United States Patent 4,209,513: Scope, Validity, Orange Book Status, and Generic Entry Risk

U.S. Patent No. 4,209,513 is a formulation patent directed to trimethoprim-sulfamethoxazole tablets. Its claims require a specific combination of active-ingredient particle size, excipient ranges, tablet hardness, and disintegration performance. The patent is expired under the pre-Uruguay Round patent term, so it presents no current U.S. patent barrier to generic manufacture, ANDA approval, or commercial launch.

What does U.S. Patent 4,209,513 protect?

The patent protects a physical tablet formulation rather than trimethoprim, sulfamethoxazole, or the antibacterial combination as chemical entities.

Claim	Protected subject matter	Narrowing limitation
1	Tablet containing trimethoprim and sulfamethoxazole	Active combination at 90-98% w/w; particle size of 1-40 micrometers; 1-5% granulating agent; 1-5% swelling disintegrant; hardness of about 9.8-20 kg; disintegration under nine minutes
2	Tablet of claim 1	Hardness narrowed to about 9.8-14.8 kg
3	Tablet of claim 1	Disintegration narrowed to 45 seconds to about five minutes

The patent therefore covers a high-drug-load immediate-release tablet with controlled mechanical strength and rapid disintegration. It does not claim every tablet containing the two active ingredients.

The independent claim requires satisfaction of all material limitations. A product that contains trimethoprim and sulfamethoxazole but uses a different particle-size distribution, a different disintegrant level, or hardness outside the claimed range would have a potential noninfringement position, subject to claim construction and the doctrine of equivalents.

How should claim 1 be construed?

Active-ingredient concentration

Claim 1 requires the trimethoprim-sulfamethoxazole combination to constitute 90-98% of the tablet by weight. This is a narrow high-load formulation requirement.

The claim does not expressly recite a specific trimethoprim-to-sulfamethoxazole ratio. Commercial co-trimoxazole products commonly use a 1:5 ratio, such as 80 mg trimethoprim with 400 mg sulfamethoxazole in a single-strength tablet and 160 mg with 800 mg in a double-strength tablet. The claim language supplied here appears to define the combination by total weight rather than by a fixed ratio.

The absence of a stated ratio creates an important construction issue. A court could read "combination of trimethoprim with sulphamethoxazole" broadly, provided both ingredients are present. A specification-based construction could impose a disclosed or commercially intended ratio if the patent repeatedly treats the combination as a defined co-trimoxazole composition.

Particle size

The active combination must have a particle size of 1-40 micrometers. Particle size is a composition-defining limitation, not merely a manufacturing preference.

Key questions for an infringement analysis include:

Whether the range applies to each active ingredient separately or to the combined active material.
Whether the measurement concerns D50, mean particle size, maximum particle size, or another distribution parameter.
Whether agglomerated particles are measured before or after granulation.
Which analytical method and sampling procedure are used.
Whether the 1-40 micrometer range covers substantially all particles or a statistical portion of the batch.

The claim does not specify a measurement percentile in the language provided. That omission could create evidentiary disputes, particularly where the accused product has a broad particle-size distribution.

Granulating agent

The claim requires 1-5% w/w of a granulating agent. The term is functional and potentially broad. Examples in tablet manufacture can include povidone, starch paste, hydroxypropyl cellulose, hydroxypropyl methylcellulose, gelatin, and related binders.

A product using a binder at less than 1% or more than 5% would fall outside the literal numerical range. A product using a dry binder rather than a liquid granulating agent would raise a classification issue. The relevant question would be whether the material performs the claimed granulating function in the accused formulation.

Disintegrating agent

The disintegrant must be present at 1-5% w/w and have a swelling capacity of 5 to about 25 ml/g. The swelling-capacity limitation materially narrows the claim.

This limitation is directed to the physical behavior of the disintegrant, not simply its identity. Potentially relevant materials could include starches and other swelling excipients, depending on the test method used in the patent specification.

An accused product could challenge infringement by showing that:

Its disintegrant is outside the 5-25 ml/g swelling range.
The disintegrant is present outside the 1-5% range.
The measured swelling value changes under the testing method used.
The material is a wicking or effervescent disintegrant rather than a swelling disintegrant.

Tablet hardness

Claim 1 requires hardness of about 9.8-20 kg. Claim 2 narrows that range to about 9.8-14.8 kg.

Tablet hardness is not normally a fixed intrinsic property. It can vary with tablet orientation, tester model, compression speed, storage humidity, tablet dimensions, and measurement timing. The use of "about" gives some tolerance, but it does not eliminate the need to establish a defensible measurement protocol.

Claim 2 is narrower than claim 1 but may be commercially significant because a tablet compressed within the claim 2 range would also satisfy claim 1, assuming all other limitations are met.

Disintegration time

Claim 1 requires disintegration in less than nine minutes. Claim 3 narrows this to 45 seconds to about five minutes.

Claim 3 excludes tablets disintegrating in less than approximately 45 seconds, depending on the interpretation of "from 45 seconds." It also excludes tablets taking more than approximately five minutes. The claim therefore targets a defined rapid-disintegration window rather than the broadest possible quick-disintegration formulation.

The test method is central. USP disintegration testing, apparatus configuration, medium, temperature, and endpoint determination can affect the result. A product that passes under one test protocol but not another could generate a claim-construction dispute.

Does "comprises" allow additional ingredients?

Yes. The use of "comprises" generally makes the claim open-ended. A tablet may contain additional excipients, coatings, lubricants, glidants, colorants, stabilizers, or processing aids, provided the product still contains every required element.

The percentages create a separate issue. The listed components do not necessarily account for 100% of the tablet because the ranges are independent and the claim is open-ended. For example, a tablet could contain:

90-98% active combination;
1-5% granulating agent;
1-5% disintegrant; and
additional excipients.

A formulation with more than one granulating agent or more than one disintegrant could also raise aggregation questions. Courts would likely assess whether the combined materials collectively fall within the claimed functional and numerical categories.

What are the likely validity issues?

Anticipation

A prior-art tablet would need to disclose every limitation of claim 1 in a single reference, including the particle-size range, excipient concentrations, swelling capacity, hardness, and disintegration time.

The strongest anticipation argument would require a contemporaneous co-trimoxazole tablet reference that expressly or inherently disclosed the same manufacturing and performance parameters. A reference merely disclosing trimethoprim-sulfamethoxazole tablets would not necessarily anticipate the claim.

Obviousness

The obviousness analysis would focus on whether a skilled formulator would have combined:

micronized or finely divided active ingredients;
a high active load;
a conventional granulating agent;
a swelling disintegrant;
compression within the claimed hardness range; and
a rapid disintegration target.

The numerical combination may provide the principal patentability distinction. A challenger could argue that optimizing tablet hardness and disintegration is routine formulation work. The patent owner could respond that the claimed parameter combination solves a specific high-dose manufacturing or dissolution problem and produces an unexpected balance between mechanical strength and rapid disintegration.

Indefiniteness and measurement issues

Potential vulnerabilities include:

The meaning of "about" in the hardness and disintegration ranges.
The test method for swelling capacity.
The analytical definition of particle size.
Whether "combination" refers to the individual active ingredients or a preformed mixture.
Whether hardness must be measured at release, after storage, or at another specified time.

These issues could affect enforceability, but they have no practical present-day effect because the patent term has expired.

When did U.S. Patent 4,209,513 expire?

U.S. Patent 4,209,513 issued on June 24, 1980. Because it was granted before the change to a patent term measured from the earliest effective nonprovisional filing date, its ordinary term was 17 years from issuance. On that basis, the patent expired on June 24, 1997, absent an unusual terminal disclaimer or other term adjustment.

Event	Date
Patent issued	June 24, 1980
Ordinary pre-1995 term	17 years from issuance
Estimated expiration	June 24, 1997
Current enforceability	None in the United States

The patent cannot presently support an infringement action, block an ANDA, or require a Paragraph IV certification.

What is the Orange Book status of U.S. Patent 4,209,513?

U.S. Patent 4,209,513 is not a current Orange Book barrier for trimethoprim-sulfamethoxazole products.

The FDA Orange Book lists patents and exclusivity information associated with approved drug products, but an expired formulation patent does not create a current period of listed-patent protection. Current generic applicants for trimethoprim-sulfamethoxazole tablets would not face a live patent-term obligation based on this patent.

FDA approval of trimethoprim-sulfamethoxazole tablets has long supported generic products through the ANDA pathway. The relevant regulatory issue is therapeutic equivalence and product quality, not continued exclusivity under Patent 4,209,513. FDA Orange Book data should be checked by specific reference-listed drug, dosage form, and strength because listings can differ across NDAs and product presentations. (FDA, 2024a)

Were Paragraph IV challenges required?

A Paragraph IV certification would have been relevant only while the patent was listed and unexpired for the applicable reference-listed drug.

For present purposes:

Patent 4,209,513 is expired.
It cannot create a current Paragraph IV launch risk.
A new ANDA applicant would not need to challenge an unexpired right under this patent.
Any historical Paragraph IV litigation would have no remaining effect on the patent's exclusionary term.

Public patent and FDA records should not be treated as evidence that a particular generic company challenged this specific patent unless the relevant ANDA litigation docket, FDA correspondence, or court filing identifies Patent 4,209,513 by number.

What patent landscape surrounds trimethoprim-sulfamethoxazole?

The relevant landscape has four categories.

Active-ingredient patents

Patents covering trimethoprim or sulfamethoxazole as chemical compounds are historically important but are expired. They do not create a current U.S. composition-of-matter barrier.

Combination-product patents

Patents covering the combination, dosage ratio, or therapeutic use are also generally historical for this mature antibacterial product. Any surviving rights would need to be verified independently by patent number and claim scope.

Formulation patents

Patent 4,209,513 is part of the formulation layer. It focuses on tablet manufacture and performance rather than pharmacological activity. Related formulation patents could address:

granulation processes;
particle-size reduction;
dissolution control;
tablet coatings;
dosage strengths;
taste masking;
stability;
suspension formulations; or
manufacturing equipment and process conditions.

For generic entry, formulation patents are relevant only if unexpired and listed or otherwise enforceable. Patent 4,209,513 is not.

Method-of-use patents

Method-of-use claims could cover treatment of bacterial infections, Pneumocystis jirovecii pneumonia, urinary tract infections, respiratory infections, or other indications. Those claims would be distinct from the tablet claims at issue. A generic applicant could address them through a Paragraph IV certification or, where applicable, a section viii statement for an approved use not covered by the proposed label.

Are biosimilars relevant to this product?

No. Trimethoprim-sulfamethoxazole is a small-molecule combination product, not a biologic. Biosimilar provisions under the Public Health Service Act do not govern its generic competition.

Competition proceeds through the ANDA framework under the Federal Food, Drug, and Cosmetic Act. The relevant regulatory issues include pharmaceutical equivalence, bioequivalence, manufacturing controls, dissolution, impurities, stability, and labeling. (FDA, 2024b)

How strong is the patent estate for this formulation?

The historical claim set was technically specific but commercially narrow.

Factor	Assessment
Claim category	Tablet formulation
Active-ingredient protection	None
Particle-size requirement	Narrow and potentially difficult to prove
Excipient requirements	Narrow numerical ranges
Hardness limitation	Measurement-dependent
Disintegration limitation	Test-method dependent
Claim breadth	Moderate to narrow
Current enforceability	None because expired
Generic entry risk today	No patent-based barrier from this patent

During its term, the patent could have supported an infringement theory against a tablet reproducing the claimed physical and performance parameters. It would not necessarily have blocked a product using a different granulation process, excipient system, particle-size profile, hardness range, or disintegration profile.

The strongest commercial protection would have existed against close copies of the patented tablet architecture. The weakest protection would have applied to alternative formulations that delivered the same active ingredients without satisfying the claimed ranges.

What generic launch scenarios existed?

While the patent was unexpired, a generic manufacturer could have pursued several strategies:

Literal design-around: Use an active particle size outside 1-40 micrometers.
Excipient design-around: Use a granulating agent or disintegrant outside the claimed concentration range.
Functional design-around: Use a non-swelling disintegration mechanism.
Mechanical design-around: Manufacture tablets outside the claimed hardness range.
Performance design-around: Target disintegration outside the claim 1 or claim 3 range.
Paragraph IV challenge: Assert invalidity, unenforceability, or noninfringement if the patent was listed and unexpired.
Post-expiration launch: Enter without addressing the expired patent.

Today, the seventh scenario applies. Generic manufacturers can develop trimethoprim-sulfamethoxazole tablets without obtaining a license under Patent 4,209,513.

Did the patent create licensing or settlement exposure?

No current licensing obligation arises from an expired U.S. patent. A historical license or settlement could have affected a specific manufacturer during the patent term, but the patent number alone does not establish the existence, parties, royalty terms, launch date, or geographic scope of any private agreement.

Any analysis of a historical settlement would require a specifically identified litigation docket, consent judgment, SEC disclosure, or settlement filing. The patent's expiration independently removes its present U.S. exclusionary effect.

What is the geographic scope?

U.S. Patent 4,209,513 covered the United States only. It did not create rights in Canada, Europe, Japan, or other markets.

Foreign counterparts would require separate review of:

priority applications;
national-stage or Paris Convention filings;
grant status;
expiration dates;
supplementary protection certificates;
oppositions or revocation actions; and
local claim amendments.

A U.S. patent expiration date cannot be applied automatically to foreign family members. For this mature product, however, any remaining foreign risk would have to arise from a separate, still-valid national patent.

Key Takeaways

U.S. Patent 4,209,513 claims a high-drug-load trimethoprim-sulfamethoxazole tablet.
The central limitations are 1-40 micrometer active particle size, 1-5% granulating agent, 1-5% swelling disintegrant, specified hardness, and rapid disintegration.
Claims 2 and 3 narrow the hardness and disintegration windows.
The claims are formulation claims, not composition-of-matter claims.
The patent issued June 24, 1980 and ordinarily expired June 24, 1997.
It presents no current Orange Book, Paragraph IV, litigation, licensing, or generic-entry barrier in the United States.
No biosimilar pathway applies because the product is a small-molecule antibacterial combination.
Historical validity disputes would have centered on obviousness, numerical optimization, particle-size measurement, swelling-capacity testing, and the meaning of "about."
Current competitive risk comes from generic manufacturing and regulatory execution, not from this patent.

FAQs

Can a generic tablet use the same trimethoprim-sulfamethoxazole ratio?

Yes. Patent 4,209,513 does not expressly state a fixed ratio in the claims provided. The product must still satisfy the remaining claimed formulation limitations if infringement is assessed during the patent term.

Does the patent cover liquid trimethoprim-sulfamethoxazole suspension?

No. The claims are directed to a tablet. A suspension, capsule, powder, or other dosage form would not literally fall within the tablet claims.

Does tablet hardness alone determine infringement?

No. Hardness is only one required limitation. The active concentration, particle size, granulating agent, disintegrant, swelling capacity, and disintegration time must also be evaluated.

Could a disintegrant with the same commercial name fall outside the claim?

Yes. The claim includes a swelling-capacity limitation. The relevant issue is the measured property under the applicable test method, not merely the excipient's trade or compendial name.

Can this expired patent be cited against a new ANDA?

It can appear in prior-art or historical patent analysis, but it cannot support a current patent certification obligation or prevent FDA approval on patent-term grounds.

References

U.S. Patent and Trademark Office. (1980). U.S. Patent No. 4,209,513. Google Patents. https://patents.google.com/patent/US4209513
U.S. Food and Drug Administration. (2024a). Approved drug products with therapeutic equivalence evaluations. Orange Book. https://www.fda.gov/drugs/drug-approvals-and-databases/approved-drug-products-therapeutic-equivalence-evaluations-orange-book
U.S. Food and Drug Administration. (2024b). Abbreviated new drug application (ANDA). https://www.fda.gov/drugs/types-applications/abbreviated-new-drug-application-anda
U.S. Food and Drug Administration. (2024c). Drugs@FDA: FDA-approved drugs. https://www.accessdata.fda.gov/scripts/cder/daf/

Foriegn Application Priority Data
Foreign Country	Foreign Patent Number	Foreign Patent Date
United Kingdom	6758/74	Feb 14, 1974

Country	Patent Number	Estimated Expiration	Supplementary Protection Certificate	SPC Country	SPC Expiration
Austria	344915	⤷ Start Trial
Austria	A108475	⤷ Start Trial
Australia	468465	⤷ Start Trial
Australia	7817075	⤷ Start Trial
>Country	>Patent Number	>Estimated Expiration	>Supplementary Protection Certificate	>SPC Country	>SPC Expiration

Summary for Patent: 4,209,513