Gene therapy meets RNA interference—and patients

More than a decade ago David Suhy and his colleagues began developing the drug that would become known as TT-034. Targeted at patients with hepatitis C virus, the drug had a double whammy of distinction.

First, it works by exploiting RNA interference—the recently discovered mechanism for turning down the expression of proteins, found throughout the plant and animal kingdoms. RNAi acts through double-stranded RNAs called short interfering RNAs (siRNAs), which can gum up the works for expression of a specific protein that complements one strand of the siRNA.

Second, and unlike most other RNAi drugs then feverishly under development, TT-034 doesn’t work by introducing siRNAs themselves into an organ or the bloodstream. Instead, TT-034 embeds a small DNA molecule known as a plasmid inside an adeno-associated virus (AAV). The virus is engineered to end up in liver cells where hepatitis C lives, and not to replicate there. The DNA is engineered to generate short hairpin RNAs, which are released into the liver cell’s cytoplasm and reshuffled into siRNAs that stop the expression of hepatitis C proteins.

Six weeks ago, after years in the lab and exhaustive tests in mice, a patient at the Duke Clinical Research Unit received an initial infusion of TT-034.

“This is the first time somebody has gone in with a viral vector encoding for shRNA directly into the patient,” Suhy told me the evening the trial kicked off, as he waited for a flight home from his trip to Duke. The TT-034 study is also one of the first efforts to deliver agents via AAV.

With this gene therapy approach, “instead of degrading like a typical small molecule drug once you get it inside the cell, the DNA plasmid basically gets the cell to produce its own therapeutic,” pointed out Suhy, senior VP for R&D at Benitec Biopharma. Animal studies showed that TT-034’s introduced genes keep churning out therapeutic for months.

TT-034 also is designed to express three shRNAs that target three independent sites on the hepatitis C viral genome. “Viruses replicate very sloppily, and when they do, they tend to develop mutations that can oftentimes overcome single treatment applications,” he explained. “Three independent shRNAs targeting three independent sites makes it less likely that the virus would be able to mutate spontaneously against all three sites.”

“It’s a slow-and-go study,” said Suhy. “Once TT-034 goes in and sets up shop in the liver, if there’s a serious adverse event we won’t be able to withdraw the compound.”

So TT-034 is being tested in a very cautious dose-escalation safety trial among 14 hepatitis patients who have failed their previous therapies, with clinical researchers boosting doses very slowly from a tiny initial infusion for the first two patients. “The overall goal is to just make sure that the compound is well tolerated, that the patients respond well to it, that there are no serious adverse events,” he said. “If there’s efficacy on top of it, that’s gravy on the mashed potatoes.”

Because the agent is a form of gene therapy, FDA regulatory hurdles will be higher than for the drugs with which it hopes to compete, Suhy acknowledges.

On the upside, treatment in a one-time shot may be a big winner since it doesn’t depend on patients taking their pills every day.

With current oral drugs, “if these affected individuals go on a drug holiday because they don’t feel well, they forgot their medicine, they’re going to Tahiti for a week or they just don’t want to feel terrible, they can easily blow their therapeutic treatment because they’re not maintaining that steady-state [drug] level and the virus mutates,” he explained. “Having a one-and-done treatment gives you the ability to take patient compliance out of the equation altogether.”

“It’s an exciting time for the company and for RNAi in general,” Suhy added as he began boarding his flight. “A multitude of RNAi programs are pushing forward into the clinic, and everyone’s success is everyone’s success collectively.”