Institute for Protein Design

September 19, 2013

Rosetta designed protein (Toca 511), now offering hope to brain cancer patients.

September 19, 2013

Brain cancer is a serious unmet medical challenge, and Washington state is one of the leading research clusters working on glioblastoma.  Here we report on how RosettaDesign proteins are being used to treat brain cancer!  It’s an amazing story.

Protein design holds tremendous promise for therapeutic application, and the Institute for Protein Design is closely tracking the progress of Rosetta designed proteins that enter clinical trials.  One of these is a thermostabilized cytosine deaminase from yeast (Figure 1) that was initially developed for anticancer therapy by Dr. Margaret Black at Washington State University, and then further engineered for improved stability and activity by Aaron Korkegian (now working at IDRI) during his PhD studies in the laboratory of Dr. Barry Stoddard at FHCRC, a long time collaborator of Dr. David Baker’s group at the University of Washington.

Figure 1. PDBID:1YSB Structure of the yeast cytidine deaminase (yCD), triple mutant (Ala23Leu, Ile140Leu, Val108Ile magenta).  The enzyme is a homo-dimeric (cyan and green) enzyme, requiring Zn++ for its active site function (grey spheres)

Figure 1. PDBID:1YSB Structure of the yeast cytidine deaminase (yCD), triple mutant (Ala23Leu, Ile140Leu, Val108Ile magenta). The enzyme is a homo-dimeric (cyan and green) enzyme, requiring Zn++ for its active site function (grey spheres)

Now ~8 years later, the code for a protein with features like the thermostabilized Rosetta designed yCD has been incorporated into a novel retroviral gene therapy replicating vector, “Toca 511”, by San Diego based Tocagen, Inc. who are testing it in an innovative clinical trial to help stop the tumors in brain cancer patients.  The way it works is kind of tricky, and like most cancer treatments, it was first tried in mice with human tumor xenografts as reported here and here, and shown to have clear proof of concept before trying it in patients.  Called pro-drug gene therapy (PGT), the treatment involves gene therapy encoding thermostabilized cytosine deaminase together with a pro-drug oral therapy that for cancer cells is “to die for” (Illustrated in Figure 2).”

Figure 2.  Pro-drug gene therapy illustration.

Figure 2. Pro-drug gene therapy illustration.

 

 

Briefly, Toca 511 is injected into brain tumors where it instructs the cells to produce a triple mutant thermostabilized yeast cytosine deaminase protein similar to the one that the Stoddard, Baker and Black groups reported back in 2005.  After allowing time for Toca 511 to spread through the tumor, each patient begins a course of an extended-release oral tablet containing 5-FC (5-fluorocytosine) a well-tolerated anti fungal agent serving as a pro-drug in this case.  The Toca 511 enzyme converts the antifungal 5-FC into the potent anti-cancer drug 5-FU (5-fluorouracil), the active drug that is a suicide inhibitor and works through irreversible inhibition of thymidylate synthase, which interferes with DNA replication, leading to cancer cell death.

This pro-drug activator gene therapy is offering hope for patients, and according to Tocagen staff, it has been used in the treatment of over 50 people thus far.   We at the Institute for Protein Design wish Tocagen and the brain cancer patients all the best of outcomes!

This article was Authored by Dr. Lance Stewart, Sr. Director of Strategy (ljs5@uw.edu) at the Institute for Protein Design, with kind input and guidance from the researchers noted in this article, along with Tocagen representative Dr. Douglas Jolly, and with the aid of web resources linked throughout this posting.