Addition of histone deacetylase inhibitors in combination therapy.

Our understanding of the biology of cancer has undoubtedly improved in the last decade. Although remarkable progress has been achieved in the treatment of cancer, much remains to be learned about the delivery of therapeutic agents, particularly with regard to the optimal combination and timing of biologic agents with cytotoxic therapy. Not long ago, the more is better strategy exemplified by high-dose chemotherapy (often resulting in increased toxicity) dominated the research agenda and clinical practices. With the option of biologically targeted therapy, novel agents have driven innovation on the accurate measurement of clinical responses and relevant biologic parameters. Current use of agents targeting epigenetic changes exemplifies this trend in clinical research. Encouraging response rates in patients receiving agents that target epigenetic marks drives continued efforts to identify key laboratory correlates that might help to deliver drugs in an optimal manner and to identify the key biologic features to elucidate a more exact mechanism of action. Histone deacetylase (HDAC) inhibitors represent some of the most promising epigenetic treatments for cancer because they have been proven to reactivate silenced genes and have pleiotropic antitumor effects selectively in cancer cells. The acetylation state of histones is reversibly regulated by two sets of enzymes, histone acetyltransferases (HATs) and HDACs. The activity of these two enzymes regulates, in part, the chromatin architecture; HAT is associated with the transcriptionally active state (euchromatin), and HDAC is associated with the transcriptionally repressed state (heterochromatin). The recruitment of HDAC by DNA methyltransferase and by specific methyl-binding proteins represses transcription by inducing a transcriptionally repressed state of chromatin. Acetylation of the histone can be mediated by multiprotein complexes containing HATs along with inhibition of HDAC activity. Acetylation neutralizes the positive charge associated with the amino group on conserved lysine residues in the histone tails, thereby facilitating access of a variety of factors to DNA. HDACs are also involved in the regulation of biologic functions including cell growth, differentiation, and oncogenesis. HDAC inhibitors have shown promising significant in vitro and in vivo activity against a variety of hematologic and solid tumor model systems. HDAC inhibitors, such as valproic acid (VPA), vorinistat, MS-275, FK228, sodium phenylbutyrate, and others, hinder HDACs with varying degrees of class specificity and directly result in histone hyperacetylation with potential facilitation to the euchromatic state. This mediates a consequent increase in tumor suppressor gene re-expression and may explain part of the anticancer effect of these agents. Active investigation is ongoing to evaluate combinations of HDAC inhibitors with DNA methyltransferase inhibitors such as azacitidine to enhance gene reexpression and clinical activity. Combining these agents with cytotoxic therapy is a rational progression because treatment with an HDAC inhibitor will offer improved access for cytotoxic agents to the target DNA/protein complex. Several preclinical reports have shown that HDAC inhibitors synergize with cytotoxic agents, such as DNA topoisomerase II inhibitors, taxanes, and mitomycin, as well as biologic agents, such as imatinib and gemtuzumab. These studies involve combination treatment of many cancer cell lines, including acute leukemia and colon, breast, endometrial, and thyroid cancer cell lines, and demonstrate the potential impact that inhibiting HDACs can have across all cancer types. When combining agents, the sequence of their delivery may have a profound impact on outcome. In xenograft models, as well as in vitro, synergistic induction of cell death by VPA pretreatment before topoisomerase II inhibitor administration was abrogated when VPA was administered concurrent with or after the topoisomerase II inhibitor. Another challenging aspect in approaching the combination of biologic and cytotoxic therapy is identifying the optimal doses for combination and whether this should be identified as the traditional maximum-tolerated dose (MTD) or as the optimal biologic dose. This challenge faces delivery of many current novel cancer regimens and is being addressed in innovative clinical trial designs. In the case of DNA methyltransferase inhibitors and HDAC inhibitors, a variety of studies suggest that lower doses may lead to superior reversal of methylation and potentially result in improved clinical outcome. Examples of this are improved responses seen in patients with hematologic diseases receiving lower doses of demethylating agents like azacitidine with or without HDAC inhibitors and the statistically significant reduction of prostate tumor growth observed in a xenograft model after treatment with a chronic low-dose HDAC inhibitor, independent of androgen regulation. The article in this issue by Munster et al is an important contribution to the clinical understanding of the role of HDAC inhibitors in combination therapy for solid tumors. This phase I study administered escalating doses of the HDAC inhibitor VPA on days 1 through 3 with epirubicin on days 3 to 44 in patients with solid tumor malignancies. Although histone acetylation was seen in the peripheral mononuclear cells after patients received VPA, increasing the VPA dose to the MTD did not seem to increase clinical response rate or JOURNAL OF CLINICAL ONCOLOGY E D I T O R I A L VOLUME 25 NUMBER 15 MAY 2