Date Published: March 25, 2019
Publisher: Public Library of Science
Author(s): Dmitri Rozanov, Anton Cheltsov, Aaron Nilsen, Christopher Boniface, Isaac Forquer, James Korkola, Joe Gray, Jeffrey Tyner, Cristina E. Tognon, Gordon B. Mills, Paul Spellman, Aamir Ahmad.
To determine the target of the recently identified lead compound NSC130362 that is responsible for its selective anti-cancer efficacy and safety in normal cells, structure-activity relationship (SAR) studies were conducted. First, NSC13062 was validated as a starting compound for the described SAR studies in a variety of cell-based viability assays. Then, a small library of 1,4-naphthoquinines (1,4-NQs) and quinoline-5,8-diones was tested in cell viability assays using pancreatic cancer MIA PaCa-2 cells and normal human hepatocytes. The obtained data allowed us to select a set of both non-toxic compounds that preferentially induced apoptosis in cancer cells and toxic compounds that induced apoptosis in both cancer and normal cells. Anti-cancer activity of the selected non-toxic compounds was confirmed in viability assays using breast cancer HCC1187 cells. Consequently, the two sets of compounds were tested in multiple cell-based and in vitro activity assays to identify key factors responsible for the observed activity. Inhibition of the mitochondrial electron transfer chain (ETC) is a key distinguishing activity between the non-toxic and toxic compounds. Finally, we developed a mathematical model that was able to distinguish these two sets of compounds. The development of this model supports our conclusion that appropriate quantitative SAR (QSAR) models have the potential to be employed to develop anti-cancer compounds with improved potency while maintaining non-toxicity to normal cells.
Despite the advances achieved in the detection and treatment of early cancer that have contributed to declining cancer-specific mortality in the United States, metastatic cancer remains in most cases an incurable disease. In this context, identifying new drugs and designing more efficacious and safe cancer treatments to prevent relapse in patients and to treat metastatic disease are clearly needed to provide an impact on cancer mortality rates.
Recently, we published that NSC130362 inhibits GSR and, as a consequence, induces ROS and subsequent apoptosis in cancer cells but not in normal human hepatocytes as a model for hepatotoxicity . Elevated ROS as compared to normal cells have been detected in almost all cancers. ROS promote tumor development and progression. To cope with the increased ROS, tumor cells also express increased levels of antioxidant proteins. A goal for ROS-based cancer therapy will be the ability to either target the antioxidant proteins that are used by tumor cells to detoxify from ROS or affect ROS producing pathways that result in increased ROS production. By achieving this goal, the ROS-based treatment could result in selective induction of apoptosis in cancer cells [1–7]. The aim of this study was two-fold: (1) to identify specific biological activities or structural features, which are responsible for the observed selective anti-cancer activity and (2) to determine whether a QSAR model could be designed for the development of potent anti-cancer compounds that are non-toxic toward normal cells. The NIH has defined several necessary conditions that should be met for validation of a compound to be target for optimization studies. Some of these conditions such as reproducible response in different assay types and adequate anti-tumor activity were addressed in our previous work . To further validate NSC130362 as a candidate molecule for further characterization and/or development, we performed additional cell-based assays. ROS inducers and GSR inhibitors are also considered as drug sensitizers [22–24, 37]. Indeed, we showed that NSC130362 potentiated anti-tumor potency of all tested pancreatic and prostate cancer drugs and decreased EC50 (cytotoxicity) of several kinase inhibitors by up to 3-logs (1000 times) in leukemia cells directly ex vivo from patients. The ability to potentiate the cytotoxic activity of various anti-cancer agents is a valuable property of NSC130362. This conclusion is based on the fact that under treatment, cancer cells often adapt and develop resistance to anti-cancer drugs. To maximally exploit chemotherapy as a therapeutic strategy, it would be meaningful to combine anti-cancer agents with compounds that target these adaptive mechanisms. One of the mechanisms of adaptation that can confer chemoresistance is upregulation of antioxidant pathways. In agreement, there is accumulating evidence showing improved treatment outcome when anti-cancer drugs are combined with ROS-inducing agents [22–24]. In line with this evidence, our data also suggest that anti-cancer activity of the NSC130362/ATO combination is affected by the reducing environment inside cancer cells. It may be the case that targeting GSR activity, which controls the reducing environment in cancer cells, makes them susceptible to ATO and thus inducing apoptosis by altering redox-sensitive proteins and enzymes . The reason that the effect of hypoxic conditions on the activity of the NSC130362/ATO combination is not pronounced is likely because ETC can function efficiently at oxygen levels as low as 0.5% . However, we also noticed that NCS130362 had negative effect on the action of some compounds as seen in Fig 3A and Supporting Information S1 Table. We think that NSC130362 could have negative effect on the action of some compounds because these compounds could exert negative effect on cancer cells by their antioxidant activities . Because NSC130362 induces ROS and subsequent oxidative stress, its action could counteract the action of antioxidants.