- Arbutus Biopharma, a Pennsylvania-based developer of hepatitis B drugs, will stop testing of an experimental treatment for the liver disease following two cases of acute hepatitis in healthy volunteers enrolled into a Phase 1a/1b study.
- As a result, Arbutus will no longer begin a combination study of the drug, called AB-506, with its more advanced candidate AB-729. That trial was expected to begin in the second half of next year.
- Shares in Arbutus slumped by nearly 25% in response to the news Friday, putting them down about 70% on the year.
AB-506 made it through the first portion of testing in healthy volunteers without apparent incident earlier this year, spurring the company to administer the drug to patients with hepatitis B virus.
Preliminary results from those study participants, released in July, showed four experienced liver enzyme spikes that resolved following treatment discontinuation or completion of testing.
In hopes of better understanding those increases, Arbutus began giving AB-506 to healthy volunteers over four weeks rather than two. The two observed cases of acute hepatitis occurred in that 28-day testing portion of the Phase 1a study.
The toxicity concerns derailed Arbutus plans to start a Phase 2 dose-finding study of AB-506 this year, and set back the company's combination plans as well.
"While we are disappointed in these recent clinical findings, we have a number of oral follow-on capsid inhibitor compounds with distinct chemical scaffolds that we believe have the potential to contribute to the inhibition of HBV replication as part of a combination regimen," said Michael Sofia, Arbutus' chief scientific officer, in a statement.
Sofia said Arbutus plans to select one of those compounds to advance into IND-enabling studies by December.
AB-506 is part of a class of drugs known as capsid inhibitors, which work by blocking the assembly of the protein shells protecting viruses. Doing so disrupts viral replication, according to Arbutus.
AB-729, one of the drugs Arbutus planned to pair with AB-506, is the company's most advanced asset for hepatitis B and works differently by interfering with RNA.
Initial safety and efficacy data from healthy subjects and people with chronic hepatitis B are expected in the first quarter of 2020.
Once worth near $1 billion, Arbutus' market value has crumbled over the past four years. And after a year in which share prices fell from $9 a piece to just over $1, the company is now worth about $60 million.