Prevalence of Resistance-Associated Bruton Tyrosine Kinase (BTK) C481 Mutations By Prior Treatment Status Among Patients with Chronic Lymphocytic Leukemia (CLL)/Small Lymphocytic Lymphoma (SLL): A Real-World Observational Study

Introduction: BTK mutations, particularly at C481, are an important cause of acquired resistance to covalent BTK inhibitors (BTKi) among pts with CLL/SLL. Almost two-thirds of pts whose disease progresses on a covalent BTKi have a BTK C481 mutation detectable at progression. Furthermore, BTK C481 mutations have been detected up to 28 mo before progression among pts with CLL/SLL treated with a covalent BTKi. The early identification of pts who are at risk for developing covalent BTKi resistance is critical to optimize clinical treatment of pts with CLL/SLL. In this cross-sectional study, the prevalence of BTK C481 mutations was evaluated in a real-world population of patients with CLL/SLL.

Methods: Patients with CLL/SLL undergoing standard-of-care treatment at Aarhus University Hospital and Gødstrup Hospital in Denmark were recruited. Eligible patients were ≥18 y old, had a confirmed diagnosis of CLL/SLL, provided a whole blood sample during their clinical visit, and had CLL cells detectable by flow cytometry. DNA was assayed from newly collected blood samples for 6 BTK C481 mutations known to confer resistance to covalent BTKi (c.1442G>C, c.1441T>A, c.1442_1443delinsCT, c.1442G>T, c.1441T>C, c.1442G>A), using droplet digital polymerase chain reaction with assay sensitivities of 0.1% variant allele frequency or less depending on the variant. A patient was considered to have a BTK C481 mutation “detected” if any of the variants were present above the limit of detection. Demographic, clinical, and treatment characteristics were extracted from medical records. Patients were grouped by treatment status at the time of blood collection: treatment naive (group 1), treated with therapies other than a BTKi (BTKi naive; group 2), and previously treated with or currently receiving a covalent BTKi (BTKi experienced; group 3). The primary objectives were to determine the presence of BTK C481 mutations in pts who had not received prior BTKi and the natural history of BTK C481 mutations in BTKi-experienced pts with CLL/SLL. Demographic and clinical variables were summarized descriptively. The prevalence of BTK C481 mutations was calculated and reported as a percentage with estimated 95% CIs.

Results: Between October 2021 and December 2022, 130 patients with clinical and BTK C481 mutation data available were enrolled (group 1, n = 53; group 2, n = 29; group 3, n = 48). The median (range) age at diagnosis was 66 y (48-85) in group 1, 66 y (37-86) in group 2, and 62 y (43-78) in group 3. The median (range) age at first therapy was 72 y (39-86) in group 2 and 65 y (44-80) in group 3. In group 1, 38 pts (72%) had Rai stage 0 disease at diagnosis compared with 8 pts (28%) in group 2 and 13 (27%) in group 3. IGHV was unmutated in 8 pts (15%) in group 1, 13 (45%) in group 2, and 33 (69%) in group 3. In group 2, 18 pts (62%) had received 1 prior line of therapy, 4 (14%) had received 2 prior lines, and 4 (14%) had received ≥3 prior lines (not including therapy received at the time of blood collection). In group 3, 17 pts (35%) had received 1 prior line of therapy, 12 (25%) had received 2, and 11 (23%) had received ≥3. BTK C481 mutations were detected in 0 patients (0%) in group 1, 0 pts (0%) in group 2, and in 10 pts (21%; 95% CI, 10-35) in group 3. Of the 10 pts with detectable BTK C481 mutations, 5 had received a BTKi as prior therapy and 5 were receiving a BTKi at the time of blood collection (Table). The median cumulative exposure to a covalent BTKi in group 3 was 27 mo (range, 1-67), with 32 pts having a cumulative exposure of ≤36 mo. BTK C481 mutations were only detected among the 16 pts with cumulative exposure to a covalent BTKi of >36 mo. Of 37 pts who were receiving BTKi therapy at the time of blood collection, 5 (14%) had a detectable BTK C481 mutation.

Conclusions: In this real-world cross-sectional study, BTK C481 mutations were observed in 21% of BTKi-experienced pts with CLL/SLL, all of whom had a cumulative exposure to BTKis of >36 mo. Because two-thirds of BTKi-experienced pts had <36 mo of BTKi exposure, the prevalence of BTK C481 mutations may be higher in populations with longer BTKi exposure. Further analyses of the natural history of BTK C481 mutations will help identify patients who are at risk of covalent BTKi resistance. Recruitment for this study is ongoing.