US Patent 4,208,398: Scope, Claims, Expiration, and Technetium-99m Patent Landscape

US Patent 4,208,398 covers technetium-99m complexes with selected sulfur-containing ligands, including dimercaptosuccinic acid, mercaptoacetic acid, mercaptosuccinic acid, mercaptopropionic acid, thioglucose, thiolactic acid, and 3-mercaptopropylsulfonic acid. The claims also cover selected injectable compositions and complexes prepared using stannous chloride or another pharmaceutically acceptable reducing agent.

The patent issued in 1980 and, under the pre-Uruguay Round patent term applicable to the patent, expired no later than 17 years after issuance. Its enforceable term therefore ended in 1997, absent an unusual term adjustment or extension. The patent does not present a current US blocking right against technetium-99m succimer, technetium-99m mercaptoacetyltriglycine, or other modern radiopharmaceutical products.

What does US Patent 4,208,398 cover?

The patent covers three related subject-matter categories:

A technetium-99m complex with specified sulfur-containing ligands.
A pharmaceutical composition containing the complex or complexing agent in an isotonic parenteral diluent.
A technetium-99m complex prepared with a reducing agent, particularly stannous chloride.

The central technical concept is the formation of a reduced technetium-99m coordination complex. Pertechnetate, generally supplied as sodium pertechnetate, contains technetium in the +7 oxidation state. A reducing agent such as stannous chloride converts technetium to a lower oxidation state so that it can bind to sulfur-containing ligands.

The patent's most commercially important claim is claim 5, which narrows claim 1 to dimercaptosuccinic acid, commonly called DMSA or succimer. Technetium-99m DMSA became an established renal cortical imaging radiopharmaceutical.

Patent identification and term

Item	Information
Patent	US 4,208,398
Subject	Technetium-99m complexes
Issue date	1980
Principal technology	Reduced technetium-99m complexes with sulfur-containing ligands
Important ligand	Dimercaptosuccinic acid, or DMSA
Patent term	Pre-1995 US patent term, generally 17 years from issue
Expected expiration	1997
Current status	Expired by lapse of statutory term
Current Orange Book blocking effect	None based on this expired patent

The patent should be distinguished from later patents covering kit composition, stabilizers, synthesis methods, radiolabeling protocols, imaging indications, and alternative technetium ligands. Those later rights may have affected commercial products during their terms, but they do not extend the enforceable life of US 4,208,398.

How many claims are in US Patent 4,208,398?

The patent has 13 claims. Claims 1, 9, 11, and 12 are independent claims. The other claims depend from those broader claims.

Claim	Claim type	Principal limitation
1	Independent composition	Tc-99m complex with one of seven listed ligands
2	Dependent	1 to 100 mCi of Tc-99m per mg of complexing agent
3	Independent or improperly dependent composition	Isotonic solution of complexing agent in parenteral diluent
4	Dependent	Sterile aqueous isotonic saline
5	Dependent	DMSA as complexing agent
6	Dependent	Thioglucose
7	Dependent	Mercaptoacetic acid
8	Dependent	3-mercaptopropanoic acid
9	Independent composition	Four ligands, prepared with stannous chloride
10	Dependent	DMSA under claim 9
11	Independent composition	Four ligands, prepared with a pharmaceutically acceptable reducing agent
12	Independent composition	Reduced Tc-99m with three ligands, reduced by stannous chloride
13	Dependent	DMSA under claim 12

What compounds are protected by claim 1?

Claim 1 lists the following ligand classes:

Dimercaptosuccinic acid
Mercaptosuccinic acid
Mercaptoacetic acid
Mercaptopropionic acid
3-Mercaptopropylsulfonic acid
Thioglucose
Thiolactic acid

The claim is directed to the complex itself, not merely to the uncomplexed ligand. A product would need to contain technetium-99m coordinated with one of the recited compounds to fall within the ordinary technical scope of the claim.

The claim does not expressly require:

A particular oxidation state of technetium
Stannous chloride
A particular pH
A particular buffer
A particular vial or kit format
A particular imaging indication
A defined chemical structure for the resulting technetium coordination complex
A specific route of administration

Those omissions make claim 1 broad in subject matter. They also create potential validity and construction issues because technetium chemistry is isotope-, oxidation-state-, ligand-, pH-, and preparation-dependent.

DMSA and technetium-99m succimer

DMSA is the most commercially significant ligand in the patent. A technetium-99m DMSA product is designed for renal cortical imaging, including evaluation of renal cortical defects and relative renal function.

Claim 5 specifically identifies DMSA. Claims 10 and 13 also separately identify DMSA in compositions prepared with stannous chloride or another reducing agent. These dependent claims provide narrower DMSA-specific coverage but do not extend the patent term beyond the expiration of the patent.

FDA-approved technetium Tc-99m succimer products are generally supplied as reagent kits. The kit is combined with sodium pertechnetate Tc-99m to produce the injectable radiopharmaceutical. The FDA-approved labeling for technetium Tc-99m succimer identifies renal cortical imaging as the principal use and describes the use of a reducing component in the kit chemistry (U.S. Food and Drug Administration, n.d.-a).

What is the scope of claim 2?

Claim 2 requires a radioactive concentration of approximately 1 to 100 millicuries of Tc-99m for each milligram of complexing agent.

This is a quantitative limitation. A product outside that stated activity-to-mass range would have a defense against literal infringement of claim 2, even if it potentially fell within claim 1. The limitation is measured against the amount of complexing agent, not necessarily the total vial mass or total formulation volume.

The limitation is also operationally difficult because Tc-99m has a short physical half-life of approximately six hours. The activity-to-mass ratio changes with time as radioactivity decays while the ligand mass remains substantially constant. A product can therefore fall within or outside the numerical range depending on when activity is measured.

Claim 2 does not independently cover a product that lacks the claim 1 complex. It is dependent on the claim 1 subject matter.

What formulations are protected by claims 3 and 4?

Claims 3 and 4 concern pharmaceutical compositions.

Claim 3 refers to an isotonic solution of the complexing agent in a pharmaceutically acceptable parenteral diluent. Claim 4 narrows the diluent to sterile aqueous isotonic saline.

The drafting creates an important distinction between:

A preformed radioactive Tc-99m complex;
A nonradioactive ligand formulation intended to be labeled later; and
A complete injectable product containing the final Tc-99m complex.

Claim 3 uses the phrase "isotonic solution of complexing agent according to claim 1." Because claim 1 is directed to a Tc-99m complex, the language can be read as requiring a solution containing the claimed complex. It can also be read as referring to the uncomplexed ligand. The specification and prosecution history would control the precise construction.

In commercial kit practice, a vial commonly contains the ligand, reducing agent, buffers, and excipients in a nonradioactive form. Sodium pertechnetate Tc-99m is added later. A kit containing only the nonradioactive ingredients may not literally satisfy a claim requiring a Tc-99m complex, depending on claim construction and the patent's written description.

How do claims 9 through 13 differ from claim 1?

Claims 9 through 13 narrow the ligand list and add preparation limitations.

Claim 9

Claim 9 covers a Tc-99m complex with:

Mercaptoacetic acid
Dimercaptosuccinic acid
Mercaptosuccinic acid
Mercaptopropionic acid

The complex must be prepared in the presence of stannous chloride.

This claim excludes the claim 1 ligands thioglucose, thiolactic acid, and 3-mercaptopropylsulfonic acid.

Claim 11

Claim 11 covers the same general four-ligand group but substitutes a broader reducing-agent limitation. The complex must be prepared in the presence of a pharmaceutically acceptable reducing agent for pertechnetate.

Stannous chloride would fall within this language, but the claim is not limited to stannous chloride.

Claim 12

Claim 12 covers reduced Tc-99m with:

Mercaptoacetic acid
Dimercaptosuccinic acid
Mercaptopropionic acid

The technetium must be reduced by stannous chloride. Mercaptosuccinic acid is omitted from this claim.

Claim 13

Claim 13 narrows claim 12 to DMSA.

The claim structure indicates that the patent applicant regarded DMSA and the stannous-chloride labeling procedure as particularly valuable. The claims also reflect the state of radiopharmaceutical development at the time: broad ligand coverage in claim 1, followed by narrower compositions and preparation conditions.

Does claim 8 cover 3-mercaptopropanoic acid?

Claim 1 recites 3-mercaptopropylsulfonic acid, while claim 8 recites 3-mercaptopropanoic acid. These are different chemical names.

That inconsistency matters. Claim 8 purports to depend on claim 1 but identifies a compound not expressly listed in claim 1. Several interpretations are possible:

Claim 8 contains a drafting error.
The term in claim 1 was intended to refer to the compound in claim 8.
The compounds were intended to be separate species, with claim 8 relying on an implicit broader disclosure.
The claim is indefinite or lacks proper antecedent basis.

The patent specification and prosecution history would be required to resolve the discrepancy. On the face of the claims, claim 8 has a formal dependency problem and is materially narrower than claim 1.

When did US Patent 4,208,398 lose exclusivity?

US 4,208,398 lost patent exclusivity in 1997 under the pre-1995 US patent-term regime.

The Patent Term Adjustment and Patent Term Extension provisions created by later statutes generally do not convert an expired pre-1995 patent into a currently enforceable patent. Patent term extension under 35 U.S.C. § 156 is also product-specific and subject to statutory requirements. No current FDA exclusivity right can revive the expired patent.

The practical result is:

Exclusivity category	Status
Patent exclusivity under US 4,208,398	Expired
Current enforceable claim right	None
FDA new-drug exclusivity	Separate from this patent
Orphan-drug exclusivity	Not created by this patent
Orange Book patent listing effect	No current blocking effect
Right to practice the claimed chemistry	Not restricted by this expired patent

Patent expiration does not eliminate regulatory obligations. A manufacturer still needs an appropriate FDA pathway, an approved application where required, compliance with radioactive-drug regulations, and compliance with manufacturing and quality standards.

What is the Orange Book status of this patent?

US 4,208,398 should not be treated as a current Orange Book barrier.

The FDA Orange Book lists patents submitted for approved drug products where the patent claims the drug substance, drug product, or approved method of use. A patent that expired decades ago does not create a current Paragraph IV litigation barrier. Orange Book records can still provide historical information, but an old listing does not restore enforceability (U.S. Food and Drug Administration, n.d.-b).

Technetium-99m reagent kits also raise product-classification issues. The relevant commercial product may be a kit, a finished radiopharmaceutical, or both. The FDA labeling and application history are more important for current regulatory analysis than the expired patent.

Were there Paragraph IV challenges to US 4,208,398?

A Paragraph IV certification is relevant only while an Orange Book-listed patent creates a live patent issue for an abbreviated new drug application.

Because US 4,208,398 expired in 1997, it cannot support a current Paragraph IV litigation strategy. Any historical challenge would have become moot as to this patent after expiration unless damages, claim construction, or other retrospective issues remained.

No current generic launch needs to wait for this patent. A manufacturer evaluating a technetium-99m succimer product would instead examine:

FDA approval requirements for the specific product;
Any unexpired formulation or kit patents;
Manufacturing-process patents;
Trade-secret controls over kit manufacture;
Drug-master-file and supplier dependencies;
Radioactive-material licensing;
Product-specific exclusivity;
Current patent listings for the approved reference product.

What is the biosimilar risk for this patent?

Biosimilar law is not relevant to US 4,208,398.

Technetium-99m complexes are small-molecule radiopharmaceuticals, not biological products regulated through the biosimilar pathway under section 351(k) of the Public Health Service Act. A competing product would generally be analyzed under the small-molecule drug framework, an NDA pathway, an abbreviated pathway where available, or another FDA pathway applicable to the product.

The relevant competitive issue is generic or alternative radiopharmaceutical entry, not biosimilar entry.

How strong is the patent estate for technetium-99m DMSA?

The historical patent estate represented by US 4,208,398 was broad at the ligand level but limited in several practical respects.

Strengths at the time of filing

Claim 1 covered multiple sulfur-containing ligands.
Claims 5, 10, and 13 specifically targeted DMSA.
Claims 9, 11, and 12 addressed the use of reducing agents.
The claims covered the radiolabeled complex rather than only a research method.
Claims 3 and 4 reached injectable solution formats.

Weaknesses and limitations

The patent did not cover every technetium radiopharmaceutical.
The claims were limited to listed ligands.
The activity range in claim 2 was narrow and time-sensitive.
Claim 8 contains a chemical-name inconsistency.
The compositions and kit language could create construction issues.
The patent expired in 1997.
Later products could use different ligands, chelators, stabilizers, or manufacturing methods.

The current legal strength is zero because the patent term has ended. Its remaining value is historical, freedom-to-operate context, and evidence of early radiopharmaceutical development.

What later patent categories affect technetium-99m radiopharmaceuticals?

Current freedom-to-operate analysis should focus on later patent families rather than US 4,208,398.

Formulation and kit patents

Later patents may cover:

Lyophilized reagent kits;
Specific stannous-ion concentrations;
Buffers and pH ranges;
Antioxidants and stabilizers;
Bulking agents;
Nitrogen or inert-gas packaging;
Vial closure systems;
Reconstitution procedures;
Radiochemical purity specifications.

Method-of-use patents

Radiopharmaceutical patents can claim specific clinical uses, such as:

Renal cortical imaging;
Differential renal function;
Detection of renal scarring;
Tumor imaging;
Infection imaging;
Skeletal imaging;
Cardiac perfusion;
Receptor-targeted imaging.

A method-of-use patent does not necessarily block the composition itself after expiration of other rights. Its effect depends on the approved indication, claim language, and the seller's labeling and inducement exposure.

Alternative chelator patents

Later technetium technologies include compounds such as:

Mercaptoacetyltriglycine, or MAG3;
HYNIC-based ligands;
Tricarbonyl technetium complexes;
Nitrido and oxo technetium complexes;
Bis(thiosemicarbazone) systems;
Peptide and antibody-linked technetium chelators.

These technologies generally fall outside the literal ligand list in US 4,208,398.

Manufacturing and quality-control barriers

Commercial entry can remain difficult even when patent rights have expired. Relevant barriers include:

Access to validated technetium generators;
Reliable supply of sodium pertechnetate;
Sterile aseptic filling;
Short shelf life;
Radiochemical purity testing;
Endotoxin and sterility controls;
Radioactive-material licensing;
Hospital pharmacy distribution;
Batch release near the point of use.

These are regulatory and operational barriers, not surviving rights under US 4,208,398.

How does US 4,208,398 compare with modern technetium-99m products?

Issue	US 4,208,398	Modern Tc-99m product landscape
Core chemistry	Sulfur-containing ligands	Broad range of chelators and ligand systems
Principal commercial example	Tc-99m DMSA	DMSA, MAG3, sestamibi, tetrofosmin, and others
Reducing agent	Stannous chloride or broader reducing agent	Product-specific kit chemistry
Dosage form	Complex and injectable solution	Mostly kit-based radiopharmaceutical preparation
Patent status	Expired	Product-specific rights vary
FDA pathway	Historical NDA and radiopharmaceutical regulation	Current NDA, ANDA, or applicable pathway
Biosimilar relevance	None	None for small-molecule Tc-99m products
Main current risk	No patent blocking risk from this patent	Later patents, regulatory approval, manufacturing, and supply

What generic launch risks exist for technetium-99m DMSA?

US 4,208,398 does not prevent launch. The principal risks are regulatory and commercial:

The proposed product must meet FDA requirements for identity, strength, quality, purity, sterility, and radiochemical performance.
The sponsor must establish comparability to the relevant approved product or otherwise support safety and effectiveness.
The product requires a dependable source of technetium-99m or sodium pertechnetate.
The short half-life limits national distribution and favors regional manufacturing.
Hospital nuclear pharmacies may rely on established suppliers and validated workflows.
Any later unexpired patent covering a formulation, kit, indication, or manufacturing process must be reviewed separately.
A product label that promotes a patented indication can create method-of-use exposure even if the active complex is unpatented.

For DMSA specifically, the expired patent removes the historical composition barrier. It does not eliminate the need to develop a compliant commercial kit or finished radiopharmaceutical.

Are there licensing deals associated with US 4,208,398?

The patent claims do not establish a current licensing obligation. After expiration, a license under the patent is generally unnecessary for practicing the claimed chemistry.

Public FDA product labeling typically identifies the manufacturer and product sponsor, not the complete chain of historical patent licenses. A commercial relationship involving a manufacturer, radiopharmacy, generator supplier, or kit distributor should not be inferred solely from the patent.

The practical licensing issues today are more likely to involve:

Generator supply;
Contract manufacturing;
Hospital distribution;
Drug-master-file access;
Trademarks and branding;
Know-how for sterile kit manufacture;
Current patents unrelated to US 4,208,398.

Key Takeaways

US Patent 4,208,398 claims Tc-99m complexes with specified sulfur-containing ligands.
DMSA is the most important claimed ligand and appears in claims 5, 10, and 13.
Claims 3 and 4 address isotonic parenteral formulations, including sterile saline.
Claims 9 through 13 add stannous chloride or another reducing-agent limitation.
Claim 8 contains a nomenclature and dependency inconsistency involving 3-mercaptopropylsulfonic acid and 3-mercaptopropanoic acid.
The patent expired in 1997 under the pre-1995 US patent-term regime.
It creates no current Orange Book, Paragraph IV, biosimilar, or patent-blocking risk.
Current diligence should focus on later kit, formulation, manufacturing, method-of-use, and regulatory requirements.
Technetium-99m DMSA is a small-molecule radiopharmaceutical, so biosimilar analysis is not applicable.
The main modern entry barriers are FDA approval, radioactive-drug manufacturing, sterile quality systems, technetium supply, and short product shelf life.

FAQs

Is technetium-99m DMSA still protected by US Patent 4,208,398?

No. The patent expired in 1997 and does not currently block manufacture, development, or sale of a technetium-99m DMSA product.

Does the patent cover the DMSA kit before technetium is added?

The answer depends on claim construction. Claims directed to the Tc-99m complex may not literally cover a vial containing only nonradioactive DMSA and reducing components. Claims 3 and 4 create additional formulation issues because they refer to an isotonic solution of the complexing agent.

Can a company launch a generic Tc-99m DMSA product without a Paragraph IV certification?

This expired patent cannot support a current Paragraph IV challenge. A sponsor must still address any other unexpired patents listed for the relevant reference product and satisfy the applicable FDA requirements.

Is technetium-99m succimer a biologic requiring a biosimilar application?

No. Technetium-99m succimer is a small-molecule radiopharmaceutical. The biosimilar pathway is not the applicable framework.

Does the expired patent provide prior-art protection for later technetium chelators?

No. The patent may be relevant prior art, but it does not confer exclusivity over later chelators such as MAG3, HYNIC compounds, or tricarbonyl technetium systems. Later patentability depends on novelty, nonobviousness, written description, enablement, and claim scope.

References

U.S. Patent and Trademark Office. (1980). U.S. Patent No. 4,208,398, Technetium-99m complexes.
U.S. Food and Drug Administration. (n.d.-a). Technetium Tc 99m succimer injection: Prescribing information.
U.S. Food and Drug Administration. (n.d.-b). Approved drug products with therapeutic equivalence evaluations.
35 U.S.C. § 154. Patent term.
35 U.S.C. § 156. Extension of patent term for regulatory review.
21 U.S.C. § 355. New drug applications and abbreviated applications.

Country	Patent Number	Estimated Expiration	Supplementary Protection Certificate	SPC Country	SPC Expiration
Australia	1320976	⤷ Start Trial
Australia	475839	⤷ Start Trial
Australia	520168	⤷ Start Trial
Australia	6815174	⤷ Start Trial
>Country	>Patent Number	>Estimated Expiration	>Supplementary Protection Certificate	>SPC Country	>SPC Expiration

Summary for Patent: 4,208,398