U.S. Patent 5,731,296: Scope, Claims, Expiration, and Adenosine Patent Landscape

U.S. Patent No. 5,731,296 covers continuous intravenous administration of adenosine at weight-based infusion rates intended to produce selective arterial vasodilation without significant venous dilation and without dipyridamole pretreatment. The patent issued March 24, 1998, and its ordinary 20-year term has expired. The claims therefore have historical importance but no current blocking effect in the United States, absent an unusual term adjustment or enforceable continuation patent.

What does U.S. Patent 5,731,296 protect?

The patent protects a treatment method, not adenosine itself, an adenosine formulation, an infusion pump, or a particular dosage form. The central technical concept is administration of adenosine at a rate intended to dilate arteries while avoiding substantial venous dilation.

The core limitations are:

The patient is human.
Adenosine is administered continuously into the bloodstream.
The administration is generally intravenous.
The dose is weight-based.
The rate is at or below a specified threshold or within a specified range.
The result is selective arterial vasodilation without significant venous dilation.
The patient has not received dipyridamole pretreatment.
Certain claims require general anesthesia or surgery.
Claim 9 focuses on reduced afterload without reduced preload.

The patent does not claim every use of adenosine. A use would need to satisfy the claimed administration method and the relevant physiological result.

Claim-by-claim scope

Claim	Principal subject matter	Dose limitation	Patient or procedure limitation
1	Selective arterial vasodilation	0.35 mg/kg/min or less	Human patient; no dipyridamole
2	Claim 1	Same as claim 1	Anesthetized patient undergoing surgery
3	Selective arterial vasodilation by IV administration	About 0.05 to about 0.30 mg/kg/min	Human patient; no dipyridamole
4	Claim 3	About 0.05 to about 0.30 mg/kg/min	Anesthetized patient undergoing surgery
5	Surgical improvement method	0.35 mg/kg/min or less	General anesthesia
6	Claim 5	About 0.05 to about 0.30 mg/kg/min	Surgical method
7	Selective arterial vasodilation	0.01 to 0.15 mg/kg/min	Human patient; no dipyridamole
8	Selective arterial vasodilation by IV administration	About 0.20 to about 0.35 mg/kg/min	Anesthetized human patient
9	Reduced afterload without reduced preload	0.35 mg/kg/min or less	Human patient; no dipyridamole

Claims 1, 3, 7 and 8 are independent claims. Claims 2, 4 and 6 add anesthesia or surgical limitations. Claim 5 is framed as an improvement to a surgical method, while claim 9 adds the hemodynamic result of reduced afterload without reduced preload.

How do the dose ranges overlap?

The claims create overlapping coverage around several dose bands:

Dose band	Potentially relevant claims
0.01 to 0.05 mg/kg/min	Claim 7
0.05 to 0.15 mg/kg/min	Claims 3 and 7
0.15 to 0.20 mg/kg/min	Claim 3
0.20 to 0.30 mg/kg/min	Claims 3 and 8, where anesthesia applies to claim 8
0.30 to 0.35 mg/kg/min	Claims 1, 5, 8 and 9, with claim-specific limitations
Above 0.35 mg/kg/min	Outside the express numeric limits of claims 1, 5, 8 and 9

The phrase "about" in claims 3 and 8 creates a potential claim-construction issue. Courts generally evaluate "about" in view of the specification, technical measurement variability, and the invention’s context. The term does not automatically extend the range to any clinically similar dose.

The claims also use different endpoints. A dose within the numerical range is not sufficient by itself. The claimed physiological effect, such as selective arterial vasodilation or reduced afterload without reduced preload, must also be established where that limitation applies.

What technical problem does the patent address?

Adenosine is a short-acting endogenous purine nucleoside that produces vasodilation and has a very short plasma half-life. Its hemodynamic effect depends on dose, route, infusion rate, receptor activity, patient condition and concomitant medication.

The patent distinguishes arterial vasodilation from venous dilation. Arterial dilation can reduce systemic vascular resistance and afterload. Venous dilation can reduce venous return and preload. The claimed method seeks to reduce afterload while preserving preload.

Dipyridamole is relevant because it inhibits adenosine uptake and can potentiate adenosine’s cardiovascular effects. The claims expressly require the absence of dipyridamole pretreatment. This limitation narrows the claims and separates the claimed method from protocols that deliberately increase adenosine exposure through pharmacological potentiation.

The surgical claims add general anesthesia or an operation. These limitations may have commercial relevance for intraoperative blood-pressure management, controlled reduction of afterload, and surgical hemodynamic control. They also make the claims narrower than a general claim directed to all IV adenosine administration.

When did U.S. Patent 5,731,296 expire?

U.S. Patent 5,731,296 issued on March 24, 1998. The patent’s ordinary term was governed by the 20-year term measured from the applicable nonprovisional U.S. filing date under the Uruguay Round Agreements Act framework. Public patent records identify the patent as expired before the present date, with an ordinary expiration in approximately 2014. The patent is therefore not an active U.S. exclusion right. (U.S. Patent No. 5,731,296, 1998; USPTO, n.d.)

Event	Date or status
U.S. patent application	Filed before issuance; ordinary term governed by post-1995 rules
Patent issued	March 24, 1998
Patent number	5,731,296
Present status	Expired
Approximate ordinary expiration	2014
Current blocking right	None from this patent alone

A patent expiration analysis should distinguish this patent from later continuations, divisionals, foreign counterparts and unrelated adenosine patents. An expired parent patent does not eliminate rights in a later-filed continuation with a separate unexpired term.

What is the Orange Book status of U.S. Patent 5,731,296?

U.S. Patent 5,731,296 is not an active Orange Book barrier for current adenosine generic entry. The patent claims methods of treatment and was not the type of active patent that would presently block approval of an abbreviated new drug application.

Adenosine is a small-molecule drug, so the relevant abbreviated pathway is an ANDA rather than a biosimilar application. The patent does not cover:

adenosine active pharmaceutical ingredient composition;
a proprietary adenosine salt;
a particular injectable formulation;
a vial or prefilled syringe;
a catheter or infusion pump;
a manufacturing process;
a stability package;
a pharmaceutical composition claim.

The Orange Book analysis therefore turns primarily on listed patents associated with specific reference products, not on this expired method patent. FDA’s Approved Drug Products with Therapeutic Equivalence Evaluations is the controlling source for current Orange Book listings. (U.S. Food and Drug Administration, n.d.-a)

Are there Paragraph IV challenges associated with this patent?

No current Paragraph IV risk arises from U.S. Patent 5,731,296 because the patent has expired. A Paragraph IV certification is relevant when an ANDA applicant challenges a listed patent that remains in force. An expired patent does not support the automatic 30-month stay applicable to a timely patent-infringement action involving a listed patent.

Historically, an applicant could have challenged the patent while it was active if the patent had been listed for the relevant reference product. The principal possible grounds would have included:

noninfringement based on a dose outside the claimed range;
lack of continuous administration;
lack of intravenous administration where required;
absence of the claimed physiological effect;
use of dipyridamole pretreatment;
invalidity for anticipation or obviousness;
indefiniteness of "significant venous dilation";
written-description or enablement challenges.

Because the patent is expired, those issues now have limited commercial significance except in damages, historical litigation, or patent-estate diligence.

How strong was the patent estate for adenosine vasodilation?

The patent estate represented by U.S. Patent 5,731,296 was narrow in subject matter but potentially meaningful in a specialized clinical setting.

Strengths

The principal strengths were:

specific weight-based dosing;
continuous bloodstream administration;
express exclusion of dipyridamole pretreatment;
physiological distinction between arterial and venous effects;
surgical and anesthesia-dependent claims;
a separate afterload/preload claim in claim 9.

The dose limitations could have made literal infringement easier to evaluate when an accused protocol used a defined infusion rate. The claims also targeted a clinical technique rather than a broad chemical compound, reducing the risk that the patent would be invalidated merely because adenosine itself was known.

Weaknesses

The patent had several structural limitations:

It did not control adenosine as a molecule.
It did not cover every cardiovascular use of adenosine.
Several claims required proof of a physiological outcome.
"Significant venous dilation" and "selectively vasodilating" could create factual and claim-construction disputes.
Hospital use may involve multiple actors, complicating direct-infringement theories.
A protocol using bolus administration rather than continuous infusion would generally fall outside the express method.
A dose above the specified ceiling would fall outside the literal numeric limitation.
Use with dipyridamole would fail the no-pretreatment limitation.

The commercial value was therefore dependent on adoption of a specific infusion protocol, not on control of the broader adenosine market.

What formulations and manufacturing methods are protected?

None are protected by the asserted claims.

The patent claims administration of adenosine in a patient. It does not claim:

adenosine injection composition;
concentration ranges in a vial or bag;
pH or excipient system;
preservative-free packaging;
ready-to-use infusion products;
manufacturing or purification;
formulation stability;
container-closure systems.

A competitor could therefore develop or market a different adenosine formulation without infringing these claims, provided its clinical use did not practice the claimed method. Conversely, a generic injectable product could theoretically be used in an infringing manner even though the product itself was not covered by the patent. That distinction is important under induced-infringement and label-based theories.

What generic launch risks exist?

The patent itself creates no current generic launch risk because it is expired. A current generic or hospital supplier would instead assess:

FDA approval of the proposed adenosine injection.
Reference-product exclusivity.
Active Orange Book patents for the relevant listed product.
Formulation, container and manufacturing patents.
Labeling that encourages the claimed use.
State-law and institutional procurement requirements.
Supply and quality requirements for sterile injectable products.

The product is a small molecule, so biosimilar risk is not applicable. Competition is more likely to arise from generic adenosine injection, compounded or hospital-supplied products where permitted, and alternative pharmacologic or mechanical approaches to hemodynamic control.

Which companies challenged or commercialized the relevant technology?

The patent record alone does not establish a current commercial challenge landscape. The technology concerns a clinical administration method rather than an exclusive adenosine product platform.

The relevant competitive groups are:

Competitive group	Relevance
Generic injectable manufacturers	May supply adenosine products without relying on an active method patent
Reference-product sponsors	May retain product-specific regulatory or formulation rights
Hospital pharmacies	May control administration protocols and procurement
Anesthesia and critical-care providers	May determine whether the claimed infusion method is used
Competing vasodilator suppliers	May offer alternative agents for afterload reduction
Device manufacturers	May supply pumps or administration systems, none covered by the claims

No biosimilar manufacturers are relevant because adenosine is a chemically synthesized small molecule, not a biologic.

What litigation and settlement issues affect the patent?

U.S. Patent 5,731,296 is expired, and no current enforcement action based solely on this patent can block future U.S. market entry. The patent’s historical litigation value would have depended on whether an accused party administered adenosine:

continuously;
intravenously;
at a covered weight-based rate;
without dipyridamole pretreatment;
to achieve the claimed arterial or afterload effect;
in an anesthetized or surgical patient where required.

A settlement involving an expired patent would not create a current market exclusion unless it included independent contractual restrictions. No active settlement-based market restriction is established by the patent claims themselves.

How does this patent compare with composition and formulation patents?

Patent category	Coverage	Commercial leverage after expiration
U.S. 5,731,296 method claims	Clinical administration and hemodynamic result	None from this patent
Composition patents	Drug product or active ingredient composition	Depends on term and listing
Formulation patents	Concentration, excipients, stability, packaging	Can affect ANDA design
Manufacturing patents	Synthesis, purification or sterile processing	Can affect supply chain
Device patents	Pump, catheter or delivery system	Can affect administration hardware
Method-of-use patents	Specific disease or patient population	May affect labeling and induced infringement

The patent’s commercial weakness compared with a composition patent is that competitors could sell adenosine while avoiding the claimed method. Its possible strength during its term was clinical specificity: a competitor using the same infusion protocol in surgery could face method-of-use exposure even without copying a formulation.

Key Takeaways

U.S. Patent 5,731,296 covers continuous, weight-based adenosine administration for selective arterial vasodilation.
The claims require no dipyridamole pretreatment and, in several claims, require intravenous delivery.
The principal dose ceiling is 0.35 mg/kg/min, with narrower ranges from 0.01 to 0.15 mg/kg/min, 0.05 to 0.30 mg/kg/min and about 0.20 to 0.35 mg/kg/min.
Claims 2, 4, 5, 6 and 8 focus on anesthesia or surgery.
Claim 9 targets reduced afterload without reduced preload.
The patent does not cover adenosine composition, formulation, manufacturing or infusion hardware.
The patent issued March 24, 1998, and expired around 2014 under its ordinary patent term.
It presents no current U.S. Orange Book, Paragraph IV or generic-entry barrier.
Biosimilar analysis is inapplicable because adenosine is a small molecule.
Any current freedom-to-operate review must focus on later continuations, unrelated formulation patents, active product patents and FDA labeling.

FAQs About U.S. Patent 5,731,296

Does a dose above 0.35 mg/kg/min avoid every claim?

No. It avoids the express ceiling in claims 1, 5, 8 and 9, but the answer depends on the full claim set, any later patent and whether another claim has a different range.

Does a bolus injection infringe the patent?

Generally, a bolus-only protocol would not meet the claims requiring continuous administration. A combined bolus and continuous infusion would require analysis of the complete protocol.

Does using dipyridamole create infringement?

Dipyridamole pretreatment would generally prevent satisfaction of the express no-pretreatment limitation in the asserted claims. It could remain relevant to other patents or claims not containing that limitation.

Is adenosine stress testing covered by this patent?

Not necessarily. Stress-testing protocols may use adenosine for coronary vasodilation, but infringement would require all relevant limitations, including the claimed continuous administration, dose range and arterial-versus-venous physiological result.

Can a generic manufacturer rely on expiration of this patent?

Yes, as to this patent. A generic manufacturer must still assess active patents, regulatory exclusivity, formulation rights, manufacturing patents and labeling issues associated with the specific adenosine reference product.

References

U.S. Patent No. 5,731,296. (1998). Method of selectively vasodilating arteries using adenosine. United States Patent and Trademark Office.
U.S. Food and Drug Administration. (n.d.-a). Approved drug products with therapeutic equivalence evaluations. U.S. Department of Health and Human Services.
U.S. Food and Drug Administration. (n.d.-b). Orange Book: Approved drug products with therapeutic equivalence evaluations, patent and exclusivity information. U.S. Department of Health and Human Services.
United States Patent and Trademark Office. (n.d.). Patent term calculator and patent term provisions. U.S. Department of Commerce.

Country	Patent Number	Estimated Expiration	Supplementary Protection Certificate	SPC Country	SPC Expiration
Austria	176154	⤷ Start Trial
Austria	84419	⤷ Start Trial
Australia	5065585	⤷ Start Trial
>Country	>Patent Number	>Estimated Expiration	>Supplementary Protection Certificate	>SPC Country	>SPC Expiration

Details for Patent: 5,731,296

Summary for Patent: 5,731,296