Background: Advantages of covalent Bruton Tyrosine Kinase inhibitor (cBTKi) treatment (tx), as compared to BCL2 inhibitor (BCL2i) containing regimens, include ease of initial administration without a requirement for frequent laboratory monitoring, even in patients (pts) with bulky disease. However, limitations of cBTKi monotherapy include acquired resistance, cumulative toxicity and the financial burden associated with continuous administration. CLL pts treated on the E1912 study who discontinued (dc) ibrutinib for reasons other than progression of disease (PD) had a median progression free survival (PFS) of ~2.7 years from ibrutinib dc, suggesting that a time-limited cBTKi strategy may be feasible for some pts(Shanafelt et al. Blood Advances 2025). We hypothesized that acalabrutinib (acala) and obinutuzumab (O) could be administered as a time-limited initial tx for CLL with duration guided by minimal residual disease (MRD).

Methods: This is an ongoing phase II, multicenter, investigator-initiated study of acala and O for previously untx pts with CLL/small lymphocytic lymphoma (SLL) requiring tx per iwCLL 2018 criteria (NCT04722172). Pts with TP53 aberrancy are excluded. Pts receive acala 100 mg twice daily and O during 28-day cycles (C) 2-7. Undetectable MRD is defined with a 10-4 sensitivity in the peripheral blood (PB) by ClonoSeq next generation sequencing (uMRD4). Pts undergo MRD testing at C13; if uMRD4 at C13 with a complete response (CR) or partial response (PR), pts dc acala and enter tx free observation (TFO). If detectable MRD4 (dMRD4) at C13, pts continue acala until uMRD4, with repeat MRD testing occurring at C16, 19, 22 and 25. All pts dc acala after a maximum of 26C and enter TFO, even if dMRD4. If pts have PD meeting iwCLL criteria for tx during TFO, re-tx consists of acala + 6 cycles of O. Pts are followed for 13C of re-tx. Primary endpoint is 36-month PFS.

Results: 55 pts initiated tx. Baseline characteristics include: median age 67, 64% male, 47% LN ≥ 5 cm, 73% IGHV unmutated. Best overall response rate (ORR) was 95% (CR: 4, PR: 48, SD: 0, PD: 1, unknown: 2). Best uMRD4 rate at any time during the study was 49% (n=27 pts, intention-to-treat, IIT). 5 pts dc tx prior to C13 MRD testing (2 pt decision, 2 AE, 1 due to PD after ~4C). At C13, 22 pts (40% of IIT) were uMRD4; 21 entered TFO after 13C and 1 pt had 19C tx prior to TFO. 28 pts had dMRD4 (51% ITT) at C13. Between C13 and 26, 2 pts became uMRD4 and entered TFO, 1 pt died (COVID-19), and 2 pts stopped acala for tx of other cancers. 21 pts completed 26C acala per protocol prior to entering TFO; 2 additional pts had PD at C25 and EOT. Median tx duration was 19C. In univariate analyses, age, del11q, del13q, IGHV mutational status and baseline LN ≥ 5 cm were not predictive of uMRD4 at C13.

At a median follow-up of 38 months, the median PFS for the overall cohort is 42 months (95% confidence interval (CI): 37, not reached), with an estimated 36-month PFS of 69% (95% CI: 57%, 84%). At data cutoff, of 46 pts entering TFO, 29 pts remained in TFO without PD, 7 pts had PD during TFO but had not met iwCLL criteria for re-tx, and 10 pts had PD requiring tx (5 pts re-tx pe protocol, 5 came off study for non-protocol tx approach). The overall median TFO period (measured from acala dc to PD) was 25 months (95% CI: 21 – not reached, n=46 pts).

5 pts have initiated re-tx with acala + O (1 pt best response PR and uMRD4; 4 ongoing tx). Median time to re-tx was 18 months (range 6-34, n=5 pts).

Conclusions: This study is proof-of-concept that cBTKi may be administered as a time-limited tx without the addition of a BCL2i. While the initial PFS is shorter than historical studies examining continuous cBTKi, of patients entering TFO after 1-2 years of acala, the median duration of TFO until PD is 25 months, and longer follow-up will provide re-tx outcomes. Potential advantages to MRD-guided, time-limited cBTKi tx include time off tx for pts, limiting acquired resistance, the ability to re-challenge with cBTKi at progression, and reserving BCL2i for third-line or later tx.

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2025
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