Malignancies of B-cells have been treated using a variety of strategies to interfere with malignant cell survival pathways. Recent approaches include the development of targeted therapies in cancer treatment, particularly that of the class of targeted protein degradation (TPD), which is conceptually distinct from conventional enzyme inhibition by creating degradation of the targeted protein.
BGB-16673 (Tacabrutideg) is a novel, investigational Bruton's tyrosine kinase (BTK) protein degrader being studied in patients with B-cell malignancies, including those whose diseases have relapsed or are refractory following treatment with covalent BTK inhibitors. Early-phase studies are assessing its safety, efficacy, and potential treatment uses in patients with B-cell malignancies.
Understanding the Role of BTK in B-Cell Malignancies
BTK is a key component of B-cell receptor (BCR) signaling pathways. In B cells, these signaling pathways control activation, proliferation, migration, and survival. In malignantly transformed B cells, dysregulation of these same pathways supports the growth advantage of tumor cells.
Chronic lymphocytic leukemia (CLL), small lymphocytic lymphoma (SLL), and mantle cell lymphoma are all B-cell malignancies that are treated with covalent BTK inhibitors. Nevertheless, disease will progress on these drugs in many patients due to the emergence of BTK alterations that abrogate drug sensitivity.
Another reason for the research into new ways of targeting the BTK protein is that B cells from patients with B-cell malignancies can develop resistance to treatment with covalent BTK inhibitors. Some of these resistance mutations affect the BTK protein itself, and so finding a new way of targeting BTK may be effective even in cancer cells with such mutations.
What Makes Tacabrutideg Different?
Tacabrutideg is a protein degrader for BTK, meant to destroy the protein rather than to inhibit its enzyme activity using a competitive substrate. Instead, it uses the protein's own destruction pathway in the cell.
A way in which degradation of BTK could be distinct from inhibition of its enzyme activity is in the disruption of B-cell receptor signaling. In situations where BTK inhibitors, even those that are covalently attached to BTK, are no longer effective (because of mutations involving BTK, for example), degradation of the BTK protein itself is being evaluated as an alternative strategy to switch off B-cell receptor-dependent malignancy.
While a biologically distinct mechanism of action is an important aspect to consider for the development of new therapeutic approaches for cancer treatment, the clinical relevance of such an approach needs to be demonstrated in clinical trials and compared to standard treatment approaches in terms of clinical endpoints.
Current Clinical Development
The development of Tacabrutideg (BGB-16673), a novel Bruton's tyrosine kinase (BTK) protein degrader, is currently ongoing in patients with B-cell malignancies. Preliminary data have been generated in a phase 1/2 study for dose escalation and expansion in patients with the following B-cell malignancies: marginal zone lymphoma, follicular lymphoma, mantle cell lymphoma, CLL/SLL, Waldenström macroglobulinemia, diffuse large B-cell lymphoma, and the transformation of DLBCL to Richter's syndrome.
The broad program for the investigational drug Tacabrutideg will reflect the unique mechanism of action of this drug as a BTK degrader, a distinct mechanism of action compared to inhibitors of this enzyme. Because of the differences between diseases caused by aberrant BTK signaling, as well as differences between treatment settings for a single disease, each disease will need to be studied in separate trials to determine its optimal use.
Emerging Evidence in CLL and SLL
The early clinical evidence of Tacabrutideg activity has been reported among patients with relapsed or refractory CLL/SLL. Phase 1 data updated in 2026 reported an overall response rate of 85.1% (95% CI: 68.1% to 95.5%) across all doses evaluated and 94.1% (95% CI: 58.4% to 100%) at the RD of 200 mg Tacabrutideg. The reported median PFS was 24.4 months, with a median study follow-up of 25.4 months. Furthermore, 50.7% of patients remained on treatment at the data cutoff.
The study reported that the median time on study for patients was 24.4 months, with a median study follow-up of 25.4 months. Fifty-seven percent of patients remained on study at the time of the data cutoff. It is too early to tell how this agent will be positioned compared to approved therapies for CLL/SLL. The early-phase data will need to be compared to results from larger, randomized studies to fully evaluate its role in treatment strategies for patients with CLL/SLL.
Moving Toward Comparative Evidence
The comparative efficacy of Tacabrutideg to other treatments, including other BTK inhibitors (e.g., Pirtobrutinib), will be demonstrated through ongoing and planned phase 3 trials, including a study comparing Tacabrutideg to Pirtobrutinib in adults with CLL/SLL relapsed or refractory after covalent BTK inhibitor therapy.
Additional phase 3 study in CLL/SLL patients previously treated with both BTK and BCL2 inhibitors, compared to the investigators' choice. The results from these studies will be crucial in comparing protein degradation with current BTK inhibitors as well as other treatment modalities for patients with CLL/SLL who have failed previous lines of therapy.
Considering Patient Selection
An important consideration for the future use of Tacabrutideg will be selection of appropriate patient populations. The eligibility criteria of the current studies (broadly a Phase 2-like study in relapsed/refractory CLL/SLL) are noted in detail.
As one example, the Pirtobrutinib versus Tacabrutideg Phase 3 trial (NCT05097866) compares Tacabrutideg with Pirtobrutinib in patients with previously treated CLL/SLL who are relapsed/refractory after prior treatment with a covalent BTK inhibitor.
It is also important to remember that evidence generated in heavily pretreated populations with poor outcomes to prior therapy must be carefully considered in the treatment of newly diagnosed patients with CLL. When selecting a treatment, health care professionals must consider many factors related to the patient, including the patient's disease, prior treatment, performance status, comorbidities, organ function, treatment goals, and other clinical evidence.
Safety and Tolerability
There is little clinical data available for Tacabrutideg to assess safety in patients with cancer receiving treatment with this targeted therapy. Tacabrutideg was reported to have a favorable safety and tolerability profile in heavily pretreated patients in the 2026 data.
More information on the safety of Tacabrutideg is needed with longer follow-up of patients and in larger randomized studies to better compare its safety profile with other treatments for CLL/SLL. Efficacy must be balanced with treatment-related adverse events, treatment discontinuation, patient quality of life, and other factors related to long-term treatment of patients with cancer.
The Broader Future of Targeted Protein Degradation
The clinical development of Tacabrutideg may help inform the broader understanding of targeted protein degradation in cancer research. Inhibiting protein degradation could potentially allow for the targeting of additional proteins as well as providing a mechanism to combat resistance to small molecule inhibitors of proteins. Studies are also underway to evaluate the combination of Tacabrutideg with other treatments in patients with relapsed or refractory B-cell malignancies.
Looking Ahead
Ongoing clinical studies are evaluating the potential of Tacabrutideg in B-cell malignancies. It targets the key BTK protein, with a unique approach for its destruction in cancer cells, for use in patients whose disease has progressed after prior BTK-inhibitor treatments.
More research is required to establish the clinical role of Tacabrutideg, including data from ongoing and future randomized clinical trials that compare it to established treatments for specific patient populations and clinical scenarios. As additional evidence emerges and becomes mature, these data will need to be carefully delineated from standard-of-care treatment options and incorporated into clinical practice on a case-by-case basis, ultimately determining whether degradation of BTK proteins will represent a viable additional therapeutic option for B-cell malignancies.
Medical Disclaimer: The information provided in this article is intended for educational purposes only and should not be interpreted as medical advice, clinical guidelines, or a recommendation for any specific treatment.
