United States Patent 5,441,958: Scope, Claims Map, and Patent Landscape for Topical Antihistaminic Treatment of Allergic Conjunctivitis
What does US 5,441,958 claim cover?
US 5,441,958 claims a method of treating allergic conjunctivitis by topical ocular administration of a specific benzimidazole antihistamine-type compound (or its ophthalmically acceptable acid addition salts). The independent claim is narrow on (i) indication, (ii) route, and (iii) compound identity, while the dependent claims narrow further on formulation concentration and specific salt identity.
Claim set provided (verbatim scope elements)
- Method for treating allergic conjunctivitis by topically administering to an affected eye an antihistaminic effective amount of a compound selected from:
- 1-(2-ethoxyethyl)-2-(4-methyl-1-homopiperazinyl)-benzimidazole, or
- ophthalmically acceptable acid addition salts of that compound.
- Method of claim 1 where the compound is 0.0001 to 1.0 wt%.
- Method of claim 2 where the compound is 0.005 to 0.2 wt%.
- Method of claim 3 where the compound is 0.05 to 0.2 wt%.
- Method of claim 1 where the compound is the difumarate salt of 1-(2-ethoxyethyl)-2-(4-methyl-1-homopiperazinyl)-benzimidazole.
Core claim logic: the patent ties the therapeutic method to a single chemical scaffold (benzimidazole derivative) used as an ocular antihistaminic via aqueous/topical formulation with specified weight-percent bands and, in one dependent claim, a specific salt form (difumarate).
What are the enforceable boundaries of claim 1?
Indication boundary: allergic conjunctivitis only
Claim 1 is limited to “treating allergic conjuctivitis” (spelling as provided). Any therapeutic use directed to:
- non-allergic conjunctivitis,
- infectious conjunctivitis,
- dry eye disease,
- blepharitis,
- or broader “ocular allergy” claims not framed as allergic conjunctivitis,
falls outside the literal wording of claim 1 as provided.
Route boundary: topical administration to the affected eye
Claim 1 requires topically administering to an affected eye. That language tends to exclude:
- systemic dosing (oral, injectable),
- intravitreal injection,
- periocular injection (unless construed as topical ocular administration),
- contact lens drug delivery (depends on whether it is argued as topical administration; claim language says “topically administering to an affected eye,” which is usually interpreted as topical ocular application).
Pharmacologic activity boundary: “antihistaminic effective amount”
The claim uses a functional phrase: antihistaminic effective amount. This can create a factual and evidentiary question in enforcement (dose efficacy tied to antihistaminic effect), but it also limits the scope to compounds with antihistaminic therapeutic effect in the claimed indication.
Chemical identity boundary: one compound family plus salts
The compound must be selected from:
- 1-(2-ethoxyethyl)-2-(4-methyl-1-homopiperazinyl)-benzimidazole
- ophthalmically acceptable acid addition salts of that compound.
That does not appear to cover:
- free base only unless the free base falls under “selected from” (it does, as the first member of the group),
- non-salt derivatives (e.g., prodrugs, quaternary ammonium variants unless they qualify as acid addition salts),
- salts not argued as “ophthalmically acceptable,” though a defendant could attempt to argue lack of ophthalmic acceptability.
How do dependent claims narrow the composition and dosage form?
Claim 2 and claim 3 and claim 4 convert claim 1’s “effective amount” into formulation concentration windows measured in wt%.
Concentration ladder (wt%)
| Claim |
Concentration range of active compound (wt%) |
Litigation relevance |
| 2 |
0.0001 to 1.0 |
Broadens enforcement to most ophthalmic strengths while still requiring topical ocular dosage |
| 3 |
0.005 to 0.2 |
Narrows to more typical ophthalmic actives range |
| 4 |
0.05 to 0.2 |
Most protective, targeted range |
Enforcement implication: A product formulation that falls outside all stated ranges risks avoiding those dependent claims, but it can still fall within claim 1 if it uses the same compound as an antihistaminic effective amount for allergic conjunctivitis by topical ocular administration.
Salt specificity: difumarate (claim 5)
Claim 5 states the compound comprises the difumarate salt of the benzimidazole base.
This adds a second axis of narrowing:
- even if a competitor uses the same base molecule, using a different salt form can target design-around by leaving claim 5.
- even if concentration falls within claim 4, the salt switch can still avoid claim 5 while remaining exposed to claims 1-4 if the salt is still an “ophthalmically acceptable acid addition salt” under claim 1.
What does the claim language likely mean for design-around strategy?
Strongest design-around levers
- Different active chemical
- Avoids claim 1 entirely unless the new compound is argued to fall within the specific listed compound group.
- Different therapeutic indication framing
- If the drug is marketed or prescribed exclusively for non-allergic indications, enforcement depends on facts and evidence; literal scope is indication-specific.
- Different route
- Systemic antihistamine for conjunctivitis does not fit “topically administering to an affected eye.”
- Different salt form to avoid claim 5
- Salt selection alone usually does not avoid claim 1 because claim 1 covers “ophthalmically acceptable acid addition salts.”
- Concentration outside dependent windows
- This can help avoid claims 2-4 but not necessarily claim 1.
Salt substitution nuance (claim construction pressure)
Because claim 1 already includes acid addition salts, swapping salts to avoid claim 5 will not automatically avoid claim 1 unless the competitor:
- uses a salt that is argued not to be an “ophthalmically acceptable acid addition salt,” or
- uses a derivative that is not an acid addition salt, or
- claims a different chemical identity than the base compound.
What is the patent landscape likely to look like around this specific chemical and method?
The landscape for a US method patent like 5,441,958 typically clusters into three buckets:
1) Same compound, same indication, same route (direct overlaps)
These are patents that claim:
- allergic conjunctivitis,
- topical ophthalmic delivery,
- antihistaminic action,
- similar benzimidazole derivatives or acid addition salt forms.
Direct overlap risk is highest when:
- the claims use the same base compound identity,
- the dependent claims recite similar wt% windows,
- and salt forms include difumarate or broadly cover ophthalmically acceptable acid addition salts.
2) Same compound, different formulation or dosing metrics (secondary overlaps)
Even if the active compound identity is the same, competitors can pursue:
- different concentration windows (outside 0.0001 to 1.0 wt% or outside 0.005 to 0.2 wt% or 0.05 to 0.2 wt%),
- different excipient systems or viscosity/tonicity attributes (often claimed in separate patents),
- different dosing regimens (frequency, duration) rather than concentration.
Those patents can coexist with 5,441,958 if they avoid the wt% concentration bands or avoid “antihistaminic effective amount” recitation, but the independent claim 1 remains a method-of-treatment hook if the marketed product is still within the method definition.
3) Different compounds, same therapeutic class (frequent competitive adjacent art)
Allergic conjunctivitis has a crowded competitive field. Adjacent patents often include:
- H1 antihistamines in other chemical classes,
- mast cell stabilizers,
- combination therapies (antihistamine + mast cell stabilizer),
- or dual-acting agents.
If those use non-identical actives, they usually do not overlap the literal compound selection of claim 1, though they can overlap in practical clinical effect and could be used in freedom-to-operate analysis as commercial risk even if they are not infringement risks for this exact patent.
Practical infringement map: how a product would land against each claim
Claim 1 infringement checklist
A product is likely within claim 1 when it satisfies all:
- Topical ocular administration to treat allergic conjunctivitis
- Uses an active that is:
- 1-(2-ethoxyethyl)-2-(4-methyl-1-homopiperazinyl)-benzimidazole, or
- an ophthalmically acceptable acid addition salt of that base
- Administers an antihistaminic effective amount.
Claim 2-4 infringement checklist (formulation wt%)
If claim 1 is met, dependent claim exposure depends on whether the formulation’s active concentration in the administered composition falls within:
- 0.0001 to 1.0 wt% (claim 2)
- 0.005 to 0.2 wt% (claim 3)
- 0.05 to 0.2 wt% (claim 4)
Claim 5 infringement checklist (difumarate)
If the product uses the same base but uses a different salt:
- it likely avoids claim 5 (difumarate-specific), while still facing claim 1 if its salt is still an ophthalmically acceptable acid addition salt.
Where the real value of the patent sits: claim structure
US 5,441,958 is structured to maximize coverage at two levels:
- Chemical identity + route + indication (claim 1)
- This is the hardest to route around without changing the active.
- Concentration bands + salt form (claims 2-5)
- This pressures competitors on formulation strength and salt selection.
That combination is a common tactic for ophthalmic actives: lock the compound identity and therapeutic method, then capture common ophthalmic strengths and a specific salt preferred by development.
Key Takeaways
- US 5,441,958 claims a topical ocular method for allergic conjunctivitis using 1-(2-ethoxyethyl)-2-(4-methyl-1-homopiperazinyl)-benzimidazole or ophthalmically acceptable acid addition salts.
- Dependent claims lock active concentration (wt%) into 0.0001 to 1.0, 0.005 to 0.2, and 0.05 to 0.2 wt% bands.
- Claim 5 targets a specific formulation variant: the difumarate salt.
- Commercial design-around is most effective by changing active identity or route/indication framing; salt and concentration changes primarily manage risk for specific dependent claims, not claim 1.
FAQs
1) Does the patent cover the free base or only salts?
It covers the base compound and its ophthalmically acceptable acid addition salts under claim 1.
2) If a competitor uses the same base compound but a different salt, is it still potentially within claim 1?
Yes, unless the alternative salt is credibly argued to be not an “ophthalmically acceptable acid addition salt.” Claim 5 specifically requires difumarate, but claim 1 is broader.
3) What formulation strengths are explicitly claimed?
Claims 2-4 define wt% windows of 0.0001 to 1.0, 0.005 to 0.2, and 0.05 to 0.2 for the active.
4) Can systemic antihistamines avoid infringement?
Yes, on the face of claim language, because claim 1 requires topical administration to the affected eye.
5) Is the therapeutic scope limited to allergic conjunctivitis only?
Yes. The method is explicitly directed to treating allergic conjunctivitis, which anchors the indication limitation.
References
[1] United States Patent No. 5,441,958. “Method for treating allergic conjunctivitis.” (Claims as provided).
