A synergistic suppression of aromatase by letrozole and the DAC inhibitor panobinostat (LBH589), an inhibitory modulator of aromatase gene expression.

Abstract

Abstract Abstract #3062 Background. The human aromatase gene contains nine translated exons (II-X) and at least eight tissue specific untranslated exon Is (I.1, I.2, I.3, I.4, I.5, I.6, I.7 and PII). Eight different promoters are located immediately upstream of the corresponding exon I's, and each promoter is regulated by different mechanisms. Previous studies have revealed that exons I.3 and PII are the major exon Is in aromatase mRNA isolated from breast cancer tissue, indicating that aromatase expression in breast cancer is mainly driven by promoters I.3 and II. In normal breast stromal cells and bone tissue, promoter I.4 is the major promoter driving aromatase expression. Furthermore, breast intra-tumor estrogen levels have been shown to play more important roles than circulating estrogens. Therefore, the potential for selective suppression of aromatase expression/estrogen biosynthesis in breast cancer tissues through the down-regulation of breast tumor-specific I.3/II promoters would be a novel approach to possible reduce side effects associated with whole-body suppression of estrogen biosynthesis achievable with aromatase inhibitors (AIs). Results. Our recent research has found that the DAC inhibitor panobinostat is a potent inhibitor of aromatase expression (IC50 = 15 nM; panobinostat is 40 times more potent than SAHA). Panobinostat has been previously found to induce the degradation of proteins driving cancer growth and is currently in Phase II/III clinical development in patients with cutaneous T-cell lymphoma (CTCL). Our results also indicate that panobinostat selectively suppresses the promoters I.3/II of the human aromatase gene. Furthermore, using the H295R cell culture model, we observed that to achieve the same degree of inhibition of aromatase activity, the needed concentration of letrozole (LET; an AI), in the presence of 25 nM panobinostat, was one fifth that required in the absence of panobinostat. Aromatase expression in H295R cells is selectively driven by promoters I.3/II. In addition, cell culture experiments were performed using a H295R/MCF7 co-culture model that demonstrated the synergistic interaction of panobinostat + LET in suppressing the proliferation of hormone-responsive breast cancer cells. Finally, our preliminary results indicate that panobinostat down-regulates the activity of aromatase promoters I.3/II through an epigenetic mechanism, possibly via the acetylation of C/EBPδ which upregulates breast cancer-specific aromatase promoters I.3/II. Discussion. Panobinostat is the first drug identified to selectively suppress the breast cancer-specific aromatase promoters I.3/II and, not other promoters used to drive aromatase expression in non-cancerous tissues. While experiments are being performed to clearly define the molecular mechanisms involved, the results demonstrating synergistic interactions between panobinostat and LET in modifying breast cancer cell proliferation may hold potential value for future clinical evaluation of these agents in combination. Citation Information: Cancer Res 2009;69(2 Suppl):Abstract nr 3062.