Combinatorial Effect of HDAC6i and Ibrutinib Therapy in CLL Murine Model

Ibrutinib, a Bruton tyrosine kinase (BTK) inhibitor is now FDA approved for front line and relapsed and refractory CLL. Although this drug has been shown to be successful in controlling the disease, most patients only reach partial responses (PR) likely secondary to activation of alternative and redundant BCR signaling pathways. In the past several years, epigenetic changes in CLL have gained special attention (Tong et al 2010) because of their intricate interplay with previously described genetic events and its active role in the regulation of pathogenesis and immune-related pathways (Stilgenbauer et al 2002). Recent publications have described the differential expression of specific HDACs in CLL, as well as the impact of global histone deacetylase activity and its relation with progression and overall survival (Van Damme et al 2014, Yang et al 2015).

Previously, we had reported that expression of HDAC6 is increased in CLL patient samples and cell lines. We have also demonstrated that modulation of this HDAC, effects cell proliferation and viability in CLL cell lines. Additionally, treatment with HDAC6i (ACY-738 - a potent and selective HDAC6i) demonstrated increased overall survival in euTCL1 mice, a murine CLL model. Treatment of euTCL1 mice (both adoptive transfer and aging model) with ACY-738 as a single agent resulted in 1) overall survival advantage, 2) reduction of tumor burden, 3) reduction in PD-L1 expression in the malignant B cell population and 4) decreased circulating Treg numbers. Furthermore, we demonstrated that in an in vitrostudy, treatment of HDAC6i with ibrutinib in CLL cell lines render strong synergistic cell killing. In our current study of this combinatorial approach, using the adoptive transfer euTCL1 model receiving Ibrutinib in drinking water and ACY-738 in feed, we demonstrate a further decrease in tumor burden when compared to single agent treatment with either compound alone. This observed effect on tumor burden occurred in conjunction with decreases in co-inhibitory molecules and circulating Treg frequency. The combination was well tolerated and no significant toxicity was observed.

Since aberrant over-expression of HDAC6 in CLL cell lines and patient's samples have already been demonstrated (Van Damme et al 2012, Sahakian et al 2012) we sought to understand its mechanistic role in BCR survival pathways of CLL. Our studiesin normal B cells isolated from C57BL/6 and HDAC6KO mice, demonstrated a reduction in phosphorylation of BTK, ERK, and AKT. Similar results were observed when we compared euTCL1 to euTCL1/HDAC6KO B cells. Additionally,we observed decreased phosphorylation of ERK and SYK in MEC2-HDAC6KD cells when compared to parental control CLL cells. Moreover, RNA-Seq studies of the eu-TCL1/HDAC6KO versus euTCL1 B cells showed several key BCR signaling proteins altered by the deletion of HDAC6.

In conclusion, these results from our preclinical CLL models suggest that combinatorial therapy of Ibrutinib plus HDAC6i show synergistic inhibition of BCR signaling and therefore a better overall treatment outcome.