Clinical Characteristics and Outcome in a Cohort of CLL Patients with BTK T474 Gatekeeper Mutation

Introduction:Targeting Bruton tyrosine kinase (BTK) has revolutionized Chronic Lymphocytic leukemia (CLL) outcomes with demonstrated durable remissions. However, their continuous use results in the development of drug resistance, often mediated by mutations in the drug binding sites on the BTK protien, with the C481S mutation seen in the majority of patients. Several non-covalent BTKi (ncBTKi) have been developed to overcome this mutation including pirtopbrutinib which has demonstrable efficacy leading to its FDA approval for marketing in BTKi/BCL-2 inhibitor treated CLL. Unfortunately, additional mutations develop rarely with cBTKI and much more commonly with ncBTKi resistance, such as the gatekeeper mutation T474, and kinase domain mutation L528. The BTK T474 mutation causes structural changes in the BTK protein that hinder drug access to any site in the protein. Outcomes and data are available for the more common C481 mutation but little to no data is available on the clinical characteristics, behavior, and outcome of patients(pts) whose CLL acquires a T474 mutation. Here we describe 17 pts who acquired this mutation, which represent the largest reported cohort thus far.

Methodology: This is a retrospective study of 17 pts at two academic centers who were treated with BTKi and developed the T474” gatekeeper” mutation. We collected pt, disease, and treatment (tx) characteristics. The median overall survival (mOS)and progression free survival (mPFS) was measured from the date of T474 mutation development using the Kaplan-Meier method. A Cox regression model was used to identify prognostic factors associated with survival.

Results: A total of 17 pts were identified. The median age at CLL diagnosis was 52.9 (range 43.3-73.8). A variety of BTK T474 submutations were seen including 14 (82.3%) T474I, 2 (11.8%) T474F, and 1(5.9%)T474L. T474 mutation was noted more frequently among IgVH unmutated (14 of 17, 82%) as compared to mutated 3 (18%) pts (P value <0.001)). As for FISH, cytogenetic alterations and molecular characteristics, 13(76%) had del13q, 7(41%) had del 17p, 6 (35%) had del 11q, and 4(24%) had trisomy 12 on FISH panel. 8 pts (47%) had complex karyotyping. The most frequent non-BTKi somatic mutations were SF3B1 in 7 pts, followed by TP53 in 6 pts, NOTCH1 in 5 pts, BCOR in 4 pts, and SETD2 in 3 pts. The median number(no) of BTKi therapy received at time of T474 detection was 2 (range 1-4). 6 (35.2%) pts developed the T474 mutation while on Ibrutinib, 5(29.4%) were on acalabrutinib, 5(29.4%) were on pirtobrutinib, and 1(6%) was on investigational ncBTKI. Pts were on BTKi tx for an average of 32 months(mo) (range 4-69) before developing the T474 mutation. 3 pts developed Richter’s transformation after T474 detection. Of the 17 pts, 15 (88%) had concurrent C481 mutations (C481S/F/R/Y) and 4 (24%) had concurrent PLCG2 mutations(combination of PLCG2 L895F, D993H, D3334H, M11411K , S707F). All 17 pts had tx failure after T474 detection. The average BTK T474 variant allelic frequency (VAF) at tx failure was 21% (range 1%-88%), and the average time from T474 detection until tx failure was 6.4 mo(ranging 0.9- 17). At time of BTKi tx failure, 9 pts (of 17 ) had the T474 at higher VAF than C481 or the C481 was present at the same or decreasing VAF from a prior BTKi tx making the T474 mutation the likely cause of failure. 7 pts had a new or an existing C481 detected at higher VAF than T474. 1 pt had PLCG2 mutation at a higher VAF than T474. The median no of tx lines received after tx failure due to the T474 was 1 (range 1-6). Venetoclax was the most common post failure therapy used in 7 pts with mPFS of 28 months (range 7-39) followed by PI3K in 2 pts with mPFS of 10 months, cBTKI in 2 pts with mPFS of 7 months, pirtobruitnib in 1 pt with PFS of 44 months (still ongoing ) and CAR-T in 1 pt with PFS of 2 months. 3 pts received investigational therapies with ongoing response. Median OS after T474 development was 39 months for this cohort.

Conclusion: Here we describe patients and disease characteristics in a cohort of CLL pts who developed a BTK T474 mutation in their CLL while on BTKi tx. The number of pts included in this series is derived predominately from the pre-pirtobrutinib era and includes only a small subset of high risk double refractory (BTKi/BCL2 inhibitor) CLL. Nonetheless, this provides justification for considering patients with T474 gatekeeper mutations as a high-risk population that will continue to expand with FDA approval of pirtobrutinib.