United States Drug Patent 5,298,520: Scope, Claims, Expiration, and Rizatriptan Patent Landscape

U.S. Patent No. 5,298,520 covers rizatriptan, the active pharmaceutical ingredient in Maxalt and Maxalt-MLT. Its claims cover the rizatriptan free base, specified salts, pharmaceutical compositions, and therapeutic use for migraine and related headache disorders. The patent issued on March 29, 1994, and its ordinary 17-year patent term expired on March 29, 2011. The patent therefore no longer creates an enforceable U.S. exclusivity barrier for rizatriptan products.[1]

What compound does U.S. Patent 5,298,520 protect?

The patent claims N,N-dimethyl-2-[5-(1,2,4-triazol-1-ylmethyl)-1H-indol-3-yl]ethylamine, which is rizatriptan.

Rizatriptan is a selective serotonin 5-HT1B/1D receptor agonist used primarily for the acute treatment of migraine attacks. The marketed drug is generally supplied as rizatriptan benzoate, although the patent claims both the free base and several expressly identified salts.[2]

Attribute	Description
Active ingredient	Rizatriptan
Chemical name in claim 1	N,N-dimethyl-2-[5-(1,2,4-triazol-1-ylmethyl)-1H-indol-3-yl]ethylamine
Drug class	Triptan; serotonin 5-HT1B/1D agonist
Original innovator	Merck & Co.
U.S. patent	5,298,520
Issue date	March 29, 1994
FDA product	Maxalt and Maxalt-MLT
Patent term under historical 17-year rule	March 29, 2011
Current status	Expired

The molecule has no claimed stereochemical limitation. Claim 1 therefore covers the named rizatriptan compound without requiring a particular stereoisomer or chiral resolution.

What does claim 1 of U.S. Patent 5,298,520 cover?

Claim 1 is the core compound claim. It covers:

Rizatriptan itself; and
A pharmaceutically acceptable salt of rizatriptan.

The claim is directed to the chemical entity, not to a particular dose, route of administration, formulation, indication, or commercial product. A product containing rizatriptan would have fallen within the literal scope of claim 1 regardless of whether it was supplied as a tablet, orally disintegrating tablet, capsule, solution, or another dosage form.

The claim does not expressly cover:

Other indole derivatives;
Triazole regioisomers with a different attachment pattern;
A prodrug that is chemically different from rizatriptan;
A metabolite of rizatriptan;
A different active ingredient in the triptan class;
A formulation that contains no rizatriptan;
Manufacturing processes not otherwise claimed.

Because the claim names a specific compound, it is a composition-of-matter claim rather than a broad genus claim. Composition claims generally provide the strongest form of pharmaceutical patent protection because they reach the active ingredient across multiple formulations and indications.

Which rizatriptan salts are protected by claim 2?

Claim 2 narrows claim 1 to four specified salts:

Salt identified in claim 2	Commercial or technical significance
Oxalate	Covered salt form; not the principal marketed U.S. form
Succinate	Covered salt form; not the principal marketed U.S. form
Benzoate	Commercially important rizatriptan salt used for Maxalt products
Hydrochloride	Covered salt form; not the principal marketed U.S. form

Claim 2 is narrower than claim 1. A salt that is pharmaceutically acceptable but does not fall within the four listed salts may still be covered by claim 1, but it would not fall within claim 2 unless it also satisfies the claim's express salt limitation.

The benzoate limitation is commercially significant because rizatriptan benzoate is the active pharmaceutical form identified in FDA product labeling for Maxalt and generic rizatriptan products.[2,3]

What pharmaceutical compositions are covered by claims 3 and 4?

Claim 3 covers a pharmaceutical composition containing:

A therapeutically effective amount of rizatriptan or a pharmaceutically acceptable salt; and
A pharmaceutically acceptable carrier or excipient.

Claim 4 narrows claim 3 to compositions containing the oxalate, succinate, benzoate, or hydrochloride salt.

The composition claims can reach a finished dosage form when the formulation contains the claimed active ingredient and a conventional pharmaceutical carrier. Covered dosage forms could include tablets, orally disintegrating tablets, capsules, powders, or other solid or liquid presentations, provided the claim elements are satisfied.

The claims do not require:

A particular dosage strength;
A specific excipient;
A particular dissolution profile;
A particular manufacturing process;
A particular release mechanism;
A specific brand name;
A particular route of administration.

The absence of a formulation limitation makes claim 3 materially broader than a later patent directed to a specific orally disintegrating tablet technology. Once claim 3 expired, it ceased to provide an enforceable product-formulation barrier.

What methods of treatment are covered by claims 5 and 6?

Claim 5 covers administering rizatriptan or a pharmaceutically acceptable salt to treat:

Migraine;
Cluster headache;
Chronic paroxysmal hemicrania;
Headache associated with vascular disorders;
Tension headache; and
Pediatric migraine.

Claim 6 narrows claim 5 to the four expressly identified salts.

These are method-of-treatment claims. They require both the administration of the claimed compound and treatment of a listed condition. Claim 5 does not specify:

Dose;
Dosing frequency;
Administration route;
Patient age, except where pediatric migraine is the stated indication;
Treatment duration;
Combination therapy;
A particular formulation.

The method claims are therefore broad in therapeutic scope but narrower than the compound claim in one respect: they require conduct directed to treatment of an enumerated disorder. A commercial product containing rizatriptan would not infringe claim 5 merely because it contains rizatriptan. The claim requires the claimed treatment method.

How strong was the patent estate for rizatriptan?

U.S. Patent 5,298,520 was historically strong because it combined four protection layers:

Protection layer	Claims in patent	Competitive effect
Active ingredient	Claim 1	Reached rizatriptan across products and indications
Specific salts	Claim 2	Covered benzoate and three additional salts
Pharmaceutical compositions	Claims 3-4	Reached products containing rizatriptan and excipients
Therapeutic use	Claims 5-6	Covered treatment of migraine and related headaches

The composition-of-matter claim was the principal commercial barrier. The salt, formulation, and method claims provided supplemental enforcement positions, but they depended on the same rizatriptan chemical entity.

The patent was stronger against a generic developer before expiration than afterward. After March 29, 2011, a generic applicant no longer needed to overcome the expired compound claims through a Paragraph IV certification directed to this patent.

When did U.S. Patent 5,298,520 lose exclusivity?

The patent's ordinary expiration date was March 29, 2011, calculated from its March 29, 1994, issue date under the pre-1995 U.S. patent-term regime.[1]

Event	Date
Patent issued	March 29, 1994
Original patent term	17 years from issue
Ordinary expiration	March 29, 2011
Post-expiration status	No enforceable patent exclusivity under this patent

The patent's expiration eliminated the principal U.S. patent barrier covering the rizatriptan molecule itself. FDA regulatory exclusivity and other later-issued patents must be analyzed separately from the expired patent.

What was the FDA and Orange Book status of rizatriptan?

The FDA approved Maxalt tablets and Maxalt-MLT orally disintegrating tablets for the acute treatment of migraine attacks with or without aura. The products contain rizatriptan benzoate.[2]

The Orange Book historically listed patents associated with approved rizatriptan products. An Orange Book listing identifies patents submitted by the NDA holder and does not itself determine whether every claim is valid, infringed, or enforceable. Once a listed patent expires, it no longer blocks FDA approval through patent rights.

The key regulatory distinction is:

FDA approval concerns safety, efficacy, quality, and applicable exclusivity;
Patent enforcement concerns claim scope, validity, infringement, and expiration;
Orange Book listing concerns patent information submitted for an approved drug.

Rizatriptan is a small-molecule drug. Biosimilar approval under the Public Health Service Act is therefore not the relevant pathway. Generic applicants use the ANDA pathway under the Federal Food, Drug, and Cosmetic Act.[4]

Did rizatriptan face Paragraph IV challenges?

Generic rizatriptan applicants could have used Paragraph IV certifications against unexpired Orange Book-listed patents for Maxalt products. A Paragraph IV certification asserts that a listed patent is invalid, unenforceable, or not infringed.[4]

By the time generic rizatriptan products entered the U.S. market, U.S. Patent 5,298,520 had expired. A generic applicant therefore could not obtain a meaningful market-exclusion benefit by challenging that patent after expiration. Any relevant Paragraph IV activity would have concerned later-listed patents, including formulation or dosage-form patents, rather than the expired core compound patent.

The patent did not create a current Paragraph IV risk for a conventional generic rizatriptan product.

What formulation patents affected Maxalt-MLT?

Maxalt-MLT was an orally disintegrating rizatriptan product. Such products can be protected by patents directed to:

Rapid disintegration;
Low-water or waterless tablet manufacture;
Taste masking;
Excipient combinations;
Tablet friability and mechanical strength;
Packaging and moisture control;
Oral delivery without water.

These rights are distinct from U.S. Patent 5,298,520. The 1994 patent does not claim an orally disintegrating tablet as such. It claims a composition containing rizatriptan and a carrier or excipient, without requiring the specialized technical characteristics of Maxalt-MLT.

A generic conventional tablet and a generic orally disintegrating tablet therefore present different patent analyses. The expired compound patent removes the active-ingredient barrier, while later formulation patents could have affected only products practicing their specific limitations.

What generic entry risks exist for rizatriptan?

The principal generic entry scenarios are:

Generic scenario	Patent exposure under U.S. 5,298,520	Business assessment
Rizatriptan benzoate conventional tablet	No current exposure because the patent expired	Low patent risk from this patent
Rizatriptan orally disintegrating tablet	No current exposure from the 1994 patent	Requires separate review of formulation patents
Alternative rizatriptan salt	Claim 1 historically covered pharmaceutically acceptable salts	No current exposure from this expired patent
Pediatric migraine labeling	Claim 5 historically covered pediatric migraine	Label strategy may be relevant to other live use patents
Combination product	Could have implicated composition claims if rizatriptan was present	No current exposure from this patent

The commercial threat to Maxalt arose from generic substitution after expiration of the core patent and any relevant regulatory exclusivity. Generic rizatriptan products compete principally on price, dosage form, pharmacy substitution, and payer formulary placement.

Which companies challenged or competed with Maxalt?

The competitive field included:

Generic manufacturers supplying rizatriptan benzoate;
Merck as the Maxalt innovator;
Other triptan manufacturers, including GlaxoSmithKline with sumatriptan and later triptan competitors;
Manufacturers of newer migraine therapies, including CGRP-targeted products.

Sumatriptan, naratriptan, zolmitriptan, and eletriptan are chemically distinct compounds. Their patents do not read on rizatriptan merely because they share the triptan mechanism or migraine indication.

The principal competitive distinction is therefore between:

Direct generic rizatriptan products; and
Therapeutic substitutes with separate active ingredients and separate patent estates.

What is the geographic coverage of U.S. Patent 5,298,520?

U.S. Patent 5,298,520 had territorial effect only in the United States. It did not directly control:

Canadian manufacture or sale;
European marketing;
Japanese sales;
Importation into jurisdictions other than the United States;
Foreign patent rights.

Foreign counterpart patents required separate national analysis. U.S. patent expiration did not automatically determine the expiration of corresponding foreign rights, although the same priority and filing history could have influenced their terms.

What manufacturing and intellectual-property barriers remain?

The expired patent no longer blocks manufacture of rizatriptan in the United States. Remaining barriers may include:

FDA ANDA approval;
Drug Master File support for the active ingredient;
Pharmaceutical quality systems;
Process validation;
Stability data;
Bioequivalence for the selected dosage form;
Trademark restrictions involving Maxalt;
Later formulation or process patents;
Trade secrets relating to manufacturing;
Supply-chain qualification for rizatriptan benzoate.

A process patent would need to claim a particular manufacturing route or process condition. U.S. Patent 5,298,520 does not do so in the claims supplied.

Key Takeaways

U.S. Patent 5,298,520 is the foundational U.S. rizatriptan compound patent.
Claim 1 covers rizatriptan and pharmaceutically acceptable salts.
Claim 2 specifically covers the oxalate, succinate, benzoate, and hydrochloride salts.
Claims 3 and 4 cover pharmaceutical compositions containing rizatriptan.
Claims 5 and 6 cover treatment of migraine and specified headache disorders.
The patent issued March 29, 1994, and expired March 29, 2011.
The patent is no longer an enforceable barrier to generic rizatriptan entry.
Biosimilar analysis is inapplicable because rizatriptan is a small-molecule drug.
Later formulation patents, especially those directed to orally disintegrating tablets, must be separated from the expired compound patent.
Generic rizatriptan competition is principally governed by FDA approval, dosage-form requirements, remaining patent rights, and commercial supply capability.

FAQs About U.S. Patent 5,298,520 and Rizatriptan

Does U.S. Patent 5,298,520 cover Maxalt?

Yes. The patent covers the rizatriptan active ingredient used in Maxalt products, including pharmaceutically acceptable salts and pharmaceutical compositions.

Does the patent cover rizatriptan benzoate?

Yes. Claim 2 expressly identifies the benzoate salt. Claim 1 also covers pharmaceutically acceptable salts more broadly.

Can a generic company manufacture rizatriptan after the patent expired?

Yes. Expiration removed the patent-based exclusion created by U.S. Patent 5,298,520. The manufacturer still requires FDA approval and must address any separate, unexpired patents.

Is pediatric migraine use protected by the patent?

Claim 5 expressly lists pediatric migraine. That method claim expired with the patent on March 29, 2011.

Does this patent cover sumatriptan or zolmitriptan?

No. The claims identify the specific rizatriptan molecule. Sumatriptan and zolmitriptan are different chemical compounds and require separate patent analyses.

References

United States Patent and Trademark Office. (1994). U.S. Patent No. 5,298,520: 5-(1,2,4-triazol-1-ylmethyl)-1H-indole derivatives. U.S. Department of Commerce.
U.S. Food and Drug Administration. (n.d.). Maxalt and Maxalt-MLT prescribing information.
U.S. Food and Drug Administration. (n.d.). Orange Book: Approved drug products with therapeutic equivalence evaluations.
U.S. Food and Drug Administration. (n.d.). Abbreviated new drug application (ANDA) and patent certification requirements.

Foriegn Application Priority Data
Foreign Country	Foreign Patent Number	Foreign Patent Date
United Kingdom	9102222	Feb 01, 1991
United Kingdom	9106917	Apr 03, 1991
United Kingdom	9113415	Jun 21, 1991
United Kingdom	9122451	Oct 23, 1991

Country	Patent Number	Estimated Expiration	Supplementary Protection Certificate	SPC Country	SPC Expiration
European Patent Office	0497512	⤷ Start Trial	98C0037	France	⤷ Start Trial
European Patent Office	0497512	⤷ Start Trial	C980019	Netherlands	⤷ Start Trial
European Patent Office	0497512	⤷ Start Trial	SPC/GB98/035	United Kingdom	⤷ Start Trial
European Patent Office	0497512	⤷ Start Trial	9891028-4	Sweden	⤷ Start Trial
European Patent Office	0497512	⤷ Start Trial	99C0011	Belgium	⤷ Start Trial
European Patent Office	0497512	⤷ Start Trial	28/1998	Austria	⤷ Start Trial
Austria	158582	⤷ Start Trial
>Country	>Patent Number	>Estimated Expiration	>Supplementary Protection Certificate	>SPC Country	>SPC Expiration

Details for Patent: 5,298,520

Summary for Patent: 5,298,520