List of Excipients in Branded Drug PRIMAXIN

Company	Tradename	Ingredient	NDC	Excipient	Potential Generic Entry
Merck Sharp & Dohme LLC	PRIMAXIN	imipenem and cilastatin sodium	0006-3514	SODIUM BICARBONATE

PRIMAXIN Excipient Strategy and Commercial Opportunities for Imipenem/Cilastatin

Last updated: August 3, 2026

PRIMAXIN is a sterile carbapenem antibiotic containing imipenem and cilastatin sodium. Its commercial opportunity is no longer centered on basic active-ingredient substitution. The stronger opportunities are differentiated injectable presentations, improved reconstitution and infusion handling, container-closure performance, reduced preparation time, and supply reliability. The core PRIMAXIN IV formulation uses sodium bicarbonate as a formulation excipient and is supplied as a sterile powder requiring dilution before intravenous administration. PRIMAXIN IM uses a lidocaine-containing diluent for intramuscular administration. [1]

What is PRIMAXIN and how is it formulated?

PRIMAXIN is Merck’s imipenem/cilastatin product for parenteral use. Imipenem provides antibacterial activity, while cilastatin inhibits renal metabolism of imipenem. Cilastatin is an active pharmaceutical ingredient, not an excipient.

PRIMAXIN product configurations

Product	Route	Strengths described in labeling	Core presentation
PRIMAXIN IV	Intravenous infusion	250 mg/250 mg and 500 mg/500 mg	Sterile powder in vial, diluted before infusion
PRIMAXIN IM	Intramuscular injection	500 mg/500 mg and 750 mg/750 mg presentations have been described in product information	Sterile powder with lidocaine-containing diluent
Generic imipenem/cilastatin injection	Intravenous or intramuscular, depending on product	Product-specific	Vial, premix, or other approved presentation

The IV formulation contains imipenem and cilastatin sodium equivalent to the labeled active strengths, with sodium bicarbonate used to support formulation pH and stability. The product is preservative-free. [1]

PRIMAXIN IV is reconstituted and further diluted for infusion. The label identifies compatible infusion solutions, including 0.9% sodium chloride and 5% dextrose, while compatibility restrictions apply to some solutions, including lactated Ringer’s solution. [1]

What excipients are used in PRIMAXIN?

The principal disclosed formulation excipients are sodium bicarbonate in PRIMAXIN IV and lidocaine hydrochloride in the IM presentation’s diluent.

Sodium bicarbonate in PRIMAXIN IV

Sodium bicarbonate has several formulation functions:

• It helps control the pH of the dry formulation and reconstituted solution.
• It supports chemical stability of imipenem, which is sensitive to aqueous degradation.
• It allows the product to be supplied as a sterile powder rather than a ready-made aqueous solution.
• It contributes to the ionic environment of the reconstituted product.

The amount of sodium bicarbonate is formulation-specific and should be taken from the applicable approved label and chemistry, manufacturing and controls record. Public product labeling identifies sodium bicarbonate as an ingredient, but commercial development decisions require direct review of the current reference-product and generic-product regulatory files. [1]

Lidocaine hydrochloride in PRIMAXIN IM

The IM product uses a lidocaine-containing diluent to reduce injection discomfort. Lidocaine is a functional excipient in this presentation, but it also creates additional regulatory and medical-use constraints.

The lidocaine-containing presentation requires attention to:

• Hypersensitivity to amide-type local anesthetics
• Maximum lidocaine exposure
• Pediatric use
• Cardiovascular and neurologic safety
• Compatibility with the imipenem/cilastatin powder
• Separate labeling for the diluent and administration route

This excipient strategy differentiates IM from IV products but increases the regulatory burden. It also limits universal substitution because a hospital may prefer an IV-only formulation or may avoid lidocaine-containing products in selected patients.

Which excipient attributes control PRIMAXIN product performance?

The main technical challenge is imipenem stability after reconstitution. The commercial value of an excipient strategy therefore depends on the full product system, not only on the ingredient list.

Critical quality attributes

Attribute	Commercial relevance
Reconstituted-solution stability	Determines usable time after preparation
pH	Affects degradation, tolerability, and compatibility
Color and clarity	Supports pharmacy quality control
Particulate burden	Affects injectable-product acceptance
Reconstitution time	Determines nursing and pharmacy labor
Osmolality	Affects infusion tolerability
Container-closure integrity	Protects sterility and moisture-sensitive powder
Compatibility with infusion bags and tubing	Reduces administration errors
Stability in common diluents	Expands hospital usability
Sterility assurance	Required for parenteral approval

A new excipient must not reduce imipenem or cilastatin stability, generate particulates, interfere with assay methods, or create an unacceptable impurity profile. The excipient must also be justified under FDA pharmaceutical-quality expectations for injectable products. [2][3]

What excipient strategies could improve PRIMAXIN?

The most credible strategies are incremental rather than radical. A reformulator would need to improve workflow or stability without creating a new safety issue.

1. Improved dry-powder stability

The existing sterile-powder model avoids the substantial aqueous instability risk associated with a ready-to-use solution. Opportunities include:

• Optimizing buffer concentration
• Controlling residual moisture
• Using tighter moisture-barrier vials or overwraps
• Improving cake or powder morphology
• Reducing reconstitution variability
• Developing a more robust lyophilized presentation

Sodium bicarbonate is already part of the PRIMAXIN IV formulation. Replacing or supplementing it with another buffer would require comparative stability, impurity, pH, osmolality, and compatibility data.

2. Ready-to-use or extended-stability presentations

A ready-to-use or extended-use product could reduce pharmacy preparation and administration burden. This is technically difficult because imipenem degrades more rapidly in solution than in dry form.

Potential formats include:

• Frozen premixed bags
• Pharmacy-ready dual-chamber containers
• Short-use liquid vials
• Extended-stability diluted solutions
• Automated-compounding-system-compatible containers

The commercial value would be highest in hospitals with centralized pharmacy compounding, high emergency-antibiotic utilization, or limited staffing. The principal risk is that the required cold chain, freezer handling, or short in-use dating could offset the benefit.

3. Dual-chamber delivery systems

A dual-chamber vial or bag could separate the dry imipenem/cilastatin powder from the diluent until administration. This approach could provide:

• Longer shelf life before activation
• Less pharmacy manipulation
• Lower contamination risk
• More consistent reconstitution
• Better compatibility control

The device becomes part of the regulatory product. Extractables and leachables, seal integrity, activation force, dose delivery, and human-factors validation become material development requirements.

4. Infusion compatibility improvements

A product that is validated with a broader range of diluents, bags, and tubing could gain practical value. The development program would need to assess:

• 0.9% sodium chloride
• 5% dextrose
• Common extension sets and infusion lines
• Y-site administration
• Temperature exposure
• Light exposure
• Contact time
• Precipitation and particulate formation

Compatibility claims must be supported by product-specific data. A broad commercial claim cannot be inferred from the reference product’s general labeling.

5. Preservative-free multidose alternatives

A preservative-free single-dose vial is aligned with hospital injectable practice. A multidose product could reduce packaging and preparation waste, but it would introduce sterility, antimicrobial-preservation, and repeated-entry concerns. For a high-risk injectable antibiotic, the regulatory and infection-control burden would likely be substantial.

6. Improved IM presentation

The IM opportunity is narrower but distinct. A reformulated IM product could target injection-volume reduction, reduced pain, or simplified preparation. Possible approaches include:

• More concentrated powder
• Lower-volume diluent
• Alternative local-anesthetic strategy
• Ready-to-use IM syringe
• Improved reconstitution behavior

Any change involving lidocaine would require separate evaluation of local tolerability, systemic exposure, contraindications, and labeling.

What formulation patents protect PRIMAXIN?

The original PRIMAXIN formulation and basic imipenem/cilastatin composition are mature technologies. Broad composition-of-matter protection for imipenem and cilastatin is not expected to provide a meaningful current barrier to generic entry.

The commercially relevant intellectual-property categories are more likely to include:

• Specific powder formulations
• Stabilized aqueous solutions
• Lyophilization processes
• Container-closure systems
• Premixed or dual-chamber devices
• Extended-stability compositions
• Specific infusion or administration methods
• Manufacturing processes that improve impurity control

Patent protection must be assessed by claim scope and jurisdiction. A patent covering a particular buffer concentration or container system may not block a conventional generic vial. Conversely, a formulation patent with broad stability or delivery-system claims could affect a differentiated injectable product.

Patent and exclusivity position

Protection category	Current commercial significance
Imipenem composition of matter	Mature and generally no longer the primary barrier
Cilastatin composition of matter	Mature and generally no longer the primary barrier
Original PRIMAXIN formulation	Legacy protection; current enforceability requires claim-by-claim review
Product-specific formulation patents	Potentially relevant for differentiated products
Device and container patents	Relevant to dual-chamber and ready-to-use products
Method-of-use patents	Limited value for a broad antibacterial indication
Regulatory exclusivity	Historic exclusivity periods have expired
Biosimilar exclusivity	Not applicable because PRIMAXIN is a small-molecule drug

PRIMAXIN does not have biosimilar risk. The relevant competitive threat is generic imipenem/cilastatin, not a biosimilar pathway.

When did PRIMAXIN lose exclusivity and what is the generic entry risk?

PRIMAXIN’s original regulatory and patent exclusivity periods have expired. FDA-approved generic imipenem/cilastatin products have entered the market, creating conventional ANDA competition. Generic applicants can rely on the reference product’s safety and efficacy findings through the abbreviated new drug application pathway, subject to applicable patent certifications and regulatory requirements. [4]

Paragraph IV challenge risk

Paragraph IV risk is most relevant to any later-listed formulation or delivery patent, not to the historical core product alone. A generic applicant may certify that a listed patent is invalid, unenforceable, or will not be infringed. The patent holder may then bring litigation, potentially triggering a 30-month stay under the Hatch-Waxman framework, subject to statutory conditions. [5]

For a mature injectable antibiotic, commercial risk typically comes from:

• Multiple generic suppliers
• Hospital contract pricing
• Shortages or allocation events
• Differences in authorized vial sizes
• Product-specific stability claims
• Distribution reliability

The presence of a formulation patent does not automatically prevent generic entry. The key questions are whether the patent is listed in the Orange Book, whether the generic product must practice the claimed formulation, and whether a non-infringing alternative is commercially practical.

What is the Orange Book status of PRIMAXIN?

The FDA Orange Book identifies approved drug products, reference-listed drugs, patents, and exclusivity information. PRIMAXIN’s current Orange Book status should be checked by exact product name, dosage form, strength, and NDA. The relevant listing is expected to be historical for the original imipenem/cilastatin product, while current market competition is driven by approved generic products and their product-specific labeling. [4]

Orange Book review should distinguish:

• PRIMAXIN IV from PRIMAXIN IM
• Original NDA listings from generic ANDA listings
• Expired patents from active listed patents
• Product patents from method-of-use patents
• Reference-product status from current commercial availability

A formulation developer should not assume that a legacy Orange Book listing covers a newly designed premix, dual-chamber system, or alternative excipient package.

What FDA regulatory pathway applies to a new PRIMAXIN formulation?

A conventional generic with the same active ingredients, dosage form, route, strength, and essential characteristics would generally proceed through an ANDA if it can demonstrate pharmaceutical equivalence and bioequivalence or otherwise satisfy FDA’s requirements.

A materially different product may require a 505(b)(2) application. This could apply to a product with:

• A new route
• A new dosage form
• A materially different formulation
• A new delivery device
• New clinical-use claims
• A new combination involving lidocaine or another active component

The regulatory classification depends on the product’s differences and the extent to which it relies on FDA’s findings for the reference product. [6]

Data package for an excipient-led product

A differentiated injectable would typically require:

1. Comparative formulation and manufacturing data
2. Sterility and container-closure validation
3. Stability data in the final commercial package
4. Extractables and leachables assessment
5. Particulate and visible-inspection testing
6. Reconstitution and administration studies
7. Compatibility studies with labeled diluents
8. Impurity and degradation-product characterization
9. Device performance and human-factors data, if applicable
10. Clinical or bridging data if the formulation changes exposure, tolerability, or use conditions

FDA’s quality-by-design framework supports systematic evaluation of critical material attributes, critical process parameters, and design space. [2]

Which companies are challenging or competing with PRIMAXIN?

Competition comes primarily from manufacturers of generic imipenem/cilastatin injection. The market can include branded legacy supply, authorized generic arrangements, hospital-contract manufacturers, and injectable-specialty companies.

The relevant competitive factors are:

• FDA approval status
• Current supply continuity
• Manufacturing location
• Vial strength and package size
• Expiration dating
• Diluent compatibility
• Hospital purchasing contracts
• Shortage history
• Availability of premixed or pharmacy-ready presentations

Company-level competitive analysis requires current FDA approval records, wholesaler listings, and procurement data. The competitive field can change quickly because sterile injectable manufacturers may enter or exit based on capacity, pricing, and shortage conditions.

What manufacturing and IP barriers affect commercial opportunities?

The largest barrier is sterile injectable manufacturing, not the cost of sodium bicarbonate or lidocaine.

Manufacturing barriers

A commercial product requires validated control over:

• Sterile powder filling
• Aseptic processing or terminal sterilization strategy, where appropriate
• Low residual moisture
• Vial and stopper compatibility
• Reconstitution performance
• Particulate control
• Environmental monitoring
• Supply of qualified sterile containers
• Stability-indicating analytical methods

Imipenem degradation creates a narrow operating window for liquid products. A company that cannot achieve commercially useful dating, shipping stability, and in-use stability will have difficulty competing even with a technically differentiated excipient package.

Intellectual-property barriers

Potentially defensible assets include:

• A specific stabilizing excipient combination
• A defined pH range linked to improved stability
• A low-moisture lyophilized cake
• A dual-chamber container
• A validated extended-stability formulation
• A device-enabled administration method
• A manufacturing process that reduces a specified impurity

The strongest patents generally connect a defined formulation or device feature to measurable technical performance. Broad claims that merely identify a conventional excipient may face validity and design-around risk.

How strong is the PRIMAXIN patent estate?

The legacy PRIMAXIN estate is commercially weak as a barrier to ordinary generic entry because the product is mature and generic competition exists. A new patent estate could be stronger if it protects a difficult-to-reproduce stability profile, a proprietary container system, or a high-value ready-to-use presentation.

Relative strength by asset type

Asset	Relative strength for new commercial entry
Conventional sodium bicarbonate powder formulation	Low
Standard sterile vial and reconstitution process	Low
Optimized low-moisture lyophilized formulation	Moderate
Extended-stability premix	Moderate to high if claims are broad and data-supported
Dual-chamber container system	Moderate, with device design-around risk
Lidocaine-containing IM formulation	Moderate, but limited market scope
Manufacturing process with impurity reduction	Moderate if difficult to replicate
Broad antibacterial method-of-use claim	Low for generic-entry blocking

What are the main commercial opportunities for PRIMAXIN excipients?

The most attractive opportunity is a hospital-use product that lowers preparation burden without materially increasing total treatment cost.

Opportunity ranking

Opportunity	Customer value	Technical difficulty	Commercial assessment
Improved vial reconstitution	Moderate	Low to moderate	Practical near-term opportunity
Longer post-reconstitution stability	High	High	Strong value if validated
Frozen premix	High	High	Attractive but dependent on cold-chain economics
Dual-chamber bag or vial	High	High	Differentiated, device-heavy opportunity
Better infusion compatibility	Moderate	Moderate	Useful for hospital protocols
Reduced IM injection volume	Moderate	Moderate	Niche opportunity
Preservative-free multidose vial	Low to moderate	High	Regulatory and infection-control burden
Novel buffer replacement	Unclear	Moderate	Requires strong stability advantage

Revenue exposure depends on hospital procurement, contract pricing, geographic supply, and the ability to secure a differentiated regulatory label. No reliable revenue estimate follows from the public product label alone.

How does PRIMAXIN compare with other carbapenem products?

PRIMAXIN competes with generic imipenem/cilastatin and with other carbapenems such as meropenem and ertapenem. Excipient strategy differs because each product has distinct stability, administration, dosing, and hospital-use requirements.

Product	Active drug	Typical commercial differentiation
PRIMAXIN	Imipenem/cilastatin	Established dual-API carbapenem; IV and historical IM options
Meropenem products	Meropenem	Generic supply, infusion workflow, hospital formulary position
Ertapenem products	Ertapenem	Once-daily dosing and outpatient-use potential
Doripenem products	Doripenem	Limited current commercial relevance relative to leading carbapenems

A new PRIMAXIN formulation must compete against the clinical and operational advantages of alternatives. Excipient improvements alone may not overcome once-daily dosing, outpatient convenience, or hospital-contract economics.

What generic launch scenarios exist for PRIMAXIN?

Conventional generic launch

A standard vial-based generic can compete on price and supply. This is the lowest-differentiation scenario and is exposed to rapid price erosion when several manufacturers are approved.

Authorized or branded-generic launch

A manufacturer may preserve a branded price position through supply guarantees, hospital contracting, or a differentiated package. The product still faces substitution pressure unless the label or delivery system provides a meaningful operational advantage.

505(b)(2) differentiated launch

A premix, dual-chamber system, or materially different administration format could support a 505(b)(2) strategy where the formulation is not suitable for a conventional ANDA. This path may justify higher pricing but requires greater development investment and regulatory evidence.

Shortage-driven launch

Sterile injectable shortages can create temporary market openings. A manufacturer needs validated capacity, reliable raw materials, and an FDA-compliant quality system to convert a shortage into durable market share. Shortage exposure is an opportunity and a risk because pricing and supply conditions can normalize quickly.

Key Takeaways

• PRIMAXIN contains imipenem and cilastatin; sodium bicarbonate is a principal IV formulation excipient.
• The IM presentation uses lidocaine-containing diluent, creating a separate safety and regulatory profile.
• The core product has mature exclusivity and faces generic competition.
• The strongest commercial opportunities are extended-stability liquids, dual-chamber systems, improved reconstitution, and pharmacy-ready delivery.
• Imipenem’s aqueous instability is the central technical barrier.
• A new excipient package must support stability, sterility, compatibility, particulate control, and practical hospital workflow.
• Conventional sodium bicarbonate reformulation is unlikely to create a strong market position without measurable stability or handling benefits.
• PRIMAXIN has no biosimilar risk; generic imipenem/cilastatin is the relevant competitive threat.
• Formulation, device, container, and manufacturing patents are more commercially relevant than legacy composition patents.
• FDA pathway selection depends on whether the proposed product remains pharmaceutically equivalent to the reference product or introduces a material formulation or delivery change.

FAQs

Can sodium bicarbonate be removed from an imipenem/cilastatin formulation?

Removal may be technically possible, but it would require comparative stability, pH, impurity, reconstitution, and injectable-product data. The excipient cannot be removed solely because it is present at a low concentration.

Is a ready-to-use PRIMAXIN formulation commercially feasible?

It is technically feasible in principle but difficult because imipenem is less stable in aqueous solution than as a dry sterile powder. A commercially useful product would need meaningful shelf life, in-use dating, shipping stability, and acceptable cold-chain economics.

Does lidocaine make PRIMAXIN IM a combination drug?

Lidocaine is used as a functional component of the IM administration system. Its presence creates additional safety, labeling, compatibility, and regulatory considerations even when the principal therapeutic purpose is imipenem/cilastatin.

Can an excipient patent block generic imipenem/cilastatin?

Only if the generic product practices the patent claims and the patent is enforceable and relevant to the applicable regulatory filing. A generic may design around a formulation or container patent by using a different excipient concentration, package, or manufacturing process.

Which PRIMAXIN formulation has the greatest commercial opportunity?

An extended-stability, pharmacy-ready IV presentation has the broadest potential because it targets hospital labor, preparation time, and administration reliability. Its value depends on achieving stability and cost targets that justify a premium over standard generic vials.

References

1. U.S. Food and Drug Administration. (n.d.). PRIMAXIN IV and PRIMAXIN IM prescribing information. Merck Sharp & Dohme LLC.
2. U.S. Food and Drug Administration. (2009). Guidance for industry: Pharmaceutical development.
3. International Council for Harmonisation. (2009). Q8(R2): Pharmaceutical development.
4. U.S. Food and Drug Administration. (n.d.). Approved drug products with therapeutic equivalence evaluations, Orange Book.
5. U.S. Food and Drug Administration. (2015). Guidance for industry: 180-day exclusivity: questions and answers.
6. U.S. Food and Drug Administration. (2023). Applications covered by section 505(b)(2).