New FDA draft guidance may have shaved off two years of patent protection from Gilead’s just-approved hepatitis C medication Harvoni or it may have conferred a full five years of marketing exclusivity. The Wall Street Journal reports that the verdict depends on what the regulator considers the publication date for new draft guidance.

The new guidance was supposed to add clarity around marketing exclusivity for approved combination drugs. Until now, rules have been based on whether or not the combinations are comprised only of new chemical entities or if they were a mix of new and established products: drugs that were 100% new were granted five years of exclusivity. Drugs that were comprised of old and new components received three.

Drugmakers asked the FDA to make it an even five for both combination types, and argued that the five- vs three-year differential was a disincentive for them to innovate.  

The FDA seems to agree, but there’s a problem: the regulator says the the new rules will not be retroactive so drugs approved before publication cannot be grandfathered in. The catch: is the effective date linked to the date it popped up on the FDA’s website (October 10) or is it on the not-yet-publication date when it appears in the Federal Register?

The decision puts two Gilead drugs into play: the HIV medication Striblid, and the hep. C medication Harvoni, which is comprised of the established drug Sovaldi (sofosbuvir, approved December 2013) and new-entity ledipasvir.

The Journal has asked the FDA for clarification.