Metronidazole Benzoate Pharma Grade API, CAS 13182-89-3, is the 2-(2-methyl-5-nitro-1H-imidazol-1-yl)ethyl benzoate ester supplied for tablet, capsule, granule, and oral suspension dosage forms. Injectable product lines use metronidazole base, as described below. The molecular formula is C13H13N3O4, the molar mass is 275.26 g/mol, and the compendial melting range is approximately 99–102 °C. The ester is described in current pharmacopoeial monographs as a white or slightly yellow crystalline powder, practically insoluble in water and soluble in acetone and dichloromethane. Qualitative identification is by infrared absorption spectrophotometry and thin-layer chromatography against a reference standard. Assay is determined by liquid chromatography with acceptance 98.0%–102.0% on the dried basis. Related substance limits include metronidazole ≤0.5%, unspecified impurities ≤0.10%, and total impurities ≤1.0%. Loss on drying is ≤0.5% after drying at 105 °C, and residue on ignition is ≤0.1%. Residual solvent control follows Ph. Eur. 5.4 and USP <467>; microbiological quality for nonsterile use is evaluated by USP <61> and USP <62>, with total aerobic microbial count ≤1000 CFU/g, total combined yeasts and molds ≤100 CFU/g, and absence of Escherichia coli and Salmonella. The benzoate ester is not appropriate for aqueous injection; injectable metronidazole is manufactured from metronidazole base.

What Differentiates the Benzoate Ester from Metronidazole Base in Oral and Injectable Formulations?

In oral liquid formulations, the ester provides a measurable organoleptic advantage: metronidazole base exhibits intense bitterness that complicates pediatric compliance, while the benzoate ester has lower aqueous solubility and reduced taste perception. The dose conversion follows the molar-mass ratio 171.15 g/mol / 275.26 g/mol = 0.622; therefore, 200 mg metronidazole benzoate is equivalent to approximately 125 mg metronidazole. Hydrolysis in the gastrointestinal tract releases metronidazole and benzoic acid. Compared with tinidazole and secnidazole, the benzoate ester is not a separate therapeutic moiety; it is an ester prodrug of metronidazole and does not alter the anaerobic spectrum but changes manufacturability and solubility. The oral suspension monograph expresses strength as metronidazole equivalent per 5 mL, not as benzoate salt. For tablet and capsule development, the ester’s poor water solubility means that dissolution methods may require surfactant or acid media; method selection must be justified with discriminatory power.

Powder flow characterization should be executed before selecting a tablet or capsule process. USP <616> tapped density and USP <1174> powder flow methods are used to calculate the Carr index and Hausner ratio; values between 1.00 and 1.25 indicate passable flow, while values above 1.35 usually require roller compaction or wet granulation. Particle size distribution is measured by laser diffraction or analytical sieving per USP <429> / USP <786>. For low-dose tablets, segregation of fines must be controlled because the API can be cohesive; content uniformity is verified by USP <905> and dissolution by USP <711>. Bulk density and tapped density should be recorded for each lot to track milled-material variability. If angle of repose exceeds 45°, flow is classified as cohesive and direct compression should be rejected. Loss-on-drying and particle size data are used to define the design space for wet and dry granulation.

Granulation, Compression, and Blend Uniformity Control Points

Because the benzoate ester is practically insoluble in water, aqueous wet granulation can be used if the binder solution is pH-buffered and the granulation drying temperature is controlled to limit hydrolysis. High-shear granulation in a 100 L to 600 L vertical granulator typically requires impeller tip speeds of 5–10 m/s and chopper speeds of 1500–3000 rpm; however, API-specific confirmed design space data are limited. Dry granulation by roller compaction is an alternative when moisture exposure must be minimized. Lubrication with magnesium stearate at 0.5–1.0 wt% for 3–5 min is typical; over-lubrication can reduce tablet hardness and delay dissolution. Compression force settings are established during process validation using rotary tablet presses with precompression stations and paddle feeder control. For capsules, powder or granular fill is metered using dosator or tamping-pin machines; fill weight variation is controlled per USP <905>, and lock ring height must be adjusted when bulk density changes.

Tablet and capsule formulations containing metronidazole benzoate usually combine lactose monohydrate or microcrystalline cellulose as filler, crospovidone or sodium starch glycolate as disintegrant, and povidone as binder. Because the API is hydrophobic and can retard water penetration, disintegrant selection is critical; crospovidone at 2–5 wt% may improve disintegration but must be correlated with USP <711> dissolution data. Direct compression may require colloidal silicon dioxide at 0.1–0.5 wt% to improve flow. Tablets are compressed to hardness of 6–10 kp and friability ≤1.0% per USP <1216>. For granule dosage forms intended for sachet or reconstitution, fluid-bed top-spray granulation or extrusion-spheronization is used; extrusion-spheronization typically requires microcrystalline cellulose content of 20–40% as a spheronization aid, with extruder screen size selected to control granule diameter. Blend uniformity is evaluated by USP <905>, and process analytical technology may be used to monitor blend homogeneity in real time.

When Aqueous Injection-Grade Metronidazole Becomes the Technically Appropriate Form

Injectable presentations of metronidazole are formulated with metronidazole base, not the benzoate ester, because the ester has practically insoluble water solubility and would require non-aqueous solvents that are not appropriate for standard intravenous infusion. Metronidazole injection is commonly supplied as 500 mg/100 mL sterile isotonic solution, with sodium chloride for isotonicity and pH adjusted to 5.0–7.0 using hydrochloric acid and sodium hydroxide. Terminal sterilization is performed by autoclaving at 121 °C for 15 min; the solution must remain clear and free of particulates after cycling. For injectable-grade metronidazole, bacterial endotoxin testing per USP <85> and sterility per USP <71> are release tests. When the product range includes injection, the active pharmaceutical ingredient supplied is metronidazole base, not metronidazole benzoate. Master batch records must not permit direct substitution of the benzoate ester into an intravenous formula without reformulation; doing so would produce a suspension or precipitate and would fail particulate matter limits under USP <788>.

For oral suspension manufacturing, the API is dispersed rather than dissolved in the vehicle. Viscosity is adjusted with xanthan gum or microcrystalline cellulose/carboxymethylcellulose sodium; the yield value must exceed the sedimentation stress of the API to maintain physical stability. The suspension pH is maintained above 4.0 to reduce acid-catalyzed ester hydrolysis. Homogenization is performed in a high-shear mixer or colloid mill, and particle size after homogenization is checked by optical microscopy or laser diffraction. The finished suspension is typically packaged with a dosing syringe or measuring cup calibrated in 5 mL increments. The label strength is expressed as metronidazole equivalent, not benzoate salt. Syringeability and drainability of the suspension are influenced by plastic viscosity and yield stress; a Brookfield viscometer with spindle LV-4 at 12 rpm is often used for routine release viscosity.

Although the dry ester is stable under controlled room temperature storage, moisture and acidic pH accelerate ester hydrolysis to metronidazole and benzoic acid. Bulk containers should be closed tightly and stored at controlled room temperature, 20–25 °C, with excursions permitted between 15–30 °C per USP <659>. If ambient relative humidity exceeds 60%, transfer should be performed in a dry room or the material should be pre-dried before use in moisture-sensitive formulations. The API is light-sensitive; light-resistant containers are specified in the monograph. Re-test dating is assigned from stability data under ICH Q1A(R2). Stress degradation studies using acid, base, heat, and oxidation are used to validate the stability-indicating HPLC method; acid stress generates metronidazole and benzoic acid peaks, which must be separated from the active peak with resolution not less than 1.5.

Analytical method transfer for the API includes separation of metronidazole, benzoic acid, and unspecified degradation products on a reversed-phase C18 column. System suitability criteria require resolution between metronidazole and the benzoate ester of not less than 1.5, tailing factor not more than 2.0, and injection precision not more than 2.0% relative standard deviation. Forced degradation under acid, base, oxidation, heat, and light is used to verify peak homogeneity and mass balance. The method is used to certify the absence of significant hydrolytic degradation in finished granule and suspension batch release.

Bulk Handling Requires Control of Thermal and Moisture Exposure

A controlled environment is required during dispensing and sampling because the powder can develop electrostatic charge and cohesive flow. Stainless steel 316L contact surfaces and antistatic grounding are used in dispensing booths. Sieving through a 500 µm or 1000 µm screen is common before blending to break soft agglomerates. If the material is held in quarantine after sampling, the container headspace should be purged with nitrogen if prolonged storage is expected, although published stability data for nitrogen-purged metronidazole benzoate bulk is limited. The material should not be exposed to temperatures above 40 °C during processing; brief excursions can increase impurity formation. Typical packaging for bulk API is a 25 kg high-density polyethylene drum with a double low-density polyethylene liner and a desiccant; the specific packaging configuration is defined in the marketing authorization or drug master file. Cleaning validation must include verification of residual API removal from contact surfaces, with swab limits established by the acceptable daily exposure.