United States Drug Patent 4,615,876: Scope, Claims, Expiration, and Technetium-99m Patent Landscape

U.S. Patent No. 4,615,876 covers neutral, lipophilic technetium-99m macrocyclic complexes formed by reducing technetium-99m pertechnetate in aqueous solution with alkylene amine oximes. Its broadest commercial significance is the PnAO complex and related technetium-99m compounds used for diagnostic imaging. The patent issued on October 7, 1986, and its ordinary 17-year patent term expired on October 7, 2003. It no longer blocks manufacture, use, sale, or regulatory approval of products within its claim scope.

The patent is historically important because its claim 8 specifically covers the PnAO technetium-99m complex, a chemical platform related to technetium-99m exametazime, marketed in the United States as Ceretec. The patent does not create current exclusivity for Ceretec or for generic technetium-99m exametazime products.

What does U.S. Patent 4,615,876 cover?

The patent covers a class of technetium-99m complexes with five central limitations:

The complex is formed from technetium-99m pertechnetate.
The reaction occurs in aqueous solution under reducing conditions.
The ligand is an alkylene amine oxime with a two- to four-carbon alkylene group.
The resulting complex has zero charge and is lipophilic.
The complex contains an O-H-O ring-closure bond and is sufficiently stable for parenteral administration.

The claims are directed principally to the radiometal complex rather than to a finished vial, kit, imaging method, or specific disease indication. Claim 8 narrows the invention to a PnAO complex with specified coordination and stability characteristics.

Core technical features

Claim feature	Technical significance
Technetium-99m pertechnetate	The radioactive starting material, generally supplied as sodium pertechnetate
Reducing conditions	Converts technetium from the +7 oxidation state in pertechnetate to a lower oxidation state suitable for ligand coordination
Alkylene amine oxime	Bifunctional ligand system capable of forming a macrocyclic technetium complex
Two to four carbons in the alkylene group	Defines the core ligand class
Zero charge	Supports membrane penetration and lipophilicity
O-H-O ring closure	Describes a structural feature of the coordinated macrocycle
Single oxygen bonded to reduced technetium	Narrows the coordination environment
Stability in aqueous solution and in oxygen	Addresses pharmaceutical handling and parenteral administration

The claim language combines structural, process, and functional limitations. A competing compound would need to be analyzed against every limitation of the applicable claim.

How do the individual claims differ?

Claim 1 is the broad independent genus claim. It covers neutral, lipophilic technetium-99m macrocyclic complexes made from qualifying alkylene amine oximes and satisfying the specified coordination and stability requirements.

Claims 2 through 7 depend on claim 1 and narrow the ligand or substituent:

Claim	Claimed limitation	Scope
1	Alkylene amine oxime with a two- to four-carbon alkylene group	Broad genus
2	Propylene amine oxime	PnAO-type compounds
3	Lipophilic hydrocarbon substitution	Substituted ligand species
4	C6-C22 long-chain fatty acid substitution	Fatty-acid-substituted species
5	Single oxygen bonded to reduced technetium	Coordination limitation
6	Alkyl amine or polyalkylamine substitution	Amino-substituted species
7	Ethylene amine oxime	Ethylene-based species
8	Specific PnAO ligand and defined complex properties	Narrow product claim

Claim 8 is independently drafted rather than dependent on claim 1. It identifies 2,2'-(1,3-diaminopropane) bis(2-methyl-3-butanone) dioxime, commonly abbreviated PnAO. It also requires a neutral complex, an O-H-O ring closure bond, a single technetium-oxygen bond, and stability in aqueous solution when exposed to oxygen.

What is the likely scope of claim 1?

Claim 1 is broad in chemical class but constrained by its combined limitations. A covered complex would generally need to satisfy all of the following:

A technetium-99m radionuclide.
A macrocyclic or ring-closed amine oxime coordination structure.
A neutral overall charge.
Lipophilicity sufficient to distinguish the complex from highly polar or ionic technetium compounds.
The claimed O-H-O ring-closure arrangement.
Formation under the recited aqueous reducing conditions.
Stability suitable for parenteral administration.

The phrase “formed by complexing” introduces a process limitation. Under U.S. patent law, a product-by-process limitation may limit a product claim when the resulting product cannot be distinguished from the prior art except by the process, although the precise treatment depends on the claim construction and the product’s identifiable structural characteristics. The process language therefore would not automatically cover every neutral technetium complex that has similar chemistry.

The functional phrase “sufficiently stable for parenteral administration” narrows the claim, but it also creates potential claim-construction issues. Stability would likely be assessed in the context of pharmaceutical formulation, oxygen exposure, radiochemical purity, and the time period relevant to administration.

What does claim 8 cover?

Claim 8 is the commercially most significant claim because it identifies PnAO by chemical name. It requires more than simply using a PnAO ligand. The resulting technetium-99m complex must also have:

Zero charge.
An O-H-O ring closure bond.
A single oxygen bonded to reduced technetium.
Stability in aqueous solution in the presence of oxygen.

This structure is associated with the neutral, lipophilic technetium-99m complexes that can cross the blood-brain barrier. In the case of technetium-99m exametazime, the imaging mechanism involves initial brain uptake followed by intracellular conversion to more hydrophilic species that are retained in brain tissue. The commercial product is supplied as a kit for preparation of the technetium-99m complex immediately before administration. The FDA label identifies exametazime as the nonradioactive ligand component used to prepare technetium Tc-99m exametazime injection. [2]

Claim 8 would not necessarily cover every formulation containing exametazime. A formulation claim would require separate limitations concerning excipients, vial composition, stabilizers, pH, kit configuration, or reconstitution conditions. Claim 8 is directed to the radiochemical complex itself.

When did U.S. Patent 4,615,876 lose exclusivity?

The patent issued on October 7, 1986. For a pre-1995 U.S. application, the ordinary term was 17 years from issue, subject to any patent-term adjustment, terminal disclaimer, or other statutory modification. On that basis, the patent expired on October 7, 2003. [1]

Event	Date or status
U.S. patent issue	October 7, 1986
Ordinary patent term	17 years from issue
Ordinary expiration	October 7, 2003
Current status	Expired
Current patent barrier	None from U.S. Patent 4,615,876

The patent’s expiration is separate from FDA exclusivity. FDA approval, orphan-drug exclusivity, pediatric exclusivity, and patent protection operate under different statutory provisions. The expiration of this patent did not itself determine the availability of an abbreviated new drug application or a competing radiopharmaceutical kit.

What is the FDA and Orange Book status of the technology?

The FDA has approved technetium Tc-99m exametazime for diagnostic cerebral blood-flow imaging under the Ceretec label. [2] The product is a radiopharmaceutical kit requiring reconstitution with sodium pertechnetate Tc-99m before administration.

The original patent is not a current Orange Book barrier. Any Orange Book listing associated with a reference product would have to be evaluated separately from Patent No. 4,615,876. A historical patent that has expired cannot support a current Paragraph IV litigation strategy.

Regulatory issue	Assessment
Active ingredient	Technetium Tc-99m exametazime
Ligand	Exametazime, a PnAO-related amine oxime
Dosage form	Kit for preparation of an injectable radiopharmaceutical
FDA pathway for a generic equivalent	Generally an ANDA or another applicable abbreviated pathway, subject to FDA product-specific requirements
Current effect of Patent 4,615,876	No blocking patent right
Biosimilar pathway	Not applicable; this is a chemically defined small-molecule radiopharmaceutical, not a biologic

The principal regulatory barriers are likely to involve pharmaceutical equivalence, radiochemical purity, sterility, kit performance, labeling, manufacturing controls, and control of radioactive materials rather than the expired patent.

Are there Paragraph IV challenges or patent litigation risks?

There is no current Paragraph IV risk arising from Patent No. 4,615,876 because the patent expired in 2003. A Paragraph IV certification against an expired patent would not create meaningful patent litigation exposure under the Hatch-Waxman framework.

Potential litigation could still arise from later patents covering:

A specific formulation.
A stabilizer system.
A kit configuration.
A manufacturing or reconstitution process.
A purification method.
A specific imaging indication.
A new dosage regimen.
A different technetium coordination complex.
A device or automated compounding system.

Those rights would need to be identified separately from the original compound patent. The existence of Ceretec approval does not establish that all later patents are invalid or expired.

No active settlement agreement is necessary to explain the current status of Patent No. 4,615,876. The patent’s ordinary expiration independently removes the original patent barrier.

What formulation patents protect technetium-99m exametazime?

Patent No. 4,615,876 does not claim a complete pharmaceutical formulation in the conventional sense. It does not specify a particular lyophilized vial composition, buffer system, excipient concentration, stabilizer package, container closure, or reconstitution device.

A later formulation patent could potentially protect a commercial kit even after the compound patent expires. The relevant claim categories would include:

Lyophilized kit claims

These may cover a vial containing exametazime or a related ligand with reducing agents, buffers, stabilizers, bulking agents, and other excipients. The claim may require specific concentration ranges or a particular order of addition.

Reconstitution claims

A patent may cover the method of adding sodium pertechnetate Tc-99m to a lyophilized vial, controlling reaction time and temperature, or limiting oxygen exposure.

Stability claims

A formulation patent may claim a product that maintains radiochemical purity for a defined period after reconstitution. Such claims are distinct from claim 8’s requirement that the complex remain stable in aqueous solution when exposed to oxygen.

Manufacturing claims

Later patents may cover synthesis and purification of the nonradioactive ligand, control of stereoisomers, precursor preparation, or kit filling and sterilization.

The existence of these possible categories does not establish that a particular later patent remains enforceable. The original patent should not be treated as a substitute for a current family-by-family freedom-to-operate search.

How does this patent compare with competing technetium-99m estates?

Patent No. 4,615,876 belongs to the class of early technetium radiopharmaceutical patents that claimed coordination chemistry and diagnostic use. Other technetium-99m products were protected through different ligand structures and imaging applications.

Product or platform	Representative chemistry	Relationship to Patent 4,615,876
Technetium-99m exametazime	PnAO-related neutral amine oxime	Closest commercial relationship
Technetium-99m bicisate	ECD-type lipophilic complex	Different ligand platform
Technetium-99m sestamibi	Cationic technetium complex	Different charge and coordination chemistry
Technetium-99m tetrofosmin	Phosphine-containing complex	Different ligand and imaging platform
Technetium-99m DTPA	Ionic chelate	Outside the claimed neutral lipophilic macrocycle class

The patent’s strongest historical position was its combination of neutral charge, lipophilicity, macrocyclic amine oxime chemistry, and PnAO specificity. Its current commercial strength is zero because the patent has expired.

How strong was the patent estate?

The original patent had meaningful historical breadth but limited present-day value.

Historical strengths

Claim 1 attempted to cover a genus of neutral, lipophilic technetium-99m amine oxime complexes.
Claim 8 identified PnAO directly.
The claims tied the chemistry to diagnostic radionuclide use and parenteral stability.
The claimed characteristics corresponded to properties important for brain imaging.

Structural weaknesses

The genus covered multiple ligand substitutions and may have raised written-description and enablement issues depending on the evidence available in the specification.
“Lipophilic” and “sufficiently stable” are context-dependent terms.
The process language may create disputes over whether structurally similar products made by different methods fall within the claims.
The O-H-O ring-closure limitation requires careful analysis of the actual coordination structure and stereochemistry.
A prior-art reference disclosing neutral technetium amine oxime complexes could have supported anticipation or obviousness arguments.

Current strength

The patent has no remaining exclusionary strength in the United States. Its value is historical, technical, and evidentiary. It may remain relevant for:

Prior-art analysis.
Inventorship and ownership history.
Interpretation of later patent claims.
Product-development history for exametazime.
Assessment of whether later patents claim genuinely new subject matter.

What generic launch risks remain after patent expiration?

A generic or competing manufacturer would face a lower patent risk from the original compound claim but could still face practical launch barriers.

Risk category	Current assessment
Expired compound patent	No blocking effect
Active formulation patents	Must be checked separately
Method-of-use patents	Possible but likely narrower than the original compound claims
Orange Book patents	Must be reviewed for the current reference product
FDA approval	Requires product-specific regulatory compliance
Radioactive-material licensing	Required under applicable federal and state regimes
Sterile manufacturing	Significant operational requirement
Kit stability	Important chemistry and quality-control issue
Supply of molybdenum-99/technetium-99m	Commercial and supply-chain constraint
Biosimilar competition	Not applicable

The principal commercial advantage of an entrant would be freedom from the expired composition-of-matter patent. The principal disadvantages would be limited market size, short radioactive half-life, specialized distribution, hospital nuclear-medicine infrastructure, and the need to demonstrate reliable kit performance.

What is the geographic coverage of the patent landscape?

U.S. Patent 4,615,876 has no current U.S. term. Foreign counterparts may have had different filing dates, prosecution histories, patent terms, or lapse events. A global analysis should separate:

United States patent expiration.
European national or regional patents.
Canadian, Japanese, and Australian equivalents.
Patent applications that never granted.
Continuations and divisionals.
Later patents directed to formulations or manufacturing.

The expiration of the U.S. patent does not establish the status of foreign family members. Geographic freedom to operate therefore depends on national records and the exact product configuration.

Key Takeaways

U.S. Patent 4,615,876 covers neutral, lipophilic technetium-99m macrocyclic amine oxime complexes.
Claim 8 specifically targets the PnAO technetium-99m complex.
The claimed chemistry is closely related to technetium-99m exametazime used in Ceretec.
The patent issued on October 7, 1986, and its ordinary term expired on October 7, 2003.
The patent does not provide current U.S. exclusivity.
It is not a current basis for Paragraph IV litigation or a patent block to generic entry.
Later formulation, kit, manufacturing, method-of-use, or device patents could create separate risks.
Biosimilar analysis is inapplicable because exametazime is a chemically defined small-molecule radiopharmaceutical.
Current commercial barriers are more likely to arise from FDA requirements, sterile radiopharmaceutical manufacturing, radioactive-material licensing, supply, and market scale.

FAQs

Does Patent 4,615,876 cover Ceretec?

It covers the underlying class of technetium-99m amine oxime complexes and specifically recites PnAO in claim 8. Ceretec contains technetium Tc-99m exametazime, a PnAO-related complex. The patent expired and no longer blocks Ceretec or a competing product.

Can a company make technetium-99m exametazime after 2003?

The expired patent does not prevent manufacture. A company must still satisfy FDA requirements, radioactive-material controls, sterile manufacturing standards, and any later unexpired patent claims.

Is PnAO the same as exametazime?

PnAO is the chemical ligand platform identified in the patent. Exametazime is the pharmaceutical active ligand associated with technetium Tc-99m exametazime and is commonly described as a PnAO-related compound. Exact identity should be assessed from the applicable chemical structure and regulatory specification.

Does the patent cover all technetium-99m brain-imaging agents?

No. Its claims are limited to specified neutral, lipophilic macrocyclic amine oxime complexes with defined coordination and stability characteristics. Other technetium-99m brain-imaging agents using different ligands or charge states fall outside the claims unless they independently satisfy every limitation.

Does patent expiration eliminate the need for an FDA filing?

No. Patent expiration removes one intellectual-property barrier. It does not eliminate the need for an appropriate FDA submission, validated manufacturing process, radiochemical testing, sterility controls, labeling compliance, or radioactive-material licensing.

References

U.S. Patent and Trademark Office. (1986). U.S. Patent No. 4,615,876, Technetium-99m complexes.
U.S. Food and Drug Administration. (n.d.). Ceretec (technetium Tc 99m exametazime) kit: Prescribing information.
U.S. Food and Drug Administration. (n.d.). Orange Book: Approved drug products with therapeutic equivalence evaluations.
U.S. Code, 35 U.S.C. §§ 154, 156, and 271.
U.S. Food and Drug Administration. (n.d.). Drugs@FDA: FDA-approved drugs database.

Country	Patent Number	Estimated Expiration	Supplementary Protection Certificate	SPC Country	SPC Expiration
Australia	2711884	⤷ Start Trial
Australia	564636	⤷ Start Trial
Canada	1231715	⤷ Start Trial
Czechoslovakia	271306	⤷ Start Trial
>Country	>Patent Number	>Estimated Expiration	>Supplementary Protection Certificate	>SPC Country	>SPC Expiration

Details for Patent: 4,615,876

Summary for Patent: 4,615,876