US Patent 4,866,168: Scope, Claims, Expiration, and Hematoporphyrin Derivative Patent Landscape
US Patent 4,866,168 protects a purification process for tumor-localizing hematoporphyrin derivative, particularly the isolation of fluorescent, photosensitizing porphyrin aggregates with molecular weight of at least approximately 10,000 daltons. The patent also covers a composition prepared by that process.
The patent issued on September 12, 1989. Under the pre-Uruguay Round Agreement Act patent-term rule applicable to the patent, its 17-year term from issuance expired on September 12, 2006. The patent therefore no longer creates an enforceable US exclusivity barrier. Its principal commercial relevance is historical: it relates to the purification and manufacture of porfimer sodium and earlier hematoporphyrin derivative products used in photodynamic therapy.
What does US Patent 4,866,168 cover?
The patent covers a process that transforms a hematoporphyrin derivative preparation into a tumor-localizing porphyrin aggregate fraction. The essential technical steps are:
- Start with hematoporphyrin derivative prepared by treating hematoporphyrin hydrochloride with acetic acid and sulfuric acid.
- Raise the pH of the aqueous preparation to between 6.5 and 12.
- Form or enrich aggregates having a molecular weight of at least approximately 10 kDa.
- Separate those aggregates from the remaining hematoporphyrin derivative material.
- Recover a porphyrin composition that is fluorescent, photosensitizing, and preferentially localizes in tumor cells.
The patent does not broadly claim every hematoporphyrin derivative, every porphyrin photosensitizer, or every photodynamic therapy method. Its central subject is a specific aggregate-enrichment and separation process.
Claim 1: the principal process claim
Claim 1 is the broadest independent process claim. It requires all of the following:
| Claim element |
Scope |
| Starting material |
A hematoporphyrin derivative preparation |
| Starting-material history |
The preparation must have been made by treating hematoporphyrin hydrochloride with acetic acid and sulfuric acid |
| Medium |
Aqueous medium |
| pH adjustment |
pH raised to 6.5 to 12 |
| Product fraction |
Aggregates of approximately 10 kDa or greater |
| Separation |
Aggregates separated from the remainder of the preparation |
| Product characteristics |
Fluorescent, photosensitizing, tumor-localizing, and retained in tumor cells longer than in normal tissues |
| Composition language |
The recovered composition consists essentially of the specified porphyrin aggregates |
A process that omits the specified acid treatment, does not raise pH into the claimed range, or does not separate the 10 kDa-or-greater fraction would have a substantial noninfringement position under the literal language of claim 1.
The phrase "consisting essentially of" permits incidental components that do not materially affect the claimed porphyrin aggregate properties. It excludes ingredients that materially change the composition's fluorescence, photosensitizing activity, tumor localization, aggregate profile, or retention characteristics.
How do the dependent claims narrow the patent scope?
Claims 2 through 9 add operational limitations to claim 1. A process must satisfy claim 1 and the additional limitation of the relevant dependent claim.
| Claim |
Additional limitation |
Practical effect |
| 2 |
pH approximately 9.5 |
Narrows the pH adjustment to the preferred operating point |
| 3 |
Filtration retaining material above 10,000 molecular weight |
Covers ultrafiltration or an equivalent size-retention step |
| 4 |
pH maintained at 6.5 to 12 during filtration |
Adds process-control requirements during separation |
| 5 |
pH maintained at approximately 9.5 during filtration |
Narrowest filtration and pH combination |
| 6 |
Separation by affinity chromatography |
Limits separation to the claimed chromatographic approach |
| 7 |
Reverse-phase column using 5-micron spheres and THF |
Narrows claim 6 to a specific column and solvent configuration |
| 8 |
C18 column, specified solvent sequence, and fourth FIG. 5 fraction |
Highly narrow process claim tied to a particular chromatographic protocol |
| 9 |
Detailed preparation of the hematoporphyrin derivative |
Requires specified acid mixing, stirring, sodium acetate precipitation, and recovery steps |
| 10 |
Composition prepared by claim 1 |
Product-by-process claim directed to the resulting porphyrin composition |
Claim 8 has the narrowest practical scope. It requires the process to use a C18 reverse-phase column, 5-micron packing, a specified methanol-water-acetic acid elution, followed by tetrahydrofuran-water elution, and recovery of the fourth component identified in Figure 5.
What does the patent require regarding 10 kDa porphyrin aggregates?
The patent is directed to high-molecular-weight porphyrin aggregates rather than a single molecular species. The claims use the terms "10 kd aggregates," "10 kd or greater," and "above 10,000" molecular weight.
This creates several claim-construction issues:
- The relevant threshold is approximately 10,000 daltons.
- The claims appear to target an aggregate fraction, not necessarily a chemically uniform compound.
- Molecular-weight measurement methodology may affect whether a material falls within the claimed range.
- The functional limitations may require the recovered material to exhibit the stated fluorescence, photosensitization, tumor localization, and retention properties.
- A process yielding a high-molecular-weight fraction for a different purpose may dispute whether the product satisfies the claimed functional characteristics.
The patent does not establish a simple purity percentage, a fixed ratio of porphyrin components, or a single molecular structure. That makes the patent potentially difficult to practice-test analytically, but it also limits the claim's reach to materials meeting the aggregate and functional requirements.
What hematoporphyrin derivative preparation is required?
Claim 1 requires a hematoporphyrin derivative preparation made by treating hematoporphyrin hydrochloride with a mixture of acetic acid and sulfuric acid. Claim 9 provides a more detailed version of that preparation:
- Add an acetic acid and sulfuric acid mixture to hematoporphyrin hydrochloride.
- Stir for one hour.
- Add the resulting mixture to a 5% sodium acetate solution.
- Form a red precipitate.
- Recover the red precipitate.
This starting-material limitation is commercially important. A manufacturer using a different synthetic route, a different acid system, or a different precipitation protocol could argue that the starting material falls outside claim 1, even if the final product contains similar porphyrin aggregates.
The claims therefore combine composition identity with manufacturing history. The historical process route is a core limitation, not an incidental example.
What formulations are protected by US 4,866,168?
The patent does not broadly claim injectable formulations, vial configurations, stabilizers, excipients, dosage strengths, or administration protocols. Its composition claim is tied to the product made by the claimed process.
The patent does not expressly claim:
- A particular sterile injectable formulation;
- A defined porfimer sodium concentration;
- A lyophilized dosage form;
- A pharmaceutical carrier;
- A dosing schedule;
- A specific light source or wavelength;
- A tumor-treatment method;
- A combination with chemotherapy or radiation.
Any protection for those features would need to arise from separate patents or regulatory exclusivity, not from the claims provided for US 4,866,168.
When did US Patent 4,866,168 lose exclusivity?
| Event |
Date |
| Patent issued |
September 12, 1989 |
| Statutory term under pre-URAA rule |
17 years from issuance |
| Expected expiration |
September 12, 2006 |
| Current status |
Expired |
| Current enforceable patent exclusivity |
None |
Because the patent issued before June 8, 1995, the applicable term was generally 17 years from grant rather than 20 years from the earliest effective nonprovisional filing date. No patent-term extension should be assumed without a specific regulatory extension record. The patent is more than 19 years past its expected expiration date.
An expired patent may remain relevant for technical history, prosecution analysis, freedom-to-operate benchmarking, and prior-art review. It cannot ordinarily support a new infringement claim for conduct occurring after expiration.
What is the Orange Book status of US 4,866,168?
US 4,866,168 should not be treated as a current Orange Book exclusivity patent.
Porfimer sodium was approved by the FDA as Photofrin under NDA 020451. The original approval covered photodynamic therapy indications, including treatment of obstructing or completely obstructing esophageal cancer in appropriate patients. The FDA labeling identifies Photofrin as porfimer sodium, a porphyrin-based photosensitizing agent. [2]
US 4,866,168 is a manufacturing and composition patent, not a conventional method-of-use patent directed to an approved indication. Its 2006 expiration also eliminates any current patent-listing value for an FDA-approved product.
Orange Book analysis should distinguish among:
- Listed patents for the approved drug;
- Unlisted manufacturing patents;
- Expired historical patents;
- Regulatory exclusivity;
- Patent protection for later formulations or new indications.
The patent does not create current Orange Book protection for porfimer sodium.
What FDA exclusivity applied to Photofrin?
FDA regulatory exclusivity is separate from patent term. Photofrin received FDA approval in 1995. Any original new-drug exclusivity associated with the approval would have expired long before the present date. The 2006 expiration of US 4,866,168 occurred more than a decade after the initial US approval.
Photofrin is a chemically defined porphyrin-based drug product rather than a therapeutic protein. A later entrant would generally evaluate an abbreviated new drug application, a 505(b)(2) application, or another FDA pathway based on product characteristics and the reference product. A biosimilar application under section 351(k) is not the ordinary pathway for porfimer sodium.
Does the patent create biosimilar risk?
No conventional biosimilar risk arises from US 4,866,168.
Porfimer sodium is not ordinarily analyzed as a monoclonal antibody, recombinant protein, or other biologic subject to a standard biosimilar competition model. The key regulatory and technical issues for a competing product would more likely include:
- Active-ingredient characterization;
- Porphyrin composition and aggregate distribution;
- Molecular-weight profile;
- Photophysical properties;
- Impurity profile;
- Manufacturing reproducibility;
- Tumor-localization and photosensitizing performance;
- FDA requirements for pharmaceutical equivalence or clinical bridging.
The patent could historically have complicated manufacture of a product with the same high-molecular-weight porphyrin fraction. Its expiration removes that patent barrier.
What generic entry risks exist for porfimer sodium?
The main post-expiration risks are technical and regulatory rather than patent-based.
A competing manufacturer would need to address the complexity of porfimer sodium as a porphyrin mixture or aggregate-containing product. The relevant barriers may include:
- Reproducible control of porphyrin aggregate size;
- Consistent high-molecular-weight fraction;
- Batch-to-batch photodynamic activity;
- Analytical comparability;
- Sterility and injectable-product controls;
- Limited market size relative to development cost;
- Clinical and commercial access to photodynamic therapy centers.
These are manufacturing and market-entry barriers, not continuing exclusivity under US 4,866,168.
A product made through a different process could avoid the expired process patent, although regulatory comparability would remain a separate issue. A product made through the exact claimed process would not face infringement liability based on this patent after expiration.
How strong was the patent estate?
Historical strength
The patent had meaningful historical value because claim 1 combined:
- A defined starting-material history;
- A pH-driven aggregation step;
- A molecular-weight threshold;
- A separation step; and
- Functional tumor-localization properties.
That combination would have made the claim more difficult to avoid through minor changes than a claim directed only to a final porphyrin composition.
Weaknesses in claim scope
The principal weaknesses were:
- The claims were process-centered rather than directed to all uses of porfimer sodium.
- The starting-material history narrowed claim 1.
- The 10 kDa threshold could raise analytical and measurement questions.
- Functional language could create proof requirements in an infringement case.
- Claims 7 and 8 covered specific chromatography conditions and therefore had limited reach.
- The patent did not cover every route to producing tumor-localizing porphyrin aggregates.
- The composition claim was linked to the process of claim 1.
Current strength
| Issue |
Current assessment |
| Enforceability |
None after expiration |
| Relevance to new US launch |
Historical only |
| Orange Book value |
No current value |
| Biosimilar barrier |
Not applicable in the conventional sense |
| Manufacturing barrier |
Technical and regulatory, not patent-based |
| Prior-art value |
Potentially relevant for aggregate purification and porphyrin compositions |
| Litigation leverage |
None based solely on this expired patent |
What patent litigation or settlements affect US 4,866,168?
The patent's expiration materially reduces the commercial significance of historical litigation. No current Paragraph IV challenge is necessary to clear this patent because an abbreviated-product applicant can target the expired patent or exclude it from the certification analysis, depending on the applicable FDA listing and filing posture.
A complete litigation history would require review of PACER, USPTO assignment and maintenance records, FDA Orange Book editions, and the patent prosecution file. The patent number and claims alone do not establish a particular infringement judgment, settlement agreement, license, or covenant not to sue. The enforceability conclusion, however, is unaffected by those historical matters: an expired patent cannot support prospective infringement liability.
Which companies are competing in the porfimer sodium market?
Photofrin was historically associated with QLT Inc. and later commercial partners. The product's commercial position has been constrained by the specialized clinical infrastructure required for photodynamic therapy and by competition from other photosensitizers and treatment modalities.
Relevant competitive categories include:
- Porfimer sodium and hematoporphyrin derivative products;
- Chlorin-based photosensitizers;
- Phthalocyanine photosensitizers;
- Other approved or investigational photodynamic therapy agents;
- Endoscopic, surgical, radiation, and systemic alternatives for the same cancer indications.
The patent did not create a permanent franchise over the entire photodynamic therapy field. It addressed one important purification route and product fraction within the broader field.
Key Takeaways
- US 4,866,168 covers a process for isolating tumor-localizing porphyrin aggregates of approximately 10 kDa or greater.
- Claim 1 requires the specified hematoporphyrin hydrochloride, acetic acid, and sulfuric acid starting-material history.
- The claim also requires pH adjustment to 6.5 to 12 and separation of the high-molecular-weight fraction.
- Claims 2 through 9 narrow the process through pH, filtration, chromatography, solvent, column, and precipitation limitations.
- Claim 10 is a composition-by-process claim tied to the process of claim 1.
- The patent issued September 12, 1989, and its 17-year statutory term expired September 12, 2006.
- The patent is not a current US exclusivity barrier for porfimer sodium.
- It does not create current Orange Book protection or biosimilar-style exclusivity.
- Remaining entry barriers are primarily analytical, manufacturing, clinical, regulatory, and commercial.
- Historical importance should not be confused with current enforceability.
FAQs
Is US Patent 4,866,168 still enforceable?
No. Its expected 17-year term from the September 12, 1989, issue date expired on September 12, 2006.
Does US 4,866,168 cover Photofrin itself?
It covers a process for preparing a defined high-molecular-weight hematoporphyrin derivative fraction and a composition prepared by that process. It does not broadly claim every form of Photofrin or every porfimer sodium use.
Can a company manufacture porfimer sodium using a different process?
The expired patent does not create current infringement risk. During the patent term, a different process could also have avoided specific process limitations, subject to the final product and other patent claims.
Is porfimer sodium eligible for a biosimilar application?
Porfimer sodium is not ordinarily treated as a standard biologic for section 351(k) biosimilar development. A competing product would require a pathway determined by its regulatory classification and product characteristics.
Does the patent cover photodynamic therapy methods?
Not in the claims provided. The claims focus on preparation and the resulting composition. Treatment methods, light parameters, dosing schedules, and combination therapies would require separate claim language.
References
-
United States Patent and Trademark Office. (1989). Purified hematoporphyrin derivative, U.S. Patent No. 4,866,168.
-
U.S. Food and Drug Administration. (n.d.). Photofrin (porfimer sodium) prescribing information. Drugs@FDA.
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U.S. Food and Drug Administration. (n.d.). Approved drug products with therapeutic equivalence evaluations. Orange Book.
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35 U.S.C. ยง 154. (2024). Contents and term of patent; provisional rights.
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21 C.F.R. pt. 314. (2024). Applications for FDA approval to market a new drug.
