Polymyxin B sulfate received for sterile injectable conversion is assayed against Ph. Eur. 2.7.2 and USP <81> before release from quarantine, because fermentation-derived polymyxin fractions shift lot-to-lot potency by up to 4%. The API is a hygroscopic, cationic lipopeptide; warehousing above 60% RH causes caking and loss of flow, so incoming lots measuring above 3.0% moisture are pre-dried in a vacuum tray dryer at 40–45°C for 4 h. The formulation is expressed in international units rather than simple weight: a 500,000 IU unit dose is calculated as target units divided by lot potency. At a representative lot potency of 8,200 IU/mg, the API charge is 60.98 mg per vial; at the lower compendial potency of 6,000 IU/mg, the charge becomes 83.33 mg per vial. Mannitol or lactose monohydrate is added as a bulking agent to maintain a uniform lyophilised cake, typically representing 70–90% of total cake mass, with the exact ratio adjusted so reconstitution to 5 mL yields 100,000 IU/mL. The conversion line operates under EU GMP Annex 1: API and excipients are dissolved in Water for Injection at 15–20°C, pH is adjusted to 5.5–6.0 with dilute hydrochloric acid, and the solution is clarified through a 0.45 μm prefilter before two 0.22 μm PVDF sterilising-grade filters in series. Filling proceeds into 10 mL Type I borosilicate vials in a Grade A zone under Grade B background, using a peristaltic filling pump with ±2% fill volume tolerance. Lyophilisation uses a primary drying plateau at −20°C and 0.1 mbar for 24 h, followed by secondary drying at 25°C for 8 h; terminal steam sterilisation is avoided because polymyxin B peptide bonds degrade under autoclave conditions. The terminal product is a sterile lyophilised powder for reconstitution and parenteral administration in cattle, horses, sheep, and other species where registered; formulation developers avoid sodium carboxymethylcellulose and phosphate buffers above 50 mM because the cationic peptide forms electrostatic precipitates with polyvalent anionic species.

Why Does Granule Potency Fade When Polymyxin B Is Wet-Granulated on Lactose?

Oral powder and granule conversion for pre-ruminant calves, neonatal piglets, and foals is used primarily for enteric colibacillosis; polymyxin B has poor intestinal absorption, so systemic bioavailability is negligible and the formulated product acts locally in the gut lumen. The active payload is potency-adjusted to 100,000–150,000 IU/g in the finished granule, with the API charge calculated as target units per gram multiplied by batch size and divided by the assayed lot potency. The carrier is lactose monohydrate or dextrose monohydrate, and the wet binder is purified water at 5–8% w/w or povidone K30 solution at 10% w/w; anionic binders such as sodium carboxymethylcellulose are omitted because the cationic peptide forms insoluble complexes. Processing uses a high-shear granulator at 1,500–2,500 rpm impeller speed and 1,500 rpm chopper speed for 3–5 min, followed by wet massing through a 1.0 mm sieve. Granules are dried in a fluid-bed dryer at 45–55°C inlet air to a final moisture of ≤ 3.0%; sieve analysis under Ph. Eur. 2.9.12 should show D90 < 1.2 mm to limit segregation in sachets. Compliance rests on EU Regulation 2019/6 for veterinary medicinal product authorisation, Ph. Eur. 5.1.4 for non-sterile microbiological quality, and VICH GL11 for stability testing; batch release includes assay by Ph. Eur. 2.7.2 and content uniformity following Ph. Eur. 2.9.40. Terminal finished dosage forms are aluminium-laminate sachets for oral suspension or direct oral granule administration, with in-use dilution directions printed for farm use.

Intramammary Suspension Phase Separation and Polymyxin B Sulfate Cationic Bridging

Intramammary infusion for lactating and dry-cow mastitis requires the API to remain uniformly suspended through the cannula delivery path; settling between manufacture and quarter infusion is a production-scale failure when phase separation occurs. Polymyxin B sulfate is water-soluble and is therefore placed in an aqueous rather than oil-continuous vehicle for this route, because oil-continuous formulations can accumulate the peptide at oil-water interfaces and reduce quarter delivery. The active concentration is typically 10,000 IU/mL, delivering 100,000 IU per 10 mL intramammary syringe; colloidal silicon dioxide at 0.25–0.6% w/v or microcrystalline cellulose with a non-ionic wetting agent provides suspending structure, while anionic rheology modifiers such as carbomer and sodium alginate are excluded because they bridge with the cationic peptide and create non-redispersible sediment. The vehicle is sterilised by autoclave at 121°C for 15 min, after which the polymyxin B solution is sterile-filtered through 0.22 μm PVDF and aseptically combined with the cooled vehicle. Filling uses a continuous-motion piston filler into low-density polyethylene intramammary cannula syringes with ±2% fill tolerance; syringes are sealed with tamper-evident caps and checked for leakers by vacuum decay. Compliance anchors are Ph. Eur. 2.6.1 and USP <71> for sterility, VICH GL18(R2) for residual solvents, and EU Regulation 2019/6 for the authorised veterinary medicinal product. The terminal product is a sterile intramammary suspension in a single-use 10 mL syringe for infusion into the affected quarter.

For pseudomonal otitis externa in dogs, polymyxin B sulfate is formulated as an otic solution or suspension in which the main manufacturability issue is pH drift during shelf storage at 25°C/60% RH. This route often uses polymyxin B sulfate at 10,000 IU/mL, neomycin sulfate at 3.5 mg/mL, and dexamethasone sodium phosphate or base at 1.0 mg/mL; the ratio is dictated by registered monographs in the destination market and by antimicrobial combination stability rather than simple API solubility. The vehicle is a co-solvent system of propylene glycol, glycerin, and Water for Injection, with benzalkonium chloride at 0.01% w/v as preservative; non-ionic poloxamer 407 at 0.2–0.5% w/v is preferred for suspension stabilisation because anionic surfactants such as sodium lauryl sulfate precipitate the cationic peptide. Compounding proceeds under low-shear mixing at 20–25°C to avoid air entrapment, and the pH is adjusted to 5.0–6.0 using dilute hydrochloric acid before bulking. The solution is clarified through a 0.45 μm filter and filled into 10 mL or 15 mL dropper bottles; where otic products are non-sterile, the microbiological specification follows Ph. Eur. 5.1.4 rather than Ph. Eur. 2.6.1, but registered sterile otic preparations must comply with EU GMP Annex 1. The terminal product is an otic solution or suspension for instillation into the external ear canal of dogs and cats; it is not interchangeable with ophthalmic preparations because preservative concentration and tonicity targets differ.

Ophthalmic Preparation Is Governed by the Same Sterility Standards as Parenterals but with Additional Osmolality Controls

Ophthalmic solutions of polymyxin B sulfate are used for corneal and conjunctival Gram-negative infections in horses, dogs, cats, and food-producing animals under veterinary supervision; the route demands sterility, particulate control, and physiological osmolality. The active concentration is generally 5,000–10,000 IU/mL, with 10,000 IU/mL reserved for severe equine keratitis associated with Pseudomonas aeruginosa; sodium chloride or mannitol is added to achieve 280–320 mOsm/kg, and the pH is maintained at 5.5–6.5 because polymyxin B stability declines above 7.0 and below 3.0. Manufacturing begins with dissolution of the API in Water for Injection at 20°C under nitrogen blanketing to reduce oxidative degradation; a 0.45 μm prefilter is placed upstream of paired 0.22 μm PVDF sterilising filters because fermentation-derived polymyxin B sulfate can shed trace insoluble polysaccharide residues that would block the final filters. Aseptic filling into low-density polyethylene ophthalmic dropper bottles or Type I glass bottles occurs under Grade A supply; the finished product is tested by Ph. Eur. 2.6.1 and USP <71> for sterility, Ph. Eur. 2.7.2 for potency, and Ph. Eur. 2.9.19 or USP <789> for particulate matter. The terminal product is an ophthalmic solution, typically 5 mL or 10 mL, for instillation into the conjunctival sac; use in food-producing species must observe the withdrawal periods specified in the reference veterinary medicinal product.

When a Medicated Premix Contains Polymyxin B Sulfate, Homogeneity Testing Must Precede Campaign Release

Medicated premix conversion is confined to jurisdictions where polymyxin B sulfate is authorised for oral mass medication in swine or poultry; it is not an EU-approved zootechnical feed additive and cannot be used as a growth promoter. The premix is manufactured on a carrier of lactose monohydrate or ground corncob at 5–10% w/w active after potency adjustment; the final feed inclusion rate is derived from the prescribed dose in mg/kg body weight, expected daily feed intake, and total feed tonnage, not from a fixed ppm value. Because uniform distribution in final feed is the safety-critical parameter, production begins with geometric dilution: the potency-adjusted API is pre-blended with carrier at a 1:10 ratio in a 25 rpm ribbon mixer for 20 min, then diluted stepwise to the final premix concentration. Homogeneity is assessed by sampling 10 g samples at 10 defined points and measuring active content by HPLC with UV detection at 215 nm; a coefficient of variation ≤ 5% is the release criterion. The process area is dedicated or subject to validated cleaning between antimicrobial campaigns because cross-contamination of non-target feed is controlled under Regulation (EU) 2019/4; feed business operators must also maintain hygiene under Regulation (EC) No 183/2005. Packing is conducted at ≤ 30% RH into 5 kg and 20 kg valve bags with desiccant-lined liners. The terminal product is a medicated premix for oral administration via feed, intended only for veterinarian-prescribed therapeutic treatment in registered species; absence of an EU maximum residue limit entry for a given species blocks that production campaign in Union markets.

Oral hard capsule and tablet conversion of polymyxin B sulfate is used for local gastrointestinal infections in companion animals when the veterinarian requires a discrete unit dose rather than a bulk oral powder. Systemic absorption is negligible, so the finished product is not intended for treating systemic infections, and formulations are designed to release the peptide in the stomach and proximal small intestine. Unit doses are generally 50,000–100,000 IU per size 2 capsule or 50,000 IU per 200 mg tablet; the API fraction after potency adjustment is 25–45% w/w in capsules and approximately 30% w/w in direct-compression tablets, with mannitol and microcrystalline cellulose as excipients. Direct compression is preferred over wet granulation because polymyxin B sulfate is hygroscopic and water-induced granulation can lower assay recovery; where flow is insufficient, roller compaction is used at 2–4 kN/cm roll pressure with a 0.8 mm ribbon mill screen. Release controls include content uniformity per Ph. Eur. 2.9.40 and USP <905>, disintegration per Ph. Eur. 2.9.1 and USP <701>, and potency per Ph. Eur. 2.7.2. Packaging uses aluminium-PVC cold-form blisters to limit moisture ingress; desiccant canisters are added when the bulk product remains open beyond 30 min under uncontrolled plant conditions. Terminal products are hard gelatin capsules or tablets for oral administration to dogs and cats; scoring is avoided for hygroscopic direct-compression formulations with low friability margins.