For direct-compression tablet applications, the received veterinary injection-grade API is controlled at intake for particle-size distribution and moisture content, because excessive fines increase segregation and high moisture causes picking and sticking on high-speed rotary presses. The target d(0.5) is normally held between 45 µm and 180 µm, with moisture not exceeding 2.0% w/w unless a pre-drying step is triggered at 40–45°C in a tray dryer for 2–4 hours. The API is pre-screened through a 500 µm stainless-steel mesh and transferred to a bin blender with microcrystalline cellulose, croscarmellose sodium, and colloidal silicon dioxide. Formulation loadings range from 5 wt% to 40 wt% for companion-animal tablets, while disintegrant level is fixed at 2 wt% to 5 wt% and lubricant level at 0.5 wt% to 1.0 wt%. Magnesium stearate is added as a final lubrication step after the main blend has reached acceptable homogeneity, because over-lubrication beyond 5 minutes at 12 rpm in a production bin blender reduces tablet hardness and slows disintegration. Compression is performed on a rotary tablet press with 10–12 station tooling, main compression force between 6 kN and 18 kN, and precompression assistance at 30% to 50% of main force to reduce lamination. Tablet hardness for chewable veterinary products is held between 50 N and 120 N, while friability is controlled at ≤1.0% according to USP <1216>. Production-scale failure modes include capping when moisture falls below 1.0% w/w, picking when moisture exceeds 2.5% w/w, and weight variability caused by hopper segregation when the blend contains a high proportion of fines.

Blend uniformity is tested under USP <905> and Ph. Eur. 2.9.40, using an acceptance value of ≤15 unless the tablet is a very low-dose product. Dissolution testing for immediate-release tablets uses USP <711> apparatus 2 at 50 rpm in 0.01 N hydrochloric acid or pH 6.8 phosphate buffer, with a typical Q value of 80% at 30 minutes. Finished tablet strengths commonly include 25 mg, 50 mg, and 100 mg scored tablets for dogs and cats; packaging is selected to limit moisture ingress when the API is hygroscopic. If relative humidity in the compression suite exceeds 60%, the direct-compression line is run with dehumidification and the hopper is fitted with a low-residence-time insert to reduce environmental moisture uptake.

What Limits Sterile-Filtration Throughput in Aqueous Injectable Solutions?

Aqueous parenteral products for cattle, swine, and companion animals are compounded by dissolving the API in Water for Injection at final concentrations commonly between 50 mg/mL and 200 mg/mL. The vessel is a jacketed 316L stainless-steel compounding tank, and an inert nitrogen overlay is applied if forced degradation data show oxidative sensitivity. pH adjustment uses 0.1 N hydrochloric acid or 0.1 N sodium hydroxide; the target pH is fixed at the point of maximum solution stability, typically within a control window of ±0.1 unit. Solution temperature is held between 20°C and 25°C during compounding to maintain filterability. Pre-filtration through a 0.45 µm polyethersulfone membrane reduces particulate load before the terminal 0.22 µm polyvinylidene fluoride sterilising filter. The critical production limit is filter capacity, not tank turnover; batch records have shown throughput loss of 30% to 50% when the solution is cooled below 15°C or when viscosity rises above 10 mPa·s at 25°C. Holding time before filtration is therefore limited to 4–8 hours to control bioburden. If terminal steam sterilisation at 121°C for 15 minutes is considered, the API must show degradation below 0.5% area in forced degradation; published data for this specific configuration is limited, so each formulation requires a thermal challenge study. Sterile filling is performed in a Grade A zone over a Grade B background, with filter integrity tested by bubble point before and after fill. The finished injectable is released for sterility according to USP <71> and Ph. Eur. 2.6.1, bacterial endotoxin according to USP <85> and Ph. Eur. 2.6.14, and particulate matter according to USP <788> and Ph. Eur. 2.9.19. Silicone tubing is avoided when the formulation contains benzyl alcohol because plasticizer extraction has been observed; fluoroelastomer or polyethylene-lined transfer lines are specified instead.

Capsule Filling and Low-Dose Oral Powder Containment

Low-dose veterinary capsules are filled as pre-blended powders rather than granulated masses when the dose is below 10 mg per unit. The API is mixed by geometric dilution in three stages to avoid carryover and improve homogeneity; final blend API content may be as low as 1 mg per capsule. Fill weight is controlled at 150–400 mg on an intermittent-motion capsule machine with tamping pins, and fill weight variation is held at ±3% relative standard deviation. Encapsulation is conducted in a negative-pressure isolator at -50 Pa when the API has occupational exposure limits; for non-potent materials, area humidity is maintained below 40% RH for gelatin capsules or 55% RH for HPMC capsules. Capsule content uniformity follows USP <905> and Ph. Eur. 2.9.40; disintegration is tested in water at 37°C according to USP <701> and Ph. Eur. 2.9.1, with complete disintegration expected within 15 minutes for immediate-release products. The finished capsule shell is sealed if the fill contains a hygroscopic API, and desiccant loading is calculated from the moisture vapour transmission rate of the packaging material.

Feed-mill premix production starts with the selection of a carrier with low free moisture content and neutral pH, typically corncob meal, calcium carbonate, or lactose in regions where lactose is cost-effective. The API is first pre-blended with the carrier at 1:10 ratio in a low-capacity drum blender or ribbon mixer, then this intermediate is diluted 1:100 into the final premix to achieve working concentrations between 50 g/kg and 200 g/kg. Mixing is performed in a horizontal ribbon mixer with working capacity at 60% to 70% of nominal volume; fill levels above 75% reduce bed turnover and create dead zones near end plates. Mix uniformity is tested by sampling at least 10 points using a sample thief, with an acceptance limit of relative standard deviation ≤5% for the active assay. The EU Regulation 2019/6 requires homogeneous distribution of veterinary medicinal products into medicated feed, and feed business operators use ISO 6497:2012 as the sampling reference. In the United States, medicated feed applications are regulated under 21 CFR 558 where the API is cleared for this route. Particle-size reduction of the API to d(0.5) 80–150 µm reduces segregation during pneumatic conveying, but over-milling below 20 µm increases dust generation and electrostatic adhesion to plastic surfaces. The finished premix is metered into final feed at a rate that delivers 5–20 mg/kg body weight per day, depending on the species, indication, and daily feed intake. Carryover control is performed by batch sequencing and wash rinses; a final rinse with coarse ground corn is used to flush the mixer discharge gate, and the flushed material is quarantined rather than reworked.

Drinking Water Solubilisation and pH Control Points

Oral solutions for flock and herd administration are compounded as concentrated stock solutions that are proportioned into drinking water at a final dilution of 0.5% to 1.0% v/v. The stock concentration is set at 10% to 20% w/v for most oral delivery systems, while the final concentration in the water line is derived from the daily water intake of the target species. Solubility is screened at 20°C over the pH range 6.5 to 7.5; if the API is a weak acid, pH raising to 8.0–9.0 with sodium carbonate or sodium bicarbonate increases solubility but shortens holding time to 24 hours due to base-catalysed hydrolysis. For weak base APIs, pH lowering to 3.5–4.5 with citric or phosphoric acid can produce solution concentrations near 100 mg/mL, but the low pH requires evaluation of metal compatibility in galvanised distribution lines. Water hardness above 300 ppm CaCO3 can precipitate phosphate buffers; water softening or selection of a non-phosphate buffer is then required. Proportioner pumps are calibrated daily to a metering accuracy of ±5%, and the stock solution is protected from light if photodegradation is significant. Incompatibility is observed with chlorinated alkaline water, which can generate oxidative degradation products for APIs containing phenolic or amine groups; sodium thiosulfate is not added to medicated water unless chemical compatibility has been demonstrated. Finished oral solution products for poultry and swine include 100 mL/L drinking-water concentrate and 200 mL/L drench solution for cattle, with pH and assay stability monitored over the labelled in-use period.

When Extrusion-Spheronisation Replaces High-Shear Granulation for Multi-Species Oral Granules

When oral granules must remain as discrete, free-flowing spheroids for top-dressing onto feed or packing into multi-dose sachets, high-shear granulation is replaced by extrusion-spheronisation. The dry blend consists of the API, microcrystalline cellulose as spheronisation aid at 20–40 wt%, and lactose or dicalcium phosphate as filler. Purified water or binder solution is added to a liquid-to-solid ratio of 35–45 wt%; the exact endpoint is determined by torque and visual mass consistency before extrusion. The wet mass is extruded through a screw extruder equipped with a 0.8–1.5 mm screen at 30–60 rpm, then discharged into a spheroniser with cross-hatch plate at 400–800 rpm for 2–6 minutes. A moisture deviation of ±2% from the predetermined endpoint forces the operation outside its acceptable window: below the lower limit the extrudate fractures, and above the upper limit the spheroniser produces oversized agglomerates and plate fouling. Drying is carried out in a fluid-bed dryer at 55–65°C until loss on drying is ≤3.0%; higher inlet temperatures cause surface crusting and slower core equilibration. Particle-size distribution is confirmed by sieve analysis according to Ph. Eur. 2.9.12 or ISO 2591-1, with a yield target of 80% or more in the specified sieve fraction. Content uniformity is tested per Ph. Eur. 2.9.40, and the finished spheroids are packaged with desiccant if the API is moisture-sensitive. Application rates for in-feed granules are calculated from the labelled dose and average daily feed intake, often in the range of 5–20 mg/kg body weight per day in swine and poultry.

Powders for reconstitution into oral suspension are manufactured as free-flowing dry blends of the API, sucrose or sorbitol as carrier, and xanthan gum as suspending agent at 0.1–0.3 wt% to slow sedimentation. The blend is filled into multi-dose bottles under controlled humidity not exceeding 30% RH, because hygroscopic excipients and the API can absorb moisture and reduce chemical stability. After reconstitution with potable water to a final volume of 50 mL or 100 mL, the suspension develops a viscosity between 50 mPa·s and 200 mPa·s at 25°C, sufficient to maintain uniform dose after 30 seconds of gentle inversion. Preservative systems, if required, are selected from methylparaben and propylparaben, but the API must be tested for preservative binding to the suspending agent. The product is used mainly for neonatal calves and piglets, where the dose volume is adjusted by body weight and delivered by syringe or drench nozzle. Compliance testing includes water content by USP <921> or Ph. Eur. 2.5.12, microbial limits by USP <61> and Ph. Eur. 2.6.12, and sedimentation volume after 1 hour and 24 hours. The in-use shelf life after reconstitution is limited to 7–14 days unless long-term preservative efficacy data support a longer period.