Source: Gilead Sciences via Google Maps.

Investors are focusing on AbbVie's (NYSE:ABBV) hepatitis C drug and its potential impact on Gilead Sciences' (NASDAQ:GILD) hepatitis C market share if it wins FDA approval later this month. However, they might also want to keep an eye on Achillion Pharmaceuticals (NASDAQ:ACHN), a small company that could report interesting midstage trial data this month.

Advancing a cure
Over the past five years, the focus for hepatitis C research has been on developing a functional cure. Prior to the launch of Vertex Pharmaceuticals' Incivek in 2011, the side effect-laden combination of peginterferon and ribavirin cured just 50% of patients. Although Incivek was better, it only cured about 80% of patients.

Gilead Sciences' launch of Sovaldi in December 2013 was game-changing. Sovaldi, which is taken orally rather than injected like previous therapies, cured 90% of patients and eliminated the use of peginterferon for many.

Equally game-changing is Gilead Sciences' second-generation hepatitis C drug, Harvoni, which won FDA approval in October. Harvoni cures up to 99% of patients and does away with both peginterferon and ribavirin. That's impressive, but at $94,500 for a 12-week treatment, it's not cheap. If AbbVie's three-pills-daily cocktail is approved and can undercut Harvoni's price, it stands a good chance of winning some market share.

I previously discussed reasons why AbbVie's cocktail could struggle to win over prescribers of Harvoni. However, regardless of AbbVie's potential short-term threat to Gilead Sciences, investors should recognize that the next battle for hepatitis C market share is more likely to be fought over treatment duration rather than cure rates bordering on 100%.

Source: Achillion Pharmaceuticals

Shortening treatment durations
Achillion is working on an adjunct therapy, ACH-3102, that can be used alongside Sovaldi to limit treatment to eight weeks or less.

During a small phase 2 trial, Achillion reported that combining ACH-3102 and Sovaldi cleared the disease in 100% of patients who received the drugs for eight weeks. That matches up nicely against Gilead's Harvoni.

Harvoni can be used for as little as eight weeks in about 40% of patients. To qualify, patients must not have been previously treated, must not have liver disease, and must have viral loads of hepatitis C that are below 6 million IU/mL.

This last requirement is particularly important to investors; nine of the 12 people who were functionally cured by ACH-3102 and Sovaldi had viral loads greater than 6 million IU/mL. That means ACH-3102 could conceivably shorten treatment duration to eight weeks for far more people than Harvoni.

That news must have captured AbbVie's attention, too. AbbVie's cocktail, which should get a yes or no decision from the FDA on Dec. 21, is dosed for 12 weeks. If Achillion's study results in larger trials show patients can be cured over a far shorter period than AbbVie can offer, then any potential market share AbbVie captures could prove be short-lived.

Possibly, even shorter
Based on its success in treating patients over an eight-week period, Achillion is studying a six-week treatment regiment for ACH-3102 and Sovaldi. Results are expected before year-end, and if they're positive, then it could ultimately be Achillion, not AbbVie, that poses a bigger threat to Gilead Sciences' Harvoni.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.