Complex Role of Histone Deacetylase Inhibitors in the Treatment of Non–Small-Cell Lung Cancer

Abstract

Although cancer was historically believed to be a result of simple genetic mutations in tumor DNA, it has since become apparent that epigenetic alteration of chromosomes, or modifications that do not involve a change in the DNA nucleotide sequence, can also result in the expression of oncogenes and tumor suppressors. Eukaryotic DNA is wound around nucleosome spools, which are composed of eight individual histone proteins. Posttranslational epigenetic modification of histones, via acetylation, methylation, and phosphorylation, increases the accessibility of gene promoter regions to transcription factors and globally upregulates gene transcription. Histone deacetylases (HDACs) oppose the acetylation of histones, promoting the condensation of chromatin and gene repression; HDACs also deacetylate a plethora of other proteins important in oncogenesis, including p53, c-Myc, NFB, HIF-1 , HSP90, and others. HDAC expression seems to be a general feature of malignancy, because many cancers display sensitivity to HDAC inhibitors, prompting the exploration of HDAC inhibitor drugs in the treatment of cancer. Two HDAC inhibitors—vorinostat (suberoylanilide hydroxamic acid [SAHA]) and romidepsin (depsipeptide)—have clinically proven antitumor activity in the treatment of cutaneous T-cell lymphoma. However, the utility of HDAC inhibition in other cancers such as non–small-cell lung cancer (NSCLC) is less established. A randomized phase II trial in NSCLC showed improved response rates when vorinostat was added to first-line carboplatin and paclitaxel. However, a subsequent phase III randomized trial was prematurely terminated because of no anticipated improvement in response rate, progressionfree survival, or overall survival. Current research on incorporating HDAC inhibitors into NSCLC treatment is focusing on their use in combination with epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors (TKIs). The EGFR TKI erlotinib is a standard treatment for NSCLC and has particularly striking activity against tumors with EGFR mutations. Multiple lines of preclinical evidence suggest a complex interplay between the sensitivity of NSCLCs to EGFR TKIs, HDAC inhibitors, and the epithelial-mesenchymal transition (EMT) state. Although all lung cancers initially arise from epithelial cells, activation of the innate EMT developmental regulatory program triggers acquisition of mesenchymal characteristics along with a propensity to invade, metastasize, and resist apoptosis. Baseline sensitivity of many EGFR wild-type NSCLC cell lines to EGFR TKIs is highly correlated with an epithelial phenotype characterized by high e-cadherin expression and low vimentin expression. Cells with a mesenchymal phenotype, characterized by high vimentin and ZEB1 transcription factor activity, can be resensitized to EGFR TKIs with HDAC inhibitors, particularly entinostat, which is thought to cause reversion to an epithelial phenotype. These experiments formed the rationale for the clinical trial published in Journal of Clinical Oncology, in which Witta et al report a randomized phase II study using the HDAC inhibitor entinostat and the EGFR TKI erlotinib in NSCLC. In contrast to pan-HDAC inhibitors like vorinostat, the oral small-molecule entinostat (SNDX-275/ MS-275) belongs to a pharmacologically distinct benzamide class of compounds and inhibits only the class I enzymes HDAC1 to HDAC3, which are highly expressed in a number of cancers. Witta et al found no additional efficacy with the addition of entinostat to erlotinib. However, a preplanned biomarker analysis revealed that the subset of patients with EGFR wild-type tumors and high e-cadherin protein expression had a significantly longer overall survival (12.2 v 5.4 months; P .03) when treated with entinostat and erlotinib compared with erlotinib alone. The clinical data thus partially recapitulate the laboratory work, suggesting that HDAC inhibitors can preserve a pre-existing epithelial phenotype and that such patients will demonstrate sensitivity to erlotinib for a longer period of time. As of yet, we have not seen evidence that HDAC inhibitors can convert a mesenchymal phenotype back to the epithelial state. There is an important distinction between lung cancers harboring EGFR mutations, which confer exquisite sensitivity to EGFR TKI treatment, and tumors that are sensitive to EGFR TKI treatment without a known EGFR mutation. Although much of the preclinical work examining EMT, HDAC inhibitors, and EGFR TKIs has been performed in EGFR wild-type models, newer data suggest analogous relationships in EGFR mutant tumors. Most importantly, in patients with EGFR mutations, emergence of EMT has been directly observed by multiple groups as a mechanism of acquired resistance to EGFR inhibitors. Because EMT is a plastic state, the driving force is unlikely to be irreversible genetic mutations but rather changes in chromatin state or another modulation in cellular signaling pathways. Additionally, there is mounting evidence that autocrine or paracrine signals can promote EMT. In one recent example, treatment of A549 (EGFR wild type) JOURNAL OF CLINICAL ONCOLOGY U N D E R S T A N D I N G T H E P A T H W A Y VOLUME 30 NUMBER 18 JUNE 2