Primidone veterinary grade API entering a solid oral dosage line is first characterized for particle size, bulk density, and flow function because the supplied material frequently fails direct compression due to cohesive arching and segregation. Laser diffraction per USP <429> is used to set a D90 window of 20 µm to 40 µm for tablet grades; coarser material above 100 µm produces inadequate content uniformity in low-dose formulations, while micronized material below 5 µm increases triboelectric charging and bin blender wall adhesion. A representative wet-granulation process for a 250 mg primidone tablet core incorporates primidone 50.0 mg, lactose monohydrate Ph. Eur. 1067 at 120–140 mg, microcrystalline cellulose Ph. Eur. 1016 at 25–40 mg, crospovidone at 7.5–10.0 mg, pregelatinized starch as binder at 5–8 mg, and magnesium stearate at 2.5–5.0 mg. The API is pre-blended with lactose in a bin blender at 12 rpm for 10 min before the binder solution is introduced; this order prevents localized overwetting of primidone particles. High-shear wet massing is performed at impeller 200–300 rpm and chopper 1500 rpm for 3–5 min, with the end point determined by watts per kilogram rather than time alone. The wet mass is passed through a 1.0 mm screen and dried in a fluid-bed dryer with inlet air at 55–60 °C to a loss on drying of 1.5–2.5%. Dried granules are milled through a 0.8 mm screen and final-blended with crospovidone and magnesium stearate. Compression is performed on a 27-station rotary tablet press with pre-compression force 5–8 kN and main compression force 12–18 kN, producing tablets of hardness 60–100 N, friability below 1.0% per USP <1216>, and disintegration below 15 min per USP <701>. Content uniformity is assessed according to USP <905>, and dissolution is evaluated in 900 mL of 0.1 N hydrochloric acid at 37 °C with USP apparatus 2 at 75 rpm; acceptance criteria follow the primidone tablet monograph rather than a generic release specification. Terminal packaging is a PVC/PVDC/aluminum blister providing a moisture barrier; tablets are not packaged in cotton-stoppered bottles when RH during packing exceeds 60% because primidone tablets can pick up moisture on long storage.

Does Direct Compression of Primidone API Survive High-Speed Rotary Tableting Without Prior Granulation?

Direct compression of primidone veterinary API is constrained by its needle-like crystal habit and low bulk density, which produce a Carr index commonly above 28% when unmilled material is blended with standard direct-compression bases. In production-scale trials on a 16-station rotary press at speeds above 50 rpm, weight variation drifts beyond ±5% because primidone-rich fines accumulate in dead zones of the feed frame and release as slugs during end-of-hopper discharge. The failure mode is observed as content uniformity RSD exceeding 6.0% in ten tablet samples, even though the blend assay meets 100.0% nominal. Particle size reduction to D90 15 µm improves uniformity but increases wall adhesion and requires colloidal silicon dioxide at 0.5–1.0% w/w to reduce electrostatic charge. Even then, the compressibility of primidone is insufficient to form a robust tablet at compression forces below 18 kN; tablets compacted with microcrystalline cellulose alone show capping at hardness above 80 N because primidone particles act as elastic bodies that recover after compression. Wet granulation or roll compaction is therefore specified for tablet strengths above 25 mg; direct compression is limited to emergency compounding of single batch sizes below 5 kg. The operational boundary for direct compression is a blend diluent ratio not exceeding 1:4 primidone to microcrystalline cellulose and a tablet press speed below 30 rpm; outside these conditions, content uniformity and hardness cannot be simultaneously maintained. Published data for direct compression of primidone veterinary formulations is limited; the above failure modes are reported for low-solubility barbiturate-like APIs and should be confirmed on the actual batch.

Injectable primidone formulations are not commercially widespread because the neutral pyrimidinedione structure has low aqueous solubility, with published solubility values below 1 mg/mL at 25 °C, and published data for a sterilized veterinary primidone injection are limited. Investigational compounding for canine use typically employs a co-solvent vehicle of propylene glycol 40–50% v/v, ethanol 96% 10–20% v/v, benzyl alcohol 1.5–2.0% v/v for multidose preservation in dogs, and water for injection q.s. to 100%. The API is dissolved at 45–50 °C under nitrogen blanketing to minimize oxidative degradation; dissolution is considered complete when visual clarity meets USP <790> visible particulate inspection. The pH of the bulk solution is maintained between 5.0 and 6.5 because alkaline conditions above pH 8.0 accelerate hydrolytic ring-opening of the primidone pyrimidinedione structure. Filtration is performed through a 0.22 µm PVDF membrane at 0.5–1.0 bar differential pressure, and the solution is filled into amber type I glass ampoules under nitrogen. Terminal sterilization by autoclaving at 121 °C for 15 min is only acceptable if pre-validation demonstrates not more than 2.0% total degradation products; if thermal stress produces precipitate or exceeds this threshold, aseptic filtration and low-temperature processing are required. Sterility is verified by USP <71>, bacterial endotoxins by USP <85>, and particulate matter by USP <788>; because the vehicle contains propylene glycol and ethanol, method suitability for membrane filtration sterility testing must demonstrate neutralization of bacteriostatic activity. Terminal product is a 50 mg/mL primidone injection for intravenous or intramuscular use in dogs. Operational limitation: this co-solvent system is incompatible with PVC infusion bags due to plasticizer extraction; use glass or polyethylene containers. For feline patients, propylene glycol and benzyl alcohol are not used at these concentrations because of species-specific toxicity risks; a separate feline formulation with reduced or alternative solvents is required.

When an Oral Solution Is Preferred for Dysphagic Canine Patients, the Manufacturing Controls Shift to pH and Co-Solvent Stability

An oral primidone solution at 10 mg/mL is manufactured by dissolving the API in a mixture of ethanol 96% 10% v/v and propylene glycol 30% v/v, followed by addition of sodium saccharin 0.2% w/v, citric acid monohydrate 0.5% w/v, and purified water q.s. to volume. The solution is adjusted to pH 5.0–6.0 with 1 N sodium hydroxide or 1 N hydrochloric acid; this pH band prevents both acid-catalyzed and base-catalyzed hydrolysis of the pyrimidinedione ring while maintaining palatability in dogs. The solution is clarified through a 1.0 µm polypropylene filter and filled into amber PET bottles with child-resistant closures. Parabens or sorbate preservatives are not added when the container closure remains intact for single-patient use, but microbial limits for non-sterile oral liquids are tested per Ph. Eur. 5.1.4 with TAMC not more than 10² CFU/g and TYMC not more than 10¹ CFU/g. Ethanol is controlled as a residual solvent according to USP <467>; because ethanol is a Class 3 solvent, the limit is 5000 ppm or justified by the formulation composition. The finished solution is stored at 25 °C/60% RH for 24 months in amber glass or PET; the main stability-indicating parameter is appearance, because precipitation of primidone at low temperature may occur if the co-solvent ratio falls below the minimum solubility threshold. Terminal product is an oral solution administered by calibrated syringe or mixed with a small amount of feed; mixing with acidic fruit juice below pH 3.5 is not recommended because the acidic environment can shorten the shelf-life after reconstitution.

Premix Homogeneity and Carryover Limits under Regulation (EU) 2019/4 for Primidone-Containing Medicated Feed

Primidone veterinary API used in a medicated feed premix is manufactured at 1% w/w or 5% w/w active concentration by adsorption onto a lactose monohydrate or wheat semolina carrier in a 500 kg ribbon blender. The API particle size for premix is specified with not less than 95% passing a 125 µm sieve, because coarser API particles produce assay variation in the final diluted feed. Mixing is validated by thief sampling at 10 positions after 8 min, 12 min, and 15 min; acceptance is an RSD below 5.0% for primidone content by HPLC. Carryover is the main process risk because primidone is metabolized to phenobarbital in dogs and can cause prolonged sedation if non-target feed is contaminated. Under Regulation (EU) 2019/4, medicated feed and intermediate products must be labeled with species-specific warnings, and cleaning validation must demonstrate that carryover of the active substance into non-target feed is below the level that causes pharmacological effects in the non-target species. For primidone, the cleaning limit is calculated from the lowest therapeutic dose and feed intake; a typical pilot-cleaning acceptance limit is 0.1% of the active concentration in the next non-target feed batch. The premix is incorporated into final feed at a ratio that depends on the prescribed dose; the final feed is then pelleted at a conditioning temperature of 65–70 °C with retention time 30–60 s. Because published data on primidone thermal recovery in pelleted feed are limited, a pilot-scale assay before and after pelletization is required for each new feed formulation. Terminal premix is packed in 25 kg multiwall paper bags with an inner PE liner and labeled for use in dogs; it is not intended for food-producing species.

For oral powders and granules supplied in sachets, primidone is converted into a sorbitol-free granule to avoid osmotic diarrhea in dogs and to improve dose flexibility for weight-adjusted dosing. A representative 25 mg/g granule contains primidone 25.0 mg, lactose monohydrate 800–850 mg, povidone K30 20 mg, sodium citrate dihydrate 20 mg, and colloidal silicon dioxide 5 mg per gram. The API is pre-milled to D90 20 µm and blended with lactose before a povidone K30 solution in purified water is added; the wet mass is passed through a 0.8 mm screen and dried in a vacuum tray dryer at 45 °C for 8 h to a final moisture not more than 2.0% by Karl Fischer titration per USP <921>. Sodium citrate is included to buffer the reconstituted granule suspension to pH 6.0–7.0; this reduces hydrolysis risk during the period of administration. Dried granules are filled into triplex laminate sachets of PET/Al/LDPE under nitrogen to maintain a moisture barrier. Sachet weight variation is assessed per Ph. Eur. 2.9.5, and content uniformity is assessed per USP <905>. Reconstitution stability in a closed container is 7 days at 2–8 °C; after reconstitution with water, sodium citrate maintains pH 6.0–7.0. Terminal product is a single-dose sachet containing 250 mg primidone in a 10 g granule base, mixed with soft food immediately before administration.

Capsule Filling Parameters and Blend Segregation in Low-Dose Primidone Formulations

Low-dose primidone capsules at 5 mg, 10 mg, and 25 mg are manufactured from a pre-blend rather than by direct API filling because the API has poor flow and segregates during capsule machine powder bed movement. Primidone is first milled to D90 20 µm and mixed with microcrystalline cellulose in a 1:3 ratio in a V-blender at 20 rpm for 15 min; this master blend reduces static adhesion and improves distribution in the final blend. Lactose monohydrate is then added to achieve a final fill weight of 200 mg, and croscarmellose sodium 3% w/w is incorporated as disintegrant. Magnesium stearate 0.5% w/w is added last and blended for only 3 min, because magnesium stearate levels above 1.0% w/w or mixing beyond 5 min prolong dissolution of primidone due to hydrophobic coating of the low-solubility API. Filling is performed on a dosator-type capsule machine with 8.0 mm dosing disc and pin height adjusted to produce a powder plug density of 0.65–0.80 g/cm³; tamping force is monitored because excessive tamping can reduce powder plug porosity and delay capsule disintegration. Weight variation for 200 mg fill is controlled to ±5%, and filled capsules are tested for content uniformity per USP <905> with acceptance value not more than 15. Dissolution is performed in 900 mL of 0.1 N hydrochloric acid with USP apparatus 2 at 75 rpm; capsule shells do not interfere with primidone UV detection at 210 nm if a suitable blank is used. Terminal product is a hard gelatin capsule packaged in high-barrier aluminum foil strips; published data for primidone capsule stability in gelatin at 40 °C/75% RH are limited, so a reduced intermediate stability commitment may be required until real-time data are available.