated. Methods: We conducted a multicenter, retrospective analysis of pts from 14 academic centers. Pts with RT diagnosed between 6/2011 to 6/2023, who had CLL-directed treatment that did not include prior CIT, with stimulated karyotype data available, were included. Pts who did not receive treatment for their CLL prior to developing RT were not included. Baseline pt, disease, and cytogenetics data were collected via chart review at time of CLL diagnosis and at the development of RT. Karyotype abn up to 20 were counted on both CLL tissue (peripheral blood or bone marrow) and RT tissue using IWGMC guidelines (Chun Leuk Res 2010). CK was categorized as CK1 (balanced abn, deletion, aneusomies) or CK2. Polyploidy was defined as ≥58 chrs. Karyotype was considered evolved if one or more chr abn were seen in a follow-up sample of CLL tissue that were not seen at CLL dx.TP53 disruption was defined as presence of del(17p) or TP53 mutation. Overall survival (OS) was measured from RT dx and estimated using the Kaplan-Meier method. Cox regression model was used to identify prognostic factors associated with OS. Results: A total of 64 eligible pts were identified. Median age at RT dx was 65 years (range 40-89) and 71.9% of pts were male. Of those evaluated (n=49), 79.6% had unmutated IGVH. Median number of therapies for CLL prior to RT was 1 (range 1-5) and included BTK inhibitor (n=37), BCL2 inhibitor (n=3), both (n=18), or other non-CIT treatment (n=6). At CLL diagnosis, 54.7% of pts had a CK, and the median number of chr abn was 3 (range 0-20). Of those with CK, 96.9% had a CK2, and 6.3% had a polyploid subclone. TP53 disruption was found in 46% of pts. At time of RT, chr analysis of CLL tissue was available for 38 pts and showed 78.9% had a CK, and the median number of chr abn was 8 (range 0-20). Polyploidy was found in 29.6%. TP53 disruption (46 pts evaluated) was found in 52.2%. Karyotype evolution from dx (35 pts evaluated) occurred in 74.3% of pts. In RT tissue, of those evaluated (n=29), 96.6% were clonally related to the preceding CLL. MYC translocation (45 pts evaluated) occurred in 17.8%. CK (20 pts evaluated) was found in 70%, with a median number of chr abn of 8 (range 0-20). Of those with CK, 33.3% had a polyploid subclone. With a median follow-up of 17.8 months (range 2.1-83.0) there were 42 deaths, with a median OS of 14.3 months (range 6.9-29.1). For this subset of patients with karyotype data available who would go on to develop RT, an exploratory univariable analysis for OS from RT found that increasing number of chr abn as a continuous variable at CLL dx was associated with worse OS (HR 1.10, 95% CI 1.02-1.19, p=0.01). Similarly, at RT, increasing number of chr abn in CLL tissue (HR 1.09, 95% CI 1.02-1.16, p=0.01) and in RT tissue (HR 1.09, 95% CI 1.01-1.18, p=0.03) was also prognostic of survival. Karyotype evolution in CLL tissue at RT (HR 3.02, 95% CI 1.02-8.94, p=0.05) was borderline significant for worse OS. Conclusions: Our multi-center study identified increasing number of chr abn at CLL diagnosis and at time of RT, and clonal evolution in the CLL tissue, are associated with worse OS in pts with RT whose CLL was treated without CIT. Limitations include incomplete cytogenetics data across the cohort and potential variability across institutions in cytogenetics laboratory practices. This provides further evidence for the clinical utility of chr analysis in the modern era of CLL therapy.

Introduction: Bruton tyrosine kinase (BTK) inhibitors are effective therapies for the treatment of chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL). Their use can be limited by intolerance and/or acquired resistance, due to BTK mutations. BGB-16673 is a bivalent small molecule that induces BTK degradation by binding specifically to BTK and the E3 ligase. The E3 ligase catalyzes the transfer of ubiquitin molecules to BTK which marks BTK for destruction by the proteasome. In preclinical models, BGB-16673 degraded wild-type and mutant forms of BTK associated with resistance to covalent and noncovalent BTK inhibitors, leading to tumor suppression. CaDAnCe-101 (BGB-16673-101, NCT05006716) is an ongoing, open-label, first-in-human, phase 1/2 study designed to evaluate BGB-16673 monotherapy in patients with B-cell malignancies. Here, updated results in patients with relapsed or refractory (R/R) CLL/SLL enrolled in the phase 1 portion are presented. Methods: Eligiblepatients must have confirmed R/R CLL/SLL (≥2 prior therapies), an ECOG performance status of 0-2, and adequate organ function. In the US, EU, and Australia, patients must have previously received a covalent BTK inhibitor (cBTKi). BGB-16673 was dosed once daily orally in 28-day cycles. Dose escalation used a Bayesian optimal interval design (6 planned dose levels, 50-600 mg once daily). Primary objectives were to assess safety/tolerability (CTCAE v5.0 and iwCLL hematologic toxicity criteria), and to establish the maximum tolerated dose (MTD) and the recommended phase 2 dose. Dose-limiting toxicities (DLTs) were assessed in the first 4 weeks (cycle 1). A secondary objective was to evaluate the overall response rate (ORR, iwCLL 2018 criteria or 2014 Lugano criteria for SLL), with the first assessment occurring after 12 weeks of treatment. Results: As of May 24, 2024, 49 patients with CLL were enrolled and treated (50 mg, n=1; 100 mg, n=5; 200 mg, n=16; 350 mg, n=15; 500 mg, n=12). The median age was 70 years (range, 50-91 years), and the median number of prior therapies was 4 (range, 2-10), including prior cBTKis (n=45 [92%]), BCL2 inhibitors (n=42 [86%]), and noncovalent BTK inhibitors (ncBTKis; n=12 [24%]). Of tested patients, 63% (31/49) had del(17p) and/or TP53 mutation and 82% (32/39) had unmutated IGHV. The median follow-up was 7.9 months (range, 0.3-23.1 months). Ninety-six percent of patients reported any-grade treatment-emergent adverse events (TEAEs; grade ≥3, 57%), of which the most common (≥25%) were fatigue (35%; grade ≥3, 2%), contusion (29%; no grade ≥3), and diarrhea (27%; grade ≥3, 2%). The most common grade ≥3 TEAEs (≥10%) were neutropenia/neutrophil count decreased (20%) and pneumonia (10%). One patient (2%) each experienced hypertension (grade 1), febrile neutropenia (in the context of COVID-19 pneumonia and norovirus diarrhea), and major hemorrhage. No atrial fibrillation was observed. Three patients (6%) experienced a TEAE that led to dose reduction. One DLT occurred in 1 patient at 200 mg (grade 3 maculopapular rash on day 27; decreased to grade 1 after 5-day hold; patient continues on treatment). The MTD was not reached. Three patients had TEAEs that led to death (septic shock, bronchopulmonary aspergillosis/cerebral aspergillosis, and pneumonia in the context of disease progression; n=1 each); none of the deaths were considered related to treatment. In 49 response-evaluable patients, the ORR (partial response with lymphocytosis or better) was 78% (38/49), and the CR/CR with incomplete hematologic recovery rate was 4% (n=2). At 200 mg, the ORR was 94% (15/16) including the 2 CRs. Median time to first response was 2.8 months (range, 2.6-8.3 months). Seventeen patients remained on treatment for ≥9 months and all 17 have ongoing responses. Responses were seen at the lowest dose, as well as in patients previously treated with a cBTKi, ncBTKi, double- (cBTKi and BCL2i) and triple- (cBTKi, BCL2i, ncBTKi) exposed patients, and in patients with and without BTK mutations. Conclusions: Emerging data from this ongoing, first-in-human study demonstrate that the novel BTK degrader BGB-16673 has a tolerable safety profile and shows promising and deep overall responses in heavily pretreated patients with R/R CLL/SLL, including those with prior BTK inhibitor treatment and BTK resistance mutations.