US Patent 6,037,157: Scope, Claim Analysis, Expiration, and Ritonavir Patent Landscape

US Patent 6,037,157 covers the use of ritonavir to inhibit cytochrome P450 metabolism, increase blood levels, and improve the pharmacokinetics of co-administered drugs. Its principal commercial significance was the use of ritonavir as a pharmacokinetic enhancer for HIV protease inhibitors, especially saquinavir. The patent issued on March 14, 2000, and its U.S. patent term has expired. It no longer blocks generic or branded products in the United States.

What does US Patent 6,037,157 protect?

The patent protects methods, rather than a ritonavir composition, formulation, manufacturing process, or particular fixed-dose combination.

Its claim structure has three operative categories:

Claim category	Claims	Protected subject matter
Pharmacokinetic improvement	1-8	Administering ritonavir with a CYP-metabolized drug to improve pharmacokinetics
Increased blood levels	9-16	Administering ritonavir with a CYP-metabolized drug to increase human blood levels
CYP inhibition	17-18	Administering ritonavir to a human or contacting ritonavir with CYP enzyme

The patent identifies numerous candidate drugs, but the narrowest claims focus on four compounds:

Saquinavir
VX-478, the development code for amprenavir
MK-639, the development code for indinavir
AG1343, the development code for nelfinavir

The patent is commonly associated with the concept of “ritonavir boosting,” in which a low or subtherapeutic amount of ritonavir inhibits CYP3A-mediated metabolism of another drug and increases systemic exposure.

How broad are the independent claims?

Claim 1: Pharmacokinetic improvement

Claim 1 requires:

A drug metabolized by cytochrome P450 monooxygenase;
Ritonavir or a pharmaceutically acceptable salt;
Administration of the combination;
Administration to a human;
A therapeutically effective amount; and
Improvement in the drug’s pharmacokinetics.

The claim is functionally drafted. It does not require a particular dose, dosage ratio, dosing schedule, formulation, route of administration, CYP isoform, or numerical pharmacokinetic increase.

The claim is broad in its choice of co-administered drugs but narrower than a claim covering every CYP inhibitor. The enhancer must be ritonavir, and the method must involve treatment of a human.

Claim 9: Increased human blood levels

Claim 9 is similar to claim 1 but uses increased blood levels as the result. This limitation is potentially easier to establish experimentally because it can be demonstrated through plasma concentration, Cmax, AUC, trough concentration, or related pharmacokinetic measurements.

The claim does not state how much blood-level increase is required. A patent dispute would therefore likely focus on whether the accused regimen produces a clinically meaningful increase and whether that increase results from the claimed ritonavir administration.

Claim 17: Human CYP inhibition

Claim 17 covers administering ritonavir to a human in an amount effective to inhibit cytochrome P450 monooxygenase.

This claim does not require a second drug. It is therefore conceptually broader than claims 1-16 in one respect, although it still requires:

Human administration;
An amount effective to inhibit CYP;
Ritonavir or a pharmaceutically acceptable salt.

The claim could reach use of ritonavir as a standalone pharmacokinetic modifier if the administration produces CYP inhibition.

Claim 18: In vitro or non-human enzyme contact

Claim 18 covers contacting CYP monooxygenase with ritonavir in an amount effective to inhibit the enzyme. It does not require human administration.

The claim is directed to an activity-based assay or biochemical use. It can potentially cover laboratory contact between ritonavir and the enzyme, subject to ordinary infringement requirements and the statutory treatment of research activities.

Which claims are strongest commercially?

Claims 5 and 13 are the most commercially focused claims because they specifically identify saquinavir. Saquinavir was the first HIV protease inhibitor associated with clinically important ritonavir boosting.

Claims 6 and 14 identify VX-478, later developed as amprenavir. Claims 7 and 15 identify MK-639, later developed as indinavir. Claims 8 and 16 identify AG1343, later developed as nelfinavir.

The broader claims 1 and 9 may cover other CYP-metabolized drugs, including drugs not individually named in the dependent claims. Their practical scope depends on construction of:

“Drug which is metabolized by cytochrome P450 monooxygenase”
“Improving the pharmacokinetics”
“Increasing human blood levels”
“Therapeutically effective amount”
“Amount effective to inhibit”

The dependent claims do not necessarily limit the independent claims to HIV drugs. They provide progressively narrower species and drug-specific embodiments.

What drugs are listed in US Patent 6,037,157?

The patent’s listed compounds fall into several groups.

Compound or code	Commercial or scientific identity
Saquinavir	HIV protease inhibitor; Invirase and Fortovase
VX-478	Amprenavir
MK-639	Indinavir
AG1343	Nelfinavir
FK-506	Tacrolimus
Rapamycin	Sirolimus
Taxol	Paclitaxel
Taxotere	Docetaxel
Clarithromycin	Macrolide antibiotic
Cyclosporine	Immunosuppressant
A-77003, A-80987	Experimental HIV protease inhibitors
DMP-323	Experimental HIV protease inhibitor
BILA compounds	Experimental antiviral compounds
BMS 186,318	Experimental pharmaceutical compound
SC-52151, SC-629	Experimental protease inhibitors
KNI-272	Experimental protease inhibitor
CGP 53437, CGP 57813	Experimental antiviral compounds
U-103017	Experimental compound

The presence of a compound in the specification or a dependent claim does not by itself establish that a later commercial product infringes. The product must satisfy every limitation of an asserted claim.

What formulations are protected by the patent?

US 6,037,157 does not claim a specific formulation. It does not require:

A tablet;
A capsule;
An oral solution;
A fixed-dose combination;
A particular excipient;
A particular ritonavir-to-co-drug ratio;
A sustained-release formulation;
A liquid-filled soft-gel formulation;
A specific particle size or polymorph.

A product may be relevant to the method claims if it is used in a claimed regimen, but the patent does not independently exclude manufacture or sale of a ritonavir dosage form.

This distinction matters for later products such as ritonavir tablets, oral solutions, and co-packaged HIV regimens. Separate formulation, salt, polymorph, combination-product, and manufacturing patents would have to be analyzed independently.

When did US Patent 6,037,157 expire?

Patent-term timeline

Event	Date
Patent issued	March 14, 2000
Earliest claimed priority period	1996
U.S. patent term expiration	March 2017
Current status	Expired

The patent’s term was governed by the post-1995 twenty-year patent term measured from the applicable nonprovisional filing date, subject to any patent-term adjustment or terminal-disclaimer effects. Public patent databases identify the patent as expired by term.

The patent therefore cannot support a current U.S. injunction against a generic ritonavir product or a product using ritonavir to enhance another drug.

What was the FDA and Orange Book significance?

Ritonavir was approved by FDA as Norvir in 1996. Its original regulatory role was as an HIV protease inhibitor. Abbott later used ritonavir at lower doses as a pharmacokinetic enhancer in combination with other antiretrovirals.

The FDA approval history includes:

Norvir capsules and oral solution;
Later ritonavir tablet formulations;
HIV treatment regimens using ritonavir with other antiretrovirals;
Combination regimens involving protease inhibitors.

US 6,037,157 was historically relevant to the method of using ritonavir as a booster. It did not create FDA regulatory exclusivity and did not substitute for New Drug Application approval.

Orange Book status

The Orange Book distinguishes approved products, patents, and regulatory exclusivity. A method-of-use patent can be listed with an associated use code when it claims an approved indication or method of use.

Because US 6,037,157 has expired, it has no current blocking effect even if it appeared historically in FDA patent records. Any historical Orange Book listing would not create present-day exclusivity.

Current market entry analysis must instead focus on:

Unexpired patents listed for the relevant product;
FDA-approved labeling;
Drug-specific exclusivity;
Any pediatric exclusivity;
Patent certifications submitted under an abbreviated application.

Were Paragraph IV challenges relevant?

A Paragraph IV certification is used when an abbreviated new drug application applicant asserts that a listed patent is invalid, unenforceable, or will not be infringed.

For US 6,037,157, the commercial importance of a Paragraph IV challenge was concentrated in the period before expiration. A generic applicant targeting a listed ritonavir product or a use involving ritonavir boosting could have faced:

A patent-certification dispute;
A 30-month stay if the statutory conditions were met;
A patent infringement action under 35 U.S.C. § 271(e)(2);
A dispute over whether the proposed labeling induced infringement.

After expiration, a Paragraph IV challenge to this patent has no practical value because the patent no longer supplies a live exclusionary right.

The relevant litigation questions would have included whether a generic label instructed administration of ritonavir with a CYP-metabolized drug, whether a formulation was sold for that purpose, and whether the proposed use satisfied the human-treatment limitations.

What patent litigation affected ritonavir boosting?

Ritonavir-related patent disputes generally divided into four groups:

Core ritonavir compound patents;
Ritonavir formulations and dosage forms;
Use of ritonavir as a pharmacokinetic enhancer;
Specific boosted combinations, such as lopinavir/ritonavir or other protease-inhibitor regimens.

US 6,037,157 belongs to the third category.

The most important infringement theory would have been induced infringement. A manufacturer does not necessarily administer the drug itself, so liability could depend on evidence that the manufacturer intentionally encouraged patients or healthcare professionals to practice the claimed method through labeling, promotional materials, dosing instructions, or product design.

A direct-infringement theory would focus on the physician, healthcare provider, or patient administering the combination. Joint-infringement and divided-infringement issues could arise where different parties supplied ritonavir and the co-administered drug.

No current enforcement action can be based on this expired patent.

How strong was the patent estate?

Technical strength

The patent had high strategic value because it claimed a general biological mechanism with broad practical applications. Its claims were not limited to one molecular combination or one disease indication.

Its technical strengths were:

Early recognition of ritonavir’s CYP inhibitory effect;
Broad claim language;
Claims covering both pharmacokinetic improvement and blood-level increases;
Separate human-use and enzyme-contact claims;
Specific coverage of major HIV protease inhibitors.

Legal and enforcement limitations

The claims also had limitations:

They require ritonavir, not another CYP3A inhibitor;
Most treatment claims require human administration;
The claims require a drug metabolized by CYP;
Result limitations could create proof issues;
Claims 1-16 require administration of a combination;
The claims do not cover all uses of ritonavir;
The patent does not independently cover formulation, manufacture, or composition.

The patent’s current legal strength is zero as an exclusionary right because the term has expired. Its historical strength was materially higher during the period when ritonavir boosting became clinically established.

How does US 6,037,157 compare with later booster patents?

Patent category	Typical claim focus	Relationship to US 6,037,157
Ritonavir compound patents	Chemical compound, salts, preparation	Earlier and structurally different
Ritonavir formulation patents	Tablet, solution, excipient, stability	Separate protection
Ritonavir boosting patent	CYP inhibition and increased exposure	US 6,037,157’s principal category
Lopinavir/ritonavir patents	Specific fixed combination and HIV treatment	Narrower product-specific protection
Cobicistat patents	Alternative CYP3A inhibitor and booster	Separate active ingredient and method estate
Darunavir or atazanavir combination patents	Specific protease inhibitor regimens	Drug-specific, later-generation protection
Manufacturing patents	Synthesis, purification, crystallization	Do not depend on the method claims here

Cobicistat is the closest commercial comparator from a pharmacokinetic-enhancer perspective. It was developed specifically as a CYP3A inhibitor without antiviral activity comparable to ritonavir. Its patents generally concern cobicistat, combinations, formulations, and treatment regimens, not the use of ritonavir itself.

What generic entry risks exist today?

US 6,037,157 creates no present generic-entry risk because it has expired. A current generic or biosimilar analysis should not treat this patent as an active barrier.

Generic risk

For ritonavir and ritonavir-containing products, current risks may arise from:

Unexpired composition or formulation patents;
Product-specific combination patents;
Regulatory exclusivity;
Labeling that includes protected uses under other patents;
Trade-secret manufacturing processes;
Patent rights outside the United States.

US 6,037,157 does not block an abbreviated application solely because the proposed product contains ritonavir.

Biosimilar risk

Biosimilar analysis is not applicable to ritonavir. Ritonavir is a chemically synthesized small-molecule drug regulated through the generic-drug pathway, not the biologics license application pathway.

What geographic coverage does the patent provide?

US 6,037,157 provides rights only in the United States. It does not establish protection in:

Europe;
Canada;
Japan;
China;
India;
Brazil;
Other national jurisdictions.

Foreign counterparts would require separate analysis of national application numbers, prosecution, validity, term, supplementary protection certificates, and litigation. Expiration of the U.S. patent does not determine the status of foreign rights.

What licensing deals are associated with the patent?

The patent was developed in the Abbott Laboratories ritonavir research program. The commercial history of ritonavir includes collaborations and licensing arrangements involving HIV products, but a license to use ritonavir or a branded combination does not automatically transfer ownership of US 6,037,157.

The patent itself should be distinguished from:

Commercial distribution licenses;
Patent settlements;
Co-development agreements;
Supply agreements;
Licenses covering specific protease inhibitors;
Rights relating to later combination products.

A patent assignment or license must be confirmed from the USPTO assignment and recorded-interest records. Commercial use of ritonavir by a third party does not establish a license to the patent.

Key Takeaways

US 6,037,157 covers ritonavir use as a CYP450-mediated pharmacokinetic enhancer.
Claims 1-16 cover co-administration with CYP-metabolized drugs.
Claims 17 and 18 cover CYP inhibition by ritonavir in humans and in enzyme-contact settings.
Saquinavir is the most commercially significant specifically claimed drug.
The patent does not claim ritonavir itself, a formulation, a fixed-dose combination, or a manufacturing process.
The patent issued March 14, 2000, and expired in March 2017.
It has no current U.S. blocking effect against generic ritonavir or ritonavir-boosted products.
Paragraph IV significance was historical and ended with patent expiration.
Biosimilar analysis does not apply because ritonavir is a small molecule.
Current freedom-to-operate work must examine later formulation, combination, manufacturing, and product-specific patents.

FAQs

Does US Patent 6,037,157 cover all CYP3A inhibitors?

No. The claims require ritonavir or a pharmaceutically acceptable salt of ritonavir. They do not cover cobicistat, ketoconazole, itraconazole, or other CYP3A inhibitors unless a separate patent applies.

Does the patent cover Paxlovid?

No. Paxlovid contains nirmatrelvir and ritonavir, but US 6,037,157 is expired and does not provide current patent protection for that product. Paxlovid analysis depends on later patents and regulatory exclusivity.

Can an expired ritonavir patent be used in a patent-infringement lawsuit?

No. Expiration eliminates the patent owner’s right to obtain prospective infringement remedies based on conduct occurring after expiration. Historical damages issues can involve separate timing and litigation questions.

Does the patent cover low-dose ritonavir used only as a booster?

The claim language is broad enough to encompass ritonavir used to inhibit CYP metabolism or increase exposure, provided every claim limitation is met. The patent’s expiration means that this historical scope has no current U.S. exclusionary effect.

Is saquinavir plus ritonavir still protected by this patent?

No. Claims 5 and 13 specifically identify saquinavir, but the patent expired in March 2017. Any current protection would have to come from another unexpired patent or regulatory exclusivity.

References

U.S. Patent and Trademark Office. (2000). U.S. Patent No. 6,037,157, Methods for improving the pharmacokinetics of a drug using ritonavir. Washington, DC: U.S. Department of Commerce.
U.S. Food and Drug Administration. (1996). Norvir (ritonavir) approval history and prescribing information. Silver Spring, MD: U.S. Department of Health and Human Services.
U.S. Food and Drug Administration. (2024). Approved drug products with therapeutic equivalence evaluations, commonly known as the Orange Book. Silver Spring, MD: U.S. Department of Health and Human Services.
U.S. Code, 35 U.S.C. §§ 154, 271(e)(2), and 271(b).
U.S. Patent and Trademark Office. (2024). Patent term adjustment and patent term calculation resources. Washington, DC: U.S. Department of Commerce.

Country	Patent Number	Estimated Expiration	Supplementary Protection Certificate	SPC Country	SPC Expiration
Austria	225186	⤷ Start Trial
Austria	392895	⤷ Start Trial
Australia	6342096	⤷ Start Trial
Australia	722812	⤷ Start Trial
Canada	2224738	⤷ Start Trial
Germany	69624136	⤷ Start Trial
>Country	>Patent Number	>Estimated Expiration	>Supplementary Protection Certificate	>SPC Country	>SPC Expiration

Details for Patent: 6,037,157

Summary for Patent: 6,037,157