Cefquinome sulfate, a semisynthetic fourth-generation cephalosporin, is supplied as a veterinary-grade active pharmaceutical ingredient for compounding into tablets, injections, capsules, powders, granules, premixes, and oral solutions. The free base has the molecular formula C23H24N6O5S2 and a calculated molecular mass of 528.60 g/mol; the sulfate salt, CAS 118443-89-3, has a molecular mass of 626.68 g/mol and is the common commercial form for veterinary dosage forms. The API is released under ICH Q7 conditions in two controlled grades: CQ-VAPI-NS, a non-sterile low-moisture powder for oral solid dosage forms and feed premixes, and CQ-VAPI-ST, a sterile micronized grade for injectable liquids. The methoxyimino-aminothiazole side chain at C-7 and the quaternary tetrahydroquinolinium substituent at C-3 distinguish the molecule from ceftiofur and cefalexin; the C-3 structure provides zwitterionic character that increases outer-membrane porin permeability in Gram-negative veterinary pathogens. Assay and related substances are determined by liquid chromatography according to Ph. Eur. 2.2.29, and residual solvents are screened by headspace gas chromatography according to Ph. Eur. 2.4.24.

What Distinguishes a Fourth-Generation Cephalosporin From Ceftiofur and Cefalexin?

Placement within the fourth generation rests on the combination of a methoxyimino group at C-7, an aminothiazole ring, and a quaternary C-3 heterocycle. The methoxyimino group suppresses hydrolysis by common plasmid-encoded and chromosomal β-lactamases. The quaternary tetrahydroquinolinium substitution forms an internal salt, and the zwitterionic state facilitates diffusion through OmpF and OmpC porins in Escherichia coli and related Enterobacterales. In contrast, cefalexin is a first-generation cephalosporin with a simple methyl C-3 substituent and is inactivated by many Gram-negative β-lactamases; its clinical use is limited primarily to susceptible staphylococcal and streptococcal pathogens. Ceftiofur, a third-generation cephalosporin, carries a furan-carbonyl thioester at C-3 and has broad Gram-negative activity, but it is metabolized to desfuroylceftiofur after administration, complicating tissue-residue interpretation. Cefquinome does not rely on thioester prodrug activation and retains parent-drug activity until β-lactam ring hydrolysis occurs. The residue marker difference is operationally important for LC-MS/MS method development under Commission Decision 2002/657/EC; ceftiofur-based methods target desfuroylceftiofur, whereas cefquinome methods target the intact molecule. Table 1 summarizes structural and functional boundaries.

Regulatory dosing and susceptibility boundaries must not be inferred from an API specification alone. Cefquinome is authorized in the European Union under Regulation (EC) No 470/2009 for the establishment of maximum residue limits; users must verify current MRL entries before assigning a withdrawal period. Clinical breakpoints for bovine and porcine respiratory pathogens are maintained in CLSI VET01S; interpretation of susceptibility requires regional MIC distributions because cefquinome is critically important for antimicrobial stewardship and is reserved for severe infections where narrow-spectrum agents are inadequate.

For tablet and capsule operations, the non-sterile grade is blended with microcrystalline cellulose, lactose monohydrate, crospovidone, and sodium starch glycolate. Direct compression of raw cefquinome sulfate is not recommended because the micronized powder exhibits poor flow and high electrostatic charge; dry processing is therefore performed by slugging or roller compaction. If relative humidity in the processing suite exceeds 60%, the active and lubricant are pre-dried before weighing, because the sulfate salt is hygroscopic and moisture uptake alters assay and flow. Aqueous wet granulation is restricted because the β-lactam ring hydrolyzes in acidic or strongly alkaline granulating fluid. Where wet granulation cannot be avoided, the binder solution is cooled to 10–15 °C, and vacuum drying is conducted at product temperatures not exceeding 40 °C until residual water is below 1.5% w/w by Karl Fischer titration (Ph. Eur. 2.5.12). The dried granulation is screened through a 0.8 mm mesh and compressed on a rotary tablet press with precompression. Tablet crushing strength is formulation-dependent and is fixed by the finished-product dissolution profile; disintegration is confirmed according to Ph. Eur. 2.9.1. The active is incompatible with primary amines, strong oxidizers, and concentrated alkali; aminophylline or sodium bicarbonate should not be included in the same granulation without forced-degradation compatibility data. Capsule filling is performed with automatic dosator or tamping-pin equipment; machine speeds above 50,000 capsules/h may increase electrostatic segregation in high-magnesium-stearate formulations, but published data for cefquinome sulfate capsule segregation at these speeds is limited.

Coating of cefquinome tablets is limited to non-aqueous or low-moisture film-coating systems. Aqueous film-coating pans are operated with inlet air at 40–50 °C and tablet-bed temperature below 35 °C; extended exposure to higher temperatures increases related-substance formation. The coating polymer is typically hydroxypropyl methylcellulose with polyethylene glycol as plasticizer. Sugar coating is not recommended because it requires high-moisture syrup layers and long drying cycles. Capsule shells are selected from low-moisture gelatin or hydroxypropyl methylcellulose; capsule filling rooms are maintained below 45% RH and 20 °C for moisture-sensitive grades.

Sterile Injectable Specifications and Endotoxin Loading Limits

Sterile cefquinome sulfate for injection is not suitable for terminal gamma irradiation in most manufacturing routes because free-radical generation in the β-lactam ring increases open-ring impurities. The sterile grade is aseptically crystallized and micronized, and filled under Grade A laminar airflow in accordance with EU GMP Annex 1 and ISO 14644-1 class 5. The active is passed through an air-jet mill with inert gas feed; the resulting particle size distribution is controlled to a D90 of 15 µm or less when the finished product is presented as a powder for reconstitution, and to D90 ≤ 20 µm where a less finely divided powder is required for reconstitution. The vial stopper is selected for low moisture vapor transmission; butyl elastomer closures with fluoropolymer coating limit sorption of cefquinome. Product-contact surfaces are specified as electropolished 316L stainless steel; carbon steel and titanium alloys are not recommended because trace metal ions catalyze β-lactam hydrolysis. Bacterial endotoxin control is dose-dependent. The limit is calculated as K/M, where K is the threshold pyrogenic dose in endotoxin units per kilogram and M is the maximum patient dose in milligrams per kilogram per hour. For parenteral finished products, K is commonly 5 EU/kg; if the maximum therapeutic dose is 1 mg/kg, the API endotoxin release limit for that formulation would be 5 EU/mg, but many manufacturers apply a tighter internal limit of 0.50 EU/mg to accommodate multi-dose vials and storage. Subvisible particulate matter is controlled at the finished drug product stage by light obscuration per Ph. Eur. 2.9.19; the API contributes particles if crystallization mother liquors are inadequately washed. The injectable solution pH is adjusted to 4.5–5.5 with trometamol or dilute sodium hydroxide, then filtered through a 0.2 µm sterilizing-grade membrane filter before aseptic filling.

For intramammary presentations, cefquinome is suspended in an oily vehicle or gel matrix; the API particle size is adjusted to prevent needle clogging for the 5 mL syringe. The vehicle must be free of peroxides because peroxide impurities in oil can oxidize the thiazole ring. Aluminum and glass syringes are preferred over certain plastic materials due to sorption at low fill volumes. The non-sterile oral solution is prepared only at the point of administration in suitably purified water; if storage is unavoidable, the solution is kept at 2–8 °C and used within 24 h to avoid assay loss above pH 6.5.

When the Sulfate Salt Is Blended Into Oral Powders and Premixes

When the sulfate salt is blended into oral powders and premixes, low-shear geometric dilution is preferred over high-shear processing because the micronized active becomes electrostatically charged and adheres to stainless steel surfaces. A two-stage premix is prepared at 1.0% w/w active concentration using lactose monohydrate or ground corn cob as the carrier before incorporation into the final feed. The carrier moisture content is held below 10% w/w; moisture above 12% w/w increases the risk of β-lactam ring hydrolysis during storage. Mixing is performed in a ribbon blender at 15 rpm to 25 rpm for 15 min; the exact mixing time is qualified by near-infrared spectroscopy or HPLC with a root mean square error of prediction below 5%. Granules for oral solutions are produced by dry granulation or by fluid-bed top spray using a binder solution containing povidone and mannitol. The top-spray process must keep the inlet air temperature below 50 °C; product temperature above 60 °C accelerates degradation of the β-lactam and increases the amount of open-ring impurity. The premix should be used in acidified feed only when the feed pH is verified above 3.5, because prolonged exposure to gastric-mimetic acid conditions dissolves the active and reduces systemic absorption. Pelleting temperatures above 70 °C can reduce assay by more than 10% due to thermal β-lactam ring opening; low-temperature conditioning below 60 °C or post-pelleting liquid spray application is required for heat-exposed feed. The final premix is packaged in multi-wall paper bags with an inner polyethylene liner; the liner is sealed under nitrogen. This package is not suitable for long-term storage above 25 °C; storage at 40 °C and 75% relative humidity for 6 months is used as an accelerated condition only for stability screening.