Ibrutinib Response in WM Linked to Clonal Evolution, T-Cell Dynamics

Unraveling Ibrutinib Response and Resistance in Waldenström Macroglobulinemia

Waldenström macroglobulinemia (WM) is a B-cell lymphoproliferative disorder marked by lymphoplasmacytic cell infiltration in bone marrow and other tissues, often causing symptomatic anemia or hyperviscosity [1, 2]. While Bruton tyrosine kinase (BTK) inhibitors like ibrutinib have significantly improved outcomes, clinical responses are not uniform [3, 4]. A subset of patients develops primary or acquired resistance, and adverse events such as atrial fibrillation can limit its use [5, 6, 7, 3]. Even with newer agents like zanubrutinib offering an improved safety profile, the biological drivers of variable ibrutinib efficacy remain a critical knowledge gap for clinicians [8, 9, 10, 11, 12]. A recent prospective study provides a high-resolution map of the molecular events within both tumor cells and the immune microenvironment that dictate treatment success or failure, offering potential new strategies for patient stratification and therapy [13].

Tracking Tumor Clonal Evolution Under Ibrutinib

To define the molecular determinants of ibrutinib response, investigators conducted a prospective phase II trial (ClinicalTrials.gov identifier: NCT02604511) of ibrutinib monotherapy in treatment-naïve patients with WM. The study analyzed 74 sequential bone marrow aspirates from 17 patients, collected from baseline through 48 treatment cycles. Each sample underwent single-cell multi-omics, a powerful technique that examines the genomic and transcriptomic profiles of individual cells, providing a granular view of cellular heterogeneity that is lost in bulk tissue analysis. This approach allowed for the precise tracking of malignant B/plasma cell populations over time. The analysis revealed that these malignant clones followed one of three distinct evolutionary patterns under the selective pressure of ibrutinib. The first, termed "evolution," involved an initial contraction of the tumor clone followed by a later expansion accompanied by increasing genomic complexity. The second, "devolution," showed the opposite: an early clonal expansion followed by a late contraction and genomic simplification. The third pattern, "no-evolution," was characterized by a stable clonal architecture. These neurobiological trajectories were directly linked to clinical outcomes: the "evolution" pattern was strongly associated with disease progression, whereas the "devolution" pattern correlated with a durable clinical response.

A Predictive Biomarker and Actionable Target: The WIP Score and LYN

Building on the analysis of clonal dynamics, the researchers sought to develop a tool to forecast treatment outcomes from the outset. By performing transcriptomic profiling, which measures gene expression activity, on resistant tumor clones, they developed and validated the Waldenström's Ibrutinib Prediction (WIP) score. This molecular signature demonstrated a capacity to predict a patient's response to treatment at baseline, raising the possibility of prospectively identifying individuals who may require alternative or combination therapies. Delving deeper into the gene signature composing the WIP score, the protein LYN emerged as a key regulator of drug sensitivity. In laboratory models, inhibiting or reducing the expression of LYN significantly increased the sensitivity of WM cells to ibrutinib. This finding is clinically relevant because it points to a rational therapeutic combination, suggesting that co-administering a LYN inhibitor with ibrutinib could potentially prevent or overcome tumor-intrinsic resistance.

T-Cell Dysfunction in Ibrutinib Resistance

The investigation extended beyond the tumor itself to characterize the immune microenvironment, revealing that treatment failure is not solely a tumor-driven process. In patients whose disease progressed, researchers observed a significant post-treatment expansion of a specific T-cell subset: GZMB⁺ CD8⁺ effector-memory (TEM) cells. This immune alteration was not an independent event; it occurred in concert with the malignant "evolution" pattern, suggesting a coupled mechanism of tumor adaptation and immune escape. A closer look at these expanded T-cells revealed profound functional impairment. They showed persistently suppressed cytotoxic programs, including reduced expression of granulysin (GNLY), a molecule essential for killing target cells. The cells also adopted a de-differentiated, memory-like state, losing their mature killing capacity. Furthermore, they had elevated expression of PDCD1, the gene encoding the immune checkpoint protein PD-1, and reduced T-cell receptor (TCR) diversity, indicating a narrowed repertoire for recognizing malignant cells. These features collectively paint a picture of T-cell exhaustion, a state where the immune system's primary cancer-fighting cells become ineffective, contributing to ibrutinib resistance.

Clinical Implications and Future Directions

This study provides the first single-cell framework detailing how both tumor clonal evolution and T-cell dysfunction contribute to ibrutinib outcomes in WM. By analyzing 74 bone marrow aspirates from 17 patients, the research moves beyond a static view of resistance to a dynamic model of tumor-immune co-evolution under therapy. For the practicing physician, the findings present two major advances. First, the development of the Waldenström's Ibrutinib Prediction (WIP) score offers a potential biomarker to stratify patients at diagnosis, helping to tailor initial treatment decisions. Second, the study identifies two distinct and actionable mechanisms of resistance. The discovery of LYN as a key mediator of tumor-intrinsic resistance provides a clear rationale for exploring LYN inhibitors in combination with ibrutinib. In parallel, the characterization of T-cell exhaustion, marked by elevated PDCD1 expression and impaired cytotoxicity, suggests that immune checkpoint inhibitors or other immune-modulating agents could be used to restore anti-tumor immunity and improve responses in patients with WM.

References

1. Nakov P, Drandi D, Kaiser L, et al. Determining the Mutational Landscape of Waldenström's Macroglobulinemia By Liquid Biopsy: Results of the Prospective Ecwm-2 Trial of the European Consortium for Waldenström's Macroglobulinemia. Blood. 2024. doi:10.1182/blood-2024-204645

2. Ferrante M, Drandi D, Zibellini S, et al. Prospective Evaluation of Minimal Residual Disease in Waldenström Macroglobulinemia across Different Tissues and Treatments: Results of the “BIO-WM” Trial of the Fondazione Italiana Linfomi (FIL). Blood. 2023. doi:10.1182/blood-2023-181731

3. Castillo JJ, Gustine J, Meid K, et al. Response and Survival Outcomes to Ibrutinib Monotherapy for Patients With Waldenström Macroglobulinemia on and off Clinical Trials. HemaSphere. 2020. doi:10.1097/hs9.0000000000000363

4. Roskoski R. Bruton protein-tyrosine kinase (BTK) FDA-approved small molecule inhibitors used for the management of neoplastic and inflammatory disorders. Pharmacological Research. 2026. doi:10.1016/j.phrs.2026.108187

5. Yun S, Vincelette ND, Acharya U, Abraham I. Risk of Atrial Fibrillation and Bleeding Diathesis Associated With Ibrutinib Treatment: A Systematic Review and Pooled Analysis of Four Randomized Controlled Trials.. Clinical lymphoma, myeloma & leukemia. 2017. doi:10.1016/j.clml.2016.09.010

6. Castillo JJ, Moreno DF, Arbelaez MI, Hunter ZR, Treon SP. CXCR4 mutations affect presentation and outcomes in patients with Waldenström macroglobulinemia: A systematic review.. Expert review of hematology. 2019. doi:10.1080/17474086.2019.1649132

7. Gustine J, Meid K, Dubeau T, Treon SP, Castillo JJ. Atrial fibrillation associated with ibrutinib in Waldenström macroglobulinemia. American Journal of Hematology. 2016. doi:10.1002/ajh.24366

8. Zinzani PL, Williams R, Xue M, et al. A Meta-analysis Investigating Response Rates with Continuous Bruton Tyrosine Kinase Inhibitor Monotherapies in the Treatment of B Cell Lymphomas. Oncology and Therapy. 2026. doi:10.1007/s40487-026-00425-y

9. Tam CS, Opat S, D’Sa S, et al. A randomized phase 3 trial of zanubrutinib vs ibrutinib in symptomatic Waldenström macroglobulinemia: the ASPEN study. Blood. 2020. doi:10.1182/blood.2020006844

10. Tam C, García‐Sanz R, Opat SS, et al. ASPEN: Long-term follow-up results of a phase 3 randomized trial of zanubrutinib (ZANU) versus ibrutinib (IBR) in patients with Waldenström macroglobulinemia (WM).. Journal of Clinical Oncology. 2022. doi:10.1200/jco.2022.40.16_suppl.7521

11. Castillo J, Tam C, García-Sanz R, et al. IBCL-117 ASPEN: Long-Term Follow-Up Results of a Phase 3 Randomized Trial of Zanubrutinib vs Ibrutinib in Patients With Waldenström Macroglobulinemia.. Clinical Lymphoma, Myeloma & Leukemia. 2022. doi:10.1016/s2152-2650(22)01551-8

12. Tedeschi A, Tam CS, Owen RG, et al. Health-related quality of life in patients with Waldenström macroglobulinemia: results from the ASPEN trial.. Future oncology (London, England). 2024. doi:10.1080/14796694.2024.2355079

13. Sun H, Pistofidis RS, Liu S, et al. Evolution of tumor subclones and T-cell dynamics underlie variable ibrutinib responses in Waldenström macroglobulinemia. Blood. 2026. doi:10.1182/blood.2025032268