Salix Pharmaceuticals vs. Ajanta Pharma: Rifaximin Patent Dispute Settles in 54 Days

Case Overview

In a swift resolution that underscores the pharmaceutical industry’s preference for negotiated outcomes over protracted litigation, **Salix Pharmaceuticals, Inc. v. Ajanta Pharma Limited** (Case No. 1:25-cv-17780) concluded via confidential settlement just 54 days after filing. The case, heard in the **U.S. District Court for the District of New Jersey**, centered on three patents protecting **rifaximin**—the active ingredient in blockbuster gastrointestinal drug **Xifaxan® 550 mg tablets**—and their application to treating **irritable bowel syndrome (IBS)**.

Filed November 20, 2025, and closed January 13, 2026, the rapid resolution reflects both the commercial stakes surrounding one of the most litigated drug franchises in Paragraph IV history and the strategic calculus that drives ANDA-related patent infringement settlements. For patent attorneys, IP professionals, and R&D teams operating in the pharmaceutical space, this case offers meaningful signals about **rifaximin patent litigation strategy**, portfolio enforcement, and the enduring value of polymorph and method-of-treatment patent estates.

The Parties

The Patents at Issue

Three U.S. patents formed the basis of this infringement action, protecting the active ingredient rifaximin and its application to treating irritable bowel syndrome (IBS). These patents are registered with the U.S. Patent and Trademark Office (USPTO) and represent overlapping layers of protection.

• • US8193196B2 — Directed to polymorphous forms of rifaximin, their production processes, and pharmaceutical preparations.
• • US11779571B2 — Covering methods for treating IBS with rifaximin formulations.
• • US11564912B2 — Similarly directed to methods for treating IBS, broadening the method-of-treatment claim estate.

Together, these patents represent overlapping layers of protection spanning composition, formulation, and therapeutic application—a textbook **patent thicket** strategy protecting Xifaxan®’s market exclusivity.

The Accused Product

Ajanta Pharma’s ANDA product—a generic rifaximin 550 mg tablet—was alleged to infringe these patents by virtue of its proposed use in treating IBS, its rifaximin polymorph composition, and its pharmaceutical formulation. Xifaxan® 550 mg is FDA-approved for IBS with diarrhea (IBS-D) and hepatic encephalopathy, making it a high-revenue target for generic competition.

Legal Representation

Salix and its co-plaintiffs were represented by **Morgan, Lewis & Bockius, LLP**, with attorney **Harvey Bartle IV** serving as lead counsel. Morgan Lewis is a dominant force in Hatch-Waxman pharmaceutical patent litigation, lending significant strategic weight to plaintiff’s enforcement posture. No defendant counsel information was disclosed in available case records.

Litigation Timeline & Procedural History

The case was filed in the **District of New Jersey**—the preeminent venue for Hatch-Waxman pharmaceutical patent litigation given its familiarity with ANDA disputes, proximity to major pharmaceutical operations, and experienced judicial docket. Chief Judge **John F. Murphy (EDPA)** presided over the matter.

The 54-day resolution is notably rapid even by Hatch-Waxman standards, where the statutory 30-month stay following an ANDA Paragraph IV certification typically creates a structured negotiation window. No claim construction hearings, summary judgment motions, or trial-level proceedings were recorded before settlement, indicating that substantive litigation activity was minimal—consistent with early-stage settlement negotiations running parallel to or immediately following the complaint.

The Verdict & Legal Analysis

Outcome

The case resolved through a **Confidential Settlement and License Agreement** between all plaintiffs (Salix Pharmaceuticals, Inc., Salix Pharmaceuticals, Ltd., Alfasigma S.p.A., and Bausch Health Ireland Limited) and defendant Ajanta Pharma Limited. The parties stipulated to dismissal, and the Clerk of Court was directed to enter the Stipulation of Dismissal on January 13, 2026 (Document 7, Filed 01/13/26).

Specific financial terms, royalty rates, and any authorized generic provisions were not publicly disclosed, consistent with standard practice in confidential pharmaceutical patent settlements. Whether the agreement includes a market entry date for Ajanta’s generic product—a commercially critical term in ANDA settlements—remains unknown from public filings.

Verdict Cause Analysis

This was a **Hatch-Waxman Act infringement action** triggered by Ajanta’s ANDA filing with a Paragraph IV certification, asserting that the listed Orange Book patents are invalid, unenforceable, or not infringed by Ajanta’s proposed generic product. The filing of such a certification constitutes a legal act of infringement under 35 U.S.C. § 271(e)(2), automatically enabling brand manufacturers to initiate litigation and invoke the 30-month regulatory stay.

The substantive legal questions—validity of the polymorph patents, scope of method-of-treatment claims, and whether Ajanta’s ANDA product would infringe—were never adjudicated on the merits. The rapid settlement suggests that either: (a) both parties assessed litigation risk and preferred commercial certainty; (b) Ajanta secured an entry date sufficiently favorable to justify settlement; or (c) the strength of the patent portfolio discouraged further challenge. The involvement of Morgan Lewis, known for aggressive pharmaceutical IP enforcement, may have accelerated defendant’s settlement calculus.

Legal Significance

The patents at issue—particularly the **polymorph patent (US8193196B2)** and the paired **method-of-treatment patents (US11779571B2 and US11564912B2)**—reflect a multi-layered Orange Book listing strategy that pharmaceutical innovators use to maximize exclusivity duration. Polymorph patents covering crystalline forms of active pharmaceutical ingredients have faced significant invalidity challenges on **obviousness grounds** (following *KSR International Co. v. Teleflex Inc.*), making early settlement a prudent risk-management choice for both sides.

The co-plaintiff structure—spanning Alfasigma S.p.A. as the originating Italian IP holder, Salix as the NDA holder, and Bausch Health entities as commercial rights holders—illustrates the complexity of **pharmaceutical patent licensing chains** and the importance of ensuring all rights holders join infringement actions to establish standing.

Strategic Takeaways

For Patent Holders: Multi-layered patent portfolios combining composition, polymorph, and method-of-treatment patents create durable enforcement leverage. Listing multiple patents in the Orange Book extends the strategic window for settlement negotiation. Engaging premier Hatch-Waxman litigation counsel (such as Morgan Lewis) signals enforcement seriousness and can accelerate favorable resolution.

For Generic Manufacturers (Accused Infringers): Early settlement assessment—particularly where method-of-treatment patents carry strong clinical evidence and polymorph patents have survived prior challenges—can minimize litigation costs while securing commercially viable entry dates. Paragraph IV certifications against established blockbuster portfolios require robust invalidity analysis before filing.

For R&D Teams: Therapeutic method patents tied to FDA-approved indications (IBS-D treatment with rifaximin) represent durable IP even as composition patents age. R&D leaders developing competing GI therapeutics should conduct **Freedom to Operate (FTO) analyses** covering both active pharmaceutical ingredient polymorphs and method-of-treatment claims across the Xifaxan® Orange Book listing.

Industry & Competitive Implications

Xifaxan® has been one of the most heavily litigated drug products in the U.S. pharmaceutical patent landscape, with Bausch Health and Salix defending the franchise across multiple ANDA filers. The rapid settlement with Ajanta Pharma continues a pattern of negotiated resolutions that have, to date, preserved significant branded market exclusivity for Xifaxan® 550 mg.

For the broader **pharmaceutical patent litigation market**, this case reinforces several trends: the District of New Jersey remains the dominant ANDA battleground; multi-entity plaintiff structures are increasingly common in licensing-heavy pharmaceutical IP; and confidential settlements with undisclosed entry dates continue to be the modal outcome in high-value Hatch-Waxman disputes.

Generic manufacturers entering the rifaximin 550 mg space face a well-resourced, multi-patent enforcement apparatus. Companies with pending ANDAs for rifaximin products should closely monitor public settlement terms as they emerge and assess their own Paragraph IV certification strategies accordingly.