Chemotherapy is a standard treatment for patients

Chemotherapy is a standard treatment for patients with advanced disease, including nonresectable, recurrent, or metastatic tumors. Due to chemoresistance, the majority of patients die within three years (Papaspyrou et al., 2011). Indeed, cisplatin and other platinum-based therapies do not present clear benefits to patient\'s survival rates (Adelstein et al., 2012). Intrinsic or acquired resistance limits the efficacy of the cisplatin, as evidenced by 30% of patients with primary disease, and >70% of relapsed patients, showing resistance to cisplatin (Murdoch, 2007). Several mechanisms contribute to cisplatin chemoresistance, including alterations in molecular signaling underlying drug influx, drug metabolism, apoptosis, cell survival regulators and DNA repair (Shen et al., 2012; Huang et al., 2014). Recent studies show that cancer stem beta lactamase inhibitor (CSCs) are involved in tumor development, chemoresistance and radioresistance of different tumors including head and neck cancers (Le et al., 2014). CSCs, a small fraction of the tumor population, are responsible for tumor initiation and growth. CSCs are quiescent, self-renew, and are inherently resistant to chemotherapy and radiotherapy thereby contributing to treatment failure (Dragu et al., 2015; Pozzi et al., 2015; Vidal et al., 2014; Qiu et al., 2015). Conventional chemotherapy targets proliferating cancer cells but fails to target dormant or slow cycling cancer cells. Initially, chemotherapy and radiotherapy destroy the majority of cancer cells, leading to reduced tumor volume. However, the CSCs that evade treatment eventually undergo cell division resulting in disease relapse (Liu et al., 2015b). Therapeutic strategies that target CSCs have the potential to improve overall survival by decreasing the incidence of tumor relapse. One strategy to reduce CSCs is via global acetylation of histones (Shukla and Meeran, 2014; Wagner et al., 2016; Guimaraes et al., 2016). Vorinostat (Suberoyanilide hydroxamic acid, SAHA) is an efficient pan-class I and class II Histone Deacetylase inhibitor approved in 2006 for the treatment of lymphoma (Marks, 2007; Park et al., 2016). Recent studies have demonstrated that Vorinostat has significant anti-tumor activity for many solid tumors (Khan and La Thangue, 2012; Kumar et al., 2015). Vorinostat is currently been used as a single agent in phase II clinical trial for adenoid cystic carcinoma but it is showing poor response rate (Chae et al., 2015). However, Vorinostat has been efficiently used in combination with other chemotherapeutic agents in several solid tumors (Park et al., 2016; Gumbarewicz et al., 2016; Pan et al., 2016; Pettke et al., 2016; Yoo et al., 2016; Pili et al., 2017) and as an efficient sensitizer for salivary gland tumor (Guimaraes et al., 2016). In this study, we investigated the efficacy of combined therapy with Vorinostat (an FDA-approved histone deacetylation inhibitor - HDACi) and cisplatin in reducing CSCs and tumor cells from ACC primary cells. We used the population of CSCs as a biological marker for tumor resistance to therapy. Surprisingly, cisplatin enhanced the accumulation of CSCs in ACC patient-derived xenografts (PDX) and primary cell lines. In contrast, Vorinostat reduced the number of CSCs in PDX samples and from primary tumor cells 8h after treatment. However, prolonged administration of Vorinostat caused an increase in CSCs in ACC primary cells despite an overall reduction in tumor cell viability. We found that the combination of Vorinostat and cisplatin efficiently depleted CSCs and reduced primary tumor cell viability via cellular senescence. Collectively, our findings suggest that combined therapy using Vorinostat and cisplatin may be a viable strategy to prevent the development of tumor resistance in ACC tumor cells.
Materials and methods
Results
Discussion ACC is one of the most common salivary gland cancers, second only to mucoepidermoid carcinoma. Almost one-quarter of all patients diagnosed with malignant salivary gland tumors have ACC, which originates from the intercalated duct of all major and minor salivary glands in the oral cavity. The histological features of ACC vary from a solid cellular growth to tubular or cribriform patterns that are comprised of epithelial and myoepithelial cells. The incidence of ACC is higher during the 5th and 6th decades of life, and clinical progression is considerably slow. Such indolent growth is reflected in the high survival rates within the first 5years of diagnosis (70–90%), particularly when compared to other malignancies. However, the long-term prognosis of patients with ACC is poor, with a survival rate of 35–40% after 15years and 10% after 20years. Moreover, the therapies that target ACC are not standardized (Chae et al., 2015) and are often ineffective in treating advanced or metastatic disease (Dillon et al., 2016). There have been few advances in understanding the biology of ACC because disease incidence is low, resulting in a small number of samples available for use in research. Furthermore, tumor cell lines are difficult to establish due to the heterogeneous cellular composition of ACC tumors. Recent advances in patient-derived xenografts (PDX) for ACC resulted in the development of primary cultures of ACC tumors derived from PDX mice (Warner et al., 2016).