Design of novel anticancer agents targeting cellular stress response pathways

2016-08-07Cancer cells exploit cellular stress response mechanisms, such as the unfolded protein response and oxidative stress response, to cope with unfavorable growth conditions and to survive through cytotoxic treatments. Cellular stress response pathways are therefore attractive targets for developing novel anticancer agents. We have identified novel small‐molecule compounds that target two players of the cellular stress response pathways—Glutathione S transferase omega 1 (GSTO1) and the 78 kDa glucose‐regulated protein (GRP78). ❧ GSTO1 is an atypical GST isoform, with an active‐site cysteine, that is involved in oxidative stress response. GSTO1 is overexpressed in several cancers and has been implicated in drug resistance. We show that silencing GSTO1 significantly impairs cancer cell survival and proliferation validating GSTO1 as a new target in oncology. Through extensive screening and hit optimization, we have identified a series of chloroacetamide‐containing small‐molecule compounds that are potent GSTO1 inhibitors with activity in the nanomolar range. Crystal structures and biochemical studies revealed a unique reversible covalent binding to the active site cysteine. Potent GSTO1 inhibitors suppressed the growth of cancer cells, enhanced the cytotoxic effects of cisplatin, and inhibited tumor growth without apparent systemic toxicity. GSTO1 inhibitors also blocked IL‐1β secretion from activated monocytes. Our findings demonstrate the therapeutic utility of selective GSTO1 inhibitors as anticancer and anti‐inflammatory agents. ❧ We also have identified small‐molecule inhibitors of the 78 kDa glucose regulated protein (GRP78) chaperone—a key mediator of the unfolded protein response. As a molecular chaperone belonging to the HSP70 family, GRP78 regulates protein quality control and relieves proteotoxic stress. In addition to providing a growth advantage to cancer cells, GRP78 induction can lead to drug resistance. Suppressing GRP78 levels using siRNA has been shown to slow cancer cell progression and overcome drug resistance. Another approach to inactivating GRP78 is by inhibiting its ATPase activity that has not received much attention. Here, we have identified novel classes of small‐molecule inhibitors of GRP78 ATPase activity through a ligand‐based drug design approach. Based on the initial hits, we further optimized several analogues with more potent activity for GRP78 inhibition. The lead compounds induced CHOP significantly under conditions of ER stress and activated autophagy. These compounds induced cancer cell death in a panel of breast cancer cell lines and increased sensitivity to chemotherapeutic agents, further supporting the role for GRP78 in cancer survival and drug resistance. The lead compounds also caused degradation of HSP70 client proteins. In vivo studies in a breast cancer xenograft mice model further demonstrated efficacy of these compounds in impeding tumor growth alone and in combination with standard chemotherapeutic agent, doxorubicin. Taken together, compounds identified in this dissertation provide novel approaches to target cellular stress pathways in cancer and will serve as initial leads for further optimization into more potent compounds