US Patent 8,158,644: Rifaximin Polymorph Composition Claims, Expiration, Litigation and Generic Risk

US Patent 8,158,644 protects solid oral pharmaceutical compositions containing specified crystalline forms of rifaximin. Its core coverage is directed to rifaximin polymorphs Form α, Form β and Form γ, identified by X-ray powder diffraction peaks and controlled water-content ranges. The patent is commercially important because a generic rifaximin product can infringe without copying a particular brand formulation if its finished dosage form contains the claimed polymorphic material.

The patent was assigned to Alfa Wassermann S.p.A., the originator of Xifaxan. It issued on April 17, 2012, from an application claiming priority to a 2008 filing. Its ordinary patent term is expected to extend to approximately June 2029, subject to the official USPTO term calculation and any terminal disclaimer or patent-term adjustment reflected in the patent record.[1]

What does US Patent 8,158,644 protect?

The patent protects a solid pharmaceutical composition that contains rifaximin in one or more defined polymorphic forms, together with a pharmaceutically acceptable excipient or carrier.

The independent claim structure is:

Claim	Protected subject matter	Required polymorph	Water content
1	Solid pharmaceutical composition	Form α	3.0% to 4.5%
9	Solid pharmaceutical composition	Form β	4.5% to 5.0%
15	Solid pharmaceutical composition	Form γ	0% to 1%

Each independent claim requires two principal technical elements:

A rifaximin crystal form identified by specified XRPD peaks.
A defined water-content range for that form.

The claims do not require a specific rifaximin dose, indication, release profile, manufacturing process or commercial brand. The claims instead focus on the physical state of rifaximin in the finished solid composition.

How are the rifaximin polymorphs defined?

The claimed XRPD identifiers are:

Polymorph	Claimed XRPD peaks, approximately	Claimed water content
Form α	7.4°, 19.7°, 21.0° and 22.1° 2-theta	3.0% to 4.5%
Form β	5.4°, 9.0° and 20.9° 2-theta	4.5% to 5.0%
Form γ	5.0°, 7.1° and 8.4° 2-theta	0% to 1%

The use of "about" before the diffraction angles introduces an issue of claim construction. The patent does not expressly provide, in the claim text supplied, a fixed numerical tolerance. In an infringement dispute, the court would likely examine the specification, analytical method, instrument variation and expert evidence to determine the allowable deviation.

Water content is a separate limitation. A product may show the relevant XRPD peaks but fall outside the claim if its water content is outside the applicable range. Conversely, a product within the water range may not infringe unless its XRPD pattern identifies the claimed polymorph.

How broad are the Form α, Form β and Form γ claims?

The independent claims are narrower than a general claim to rifaximin. They require a specific polymorph and a specific water-content range. They are broader than claims limited to one tablet strength or one named indication.

Form α coverage

Claim 1 covers a solid composition containing Form α with 3.0% to 4.5% water. Claims 2 and 3 narrow the water content to 3.0% and 4.5%, respectively.

Claim 1 does not require Form α to be the only rifaximin form in the composition. This matters because the claim expressly permits dependent claims 7 and 8, which add Form β or Form γ. A composition containing Form α and Form β can therefore fall within claim 7 if it satisfies all limitations of claim 1.

Form β coverage

Claim 9 covers a composition containing Form β with 4.5% to 5.0% water. Claims 10 and 11-14 add narrower excipient, dosage-form and mixed-polymorph limitations.

The Form β claim has a potentially important boundary issue. Form β begins at 4.5% water, while Form α extends through 4.5%. A product measured at or near 4.5% may create a factual dispute depending on the applicable analytical precision and whether the composition contains one form or a mixture.

Form γ coverage

Claim 15 covers Form γ containing 0% to 1% water. Claims 16 and 17 narrow the range to 0% and 1%.

The Form γ claims are directed to a relatively dry material. A generic manufacturer that uses drying, milling, granulation or storage conditions that reduce water content could move its product into the claimed range even if the initial active pharmaceutical ingredient had a different moisture level.

What formulations are protected by US 8,158,644?

The patent claims a wide set of solid oral dosage forms, including:

Coated and uncoated tablets
Hard and soft gelatin capsules
Sugar-coated pills
Lozenges
Wafer sheets
Pellets
Powders in sealed packets

Claim 5 applies these dosage-form limitations to the Form α composition. Claim 12 applies them to Form β, and claim 19 applies them to Form γ.

The listed excipients include:

Colloidal silicon dioxide
Hydroxypropyl methylcellulose
Cellulose
Microcrystalline cellulose
Propylene glycol
Sodium starch glycolate
Talc

Claims 4, 11 and 18 also identify broad functional excipient categories such as diluents, binders, lubricants, disintegrants, coloring agents, flavoring agents and sweeteners.

These dependent claims do not necessarily create independent commercial barriers if the independent claim is already satisfied. For example, a tablet containing Form α and microcrystalline cellulose may infringe claim 1 without infringing claim 6. Claim 6 is narrower because it requires one of the specifically identified ingredients.

Does the patent cover Xifaxan tablets?

The patent is relevant to solid oral rifaximin products, including the tablet and capsule formats associated with Xifaxan. Xifaxan is marketed by Salix Pharmaceuticals, a subsidiary of Bausch Health, for hepatic encephalopathy, irritable bowel syndrome with diarrhea and travelers' diarrhea.[2]

The patent does not require the product to be called Xifaxan or to contain the Xifaxan label dose. A generic 200 mg or 550 mg product could face a product claim if its rifaximin material satisfies the relevant XRPD and water-content limitations.

The practical infringement question is therefore analytical rather than merely label-based:

What XRPD peaks appear in the finished dosage form?
What water content does the rifaximin component have?
Is the product a single polymorph or a mixture?
Are the measurements taken from the active ingredient before formulation or from the finished composition?
Does the accused product meet the construction of "about" for each XRPD peak?

When does US Patent 8,158,644 lose exclusivity?

The patent is expected to expire in approximately June 2029 based on its priority and filing history.[1] The exact expiration date should be taken from the USPTO patent-term calculation, not inferred solely from the issue date.

The patent is separate from FDA regulatory exclusivity. Xifaxan's original new-chemical-entity exclusivity expired years ago. Later approvals for additional indications received separate periods of regulatory exclusivity, including three-year exclusivity associated with certain supplemental approvals. Those FDA periods do not extend the patent term.

Protection type	Relevance to Xifaxan
NCE exclusivity	Expired
Later clinical-investigation exclusivity	Applied to certain supplemental approvals
Patent 8,158,644	Polymorph-based solid composition protection, expected to run into approximately 2029
Patent-term extension	Must be confirmed in the official patent and Orange Book records
Pediatric exclusivity	Separate six-month extension only if awarded and applicable

What is the Orange Book status of US 8,158,644?

FDA Orange Book listings connect approved drug products with patents that may be subject to a Paragraph IV certification. Rifaximin products approved under NDA 021361 have historically been associated with multiple patents covering composition, formulation, polymorph and method-of-use subject matter.[3]

The commercial significance of an Orange Book listing is procedural. An ANDA applicant must address each listed patent through a certification. A Paragraph IV certification alleges that the patent is invalid, unenforceable or will not be infringed. If the patent owner timely files suit, the filing can trigger a 30-month stay of ANDA approval under the Hatch-Waxman Act.[4]

A listed polymorph patent can create a different defense profile from a method-of-use patent. A generic applicant may carve out a patented indication, but it cannot avoid a composition claim merely by omitting the indication from its label if the marketed product contains the claimed polymorph.

Which companies are challenging Xifaxan patent protection?

The principal public challenge has come from Norwich Pharmaceuticals and its generic rifaximin development program. Salix and related entities litigated against Norwich over Xifaxan patents in the District of Delaware. The litigation focused heavily on patents covering rifaximin formulations and methods of treating hepatic encephalopathy and IBS-D.[5]

The Federal Circuit in 2023 addressed the validity of several Xifaxan patents, including patents asserted against Norwich. The court held that certain claims were invalid for obviousness while treating other claims differently based on the asserted patent and claim scope.[6]

The reported Norwich decisions do not eliminate all Xifaxan patent risk, and they do not automatically invalidate US 8,158,644. Patent-by-patent analysis remains necessary. A decision invalidating a formulation or method patent does not remove a separate polymorph composition patent unless that patent was adjudicated and invalidated.

What Paragraph IV risks exist for a rifaximin generic?

A generic applicant seeking approval before the expiration of the patent would likely evaluate several certification strategies:

Strategy	Effect on risk
Paragraph III certification	Defers approval until patent expiration
Paragraph IV certification	Creates an invalidity, unenforceability or noninfringement dispute
Section viii statement	Removes a patented indication where legally available
Design-around formulation	Attempts to avoid the claimed polymorph or water range
Alternative crystal form	May avoid this patent but trigger other patent claims
Amorphous or nonclaimed material	May reduce risk if technically stable and manufacturable

For this patent, a design-around must address the XRPD and moisture limitations together. Changing only the excipient is unlikely to avoid claim 1, 9 or 15 if the same polymorph and water range remain present.

A generic product could attempt to use:

A different rifaximin solid form
A polymorph outside the claimed water range
An amorphous form
A formulation in which the claimed form is not present in the finished dosage form
A process that converts the active into a nonclaimed form before manufacture

Each strategy creates technical risks. Rifaximin polymorphs can differ in stability, solubility, dissolution, moisture sensitivity and bioavailability. A design-around that avoids the patent but changes product performance may require additional pharmaceutical development and regulatory support.

How strong is the patent estate for rifaximin?

US 8,158,644 is technically specific but commercially meaningful. Its strength depends on four factors.

Claim clarity

The XRPD peaks provide an objective identity test. That supports enforcement compared with a claim defined only by processing conditions. The "about" language, however, may generate disputes over measurement tolerance.

Product detectability

Polymorph and water-content claims are easier to investigate than purely process-dependent claims. Analytical testing of an accused tablet, capsule or active ingredient can produce evidence relevant to infringement.

Breadth

The claims cover multiple dosage forms and excipient classes. They are not limited to Xifaxan's exact commercial composition. The claims remain limited by the named polymorph and moisture window.

Validity exposure

Polymorph patents can face obviousness and enablement challenges. A challenger may argue that the claimed crystal forms, hydration states or moisture ranges were predictable from known rifaximin solid-state work. The patent owner would likely rely on crystallographic distinctions, stability, manufacturability and unexpected pharmaceutical properties.

The patent's strongest enforcement position is against a generic that uses the same polymorph and moisture specification. Its weaker position is against a product using a demonstrably different solid form outside the claimed water range.

What manufacturing and intellectual-property barriers affect generic entry?

The principal manufacturing barrier is solid-state control. The manufacturer must control:

Crystallization conditions
Solvent and water activity
Drying temperature and duration
Milling energy
Granulation conditions
Compression and coating
Packaging humidity
Stability during shelf life

Water content may change during processing and storage. A batch that avoids the claim at release could move into a claimed range during stability testing. The reverse can also occur. A generic manufacturer therefore needs batch-specific XRPD and moisture controls, not only an API certificate of analysis.

The patent also creates an evidentiary issue. Testing the isolated API may not establish the composition of the finished dosage form if manufacturing changes the solid state. Conversely, testing only the finished product may not identify the original manufacturing pathway. Both product composition and process records may become relevant in litigation.

How does US 8,158,644 compare with other Xifaxan patents?

Patent category	Typical protected subject matter	Generic design-around potential
Polymorph patent, including US 8,158,644	Crystal form, XRPD pattern and moisture content	Use another form or moisture range
Formulation patent	Excipients, dissolution, release or dosage composition	Modify excipients or formulation architecture
Method-of-use patent	Treatment of hepatic encephalopathy, IBS-D or travelers' diarrhea	Label carve-out, where permitted
Manufacturing patent	Crystallization, drying or isolation process	Use an alternative process
Combination or regimen patent	Dose, administration schedule or adjunctive therapy	Modify regimen or seek indication carve-out

The polymorph patent can be more difficult to avoid than a method-of-use patent because the product itself, rather than the label, is the infringement focus. It can also be easier to test analytically than a process patent.

What is the revenue exposure from delayed generic entry?

Xifaxan is one of Bausch Health's largest products. Annual Xifaxan revenue has been approximately $2 billion in recent reporting periods, although the precise amount varies by year and reporting basis.[7]

A successful generic launch would likely create price erosion and volume migration. The timing would depend on:

Whether the generic receives tentative or final approval
The number of approved competitors
Whether a first filer obtains 180-day exclusivity
The scope of any litigation settlement
Whether the launch covers 550 mg tablets, 200 mg tablets or both
The ability to avoid formulation and method-of-use patents

A polymorph patent expiring around 2029 may therefore remain relevant after earlier method and formulation patents are invalidated, settled or carved out. The actual commercial barrier is the intersection of all enforceable patents listed for the relevant NDA and dosage form.

Key Takeaways

US Patent 8,158,644 covers solid rifaximin compositions containing Form α, Form β or Form γ.
The claims require both specified XRPD peaks and a defined water-content range.
Form α is claimed at 3.0% to 4.5% water, Form β at 4.5% to 5.0%, and Form γ at 0% to 1%.
The patent covers tablets, capsules, lozenges, pellets, powders and other oral solid formats.
Changing excipients alone may not avoid infringement.
A generic design-around would likely require a different solid form or water-content profile.
The patent is expected to remain relevant until approximately June 2029, subject to the official USPTO term calculation.
Xifaxan litigation involving Norwich challenged several patents, but those decisions do not by themselves eliminate US 8,158,644.
No biosimilar pathway applies because rifaximin is a small-molecule drug. Generic entry proceeds through the ANDA pathway.
Xifaxan's multibillion-dollar annual revenue makes polymorph, formulation and method-of-use patents commercially material.

Frequently Asked Questions

Does US 8,158,644 cover rifaximin as an active ingredient generally?

No. It covers solid compositions containing specified rifaximin polymorphs with defined XRPD peaks and water-content ranges.

Can a generic avoid US 8,158,644 by using the same rifaximin dose?

No. Dose selection alone does not determine infringement. The relevant issue is whether the product contains a claimed polymorph within the required moisture range.

Does a mixture of rifaximin polymorphs infringe?

Potentially. Claims 7, 8 and 14 expressly address compositions containing more than one polymorphic form. A mixture may also infringe an independent claim if it contains the claimed form and satisfies the other limitations.

Is Form γ necessarily outside the Form α and Form β claims?

Not necessarily. Form γ is defined by different XRPD peaks, but a composition containing multiple forms could implicate dependent claims if it also contains Form α or Form β within the required water range.

Can an ANDA applicant use a Paragraph IV certification against this patent?

Yes, if the patent is listed for the relevant reference product and the applicant seeks approval before patent expiration. The applicant would need to allege that the patent is invalid, unenforceable or not infringed under the Hatch-Waxman framework.

References

United States Patent and Trademark Office. (2012). U.S. Patent No. 8,158,644, Rifaximin polymorphs and pharmaceutical compositions.
U.S. Food and Drug Administration. (2024). Xifaxan (rifaximin) prescribing information. Salix Pharmaceuticals.
U.S. Food and Drug Administration. (2024). Approved drug products with therapeutic equivalence evaluations: Orange Book.
21 U.S.C. § 355(j); 35 U.S.C. § 271(e).
Salix Pharmaceuticals, Ltd. v. Norwich Pharmaceuticals Inc., litigation concerning Xifaxan patents, U.S. District Court for the District of Delaware.
Salix Pharmaceuticals, Ltd. v. Norwich Pharmaceuticals Inc., 2023 Federal Circuit decisions concerning Xifaxan patent validity.
Bausch Health Companies Inc. (2024). Annual report and Form 10-K.

Foriegn Application Priority Data
Foreign Country	Foreign Patent Number	Foreign Patent Date
Italy	M12003A002144	Nov 07, 2003

Country	Patent Number	Estimated Expiration	Supplementary Protection Certificate	SPC Country	SPC Expiration
Argentina	043547	⤷ Start Trial
Argentina	081991	⤷ Start Trial
Argentina	081992	⤷ Start Trial
Austria	E361927	⤷ Start Trial
>Country	>Patent Number	>Estimated Expiration	>Supplementary Protection Certificate	>SPC Country	>SPC Expiration

Details for Patent: 8,158,644

Summary for Patent: 8,158,644