United States Patent 5,977,099: Mirtazapine and SSRI Combination Patent Scope, Expiration, and Litigation Risk

US Patent 5,977,099 covers the use of mirtazapine, alone or as a pharmaceutically acceptable salt, together with an SSRI for treating depression. Its broadest commercial relevance is the combination of mirtazapine and fluoxetine, although the issued claims also identify fluvoxamine, citalopram, sertraline, paroxetine, and other SSRIs.

The patent is no longer an enforceable exclusivity barrier. It issued on November 2, 1999, and its statutory term expired no later than 20 years from the applicable nonprovisional filing date, subject to any patent-term adjustment. The patent is therefore expired by 2026. It does not create current patent protection for standalone mirtazapine, fluoxetine, or other SSRI products.

What does US Patent 5,977,099 protect?

The patent protects a pharmacological combination rather than a new mirtazapine molecule, a new SSRI molecule, or a standalone dosage form.

The core inventive concept is the use of mirtazapine with an SSRI to treat depression. The claims cover four principal categories:

Claim category	Claims	Protected subject matter
Combination	1-3	Mirtazapine plus at least one SSRI
Pharmaceutical composition	4, 7-8	Combination with one or more pharmaceutically acceptable carriers
Treatment method	5, 9-12	Treating depression in an animal with the combination or composition
Package or kit	6, 13	Separate dosage units containing mirtazapine and an SSRI

Claim 1 is the broadest independent combination claim. It requires:

Mirtazapine or a pharmaceutically acceptable salt; and
At least one SSRI or a pharmaceutically acceptable salt.

The claim does not require the two agents to be in the same tablet. That distinction is important because claim 6 separately covers a package containing separated dosage units.

How broad is claim 1?

Claim 1 is broad in therapeutic and product configuration terms. It does not specify:

A particular dose of mirtazapine;
A particular dose of the SSRI;
A dosing interval;
A fixed-dose tablet;
A particular route of administration;
A specific patient population;
Treatment-resistant depression;
A particular formulation technology;
A synergistic effect;
A requirement that the two drugs be administered simultaneously.

The phrase “comprising” generally makes the claim open-ended. A product or treatment containing mirtazapine and an SSRI could fall within the literal scope even if it also contains other active ingredients, subject to claim construction and other validity or infringement defenses.

The claim also extends to pharmaceutically acceptable salts. A defendant could not ordinarily avoid claim 1 merely by using a qualifying salt of mirtazapine or a qualifying salt of the SSRI.

The principal limitation is the requirement that the second active agent be an SSRI. A combination of mirtazapine with an antidepressant outside the SSRI class would not literally satisfy claim 1, although other patent or regulatory issues could apply.

Which SSRIs are expressly identified?

Claims 2, 7, and 9 list the following compounds:

SSRI named in the patent	Claim coverage
Zimeldine	Claims 2, 7, 9
Fluoxetine	Claims 2, 3, 7, 8, 9, 10, 13
Fluvoxamine	Claims 2, 7, 9
Citalopram	Claims 2, 7, 9
Cericlamine	Claims 2, 7, 9
Femoxetine	Claims 2, 7, 9
Ifoxetine	Claims 2, 7, 9
Cyanodothiepin	Claims 2, 7, 9
Sertraline	Claims 2, 7, 9
Paroxetine	Claims 2, 7, 9
Litoxetine	Claims 2, 7, 9

Claim 1 is not limited to this list if “SSRI” is construed according to the specification and ordinary pharmaceutical meaning. Claims 2, 7, and 9 narrow the scope to the listed compounds. Claim 3 narrows the combination to mirtazapine and fluoxetine.

Some listed compounds, such as zimeldine, femoxetine, ifoxetine, cyanodothiepin, and litoxetine, have limited or no meaningful current commercial relevance in the United States. The practical market scope was therefore narrower than the literal claim language.

What does the fluoxetine coverage mean?

Fluoxetine is the most commercially significant species in the claims. Claims 3, 8, 10, and 13 specifically target mirtazapine plus fluoxetine.

The patent covers several possible commercial arrangements:

A single composition containing both active ingredients;
Separate mirtazapine and fluoxetine dosage units sold or supplied together;
A treatment regimen using both agents;
A package containing separate dosage units;
A pharmaceutical composition containing the combination and carriers.

The claims do not require a fixed-dose combination tablet. A kit containing separately packaged Remeron or generic mirtazapine and Prozac or generic fluoxetine could have been relevant during the patent term if the package was intentionally configured and supplied for the claimed combination.

What are the treatment-method claims?

Claim 5 covers treating depression in an animal with a therapeutically effective amount of the combination. In pharmaceutical patent practice, “animal” ordinarily includes humans unless the specification or prosecution history establishes a narrower meaning.

Claims 9 and 10 narrow claim 5 to the listed SSRIs and to fluoxetine, respectively. Claim 11 covers treatment using the pharmaceutical composition of claim 4.

The method claims create several potential infringement theories:

Conduct	Potential relevance during patent term
Prescribing mirtazapine and fluoxetine together for depression	Potential direct or induced infringement issue
Marketing a fixed-dose mirtazapine/fluoxetine product	Potential composition and method infringement
Selling a coordinated package of separate dosage units	Potential package and combination infringement
Labeling a product for adjunctive use with an SSRI	Potential induced-infringement issue
Manufacturing mirtazapine alone	Outside the claims
Manufacturing fluoxetine alone	Outside the claims
Prescribing mirtazapine with a non-SSRI antidepressant	Outside the literal claims

The claims do not expressly require treatment-resistant depression, inadequate response to prior therapy, or a particular sequence of administration. Those limitations should not be added to the claims without support from the specification or prosecution history.

What is the scope of the package claims?

Claim 6 covers a package containing separated dosage units. At least one dosage unit must contain mirtazapine, and at least one other dosage unit must contain an SSRI.

Claim 13 narrows claim 6 to fluoxetine as the SSRI.

This is commercially significant because the patent was drafted to reach a coordinated combination package even where the drugs are not co-formulated. A company could therefore face a different claim analysis for:

A conventional fixed-dose combination tablet;
A blister pack containing separate tablets;
A starter pack;
A physician sample package;
A co-branded kit;
A prescription bundle with a combination-directed label.

The claim is less likely to reach ordinary, unrelated sales of mirtazapine and fluoxetine made without a package, label, instruction, or other evidence tying the products together as a combination.

Is claim 12 legally significant?

Claim 12 states:

“The method of claim 11, which comprises treating said animal with a therapeutically effective amount of the pharmaceutical composition of claim 11.”

Claim 11 is itself a method claim, not a pharmaceutical composition claim. The reference to “the pharmaceutical composition of claim 11” creates an apparent dependency and drafting defect.

The issue could support an invalidity or claim-construction argument based on improper dependency, indefiniteness, or lack of clear antecedent basis. The effect would depend on the patent’s prosecution history, claim-construction record, and applicable law. Claim 12 should not be treated as a clean, independently reliable scope expansion. Claims 1, 3, 4, 5, 6, 8, 10, 11, and 13 carry greater practical importance.

When did US Patent 5,977,099 lose exclusivity?

Event	Date or status
Patent issued	November 2, 1999
Patent term framework	Generally 20 years from the applicable nonprovisional filing date
Patent status	Expired
Current enforceability	No enforceable patent exclusivity
Current Paragraph IV significance	None as a live blocking patent
Biosimilar relevance	None; the patent concerns small-molecule drugs

The precise terminal date should be taken from the USPTO patent record, including any patent-term adjustment or terminal disclaimer. Regardless of the precise adjustment calculation, the patent’s term has ended. The patent cannot presently block launch of a mirtazapine/SSRI product.

The patent’s expiration does not eliminate separate rights in later patents covering:

Orally disintegrating mirtazapine;
Extended-release formulations;
Fixed-dose combination formulations;
New salt or crystal forms;
Manufacturing processes;
Stability improvements;
Specific dosing regimens;
New depression or anxiety indications;
Combination use with agents other than those covered by this patent.

What was the Orange Book status?

US Patent 5,977,099 should not be treated as a current Orange Book barrier to generic mirtazapine or fluoxetine.

The Orange Book lists patents associated with approved drug products when the patent meets the statutory and regulatory listing requirements. A patent directed to a combination therapy does not automatically block an ANDA for either individual component.

The relevant distinctions are:

Product or regulatory target	Effect of US 5,977,099
Generic mirtazapine monotherapy	No current blocking effect
Generic fluoxetine monotherapy	No current blocking effect
Generic sertraline or paroxetine monotherapy	No current blocking effect
Fixed-dose mirtazapine/fluoxetine product	Historical combination-patent relevance, but patent expired
505(b)(2) combination product	Historical certification or patent-status relevance, but no current enforceable barrier
New indication involving mirtazapine plus an SSRI	No current barrier from this expired patent

An Orange Book listing, if one existed historically, would not extend the patent term. FDA listing and patent enforceability are separate questions. The current Orange Book and USPTO status records control the regulatory and legal assessment. [FDA, 2025; 21 C.F.R. § 314.53]

Could the patent have supported a Paragraph IV challenge?

During the patent term, an ANDA applicant seeking approval for a product whose labeling or formulation implicated the claims could have considered a Paragraph IV certification.

A Paragraph IV position could have challenged:

Validity, based on obviousness or anticipation;
Claim construction, particularly the meaning of “SSRI” and “combination”;
Written description or enablement;
The enforceability of method or package claims;
The relevance of the claims to the proposed ANDA product;
The absence of a fixed-dose combination or coordinated package;
The lack of an indication for combined treatment in the proposed labeling.

Today, a Paragraph IV challenge to this patent has no practical blocking function because the patent is expired. An applicant would generally address any still-listed patents or regulatory exclusivities associated with the proposed product, not an expired combination patent.

What prior-art and validity issues would have mattered?

The patent’s validity would likely have turned on whether the claimed combination was nonobvious over:

Mirtazapine monotherapy disclosures;
SSRI monotherapy disclosures;
General combination-antidepressant literature;
Prior use of mirtazapine with serotonergic agents;
Motivation to combine antidepressants for treatment-resistant depression;
Evidence of unexpected efficacy or tolerability;
The scope of the specification’s disclosure for the listed SSRIs.

The broad genus in claim 1 creates a potential written-description and enablement pressure point if the specification did not adequately support the full range of SSRIs or pharmaceutically acceptable salts. The narrower fluoxetine claims would generally present a more focused validity analysis.

A challenger would also distinguish between:

Predictable co-administration of two known antidepressants;
A demonstrated pharmacological interaction;
A clinically meaningful improvement over either agent alone;
A specific dosing or treatment population disclosed in the patent.

The patent’s current expiration makes these validity questions historical rather than launch-blocking.

Which companies challenged or licensed the patent?

No current, enforceable litigation or settlement risk arises from US Patent 5,977,099 because the patent has expired. The claim set alone does not establish that a particular generic company filed a Paragraph IV certification, entered a settlement, or obtained a license.

The original patent owner and any historical licensing arrangements should be confirmed from assignment records, litigation dockets, and transaction documents. Patent ownership may also have changed through corporate reorganizations involving the original Organon/Akzo Nobel business. The patent record, rather than the claim text, controls the chain of title.

No license is needed today to practice the expired claims. A license could still be relevant for separate, unexpired patents in a later formulation, manufacturing process, or branded combination product.

How strong is the patent estate?

The patent estate is strong as a historical combination patent but weak as a current commercial barrier because the patent has expired.

Factor	Assessment
Breadth of combination claim	Broad
Fluoxetine coverage	Explicit and commercially relevant
Separate dosage-unit coverage	Expressly covered
Method-of-treatment coverage	Broad but dependent on treatment and causation facts
Fixed-dose formulation protection	Not specifically required
Standalone mirtazapine protection	None
Standalone SSRI protection	None
Current enforceability	None
Biosimilar relevance	None
Manufacturing barrier	None apparent from the claims
Current generic-launch risk	No risk from this patent
Follow-on patent risk	Must be assessed separately

The claims are broader than a conventional formulation patent because they reach the therapeutic relationship between two active ingredients. They are narrower than a molecule patent because they do not cover mirtazapine or an SSRI independently.

What generic launch scenarios existed?

During the term, the principal launch scenarios were:

Generic mirtazapine monotherapy

This would not infringe the combination claims merely because patients might later receive an SSRI. Product liability would depend on labeling, promotion, and the facts surrounding any alleged induced infringement.

Generic fluoxetine monotherapy

The same analysis applies. A generic fluoxetine product would not practice the claimed combination by being sold alone.

Fixed-dose mirtazapine/fluoxetine product

This would have been the highest-risk scenario because it directly practices claims 1, 3, 4, 8, and potentially 10 or 11.

Coordinated kit

A package containing separate mirtazapine and fluoxetine units could have implicated claims 6 and 13, particularly if the package instructions directed combined use.

New combination product through 505(b)(2)

A sponsor seeking approval for a combination product or an adjunctive-treatment indication would have faced historical patent-certification and litigation issues. Those issues no longer arise from the expired patent.

How does this patent compare with formulation and method-of-use patents?

US 5,977,099 is primarily a combination-use and combination-package patent.

Patent type	Typical scope	Relationship to US 5,977,099
Compound patent	Covers the active pharmaceutical ingredient	Different; this patent does not cover mirtazapine alone
Salt or polymorph patent	Covers a particular solid form	Not addressed by the claims provided
Formulation patent	Covers excipients, release profile, or dosage form	Claim 4 is broad but does not require a specific formulation
Method-of-use patent	Covers treatment of a disease or patient population	Claims 5 and 11 are method claims
Combination patent	Covers two or more active ingredients	This is the principal category
Package or kit patent	Covers coordinated separate dosage units	Claims 6 and 13 expressly address this category
Manufacturing patent	Covers a process for producing the drug	No manufacturing process appears in the claims

A current freedom-to-operate review must therefore search beyond the patent number for later patents involving mirtazapine formulations, combination products, dosing regimens, and manufacturing processes.

Key Takeaways

US Patent 5,977,099 covers mirtazapine combined with an SSRI for depression.
The broadest claim is claim 1, which does not require a fixed-dose product or simultaneous administration.
Fluoxetine is expressly covered by claims 3, 8, 10, and 13.
Separate mirtazapine and SSRI dosage units are addressed by claims 6 and 13.
The patent does not cover mirtazapine or any SSRI as a standalone product.
Claim 12 contains an apparent dependency and antecedent-basis defect.
The patent has expired and is not a current launch barrier.
Paragraph IV, Orange Book, and litigation risks must be assessed against any later, unexpired patents.
Biosimilar analysis is inapplicable because mirtazapine and SSRIs are small-molecule drugs.
The most important current risks would arise from later formulation, dosing, combination-product, or manufacturing patents.

FAQs

Does US Patent 5,977,099 cover Remeron alone?

No. The claims require mirtazapine together with at least one SSRI. Standalone mirtazapine is outside the literal scope of the combination claims.

Does the patent cover mirtazapine combined with sertraline?

Historically, yes. Sertraline is expressly listed in claims 2, 7, and 9. The patent is now expired.

Does a prescription for mirtazapine and fluoxetine infringe the patent?

The combination could have implicated the method claims during the patent term, depending on the facts, labeling, prescribing conduct, and applicable infringement standards. The expired patent creates no current infringement liability.

Is a mirtazapine/fluoxetine fixed-dose tablet blocked by this patent?

No. US Patent 5,977,099 has expired. A fixed-dose tablet would need to be evaluated against any later unexpired formulation or combination patents.

Are biosimilars relevant to this patent?

No. Mirtazapine and fluoxetine are small-molecule drugs regulated through the ANDA or, in some cases, 505(b)(2) pathways. The relevant competitors are generic-drug applicants, not biosimilar sponsors.

References

U.S. Patent No. 5,977,099. (1999). Combination of mirtazapine and a selective serotonin reuptake inhibitor. United States Patent and Trademark Office.
U.S. Food and Drug Administration. (2025). Approved drug products with therapeutic equivalence evaluations. U.S. Department of Health and Human Services.
United States Code, 35 U.S.C. § 154. Patent term.
Code of Federal Regulations, 21 C.F.R. § 314.53. Submission of patent information.
United States Code, 21 U.S.C. § 355. New drugs and abbreviated new drug applications.

Foriegn Application Priority Data
Foreign Country	Foreign Patent Number	Foreign Patent Date
	96201703	Jun 19, 1996

Country	Patent Number	Estimated Expiration	Supplementary Protection Certificate	SPC Country	SPC Expiration
Argentina	007609	⤷ Start Trial
Austria	213159	⤷ Start Trial
Australia	2612997	⤷ Start Trial
>Country	>Patent Number	>Estimated Expiration	>Supplementary Protection Certificate	>SPC Country	>SPC Expiration

Details for Patent: 5,977,099

Summary for Patent: 5,977,099