Combining Epigenetic and Cytotoxic Therapy in the Treatment of Solid Tumors

DNA methylation is a clonally inherited mechanism of gene silencing that occurs when a methyl group becomes covalently attached to the 5-carbon position of a cytosine residue at a “CpG” site. This epigenetic process occurs during S phase and is catalyzed by the DNA methyltransferase (DNMT) family of enzymes. Genes with heavily methylated promoter regions cannot be transcribed and are effectively silenced. In cancer cells, many genes, including tumor suppressor genes, are abnormally silenced by DNA methylation. In addition to potentially modulating cancer biology, DNA methylation and associated silencing has been shown to be involved in the development of drug resistance, prompting investigations of the use of hypomethylation therapy to resensitize malignant cells to classical cytotoxic agents. The flattening of the dose-response curve at higher drug doses for many cytotoxic agents suggests that downregulation of expression of factors required for tumor cell killing may be at least as important as overexpression of resistance factors in limiting the ability to cure advanced epithelial tumors. Decitabine is a cytosine analog that is a powerful hypomethylating agent, especially at lower doses. Previous studies have shown that DNA in treated cells becomes progressively hypomethylated after each round of transcription in the presence of decitabine. Gene-specific hypomethylation by decitabine may be responsible for the reactivation of tumor suppressor genes and tumor antigens that are frequently silenced by methylation in neoplasia. Decitabine has shown significant promise in the treatment of hematopoietic malignancies, and is an approved therapy for the treatment of myelodysplastic syndrome (MDS). Its track record in solid tumors has been less impressive. During the last two decades, a handful of clinical trials investigating the use of decitabine in solid tumors have reported high toxicity, particularly myelosuppression, and meager response rates. Trials investigating the combination of decitabine with cisplatin in solid tumors have been similarly disappointing. However, it is important to note that the majority of these studies used dose and schedule combinations that are now recognized to be suboptimal. Clinical observations in MDS have shown that low-dose but highintensity, multiday, and multicycle administration are all key factors in achieving responses to decitabine. The reported solid tumor trials generally used higher, more toxic doses of decitabine, single day administration for one cycle only, and, in the case of combination therapy, concurrent administration of decitabine with cytotoxic agents. Several years ago, Plumb et al showed that resistance to carboplatin in ovarian cancer cells was mediated by hypermethylation and loss of function of the MLH1 mismatch repair gene, and that decitabine resensitized the cells to the platinum compound through hypomethylation induction. In this issue of the Journal of Clinical Oncology, Appleton et al test this concept in a trial combining decitabine and carboplatin in advanced solid tumors. This dose-finding trial uses a series of doses of decitabine that, per cycle, all fall within the range of low doses shown to induce hypomethylation in vitro and in vivo. Furthermore, decitabine was administered 8 days before initiation of cytotoxic therapy, in keeping with preclinical models. The investigators conducted two separate dose escalations of decitabine, the first with carboplatin fixed at area under the concentration time curve (AUC) 5 and the second at AUC 6, concluding that the recommended phase II dosing for this combination is decitabine 90 mg/m administered on day 1 followed by carboplatin AUC 6 on day 8 of a 28-day cycle. Of the 30 patients assessable for response, one patient with melanoma had a partial response and three other patients had stable disease. The majority of responses clustered at the recommended combination dose. A strength of the current report resides in the careful correlative studies. DNA from peripheral-blood mononuclear cells (PBMCs), buccal cells, and tumor tissue was assessed for changes in DNA methylation and gene expression. Consistent with previous studies, hypomethylation in PBMCs was most pronounced in the middle of the cycle. Of some concern, methylation rapidly recovered to baseline by day 20. In studies where decitabine was administered for 5 to 10 days per cycle, recovery did not happen until after day 30, affording a longer therapeutic window for hypomethylation. Also, prior studies have shown a continued linear increase in methylation nadirs as decitabine dose was titrated up to 200 mg/m/cycle, a dose that could not be reached in this study because of toxicity. This may or may not be relevant in light of the fact that, to date, the degree of hypomethylation in PBMCs has not been shown to correlate with disease response; therefore, pushing decitabine to the high end of the low-dose spectrum may not be important. Moreover, what has been shown to be correlated with response in clinical trials in MDS is the number of cycles of decitabine administered. This finding may be explained by the JOURNAL OF CLINICAL ONCOLOGY E D I T O R I A L VOLUME 25 NUMBER 29 OCTOBER 1