PF359 TP53 CLONAL AND SUBCLONAL ARCHITECTURE IN CHRONIC LYMPHOCYTIC LEUKEMIA PATIENTS UNDER IBRUTINIB TREATMENT
Authors: L. Cafforio, L.V. Cappelli, I. Del Giudice, C. Ilari, S. Raponi, M.S. De Propris, P. Mariglia, F.R. Mauro, A. Piciocchi, G. Giuliani, M. Vignetti, A. Guarini, R. Foà
Journal: HemaSphere (2019)

Background: Ibrutinib (IBR) is active in chronic lymphocytic leukemia (CLL) patients (pts) with TP53 aberrations. Aims: To investigate the dynamics of TP53 mutations (TP53 ‐mut) in treatment‐naïve and relapsed CLL pts treated with IBR. Methods: Fifty‐four pts (33 unmutated, 21 mutated IGHV; 7/50 del17p) underwent a longitudinal TP53 monitoring (234 samples), by ultra‐deep sequencing. Forty pts received IBR+rituximab (IBR+RTX) front‐line in the GIMEMA LLC1114 protocol (NCT02232386) (median IBR exposure: 32 months; range 14–44) (cohort 1), while 14 pts received IBR single agent after a median of 1.5 (range 1–4) lines of chemoimmunotherapy (IBR exposure: 2.1 to 4 years in 12 pts) (cohort 2). Samples were sequenced to obtain a 5000X coverage/base on a MiSeq Illumina platform. Mutations were checked on the IARC TP53 database. VAF was corrected to cancer cell fraction (CCF) by the % of CD19+/CD5+ cells. Results: In cohort 1, at baseline (T0) 16/40 (40%) pts showed a total of 22 TP53 mutations (1.4 TP53 ‐mut/pt; range: 1–5) and 24/40 (60%) were wild‐type (WT). Of 22 mutations, 16 (73%) were clonal (mean VAF: 60.3%; range: 18–94%) and 6 (27%) subclonal (mean VAF: 5.3%; range: 2.1–9.2%). According to CCF, TP53‐ mutated pts followed 4 patterns (Figure): 1) clonal TP53 ‐mut present from T0 and persisting clonal with a stable CCF (n = 6); 2) clonal TP53 ‐mut present from T0 and persisting clonal with a decreasing CCF (n = 6); 3) subclonal TP53 ‐mut evolving to clonal (n = 1); 4) subclonal TP53 ‐mut disappearing (n = 1). In addition, 2 cases showed coexisting clonal and subclonal TP53 ‐mut at T0 without a distinct mutational pattern. Overall, 9 TP53 ‐mut (41%) (8 clonal+1 subclonal) persisted stable, 7 (32%) clonal decreased, 1 (4%) subclonal increased, 5 (23%) (2 clonal+3 subclonal) were lost and none emerged. In the 24 TP53‐ WT pts, no novel mutation was acquired over time. In cohort 2, prior to IBR 10/14 (71%) pts showed a total of 31 TP53 mutations (3.1 TP53 ‐mut/pt; range: 1–11) and 4/14 (29%) were WT. Of 31 mutations, 11 (35.5%) were clonal (mean VAF: 31.9%; range: 10.5–78.8%) and 20 (64.5%) were subclonal (mean VAF: 2.9%; range: 0.9–6.8%). After IBR, 17 TP53 ‐mut (55%) (7 clonal+10 subclonal) persisted stable, 2 (6.5%) clonal decreased, 11 (35.5%) (2 clonal+9 subclonal) were lost, 1 (3%) subclonal evolved to clonal and 2 novel subclonal emerged. No mutation was identified in the 4 WT pts over time. Overall, in both cohorts, most of TP53 ‐mut remained stable (41% vs 55% in cohort 1 and 2, respectively), some decreased (32% vs 6.5%) and some others were lost (23% vs 35.5%) (p = NS). Notably, while no novel mutation arose in cohort 1, 2 novel mutations emerged in cohort 2. Although the lymphocyte count significantly decreased during IBR+RTX/IBR exposure (cohort 1: 39.1x10 9 /L vs 11.1x10 9 /L, p &lt; 0.017; cohort 2: 48.5x10 9 /L vs 15.3x10 9 /L, p = 0.015), the mean CCF of the existing mutations remained stable on treatment (cohort 1: 62.4% vs 45.3%, p = NS; cohort 2: 16.9 % vs 13.02%; p = NS). Summary/Conclusion: In most TP53 ‐mutated pts, IBR appears to decrease the TP53 clonal and subclonal numerosity and complexity both when used front‐line or as a subsequent line of therapy. The emergence of novel mutations or the expansion of a pre‐existing mutation is a rare event. In TP53 WT pts, treatment with IBR never induced the emergence of novel TP53 ‐mut after an exposure of more than 2 years. The significant decrease of lymphocytosis with stable CCF, proves the IBR effectiveness both on TP53 mutated and WT cells, regardless of previous therapies. image