Oxyquinoline Sulfate Veterinary Grade API for Tablets / Injections / Capsules / Powders / Granules / Premix / Solutions is chemically defined as bis(8-hydroxyquinolinium) sulfate, CAS 134-31-6, molecular formula C18H16N2O6S, relative molecular mass 388.41. It is supplied as a pale-yellow to tan crystalline powder with a reported melting endotherm in the region of 175–178°C when determined by differential scanning calorimetry at a heating rate of 10°C/min under ISO 11357-1:2023. The sulfate salt is selected for veterinary formulations because its aqueous solubility is substantially higher than that of the free base below pH 5.5; as pH is increased, free 8-hydroxyquinoline precipitates and reduces the available dissolved antimicrobial fraction. Antimicrobial activity is primarily chelation-driven, sequestering divalent cations such as iron(II), zinc(II), and copper(II) from metalloproteins involved in microbial respiration and biofilm formation. No global machine-readable model designation is assigned to this veterinary-grade active pharmaceutical ingredient under current compendial nomenclature; traceability is controlled through the batch certificate of analysis, the supplier’s internal grade code, and the applicable monograph reference. Storage in light-resistant containers at ≤25°C and ≤65% relative humidity is recommended because prolonged ultraviolet exposure causes surface darkening without necessarily altering assay potency.

Release testing for the veterinary-grade API is aligned with the USP monograph for Oxyquinoline Sulfate where the regional filing references USP; identification is performed by infrared absorption spectrophotometry, and assay is performed by non-aqueous titration with perchloric acid. Typical compendial assay acceptance is 98.0% to 101.0% on the dried basis. Loss on drying after 3 h at 105°C is generally controlled at ≤0.5% w/w, and residue on ignition is controlled at ≤0.1% w/w. Elemental impurities are evaluated under USP <232>/<233> or Ph. Eur. 5.20, with particular attention to residual iron, zinc, and copper arising from process equipment because these metals complex with the API. Microbial quality for non-sterile oral and topical dosage forms follows USP <61> and USP <62>; specified absence of Escherichia coli, Salmonella species, Staphylococcus aureus, and Pseudomonas aeruginosa is typical. Residual solvents, if the synthesis route uses acetone or isopropanol, are controlled under Ph. Eur. 5.4 and VICH GL18. Particle-size distribution is not fixed by the monograph and must be established by the finished-dose manufacturer using laser diffraction per ISO 13320:2020; dry-blend grades commonly target D90 ≤150 µm and solution grades target D97 ≤75 µm.

Which stability-indicating methods distinguish sulfate counterion dissociation from free-base precipitation?

Aqueous formulations of Oxyquinoline Sulfate are sensitive to pH because the sulfate counterion dissociates in neutral-to-alkaline media, producing the poorly soluble free base. A stability-indicating reversed-phase HPLC method with a C18 column, phosphate buffer at pH 3.0, and acetonitrile gradient separates the protonated oxyquinolinium peak from the neutral free base and oxidative degradation products. Detection at 254 nm is suitable for quantifying free base at 0.1% area in spiked stressed samples. Forced degradation studies should include acidic, alkaline, oxidative, thermal, and photolytic conditions according to Ph. Eur. 5.2.2 or the corresponding VICH stability guidance for veterinary drug substances. Non-aqueous titration with perchloric acid is acceptable for release assay but does not discriminate free base from sulfate-associated active species, so the chromatographic method is required for stability evaluation. During sample preparation, amber glass volumetric flasks are used to limit photodegradation, and iron or copper stir bars are not used because the chelating action contaminates the sample with redox-active metal complexes.

For tablet and capsule unit-dose forms, oxyquinoline sulfate is typically dry-blended at 5–20% w/w active concentration in a bin blender or twin-shell blender operated at 60–75% of gross capacity. The API may be passed through a cone mill fitted with an 800 µm screen before blending to reduce agglomerates and improve content uniformity. A direct compression matrix containing microcrystalline cellulose, lactose monohydrate, sodium starch glycolate, and colloidal silicon dioxide is generally suitable; tablet compression on a rotary press is often run at 8–15 kN for a total tablet weight of 250 mg. Disintegration is monitored by USP <701> or Ph. Eur. 2.9.1 in water at 37°C, with a typical acceptance limit of ≤15 min. Capsule filling uses automatic dosator or tamping machines with fill weight variation controlled at ±3%; content uniformity is evaluated by USP <905> or Ph. Eur. 2.9.40. For wet granulation, a high-shear granulator may be used with an aqueous binder solution, but granulation pH should remain below 5.5 to prevent free-base precipitation; fluid-bed drying at inlet air temperature not exceeding 60°C minimizes thermal discoloration.

Medicated premix manufacture imposes stricter mixing validation than simple powders because the API is often present at inclusion rates below 1 kg/tonne in final feed. A staged dilution sequence is used: first a 1:10 dilution with ground corn or lactose carrier in a ribbon mixer or paddle mixer, followed by 1:100 dilution into the complete premix. Sampling of 10 locations per batch after 10 min, 15 min, and 20 min mixing determines the blend time at which HPLC assay relative standard deviation reaches ≤5.0%. Electrostatic adhesion to stainless steel mixer walls is observed when ambient relative humidity falls below 30%; conditioning the blend room to 45–55% relative humidity reduces wall losses. Contact with uncoated brass, copper, or galvanized equipment is not recommended because extractable transition metals form colored chelates and reduce free antimicrobial activity. Stainless steel 316L contact surfaces and non-metallic gaskets are preferred for bulk handling.

When injectable solutions must meet bacterial endotoxin and particulate limits, what defines the operating window?

Injectable-grade Oxyquinoline Sulfate is dissolved in Water for Injection at a concentration that depends on the intended clinical dose; solution pH is adjusted to 4.0–5.0 with dilute hydrochloric acid or citric acid before filtration. Sterile filtration through a 0.22 µm polyvinylidene fluoride membrane is performed after the solution has cooled to 15–25°C. Filter integrity testing is conducted before and after filtration per ISO 17025-accredited site procedures and the filled product is tested for subvisible particulates by USP <788> or Ph. Eur. 2.9.19. Bacterial endotoxin testing by USP <85> or Ph. Eur. 2.6.14 should support an acceptance limit not greater than 0.5 EU/mg if the route is intravenous or intramuscular. Terminal autoclaving at 121°C for 15 min is applicable only when the solution pH is maintained below 5.5; at neutral or alkaline pH, autoclaving accelerates free-base precipitation and creates visible turbidity. Nitrogen purging during filling into amber ampoules limits oxidative discoloration.

Chelation-dependent preservation in drinking-water solutions and medicated feed premixes

In drinking-water solutions, oxyquinoline sulfate functions as both an antimicrobial and a metal-chelating stabilizer, but hard water containing high concentrations of calcium and magnesium can partially consume the chelating capacity. Water-quality testing by inductively coupled plasma optical emission spectrometry is advisable before large-scale use; total hardness above 200 mg/L as CaCO₃ may require pH adjustment to 4.5–5.5 and increased API concentration based on bioequivalence or efficacy data. Stock solutions should be prepared in polyethylene or glass-lined tanks, not bare steel or copper, and should be protected from light. In feed premixes, the chelating action of the API can bind trace minerals added as nutritional supplements; therefore, the addition point for iron, zinc, and copper mineral premixes should be separated from the oxyquinoline sulfate addition point or the minerals should be supplied in protected forms with limited free ion availability. Published data for this specific interaction in complete feed matrices are limited, so mixing validation under representative production conditions is required.

Oxyquinoline Sulfate differs from oxyquinoline free base in aqueous solubility and process suitability. The free base, CAS 148-24-3, is better adapted to ointments, topical sprays, and petrolatum-based veterinary preparations, whereas the sulfate salt is preferred for oral liquids, medicated drinking water, injectable solutions, and solid dosage forms requiring aqueous granulation. Chlorquinaldol, CAS 72-80-0, carries both a 2-methyl substituent and 5,7-dichloro substitutions; these increase lipophilicity and local persistence in mastitis ointments but reduce aqueous process flexibility. Clioquinol, CAS 130-26-7, contains iodine and chlorine substituents, enhancing its antimicrobial spectrum in topical therapy but introducing iodine-related residue and stability concerns. Unlike oxolinic acid, a quinolone antibiotic with systemic activity linked to DNA gyrase inhibition, oxyquinoline sulfate does not belong to the quinolone antibacterial class and is not considered interchangeable with oxolinic acid in veterinary therapy. Oxyquinoline sulfate is non-halogenated, which reduces the likelihood of dehalogenation byproducts in aqueous solution, but the molecule remains susceptible to photodegradation and to precipitation of the free base if the formulation pH rises above 5.5.

For oral solution compounding, the API is dissolved in purified water at 0.5–2.0% w/w after pH adjustment with citric acid or hydrochloric acid; mixing is performed in a stainless steel or glass-lined vessel with a low-shear propeller agitator at 100–200 rpm. Continuous stirring over 30–60 min is typical to complete dissolution at 20–25°C. Filtration through a 10 µm polypropylene depth filter may be used before packaging into amber high-density polyethylene bottles with tamper-evident caps. The final solution should be checked visually for clarity and filter-clogging particles; any change in color from pale yellow to brown indicates oxidative degradation and should trigger batch rejection rather than rework.