Executive summary
US Drug Patent 5,427,798 is a formulation-and-performance patent focused on controlled/sustained-release bupropion hydrochloride tablets using hydroxypropyl methylcellulose (HPMC) with narrowly bounded polymer:bupropion ratios, surface-area-to-volume (SA:V) ranges, specific water release kinetics (percent released at 1/4/8 hours), and a stability shelf-life window under defined temperature/humidity. Dependent claims add glycine hydrochloride or cysteine hydrochloride as excipient modulators, optional instant-release film coating, dose- and SA:V-specific embodiments (50/100/150 mg), and human plasma level banding over 24 hours. The enforceable scope is therefore concentrated on tablets that meet the claimed HPMC-based release profile and related structural/performance parameters, not on bupropion generally or other release technologies.
US Patent 5,427,798 scope: what does each claim actually cover?
Claim 1: core protected invention (HPMC + specific release kinetics + SA:V + stability)
Independent Claim 1 defines a controlled sustained release tablet with these essential elements:
- Active and dosage range: 25 to 500 mg bupropion hydrochloride.
- Polymer: hydroxypropyl methylcellulose (HPMC).
- Polymer:bupropion ratio: amount of HPMC “to one part of bupropion hydrochloride being 0.19 to 1.1.”
- Geometry parameter: tablet surface-to-volume ratio 3:1 to 25:1 cm⁻¹.
- Shelf-life requirement: “at least one year at 59° to 77°F and 35 to 60% RH.”
- Release kinetics in water:
- 20% to 60% in 1 hour
- 50% to 90% in 4 hours
- ≥75% in 8 hours
Practical scope interpretation
- “Controlled sustained release” is tied to objective lab outcomes: percent released at 1/4/8 hours in water.
- “Surface to volume ratio” is a structural constraint.
- Shelf-life constraint in defined storage conditions adds another objective gate for infringement analysis where formulation stability evidence is available.
Claim 2: excipient modifier (glycine HCl or cysteine HCl)
Depends on Claim 1 and adds:
- Glycine hydrochloride or cysteine hydrochloride at 0.027 to 0.27 parts by weight relative to bupropion hydrochloride.
This claim expands coverage to tablets meeting the Claim 1 performance/structure while additionally incorporating one of the specified amino-acid salts in the specified relative amount.
Claim 3: admixture form (particles + both excipients)
Independent-like dependent from Claim 1 language: admixture of:
- 25 to 500 mg bupropion hydrochloride particles
- HPMC
- glycine HCl or cysteine HCl
- with SA:V 3:1 to 25:1 cm⁻¹
Claim 3 is significant because it explicitly requires bupropion particles (not merely dissolved drug) in an admixture system with the excipient(s).
Claim 4: outer instant-release film coating option
Depends on Claim 1:
- optional instant release film coating as an outer coating.
This suggests at least some claim coverage for layered release strategies where the core maintains the sustained-release profile, while the outer layer gives an initial dose fraction.
Claims 5–6–7–8: narrower dependent embodiments
- Claim 5: same as Claim 2 but narrower amino-acid salt amount: 0.05 to 0.162 parts by weight.
- Claim 6: admixture with 50 to 150 mg bupropion, HPMC, cysteine HCl, and SA:V 7:1 to 16:1 cm⁻¹.
- Claim 7: similar but allows glycine HCl, SA:V 7:1 to 16:1 cm⁻¹, dose 50 to 500 mg.
- Claim 8: film coating for the Claim 6/7 tablets.
These dependent claims create multiple “entry points” into infringement depending on the exact formulation and dose.
Claims 9–12: dose-specific SA:V bands
These are formulation geometry sub-clauses for Claim 6/7:
- Claim 9: about 50 to 150 mg bupropion; SA:V 7:1 to 16:1 cm⁻¹
- Claim 10: about 50 mg; SA:V 13:1 to 16:1 cm⁻¹
- Claim 11: about 100 mg; SA:V 9:1 to 12:1 cm⁻¹
- Claim 12: about 150 mg; SA:V 7:1 to 10:1 cm⁻¹
These can matter in litigation because SA:V is not a generic property and is product-size/product-shape dependent.
Claims 13–16: human plasma exposure matching (FIG. 4–6)
These claims add in vivo performance constraints:
- Claim 13: admixture of 50 mg bupropion HCl + HPMC; after oral dosing to adult men, plasma levels of bupropion (free base) fall within a minimum-to-maximum band “as shown in FIG. 4 over 24 hours.”
- Claim 14: same for 100 mg, tied to FIG. 5.
- Claim 15: same for 150 mg, tied to FIG. 6.
- Claim 16: film coating provided “thereover” for Claims 13–15.
Practical scope interpretation
- This is not merely “intended use.” It is an objective performance envelope that requires that the accused product’s in-human profile falls within the claimed band.
- If a challenger makes bioequivalent design changes, even with the same in vitro release kinetics, they may still fall outside these plasma bands.
Claims 17–19: “means for releasing” with dose-specific percent-release ranges
These claims are directed to “means comprising HPMC” rather than enumerating SA:V and shelf-life like Claim 1. They require:
- Claim 17: tablet containing 50 mg bupropion HCl; HPMC as release means; release in distilled water:
- 30% to 50% at 1 hour
- 70% to 95% at 4 hours
- ≥80% at 8 hours
- Claim 18: 100 mg; release:
- 25% to 45% at 1 hour
- 60% to 85% at 4 hours
- ≥80% at 8 hours
- Claim 19: 150 mg; release:
- 25% to 45% at 1 hour
- 60% to 85% at 4 hours
- ≥80% at 8 hours
Claim architecture note
- The “means” language can be read as HPMC-based sustained release achieving those quantitative dissolution/release performance bounds, aligning them as an enforceable formulation-performance combination.
What is the patent landscape around US 5,427,798?
What the claim set signals about competitive positioning
US 5,427,798 is the kind of patent that is typically used to block:
- generic sustained-release bupropion tablets built around HPMC matrix systems, and
- products with similar release curves in water/distilled water within narrow percent windows at 1/4/8 hours.
Because the claim set includes SA:V and shelf-life (Claim 1) and human plasma bands (Claims 13–15), the strongest infringement cases tend to be against products that preserve the inventor’s design space, not merely “bupropion sustained release” in general.
Where carve-outs are most likely to occur
From the claims alone, noninfringement design space would often be created by moving at least one hard parameter outside the claimed window, such as:
- changing release technology away from HPMC matrix behavior, or
- altering release curve so that percent released at 1 hour or 4 hours falls outside the claimed ranges, or
- changing tablet geometry so SA:V is outside the bands, or
- changing stability so shelf-life conditions do not qualify, or
- for the FIG-based claims, producing a human plasma curve outside the FIG envelope.
How strong is the patent estate for competitors seeking generic or AB-rated entry?
Claim strength drivers
US 5,427,798 claim language is built around measurable and testable parameters:
- In vitro release kinetics (percent at 1/4/8 hours in water/distilled water)
- SA:V structural constraint
- Shelf-life defined under specific temperature and humidity ranges
- In vivo plasma exposure bands over 24 hours (FIG 4–6)
- Defined excipient salt options (glycine HCl/cysteine HCl) and amounts
- Optional film coating (a potential design-around area but also a coverage path)
These objective limitations can increase the ability to establish or refute infringement with dissolution data and, for FIG-based claims, pharmacokinetic evidence.
Claim scope compression
The presence of multiple narrow ranges means:
- A product can avoid Claim 1 by being outside any of the tight constraints (polymer ratio, SA:V, kinetics, shelf-life).
- Dependent claims can add additional constraints that can still be avoided even if Claim 1 is met.
Litigation leverage points
In an infringement dispute, the most leverage is typically concentrated in:
- The 1-hour and 4-hour release windows (first two timepoints constrain curve shape).
- The SA:V range (geometry changes are often controllable via tooling and formulation bulk properties).
- The FIG plasma bands (if asserted, they require human PK alignment).
What patents are likely co-pending or related (family context) for this chemistry?
This cannot be fully and accurately mapped from the claim text alone. A full “landscape” requires at least one of: prosecution history, patent family publications, or the Orange Book listing and associated reference patents. With only the claims provided, an exhaustive family map would risk being incomplete or incorrect.
Timeline: when does exclusivity end for US 5,427,798?
A precise exclusivity/expiration timeline cannot be computed from the claim text alone. It requires:
- filing date,
- priority date,
- term adjustments,
- patent term extension (if any),
- and whether there are overlapping listed patents for the relevant FDA product.
FDA Orange Book status and generic entry risk for bupropion sustained release
A defensible Orange Book status assessment is not possible from claim language alone. Determining:
- whether US 5,427,798 is listed for the relevant NDA for bupropion sustained release,
- and which strength and dosage forms are covered,
requires Orange Book listing data and label-NDA mapping.
Key takeaways
- US 5,427,798 protects a specific HPMC-controlled sustained-release bupropion hydrochloride tablet design space defined by polymer ratio, SA:V, dissolution/release kinetics at 1/4/8 hours, and stability under defined conditions (Claim 1).
- Excipient amino-acid salts (glycine HCl or cysteine HCl) in specified ratios are covered by multiple dependent claims.
- Tablet geometry (SA:V) is a major differentiator; multiple dependent claims specify SA:V bands by dose.
- In vivo PK banding tied to FIG. 4–6 adds an additional enforceability layer beyond dissolution.
- For competitive products, the strongest noninfringement path is to move at least one critical parameter outside the claimed windows, especially the 1-hour/4-hour release percentages, SA:V, or the human plasma band requirements.
FAQs
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Does US 5,427,798 cover all bupropion sustained-release tablets?
No. It requires bupropion hydrochloride tablets formulated with hydroxypropyl methylcellulose and meeting specific release kinetics and, in Claim 1, SA:V and stability criteria.
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What dissolution timepoints matter most under the independent claim?
Claim 1 constrains 1 hour (20% to 60%), 4 hours (50% to 90%), and 8 hours (≥75%) release in water.
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Can a film-coated product still fall within the patent?
Yes. Claims 4 and 8 and Claim 16 explicitly include embodiments with an instant-release film coating layered over the sustained-release tablet.
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How do the FIG-based claims increase infringement risk?
Claims 13–15 require that, after oral dosing, plasma free-base bupropion levels in adult men fall within minimum-to-maximum bands shown in FIG. 4–6 over 24 hours, making it harder to rely only on in vitro testing.
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Are glycine hydrochloride and cysteine hydrochloride interchangeable under the claims?
They are interchangeable as options in the relevant dependent claims, where the language covers either glycine hydrochloride or cysteine hydrochloride in stated relative amounts.
References
- United States Patent No. 5,427,798. “Controlled sustained release tablets comprising bupropion hydrochloride and hydroxypropyl methylcellulose.” (Claims as provided in prompt).
