Introduction
Despite the introduction of newer therapeutic protocols, mortality rates associated with head and neck squamous cell carcinoma (HNSCC) have remained largely unchanged over the last four decades [1]. Concurrent radiation and chemotherapy have become the standard for adjuvant therapy after surgical ablation, as well as the definitive treatment of HNSCC in select cases. Systemic platinum-based chemotherapy, namely, cisplatin, remains a first-line agent due to its radiosensitizing and cytotoxic effects [2], [3]. Unfortunately, chemotherapy and radiation-based treatments have been associated with significant toxicity, particularly in patients receiving concurrent chemoradiotherapy [4]. HNSCC has shown marked resistance to radiation and cisplatin in many cases, and treatment resistance requiring dose escalation and resultant toxicities continues to be problematic, highlighting a need for the development of novel therapies that effectively treat this disease and its cisplatin resistance [5], [6].
Cancer stem cells (CSCs) represent a subpopulation of cells within a tumor that have the ability of self-renewal and regeneration. Recent literature suggests that this population of cells is thought to significantly contribute to tumor proliferation, invasion, metastatic potential, and resistance to drug therapy [7], [8], [9], [10], [11]. This subpopulation may lie quiescent for periods of time as well as harbor protective mechanisms against cellular damage, and is felt to be responsible for the majority of tumor growth and metastatic potential in HNSCC [11], [12]. Furthermore, CSCs have been shown to contribute to resistance against chemotherapeutic agents, including platinum-based regimens along with external beam radiation [5], [13]. Nor et al. using an HNSCC xenograft model observed that cisplatin treatment increased the fraction of CSCs as defined by the ALDHhigh/CD44high populations, again implicating this cellular subpopulation in resistance to current standard-of-care therapies [14]. Thus, it seems to reason that in order to more effectively treat or abrogate chemo- and drug resistance in HNSCC, this process should involve some level of targeting of the CSC population.
Heat shock protein (Hsp) 90 is a molecular chaperone protein that regulates several “client” proteins involved in cancer development, including proteins involved in pathways critical for cell growth, invasiveness, and survival [15]. Numerous proteins implicated as critical for CSCs’ development are also dependent on Hsp90. This suggests a high therapeutic potential for Hsp90 inhibitors as they can simultaneously suppress multiple oncogenic pathways involving the bulk tumor cell population of a cancer as well as its CSCs. Use of first- and second-generation Hsp90 inhibitors targeting the N-terminal domain of the chaperone was restricted due to dose-limiting toxicity, resulting mainly from activation of the heat shock response leading to induction of compensatory proteins (e.g., Hsp70) with prosurvival effects [16]. Thus, early-generation N-terminal Hsp90 inhibitors have not progressed beyond early-phase clinical trials despite showing potent anticancer effects. To address limitations of N-terminal Hsp90 inhibitors, our group has developed potent, novel Hsp90 inhibitors targeting the carboxy terminus of the chaperone which blocks Hsp90 chaperone function without concurrently upregulating Hsp70 and its prosurvival effects, thus avoiding this key limitation of N-terminal inhibitors [17], [18], [19], [20], [21]. These compounds have potential to act synergistically with current standard-of-care therapies and prolong or prevent development of drug resistance [16], [22]. Hence, we hypothesized that C-terminal Hsp90 inhibitors (especially our lead compounds KU711 and KU757, chosen for their potency and selectivity for cancer cells; structures in Supplemental Figure 1) can inhibit key CSC functions including migration, invasion, self-renewal, and epithelial to mesenchymal transition (EMT); can target the miRNAs involved in CSC function; and can reduce tumor growth of HNSCC xenografts.
