Regulus Therapeutics Inc said interim mid-stage data for its hepatitis C combination drug showed it had the potential to sharply reduce the duration of the treatment to four weeks from 12 weeks.
The company's shares recorded their best day in more than 15 months, rising as much as 41 percent to $8.85 in heavy trading on Wednesday.
Regulus tested its injectable drug, RG-101, separately with FDA-approved hepatitis C drugs such as Gilead Sciences Inc's Harvoni, Johnson & Johnson's Olysio, and Bristol Myers Squibb's Daklinza.
Jefferies analyst Brian Abrahams said Gilead's hepatitis C drug sales may take a hit if Regulus's treatment turns out to be more effective in combination with drugs developed by its rivals.
Gilead, with its two blockbuster drugs Harvoni and Sovaldi, has dominated the market for hepatitis C, which affects about 185 million people worldwide. Harvoni's total sales were $13.86 billion in 2015.
However, the company has been facing growing competition from rival treatments developed by AbbVie Inc and Merck Inc.
Gilead's shares fell 2 percent to $89.22 following Regulus's announcement.
Wedbush securities analyst Liana Moussatos said she expected Regulus's drug to be priced either in line or less than the oral pills.
Regulus enrolled 79 patients with chronic hepatitis C virus and 38 of these patients were being evaluated through eight weeks of follow up.
The data showed 97 percent of these patients had a sustained reduction of the virus at eight weeks of the treatment.
Analysts called Regulus's interim data set impressive but cautioned that longer follow-up period would be needed to asses the drug's curative potential.
Regulus said it expected to report 12-week response data in the second quarter.
Most adverse events were mild to moderate though it appears there were two serious adverse events that required hospitalization, one possibly occurred after dosing, Abrahams said.
Investors will be keen to know whether there will be any other adverse effects related to the treatment during the study, Cowen and Co analyst Eric Schmidt wrote in a note.
Regulus shares were up 13 percent at $7.10 on the Nasdaq on Wednesday.
(Reporting by Rosmi Shaji in Bengaluru; Editing by Sriraj Kalluvila and Anil d'Silva)