United States Drug Patent 4,755,375: Scope, Claims, Expiration, and Patent Landscape for Technetium-99m Red-Blood-Cell Labeling

U.S. Patent No. 4,755,375 covers a multi-step method for labeling red blood cells with technetium-99m. The claimed process uses intracellular stannous ions to reduce technetium, followed by chemical removal or sequestration of extracellular tin. The patent issued on July 5, 1988, and its ordinary 17-year patent term expired on July 5, 2005.[1] The claims therefore have no current exclusionary force in the United States, although they remain relevant to historical product development, freedom-to-operate analysis, and interpretation of later radiopharmaceutical patents.

The patent is a process patent, not a broad composition-of-matter patent. Its commercial relevance was directed principally to in-vitro preparation of technetium-99m-labeled red blood cells for nuclear medicine imaging, including blood-pool imaging and applications involving damaged or heat-treated erythrocytes.

What does U.S. Patent 4,755,375 protect?

The patent protects a defined sequence for labeling red blood cells:

Introducing whole blood into a sterile container containing a lyophilized stannous-ion complex.
Allowing some stannous ion to cross the red-cell membrane.
Oxidizing substantially all extracellular stannous tin to stannic tin.
Chelating extracellular and residual plasma tin.
Adding technetium-99m pertechnetate in a membrane-permeable form.
Using intracellular stannous ion to reduce technetium to a lower-valence state that remains trapped within the cells.

The central technical concept is compartmentalization. Tin inside the erythrocyte reduces technetium after the isotope enters the cell. Tin remaining in plasma is oxidized and chelated to limit nonspecific technetium binding outside the cells.

Core claim architecture

Claim	Subject matter	Scope
1	Basic labeling method	Broadest independent process claim
2	Anticoagulant treatment before blood enters container	Dependent claim
3	Heparin as anticoagulant	Narrower dependent claim
4	Anticoagulant already present in container	Alternative anticoagulation route
5	Enumerated tin-complexing agents	Narrower chemical limitation
6	Sodium citrate	Species claim
7	Sodium glucoheptonate	Species claim
8	Reducing sugar stabilizer	Narrower stabilizer limitation
9	Dextrose	Species claim
10	Alkali-metal hypochlorite oxidant	Narrower oxidant limitation
11	EDTA, calcium disodium EDTA, or acid citrate dextrose	Chelator limitation
12	Alkali-metal pertechnetate	Technetium-source limitation
13	Combination of specified complexant, dextrose, hypochlorite, chelator, and pertechnetate	Combination claim
14	Sodium hypochlorite and sodium pertechnetate	Narrower combination claim
15	Elevated-temperature incubation that damages labeled cells	Functional-use claim
16	Claim 13 combination plus heat damage	Narrowest combination claim

Claims 1 and 13 are the principal commercial claims. Claim 1 captures the process at a functional level. Claim 13 consolidates many of the preferred ingredients into a more specific formulation and procedure.

How should claim 1 be interpreted?

Claim 1 is an open-ended process claim with a substantial-elements requirement. The phrase “consisting essentially of” limits the process to the recited steps and additional materials that do not materially affect the basic and novel characteristics of the invention.

Required elements of claim 1

A process would need to satisfy each of the following limitations to fall within the literal scope of claim 1:

1. Whole blood and red blood cells

The starting material must be a sample of whole blood containing plasma and the red blood cells to be labeled. A process using isolated erythrocytes without plasma may not satisfy this limitation literally, although claim construction would depend on the specific process and any applicable equivalents analysis.

2. Sterile container with lyophilized stannous complex

The container must contain, under sterilized conditions:

A lyophilized stannous-ion complex;
A tin-complex stabilizer; and
Tin in an amount of 5 to 1,000 micrograms.

This limitation favors a single-use, prepackaged radiopharmaceutical kit. A process in which tin is freshly added to liquid blood, or in which the stannous reagent is not lyophilized in the container, presents a stronger noninfringement position against literal claim 1.

3. Intracellular transfer of stannous ion

The blood must be incubated long enough for some stannous ion to pass through the red-cell membrane. The claim does not require that all tin enter the cells. It requires only that some of the complex or its stannous component cross the membrane.

4. Oxidation of extracellular tin

The suspension must be treated with an oxidizing agent so that substantially all stannous tin in plasma is converted to stannic tin. This is a material process step. A method that removes extracellular tin by filtration, centrifugation, immunochemical capture, or a different chemical mechanism may avoid literal infringement if it does not perform the claimed oxidation.

5. Chelation

The suspension must then be incubated with a chelating agent capable of binding both stannic and residual stannous tin in plasma. The claim focuses on extracellular tin control rather than merely adding a chelator at any point in the process.

6. Addition of membrane-permeable technetium pertechnetate

The technetium source must be a form in which pertechnetate can pass through the red-cell membrane. The claim requires an incubation period sufficient to allow cellular uptake.

7. Intracellular reduction and retention

Intracellular stannous ions must reduce technetium to a lower-valence state that does not pass back through the red-cell membrane. This functional result links the chemistry to the labeling outcome.

Which claims are broadest and which are commercially narrow?

Claim 1 has the broadest technical coverage. It does not limit the complexing agent, stabilizer, oxidant, chelator, or pertechnetate counterion to one specific compound. Claims 2 through 12 progressively narrow the process by specifying anticoagulants and reagent classes.

Claim 13 is a multi-component combination claim. It requires:

Sodium citrate or sodium glucoheptonate;
Dextrose;
An alkali-metal hypochlorite;
Disodium EDTA, calcium disodium EDTA, or acid citrate dextrose; and
An alkali-metal pertechnetate.

Claim 14 narrows claim 13 to sodium hypochlorite and sodium pertechnetate. A process using the commercial-style combination recited in claims 13 and 14 would have been the closest historical infringement target.

The supplied text uses “sodium glucophetonate” in claim 13. Claims 5 and 7 use “sodium glucoheptonate.” The latter is the chemically recognized term and is likely the intended wording. The issued patent should control any claim-construction analysis.

What formulations are protected by U.S. Patent 4,755,375?

The patent does not claim a single fixed kit formula. It covers multiple reagent alternatives and combinations.

Formulation component	Covered alternatives
Tin complexing agent	Alkali-metal citrate, alkali-metal glucoheptonate, alkaline-earth citrate, alkaline-earth glucoheptonate, tartrate, pyrophosphate, organic diphosphonate, EDTA, dimercaptosuccinic acid, DTPA
Preferred complexant	Sodium citrate or sodium glucoheptonate
Stabilizer	Reducing sugar
Preferred stabilizer	Dextrose
Oxidant	Alkali-metal hypochlorite
Preferred oxidant	Sodium hypochlorite
Chelator	Disodium EDTA, calcium disodium EDTA, or acid citrate dextrose
Technetium source	Alkali-metal pertechnetate
Preferred isotope source	Sodium pertechnetate
Tin quantity	5 to 1,000 micrograms

The claims are directed to a process using these substances. They do not independently claim a vial, a lyophilized kit, a finished labeled-cell product, or a method of diagnosing a disease.

What is the significance of the elevated-temperature claims?

Claims 15 and 16 add a step in which the labeled red blood cells are incubated at an elevated temperature to damage the cells. This limitation is directed to damaged-cell applications, including nuclear medicine procedures in which altered erythrocyte distribution or sequestration is evaluated.

The claims do not define a specific temperature in the supplied text. A temperature must therefore be high enough, under the applicable claim construction, to damage the labeled red blood cells. A process that labels intact cells without a damaging heat treatment would not satisfy claims 15 or 16.

Claims 15 and 16 are narrower than claim 1 and would have had greater vulnerability to design-around through chemical, osmotic, mechanical, or other nonthermal red-cell damage methods.

When did U.S. Patent 4,755,375 lose exclusivity?

The patent issued July 5, 1988. For a patent subject to the pre-Uruguay Round 17-year term measured from grant, the ordinary expiration date was July 5, 2005.[1] The patent is therefore expired.

Event	Date
Patent issued	July 5, 1988
Ordinary 17-year term	July 5, 2005
Current enforceability	Expired
Current U.S. patent barrier	None from this patent

Patent expiration eliminates the right to exclude others from practicing the claimed process. It does not eliminate the historical value of the patent as prior art or its relevance to patent prosecution, validity analysis, and technology lineage.

What is the Orange Book status of U.S. Patent 4,755,375?

U.S. Patent 4,755,375 is not an active Orange Book patent listing. The Orange Book lists patents submitted for approved drug products under the Hatch-Waxman framework, but the patent itself is an expired process patent concerning radiolabeling methodology.

Technetium-99m red-blood-cell products may have FDA-approved kit or radiopharmaceutical labeling, but an FDA-approved product listing does not establish that this patent remains enforceable. Orange Book status and patent validity are separate questions.[2]

What is the FDA regulatory status of technetium-99m red-blood-cell labeling?

Technetium-99m-labeled red blood cells are used as a radiopharmaceutical preparation for blood-pool imaging and related nuclear medicine procedures. FDA-approved products in this area have generally been regulated as kits or radiopharmaceutical products requiring preparation with sodium pertechnetate and use under labeled conditions.[3]

The regulatory product may include:

A lyophilized reagent vial;
Stannous compounds;
Complexing or stabilizing agents;
Chelating agents;
Directions for adding patient blood;
Directions for adding technetium-99m pertechnetate; and
Quality-control procedures for radiochemical purity and cell labeling.

FDA approval does not create patent exclusivity. The expired patent likewise does not establish regulatory approval, substitutability, or clinical equivalence.

Are there Paragraph IV challenges to this patent?

No current Paragraph IV challenge can have practical effect against U.S. Patent 4,755,375 because the patent expired in 2005. A Paragraph IV certification is relevant to an unexpired patent listed for an approved drug product. It cannot restore exclusivity after patent expiration.

Historical ANDA activity involving technetium-99m red-blood-cell kits would need to be assessed against the relevant product listing and any later patents, not against the expired claims of U.S. Patent 4,755,375.

Which companies are challenging the patent?

No current commercial challenger is required to overcome this patent. Its expiration makes a present-day patent challenge unnecessary.

The relevant competitive parties in this technology area have historically included radiopharmaceutical kit developers, hospital radiopharmacies, nuclear medicine suppliers, and manufacturers of technetium-99m blood-pool imaging products. Product competition is more likely to turn on FDA approval, reagent manufacturing, radiochemical quality, supply of technetium-99m, hospital purchasing, and reimbursement than on this expired patent.

How strong was the patent estate?

The estate was technically focused but commercially useful during its term.

Strengths

Claim 1 covered the principal sequence of intracellular tin loading, extracellular tin oxidation, chelation, and technetium uptake.
The claim covered classes of reagents rather than only one commercial formulation.
The 5- to 1,000-microgram tin range captured a broad operating window.
Claims 13 and 14 targeted a practical combination of reagents.
Claims 15 and 16 addressed damaged-cell applications separately.

Limitations

The patent claimed a method, not technetium-99m itself or a composition of matter.
Each independent process element had to be practiced.
Functional terms such as “sufficient time,” “substantially all,” and “will not pass out” could create claim-construction and proof issues.
Alternative processes that label cells without the specified extracellular oxidation and chelation sequence could avoid literal infringement.
The patent term expired in 2005.

Its present patent strength is zero as an exclusionary right, but its historical blocking position was strongest against products using the specific multi-step in-vitro labeling sequence and reagent combinations recited in claims 1, 13, and 14.

What patent litigation affects this technology?

No presently enforceable litigation position arises from U.S. Patent 4,755,375. Any historical litigation involving a technetium-99m red-blood-cell product would need to be evaluated by docket, asserted claim, accused kit formulation, and the patent family’s prosecution history.

The patent’s expiration materially changes the litigation analysis:

No injunction is available for future practice of the expired claims.
No current royalty demand can be based solely on the patent.
Past damages would be limited by applicable statutes of limitation and litigation-specific facts.
Later patents could still cover distinct kits, manufacturing methods, quality-control procedures, or clinical uses.

What generic-entry risks exist?

There is no generic-entry risk from this patent because the patent is expired. A competitor can design, manufacture, and sell a technetium-99m red-blood-cell labeling kit without obtaining a license to U.S. Patent 4,755,375.

Commercial barriers remain in other areas:

Barrier	Present relevance
FDA approval or applicable regulatory pathway	High
Sterile manufacturing	High
Radiopharmaceutical quality control	High
Technetium-99m supply and logistics	High
Hospital adoption and radiology workflow	High
Patent 4,755,375	None as an enforceable U.S. barrier
Biosimilar regulation	Not applicable
Conventional small-molecule generic substitution	Limited, product-specific

Biosimilar risk is not relevant because the product is a radiolabeled cellular preparation and not a therapeutic biologic subject to the biosimilar pathway.

How does this patent compare with later patent strategies?

U.S. Patent 4,755,375 uses a broad process claim followed by narrower reagent and use claims. Later radiopharmaceutical patent strategies commonly divide protection among:

Kit compositions;
Vial configurations;
Stabilized stannous formulations;
Radiochemical purity improvements;
Labeling efficiency;
Cell-storage conditions;
Automated preparation systems;
Quality-control methods;
Imaging indications; and
Manufacturing or fill-finish processes.

That later strategy can produce a longer commercial exclusivity profile than a single process patent. It also creates multiple potential infringement positions, while increasing prosecution and validity complexity.

What geographic coverage does the patent provide?

U.S. Patent 4,755,375 provides only U.S. patent rights. It does not establish protection in Canada, Europe, Japan, Australia, or other jurisdictions.

International protection would require separate national or regional patents. Expiration in the United States does not prove that every foreign counterpart expired on the same date, although patents filed from the same priority period would generally have reached the end of their terms by now under modern 20-year-from-filing rules.

A global freedom-to-operate review must therefore distinguish:

U.S. patent expiration;
Foreign counterpart status;
Continuations or divisionals;
Later patents covering commercial formulations; and
Regulatory exclusivity in each market.

Key Takeaways

U.S. Patent 4,755,375 covers an in-vitro method for labeling red blood cells with technetium-99m.
The technical core is intracellular stannous reduction of technetium combined with oxidation and chelation of extracellular tin.
Claim 1 is the broadest process claim.
Claims 13 and 14 target preferred reagent combinations involving citrate or glucoheptonate, dextrose, hypochlorite, EDTA or acid citrate dextrose, and pertechnetate.
Claims 15 and 16 cover elevated-temperature damage of labeled red blood cells.
The patent issued July 5, 1988, and expired July 5, 2005.
It has no current Orange Book or Paragraph IV significance as an enforceable patent.
No biosimilar issue arises.
Current market barriers are regulatory, manufacturing, radiochemical, and logistical rather than patent-based under this patent.
Later patents, continuations, foreign counterparts, and product-specific regulatory exclusivities must be reviewed separately.

FAQs

Can a company practice the claimed technetium-99m red-blood-cell labeling method today?

Yes. The U.S. patent expired in 2005, so the claimed method is no longer subject to exclusion by this patent.

Does the patent cover a technetium-99m red-blood-cell kit itself?

Not as a standalone product claim. The patent claims methods. A kit could be relevant to infringement only if its use results in practice of the claimed method.

Does using sodium citrate and dextrose create infringement risk?

Not under this expired patent. During its term, those ingredients could have supported claims 6, 9, 13, or 14 when combined with the other required process steps.

Are heat-damaged red blood cells protected separately?

Claims 15 and 16 address elevated-temperature incubation that damages the labeled cells. Those claims are narrower and expired with the patent.

Is a license needed to commercialize a competing red-blood-cell labeling kit?

No license is required under U.S. Patent 4,755,375. A separate review of later patents, regulatory approvals, trademarks, manufacturing rights, and foreign patent rights may produce different results.

References

U.S. Patent No. 4,755,375. (1988). Method of labeling red blood cells with technetium-99m. United States Patent and Trademark Office.
U.S. Food and Drug Administration. (2023). Approved drug products with therapeutic equivalence evaluations. FDA.
U.S. Food and Drug Administration. (n.d.). Drugs@FDA: FDA-approved drugs. FDA.

Details for Patent: 4,755,375

Summary for Patent: 4,755,375