US 4,251,510: What the Claims Actually Require, and How the Landscape Looks

What does US 4,251,510 claim, in operational terms?

US 4,251,510 claims a sterile injectable aqueous formulation built around a specific kind of human plasma protein fraction, with quantified limits on Factor XIIa activity and kinin-generation/kinin content, plus constraints on the fraction’s protein composition. The claims are formulation-and-impurity centric, not a new manufacturing unit-operation claim.

Claim 1: Core specification that drives novelty and infringement

Claim 1 requires all of the following in the final injectable solution:

Sterility and aqueous injection vehicle
- “A sterile aqueous solution … suitable for injection.”
Dose and concentration anchor
- About 5 grams of Plasma Protein Fraction (Human) per 100 mL of solution.
- This implies ~5% w/v (aligned to dependent claim 2).
Factor XIIa impurity limit (activity-based)
- No greater than about 16 milli-units per milliliter Factor XIIa and fragments thereof capable of activating prekallikrein.
Kinin-related impurity limits (equivalents and absolute)
- No greater than about 10 nanograms per milliliter bradykinin equivalents per milliliter of kininogen.
- Less than about 10 nanograms per milliliter of bradykinin.
Defined fraction composition and profile
- The Plasma Protein Fraction (Human) is a mixture of constituent plasma proteins isolated from human plasma by a modified Cohn fractionation process.
- Composition constraints:
  - At least 83% albumin
  - No more than 17% alpha and beta globulins (globulins with “molecular weights similar to that of albumin”)
  - No more than 1% gamma globulin
Process is referenced through product definition
- “Modified Cohn fractionation process” defines the type of fraction, not the exact steps in claim language.
- This matters for enforcement: courts often look at whether an accused product meets the claim-imposed composition and impurity parameters, not whether its steps match the specification.

Claim 2: Tightens concentration to the claimed dose level

Protein concentration is about 5 percent.

Claim 3: Adds a stabilizer from a narrow, named set

Stabilizer selected from:
- sodium acetyltryptophan
- sodium caprylate

Practical takeaway: US 4,251,510 is best viewed as an “albumin fraction” injectable with controlled activation of the contact/kinin pathway (Factor XIIa → prekallikrein activation → bradykinin/kinins), plus a specific composition profile and optional stabilizers.

What do these claims try to solve, mechanistically?

The claim language encodes a product-quality target:

Lower Factor XIIa activity limits initiation of the contact activation system.
The bradykinin equivalents and bradykinin limits cap the final kinin burden.
The albumin-dominant composition with restricted globulins and gamma globulin reduces protein species associated with contaminant carryover or kinin system activation propensity.
The defined manufacturing heritage (“modified Cohn”) indicates a fractionation approach designed to reduce biologically active impurities.

This is consistent with the typical safety risk management objective in plasma-derived injectables: preventing kinin-mediated reactions, often described in the industry around bradykinin and contact activation.

What must an accused product prove to infringe, claim-by-claim?

Claim 1 infringement checklist (all elements)

An accused sterile aqueous human plasma protein fraction product must satisfy each of these elements:

Element in Claim 1	Claim requirement (hard numbers)	Infringement implication
Form	sterile aqueous solution suitable for injection	product must be a sterile injectable formulation
Dose	~5 g per 100 mL	~5% w/v in the administered solution
Factor XIIa impurity	≤ ~16 milli-units/mL Factor XIIa + fragments that activate prekallikrein	activity-based assay and definitional scope of “fragments capable”
Bradykinin burden	≤ ~10 ng/mL bradykinin equivalents of kininogen	equivalency assay linked to kininogen substrate
Bradykinin content	< ~10 ng/mL bradykinin	direct bradykinin measurement threshold
Fraction type	plasma proteins via modified Cohn fractionation	product must be the defined albumin-rich fraction
Composition	≥83% albumin; ≤17% alpha/beta globulins (MW similar to albumin); ≤1% gamma globulin	composition-based product profile

Claim 2 infringement refinement

If a product is not at ~5% protein concentration, it may not meet claim 2, but claim 1 still allows “about” 5 g/100 mL. The practical infringement question becomes whether a given formulation is still within “about” under claim construction.

Claim 3 infringement refinement

If the product uses a stabilizer not in the two named options, claim 3 is narrower and may not be met even if claim 1 is.

Key enforcement dynamic: for plasma-derived fraction products, infringement disputes often reduce to whether the accused product’s measured assays hit the specified impurity thresholds and whether its composition matches the claimed albumin/globulin/gamma limits. Manufacturing route matters only if it affects those outcomes or is used to interpret “modified Cohn fractionation.”

What is the novelty posture of these claims?

US 4,251,510 does not claim the existence of an albumin fraction in general. It claims a specific formulation with specific impurity specifications tied to a specific fraction profile and a dose/concentration.

How the claim text positions novelty

The defining differentiators are:
1. Quantified Factor XIIa and kinin limits
2. Albumin-rich fraction profile (83% albumin; restricted globulins)
3. Modified Cohn fractionation heritage
4. Sterile injectable format at ~5%

This is a typical “product-by-process-plus-specs” pattern: the product is functionally defined by impurity limits and compositional constraints, with process language used for fraction identification.

Where does the landscape likely concentrate: albumin products and kinin-control specifications

This patent sits in a known commercial and regulatory space: plasma-derived albumin products and other plasma fractions that have historically confronted contamination-driven adverse events. Even when the brand names differ, the functional control target (contact system and bradykinin formation) tends to recur across formulations.

Competitive positioning inside the albumin/kinin control space

In the broader US market, “human albumin” and related plasma fractions typically vary by:

method of fractionation (e.g., Cohn-derived variants),
purification steps and inactivation/removal controls,
excipients and stabilizers,
packaging and sterile filtration,
and specifications for biologically active contaminants (including kallikrein/kinin pathway elements).

US 4,251,510 targets a narrow slice:

5% solution
albumin-rich fraction with capped globulins/gamma
and quantified kinin pathway limits.

How do similar patents and prior art usually attack these claim elements?

Even without listing specific references here, the landscape analysis for this claim set typically falls into three predictable attack vectors:

Published plasma fraction specifications
- If earlier patents or literature describe a modified Cohn-derived albumin fraction with impurity control for the Factor XIIa/prekallikrein axis and bradykinin endpoints in sterile aqueous injectable form, claim 1’s novelty erodes.
Equivalence of impurity testing to known specs
- If prior art uses different measurement units or assay definitions that map to the same biological endpoint, the thresholds may be argued as already disclosed.
“About” and ranges
- Claim 1 uses “about,” which can be used by defendants to fit products slightly outside exact figures while still argued within claim scope.
- Conversely, plaintiffs can argue that the prior art disclosure includes the same “about” region.

Critical point for this patent: the strongest claims are those that are tight on impurity metrics and composition percentages. If prior art discloses those values or clearly overlapping ranges, invalidity risk increases sharply.

What about claim 3: stabilizer selection between two named options?

Claim 3 is a classic narrow dependent claim:

It limits the formulation to stabilizers from a two-member set:
- sodium acetyltryptophan
- sodium caprylate

Landscape implication

Stabilizer selection is common across albumin formulations. Many competitors historically use stabilizers in protein injectables to reduce denaturation and adsorption. If either named stabilizer is already used in the same type of 5% albumin fraction formulation with comparable impurity control, claim 3 becomes more vulnerable. Its enforceability depends on whether claim 1 is itself already met or already disclosed in the prior art and whether the stabilizer addition is already in the same configuration.

Where do these claims sit in terms of enforceability risk?

Claim fragility comes from three constraints that must all be satisfied

Hard numeric thresholds for Factor XIIa/kinins
Specific composition profile (albumin ≥83%, gamma ≤1%)
Dose/concentration (about 5 g/100 mL)

If the market has moved toward different specs (for instance tighter than the patent or using different fraction compositions), infringement may narrow to a smaller set of products. For invalidity, however, if even one earlier disclosure comes close on impurity targets and fraction composition, the patent’s “clean room” shrinks.

Composition gating can be both strength and weakness

Strength: “≥83% albumin” and “≤1% gamma globulin” can exclude many fraction variants.
Weakness: if competitors measure broadly similar composition profiles (even if they use different characterization methods), they can still be accused of meeting the thresholds.

What is the most actionable way to read US 4,251,510 for business decisions?

1) Target the assays, not the marketing labels

For freedom-to-operate or post-grant strategy, the technical enforcement center is:

Factor XIIa activity assay at or below 16 milli-units/mL
bradykinin equivalents associated with kininogen at or below 10 ng/mL
bradykinin content below 10 ng/mL
plus compositional percentages (albumin/globulin/gamma)

A product meeting the composition and dose but not the impurity endpoints is outside claim 1.

2) Treat “modified Cohn fractionation” as a claim construction lever

Because claim 1 does not enumerate specific process steps, “modified Cohn fractionation” will likely be interpreted by what fraction types qualify in industry practice and by whether the product matches the defined compositional outcome. That means process-dependent patents may still be invalidated or avoided by demonstrating that another process yields the same final profile.

3) Stabilizer strategy only matters if claim 1 already aligns

If a competitor’s product satisfies claim 1, then adding or removing the named stabilizer options becomes the remaining differentiator for claim 3 only. If claim 1 fails, claim 3 is irrelevant.

Key Takeaways

US 4,251,510 is a formulation-specification patent: sterile injectable aqueous albumin-rich plasma fraction at ~5% with capped Factor XIIa activity and capped bradykinin/kinin equivalents, plus composition constraints (albumin ≥83%, gamma ≤1%) tied to a “modified Cohn” fraction type.
Claim 1 is the core and is narrow only in ways that matter for infringement: impurity endpoints, composition percentages, and ~5% concentration.
Claim 2 is concentration tightening with “about” leaving some interpretive room.
Claim 3 is narrow and stabilizer-dependent: only two named stabilizers qualify; stabilizer choice matters only if claim 1 is already met.
Landscape risk concentrates on any prior disclosures that provide the same albumin-rich fraction composition plus comparable control of Factor XIIa and bradykinin endpoints in sterile injectable solutions.

FAQs

1) What ingredient “controls” infringement in US 4,251,510?

The “control” is the combination of the plasma fraction type and its quantified impurity endpoints: Factor XIIa activity (≤16 milli-units/mL), bradykinin equivalents (≤10 ng/mL), and bradykinin (<10 ng/mL), plus compositional limits (albumin ≥83%, gamma ≤1%) at ~5% protein.

2) Does the patent protect the manufacturing method directly?

The claims reference “modified Cohn fractionation” to define the fraction type but do not enumerate manufacturing steps in the claim text. The enforceable boundaries are the product outcomes described by composition and impurity thresholds.

3) If a competitor uses the same albumin fraction but different stabilizers, is it outside the patent?

It may still infringe claim 1 if all claim 1 elements match. Different stabilizers only affect claim 3, which is limited to sodium acetyltryptophan and sodium caprylate.

4) What is the practical meaning of “about 5 grams per 100 milliliters”?

It sets the formulation protein concentration to around 5% w/v. “About” expands the boundary beyond a single exact value, but infringement will still be anchored to the overall claimed concentration region.

5) Which assays matter most in disputes for this patent?

Disputes will focus on the specified endpoints tied to the kinin/contact system: Factor XIIa activity capable of activating prekallikrein, and quantified bradykinin and bradykinin equivalents in the injectable product, together with albumin/gamma composition profiling.

References

United States Patent 4,251,510. “Plasma protein fraction (human) and process.” (Claims text provided in prompt).

Applicant	Tradename	Biologic Ingredient	Dosage Form	BLA	Approval Date	Patent No.	Expiredate
Grifols Therapeutics Llc	PLASMANATE	plasma protein fraction (human)	Injection	101140	October 02, 1958	4,251,510	1999-08-15
>Applicant	>Tradename	>Biologic Ingredient	>Dosage Form	>BLA	>Approval Date	>Patent No.	>Expiredate

Patent: 4,251,510

Summary for Patent: 4,251,510