US Patent 5,602,116: Scope, Claims, Expiration, and Vitamin D Analog Patent Landscape

US Patent No. 5,602,116 covers methods of lowering or maintaining reduced serum parathyroid hormone, or PTH, in patients with secondary hyperparathyroidism associated with end-stage renal disease, or ESRD. The patent is directed to therapeutic use of selected 1-alpha-hydroxylated vitamin D analogs, including doxercalciferol, also known as 1-alpha-hydroxyvitamin D2.

The patent does not claim a new chemical compound in isolation. Its principal protection is method-of-treatment protection covering administration of specified vitamin D analogs to human patients with ESRD-related secondary hyperparathyroidism. The patent term has expired, eliminating current US exclusionary rights under the patent.

What does US Patent 5,602,116 cover?

The broadest independent claims cover two related treatment concepts:

Claim	Core subject matter	Principal limitation
1	Treating ESRD patients with secondary hyperparathyroidism	Administering an effective amount of a defined vitamin D analog to lower or maintain lowered serum PTH
9	Treating pathological effects of secondary hyperparathyroidism	Oral administration of selected vitamin D analogs in an amount sufficient to lower serum PTH over time
2	Specific analogs	1-alpha-OH-vitamin D2, 1-alpha-OH-vitamin D4, or 1-alpha,24(R)-(OH)2-vitamin D4
3	Dose	Approximately 1 to 100 micrograms per week
4	Oral formulation	Analog dissolved in an ingestible, nontoxic liquid vehicle and administered in encapsulated form
5-6	Parenteral administration	Subcutaneous, intramuscular, intravenous, nasopharyngeal, mucosal, or transdermal delivery
7	Specific analog	1-alpha-OH-vitamin D2
8	Specific analog	1-alpha,24-(OH)2-vitamin D2 as written

The patent’s practical commercial relevance centers on claim 7 and the analog identified in claim 2 as 1-alpha-OH-vitamin D2. That compound is doxercalciferol, the active ingredient in Hectorol.

What chemical compounds fall within the claimed formula?

The formula in claim 1 defines a vitamin D analog with a 1-alpha-hydroxyl group and specified vitamin D side-chain variations. The claim language permits:

A saturated or unsaturated relationship involving carbon atoms C22 and C23.
Hydrogen or hydroxyl at the R1 position.
A limitation that R1 must be hydrogen when C22-C23 is a carbon-carbon double bond.

The formula is functional and structural rather than product-specific. It is intended to encompass several vitamin D2 and vitamin D4 analogs.

The principal named compounds are:

Compound	Relation to patent claims	Commercial relevance
1-alpha-OH-vitamin D2	Expressly recited in claim 2 and separately claimed in claim 7	Doxercalciferol; active ingredient in Hectorol
1-alpha-OH-vitamin D4	Expressly recited in claim 2	No comparable current US commercial importance
1-alpha,24(R)-(OH)2-vitamin D4	Expressly recited in claim 2	Research and historical patent relevance
1-alpha,24-(OH)2-vitamin D2	Recited in claim 8 as written	Claim-drafting and scope issue requiring attention

Doxercalciferol is converted in vivo to active vitamin D metabolites and is used to suppress excessive PTH secretion in chronic kidney disease. The FDA-approved product label identifies Hectorol capsules and injection as doxercalciferol products for management of secondary hyperparathyroidism in specified chronic kidney disease populations.[1]

How broad is independent claim 1?

Claim 1 has four material limitations:

The patient must be a human.
The patient must suffer from hyperparathyroidism secondary to ESRD.
The treatment must administer an effective amount of the claimed vitamin D analog.
The treatment must lower or maintain lowered serum PTH.

The claim is therefore narrower than a general claim to vitamin D analog treatment. It does not cover:

Primary hyperparathyroidism;
Hyperparathyroidism unrelated to ESRD;
Vitamin D deficiency without secondary hyperparathyroidism;
Patients without a human subject;
Administration of a non-covered vitamin D analog;
Administration that does not satisfy the claimed PTH-lowering result.

The disease etiology is central. A treatment involving doxercalciferol for another indication would not necessarily meet the claim’s ESRD-related secondary hyperparathyroidism limitation.

The claim also requires a therapeutic outcome, but it does not specify a numerical PTH threshold, a fixed treatment duration, or a particular assay. That drafting approach gives the claim flexibility across treatment regimens but can create disputes over causation, baseline PTH, treatment response, and the meaning of “maintain lowered” levels.

What does claim 9 add to the patent scope?

Claim 9 is a separate independent method claim and is potentially important because it uses a different structure from claim 1.

It requires:

Treatment of a human;
A human in need of treatment;
The purpose of alleviating or preventing pathological effects of secondary hyperparathyroidism;
Oral administration;
One of three specified analogs;
An amount sufficient to lower serum PTH as measured over time after ingestion.

Claim 9 is narrower in route of administration because it requires oral delivery. It may be broader in therapeutic objective because it refers to alleviating or preventing pathological effects, rather than expressly requiring the patient to have ESRD in the body of the claim. The phrase “secondary hyperparathyroidism” remains a central disease limitation, and the specification and prosecution history would be relevant to determining whether ESRD is imported into the claim through the claim’s construction.

Claims 1 and 9 should be analyzed separately. A product or treatment regimen might fall outside claim 1 because of the patient population or formulation but still require analysis under claim 9 if it involves oral administration of one of the listed analogs.

What is the significance of claims 2, 7, and 8?

Claims 2, 7, and 8 narrow the chemical scope.

Claim 7 is the most commercially important species claim because it specifically covers 1-alpha-OH-vitamin D2. That compound corresponds to doxercalciferol.

Claim 8 presents a textual inconsistency. Claim 2 lists 1-alpha,24(R)-(OH)2-vitamin D4, while claim 8 states 1-alpha,24-(OH)2-vitamin D2. The difference between vitamin D2 and vitamin D4 is material because the side chains differ. The difference between the R-specific form and an unspecified stereochemical form may also affect scope.

The legal effect of the inconsistency depends on the issued patent text, prosecution history, claim dependency, and any correction or reexamination record. A claim chart should reproduce the issued language exactly and should not silently harmonize claim 8 with claim 2.

What dosage and administration routes are protected?

Claim 3 recites a dosage of 1 to approximately 100 micrograms per week. The claim is dependent on claim 2, so it is limited to one of the analogs identified in claim 2.

The dosage range is broad. It covers weekly doses across two orders of magnitude, but it does not expressly require:

Daily dosing;
A loading dose;
A particular dose-adjustment protocol;
A specific serum calcium target;
A specified PTH reduction percentage.

Claim 4 addresses an oral encapsulated formulation in which the analog is in solution in a liquid vehicle that is ingestible and nontoxic. This is a formulation and dosage-form limitation, not a composition-of-matter claim to the analog itself.

Claims 5 and 6 cover parenteral and other absorption routes, including:

Subcutaneous injection;
Intramuscular injection;
Intravenous injection;
Nasopharyngeal absorption;
Mucosal absorption;
Transdermal absorption.

These claims are method claims. They do not independently claim a vial, syringe, patch, capsule, or finished pharmaceutical composition. A competing product would need to be analyzed based on the administered route and treatment conduct, not merely its existence in the marketplace.

Does the patent cover Hectorol and doxercalciferol?

Yes. Doxercalciferol is 1-alpha-hydroxyvitamin D2, the compound expressly identified in claims 2 and 7. The patent’s treatment claims therefore read directly on administration of doxercalciferol for the claimed ESRD-related secondary hyperparathyroidism use, subject to the remaining limitations.

Hectorol was approved by the FDA as a doxercalciferol product. The FDA label describes oral capsules and an injectable formulation and identifies secondary hyperparathyroidism associated with chronic kidney disease as the therapeutic use.[1]

The patent did not provide a perpetual monopoly over doxercalciferol. Its protection depended on:

The patent’s unexpired term;
The claimed patient population;
The claimed therapeutic purpose;
The claimed route;
The claimed dose, where applicable;
The ability to establish infringement by the manufacturer, provider, or user.

When did US Patent 5,602,116 lose exclusivity?

US Patent 5,602,116 has expired. Its enforceable US patent term ended in approximately 2015, based on the patent’s statutory term and relevant filing history recorded in USPTO and patent database records.[2][3]

Event	Date or status
US patent number	5,602,116
Issue date	February 11, 1997
Patent type	Utility patent
Subject matter	Vitamin D analog treatment methods
Principal commercial analog	Doxercalciferol
Patent status	Expired
Approximate end of enforceable term	2015
Current Paragraph IV leverage	None under this expired patent

An expired method patent cannot support a current US patent infringement suit for post-expiration conduct. It also cannot block an ANDA applicant today through a live patent listing, although historical litigation and settlement records may remain relevant to market-entry analysis.

Patent expiration is distinct from FDA regulatory exclusivity. FDA exclusivity may have expired earlier or on a different date. Regulatory exclusivity and patent term must be assessed separately.

What was the Orange Book status of the patent?

The relevant product is Hectorol, not the patent itself. The FDA Orange Book historically listed patents associated with approved doxercalciferol products, subject to the sponsor’s certification and listing practices.[4]

A method-of-use patent can be listed in the Orange Book only if it meets the statutory and regulatory requirements for listing in connection with an approved drug product. Listing does not expand the patent’s claim scope. It gives an ANDA applicant a regulatory notice and certification framework.

For an expired patent:

It no longer creates a current blocking patent right.
A new ANDA applicant would not face a live Paragraph IV challenge directed to that patent.
The patent’s historical listing may remain relevant to understanding past generic-entry timing.
Current Orange Book status should be checked against the FDA’s latest publication rather than inferred from older litigation records.

The Orange Book separates patents from exclusivity. A product may have no unexpired listed patent but still have, or previously have had, regulatory exclusivity.

Were there Paragraph IV challenges to doxercalciferol?

Doxercalciferol was subject to generic competition after FDA approval of Hectorol. Generic applicants could challenge listed patents through Paragraph IV certifications under the Hatch-Waxman Act.

The legal significance of a Paragraph IV certification depends on:

The specific patent listed for the reference product;
The ANDA product’s proposed labeling;
Whether the applicant alleged noninfringement, invalidity, or unenforceability;
Whether the patent remained unexpired at the time of certification;
Whether the innovator filed suit within the statutory period.

Because US Patent 5,602,116 is expired, it is no longer a current barrier to generic doxercalciferol approval. Historical litigation records may involve other patents, listed uses, product patents, or settlement terms. The existence of a Paragraph IV challenge does not establish that the challenged patent was invalid or that the generic product was noninfringing.

How strong was the patent estate for doxercalciferol?

The patent estate was strongest during the period when 5,602,116 was unexpired and doxercalciferol was commercially protected by the combination of:

A species claim to 1-alpha-OH-vitamin D2;
Broader analog claims;
Oral and parenteral treatment claims;
A dosage-range claim;
A disease-specific indication tied to secondary hyperparathyroidism;
FDA-approved labeling aligned with the claimed treatment.

Its principal weaknesses were structural:

The patent did not claim doxercalciferol as a new chemical compound.
The claims were limited to particular clinical uses.
Infringement required proof of treatment conduct and patient population.
The patent had no current force after expiration.
Claim 8 contains an apparent species and stereochemistry inconsistency.
A competing manufacturer could potentially design around route, indication, patient population, or analog selection while the patent was active.

The patent was therefore commercially meaningful but narrower than a composition-of-matter patent.

How does this patent compare with competing vitamin D analog patents?

The relevant competitive products included doxercalciferol, paricalcitol, calcitriol, and other vitamin D receptor-active therapies.

Product or class	Active substance	Typical patent protection profile	Relationship to US 5,602,116
Hectorol	Doxercalciferol	Method and product-related protection	Directly aligned with the claimed 1-alpha-OH-vitamin D2 use
Zemplar	Paricalcitol	Separate composition, formulation, and use patents	Generally outside the claimed chemical species
Rocaltrol and generic calcitriol	Calcitriol	Older compound and formulation history	Not the claimed 1-alpha-hydroxyvitamin D2/D4 species
Vitamin D analog research products	Various analogs	Compound, use, and formulation patents	Scope depends on structural relationship to formula (I)

Paricalcitol is not doxercalciferol and would not ordinarily fall within the expressly named species claims. A direct infringement analysis would require comparison of paricalcitol’s structure with the formula in claim 1 and evaluation of the treatment limitations. Therapeutic similarity alone is insufficient.

What generic entry risks exist today?

For US Patent 5,602,116, current generic-entry risk is no longer patent-driven because the patent has expired.

Current commercial risks relate to:

FDA approval of generic doxercalciferol capsules or injection;
Bioequivalence and product quality;
Manufacturing cost;
Distribution contracts;
Reimbursement and dialysis-provider purchasing;
Remaining patents, if any, covering later formulations or products;
Labeling and indication differences;
State substitution rules.

Doxercalciferol is a small-molecule drug, not a biologic. Biosimilar regulation under the Public Health Service Act does not apply. Competitors proceed through the ANDA pathway or, for products that do not qualify as therapeutically equivalent, another applicable FDA pathway.[5]

What manufacturing and intellectual-property barriers remain?

US Patent 5,602,116 does not claim a manufacturing process. It creates no current process barrier because it is expired.

Potential barriers may still arise from:

Stereochemical control of vitamin D analog synthesis;
Purity and impurity specifications;
Stability of the active ingredient;
Light and oxygen sensitivity;
Soft-gel or liquid-fill encapsulation;
Injectable formulation stability;
Analytical methods;
Drug-master-file or supplier dependence;
Regulatory requirements for complex formulations.

These are commercial and regulatory barriers rather than continuing rights under the expired patent. A manufacturer may use a different synthesis route and still need to demonstrate pharmaceutical equivalence, bioequivalence, stability, and compliance with current FDA quality requirements.

What litigation and settlement issues matter?

Historical litigation involving Hectorol or doxercalciferol should be divided into four categories:

Litigation under US 5,602,116 itself;
Litigation under later or related patents;
Hatch-Waxman litigation following Paragraph IV certifications;
Commercial disputes involving licensing, supply, distribution, or settlement agreements.

An expired patent can remain relevant to historical market-entry analysis, but it cannot support a current infringement claim for acts occurring after expiration. Settlement agreements may have governed entry dates before expiration, but those agreements do not revive an expired patent or create a new patent term.

A reliable litigation review should match each case to the asserted patent number, filing date, disposition, settlement terms, and generic launch date. Patent-family references alone are insufficient because related patents can have materially different claim scope and expiration dates.

How does geographic coverage affect the analysis?

US Patent 5,602,116 provided rights only in the United States. It did not directly control:

Canadian manufacture or sale;
European marketing;
Japanese distribution;
Indian production;
Export transactions lacking a US statutory nexus.

Foreign counterparts required separate analysis of national validity, claim scope, patent term, supplementary protection certificates, and litigation history. A US patent expiration date does not establish foreign freedom to operate.

For US activities, possible territorial issues include:

Manufacture in the United States;
Importation of a product made abroad;
US sale or offer for sale;
Administration to US patients;
Inducement based on US-approved labeling.

Key Takeaways

US Patent 5,602,116 is a method-of-treatment patent for vitamin D analog therapy in secondary hyperparathyroidism.
Its primary commercial species is 1-alpha-hydroxyvitamin D2, or doxercalciferol.
Claims 1 and 9 are independent treatment claims with different disease, route, and outcome limitations.
Claims 2, 7, and 8 narrow the scope to named vitamin D2 and vitamin D4 analogs.
Claim 8 contains an apparent inconsistency involving vitamin D2 versus vitamin D4 and specified versus unspecified stereochemistry.
Claim 4 covers an oral encapsulated solution formulation, while claims 5 and 6 cover parenteral and other administration routes.
The patent is expired, with enforceable US protection ending approximately in 2015.
It is no longer a current barrier to generic doxercalciferol entry.
Doxercalciferol is a small molecule and is subject to generic, not biosimilar, competition.
Any current freedom-to-operate analysis must focus on later patents, regulatory requirements, manufacturing controls, and commercial execution.

FAQs About US Patent 5,602,116

Is US Patent 5,602,116 a composition-of-matter patent?

No. Its principal claims cover methods of treating secondary hyperparathyroidism with defined vitamin D analogs. It does not broadly claim doxercalciferol as a standalone chemical compound.

Does US Patent 5,602,116 cover Hectorol?

The patent claims cover administration of doxercalciferol, the active ingredient in Hectorol, when the claimed patient, indication, route, and therapeutic-result limitations are met. The patent has expired.

Can a company now launch generic doxercalciferol despite this patent?

Yes, this patent no longer creates a current US patent barrier. FDA approval, applicable Orange Book listings, other unexpired patents, and ANDA requirements remain relevant.

Does paricalcitol infringe this patent?

Not based solely on its use for secondary hyperparathyroidism. Paricalcitol is a different vitamin D analog. Infringement would require a detailed structural and method comparison against the issued claims.

Does the patent cover treatment of primary hyperparathyroidism?

The claims are directed to secondary hyperparathyroidism. Treatment of primary hyperparathyroidism would not ordinarily satisfy the express disease limitation without a claim-construction basis supported by the patent record.

References

U.S. Food and Drug Administration. (2020). Hectorol (doxercalciferol) prescribing information. FDA.
United States Patent and Trademark Office. (1997). U.S. Patent No. 5,602,116: Methods of treating secondary hyperparathyroidism using vitamin D analogs. USPTO.
Google Patents. (n.d.). US5602116A: Methods of treating secondary hyperparathyroidism using vitamin D analogs. Google.
U.S. Food and Drug Administration. (2024). Approved drug products with therapeutic equivalence evaluations. FDA.
U.S. Food and Drug Administration. (2024). Abbreviated new drug application (ANDA) process. FDA.

Country	Patent Number	Estimated Expiration	Supplementary Protection Certificate	SPC Country	SPC Expiration
Argentina	247817	⤷ Start Trial
Austria	114471	⤷ Start Trial
Austria	250566	⤷ Start Trial
Austria	258796	⤷ Start Trial
Austria	347366	⤷ Start Trial
Australia	2002322346	⤷ Start Trial
Australia	2002346596	⤷ Start Trial
>Country	>Patent Number	>Estimated Expiration	>Supplementary Protection Certificate	>SPC Country	>SPC Expiration

Details for Patent: 5,602,116

Summary for Patent: 5,602,116