A Phase I II study of paclitaxel plus carboplatin in combination with vorinostat is cur rently underway in Denmark for patients with advanced, recurrent, platinum sensitive selleck chemicals Imatinib Mesylate epithelial OC. Further trials with correlative studies focusing on the BRCA1 pathway are needed to define a subset of the patient population which is most responsive to HDAC inhibitors. There are several limitations to this study which merit consideration. Firstly, we recognize that studying the mechanism of BRCA1 down regulation by an HDAC inhi bitor exclusively in cancer cell lines provides limited data that requires further exploration in an in vivo model. This will allow the involvement of extracellular components, such as the hormone estrogen, which has been shown to play a role in BRCA1 function.

Secondly, we and others have observed a lack of correlation between the BRCA1 mRNA and protein levels. This can be partly explained by the expression level of BRCA1 which oscil lates with the cell cycle and is regulated by both transcrip tion and protein stability. BRCA1 protein can be degraded by BARD1 in S phase through the ubiquitin pro teolysis pathway, thus unbalancing the mRNA to protein ratio. Discrepancies between BRCA1 mRNA and pro tein can also be due to experimental limitations. Western blot analysis using the C terminal BRCA1 antibody cap tures all splice variants of the gene but is unable to detect truncated forms. Furthermore, BRCA1 11b, a splice variant abundantly expressed in many cells, is not captured by the primers designed to cross the exon 11 12 boundary, which are used to measure mRNA levels by RT PCR in our study.

Thirdly, we propose that the enhanced sensitivity to cisplatin seen by HDAC inhibition is mediated though a BRCA1 mechanism although we are unable to provide direct evidence for this correlation. However, there is evidence in other reports that BRCA1 plays an essential role in inducing apoptosis in response to DNA damaging agents in breast cancer cell line models. Inhibiting BRCA1 protein in MCF 7 cells increased cispla tin sensitivity and depleted BRCA1 protein expression by siRNA inhibited activation of the apoptotic pathway in response to DNA damaging treatment. Furthermore, BRCA1 transcription is known to be activated by the tran scription factor E2F1. E2F1 protein levels were depleted with valproic acid exposure in prostate cancer cell lines and Entinostat valproic acid reduced E2F1 binding to the BRCA1 promoter, thus providing insight into a mechan ism for the down regulation of the BRCA1 gene by HDAC inhibition. This study suggests that treatment with an HDAC inhibitor enhances the cytotoxicity of cisplatin therapy in ovarian and breast cancer cells and that this increased sensitivity may be mediated by a BRCA1 mechanism.