US Patent 8,642,573: Scope, Claims, Expiration, Litigation, and Rifaximin Patent Landscape

US Patent No. 8,642,573 protects a specific rifaximin regimen for maintaining remission of hepatic encephalopathy: approximately 1,000 to 1,200 mg per day for at least 12 months. The commercially relevant embodiment is Xifaxan 550 mg administered twice daily, or 1,100 mg per day. The patent is a method-of-use patent, not a composition-of-matter patent. Its reported expiration date is February 12, 2029, subject to the controlling USPTO and FDA records.[1,2]

The patent has high commercial relevance because it covers the FDA-approved hepatic encephalopathy regimen for rifaximin and has been asserted against generic rifaximin products seeking approval for the same indication.[3,4]

What does US Patent 8,642,573 protect?

The patent protects long-term administration of rifaximin to maintain remission of hepatic encephalopathy. Its core limitations are:

Limitation	Claim requirement
Disease	Hepatic encephalopathy
Patient status	A subject in need of treatment; dependent claim 3 specifies remission before treatment
Active ingredient	Rifaximin
Daily dose	About 1,000 mg to about 1,200 mg
Duration	At least 12 months
Therapeutic outcome	Maintenance of remission
Commercial dose	550 mg twice daily, totaling 1,100 mg daily
Optional co-therapy	Lactulose
Remission endpoint	Conn score of 0 or 1

Claim 1 is the independent claim. Claims 2 through 8 add narrower limitations. The commercial center of gravity is the combination of claims 1, 6, and 7: approximately 1,100 mg per day, administered as 550 mg twice daily, for at least 12 months to maintain remission of hepatic encephalopathy.

How should the eight claims be interpreted?

Claim 1: Long-term maintenance of hepatic encephalopathy remission

Claim 1 requires all of the following:

The patient has, or is at risk of, hepatic encephalopathy.
Rifaximin is administered.
The daily dose is approximately 1,000 to approximately 1,200 mg.
Treatment continues for 12 months or longer.
The purpose or result is maintaining remission.

The claim does not require lactulose, a particular tablet strength, a specific dosing schedule, or a specific definition of remission. It does require a maintenance regimen rather than short-term treatment of an acute episode.

The phrase "about" creates numerical flexibility. A court would ordinarily assess the scope of "about 1,000 mg to about 1,200 mg" in view of the specification, prosecution history, technical meaning, and evidence concerning dose equivalence. The safest commercial mapping is 1,100 mg per day, which is centered within the stated range.

Claim 2: Combination with lactulose

Claim 2 requires administration of lactulose in addition to the rifaximin regimen. Lactulose is commonly used in hepatic encephalopathy management, but the claim does not state a particular lactulose dose, formulation, route, or administration schedule.

A product label that recommends rifaximin with lactulose can create a direct or induced infringement issue if the other claim limitations are present. A label that merely permits lactulose without directing or encouraging its use presents a different infringement analysis.

Claim 3: Patient already in remission

Claim 3 narrows claim 1 by requiring that the patient be identified as being in remission before rifaximin administration. This limitation distinguishes maintenance therapy from treatment initiated during an active episode.

The claim creates a factual issue concerning the timing and documentation of remission. Medical records, Conn scores, hospitalization records, and prescribing instructions may be relevant to whether the patient was in remission before the regimen began.

Claim 4: Treatment for 12 to 24 months

Claim 4 narrows the duration to approximately 12 to approximately 24 months. It provides a more defined treatment window than claim 1 but does not cover treatment extending beyond the approximate 24-month upper boundary unless the claim construction permits such treatment within the ordinary meaning of "about."

Claim 5: Treatment until death

Claim 5 covers administration continuing until the patient dies. This is an unusually narrow factual limitation. The claim is unlikely to be infringed merely because a label authorizes indefinite treatment. The patient would need to receive the claimed regimen until death, or the applicable legal theory would need to address instructions or inducement rather than completed administration.

Claim 6: Approximately 1,100 mg per day

Claim 6 narrows the dose to approximately 1,100 mg per day. It is directed to the commercially relevant total daily dose for Xifaxan 550 mg tablets.

Claim 7: 550 mg twice daily

Claim 7 specifies the dosage form and schedule: 550 mg twice daily. This equals 1,100 mg per day. It is the clearest claim for evaluating a generic label that instructs patients to take one 550 mg tablet twice daily.

Claim 8: Conn score of 0 or 1

Claim 8 defines remission as a Conn score of 0 or 1. The Conn scale is used to assess the severity of hepatic encephalopathy. The claim requires the dosing and patient limitations of claims 1, 6, and 7, together with this specific remission definition.

What is the patent’s prosecution and expiration timeline?

Event	Date
Earliest reported priority date	February 12, 2009
Nonprovisional filing	February 12, 2010
Patent issued	February 4, 2014
Patent number	US 8,642,573
Reported expiration	February 12, 2029
Commercial product associated with the use	Xifaxan 550 mg tablets
FDA application	NDA 021361

The patent was issued after the FDA approval of rifaximin for hepatic encephalopathy. The relevant patent term is based principally on the earliest effective nonprovisional filing date, with any patent-term adjustment reflected in the official USPTO record. The February 12, 2029 date is also reflected in the Orange Book listing for Xifaxan-related protection.[1,2]

The patent is distinct from earlier rifaximin patents that protected the active ingredient, polymorphic forms, or broader pharmaceutical compositions. Those earlier patents do not eliminate the separate method-of-use barrier created by the hepatic encephalopathy maintenance claims.

What is the Orange Book status of US 8,642,573?

US 8,642,573 has been listed in the FDA Orange Book for Xifaxan 550 mg tablets and the hepatic encephalopathy indication. Orange Book listing gives the NDA holder a mechanism to notify the FDA of patent certification and to invoke the statutory stay associated with a qualifying Paragraph IV notice.[2]

The Orange Book does not determine ultimate validity or enforceability. It records patents submitted by the NDA holder that the FDA has accepted for listing. Patent validity, claim construction, infringement, and enforceability remain matters for the courts.

The listed patent is commercially important because an ANDA applicant seeking approval for a rifaximin product with the hepatic encephalopathy indication may need to address the patent through:

Paragraph III certification, deferring approval until patent expiration;
Paragraph IV certification, alleging that the patent is invalid, unenforceable, or not infringed;
A section viii statement, omitting a patented indication where the remaining label is legally supportable.

Which companies have challenged the Xifaxan patent estate?

Norwich Pharmaceuticals

Norwich Pharmaceuticals pursued an ANDA for rifaximin tablets and challenged Salix’s Xifaxan patent estate. Salix and related plaintiffs litigated the challenge in the District of Delaware under the Hatch-Waxman framework.

The dispute included US 8,642,573 and US 9,296,181, among other Xifaxan-related patents. The litigation addressed whether Norwich’s proposed product would infringe method-of-use claims covering hepatic encephalopathy and whether those claims were invalid, including on obviousness grounds.[3,4]

The Federal Circuit’s decision in Norwich Pharmaceuticals Inc. v. Salix Pharmaceuticals Ltd. addressed the enforceability of the listed Xifaxan patents and the scope of the proposed generic label.[4] The litigation materially delayed an unrestricted generic launch directed to the protected indication.

Other generic applicants

Rifaximin has attracted substantial ANDA interest because Xifaxan is a high-value branded gastrointestinal product. Public patent disputes and FDA records should be reviewed separately for each applicant because a generic company may pursue a carved-out label, challenge different patents, or settle on different launch terms.

A Paragraph IV filing does not itself establish that a generic product will launch. The commercial outcome depends on litigation judgment, settlement restrictions, FDA approval, manufacturing readiness, and whether the approved label includes the patented hepatic encephalopathy indication.

What litigation risks does the patent create for a generic rifaximin product?

A generic applicant faces several distinct risks.

Direct infringement

Direct infringement is most straightforward when the generic product is used according to instructions that match the claims:

rifaximin 550 mg;
twice-daily administration;
hepatic encephalopathy indication;
long-term maintenance;
treatment of patients in remission.

The product itself is not necessarily infringing merely because it contains rifaximin. The asserted claims are method claims, so use of the product according to the claimed method is central.

Induced infringement

A generic label can create induced-infringement exposure if it instructs physicians or patients to perform the claimed method. Relevant evidence can include the indication, dosing instructions, duration language, promotional materials, prescribing information, and clinical-support communications.

A narrow label carve-out can reduce this risk, but a carve-out must remove the patented method in substance rather than only delete the indication heading. FDA approval and patent noninfringement are separate issues.

Contributory infringement

Contributory infringement is less direct for a product containing rifaximin because rifaximin has substantial noninfringing uses, including other gastrointestinal indications. The analysis depends on the product’s intended uses and the evidence of special adaptation or lack of substantial noninfringing use.

Validity challenges

The principal validity theories include:

anticipation;
obviousness;
lack of written description;
lack of enablement;
indefiniteness, particularly concerning "about," remission, and duration;
double patenting;
inequitable conduct, if supported by the prosecution record.

The most plausible technical challenge is obviousness based on prior rifaximin use, known hepatic encephalopathy treatment, and the established 550 mg twice-daily dose. The patent holder’s response is that long-term maintenance of remission, rather than short-term treatment, required clinical evidence and was not necessarily predictable from the prior art.

How strong is the patent estate for Xifaxan?

The estate is strongest when evaluated as a layered portfolio rather than as a single patent.

Patent category	Commercial function	Relative risk to generic entry
Composition and formulation patents	Protect rifaximin chemical or pharmaceutical forms	Can block products using the same protected form
Hepatic encephalopathy method patents	Protect long-term maintenance treatment	High risk for a full HE label
IBS-D method patents	Protect rifaximin use for irritable bowel syndrome with diarrhea	High risk for an IBS-D label
Manufacturing patents	Protect production processes or purity controls	Relevant if the ANDA process practices the claimed method
Regulatory exclusivity	Delays approval independently of patent validity	Usually shorter than patent term

The hepatic encephalopathy claims are narrower than a composition claim but stronger against a label that directly reproduces the branded regimen. Their principal weakness is that they depend on treatment duration, patient status, and clinical outcome. Those facts may complicate proof compared with a composition claim.

What generic launch scenarios exist for rifaximin?

Launch after patent expiration

The lowest-risk pathway is approval with launch after February 12, 2029, assuming no later patent, pediatric extension, injunction, or settlement restriction applies.

Launch under a carved-out label

A generic applicant may seek approval for nonprotected indications while omitting hepatic encephalopathy. This pathway depends on the final label and the FDA’s approval of the section viii strategy.

At-risk launch

A generic company may launch after receiving FDA approval but before patent expiration if it believes the patent is invalid or not infringed. That approach exposes the company to damages, injunction risk, and potential recovery of lost sales or reasonable royalties.

Settlement-based entry

A settlement may establish an agreed launch date before patent expiration. The commercial terms may also address the scope of the label, supply arrangements, authorized generic rights, or restrictions on challenging later-issued patents. Publicly available litigation records should be used to determine whether any settlement has been entered and whether its terms are confidential.

What manufacturing and geographic barriers remain?

US 8,642,573 does not principally protect a manufacturing process. It protects a treatment method. A generic manufacturer can therefore avoid this patent only if its product and label do not induce the claimed use, or if the patent is invalid, unenforceable, expired, or not infringed.

Geographic protection is territorial. The patent affects US manufacture, sale, offer for sale, importation, and use under US patent law. Separate European, Canadian, Japanese, and other national patents must be analyzed independently. FDA approval does not grant protection outside the United States, and foreign approval does not eliminate US patent exposure.

How does US 8,642,573 compare with the core Xifaxan patents?

US 8,642,573 differs from composition patents such as US 7,045,620 and US 7,612,199 because it does not broadly cover rifaximin as a chemical or pharmaceutical product. A competitor could potentially practice the composition technology without practicing the hepatic encephalopathy maintenance method.

Its commercial force comes from the overlap between the claim limitations and the approved 550 mg twice-daily Xifaxan regimen. A product designed for the same indication and schedule faces substantially greater risk than a product limited to an unclaimed indication.

Key Takeaways

US 8,642,573 is a method-of-use patent for maintaining hepatic encephalopathy remission with long-term rifaximin.
Claim 1 covers approximately 1,000 to 1,200 mg daily for at least 12 months.
Claims 6 and 7 specifically target approximately 1,100 mg daily, administered as 550 mg twice daily.
Claim 2 adds lactulose; claim 3 requires pre-existing remission; claim 8 defines remission by a Conn score of 0 or 1.
The reported expiration date is February 12, 2029.
The patent has been listed for Xifaxan 550 mg and has been involved in Norwich Pharmaceuticals’ ANDA litigation.
A generic label that includes the hepatic encephalopathy indication, 550 mg twice-daily dosing, and long-term maintenance instructions presents the highest infringement risk.
A carved-out label may reduce risk but must be evaluated against the full label, promotional conduct, and induced-infringement precedent.
The patent is narrower than composition patents but commercially significant because it maps closely to the approved Xifaxan hepatic encephalopathy regimen.

FAQs About US Patent 8,642,573

Does US 8,642,573 cover rifaximin for IBS-D?

No. The claims provided are directed to maintaining remission of hepatic encephalopathy. Rifaximin use for IBS-D is covered, if at all, by separate patents, regulatory labeling, or other legal protections.

Does taking 550 mg of rifaximin twice daily automatically infringe the patent?

No. The dose matches claim 7, but infringement also requires the other limitations, including the hepatic encephalopathy use, long-term duration, and maintenance-of-remission purpose or result.

Can a generic sell rifaximin before February 2029?

Potentially, if it receives approval for a label that does not practice the patented method, prevails in litigation, reaches a settlement permitting entry, or launches at risk. The expiration date alone does not determine every possible launch pathway.

Is lactulose required for infringement?

No. Lactulose is required only by claim 2. Claims 1 and 3 through 8 do not require concurrent lactulose administration.

Why is the Conn score limitation commercially important?

The Conn score limitation in claim 8 links the patent to an objective clinical definition of remission. It can narrow the claim but also create evidentiary questions concerning when the score was measured and whether the patient’s treatment matched the claimed clinical condition.

References

United States Patent and Trademark Office. (2014). United States Patent No. 8,642,573, Methods of treating hepatic encephalopathy.
U.S. Food and Drug Administration. (2024). Approved drug products with therapeutic equivalence evaluations: Xifaxan (rifaximin), NDA 021361.
United States District Court for the District of Delaware. (2022). Salix Pharmaceuticals, Ltd. v. Norwich Pharmaceuticals Inc., patent litigation concerning rifaximin ANDA products.
United States Court of Appeals for the Federal Circuit. (2024). Norwich Pharmaceuticals Inc. v. Salix Pharmaceuticals Ltd., appeal concerning Xifaxan patent infringement and validity.

Applicant	Tradename	Generic Name	Dosage	NDA	Approval Date	TE	Type	RLD	RS	Patent No.	Patent Expiration	Product	Substance	Delist Req.	Patented / Exclusive Use	Submissiondate
Salix Pharms	XIFAXAN	rifaximin	TABLET;ORAL	021361-002	Mar 24, 2010	AB	RX	Yes	Yes	⤷ Start Trial	⤷ Start Trial				REDUCTION IN RISK OF OVERT HEPATIC ENCEPHALOPATHY (HE) RECURRENCE	⤷ Start Trial
>Applicant	>Tradename	>Generic Name	>Dosage	>NDA	>Approval Date	>TE	>Type	>RLD	>RS	>Patent No.	>Patent Expiration	>Product	>Substance	>Delist Req.	>Patented / Exclusive Use	>Submissiondate

Country	Patent Number	Estimated Expiration	Supplementary Protection Certificate	SPC Country	SPC Expiration
Australia	2009298389	⤷ Start Trial
Australia	2010260089	⤷ Start Trial
Australia	2010271070	⤷ Start Trial
>Country	>Patent Number	>Estimated Expiration	>Supplementary Protection Certificate	>SPC Country	>SPC Expiration

Summary for Patent: 8,642,573