SUMMARY: The American Cancer Society estimates that for 2025, about 23,690 new cases of Chronic Lymphocytic Leukemia (CLL) will be diagnosed in the US and 4460 patients will die of the disease. CLL accounts for about one-quarter of the new cases of leukemia. The average age of patients diagnosed with CLL is around 70 years, and is rarely seen in people under age 40, and is extremely rare in children. Patients with CLL often receive continuous therapy with either Brutons Tyrosine Kinase (BTK) inhibitor, time limited therapy with BCL2 inhibitor Venetoclax given along with anti-CD20 antibody Obinutuzumab, or under certain circumstances, chemoimmunotherapy.
Brutons Tyrosine Kinase (BTK) is a member of the Tec family of kinases, downstream of the B-cell receptor, and is predominantly expressed in B-cells. It is a mediator of B-cell receptor signaling in normal and transformed B-cells. BTK inhibitors inhibit cell proliferation and promote programmed cell death (Apoptosis) by blocking B-cell activation and signaling. BTK is a validated molecular target found across numerous B-cell leukemias and lymphomas including Chronic Lymphocytic Leukemia (CLL), Mantle Cell Lymphoma (MCL), and Waldenstrom Macroglobulinemia (WM).
The 3 covalent BTK inhibitors (cBTKi) presently approved by the FDA for CLL/SLL include Ibrutinib (IMBRUVICA®) Acalabrutinib (CALQUENCE®), and Zanubrutinib (BRUKINSA®). Although covalent BTK inhibitors have dramatically improved outcomes for patients with CLL or SLL, they are not curative. Despite the efficacy of covalent BTK inhibitors, treatment failure often occurs through development of resistance or intolerance.
Pirtobrutinib (JAYPIRCA®) is a next-generation, highly selective, reversible, non-covalent BTK inhibitor (BTKi), developed to reversibly bind BTK, deliver consistently high target coverage regardless of BTK turnover rate, and preserve activity in the presence of the C481 acquired resistance mutations. Pirtobrutinib is 300 times more selective in BTK inhibition versus 98% of other kinases tested in preclinical studies, and inhibits both wild type and C481-mutant BTK with equal low nM potency, and has favorable oral pharmacology. Pirtobrutinib is well tolerated and demonstrated encouraging efficacy and safety in early-phase studies, leading to FDA accelerated approval in December 2023 for patients with CLL/SLL who have received ≥2 prior lines of therapy, including both a BTKi and a BCL-2 inhibitor.
Relapsed/Refractory (R/R) Chronic Lymphocytic Leukemia (CLL) and Small Lymphocytic Lymphoma (SLL) in the post–covalent Bruton Tyrosine Kinase Inhibitor (cBTKi) setting remains a major therapeutic challenge. No prospective, randomized trials have previously evaluated treatment options in this population, and real-world data suggest poor outcomes, particularly after sequential covalent BTKi and BCL-2 inhibitor exposure.
Study Design
BRUIN CLL-321 is the first global, randomized, multicenter, Phase III trial conducted exclusively in patients with R/R CLL/SLL previously treated with a cBTKi.
- Design: Open-label, 1:1 randomization to Pirtobrutinib 200 mg PO daily (N=119) vs. investigator’s choice (IC) of Idelalisib plus Rituximab (IdelaR-N=82) or Bendamustine plus Rituximab (BR-N=37).
- Population: 238 patients; median 3 prior therapies; 50% had prior Venetoclax; high prevalence of high-risk genomic features (del[17p]/TP53 mutation ~54%, complex karyotype up to 72%).
- Endpoints: Primary endpoint was Independent Review Committee (IRC)–assessed Progression-Free Survival (PFS). Secondary endpoints included Time to Next Treatment or death (TTNT), Overall Survival (OS), Overall Response Rate (ORR), and Safety.
Patients could cross over from IC to Pirtobrutinib upon confirmed progression, and treatment beyond IRC-defined progression was permitted if clinical benefit was maintained.
Efficacy Outcomes
- PFS: Median 14.0 months with Pirtobrutinib vs. 8.7 months with IdelaR/BR (HR = 0.54; P =0.0002), representing a 46% reduction in the risk of progression or death.
- TTNT: Median 24.0 months with Pirtobrutinib vs. 10.9 months with IC (HR = 0.37), reflecting sustained clinical benefit beyond protocol-defined progression in many patients.
- OS: No statistically significant difference at final analysis (HR = 1.09), likely influenced by crossover (75.8% of eligible IC patients switched to Pirtobrutinib).
- Subgroup Benefit: PFS improvement was consistent across key high-risk subgroups, including those with del(17p)/TP53 mutation, complex karyotype, and unmutated IGHV.
Safety Profile
Pirtobrutinib was generally well tolerated:
- Grade ≥3 adverse events (AEs): 57.7% with Pirtobrutinib vs. 73.4% with IC.
- Discontinuations due to AEs: 17.2% vs. 34.9%, respectively.
- Class-related BTKi toxicities (atrial fibrillation, hypertension, major bleeding) were infrequent, with rates comparable to or lower than background incidence in CLL populations.
- No cases of Richter transformation were reported in the Pirtobrutinib arm, versus three in the IC group.
Clinical Implications
The BRUIN CLL-321 trial establishes Pirtobrutinib as a new standard of care option for patients with CLL/SLL previously treated with cBTKi, offering:
- Significant PFS improvement in a population with historically poor prognosis.
- Prolonged TTNT, which may be more reflective of real-world benefit than PFS alone.
- Favorable safety profile supporting long-term tolerability.
These findings also raise important considerations for sequencing strategies, including potential use of Pirtobrutinib prior to Venetoclax in certain patients, pending further prospective data (e.g., BRUIN-322 trial).
Takeaway for Practice:
For CLL/SLL patients progressing after cBTKi therapy, Pirtobrutinib offers a meaningful advancement, providing durable disease control with a manageable toxicity profile. BRUIN CLL-321 delivers the first randomized evidence supporting treatment in this setting, addressing a long-standing gap in the therapeutic landscape.
Phase III Trial of Pirtobrutinib Versus Idelalisib/Rituximab or Bendamustine/Rituximab in Covalent Bruton Tyrosine Kinase Inhibitor–Pretreated Chronic Lymphocytic Leukemia/Small Lymphocytic Lymphoma (BRUIN CLL-321). Sharman JP, Munir T, Grosicki S, et al. J Clin Oncol. 2025;43:2538-2549
