P670: ABSENCE OF BTK, BCL2, AND PLCG2 MUTATIONS IN RELAPSING CHRONIC LYMPHOCYTIC LEUKEMIA (CLL) AFTER FIRST-LINE TREATMENT WITH FIXED-DURATION IBRUTINIB (I) PLUS VENETOCLAX (V)

Abstract

Background: CAPTIVATE (PCYC-1142; NCT02910583) is a multicenter phase 2 study of first-line I+V in patients (pts) with CLL in 2 cohorts: Minimal Residual Disease (MRD) and Fixed Duration (FD). We previously reported that fixed-duration I+V provides deep, durable responses (Ghia et al, ASCO 2021; Wierda et al, J Clin Oncol 2021). Clinically-relevant mutations in BTK, PLCG2, and BCL2 have previously been reported in pts with progressive disease (PD) following single agent BTK or BCL2 inhibitor therapies. Such resistance-associated mutations may occur with increasing frequency after prolonged drug exposure. Fixed duration I+V is hypothesized to decrease the likelihood of developing resistance mutations and facilitate extended clinical benefit. Aims: We sought to ascertain presence or absence of resistance mutations at the time of PD in samples from pts treated with fixed-duration I+V in CAPTIVATE. Baseline genomic risk features and potentially prognostic mutations in CLL-related genes of interest were also evaluated in pts with and without subsequent PD. Methods: The analysis was conducted in 191 pts aged ≤70 y with previously untreated CLL who completed fixed-duration I+V, defined as 3 x 28d cycles of I then 12 cycles of I+V (I 420 mg/d orally; V ramp-up to 400 mg/d orally); this included pts from the FD cohort, and pts from the MRD cohort with confirmed undetectable MRD who received an additional cycle of I+V and were randomized to placebo. Samples from pts at the time of PD were evaluated for resistance mutations by targeted next-generation sequencing (NGS) using a custom-designed high-read-depth 100-gene panel (limit of detection 1% VAF, negative predictive value ≥95% assuming population prevalences <20%, for BTK, PLCG2, or BCL2 variants) in CD19-enriched peripheral blood (PB) or bone marrow. Mutations in additional genes of interest (Table) were evaluated by NGS using the Personalis ACE panel in baseline PB samples. Genomic risk features (Table) were also assessed at baseline. Mutations and genomic risk features evident at baseline were compared across PD and non-PD pts. Results: Baseline mutations in genes of interest and genomic risk features in 190 pts who completed fixed-duration I+V and with available baseline data are shown in the Table. Rates of mutations were similar between pts with (n=30) and without (n=160) subsequent PD. Of 30 pts with PD events (occurring between 16 and 53 mos after start of treatment), 22 (73%) had baseline mutations in genes of interest and 27 (90%) had ≥1 baseline genomic risk feature. Pts with PD generally had more baseline genomic risk features than pts without PD (median 3 vs 2; Wilcoxon P=0.02; analysis restricted to FD cohort pts to avoid bias). To date, 26 of 30 PD patient samples have been evaluated for mutations; no resistance-associated variants in BTK, PLCG2, or BCL2 were detected. Image:Summary/Conclusion: Earlier work has demonstrated a promising safety profile and efficacy of fixed-duration I+V in first-line CLL. Importantly, the current results show that no resistance-associated mutations in BTK, PLCG2, or BCL2 were identified in the pts with PD evaluated to date after first-line, fixed-duration I+V. The absence of such mutations at PD suggests that fixed-duration I+V treatment may offer the possibility of effective subsequent retreatment options with I and/or V and extend clinical benefit.