ACQUIRED MUTATIONS IN PATIENTS WITH RELAPSED/REFRACTORY CLL WHO PROGRESSED IN THE ALPINE STUDY.

Some CLL patients who develop progressive disease (PD) during treatment with covalent Bruton tyrosine kinase inhibitors (cBTKi) acquire pathway resistance mutations in BTK or PLCG2. Here, we report gene mutation data from paired baseline and PD peripheral blood samples from 52 patients (zanubrutinib, n=24; ibrutinib, n=28) who, at an early median follow-up time of 25.7 months, progressed on zanubrutinib or ibrutinib treatment in the ALPINE trial (NCT03734016). No BTK mutations were observed at baseline; at PD, eight patients (zanubrutinib, n=5, ibrutinib, n=3) acquired a total of 17 BTK mutations. Among BTK mutations, 82.4% (zanubrutinib, n=11/14; ibrutinib, n=3/3) were at C481. Non-C481 mutations were detected in 12.5% (3/24) of zanubrutinib-treated patients (L528W: n=2, cancer cell fraction [CCF]=9.58% and 17.6%; A428D, n=1, CCF=37.03%); these were not detected in ibrutinib-treated patients. At baseline, 48/52 patients had ≥1 driver gene mutation/s, most frequently in: NOTCH1 (n=21), TP53 (n=19), BRAF (n=10), SF3B1 (n=8), and ATM (n=8). At PD, acquired driver gene mutations were observed in one zanubrutinib-treated patient (TP53, XPO1) and five ibrutinib-treated patients (TP53, n=1 patient; SETD2, n=1; SF3B1, n=1; ASXL1, n=2). Baseline driver gene mutations were not associated with later development of BTK mutations, but patients with ≥2 baseline driver gene mutations were more likely to acquire BTK mutations at PD. In conclusion, patients in this data set had a short treatment duration and a low incidence of BTK mutations, suggesting that mechanisms other than BTK/PLCG2 mutations are driving most instances of early PD. NCT03734016.