United States Patent 6,559,158: Claim Scope, Methylnaltrexone Coverage, Expiration, and Patent Landscape

U.S. Patent No. 6,559,158 covers low-systemic-exposure use of a quaternary noroxymorphone derivative, principally methylnaltrexone, to prevent or treat opioid-induced side effects in patients receiving chronic opioid therapy. The commercial target is opioid-induced constipation, with additional coverage for pruritus, urinary retention, and dysphoria. The patent’s central limitations are chronic opioid exposure, a differential dose-response between chronic and nonchronic patients, and a maximum plasma concentration threshold.

The patent expired in December 2019 under the standard 20-year patent term measured from its nonprovisional filing date. It therefore does not create a current U.S. blocking right against generic methylnaltrexone or new methylnaltrexone products. Its historical importance remains high because it helped define the clinical and commercial rationale for Relistor, the methylnaltrexone product developed by Progenics Pharmaceuticals and commercialized through later licensing and acquisition arrangements. [1][2][3]

What does U.S. Patent 6,559,158 protect?

The patent protects treatment methods rather than a new chemical compound standing alone. The claims are directed to administering a quaternary derivative of noroxymorphone to an opioid-treated patient at an exposure level intended to avoid central opioid antagonism while treating peripheral opioid effects.

The principal claim elements are:

Claim feature	Scope
Active agent	A quaternary derivative of noroxymorphone
Named species	Methylnaltrexone
Patient population	A chronic opioid patient
Clinical target	Opioid-induced constipation, pruritus, urinary retention, dysphoria, or another opioid-induced side effect
Dose-response limitation	The dose works in a chronic opioid patient but is insufficient in a patient not chronically administered opioids
Pharmacokinetic limitation	Peak plasma concentration does not exceed 100 ng/mL, with dependent claims narrowing the threshold to 50, 26, 20, or 10 ng/mL
Route	Oral, enterically coated oral, non-enterically coated oral, or combined oral administration with the opioid
Dose	Approximately 0.001 to 5 mg/kg, with narrower limits below 3, 1, and 0.1 mg/kg
Dosing method	Dose selection based on the duration of opioid administration

The claims do not broadly cover every use of methylnaltrexone. A use must satisfy the specified patient, therapeutic, dosing, and pharmacokinetic conditions.

How are the independent claims structured?

Claims 1, 31, and 36 through 39 are the principal independent claims.

Claim 1: low-exposure treatment of opioid side effects

Claim 1 requires:

A chronic opioid patient.
Administration of a quaternary noroxymorphone derivative.
Treatment or prevention of an opioid-induced side effect.
A dose effective in the chronic opioid patient.
A dose insufficient to treat the same side effect in a nonchronic opioid patient.
A peak plasma concentration not exceeding 100 ng/mL.

The claim is broad as to the quaternary derivative and side effect, but the functional dose comparison materially narrows its reach. A potential accused method would need to show that the dose is effective in the chronic opioid population while not effective in the specified comparator population.

Claim 31: duration-based dose selection

Claim 31 adds a dosing algorithm. The practitioner must:

determine how long the patient has received opioids;
select a quaternary noroxymorphone derivative dose based on that duration;
use a dose for chronic opioid patients at least 10% lower than the dose selected for nonchronic patients; and
administer the selected dose while maintaining the 100 ng/mL peak plasma limit.

Claims 32 through 35 narrow the required reduction to at least 20%, 30%, 50%, or 70%.

This claim is more difficult to read on a conventional fixed-dose product unless the prescribing process actually differentiates the dose based on opioid-treatment duration. A product label that merely provides a standard dose may not satisfy the claim’s explicit dose-selection limitation.

Claims 36 through 39: symptom-specific claims

These claims separately target:

constipation;
urinary retention;
dysphoria; and
pruritus.

Claims 36 and 37 use "quaternary derivative of noroxymorphone." Claims 38 and 39 use the apparent typo "quaternary derivative of noroxymorphane." The claims also contain drafting errors, including the reference to reducing dysphoria in claim 39 even though the claim identifies pruritus as the target symptom.

Those errors would have affected claim-construction and validity analysis while the patent was enforceable. They do not restore enforceability after expiration.

What is the commercial relevance of methylnaltrexone?

Methylnaltrexone is a peripherally acting mu-opioid receptor antagonist. Its quaternary ammonium structure limits passage across the blood-brain barrier relative to centrally acting opioid antagonists. The commercial objective is to reverse peripheral opioid effects, especially gastrointestinal opioid activity, without materially reducing central analgesia.

The main branded product associated with this patent family is Relistor, containing methylnaltrexone bromide. FDA approvals include subcutaneous and oral dosage forms for opioid-induced constipation in specified patient populations. The FDA label identifies methylnaltrexone bromide as the active ingredient and describes use in patients receiving opioids for chronic pain and in patients with advanced illness or active cancer pain. [4]

The patent claims map most closely to:

oral methylnaltrexone;
enterically coated methylnaltrexone;
non-enterically coated methylnaltrexone;
low-dose administration;
chronic opioid-induced constipation; and
peripheral side-effect treatment without intended central opioid withdrawal.

What formulations are protected by U.S. Patent 6,559,158?

The patent contains formulation and route limitations, but it is not principally a formulation-composition patent.

Formulation or route	Claims implicated	Commercial significance
Oral methylnaltrexone	7, 15-19, 25-27	Directly relevant to oral Relistor
Enterically coated oral form	5, 20, 24, 27	Covers protection of the active ingredient through upper gastrointestinal delivery
Non-enterically coated oral form	6, 7, 25	Covers immediate or non-enteric oral administration
Opioid and antagonist in one oral dose	23-24	Relevant to combination products
Parenteral administration	Not expressly required by the listed claims	Potentially within broader method claims if all other limitations are satisfied

The patent does not by itself establish protection for a particular tablet excipient system, coating polymer, tablet architecture, manufacturing process, or release profile. Those matters would normally be addressed through separate formulation, composition, or process patents.

How narrow are the plasma-concentration limitations?

The Cmax limitations are central to the patent’s technical distinction. The claims progressively narrow the maximum peak plasma concentration:

Claim	Maximum peak plasma concentration
1, 31, 36-39	100 ng/mL
12, 16, 28, 40	50 ng/mL
17, 43	26 ng/mL
13, 18, 29, 41	20 ng/mL
14, 19, 30, 42	10 ng/mL

These limitations raise several enforcement questions:

the biological matrix used for testing;
whether the concentration is total or unbound drug;
the sampling time and pharmacokinetic method;
whether the threshold is measured in a single patient, a mean population value, or a specified percentile;
whether a formulation that produces variable Cmax values satisfies a maximum limitation; and
whether the claimed value is evaluated under labeled use or actual clinical use.

A product could avoid a claim with a 10 ng/mL ceiling while potentially remaining within a broader 20, 50, or 100 ng/mL claim. Conversely, a formulation with a higher Cmax may avoid the listed claims even if it treats opioid-induced constipation.

Does the patent cover all methylnaltrexone products?

No. The claims require a particular clinical use and patient context.

A methylnaltrexone product would generally need to be evaluated against these claim limitations:

The patient must be chronically administered opioids.
The treatment must address an opioid-induced side effect.
The dose must be effective in the chronic opioid patient.
The same dose must be insufficient in the nonchronic comparator population.
The relevant Cmax must remain below the applicable threshold.
Any additional limitations, such as oral delivery, enteric coating, or dose reduction, must also be satisfied.

A generic product with the same active ingredient can have a different infringement profile depending on its approved labeling, formulation, pharmacokinetics, and induced-use evidence. Under U.S. patent law, a generic manufacturer may face liability based on the uses encouraged by its label even when the product has other noninfringing uses. The governing legal framework includes 35 U.S.C. §271(e)(2) for ANDA-related acts and §271(b) for induced infringement.

When did U.S. Patent 6,559,158 lose exclusivity?

U.S. Patent 6,559,158 expired in December 2019, based on the 20-year term from its nonprovisional filing date. The expiration date is distinct from FDA regulatory exclusivity and from other patents in the methylnaltrexone estate. [1][2]

Rights category	Status
U.S. Patent 6,559,158	Expired in December 2019
Patent-based enforcement under this patent	No longer available after expiration
Small-molecule FDA exclusivity	Separate from patent term
Biosimilar exclusivity	Not applicable
Other methylnaltrexone patents	Must be assessed separately by product, formulation, and jurisdiction

The patent’s expiration means that a company does not need a license to practice the expired claims in the United States. A license may still be relevant to unexpired continuation patents, foreign patents, know-how, trademarks, data rights, or commercial arrangements.

What was the Orange Book status of the methylnaltrexone products?

Relistor is a small-molecule drug product, not a biologic. Its relevant FDA pathway is the NDA and ANDA framework, not the biosimilar pathway.

The Orange Book historically listed patents associated with Relistor. Patent listings for a drug product do not automatically establish validity or infringement. They identify patents submitted by the NDA holder for FDA listing and provide the framework for ANDA patent certifications. [3]

For an ANDA applicant, the principal certification possibilities are:

Paragraph I: no patent information has been submitted;
Paragraph II: the listed patent has expired;
Paragraph III: the applicant will wait until patent expiration; or
Paragraph IV: the listed patent is invalid, unenforceable, or will not be infringed.

Because U.S. Patent 6,559,158 expired in 2019, it no longer supports a current Paragraph III delay. A later applicant would have had to address any unexpired Orange Book-listed patents separately.

Which companies developed or commercialized methylnaltrexone?

The commercial history centers on Progenics Pharmaceuticals, with later involvement by Salix Pharmaceuticals and Bausch Health.

Company	Role
Progenics Pharmaceuticals	Developer and patent holder or commercial rights holder associated with methylnaltrexone
Wyeth and related commercial partners	Earlier development and commercialization relationships associated with methylnaltrexone
Salix Pharmaceuticals	U.S. commercialization and licensing platform for Relistor
Bausch Health	Parent company of Salix and current commercial owner of the Salix portfolio

Progenics entered licensing and collaboration arrangements relating to methylnaltrexone development and commercialization. Salix acquired U.S. rights to Relistor before the product became part of the Bausch Health portfolio. The commercial value of the patent was therefore linked to a broader package of patents, regulatory approvals, clinical data, manufacturing rights, and brand rights rather than to Patent 6,559,158 alone. [5][6]

What patent litigation affected methylnaltrexone?

The main litigation risk involved ANDA applicants seeking approval for generic methylnaltrexone products and challenges to patents listed for Relistor. The relevant disputes concerned patent validity, infringement, and the timing of generic entry.

Patent 6,559,158 was a historical foundational patent. By the time generic competition became commercially relevant, the litigation analysis would have focused on:

whether the patent had expired;
whether later continuation or formulation patents remained in force;
whether the proposed generic label induced practice of a patented method;
whether the generic formulation met the claimed Cmax limits;
whether the ANDA product included oral or parenteral indications; and
whether any settlement restricted or delayed launch.

An expired patent cannot support a current injunction or damages claim for post-expiration conduct. Historical litigation involving the patent remains relevant to the development of the patent family and to assessing how later patents were drafted, but it does not create current exclusivity.

How strong was the patent estate?

The estate was strongest when evaluated as a portfolio rather than as Patent 6,559,158 in isolation.

Strengths

It identified a specific clinical problem with substantial opioid-use prevalence.
It linked methylnaltrexone to peripheral opioid side-effect treatment.
It included both broad method claims and narrower oral, enteric, dose, and Cmax claims.
It covered constipation and several additional opioid-induced side effects.
It included a dose-selection concept based on chronic opioid exposure.
It aligned with the clinical positioning of Relistor.

Weaknesses

The claims depended heavily on functional language such as "sufficient" and "insufficient."
"Chronic opioid patient" is a patient-status limitation that may require factual and clinical proof.
The nonchronic comparator requirement complicates infringement analysis.
Cmax thresholds require reproducible pharmacokinetic testing.
The broad genus of quaternary noroxymorphone derivatives creates written-description and enablement questions.
Claims 38 and 39 contain apparent chemical and drafting errors.
The patent was limited to methods and did not independently block all methylnaltrexone compositions or manufacturing processes.
The patent is expired.

The practical strength of the patent was therefore highest during its enforceable term and in combination with later patents covering formulations, dosing regimens, polymorphs, manufacturing, or specific product presentations.

What generic entry risks exist now?

For the United States, Patent 6,559,158 creates no current generic-entry barrier. The relevant risks are instead:

Unexpired Relistor patents listed in the Orange Book.
Continuation or divisional patents with overlapping method claims.
Formulation or manufacturing patents outside the claims supplied.
Label-induced infringement based on approved opioid-induced constipation indications.
Regulatory exclusivity or pediatric exclusivity, if applicable at the relevant filing date.
Product-specific patent disputes involving tablets, injections, prefilled syringes, or other delivery systems.
Commercial barriers, including manufacturing scale, API sourcing, reimbursement, and physician adoption.

Because methylnaltrexone is a small molecule, biosimilar litigation is not the relevant framework. The competitive threat comes from ANDA-approved generics, authorized generics, alternative peripherally acting mu-opioid receptor antagonists, and other constipation treatments.

How does methylnaltrexone compare with competing opioid-induced constipation drugs?

Product	Active ingredient	Regulatory class	Primary differentiation
Relistor	Methylnaltrexone bromide	Small-molecule NDA	Peripheral opioid antagonism; oral and subcutaneous presentations
Movantik	Naloxegol	Small-molecule NDA	Oral PEGylated naloxol derivative
Symproic	Naldemedine	Small-molecule NDA	Oral peripherally acting mu-opioid receptor antagonist
Entereg	Alvimopan	Small-molecule NDA	Restricted-use opioid bowel dysfunction product, principally in postoperative settings

Patent risk is product-specific. A competitor does not need to practice the claims of Patent 6,559,158 merely because it treats opioid-induced constipation. The active chemical, formulation, indication, dosing, pharmacokinetics, and patent family must be compared claim by claim.

What manufacturing and geographic barriers remain?

U.S. expiration does not eliminate foreign rights. The corresponding international patent family may have expired at different times because national filing dates, patent-term adjustments, supplementary protection certificates, and prosecution histories vary by jurisdiction.

A global freedom-to-operate review should separate:

United States composition and method patents;
European national or unitary rights;
Canadian, Japanese, and Australian counterparts;
formulation and dosage-form patents;
API synthesis and salt-form patents;
manufacturing-process patents;
regulatory data protection; and
trademark rights in Relistor and related product names.

Manufacturing barriers may survive the expiration of a use patent. A supplier could still face rights covering a particular methylnaltrexone salt, crystal form, impurity profile, process intermediate, or scalable manufacturing route.

Key Takeaways

U.S. Patent 6,559,158 is a method-of-use patent centered on methylnaltrexone and other quaternary noroxymorphone derivatives.
Its core invention is low-systemic-exposure treatment of opioid-induced side effects in chronic opioid patients.
The most important limitations are chronic opioid exposure, differential efficacy versus nonchronic patients, and Cmax thresholds of 100 ng/mL or lower.
Claims 31 through 35 add a duration-based individualized dosing algorithm.
Claims 36 through 39 target constipation, urinary retention, dysphoria, and pruritus.
The patent expired in December 2019 and does not currently block U.S. generic entry.
Relistor is a small-molecule product subject to the NDA and ANDA framework, not biosimilar regulation.
Current freedom-to-operate analysis must focus on later Relistor patents, formulation rights, manufacturing rights, Orange Book status, and label-induced infringement.
The patent was commercially important as part of a broader methylnaltrexone portfolio, but it is no longer an enforceable U.S. exclusionary right.

FAQs

Can an ANDA applicant rely on expiration of Patent 6,559,158?

Yes. After expiration, the patent cannot support a Paragraph III delay or post-expiration infringement claim. The applicant must still address any other unexpired patents listed for the relevant methylnaltrexone product.

Does Patent 6,559,158 cover naloxegol or naldemedine?

No. The claims are directed to quaternary derivatives of noroxymorphone, with methylnaltrexone as the expressly claimed species. Naloxegol and naldemedine are separate chemical entities with separate patent estates.

Does the patent cover a methylnaltrexone product used in a non-opioid patient?

Generally no. The listed claims require treatment of a chronic opioid patient or otherwise require the dose to be effective in a chronic opioid patient and insufficient in a nonchronic patient.

Could a manufacturer practice the expired claims outside the United States?

Potentially, but the answer depends on the status of corresponding national patents. U.S. expiration does not determine the term of every foreign counterpart.

Are methylnaltrexone manufacturing patents separate from Patent 6,559,158?

Yes. A manufacturing process, salt, polymorph, formulation, impurity specification, or API-production method may be protected by separate patents or regulatory rights. Those rights require independent review.

References

United States Patent and Trademark Office. (2003). U.S. Patent No. 6,559,158, Methods of treating opioid-induced side effects.
United States Patent and Trademark Office. (n.d.). Patent term adjustment and patent term calculation resources. https://www.uspto.gov/
U.S. Food and Drug Administration. (n.d.). Approved drug products with therapeutic equivalence evaluations, Orange Book. https://www.accessdata.fda.gov/scripts/cder/ob/
U.S. Food and Drug Administration. (2023). Relistor (methylnaltrexone bromide) prescribing information.
Progenics Pharmaceuticals, Inc. (various years). Annual reports and Securities and Exchange Commission filings concerning methylnaltrexone and Relistor.
Bausch Health Companies Inc. (various years). Annual reports and product information concerning Salix Pharmaceuticals and Relistor.

Country	Patent Number	Estimated Expiration	Supplementary Protection Certificate	SPC Country	SPC Expiration
Austria	467415	⤷ Start Trial
Australia	1380299	⤷ Start Trial
Australia	758416	⤷ Start Trial
>Country	>Patent Number	>Estimated Expiration	>Supplementary Protection Certificate	>SPC Country	>SPC Expiration

Details for Patent: 6,559,158

Summary for Patent: 6,559,158