MS-275 and Letrozole Treatments Inhibit Tumor Growth and Reduce Phosphomonoesters in Triple Negative MDA-MB-231 Tumors

Introduction: Histone deacetylase (HDAC) inhibitors have been found to reverse the epigenetic profile of some genes, including estrogen receptor (ER), epidermal growth factor receptor (EGFR), and retinoic acid receptor (RAR) -β2 (1). Here we have studied the metabolic effects of the HDAC inhibitor MS-275, the aromatase inhibitor letrozole, and their combination in vivo in the ER/PR/Her-2 negative (triple negative) MDA-MB-231 human breast cancer xenograft in SCID mice. The highest tumor growth inhibition was achieved with the combination treatment supporting the possibility that reactivation of the transcription of the ER by MS-275 rendered MDA-MB-231 human cell xenografts sensitive to aromatase inhibition in vivo. We observed a significant decrease of phosphocholine (PC) and total choline (tCho) with MS-275 as well with combination treatment. However, a significant decrease of phosphoethanolamine (PE) was observed for the combination treatment Materials and Methods: Twenty mice were injected with 5x10 MDA-MB-231 cells subcutaneously in the mammary fat pad of female SCID mice. Tumors were allowed to grow to approximately 5 mm before treatment was initiated. Mice were randomized and divided in 4 groups consisting of a control group which received saline alone, a second group which received MS-275 alone, a third group which received letrozole alone, and a fourth group which received both MS-275 and letrozole. MS-275 was dissolved in 30% hydroxypropyl-β-cyclodextrin to achieve the target concentration of 0.75 mg/ml. Each mouse received 100 μl (2.5 mg/kg/day) of 0.75 mg/ml of MS-275 daily for 2 week via oral administration using a feeding tube. Letrozole was dissolved in 0.3% hydroxypropyl cellulose in saline. For each treatment mice were injected daily via intraperitoneal (i.p.) injection with 100 μl (400 μg/kg) of 100 μg/ml letrozole. Tumor volume was measured twice a week for the duration of the study. Relative tumor growth was determined and compared between the groups. Once tumors reached 10 mm in size in approximately two weeks, mice were sacrificed and the tumors excised and snap-frozen in liquid nitrogen. Tumor extracts were made using dual phase chloroform-methanol method as described (2). Metabolites were estimated and compared between the groups. The Mann-Whitney U test was performed to test for statistically significant differences (p <0.05) between the groups Results and Discussion: Combination treatment with MS-275 and letrozole resulted in a significant reduction (p<0.05) of tumor growth (Figure 1). While letrozole or MS-275 alone also resulted in a reduction of tumor growth, this was to a lesser extent than the combination treatment.