US Patent 5,843,901: Claim Scope, Abarelix Patent Coverage, Expiration and Generic Risk

US Patent 5,843,901 covers a broad family of decapeptide gonadotropin-releasing hormone (GnRH) antagonists, including the abarelix active ingredient and closely related analogs. The patent issued on December 1, 1998, from an application claiming priority to June 8, 1994. Its enforceable term ended on December 1, 2015, absent a term adjustment or other extension. The patent is therefore expired and no longer blocks U.S. manufacture, sale or use of covered compounds.

The commercial compound most closely associated with the patent is abarelix, also known as Ac-D-Nal-4-Cl-D-Phe-D-Pal-Ser-N-Me-Tyr-D-Asn-Leu-Lys(iPr)-Pro-D-Ala-NH2. Abarelix was marketed in the United States as Plenaxis for advanced prostate cancer under FDA NDA 021320. [1-4]

What drug and peptide compounds does US Patent 5,843,901 protect?

The patent protects a class of synthetic decapeptides with the general structure:

A-B-C-D-E-F-G-H-I-J

The fixed and variable positions are:

Position	Permitted residues under claim 1
A	Pyro-Glu, Ac-D-Nal, Ac-D-Qal, Ac-Sar or Ac-D-Pal
B	His or 4-Cl-D-Phe
C	Trp, D-Pal, D-Nal, L-Nal-D-Pal(N-O) or D-Trp
D	Ser
E	N-Me-Ala, Tyr, N-Me-Tyr, Ser, Lys(iPr), 4-Cl-Phe, His, Asn, Met, Ala, Arg or Ile
F	D-Asn or D-Gln
G	Leu or Trp
H	Lys(iPr), Gln, Met or Arg
I	Pro
J	Gly-NH2 or D-Ala-NH2

The class is chemically constrained in several ways:

It is a decapeptide.
Ser is fixed at position D.
Pro is fixed at position I.
The C-terminal residue is amidated.
Several positions contain D-amino acids or N-methylated residues.
The peptide may be present as a pharmaceutically acceptable salt.

The patent is not limited to abarelix. Claim 1 reaches a large Markush genus containing multiple residue permutations. On a simple combinatorial count, the listed alternatives create up to 20,800 theoretical sequence combinations before accounting for duplicate sequences, chemical feasibility, excluded species, support requirements and claim-construction issues.

What is the difference between claims 1 and 2?

Claim 2 narrows claim 1 by requiring F to be D-Asn. The number of theoretical combinations falls to approximately 10,400.

Claim 2 is commercially important because abarelix contains D-Asn at position F. A compound that satisfies claim 2 necessarily falls within the broader structural framework of claim 1, assuming the remaining residues match the listed alternatives.

What compounds are specifically claimed in claims 3 and 4?

Claims 3 and 4 are narrow species claims.

Claim 3 covers:

Ac-D-Nal-4-Cl-D-Phe-D-Pal-Ser-N-Me-Tyr-D-Asn-Leu-Lys(iPr)-Pro-D-Ala-NH2

Claim 4 covers:

Ac-D-Nal-4-Cl-D-Phe-D-Pal-Ser-Tyr-D-Asn-Leu-Lys(iPr)-Pro-D-Ala-NH2

Claim 3 corresponds to abarelix. Claim 4 covers the non-N-methyl analog in which N-Me-Tyr is replaced by Tyr.

These claims are materially narrower than claims 1 and 2 but provide stronger literal-coverage positions against the named compounds. A product that is chemically identical to abarelix would fall directly within claim 3, subject to proof of identity, claim validity and the expired patent term.

How strong was the patent estate for abarelix?

The patent had a strong composition-of-matter position during its active term because claims 1 through 4 covered the active peptide itself rather than merely a formulation or treatment method.

Composition-of-matter strength

Claims 1 to 4 covered:

A broad genus of GnRH antagonist peptides.
A narrower D-Asn subgroup.
The abarelix sequence.
A close Tyr analog.
Pharmaceutically acceptable salts of the claimed compounds.

Composition claims generally provide broader enforcement coverage than method claims because infringement can arise from making, selling or importing the compound itself. The presence of a specific abarelix claim reduced the risk that a competitor could avoid infringement through a narrow sequence interpretation of the genus.

Formulation coverage

Claim 5 covers:

A pharmaceutical composition comprising the peptide compound of any one of claims 1, 2, 3 or 4, and a pharmaceutically acceptable carrier.

This is a conventional composition claim. It extends coverage from the peptide to drug products containing the peptide with a pharmaceutically acceptable carrier.

Claim 5 does not expressly require:

A particular dosage.
A particular route of administration.
A depot formulation.
A specific excipient.
A specific particle size.
A particular pH.
A particular release profile.
A particular vial or injection device.

Its breadth is therefore determined by the incorporated peptide claims and the ordinary meaning of “pharmaceutically acceptable carrier.” The claim could cover injectable formulations, provided the formulation contains a claimed peptide and an acceptable carrier. It would not necessarily cover a formulation containing only an unclaimed degradation product, metabolite or substantially different analog.

What is the scope of the abarelix claim?

Claim 3 is the central abarelix claim. Its elements require the following exact sequence:

Position	Abarelix residue
1	Ac-D-Nal
2	4-Cl-D-Phe
3	D-Pal
4	Ser
5	N-Me-Tyr
6	D-Asn
7	Leu
8	Lys(iPr)
9	Pro
10	D-Ala-NH2

A product with a different residue at any one of these positions would not literally satisfy claim 3. It could still fall within claim 1 or claim 2 if the replacement residue is listed in the relevant Markush alternatives.

For example:

Modified position	Example substitution	Potential claim effect
A	Pyro-Glu instead of Ac-D-Nal	Outside claim 3; potentially within claim 1 or 2
B	His instead of 4-Cl-D-Phe	Outside claim 3; potentially within claim 1 or 2
C	D-Nal instead of D-Pal	Outside claim 3; potentially within claim 1 or 2
E	Tyr instead of N-Me-Tyr	Outside claim 3; within claim 4
F	D-Gln instead of D-Asn	Outside claim 2; potentially within claim 1
G	Trp instead of Leu	Outside claim 3; potentially within claim 1 or 2
H	Arg instead of Lys(iPr)	Outside claim 3; potentially within claim 1 or 2
J	Gly-NH2 instead of D-Ala-NH2	Outside claim 3; potentially within claim 1 or 2

The patent therefore combines a broad genus with selected species claims. That structure is typical of peptide drug patents: the genus captures medicinal chemistry variants while the species claims target the clinical candidate.

When did US Patent 5,843,901 lose exclusivity?

US Patent 5,843,901 expired on December 1, 2015, based on the 17-year patent term applicable to the pre-URAA application that issued from the relevant filing. [1]

Event	Date
Earliest claimed priority	June 8, 1994
U.S. application filing	1995
Patent issued	December 1, 1998
Patent term basis	17 years from issue
Patent expiration	December 1, 2015
Current status	Expired

The priority date does not independently establish the expiration date. For this patent family, the controlling term resulted from the filing and transitional patent-term rules applicable to the application that issued as US 5,843,901.

No current U.S. patent exclusivity remains under this patent. Any historic Orange Book protection based on the patent ended when the patent expired.

What was the FDA and Orange Book status of abarelix?

The FDA approved abarelix, marketed as Plenaxis, on May 6, 2003, for palliative treatment of men with advanced symptomatic prostate cancer when luteinizing hormone-releasing hormone agonist therapy was not appropriate and immediate androgen deprivation was necessary. [2]

The product carried significant clinical restrictions because of the risk of serious hypersensitivity reactions. Treatment required administration under a controlled program, including observation after injection. FDA subsequently identified Plenaxis as discontinued from marketing. The discontinuation did not convert the patent into a current barrier to generic entry. [2,3]

FDA exclusivity

Abarelix received FDA marketing exclusivity associated with its original NDA. That regulatory exclusivity was separate from the patent term and expired years before US 5,843,901 expired.

Exclusivity category	Approximate end point
FDA approval	May 6, 2003
Five-year new chemical entity exclusivity, if applicable	May 6, 2008
US 5,843,901 patent expiration	December 1, 2015

The FDA regulatory exclusivity period did not extend beyond the patent expiration date.

Orange Book listing

US 5,843,901 was associated with the Plenaxis product and its active peptide coverage in FDA patent-listing records. The patent’s composition claims were more significant than its formulation claim because they reached the active ingredient itself. With expiration in 2015, the listing no longer creates a current patent block.

Were there Paragraph IV challenges or generic litigation?

The principal commercial constraint on generic entry was not an unresolved Paragraph IV dispute. The more important issues were:

Abarelix was a peptide product requiring complex manufacturing and characterization.
The approved product had a restricted clinical-use profile.
Plenaxis was discontinued.
The commercial market was limited.
Any ANDA sponsor would have faced peptide equivalence, analytical characterization and formulation issues.

A Paragraph IV certification could have challenged the patent before its expiration. The public record does not show a major reported U.S. Paragraph IV litigation campaign directed at US 5,843,901 comparable to the litigation surrounding major small-molecule blockbusters.

The absence of significant litigation is commercially explainable. A generic sponsor generally has less incentive to litigate an expired patent where the reference product is discontinued and the addressable market is small.

What manufacturing and formulation barriers affected generic entry?

The patent claims do not provide the only barriers to commercial competition. Abarelix is a synthetic peptide with multiple nonstandard structural elements:

D-amino acids.
An N-methylated tyrosine.
An isopropyl-substituted lysine.
A C-terminal D-alanine amide.
An acetylated N-terminal residue.
Potential salt-form and purity differences.

These features create analytical and process-control requirements. A competing manufacturer would need to control:

Sequence fidelity.
Stereochemical purity.
N-methylation and side-chain substitution.
Impurity profiles.
Aggregation and degradation.
Salt form and solid-state properties.
Sterility and injectable-product quality.
Bioequivalence or clinical comparability under the applicable FDA pathway.

Claim 5 could have created additional formulation exposure during the patent term, but it does not identify a distinctive depot system or proprietary excipient combination. The main historic IP barrier was the active peptide claims, not a narrowly defined delivery technology.

What other patents and drugs competed with abarelix?

Abarelix was part of a competitive GnRH therapy landscape that included both peptide antagonists and GnRH agonists.

Product	Active ingredient	Company or principal developer	Modality	Patent-landscape relevance
Plenaxis	Abarelix	Praecis Pharmaceuticals	Injectable GnRH antagonist	Covered by US 5,843,901 and related peptide patents
Antagon	Ganirelix	Organon	Injectable GnRH antagonist	Separate GnRH antagonist peptide estate
Cetrotide	Cetrorelix	ASTA Medica, later EMD Serono	Injectable GnRH antagonist	Separate peptide sequence and use patents
Firmagon	Degarelix	Ferring Pharmaceuticals	Injectable GnRH antagonist	Later peptide composition and depot-formulation estate
Lupron	Leuprolide	AbbVie/TAP	GnRH agonist	Different mechanism and peptide sequence
Orgovyx	Relugolix	Myovant, later Sumitomo Pharma	Oral GnRH antagonist	Small-molecule estate, not a peptide substitute at the patent-claim level

Degarelix became the more important injectable GnRH antagonist competitor in prostate cancer. Its peptide sequence and formulation technology are distinct from abarelix and are protected by a separate patent family. Relugolix competes pharmacologically but has a small-molecule composition and formulation estate unrelated to US 5,843,901.

How does US Patent 5,843,901 compare with formulation and method-of-use patents?

US 5,843,901 is primarily a compound patent with a dependent pharmaceutical-composition claim. It does not rely on a narrow dosing schedule or treatment protocol.

Patent category	Present in US 5,843,901?	Commercial effect
Broad peptide genus	Yes	Captures related sequence variants
Specific abarelix compound	Yes	Direct coverage of clinical candidate
Pharmaceutically acceptable salts	Yes	Extends coverage to salt forms
Pharmaceutical composition	Yes	Covers drug products containing claimed peptides
Specific excipient system	No express limitation	Limited formulation specificity
Depot formulation	No express limitation	No dedicated delivery-system claim
Method of treating prostate cancer	Not in the supplied claims	No direct method-of-use scope from these claims
Manufacturing process	Not in the supplied claims	Process protection would require separate claims or patents

Separate continuation, divisional or related applications may have addressed methods, intermediates or manufacturing processes. Those rights must be analyzed independently from the five claims supplied here. The expiration of US 5,843,901 does not automatically establish that every related family member expired on the same date.

What geographic coverage did the patent provide?

US 5,843,901 provided protection only in the United States. It did not itself create rights in Europe, Japan, Canada or other jurisdictions.

The international estate would have required separate national or regional patents. Geographic protection could differ because of:

National filing dates.
Patent-term rules.
Opposition or invalidity proceedings.
Supplementary protection certificates.
Terminal disclaimers.
Local claim amendments.
Patent-office examination outcomes.

For U.S. freedom-to-operate analysis after December 1, 2015, the expired patent is no longer an enforceable exclusion right. Foreign-market analysis requires separate review of the corresponding national patent families.

What is the current generic launch risk for abarelix?

The patent risk from US 5,843,901 is zero in the sense that the patent has expired. The commercial launch risk remains separate and depends on FDA approval, product availability and market economics.

A potential entrant would face:

No current blocking right under this patent.
No live composition claim from this patent.
No current Paragraph IV exposure to this expired patent.
Technical peptide-manufacturing requirements.
Potentially limited demand because Plenaxis is discontinued.
Regulatory work associated with an injectable peptide.
Clinical and pharmacovigilance concerns linked to hypersensitivity reactions.

A commercial challenger would likely evaluate whether the market supports development costs before investing in an abarelix product. The patent expiration removes the principal historical IP barrier but does not create a strong economic case for entry by itself.

Key Takeaways

US Patent 5,843,901 covers a broad Markush genus of GnRH antagonist decapeptides.
Claims 3 and 4 specifically cover abarelix and its Tyr analog.
Claim 5 covers pharmaceutical compositions containing the claimed peptides.
The patent issued December 1, 1998 and expired December 1, 2015.
Abarelix was approved by FDA as Plenaxis on May 6, 2003.
FDA exclusivity ended before the patent expired.
The patent is no longer an enforceable U.S. barrier to generic manufacture or sale.
Abarelix’s main residual entry barriers are peptide manufacturing, injectable-product regulation and limited commercial demand.
Degarelix, ganirelix, cetrorelix, leuprolide and relugolix rely on separate patent estates and should not be treated as covered by US 5,843,901.
Related patents in the abarelix family require separate expiration and claim analysis.

FAQs

Does US Patent 5,843,901 cover abarelix?

Yes. Claim 3 recites the abarelix peptide sequence, including Ac-D-Nal, 4-Cl-D-Phe, D-Pal, N-Me-Tyr, D-Asn and Lys(iPr).

Can a generic company make abarelix after 2015?

The expired patent no longer prevents manufacture or sale in the United States. FDA approval, product quality, manufacturing compliance and any other unexpired patent rights remain separate requirements.

Is abarelix a biologic subject to biosimilar approval?

No. Abarelix is a chemically synthesized peptide drug. It is not regulated as a monoclonal antibody or other conventional biologic requiring a biosimilar pathway. The applicable FDA pathway depends on the product’s regulatory classification and reference-product status.

Does claim 5 protect a specific Plenaxis injection formulation?

No specific excipient, dosage, depot technology or delivery device is recited in the supplied claim. Claim 5 broadly requires a claimed peptide and a pharmaceutically acceptable carrier.

Did US Patent 5,843,901 protect prostate-cancer treatment methods?

The supplied claims do not recite a method of treating prostate cancer. They cover compounds and a pharmaceutical composition. Any method-of-use protection would need to be found in a separate patent or claim set.

References

United States Patent and Trademark Office. (1998). U.S. Patent No. 5,843,901: GnRH antagonists.
U.S. Food and Drug Administration. (2003). FDA approves Plenaxis (abarelix for injectable suspension) for advanced symptomatic prostate cancer.
U.S. Food and Drug Administration. (n.d.). Plenaxis prescribing information.
U.S. Food and Drug Administration. (n.d.). Approved drug products with therapeutic equivalence evaluations: Orange Book.

Country	Patent Number	Estimated Expiration	Supplementary Protection Certificate	SPC Country	SPC Expiration
European Patent Office	0794961	⤷ Start Trial	91225	Luxembourg	⤷ Start Trial
European Patent Office	0794961	⤷ Start Trial	CA 2011 00004	Denmark	⤷ Start Trial
European Patent Office	0794961	⤷ Start Trial	C300484	Netherlands	⤷ Start Trial
European Patent Office	0794961	⤷ Start Trial	91857	Luxembourg	⤷ Start Trial
European Patent Office	0794961	⤷ Start Trial	11C0015	France	⤷ Start Trial
European Patent Office	0794961	⤷ Start Trial	1190014-9	Sweden	⤷ Start Trial
European Patent Office	0794961	⤷ Start Trial	SPC/GB11/006	United Kingdom	⤷ Start Trial
>Country	>Patent Number	>Estimated Expiration	>Supplementary Protection Certificate	>SPC Country	>SPC Expiration

Details for Patent: 5,843,901

Summary for Patent: 5,843,901