US Patent 4,906,755: Scope, Claim Construction, Expiration, and Patent Landscape

US Patent 4,906,755 is a broad chemical compound patent covering substituted 2,6-methano-2H-quinolizin-3(4H)-ones, their endo and exo stereoisomers, selected aromatic and heteroaromatic esters, and pharmaceutically acceptable acid-addition and quaternary ammonium salts. The patent issued on March 6, 1990. Under the pre-Uruguay Round patent term applicable to the grant, the ordinary US term was 17 years from issuance, producing an expected expiration date of March 6, 2007, absent a terminal disclaimer or an applicable term adjustment. The patent is therefore expired.

The claims do not identify a currently marketed FDA-approved drug, an Orange Book-listed product, or a known surviving regulatory exclusivity right. Any present-day commercial risk from this patent is therefore limited to historical freedom-to-operate analysis, patent-family diligence, foreign rights, or technical know-how not disclosed in the claims.

What does US Patent 4,906,755 protect?

The patent protects a genus of bridged quinolizinone compounds having variable oxidation states at positions represented by A and B and an aromatic or heteroaromatic ester substituent at the 8-position.

The central claimed scaffold is a hexahydro-2,6-methano-2H-quinolizin-3(4H)-one system. The claim set covers:

Protected feature	Scope
Core ring system	2,6-methano-2H-quinolizin-3(4H)-one and related reduced, oxidized, hydroxylated, oxime, methyl, aminomethyl and exocyclic methylene forms
Position represented by A	Methylene, carbonyl, hydroxy-substituted carbon, or oxime
Position represented by B	Methylene, methyl-substituted methylene, aminomethyl, or exocyclic methylene
8-position substituent	Aromatic or heteroaromatic carbonyloxy group
Heteroatom Z	Nitrogen, oxygen, or sulfur
Aromatic substituent	Substituted phenyl and heteroaromatic systems
Stereochemistry	Endo or exo configuration of the oxygen substituent
Salt forms	Pharmaceutically acceptable acid-addition salts and quaternary ammonium salts

Claim 1 is the principal genus claim. Claims 2 through 5 narrow the core structure by selecting progressively more limited formula classes. Claims 6 through 10 identify named chemical species.

How broad is claim 1?

Claim 1 is a Markush compound claim. Its breadth comes from the combination of several independently variable structural elements.

Core scaffold

The claim covers a bridged quinolizidine-derived skeleton rather than one single active ingredient. The core contains a methano bridge between positions 2 and 6 and a carbonyl-containing quinolizinone framework.

The A substituent allows four oxidation states:

A methylene group, represented by (H)(H).
A carbonyl group, represented by =O.
A hydroxylated carbon, represented by (H)(OH).
An oxime, represented by =N-OH.

The B substituent allows:

A methylene group.
A methyl-substituted methylene.
An aminomethyl group bearing dialkylamino or cyclic amino substituents.
An exocyclic methylene group.

This creates coverage over reduced, oxidized and functionalized analogues of the same bicyclic system.

Aminomethyl variation

Where B is (H)(CH2NR3R4), R3 and R4 may be:

C2-C4 alkyl groups;
Combined into a tetramethylene ring;
Combined into a pentamethylene ring; or
Combined into a morpholine-type ring represented by -CH2CH2-O-CH2CH2-.

This provision covers acyclic tertiary amines and several cyclic tertiary amines. It materially expands the genus beyond simple alkyl analogues.

Aromatic and heteroaromatic ester variation

R1 is a carbonyloxy substituent attached to the quinolizidine core. The claim permits Z to be:

NR9;
Oxygen; or
Sulfur.

The result is coverage of benzoate-type esters, heteroaromatic carboxylates, carbamates or related nitrogen-containing carbonyl derivatives, and sulfur analogues, depending on the precise illustrated formula.

The aromatic substituents R5, R6 and R8 may independently be hydrogen, halogen, C1-C3 alkyl, or C1-C3 alkoxy. R7 is more broadly substituted and may be hydrogen, amino, alkylamino, dialkylamino, C1-C3 alkoxy, or nitro.

The claim therefore reaches a large substituent matrix, including:

Alkyl-substituted benzoates;
Halogenated aromatic esters;
Alkoxy-substituted aromatic esters;
Amino-substituted aromatic systems;
Nitro-substituted systems;
Indole and indazole derivatives; and
Other oxygen- and sulfur-containing heteroaromatic analogues.

What compounds are specifically claimed in claims 6 through 10?

Claims 6 through 10 narrow the broad Markush genus to individual compounds.

Claim	Specifically identified compound
6	Endo-8-(3,5-dimethylbenzoyloxy)hexahydro-2,6-methano-2H-quinolizin-3(4H)-one
7	Endo-hexahydro-8-(3-indolylcarbonyloxy)-2,6-methano-2H-quinolizin-3(4H)-one
8	Endo-hexahydro-8-(1-methyl-3-indazolylcarbonyloxy)-2,6-methano-2H-quinolizin-3(4H)-one
9	Endo-hexahydro-8-hydroxy-2,6-methano-2H-quinolizin-3(4H)-one
10	Exo-hexahydro-8-(3-indolylcarbonyloxy)-2,6-methano-2H-quinolizin-3(4H)-one

Claims 6, 7, 8 and 10 are ester-specific claims. Claim 9 covers the corresponding 8-hydroxy parent compound rather than an aromatic ester.

The endo and exo distinctions are legally significant. A compound with the same constitutional structure but the opposite configuration may fall outside a stereospecific claim unless it is independently covered by the broader genus or another claim.

What is the claim hierarchy?

The claims form a conventional genus-to-species structure.

Claim 1: Broad genus

Claim 1 covers the full combination of:

The bridged quinolizinone core;
The A and B variable groups;
The R1 aromatic or heteroaromatic carbonyl substituent;
The specified substituent ranges;
Endo and exo configurations; and
Certain salt forms.

Claims 2 through 5: Intermediate subgenera

Claims 2 through 5 select narrower structural formulae. The text supplied does not reproduce the chemical drawings represented by the image placeholders, so the exact ring and bond limitations in those claims cannot be reconstructed solely from the text. Their legal function is clear: they restrict the genus of claim 1 to selected subclasses of the core and substituent pattern.

Claims 6 through 10: Species claims

Claims 6 through 10 identify discrete compounds. These claims are easier to evaluate for infringement because they require a defined substitution pattern and, in most instances, a defined stereochemical configuration.

What is the chemical scope of the salt language?

The patent expressly includes pharmaceutically acceptable acid-addition salts and quaternary ammonium salts.

The acid-addition salt language can cover protonated amine forms paired with pharmaceutically acceptable acids. The quaternary ammonium language covers permanently charged derivatives formed by alkylation or other quaternization of a suitable nitrogen atom.

The salt language does not automatically cover every formulation, polymorph, solvate, hydrate or crystal form. A salt must correspond to a claimed compound. A later-developed solid form may require separate patent protection if it has distinct crystalline, hydration, particle-size or formulation properties.

Does the patent claim formulations or pharmaceutical compositions?

Based on the claims supplied, no.

The provided claims are product claims directed to chemical compounds and salts. They do not expressly claim:

Tablets;
Capsules;
Injectable formulations;
Transdermal systems;
Controlled-release formulations;
Pharmaceutical compositions containing a compound;
Dosage regimens;
Treatment methods;
Combination therapy;
Manufacturing processes; or
Specific excipients.

A patent specification may disclose pharmaceutical compositions or therapeutic uses even when the issued claims do not cover them. Those disclosures do not create enforceable claim scope unless they appear in an issued claim or a separately enforceable continuation, divisional or related patent.

Does the patent claim methods of treatment?

No method-of-use claim appears in the supplied claim set.

The claims are directed to compounds and salts. They do not recite treatment of a disease, administration to a patient, dosage, route of administration, or therapeutic endpoint.

If the compounds were developed for a neurologic, gastrointestinal, ophthalmic or other indication, the underlying therapeutic use would need to be assessed through separate patent families. US Patent 4,906,755 alone does not provide method-of-use protection.

When did US Patent 4,906,755 expire?

US Patent 4,906,755 issued on March 6, 1990. For a pre-Uruguay Round US patent, the ordinary patent term was 17 years from grant under the then-applicable version of 35 U.S.C. § 154. The resulting ordinary expiration date was March 6, 2007.

Event	Date
US patent grant	March 6, 1990
Ordinary term	17 years from grant
Expected expiration	March 6, 2007
Current status	Expired

No current US patent exclusivity remains under this patent. Any patent-term-extension analysis would require an eligible regulated product, FDA approval data and a qualifying regulatory review period. The claims supplied do not establish such an extension.

What is the Orange Book status of US Patent 4,906,755?

No Orange Book listing can be attributed to this patent from the supplied claim set.

An Orange Book listing requires an approved drug product and an NDA sponsor submission identifying an applicable patent. The patent itself does not establish:

That one of the claimed compounds received FDA approval;
That a product was marketed under a proprietary name;
That the patent was listed in the Orange Book;
That the patent covered an approved indication; or
That any listed patent remained enforceable.

Because the patent expired in 2007, it would not provide current Orange Book-based blocking protection even if it had historically been listed.

Were there Paragraph IV challenges?

No Paragraph IV challenge is established by the patent claims or patent number alone.

A Paragraph IV certification applies to an ANDA applicant challenging a listed patent for an FDA-approved reference drug. It requires a corresponding NDA, Orange Book listing and ANDA filing. The available patent information does not identify such a reference product or a related ANDA dispute.

Any Paragraph IV history would have to be tied to a specific approved active ingredient and NDA, not merely to the chemical structures in the patent.

What is the current generic-entry risk?

For US Patent 4,906,755 itself, the patent-based generic-entry risk is zero because the patent expired in 2007.

Commercial entry could still face non-patent barriers, including:

Lack of an FDA-approved reference product;
Absence of an established abbreviated pathway;
Clinical development requirements;
Difficult analytical characterization;
Stereochemical purity requirements;
Low commercial demand;
Manufacturing cost;
Controlled-substance or pharmacology concerns, if applicable; and
Separate patents covering formulations, uses, processes or later-developed products.

A generic applicant cannot ordinarily rely on an ANDA pathway without a suitable reference-listed drug. If no approved reference exists, a 505(b)(2) application or full NDA pathway may be required, depending on the proposed product and regulatory strategy.

How strong is the patent estate?

The issued patent had strong chemical breadth at the time of grant but weak present-day exclusionary value.

Factor	Assessment
Genus breadth	Broad
Specific compound coverage	Strong for listed endo and exo species
Stereochemical coverage	Explicitly addressed
Salt coverage	Expressly included
Formulation coverage	Not shown
Method-of-use coverage	Not shown
Manufacturing coverage	Not shown
Remaining US term	None
Current blocking value	None
Historical design-around difficulty	Moderate to high
Current litigation leverage	None from this patent alone

The principal vulnerability of the genus would have been prior-art and enablement scrutiny across the large Markush scope. The species claims would generally have been more defensible if the specification provided preparation methods, characterization data and biological activity for each named compound.

What design-arounds would have been available?

During the patent term, potential design-around strategies would have included:

Changing the endo configuration to the exo configuration, or vice versa, where the relevant claim was stereospecific.
Altering the aromatic substitution pattern outside the stated carbon-number and substituent limits.
Replacing the claimed heteroatom Z with an unclaimed atom or linkage.
Modifying the A or B oxidation state outside the enumerated alternatives.
Using a different bridge or ring system.
Preparing an unclaimed prodrug or derivative, subject to the doctrine of equivalents.
Developing a formulation or method patent that did not practice the compound claims, although this would not authorize manufacture of a patented active compound during the patent term.

Because claim 1 is broad, a design-around would need to change at least one required structural limitation. A minor substituent change within the enumerated Markush categories would likely remain inside the claim.

What manufacturing and intellectual-property barriers remain?

The patent does not show current manufacturing exclusivity. It may, however, have disclosed synthetic routes, intermediates, reaction conditions and purification methods that could have created practical know-how advantages.

Potential technical barriers include:

Construction of the bridged quinolizidine ring;
Control of endo and exo stereochemistry;
Selective oxidation at the quinolizinone position;
Preparation of the 8-hydroxy intermediate;
Esterification with substituted aromatic or heteroaromatic carboxylic acids;
Removal of regioisomeric impurities;
Control of salt stoichiometry;
Chiral or diastereomeric purification; and
Reproducible solid-state characterization.

These are process and manufacturing considerations, not continuing patent rights under the expired grant.

Which companies challenged or licensed the patent?

The supplied information does not establish a licensing transaction, assignment chain, Paragraph IV filing, ANDA dispute, settlement agreement or patent litigation involving US Patent 4,906,755.

The patent number alone is not sufficient to attribute a license or challenge to a specific company. No such commercial event should be treated as established without a corresponding assignment record, court docket, FDA filing or transaction disclosure.

How does this patent compare with a modern pharmaceutical patent estate?

US Patent 4,906,755 is narrower in claim type than a modern commercial pharmaceutical estate. A contemporary estate may include:

Composition-of-matter claims;
Salt, polymorph and crystalline-form claims;
Formulation claims;
Method-of-treatment claims;
Dosing claims;
Combination-treatment claims;
Manufacturing-process claims;
Intermediate claims;
Device or delivery-system claims;
Pediatric or regulatory exclusivity; and
Patent-term-extension rights.

The supplied patent principally provides composition-of-matter coverage. It does not, on the available claims, create layered exclusivity around dosage form, indication, dosing schedule or manufacturing.

Key Takeaways

US Patent 4,906,755 covers substituted bridged quinolizinone compounds and selected salts.
Claim 1 is a broad Markush genus with substantial variation at the A, B and R1 positions.
Claims 6 through 10 cover named endo and exo compounds, including indole, indazole and 3,5-dimethylbenzoyl derivatives.
The patent issued on March 6, 1990, and its ordinary US term expired on March 6, 2007.
The supplied claims do not cover formulations, therapeutic methods, dosing regimens or manufacturing processes.
No current Orange Book, Paragraph IV, litigation or licensing right is established for this patent.
Current US generic-entry risk from the patent is zero because the patent is expired.
Any remaining commercial analysis must focus on later patents, regulatory status, manufacturing know-how and the existence of an approved reference product.

FAQs

Does US Patent 4,906,755 cover the free base and salts?

Yes. The claims expressly include the claimed compounds and pharmaceutically acceptable acid-addition and quaternary ammonium salts.

Does the patent cover both endo and exo stereoisomers?

Claim 1 expressly states that the oxygen substituent may have endo or exo configuration. The named species claims separately identify endo and exo compounds.

Can a company now manufacture a claim 6 compound in the United States?

The patent itself no longer blocks manufacture because its ordinary term expired in 2007. Separate later patents, regulatory requirements or non-patent rights could still affect commercialization.

Is the 8-hydroxy compound in claim 9 protected as an intermediate?

Claim 9 is a product claim to the endo 8-hydroxy compound. It is not expressly limited to use as an intermediate, although the claim’s practical value is affected by the patent’s expiration.

Does an expired patent still matter in a freedom-to-operate review?

Yes, as historical prior art and as a source of disclosed synthetic routes, but it does not create current infringement liability. Later patents may use the disclosed compounds, intermediates or methods as prior-art references.

References

United States Patent and Trademark Office. (1990). U.S. Patent No. 4,906,755. Washington, DC: U.S. Department of Commerce.
United States Code. (2023). 35 U.S.C. § 154: Contents and term of patent; provisional rights.
United States Code. (2023). 35 U.S.C. § 271: Infringement of patent.
U.S. Food and Drug Administration. (2025). Approved drug products with therapeutic equivalence evaluations. Silver Spring, MD: U.S. Department of Health and Human Services.

Country	Patent Number	Estimated Expiration	Supplementary Protection Certificate	SPC Country	SPC Expiration
European Patent Office	0266730	⤷ Start Trial	SPC/GB97/005	United Kingdom	⤷ Start Trial
European Patent Office	0266730	⤷ Start Trial	98C0014	Belgium	⤷ Start Trial
European Patent Office	0266730	⤷ Start Trial	C970043	Netherlands	⤷ Start Trial
European Patent Office	0266730	⤷ Start Trial	73/1997	Austria	⤷ Start Trial
>Country	>Patent Number	>Estimated Expiration	>Supplementary Protection Certificate	>SPC Country	>SPC Expiration

Details for Patent: 4,906,755

Summary for Patent: 4,906,755