Combined ibrutinib and venetoclax treatment vs single agents in the TCL1 mouse model of chronic lymphocytic leukemia.

The covalent inhibitor of Bruton's tyrosine kinase ibrutinib and the specific Bcl-2 inhibitor venetoclax are both highly efficacious single-agent drugs in the treatment of chronic lymphocytic leukemia (CLL). Based on their complementary modes of action, ibrutinib and venetoclax are hypothesized to act in a synergistic fashion. Currently, it is unclear whether combined treatment is indeed superior to continuous single-agent treatment and what mechanisms underlie the resistance to combination treatment. In addition, the effects of such treatment on the skewed T-cell compartment characteristic of CLL are as yet unknown. In the murine Eµ-TCL1 adoptive transfer model resembling aggressive CLL, we found that combined treatment resulted in the deepest responses, with the longest duration related to a combination of decreased proliferation and increased induction of apoptosis. In addition, alterations in T-cell subsets were most prominent after combination treatment, with increased naive cells and reduced effector memory cells. Remarkably, effects of single agents but also combination treatment were eventually interrupted by relapse, and we found downregulation of BIM expression as a plausible cause of acquired drug resistance. Nevertheless, in this murine model, the combination of venetoclax and ibrutinib has increased efficacy over single agents, accompanied by a restoration of the T-cell compartment.

Isoform-specific histone deacetylase inhibitors: the next step?

Histone deacetylases (HDACs) have emerged as attractive drug targets, particularly for neoplastic indications. This large family is divided into four classes, of which three consist of zinc-dependent enzymes, and inhibitors of these are the subject of this review. Currently, there are several inhibitors advancing through clinical trials, all of which inhibit multiple isoforms of these three classes. While promising, these compounds have exhibited toxicities in the clinic that might limit their potential, particularly in solid tumors. It may be possible to reduce some of the toxicity by specifically targeting only the isoform(s) involved in maintaining that particular tumor and spare other isoforms that are uninvolved or even beneficial. This review examines the selectivity and toxicity of HDAC inhibitors currently in clinic, comparing pan-HDAC inhibitors to Class I selective compounds. The rationale for isoform-specific inhibitors is examined. The current status of isoform-specific inhibitor development is analyzed, especially inhibitors of HDAC1, 2, 4 and 8 enzymes, and the potential clinical utility of these compounds is discussed.