Sulfaquinoxaline (SQ) is a veterinary sulfonamide antimicrobial supplied as a non-sterile active pharmaceutical ingredient (API) for downstream manufacture of tablets, injections, capsules, powders, granules, premixes, and oral solutions. The free acid, CAS 59-40-5, has the molecular formula C14H12N4O2S and a molecular weight of 300.34 g/mol; the sodium salt form is produced for aqueous dosage forms. The quinoxaline substituent at the N4 position of the sulfanilamide core separates sulfaquinoxaline from pyrimidine-substituted sulfonamides such as sulfadiazine and sulfamethazine. The compound is used mainly in poultry for coccidiosis control and in food-producing species for susceptible bacterial infections. As a weak acid, the free base is practically insoluble in water and dissolves with pH adjustment or through salt formation. This property drives the selection of the sodium salt for injectable solutions and drinking-water formulations, while the free acid or direct-compression grades are used in dry oral solids and feed premixes. Release specifications for veterinary grade sulfaquinoxaline API typically include assay, related substances, loss on drying, heavy metals, residual solvents, and particle size; the certificate of analysis should be checked against the current compendial monograph and the user’s formulation requirements.

Depending on the intended dosage form, sulfaquinoxaline is supplied in micronised free acid, crystalline free acid, spray-dried sodium salt, and granulated premix-grade forms. Micronised free acid with a D90 of 20 µm to 50 µm is used where dissolution rate is critical; crystalline free acid with a D90 of 100 µm to 150 µm is used in direct-compression dry blends; spray-dried sodium salt is used for injectable and drinking-water solutions. Premix-grade material may be coarsely ground or granulated to reduce dust and improve flow into feed mixer lines. The selected grade is not a single model but a set of physical forms that change handling, blend segregation, and dissolution behaviour.

The values in the table are representative release parameters for non-sterile veterinary API and are not a substitute for a registered specification. The current monograph may impose additional tests for sulfanilamide and related quinoxaline impurities, clarity of solution, and pH of the sodium salt solution.

What Distinguishes the Free Acid from the Sodium Salt in Aqueous Formulation?

In aqueous media, sulfaquinoxaline free acid ionises as the pH approaches the weakly acidic pKa of the sulfonamide N-H group. The free acid requires alkaline dissolution, whereas the sodium salt provides a readily soluble form for injection or drinking-water preparations. For injectable solutions, the sodium salt is dissolved in water for injection and adjusted to an alkaline pH range, commonly pH 8.5–9.5, to maintain the ionised state and prevent precipitation. The alkaline pH introduces process constraints: base-catalysed hydrolysis of the sulfonamide linkage can increase related substances, and trace iron or copper can accelerate oxidative discolouration. Formulators add disodium edetate at 0.01%–0.05% w/v and purge the solution with nitrogen. The sodium counterion contributes to osmolarity; osmolality should be measured and adjusted to isotonicity using Ph. Eur. 2.2.35 or USP 785. Phosphate buffers are usually avoided in strongly alkaline parenteral formulations because pH drift during moist-heat sterilisation can reduce solubility and raise the risk of precipitation. The free acid is more appropriate for dry oral solids, where aqueous solubility is less critical and blend homogeneity with direct-compression excipients determines content uniformity.

For tablet and capsule manufacture, sulfaquinoxaline free acid is processed by wet granulation or direct compression, depending on the dose and the selected particle-size grade. The free acid exhibits low bulk density and poor compressibility; roller-compacted or spray-dried grades with D90 ≤ 150 µm are preferred for direct-compression blends. A typical dry oral blend contains microcrystalline cellulose, lactose monohydrate, sodium starch glycolate as disintegrant, and magnesium stearate as lubricant. Tablets compressed on a rotary press are controlled for hardness, friability, disintegration, and dissolution; disintegration is assessed according to Ph. Eur. 2.9.1 or USP 701, and dissolution according to Ph. Eur. 2.9.3 or USP 711. Uniformity of dosage units is tested according to Ph. Eur. 2.9.40 or USP 905. Low-dose tablets may require geometric dilution before final blending because the active charge can be below 5% w/w. Lubrication with magnesium stearate should be limited to 0.5%–1.0% w/w and blended for 3–5 minutes to avoid over-lubrication and dissolution slowdown. Wet granulation using a high-shear granulator requires water or an aqueous binder solution; the granulate is dried to moisture content below 2.0% w/w before lubrication. Capsule filling of sulfaquinoxaline-containing blends is performed on tamping-pin or dosator machines; filled capsules are checked for weight variation and dissolution. Because the API is hydrophobic, dissolution testing in 900 mL of pH 6.8 phosphate buffer at 50 rpm using basket or paddle apparatus is a typical quality-control setting, but the final method must be justified by the registered formulation. Dissolution of sulfaquinoxaline free acid is pH-dependent; the drug dissolves more slowly at gastric pH and more rapidly under intestinal pH where ionisation increases. Therefore, dissolution media with pH 6.8 to 7.4 are more discriminatory for formulation changes than pH 1.2 media. Reduction of D90 from 150 µm to 20 µm can markedly increase dissolution rate but may worsen blend flow and tablet capping.

Premix and Granular Feed Processing Thresholds

Feed premix manufacture with sulfaquinoxaline free acid requires attention to particle-size segregation and moisture uptake. The API is commonly milled to a D90 ≤ 100 µm and blended with a mineral or organic carrier having a mean particle size of 200 µm–500 µm to reduce segregation. Double-ribbon mixers or paddle mixers are used, with blend uniformity samples taken from 10 locations and assayed for sulfaquinoxaline content; the coefficient of variation should remain below 5%. Mixing time is established by blend uniformity studies using an analytical method such as HPLC with UV detection at 254 nm. Because sulfaquinoxaline is sensitive to alkaline and oxidising conditions, premixes should not contain strongly alkaline mineral carriers such as quicklime or significant amounts of free iron. The pH of the premix aqueous slurry is typically controlled between 6.0 and 8.0 to maintain chemical stability during storage. Because feed premixes may contain hygroscopic carriers, moisture uptake above 5% w/w can accelerate hydrolysis of the sulfonamide linkage; storage in moisture-resistant liners is required. The premix is diluted into final feed at the rate prescribed by the marketing authorisation. Granular products containing sulfaquinoxaline can be produced by fluidised-bed granulation using an aqueous binder; inlet air temperature is typically held between 55 °C and 65 °C, while product temperature remains below 40 °C to avoid degradation. Granules are dried to a moisture content below 2.0% w/w and sieved to a defined particle-size range before packaging. Published data for continuous production-scale mixing of sulfaquinoxaline premixes on specific equipment configurations is limited; therefore, blend uniformity studies should be executed on the actual line to justify sampling and acceptance limits.

When the API Is Selected for Injectable Solutions Rather Than Dry Oral Powders

Injectable formulations use sulfaquinoxaline sodium rather than the free acid because the sodium salt dissolves rapidly in water for injection. The solution is typically compounded under nitrogen and adjusted to an alkaline pH range of pH 8.0–9.5 using tromethamine or sodium carbonate-based buffers. Moist-heat sterilisation at 121 °C for 15 minutes can shift pH by 0.3–0.5 units and increase related substances; the formulation is protected by headspace nitrogen and, where necessary, an antioxidant such as sodium metabisulfite at 0.1% w/v. However, sulfite-containing injectables carry a risk of hypersensitivity in some species, so alternative stabilisation with disodium edetate and nitrogen overlays is preferred. Scale-up from laboratory to production may reveal pH drift after terminal sterilisation because heat transfer in large-volume vessels is slower. For 100 mL and larger vials, the sterilisation cycle may require an equilibration time to reach 121 °C; total F0 should be measured with thermocouples placed in the coldest solution region. Over-processing can increase related substances above the acceptance limit. A nitrogen overlay during filling reduces headspace oxygen content below 2% w/w, which is controlled by headspace oxygen analysis. The filled ampoules or vials are tested for particulate matter according to Ph. Eur. 2.9.19 or USP 788, and for sterility according to Ph. Eur. 2.6.1 or USP 71. Because sulfaquinoxaline sodium solutions are light-sensitive, amber glass primary packaging is used to limit photochemical degradation. Precipitation at low temperatures is a known operational boundary; storage below 2 °C may produce crystalline deposits that do not fully redissolve without warming and agitation. The sodium salt injectable grade is not interchangeable with the free acid grade in a formulation without rebalancing the pH adjustment and osmolarity.

For oral powders and powders for solution, sulfaquinoxaline sodium is selected when the dosage form is intended for drinking water, and the free acid is selected when the dosage form is a dry feed powder. Drinking-water powders are prepared by geometric dilution of sulfaquinoxaline sodium with sorbitol, citric acid, and sodium bicarbonate. The effervescent reaction after addition to water raises pH and aids dissolution. The powder must be protected from moisture during storage because carbon dioxide release can occur prematurely if the package is not hermetically sealed. Granulation of oral powders improves flow and reduces dust generation during sachet filling; a final moisture specification below 2.0% w/w is used. The reconstituted solution should be mixed gently to avoid localised alkaline hydrolysis; in-use stability is evaluated by holding solutions at 25 °C and 5 °C for 24 hours and measuring assay and related substances. Differences between sulfaquinoxaline and other veterinary sulfonamides are most evident in solubility, elimination behaviour, and coccidiosis activity. The quinoxaline ring reduces aqueous solubility of the free acid and contributes to a different tissue-residue profile compared with sulfadiazine or sulfamethazine. In poultry, sulfaquinoxaline is frequently combined with pyrimethamine or diaveridine to potentiate folate-pathway inhibition, whereas sulfadiazine and sulfamethazine are more commonly combined with trimethoprim. These combinations are not interchangeable in premix formulations because the companion compound changes the final dosage and residue depletion kinetics.

Bulk API and finished dosage forms should be stored in well-closed, light-resistant containers at controlled room temperature, defined as 15 °C to 25 °C. Relative humidity above 60% can cause deliquescence of the sodium salt and hydrolytic degradation; desiccant systems are used in bulk packaging. The free acid is more stable in dry conditions but can darken under prolonged light exposure. Re-test periods for sulfaquinoxaline API are assigned after stability studies according to ICH Q1E or regional veterinary guideline; expiry dating of the finished product is based on long-term, intermediate, and accelerated storage conditions as described in ICH Q1A. In the European Union, sulfonamides are controlled under Regulation (EU) No 37/2010; the marker residue is the sum of all sulfonamides, with a muscle MRL of 100 µg/kg. Sulfaquinoxaline-containing formulations must therefore be used with a clearly defined withdrawal time based on the final product’s residue depletion data. The API’s impurity profile should include the corresponding sulfanilamide and quinoxaline-related substances, with acceptance limits set by the current monograph.