Robenidine hydrochloride enters tablet manufacturing as a yellow to pale yellow crystalline powder with needle-like crystal habit. Incoming pharma grade API is released with D90 particle size below 100 µm, loss on drying below 1.0%, and residual solvents controlled per Ph. Eur. 5.4. The powder exhibits poor mass flow through a 2.5 mm orifice, with Carr index values frequently above 25% and Hausner ratio above 1.34. Direct compression is limited to dry blends containing ≤15.0 wt% active because higher loads produce cone-caking in the hopper and sticking on punch tips. A representative tablet core at 12.5 wt% robenidine hydrochloride uses 68.5 wt% microcrystalline cellulose PH102, 12.0 wt% lactose monohydrate 200 M, 4.0 wt% crospovidone type A, 2.0 wt% colloidal silicon dioxide, and 1.0 wt% sodium stearyl fumarate. The blend is ribbon-compacted on a Gerteis Mini-Pactor at roll pressure 6 kN/cm, gap 2.0 mm, and roller speed 4 rpm. Milled granules are screened through 800 µm and reblended with 0.5 wt% colloidal silicon dioxide. Tablets are compressed on a Korsch XL 100 at turret speed 30 rpm, precompression force 2.5 kN, main compression force 12 kN, and ejection force not exceeding 350 N. Hardness is held at 70–90 N using a Schleuniger 4M hardness tester. Friability is kept below 0.8% after 100 revolutions per Ph. Eur. 2.9.7. Disintegration is tested in 900 mL water at 37 ± 2 °C per Ph. Eur. 2.9.1; acceptance is not more than 15 minutes. Tablet cores are sampled after compression according to Ph. Eur. 2.9.5 for mass uniformity. The terminal dosage form is a 10 mg or 25 mg oral tablet for single-animal veterinary administration. Metal detection after dedusting uses a 0.8 mm ferrous test piece and a 1.2 mm non-ferrous test piece.

Why Does Capsule Filling Require Spray-Dried Mannitol Instead of Anhydrous Lactose?

Low bulk density and triboelectric charging of robenidine hydrochloride cause dose drift in dosator-style capsule machines. Spray-dried mannitol is selected because its residual moisture is typically below 0.5% and its spherical particle surface reduces triboelectric charging compared with angular anhydrous lactose. A representative hard gelatin capsule size 3 fill mass is 180.0 mg. The active is present at 27.8 wt%, corresponding to 50.0 mg robenidine hydrochloride per capsule. The blend contains 60.0 wt% spray-dried mannitol, 8.0 wt% crospovidone, 3.0 wt% sodium lauryl sulphate, 0.8 wt% magnesium stearate, and 0.4 wt% fumed silica. Sodium lauryl sulphate is pre-dispersed in a geometric dilution before mixing; dry addition without pre-blending causes localised wetting at dissolution stage. Capsule filling runs on a Bosch GKF 1500 at 45,000 capsules/h with tamping pin force of 120 N. Powder bed temperature is maintained below 28 °C because processing above 32 °C has been observed to increase tackiness on production runs. Dissolution testing follows Ph. Eur. 2.9.3 apparatus II in 900 mL of pH 1.2 HCl at 75 rpm; Q value is 80% at 45 minutes. Capsule shells are tested for moisture ingress by Karl Fischer method per Ph. Eur. 2.5.12; moisture content is held below 4.0%. The terminal product is a 50 mg oral capsule for multi-day dosing in companion animal and production animal protocols. Empty capsule shells are size 3 hard gelatin, titanium dioxide-opacified to reduce actinic degradation of robenidine hydrochloride. Batch records require in-process mass checks every 15 minutes.

Medicated feed remains the highest-volume downstream application for robenidine hydrochloride. The EU feed additive Register established under Regulation (EC) No 1831/2003 lists 33 mg/kg complete feed for chickens for fattening and 66 mg/kg complete feed for rabbits. A commercial premix standardised to 66 g/kg robenidine hydrochloride is typical. Addition of 0.5 kg premix per 1,000 kg finished feed delivers 33 mg/kg; addition of 1.0 kg/t delivers 66 mg/kg. The premix is prepared by stepwise dilution on a double-ribbon mixer. Robenidine hydrochloride is pre-dispersed in ground maize carrier for 3 minutes, then mixed for 12 minutes. Carrier particle size is controlled between 200 µm and 800 µm to limit segregation in transport. Finished feed samples are drawn according to ISO 6497:2002, and assay variance across 10 sample points is kept below 5.0% relative standard deviation.

Feed pellets are conditioned at 70–75 °C for 40–60 seconds; retained active after pelleting should exceed 95% of the pre-pelleting assay. The terminal product is a 66 g/kg premix packaged in 25 kg multi-wall paper bags with polyethylene liner. Cross-contamination control uses a flush sequence of at least 250 kg ground maize after medicated batches. Published data for robenidine carry-over in sequential non-medicated batches is limited; operators therefore validate flush mass case-by-case. Mills that run ionophore-containing coccidiostats after robenidine must use separate production lines because electrostatic carry-over on dust filters cannot be excluded by dry flushing alone.

Oral Suspension Concentrate Rheology and Acid-Phase Sedimentation Limits

Robenidine hydrochloride is formulated as an aqueous suspension concentrate when drinking-water medication is required for species that do not consume pelleted feed reliably. The hydrochloride salt has limited aqueous solubility at neutral pH, so the concentrate is designed as a flocculated suspension rather than a solution. A representative concentrate contains 10.0% w/v robenidine hydrochloride, 0.15% w/v xanthan gum, 0.10% v/v polysorbate 80, 0.20% w/v simethicone emulsion, 0.05% w/v sodium benzoate, and citrate buffer to pH 4.2 ± 0.2. The suspending system produces a yield value above 0.8 Pa and apparent viscosity between 250 mPa·s and 450 mPa·s at 20 °C using a Brookfield RVT spindle 3 at 50 rpm. High-shear mixing is carried out on a Silverson L5T rotor-stator at 3,000 rpm for 20 minutes. Particle size after milling is controlled to D90 <30 µm, measured by laser diffraction per ISO 13320:2020. The suspension is filled into 100 mL and 1,000 mL HDPE bottles with low-density polyethylene dose cups. Deliverable volume is checked by Ph. Eur. 2.9.17. Redispersibility after 72 hours is tested by ten manual inversions; the product must return to homogeneity within 15 seconds. The terminal oral presentation is a 10% w/v oral suspension for dilution into drinking water at 1.0–2.0 mL/L depending on target robenidine intake per kg body weight. Published data for exact dose conversion in all species is limited.

When Sterile Injection Particle Size Distribution Drops Below 10 µm, Passivation of Stainless Steel Contact Surfaces Becomes Critical

Parenteral robenidine hydrochloride is a niche formulation route driven by veterinary research protocols and emergency dosing in high-value breeding stock. No Ph. Eur. monograph for robenidine injection is currently listed; formulation work therefore follows the general monograph 0520 for parenteral preparations. A representative sterile suspension contains 5.0% w/v robenidine hydrochloride, 0.5% w/v poloxamer 188, 0.15% w/v methylcellulose 400 cP, 0.9% w/v sodium chloride, and water for injection. Terminal sterilisation by moist heat at 121 °C for 15 minutes is preferred only when the suspension is filtered through a 5 µm membrane and the D50 particle size is maintained below 10 µm. Autoclaving is performed with an F0 value of 15 minutes. If terminal sterilisation cannot be used due to crystal growth, aseptic milling of gamma-irradiated robenidine hydrochloride is required in a Grade A isolator under ISO 14644-1:2015 class ISO 5. Syringeability is measured by force-to-plunge through a 21G needle; the value is kept below 15 N at 25 °C. pH after sterilisation is 4.0–5.0. Sterility testing follows Ph. Eur. 2.6.1; bacterial endotoxin limits follow Ph. Eur. 2.6.14 with water for injection controlled to <0.25 EU/mL. The terminal product is a 50 mL multi-dose vial for intramuscular injection in sheep and goat research models. Published pharmacokinetic and tissue residue data for this specific configuration is limited.

Palatable granules for direct oral administration are manufactured when tablets cannot be administered to small production animals. The granule formulation is built around sucrose spheres 600–710 µm as a core. Robenidine hydrochloride is layered from an aqueous dispersion containing 5.0 wt% active, 3.0 wt% povidone K30, and 0.5 wt% sodium lauryl sulphate in a Glatt GPCG 3 fluid-bed coater. Inlet air temperature is held at 60 °C, product temperature at 38–42 °C, and spray rate at 4.0 g/min. Layering continues until a weight gain of 100% relative to sphere mass is achieved. Granules are then dusted with 0.3% talc and sieved through 1.0 mm. The final active content is 50 mg/g; a 2.0 g sachet delivers 100 mg robenidine hydrochloride. Sachet filling is performed on a multi-lane stick-pack machine with dust extraction below 0.5 m/s to control electrostatic drift. Loss-on-drying is measured by Ph. Eur. 2.2.32 and is limited to 2.0%. The terminal product is a 2.0 g unit-dose sachet for mixing into soft food at the point of administration. Batch-to-batch variability in drug layering efficiency is monitored by near-infrared spectroscopy; the prediction model uses a standard error of prediction below 1.5 mg/g. This granule presentation is used in veterinary pharmacies where tablet splitting is prohibited due to contamination risk.