List of Excipients in Branded Drug RELISTOR

Relistor Excipient Strategy and Commercial Opportunities for Methylnaltrexone Bromide

Last updated: August 7, 2026

Relistor is a differentiated opioid-induced constipation product whose commercial value depends on two dosage-form platforms: oral methylnaltrexone bromide tablets and preservative-free subcutaneous injection. The strongest excipient opportunities are in generic substitution, oral bioavailability and tolerability optimization, prefilled-device compatibility, and differentiated 505(b)(2) products. Excipient protection alone is unlikely to provide a durable barrier unless it is tied to measurable dissolution, stability, device, or clinical-performance advantages.

What is Relistor and which dosage forms are commercially relevant?

Relistor contains methylnaltrexone bromide, a peripherally acting mu-opioid receptor antagonist. It is approved for opioid-induced constipation in adults with chronic non-cancer pain and for patients with advanced illness receiving palliative care when the response to laxative therapy has been insufficient [1,2].

Product	Strength	Route	Primary commercial use
Relistor tablets	150 mg	Oral	Opioid-induced constipation in chronic non-cancer pain
Relistor injection	8 mg/0.4 mL	Subcutaneous	Advanced illness and weight-based dosing
Relistor injection	12 mg/0.6 mL	Subcutaneous	Advanced illness and chronic non-cancer pain

The active molecule is a quaternary ammonium derivative of naltrexone. Its limited central nervous system penetration allows peripheral opioid antagonism without materially reversing central analgesia when used according to labeling [1].

Bausch Health commercializes Relistor in the United States through its Salix Pharmaceuticals business. The product originated from Progenics Pharmaceuticals and was licensed to Salix before Progenics was acquired by Xoma in 2020. Commercial rights, licensing scope, and regional ownership must be reviewed separately by jurisdiction because the U.S. business history does not establish global rights ownership [3,4].

What excipients are used in Relistor tablets?

Relistor tablets use a conventional immediate-release solid oral formulation with a film coat. FDA labeling identifies the inactive ingredients, which include common fillers, disintegrants, lubricants, coating agents, and opacifiers [1].

The tablet formulation includes:

• Microcrystalline cellulose
• Lactose monohydrate
• Croscarmellose sodium
• Magnesium stearate
• Colloidal silicon dioxide
• Hypromellose
• Polyethylene glycol
• Polyvinyl alcohol
• Talc
• Titanium dioxide

The excipient architecture supports standard high-volume tablet manufacture. Microcrystalline cellulose provides bulk and compaction. Lactose contributes additional diluent capacity. Croscarmellose sodium promotes tablet breakup, while magnesium stearate controls lubrication during compression. Silicon dioxide supports powder flow. The film-coating system protects the tablet surface and controls appearance rather than providing modified release.

What technical issues matter in the oral formulation?

Methylnaltrexone bromide presents several formulation considerations:

1. The bromide salt has high ionic character and must be evaluated for moisture sensitivity, hygroscopicity, and solid-state behavior.
2. The drug’s therapeutic effect depends on systemic exposure sufficient to reach peripheral opioid receptors, making dissolution and gastrointestinal performance commercially relevant.
3. Gastrointestinal adverse effects, including abdominal pain and diarrhea, create a commercial incentive to avoid unnecessary exposure peaks.
4. The product is administered orally at a relatively high 150 mg strength, increasing tablet-mass and manufacturability considerations.
5. Lactose-containing formulations may limit use in patients with lactose intolerance or create a differentiation opportunity for lactose-free versions.

The current immediate-release format leaves room for alternative excipient platforms, but an improved product would need a documented benefit. A simple substitution of lactose, lubricant, or disintegrant would generally be weak as a standalone regulatory or patent strategy.

What excipients are used in Relistor injection?

Relistor injection is a sterile, preservative-free solution supplied for subcutaneous administration. The formulation uses methylnaltrexone bromide in an aqueous vehicle with sodium chloride for tonicity adjustment and hydrochloric acid or sodium hydroxide for pH adjustment, with water for injection as the vehicle [2].

The injectable formulation places greater emphasis on:

• pH control
• Osmolality
• Chemical stability
• Particulate control
• Container-closure compatibility
• Extractables and leachables
• Syringe and needle performance
• Sterile manufacturing yield

The formulation is supplied in single-dose presentations. That design reduces preservative exposure and simplifies use in palliative-care and outpatient settings. A ready-to-use prefilled syringe can create practical value by reducing dose-preparation steps, particularly for home administration.

Which injectable excipient opportunities are commercially credible?

The principal opportunity is not adding more excipients. It is optimizing the existing low-excipient solution around stability and delivery.

Potential development areas include:

Opportunity	Technical objective	Commercial value
Low-sorption container system	Reduce drug loss to syringe or vial components	Better delivered-dose consistency
Silicone and tungsten control	Reduce particulate and interaction risk	Improved device quality and regulatory robustness
pH optimization	Increase chemical stability and injection tolerability	Longer shelf life or lower degradation
Ready-to-use prefilled syringe	Eliminate preparation steps	Home-use and specialty-pharmacy differentiation
Alternative primary container	Improve compatibility with autoinjectors	Device-enabled product lifecycle extension
Low-volume presentation	Reduce injection burden	Potential adherence and patient-preference benefit
Terminal sterilization assessment	Improve manufacturing economics if feasible	Lower aseptic-processing cost

Any injectable reformulation must preserve sterility, potency, appearance, particulate limits, and extractables/leachables performance. For a mature product, a formulation patent based only on routine pH or tonicity adjustment would face substantial obviousness and enablement risk.

When does Relistor lose exclusivity?

Relistor’s original U.S. new chemical entity exclusivity would have expired years ago. The injection was approved in 2008, and the oral tablet was approved in 2016 [1,2]. The original approval dates therefore do not provide a current market-exclusivity barrier.

The relevant commercial question is current patent and regulatory protection, not NCE exclusivity. Relistor may have had or may continue to have Orange Book-listed patents covering approved methods of use, formulations, or other aspects of the product. Current status must be determined from the FDA Orange Book and patent records for the specific NDA and presentation [5].

Protection category	Relistor relevance	Commercial implication
NCE exclusivity	Historical, expired	Does not block current generic development
Three-year clinical exclusivity	Potentially relevant to the 2016 tablet approval at the time of approval	Historical barrier only
Method-of-use patents	Possible	Could limit label-conforming generic use if unexpired
Formulation patents	Possible	Could protect a specific dosage form or composition
Device patents	More relevant to prefilled injection systems	May affect injectable substitution
Pediatric exclusivity	No current basis established here	Requires FDA database confirmation
Orphan-drug exclusivity	The advanced-illness indication should not be assumed to have orphan protection	Requires FDA orphan designation review

A generic applicant can seek approval through an ANDA if it can show pharmaceutical equivalence and bioequivalence. A 505(b)(2) pathway may be more appropriate for a reformulated product, a new device, a new dosage form, or a product supported by literature and bridging data.

What is the Orange Book status of Relistor?

The Orange Book is the controlling source for current listed patents, patent-use codes, exclusivity, and NDA reference-product information [5]. Relistor’s commercial analysis should separate the tablet and injection NDAs because patent listings and regulatory barriers can differ by presentation.

A review should identify:

• NDA numbers for each dosage form
• Current patent numbers listed against each NDA
• Expiration dates and pediatric extensions
• Use codes for opioid-induced constipation indications
• Whether any patents have been delisted, expired, or disputed
• Paragraph IV certification activity
• ANDA approvals or tentative approvals
• Authorized-generic arrangements

A formulation patent listed in the Orange Book does not automatically block all generic products. Its impact depends on claim scope, the patent-use code, the generic label, and the applicant’s certification strategy.

Which companies are challenging Relistor with generics?

The principal competitive threat is from generic methylnaltrexone products rather than biosimilars. Methylnaltrexone bromide is a small molecule, so biosimilar regulation under the Public Health Service Act does not apply.

Potential generic entrants may target:

• 150 mg oral tablets
• 8 mg/0.4 mL injection
• 12 mg/0.6 mL injection
• Vial presentations
• Prefilled syringes, if the reference-device requirements can be met

A complete current list of ANDA applicants, Paragraph IV challengers, approval dates, and litigation status depends on live FDA and federal court records. The economic risk remains highest for the oral tablet because it is easier to manufacture, distribute, and substitute through retail or specialty-pharmacy channels.

What Paragraph IV risks exist for Relistor?

A Paragraph IV certification alleges that a listed patent is invalid, unenforceable, or will not be infringed by the proposed generic. The filing can trigger patent litigation and a statutory 30-month stay of approval under specified conditions [6].

For Relistor, likely challenge targets would include:

• Method-of-use patents covering opioid-induced constipation
• Tablet composition or manufacturing patents
• Injection formulation patents
• Device or presentation patents, where listed and applicable

Design-around options could include a carved-out label for patented uses, an alternative excipient system, a different container closure, or a non-infringing manufacturing process. The value of a Paragraph IV challenge depends on the remaining patent term, market size, settlement terms, and the likelihood of a first-filer position.

What formulation patents could protect a Relistor follow-on product?

The strongest patent claims would connect the excipient system to a measurable technical result. Examples include:

• A specific methylnaltrexone particle-size distribution producing defined dissolution performance
• A tablet composition with a narrow ratio of diluent, disintegrant, and lubricant
• A moisture-protective coating that improves stability under defined humidity conditions
• A formulation that reduces degradation products during long-term storage
• A low-pH injectable solution with defined impurity limits
• A prefilled syringe formulation that limits adsorption or particulate formation
• A ready-to-use device with a defined delivered-volume tolerance
• A long-acting or depot formulation with controlled peripheral opioid-antagonist exposure

Weak claims would cover routine excipient substitutions without comparative data. A credible patent package should include accelerated and long-term stability, dissolution profiles, impurity characterization, container-closure data, and, where relevant, pharmacokinetic or clinical evidence.

How does Relistor compare with competing opioid-induced constipation drugs?

Relistor competes with several approved products that use different active ingredients and dosage forms.

Product	Active ingredient	Route	Key differentiation
Relistor	Methylnaltrexone bromide	Oral and subcutaneous	Oral and injectable options; palliative-care use
Movantik	Naloxegol	Oral	Once-daily oral PAMORA
Symproic	Naldemedine	Oral	Once-daily oral PAMORA
Amitiza	Lubiprostone	Oral	Chloride-channel activator, not a PAMORA
Trulance	Plecanatide	Oral	Guanylate cyclase-C agonist
Linzess	Linaclotide	Oral	Guanylate cyclase-C agonist

Relistor’s injectable option is its clearest platform distinction. Oral competition is stronger because Movantik and Symproic offer once-daily oral PAMORA therapy. Relistor’s commercial positioning is therefore sensitive to tablet cost, generic substitution, adherence, tolerability, and payer coverage.

What commercial opportunities exist for Relistor excipients?

Generic tablet opportunity

The largest near-term opportunity is a cost-efficient 150 mg tablet. A generic manufacturer could pursue a conventional immediate-release formulation using widely available excipients. Competitive advantages would come from:

• Lower tablet cost
• Robust direct-compression manufacturing
• Reduced coating complexity
• Lactose-free composition
• Improved stability in high-humidity markets
• Efficient packaging and bottle-count configuration

The principal risk is that a technically acceptable formulation may still face low margins if several generic suppliers enter simultaneously.

Reformulated oral product

A 505(b)(2) product could target a defined clinical or usability benefit:

• Lower gastrointestinal peak-related intolerance
• Improved tablet disintegration under variable gastric conditions
• Reduced tablet size
• Orally disintegrating or sprinkle formulation
• Liquid formulation for patients with swallowing difficulty
• Lactose-free or low-sodium formulation
• Combination packaging with a bowel regimen for palliative-care use

A modified-release formulation would require careful pharmacokinetic justification. Excessive delay or prolonged exposure could reduce the rapid bowel-response profile expected from a rescue-oriented product.

Injectable and device opportunity

The injectable product offers a more defensible lifecycle strategy than routine oral excipient changes. Opportunities include:

• Prefilled syringe conversion
• Autoinjector compatibility
• Lower dead-volume syringe design
• Compact home-use packaging
• Improved cold-chain tolerance
• Longer shelf life
• Fewer preparation steps for hospice providers

Device integration may create separate patentable subject matter and commercial differentiation. It also creates more complex human-factors, combination-product, and container-closure requirements.

Regional excipient strategy

Geographic expansion may require different excipient choices because of local availability, climatic stability, and regulatory expectations. A formulation designed for the United States may need additional stability work for hot and humid markets. Suppliers should qualify multiple sources for lactose, microcrystalline cellulose, croscarmellose sodium, and coating polymers to reduce supply-chain risk.

Excipient manufacturers can pursue preferred-supplier status by offering:

• Compendial grades with consistent particle-size distribution
• Low-endotoxin injectable materials
• Documentation for nitrosamine and elemental-impurity control
• Change-control support
• Regional regulatory dossiers
• Co-processed excipients for direct compression

What manufacturing and intellectual-property barriers affect market entry?

For tablets, the main barriers are bioequivalence, dissolution matching, process validation, and reliable high-dose compression. These are manageable for experienced generic manufacturers.

For injections, barriers are higher:

• Sterile fill-finish capacity
• Container-closure qualification
• Particulate control
• Syringe or vial compatibility
• Extractables and leachables testing
• Device performance
• Aseptic-process validation
• More complex approval documentation

The injection’s manufacturing complexity can delay entry even after patent barriers fall. It can also preserve price differentials between oral and injectable products.

What litigation and settlement issues should investors monitor?

Relistor diligence should monitor:

1. Federal patent cases involving the NDA holder and ANDA applicants.
2. Paragraph IV notices and 30-month-stay calculations.
3. Generic approval or tentative-approval dates.
4. Settlement agreements with authorized-launch dates.
5. Authorized-generic launches.
6. Patent delisting or use-code changes.
7. FDA labeling changes affecting indication carve-outs.
8. State substitution rules for injectable presentations.

Settlement agreements can materially change the effective generic-entry date. A nominal patent expiration date is therefore not always the practical launch date.

Key Takeaways

• Relistor contains methylnaltrexone bromide and is marketed as 150 mg tablets and preservative-free subcutaneous injection.
• The tablet uses conventional excipients, including microcrystalline cellulose, lactose, croscarmellose sodium, magnesium stearate, and film-coating materials.
• The injection uses a simple aqueous vehicle with sodium chloride and pH adjustment, placing greater value on container, device, sterility, and stability engineering.
• NCE exclusivity is historical and no longer the principal barrier to generic entry.
• Oral generic competition presents the highest substitution risk.
• Injectable reformulation and prefilled-device strategies offer stronger lifecycle-management potential.
• Excipient patents are most defensible when tied to improved stability, dissolution, impurity control, delivered dose, or device performance.
• Relistor has no biosimilar pathway because methylnaltrexone bromide is a small molecule.
• Current Orange Book listings, ANDA activity, Paragraph IV challenges, patent litigation, and settlement dates must be evaluated by NDA and presentation.
• Commercial opportunities include low-cost tablets, lactose-free formulations, orally disintegrating products, prefilled syringes, autoinjectors, and climate-stable regional products.

Frequently Asked Questions

Can methylnaltrexone be formulated without lactose?

Yes. A lactose-free tablet could use microcrystalline cellulose, mannitol, dibasic calcium phosphate, or another suitable diluent. The product would require comparative dissolution, stability, manufacturability, and bioequivalence support.

Is Relistor suitable for an autoinjector?

The subcutaneous solution may be technically compatible with an autoinjector, but the final design would require device compatibility, delivered-volume validation, human-factors testing, sterility data, and combination-product regulatory review.

Could an excipient change create a new Relistor patent?

Yes, but routine substitution is weak subject matter. A stronger patent would claim a defined composition linked to unexpected stability, dissolution, impurity, delivery, or clinical performance.

Does Relistor have biosimilar competition?

No. Methylnaltrexone bromide is a chemically synthesized small molecule. Potential competitors would use the ANDA or 505(b)(2) pathways rather than the biosimilar pathway.

Which Relistor presentation is most attractive for generic development?

The 150 mg tablet is generally the simpler target because it avoids sterile manufacturing and device qualification. The injection may offer higher technical barriers and better pricing, but it requires materially greater manufacturing and regulatory investment.

References

1. U.S. Food and Drug Administration. (2023). Relistor (methylnaltrexone bromide) tablets: Prescribing information. FDA.

2. U.S. Food and Drug Administration. (2023). Relistor (methylnaltrexone bromide) injection: Prescribing information. FDA.

3. Bausch Health Companies Inc. (2024). Annual report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934. Bausch Health Companies Inc.

4. XOMA Corporation. (2020). Progenics Pharmaceuticals acquisition and portfolio information. XOMA Corporation.

5. U.S. Food and Drug Administration. (2024). Approved drug products with therapeutic equivalence evaluations: Orange Book. FDA.

6. U.S. Food and Drug Administration. (2024). Abbreviated new drug application regulations and patent certifications. FDA.