Fighting Cancer on Her Turf: Deoxynyboquinone

 

        This past July, one of my peers at the Stritch School of Medicine’s Summer Enrichment Program introduced me to his research with DNQ.  I learned that DNQ is short for “deoxynyboquinone,” a potential anticancer agent which is depicted in the figure above. (Ace Website) During the program, a lecture given by an oncologist furthered my interest in cancer research and in the medical specialty of oncology.  I was intrigued to revisit the topic of DNQ on my own time.  I thought, "How exactly does DNQ fight cancer?  How long has it been studied for?"  I began searching for information online regarding DNQ this semester, and I found that I was continually led to an article from my home institution, the University of Illinois at Urbana-Champaign.  The article was titled “Chemistry and biology of deoxynyboquinone, a potent inducer of cancer cell death.” (Bair, et al).  The research group synthesized DNQ in a series of steps and determined how it promotes cancer cell death.  The abstract said that “Experiments performed on cancer cells grown in hypoxia and normoxia strongly suggest that DNQ undergoes bioreduction to its semiquinone, which then is re-oxidized by molecular oxygen, forming superoxide that induces cell death.”  I was unfamiliar with the terms normoxia and hypoxia, as well as semiquinone.  Knowing that these would be important in order to understand the group’s work, I decided to make those terms part of my initial information search.  One definition of semiquinone is “any class of free radicals formed as intermediates in the oxidation of a hydroquinone to a quinone.” (dictionary.com) Additionally, I found that normoxic conditions are environments with normal oxygen levels, and hypoxic conditions are oxygen deficient. (Medical Dictionary) After clarifying these words, I was able to focus on more essential questions.  For instance, what did DNQ have to do with these two particular environments?  The U of I article included the figure below, which depicts the effect of DNQ on cervical cancer cells in normoxic conditions and in hypoxic conditions.  

The graph shows DNQ’s dose dependence for inducing cell death under each of these conditions. This raised another question for me: How do these results with DNQ compare to those of other experimental anti-cancer agents?  Can all potential agents work in both conditions?  The researchers stated that DNQ is “the most potent anti-neoplastic agent that operates predominantly through this direct ROS generation, bioreduction mechanism.”  I read on.  Unlike the experimental therapeutic elesclomol, DNQ is able to induce cancer cell death under hypoxic conditions.  Elesclomol can only induce such death in normoxic conditions, conditions of ample oxygen.  I wondered: is it beneficial that DNQ can function in both?  Yes, as this allows DNQ to “fight cancer on her turf.”  For example, tumors tend to have hypoxic conditions in the interior.  DNQ can work in that environment.  Since DNQ can induce cancer cell death in both of these conditions, it is effective in handling more tumors than anticancer agents that are specific to one or the other.  Once I had a better understanding of DNQ’s ability to work in typical tumor environments, I wanted to learn more about its mechanistic actions.  I wondered, "What does it actually do?"  

         Another article by University of Illinois researchers provided me with such information.  The article discusses “reactive oxygen species” (ROS), chemically reactive molecules containing oxygen. (Genetics) The article indicates that there are increased levels of ROS in cancer cells because of increased metabolism, among other things.  The authors proposed that perhaps disrupting the ROS levels in cancer cells could serve as an anticancer strategy.  The authors stated that inhibition of this stress response or directly generating reactive oxygen species can be useful outcomes of anti-cancer drugs.  I did not fully understand how ROS could aid in killing cancer cells because I had previously studied in a Molecular and Cellular Biology course that ROS promote cell mutation and proliferation.  Later in the article, however, the authors explained that cancer cells have higher oxidative stress levels, yet are also more sensitive to increases in oxidative stress.  Increasing the ROS to a level sufficient to kill cancer cells would be helpful, as long as it is within the antioxidant capacity of healthy cells.  I began to synthesize what I read with what I had learned about DNQ.  I came to the conclusion that in the case of DNQ, increasing the level is beneficial in that it induces the oxidative stress response. (Hergenrother and Parkinson)  Another article explained that DNQ does not act alone in raising ROS levels.  NQO1, an enzyme over-expressed in most solid tumors, is involved.  DNQ generates toxic levels of ROS selectively in cancer cells through a bioreduction/ oxidation process which is mediated exclusively by this enzyme.  

         One might wonder why DNQ is not being used widely today as an anticancer strategy in humans if research supports DNQ's potential as a novel anticancer agent.  After reading additional articles, I came across an explanation.  While there have been studies with rats indicating that DNQ reduces the size of tumors in these rodents, it would be unsuccessful in humans.  This is because DNQ has a poor aqueous solubility, which requires the use of a formulation with a high concentration of 2-hydroxypropyl-Beta-cyclodextrin (HPBCD), which is shown below to the right.

This would complicate human treatment.  However, the researchers did not give up.  They knew they could find a better option for humans---a more soluble, yet equipotent derivative of DNQ. (Bair and Hergenrother).  The authors explained that a subset of derivatives have been found that are equipotent to DNQ and as much as 4-fold more soluble in water, 250-fold more soluble in organic solvents, and 9-fold more soluble in an aqueous solution of HPBCD.  Moreover, studies have shown that mice can tolerate as much as 4-fold more of the best of these derivatives than they could DNQ.  For now, research and extensive human trials must be carried out to determine if these will be successful anticancer agents as predicted.

         Overall, the articles I read answered my question as to how DNQ serves to induce cell death; its mechanism involves reactive oxygen species.  DNQ generates toxic levels of ROS selectively in cancer cells through a bioreduction/oxidation process mediated by NQO1.  Although DNQ reduces the size of tumors in a mouse, it is not used in humans because HPBCD (which is added due to DNQ's poor aqueous solubility) complicates treatment.  Instead, the future will be comprised of identifying DNQ derivatives which are more soluble yet equipotent, testing these derivatives in human clinical studies, and implementing them as an anticancer strategy once deemed safe.  In researching DNQ, I have acquired foundational knowledge that I can build upon as research on DNQ derivatives progresses.  Below is a "Jmol image" of DNQ, and the buttons can be utilized to view either a rotating or stationary image.

         Finally, as a premedical student, researching DNQ and its role in both organic chemistry and medicine has helped me to understand why organic chemistry is a required course for students with this professional goal; it has important implications in oncology and all fields of medicine.  The articles I read on DNQ highlighted the fact that the most extreme, elevated levels of reactive oxygen species are characteristic of pancreatic cancer and other cancers that are unresponsive to drugs.  While I know organic chemistry I and II are important for the Medical College Admissions Test and medical school, it was not until after making this application of the material to a topic of interest to me that I truly grasped its relevance outside of that scope.  Practicing medicine is my ultimate ambition, and seeing the importance of organic chemistry in understanding anticancer agents and drug mechanisms makes my current enrollment in organic chemistry II seem worthwhile.  If students take the time to apply course material to their passions and interests outside of class, it can make learning the material easier and more effective. 

References

            1. Ace Mechanism Calculator Website. <http://aceorganic.pearsoncmg.com/epoch-plugin/public/mechmarvin.jsp>. (Used to create figure 1).

            2. Bair, Joseph S., Rahul Palchaudhuri, and Paul J. Hergenrother. "Chemistry and Biology of Deoxynyboquinone, a Potent Inducer of Cancer Cell Death." Journal of the American Chemical Society 132.15 (2010): 5469-478. Print. 
            3. Bair, Joseph S. and Hergenrother, Paul J. "The Development of Deoxynyboquinone as a Personalized Anticancer Compound." <http://www.chemistry.illinois.edu/events/conferences/Allerton_Conference/documents/Joe_Bair_abstract.pdf>.

            4. Genetics Home Reference: Your guide to understanding genetic conditions. <http://ghr.nlm.nih.gov/glossary=reactiveoxygenspecies>.  2012.

            5. Hergenrother, Paul J. and Parkinson, Elizabeth I.  “Runaway ROS as a Selective Anticancer Strategy.” ChemMedChem 6 (2011): 1957-1959. Print. DOI: 10.1002/cmdc.201100381.  
            6. Huang, et al. "An NQO1 substrate with potent anti-tumor activity." Cancer Research. 16 April 2012.

            7. "Online Dictionary"-Reference for meaning of “semiquinone” <http://dictionary.reference.com/browse/semiquinone>.

            8. "Online Medical Dictionary"-Reference for “normoxia” and “hypoxia” conditions

<http://www.medterms.com/script/main/art.asp?articlekey=33962>.

            9. Sumei Li, et al. Pseudonocardians A-C, New Diazaanthraquinone Derivatives from a Deep-Sea Actinomycete Pseudonocardia sp. SCSIO 01299. Marine Drugs. 2011.