Selective Bcl-2 Inhibition With ABT-199 Is Highly Active Against Chronic Lymphocytic Leukemia (CLL) Irrespective Of TP53 Mutation Or Dysfunction

CLL is characterized by universal overexpression of the anti-apoptotic protein Bcl-2 which promotes inappropriate survival and chemotherapy resistance. ABT-199 is a Bcl-2 selective BH3-mimetic molecule currently in phase I clinical trials for the treatment of CLL. Del(17p) is a key negative prognostic indicator in CLL, associated with reduced response rates, progression-free and overall survival with standard therapy. Reduction or loss of TP53 function is considered the biological basis for its impact. Consistent with this, TP53 mutations in CLL lacking del(17p) also confer similar negative prognostic effects. BH3-mimetics are small molecule inhibitors that theoretically act downstream of TP53. We therefore hypothesized that ABT-199 will be equally effective in patients (pts) with CLL irrespective of TP53 function. We aimed to: (i) examine the incidence of TP53 aberrations in a heavily pretreated group of patients referred to two centers for ABT-199 therapy; (ii) determine the impact of TP53 aberrations on in vitro and clinical responses of CLL to ABT-199; and (iii) determine if CLL cells deficient in TP53 pathway function are as sensitive to ABT-199 as CLL with intact TP53 function.

This work was conducted in parallel with phase I clinical trials of ABT-199 monotherapy (NCT01328626; n = 25) or monotherapy followed by combination with rituximab (NCT01682616; n = 5) in pts with relapsed and refractory (r/r) CLL, after written, informed consent. Pts were tested for potential TP53 aberration using (i) FISH for del(17p); (ii) bone marrow (BM) immunohistochemistry (IHC) for elevated TP53 expression (>20% CLL cells staining positive on BM trephine); (iii) a 72hour in vitro nutlin-3a cytotoxicity assay for functional assessment of the TP53 pathway; and (iv) targeted next generation sequencing of the TP53 gene (with confirmatory Sanger sequencing) to detect deleterious mutations. Del(17p) and TP53 status of CLL samples at screening was determined and correlated with in vitro responses to ABT-199 after 24 hours and objective clinical responses to therapy with ABT-199 as determined within the trial protocol using iwCLL criteria.

In this r/r group of CLL pts, 13/30 (43%) had evidence of del(17p) by FISH (range 9 - 90% of cells); 15/28 (54%) had evidence of TP53 mutation by sequencing; and 7/29 (24%) had TP53 overexpression by IHC, all of whom had TP53 mutations. For 27 pts with data for all three assays, ten had no evidence of TP53 abnormality. In the 10 samples with >20% del(17p) by FISH, deleterious TP53 mutations were detected in all. For the purposes of comparing responses to ABT-199 by TP53 status, three categories were identified: negative for both del(17p) and p53 mutation, positive for one only, and positive for both (table 1). CLL cells in all groups were similarly highly sensitive in vitro to ABT-199 (ANOVA p>0.05). The overall response rate was at least 87.5% in all groups. Given that CLL is characterized by heterogeneity for TP53 aberrations within individual patients, we selected four samples with high level del(17p) and TP53 mutations for assessment of TP53 function. All showed resistance to nutlin 3a-induced cytotoxicity (median IC₅₀ 12mM [95%CI 9 – 41mM]) when compared with CLL cells without del(17p) or TP53 mutations which were 15 – 30-fold more sensitive. No differences in vitro sensitivity to ABT-199 were detected when these nutlin-3a resistant samples were compared with cells lacking TP53 mutations or del(17p).

Defects in the TP53 pathway are present in the majority of patients with heavily pretreated CLL; novel treatment for such patients must include activity against TP53 aberrant clones. CLL cells lacking TP53 pathway function have similar sensitivity to ABT-199 in vitro as do TP53 pathway intact cells, and clinical responses were observed in ≥87.5% of patients. These data suggest that ABT-199 should be evaluated alone and in combination in pts with TP53-aberrant lymphoid neoplasms.

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Anderson:Abbvie: Research Funding. Off Label Use: ABT-199 is an unlicensed drug currently in phase I and II studies in patients with chronic lymphocytic leukemia. Seymour:Roche: Honoraria, Membership on an entity’s Board of Directors or advisory committees; Genentech: Honoraria, Membership on an entity’s Board of Directors or advisory committees. Huang:Genentech: Employee of Walter and Eliza Hall Institute which recieves commercial income related to ABT-199. Other, Research Funding; Abbvie: Research Funding. Roberts:Abbvie: Research Funding; Genentech: Employee Walter and Eliza Hall Institute which recieves commercial income related to ABT-199. Other, Research Funding.