“The complete absence of treatment options for chronic Toxoplasma gondii (Tg) infection leave almost two billion people at risk for reactivated toxoplasmosis,” explains Dr. Carruthers. “Congenitally infected individuals or those with weakened immune systems are particularly vulnerable to reactivated toxoplasmosis manifested as fatal encephalitis, myocarditis or loss of vision.” As the leading cause of infectious posterior uveitis and second leading cause of foodborne deaths in the USA, reactivated toxoplasmosis could be substantially reduced in high-risk individuals by eliminating Tg tissue cysts.
“We used new genetic tools to identify an essential role for a cathepsin protease L (CPL) activity during chronic Tg infection, creating an exciting opportunity to exploit a new target for combating reactivated toxoplasmosis,” continues Dr. Carruthers. “To begin addressing this key unmet need, we identified an initial lead dipeptide nitrile CPL inhibitor based on its potential for CNS penetrance and conducted preliminary SAR studies against Tg and human cathepsin L, demonstrating that we could achieve over 100-fold improvement in selectivity for the Tg enzyme in under 20 analogs,” Dr. Larsen adds.
“A medicinal chemistry graduate student, Jeff Zwicker, has been a key contributor to this project,” notes Dr. Larsen. “First, he generated much of the preliminary data needed for the R21/R33 submission, including synthesis of new analogs in the Larsen lab, as well as testing these new analogs for CPL inhibition in the Carruthers lab. Second, he helped write the medicinal chemistry strategy for a Center for the Discovery of New Medicines pilot grant that supported much of this early work. Finally, he has recently been assisting the Center for Structural Biology in trying to obtain the molecular structure of CPL bound to inhibitors, an endeavor that has proved unexpectedly challenging.”
In the R21 phase Drs. Larsen and Carruthers plan to use structure-based design to further optimize potency and selectivity along with improving stability and permeability in test tube or cell culture models, delivering one or more potent, selective, stable, and cell-permeable leads. In the R33 phase they will evaluate and further refine PK and CNS penetrance in mice before measuring maximum tolerated dose and efficacy in an established mouse treatment model for chronic Tg infection. Both phases will feature first-of-their-kind assays for cyst viability developed for the studies. Upon successful completion, this project will yield one or more Tg CPL inhibitors effective for reducing or eliminating tissue cysts, thereby advancing a potential new solution for chronic Tg infection.
Dr. Larsen spent over 22 years in the pharmaceutical industry as a synthetic medicinal chemist before joining U-M in 2007. He is currently Director of the Vahlteich Medicinal Chemistry Core, the mission of which is to assist the University of Michigan biomedical research community in the development of biologically active small molecules, usually identified by high throughput screening (HTS). This entails the design and synthesis of new analogs to improve potency and selectivity, and the refinement of physicochemical properties necessary for good cellular and in vivo activity (including solubility, permeability, and metabolic stability). Dr. Carruthers has more than 30 years of experience defining virulence factors that eukaryotic pathogens use to establish infection, evade host immunity, and cause disease. His team is placing increasing emphasis on identifying new solutions for chronic toxoplasmosis.