Shares of Inhibitex Inc. skyrocketed a whopping 130 percent Friday after top-line data from an early hepatitis C virus (HCV) study indicated INHX-189's potential as a monotherapy, putting the Atlanta-based company hot on the trail of the interferon-free HCV regimen vanguard.

The firm's stock (NASDAQ:INHX), trading at 60 times its normal volume, gained $5.26 to close at $9.22, the highest that Inhibitex shares have closed since February 2006.

While the INX-189 data are still early – the trial is designed to test higher doses of the nucleoside polymerase inhibitor alone or in combination with ribavirin over only a seven-day period – experts couldn't help drawing comparisons to Pharmasset Inc.'s PSI-7977, which entered pivotal studies last week as part of an all-oral, interferon-free regimen. Both compounds hail from the same nucleotide class, and PSI-7977 also wowed in early trials showing efficacy without the need for interferon. (See BioWorld Today, Nov. 2, 2011.)

Pharmasset's drug also has been at the center of combination studies in partnership with big pharma. In fact, heading into the weekend's American Association for the Study of Liver Disease meeting in San Francisco, the Princeton, N.J.-based biotech announced the addition of four new treatment arms to an ongoing Phase IIa study testing PSI-7977 in combination with daclatasvir, an NS5A replication complex inhibitor from New York-based Bristol-Myers Squibb Co.

The combo route could work well for Inhibitex, too. The firm has plans to test INX-189 in Phase II studies "to include all oral combos," Chief Scientific Officer Joseph Patti said, adding that "we're engaged in a number of discussions and activities" for testing the drug in regimens with other direct-acting antivirals (DAAs).

Inhibitex has always planned on partnering INX-189, but the potential for combo treatment "obviously provides some flexibility," CEO Russell Plumb told investors.

And a partnership could help the firm speed up work on INX-189. Developing an all-oral HCV regimen is the latest goal in HCV therapy, but other firms are farther ahead in the race. Besides Pharmasset, there is Roche AG, which has been testing combinations of Pharmasset-partnered candidate RG7128 with danoprevir, a protease inhibitor originally developed at Brisbane, Calif.-based InterMune Inc.

Johnson & Johnson subsidiary Tibotec BVBA and Boehringer Ingelheim GmbH also are in late-stage trials testing protease inhibitor combos without interferon, and Foster City, Calif.-based Gilead Sciences Inc. is in trials with a four-drug regimen – non-nucleotide polymerase inhibitor GS 9190, protease inhibitor GS 9451, NS5A inhibitor GS 5885 and ribavirin – as well as three-drug combos omitting either GS 9190 or ribavirin. (See BioWorld Today, Sept. 7, 2011.)

Any one of those firms might make a promising partner for Inhibitex. And, given the interest generated by INX-189, an acquisition might not be out of the question, either. Already this year, San Diego-based Anadys Pharmaceuticals Inc., which had been seeking a partner for its non-nucleoside candidate setrobuvir, snagged a $230 million buyout by Swiss pharma Roche, and analysts are predicting further consolidation in the HCV space. (See BioWorld Today, Oct. 18, 2011.)

Should Inhibitex's drug prove just as efficacious without the addition of other DAAs, that could open up another strategic option for the small biotech, though Plumb cautioned that it's "still early."

Data from a multiple-ascending dose Phase Ib study showed that INX-189 given at 100 mg over seven days resulted in a -2.53 log10 reduction in HCV RNA. Results from the first cohort of the extension trial, as reported Friday during Inhibitex's quarterly earnings call, showed that 200 mg of IHX-189 dosed for seven days had a median HCV RNA reduction from baseline of -4.25 log10. "So clearly there's a nice dose-dependent response," Patti noted. The dose also was generally well tolerated.

The additional planned cohorts in the Phase Ib trial include 100 mg INX-189 once-daily in combination with ribavirin; 100 mg INX-189 twice-daily as a monotherapy; 100 mg INX-189 dosed with food. Patti added that it's possible higher dosing cohorts will be added in the future.

Inhibitex also anticipates a protocol amendment for a 90-patient Phase II trial initiated in September to include interferon-free treatment arms. The current protocol calls for INX-189 to be given in combination with pegylated interferon and ribavirin in chronic HCV genotype 2 and 3 treatment-naïve patients over 12 weeks. Patient recruitment in that study is expected to complete by the end of this year.

For now, the firm looks to have enough cash on hand to move into Phase II. Plumb said the Sept. 30 cash balance of $53.7 million – that includes an April offering that brought in about $50 million – should carry Inhibitex through the end of the first quarter of 2013. The firm also filed a $130 million shelf in August.

Inhibitex posted a net loss of $5.3 million, or 7 cents per share, for the third quarter. Its burn rate was about $4.2 million, though that's expected to increase going forward as work on INX-189 advances.