Sterile Ophthalmic Solutions Require Osmolality, pH, and Preservative Control

Veterinary ophthalmic solutions based on polymyxin B sulfate are compounded around a target potency of 5,000–10,000 IU/mL, with the API commonly combined with neomycin sulfate at 5.0 mg/mL or gramicidin at 0.025 mg/mL in commercial references. The bulk solution is buffered to pH 5.0–6.5, because peptide degradation and aggregation accelerate under neutral-to-alkaline conditions. Osmolality is adjusted to 280–320 mOsm/kg using sodium chloride 0.85–0.95% w/v or mannitol 4.0–5.0% w/v. Benzalkonium chloride at 0.01% w/v remains a common preservative; however, preservative efficacy testing per USP <51> is mandatory because the cationic lipopeptide can partition onto filter membranes and container surfaces, lowering the free preservative concentration in the final filled unit. The combination of benzalkonium chloride and polymyxin B sulfate requires verification of antimicrobial recovery in the presence of neutralizers, since residual quaternary ammonium compounds may interfere with microbial assay results unless neutralized with polysorbate 80 and lecithin at defined concentrations.

Filtration of the finished ophthalmic solution is conducted through 0.22 µm PVDF or PES membrane cartridges; nylon membranes are avoided because their anionic surface retains the cationic polypeptide. Terminal steam sterilization at 121°C is not assumed without pH-stability data, so aseptic filtration followed by filling into low-density polyethylene dropper bottles is the standard route. Subvisible particulate matter is controlled by light obscuration according to USP <789>, using detection thresholds at 10 µm and 25 µm. Endotoxin levels are assessed with USP <85>. In production vessels, the API is dissolved in Water for Injection at 20–25°C under low-shear impeller mixing; local alkaline spikes during pH adjustment with 0.1 N NaOH are avoided because precipitation occurs above pH 7.0 and cannot always be reversed by acidification. Stainless steel 316L tanks with spray-ball CIP are used to reduce cross-contamination with β-lactam or cephalosporin residues; hold times beyond 24 h between final filtration and filling start are qualified by stability data because the peptide can bind to metal oxide layers during extended contact.

The fill volume is targeted to 5–10 mL dropper bottles, and the closure system is qualified for extractables under USP <661.1/661.2>. In-use stability after first opening of the dropper is a critical test parameter. For multi-dose veterinary ophthalmic products, in-use dating is justified by preservative effectiveness and sterility testing and often does not exceed 28 days once opened. The final formulation is released for pH, osmolality, preservative content, potency by microbiological assay or HPLC, and subvisible particulates. Sterility is confirmed by membrane filtration in an isolator under USP <71>.

What Happens When Polymyxin B Sulfate Is Reconstituted for Intravenous Infusion?

Polymyxin B sulfate for parenteral administration is supplied as a sterile lyophilized cake in vials potently standardized to not less than 6,000 IU/mg on the dried basis, with potency confirmed by the cylinder-plate microbiological assay described in USP <81>. Reconstitution is commonly performed with Water for Injection or 0.9% sodium chloride injection; the resulting concentrate is then diluted into 5% dextrose injection for infusion. The peptide undergoes aggregation above pH 7.5 and is sensitive to elevated divalent cation concentrations, which reduce outer-membrane-disrupting activity; therefore calcium- or magnesium-containing diluents such as lactated Ringer’s solution are generally avoided. The dissolved API can display surface activity that causes foaming during manual reconstitution; on manufacturing lines, vacuum venting or slow swirl is used instead of vigorous shaking. Bulk reconstitution vessels are equipped with sanitary diaphragm valves and vent filters to control foaming and minimise droplet carryover into laminar air flow zones.

Post-reconstitution physical stability is not harmonized across compendial monographs, and published data for this specific formulation configuration are limited. Facilities typically assign an initial use window of 24 h at 2–8°C for diluted infusions until bracketing stability data justify extension under ICH Q1A(R2). Terminal sterilization of the reconstituted solution is not recommended because the peptide may undergo oxidative degradation; instead, aseptic handling and immediate transfer through 0.22 µm low-protein-binding filters are required. Particulate control is performed under USP <788>; for small-volume parenterals, subvisible counts are limited by the specific dosage volume and container type. The vial and stopper configuration must be qualified for container closure integrity per USP <1207>, because lyophilized cakes are hygroscopic and moisture ingress after vial opening leads to loss of vacuum and potency drift.

On a production scale, the primary incompatibility is not the API itself but the concurrent preparation of polymyxin B sulfate with aminoglycoside or cephalosporin solutions. Separate fluid lines and dedicated mixing vessels are required because residual antibiotic films can form insoluble complexes. The clinical nephrotoxicity of polymyxin B is documented and imposes an indirect formulation boundary: batch records must ensure no accidental addition of divalent cation-containing salts, and label storage conditions must specify controlled room temperature not exceeding 25°C with protection from light. Because the API is hygroscopic, relative humidity in the dispensing suite is maintained below 40% RH during powder handling; otherwise caking and inaccurate potency adjustment occur. These constraints are established through production-scale use of isolator-based filling lines with in-process weight checks at ±2% of target fill.

Canine and feline otitis externa formulations add polymyxin B sulfate at 10,000 IU/mL to an aqueous or oil-in-water base, often with neomycin sulfate 5.0 mg/mL and dexamethasone sodium phosphate 0.1% w/v. In this application the major formulation conflict is thickener chemistry: anionic carbomer at 0.2–0.5% w/v is generally incompatible because the cationic peptide forms electrostatic complexes that reduce gel clarity and produce a stringy precipitate. Nonionic hydroxyethylcellulose or hypromellose 0.3–0.5% w/v is substituted to achieve contact time without loss of activity. The pH is maintained at 5.5–6.5; ear canal tolerance and peptide stability overlap in this narrow range, so pH adjustment after thickener hydration is performed with 0.1 N HCl rather than citric acid to avoid buffering interactions.

Thickened otic suspensions are prepared on a rotor-stator homogenizer at 5,000–10,000 rpm with a jacketed vessel cooled to 15–20°C. The dispersed phase is passed through a 40 µm screen before filling, and particle size is monitored with laser diffraction to maintain a D90 below 75 µm. If the product is labeled sterile, USP <71> applies. For nonsterile veterinary otic products, microbial limits follow USP <61> and USP <62>, with absence of Pseudomonas aeruginosa and Staphylococcus aureus. Preservative efficacy is evaluated with USP <51> where multi-dose containers are used. The fill volume for companion animal drops is commonly 7.5–15 mL, and the dropper tip geometry is specified to prevent trauma to inflamed ear canals; extractable testing follows USP <661.1/661.2>.

In oral powder and granule directions, polymyxin B sulfate is used as a non-absorbed enteric Gram-negative agent, and the major technical problem is homogeneous dilution because the API is potent in International Units per gram and is frequently added at only 0.5–5.0% w/w of a lactose monohydrate or dextrose monohydrate carrier. The initial API is passed through a 500 µm screen, then geometrically diluted in a 50–100 L V-blender or bin blender. Blend uniformity is measured by taking 10 sampling points and determining the relative standard deviation; a threshold of CV ≤ 5.0% is used before granulation or direct filling. If granules are required, dry granulation by roller compaction is preferred over aqueous wet granulation because the hygroscopic API hydrates unevenly and forms hard agglomerates that resist size reduction. Magnesium stearate is kept below 1.0% w/w to avoid over-lubrication and a drop in bulk density.

Premix conversion into feed at the farm or feed mill must account for segregation during handling. A final medicated premix is typically diluted to 2,000–10,000 IU/g; lower concentrations require the addition of colloidal silicon dioxide at 0.1–0.3% w/w as a flow-conditioning agent. Packaging in aluminum foil laminate pouches with silica gel desiccant is used because moisture uptake above 60% RH produces caking and potency loss. Stability storage is bracketed at 25°C/60% RH and 40°C/75% RH. Published data for polymyxin B oral premix configurations are limited, so the assignable shelf life is based on internal stability protocols. Polymyxin B sulfate has no chromophoric degradation marker that allows simple HPLC stability indication, so the microbiological assay remains the release method under USP <81>. Compliance for medicated feed premixes falls under FDA 21 CFR Part 225 in the United States; in the EU, Regulation (EU) 2019/4 harmonizes requirements for medicated feed. Batch records document cleanout between incompatible antibiotic premixes; a rinse or swab limit of ≤10 ppm for cross-contamination is typical where the next product contains cephalosporins or penicillins.

If Tablet or Capsule Forms Are Specified for Non-Systemic Enteric Delivery

Polymyxin B sulfate shows negligible oral bioavailability, generally below 5%, because the cyclic polypeptide is not absorbed across intestinal mucosa; therefore tablet or capsule monographs are not intended to deliver systemic concentrations but instead function as non-absorbed enteric preparations. When tablet production is required, the API is first blended with microcrystalline cellulose and lactose monohydrate, then compacted on a roller compactor at a roll pressure of 30–60 bar to form granules with a bulk density above 0.50 g/cm³. Direct compression without granulation is generally not viable because the raw API has poor flow, low bulk density, and a tendency to adhere to punch faces under compression forces of 10–20 kN. Tablet compression is run on a rotary press with forced feeder, lower punch precompression at 5–8 kN, and main compression adjusted to produce tablet hardness of 50–100 N. Magnesium stearate is added at 0.5–1.0% w/w as a final external lubricant.

Capsule filling uses a dosator or tamping pin machine with precompacted granule plugs; target plug density is 0.6–0.8 g/cm³. Dissolution testing under USP <711> in 0.1 N HCl or simulated intestinal fluid is used mainly to confirm batch-to-batch release rather than to predict systemic absorption. An enteric coating is not required for activity but may be specified to protect the peptide from gastric acid; a methacrylic acid copolymer film at 5–7% weight gain is used when gastric bypass is desired. Alkaline fillers such as sodium bicarbonate or calcium carbonate are prohibited because the API degrades rapidly above pH 7.0. Batch records require relative humidity below 40% RH in compression and encapsulation suites to prevent punch filming and capsule shell brittleness. Published bioavailability data for veterinary polymyxin B tablets are limited, so bioequivalence is not a meaningful release criterion; content uniformity per USP <905> and assay per USP <81> serve as the primary controls.

Wound Dusting Powders, Carrier Selection, and Excipient Incompatibilities

Topical wound dusting powders combine polymyxin B sulfate, bacitracin zinc, and neomycin sulfate in a talc, maize starch, or purified kaolin base. The API concentration is usually expressed as 5,000 IU/g, and the particle size of the final powder is controlled at D90 ≤ 75 µm to adhere to moist wound surfaces without causing excessive irritation. Starch-based carriers are preferred over talc in oozing wounds because talc can form a hydrophobic barrier that traps exudate. The carrier is dried to ≤2.0% moisture before blending, and the API is incorporated by low-speed tumble blending rather than high-shear mixing to avoid electrostatic segregation of the fine polypeptide fraction. Sterility is not routinely specified for dusting powders; microbial control follows USP <61> and USP <62>, with absence of Clostridium spores required for products intended for deep or puncture wounds.

Because polymyxin B sulfate is cationic and bacitracin zinc is zinc-chelated, the combination can show incompatibility if free moisture is present; therefore desiccant-lined packaging and moisture-barrier pouches are standard. The bulk powder is filled by auger filler at 10–50 g per unit, with nitrogen flush when residual oxygen affects potency. No organic solvent granulation is used, and the operational boundary is humidity: production at relative humidity above 50% produces clumping, uneven distribution, and false potency readings. The blend is assayed at 3 sampling depths within the hopper to detect segregation; acceptance is RSD ≤ 6.0%. There are no harmonized wound powder monographs for veterinary polymyxin B, so the release specification is based on the supplier’s pharmacopeial assay and on total aerobic microbial count.