Acalabrutinib May Weaken Rituximab’s Efficacy in CLL

Although a high-frequency, low-dose rituximab regimen rapidly cleared circulating chronic lymphocytic leukemia (CLL) cells and shrank lymph nodes within the first week of treatment, adding the Bruton tyrosine kinase (BTK) inhibitor acalabrutinib appears to blunt that effect by stripping CD20 from the surface of surviving leukemia cells, according to new findings from a phase 2 clinical trial published in HemaSphere.¹ The results, drawn from serial blood samplings of 38 previously untreated patients, suggest that the current playbook for combining anti-CD20 antibodies with BTK inhibitors may need rethinking.

Only 1 of the 38 treated patients achieved an undetectable measurable residual disease response after 12 cycles of therapy, raising questions about whether adding rituximab provided any benefit over acalabrutinib alone. For a field that has invested heavily in multidrug CLL regimens, the data offer a cautionary note about the mechanics of combining antibody therapy with kinase inhibition.

How Does Low-Dose Rituximab Clear CLL Cells?

The trial (NCT03788291) tested a high-frequency low-dose approach: 50 mg of intravenous rituximab on day 1, followed by 50-mg subcutaneous doses twice weekly, with oral acalabrutinib (100 mg every 12 hours) added on day 8.² Investigators found that even a 25-mg dose of rituximab dropped circulating CLL cell counts by 85% within the first hour, driven primarily by macrophage-mediated antibody-dependent cellular phagocytosis and complement activation.¹ Palpable lymph node size shrank by 34% within the first week, and 100 mg of rituximab monotherapy reduced adenopathy by 66%.

Notably, clearance stalled after that first hour despite ample rituximab and complement available in the blood. The research team traced this to a temporary exhaustion of the innate immune system’s phagocytic capacity rather than a shortage of drug or complement.

Circulating CLL counts climbed back toward baseline by 48 hours, a rebound the study links to leukemia cells being mobilized from lymphoid tissue and into the bloodstream rather than tumor regrowth. That mobilization dynamic is a recurring theme in CLL care: BTK inhibitors such as ibrutinib and acalabrutinib are already known to push leukemia cells out of lymph nodes and into circulation, a mechanism that clinicians rely on for redistribution before killing the cells systemically.^3,4

How Might Acalabrutinib Undercut the Regimen?

Within a week, expression of MS4A1—the gene encoding CD20, rituximab’s target—fell by 52%, and CD20 protein levels on CLL cells dropped by 44%.¹ Laboratory testing of cells collected from patients confirmed this reduction translated into reduced sensitivity to rituximab- and alemtuzumab-induced antibody-dependent cellular phagocytosis.

This finding builds on earlier work from the same research group showing that ibrutinib, but not acalabrutinib, directly interferes with macrophage phagocytic function.⁵ The new in vivo data suggest a separate, indirect mechanism: rather than disabling the immune effector cells, acalabrutinib appears to reduce the target that those cells are supposed to recognize.

What Are the Cost and Care Delivery Implications?

Payers and health systems continue to grapple with the cost and sequencing implications of multidrug CLL regimens.¹ Standard doses of rituximab and obinutuzumab, often exceeding 500 mg per infusion, far surpass what this study suggested are needed to achieve maximum clearance of circulating B cells, with authors pointing to a potential opening for lower-dose, more frequent schedules that could reduce drug costs and infusion-related toxicity without sacrificing efficacy.

Real-world treatment selection in CLL is already shifting based on patient age and genetic risk factors, with clinicians increasingly weighing BTK inhibitor monotherapy, venetoclax-based combinations, and antibody therapy against each other rather than defaulting to combination regimens.⁶ Recent coverage of venetoclax-rituximab retreatment data also underscores how thoughtful sequencing and retreatment strategies can extend progression-free survival years after initial therapy.⁷

“Based on these data and previous reports, we propose that rituximab therapy in CLL patients is optimized by [high-frequency, low-dose] regimens,” the authors concluded, “which achieve maximum CLL cell clearance at dosing intervals sufficient for recovery of immune cytotoxicity.”

The study authors said, however, that further work is needed to confirm whether the CD20 suppression they observed with acalabrutinib extends to other BTK inhibitors and other CD20-targeting antibodies such as obinutuzumab and daratumumab. For now, the data suggest that clinicians and formulary committees evaluating BTK inhibitor-antibody combinations should look closely at target-antigen dynamics, not just complementary mechanisms of action, when designing or reimbursing combination regimens.

References

1. Chu CC, Heffernan CB, Jaimes-Delgadillo NC, et al. Efficacy of high-frequency low-dose rituximab and acalabrutinib in chronic lymphocytic leukemia. HemaSphere. 2026;10(7):e70430. doi:10.1002/hem3.70430
2. Wallace DS, Zent CS, Baran AM, et al. Acalabrutinib and high-frequency low-dose subcutaneous rituximab for initial therapy of chronic lymphocytic leukemia. Blood Adv. 2023;7(11):2496-2503. doi:10.1182/bloodadvances.2022009382
3. Herman SEM, Niemann CU, Farooqui M, et al. Ibrutinib-induced lymphocytosis in patients with chronic lymphocytic leukemia: correlative analyses from a phase II study. Leukemia. 2014;28(11):2188-2196. doi:10.1038/leu.2014.122
4. Brown JR, Seymour JF, Jurczak W, et al. Fixed-duration acalabrutinib combinations in untreated chronic lymphocytic leukemia. N Engl J Med. 2025;392(8):748-762. doi:10.1056/NEJMoa2409804
5. Pinney JJ, Blick-Nitko SK, Baran AM, et al. The highly selective Bruton tyrosine kinase inhibitor acalabrutinib leaves macrophage phagocytosis intact. Haematologica. 2022;107(6):1460-1465. doi:10.3324/haematol.2021.279560
6. Shaw ML. Age and genetics drive real-world CLL treatment choices. AJMC^®. April 25, 2026. Accessed July 31, 2026. https://www.ajmc.com/view/age-and-genetics-drive-real-world-cll-treatment-choices
7. Shaw ML. CLL breakthrough: 100% efficacy reported for VenR retreatment. AJMC. January 28, 2026. Accessed July 31, 2026. https://www.ajmc.com/view/cll-breakthrough-100-efficacy-reported-for-venr-retreatment