BET Inhibitors in Chronic Lymphocytic Leukemia: JQ1 Synergizes with Venetoclax in Promoting Apoptosis

Background: The bromodomain and extra terminal BET family proteins (BRD2, BRD3, BRD4 and the testis-specific BRDT) play a critical role in mediating gene transcription and have been considered as highly promising targets in several diseases including cancer. Specifically, BET inhibitors block the transcription of key oncogenes (BCL2, C-MYC and CDK6) through the displacement of BRDs and other epigenetic modifiers from chromatin. However, while BET inhibitors were consistently described as highly effective drugs in in vitro experiments, few reports and clinical trials have already demonstrated the rapid emergence of BET inhibitors resistance.

Aim: We examined the efficacy of the BET inhibitor JQ1 in CLL samples, upon short and prolonged treatments, to assess its efficacy and the potential development of resistances.

Methods: MEC-1/EHEB CLL cell lines (p53 wild-type EHEB and p53-mutated MEC-1 cell lines) and primary CD19 positive lymphocytes from CLL patients were subjected to short and prolonged treatment with the BET inhibitor JQ1. Responses were measured in terms of proliferation, cell-cycle and apoptosis.

Results: Treatment of CLL cell lines with the BET inhibitor JQ1 was associated with block of proliferation, cell cycle arrest and apoptosis. Molecularly, JQ1 treatment was associated to c-Myc down-modulation. Notably, treatment with JQ1 caused also loss of viability of primary patient-purified CLL cells. However, starting from 48 hours of JQ1 treatment in CLL cell lines, we observed a compensatory increase of BRDs mRNA levels, which was associated to the development of resistant clones after longer treatments (30 days). These data demonstrate that resistance to BET inhibitors can rapidly emerge and suggest that a combination therapy with other drugs should be explored. Interestingly, we observed that JQ1 was also able to modulate BCL-2 expression in CLL. We therefore assessed the effect of a combination therapy based on JQ1 in association with the BCL-2 inhibitor, Venetoclax. Intriguingly, co-treatment with JQ1 and Venetoclax significantly increased apoptosis demonstrating a synergic action.

Conclusions: Our data demonstrated for the first time that: i) CLL may be effectively targeted by BET inhibitors; ii) short term treatment with BET inhibitors rapidly results in the upregulation of additional BRD family members; iii) prolonged treatment is associated with the development of resistance; IV) BET proteins modulate BCL-2 and c-MYC expression also in CLL; V) the combination therapy based of JQ1 administration in association with Venetoclax is extremely effective in CLL

Discussion: Here, we show that JQ1 treatment results in growth inhibition and apoptosis of CLL cells by down-modulating the expression of the pro-survival genes, C-MYC and BCL-2. Furthermore, our data show that treatment with JQ1 is only effective for a short time while the co-treatment with Venetoclax exerts a synergistic lethality effect in CLL. These data suggest the efficacy of a combination therapy with JQ1 and Venetoclax in refractory TP53 mutated CLL patients.