High-Shear Wet Granulation Routes for Paste-API Tablets

Tablet manufacture begins with a formal as-is assay of the paste-grade veterinary API, followed by loss-on-drying according to USP <731> or Ph. Eur. 2.2.32. Because the paste may contain propylene glycol, polyethylene glycol, or water as a vehicle, the batch calculation uses the anhydrous and solvent-free active equivalent, not the gross paste mass. If residual solvents are suspected, USP <467> or Ph. Eur. 2.4.24 is applied. Direct compression is not a viable route when the paste fraction exceeds 5% w/w of the granulate, because the semi-solid phase increases punch filming and die-fill variability. The corrective process is high-shear wet granulation. The paste is diluted 1:4 to 1:10 with a preblend of microcrystalline cellulose, lactose monohydrate, croscarmellose sodium, and sodium starch glycolate. The mixture is processed in a high-shear granulator with impeller tip speed of 4–8 m/s; the paste is metered through a positive-displacement or peristaltic pump to avoid binder pooling at the vessel wall. Water or an aqueous binder solution is sprayed to reach granulate moisture of 3%–5% w/w. Over-wetting above 6% produces dense granules, increases the disintegration time beyond 15 minutes in water at 37 °C, and can slow drug release. The wet mass is milled through a 2.0 mm conical screen, dried in a fluid-bed dryer at 45–55 °C to a final loss-on-drying of 2.0%–3.5%, and then milled through a 0.8 mm screen. Compression is performed on a rotary press with precompression force of 4–8 kN and main compression force of 8–20 kN. Magnesium stearate is added at 0.5%–1.0% w/w and blended for 2–3 minutes. Tablet hardness is maintained at 50–80 N, friability below 1.0% by USP <1216> or Ph. Eur. 2.9.7, and disintegration below 15 minutes by USP <701>. Content uniformity is assessed by USP <905> or Ph. Eur. 2.9.5, and dissolution by USP <711> Apparatus II at 50–75 rpm in 900 mL of buffer. The manufacturing dossier for such tablets typically cites 21 CFR 211.84 for raw material identity, 21 CFR 211.110 for in-process blend uniformity, and 21 CFR 211.165 for finished product testing. Terminal tablets are chewable or film-coated dosage forms for dogs, cats, calves, and swine. The final formulation contains 5%–25% w/w active equivalent, 50%–70% fillers, 2%–5% disintegrant, 1%–3% binder, and 0.5%–1.0% lubricant.

For injectable presentations, a paste-grade veterinary API imposes burdens that do not exist in solid oral processing. The starting paste is not sterile, and the vehicle may contribute osmolarity, viscosity, or irritation. When the active is water-soluble at the target pH, the paste is introduced into a jacketed stainless steel vessel containing water for injection and a co-solvent blend of propylene glycol or PEG 300 at 10%–30% v/v, where the pH is adjusted with hydrochloric acid, sodium hydroxide, citrate, or phosphate buffer after the API has begun to dissolve. If the active is poorly soluble, a sterile suspension is produced. The paste is dispersed with high-shear mixing and wet-milled using zirconium oxide beads until the D50 is below 5 µm and the D90 is below 10 µm, checked by laser diffraction according to USP <429> or Ph. Eur. 2.9.31. Sterile suspensions cannot be terminally filterable; therefore the entire formulation, filling, and sealing operation is conducted aseptically in a Grade A unidirectional airflow zone within a Grade B background classified according to ISO 14644-1:2015 and EU GMP Annex 1. Solutions are passed through a 0.22 µm PVDF or PES membrane; filter integrity is tested before and after filling. Filling is performed on rotary piston pumps or peristaltic pumps under nitrogen overlay, with headspace oxygen held below 2% for oxygen-sensitive actives. Sterility is verified by USP <71> or Ph. Eur. 2.6.1; bacterial endotoxin is controlled by USP <85> or Ph. Eur. 2.6.14; particulate matter is controlled by USP <788> with limits of not more than 6,000 particles ≥10 µm and 600 particles ≥25 µm per container for small-volume injections. The formulation ratio is normally 0.5%–10% w/v active equivalent; preservative at 0.1%–0.3% w/v for multi-dose vials; antioxidant at 0.05%–0.2% w/v; and tonicity agent to 270–330 mOsm/kg. Terminal injectable products for cattle, swine, horses, sheep, and goats are filled into amber Type I glass vials or cyclic olefin copolymer syringes with butyl rubber stoppers and aluminium flip-off caps. The batch record must include the paste vehicle in the mass balance because the vehicle can alter osmolality and preservative partitioning. Published data for a specific paste vehicle combination may be limited, so the final osmolality and preservative recovery are confirmed experimentally during pilot formulation.

What Limits Powder Flow After Adsorbing a Paste API onto Microcrystalline Cellulose?

Encapsulation of a paste-grade API fails in the hopper unless the paste is first converted into a free-flowing powder. The paste is combined with microcrystalline cellulose or dibasic calcium phosphate in a planetary mixer, with the carrier charged in three equal portions. The ratio is commonly 1:2 to 1:4 paste to carrier by weight. Colloidal silicon dioxide is incorporated at 0.5%–1.0% w/w to reduce interparticle cohesion, but magnesium stearate at 0.5% w/w is added only after drying to avoid over-lubrication. The wet adsorbed mass is dried in a forced-air oven at 40–50 °C or in a vacuum dryer at not more than 60 °C under −0.08 MPa; the target loss-on-drying is below 2.0% by USP <731>. The dried powder is screened through a 0.6 mm sieve. Acceptance for automatic dosator machines is a Carr Index between 15% and 25% and a Hausner ratio below 1.35. If the Hausner ratio rises above 1.45, the powder is re-sieved and additional glidant is added in 0.25% increments. Capsule filling is conducted at 60,000–100,000 capsules/h, with weight checks every 30 minutes using a balance of ±1 mg readability. Content uniformity is controlled by USP <905> or Ph. Eur. 2.9.5; dissolution is controlled by USP <711>. The capsule shell is hard gelatin or HPMC, size 0 to size 3. The terminal product is a powder-filled capsule for companion animals or small ruminants. A representative formulation contains 15%–30% w/w paste-derived active equivalent, 60%–70% filler, 2%–4% disintegrant, 0.5%–1.0% glidant, and 0.5% lubricant.

Adsorption onto fumed silica or precipitated calcium carbonate is the first unit operation when the paste-grade API is destined for oral powders that must be dissolved or dispersed in drinking water or applied as a top dress. The paste is mixed with the silicate carrier at a 1:1 to 1:3 ratio in a planetary mixer until the mass is free-flowing. This intermediate is transferred to a V-cone or double-cone blender and blended with dextrose monohydrate, sodium citrate, and a flavouring agent at 60%–70% of blender capacity for 20–40 minutes at 15–25 rpm. The finished powder is milled through a 0.5 mm screen and filled into heat-sealed aluminium foil sachets or high-density polyethylene jars containing desiccant sachets. The active equivalent in the final powder is usually 5%–20% w/w; the carrier system is 60%–90%; citric acid or sodium citrate is 1%–5%; silicon dioxide is 0.5%–2%; and sweetener or flavour is 0.1%–1%. The critical control point is moisture: residual water above 3.0% w/w leads to sachet bloating and accelerates hydrolysis. The powder is therefore dried or blended under controlled relative humidity below 30% RH and tested by USP <731> or Ph. Eur. 2.2.32. Blend uniformity is evaluated with a sampling thief at the beginning, middle, and end of filling; the relative standard deviation of the active assay should be below 5.0%. Content uniformity references USP <905> or Ph. Eur. 2.9.5, and microbial limits are applied according to the product licence. Terminal products include water-soluble powders for poultry and swine, oral top-dress powders for horses, and electrolyte/medicated powders for calves. The paste vehicle lot concentration is recorded because it can reduce the apparent assay and change the final powder flow.

Extrusion-spheronization is selected when the paste-grade API must be converted into low-dust, free-flowing granules for direct oral administration or for proportioning into drinking water systems. The paste is combined with microcrystalline cellulose, lactose monohydrate or mannitol, and a binder solution in a planetary mixer. A typical working ratio is API paste 20%–40% w/w, microcrystalline cellulose 50%–70%, lactose 0%–10%, and binder 1%–3%. The wet mass target moisture is 25%–35% w/w. The mass is extruded through a dome or radial extruder fitted with a 0.8–1.2 mm screen at a feed rate of 20–50 kg/h. The extrudate is spheronized at 800–1200 rpm on a cross-hatch plate for 2–6 minutes, producing pellets of 0.8–1.4 mm diameter. The pellets are dried in a fluid-bed dryer at 50–60 °C until loss-on-drying is below 3.0% and then sieved. If the active is bitter, a Wurster column applies an ethylcellulose or polymethacrylate coat at a weight gain of 3%–10%. Particle-size distribution is checked by sieve analysis according to Ph. Eur. 2.9.12; not more than 15% of the pellets should fall outside the target size range. Dissolution is tested on coated granules by USP <711> Apparatus I at 100 rpm, with enteric or taste-masked coatings assessed by a two-stage dissolution protocol. The terminal granules are packaged in single-dose sachets, bulk jars, or HDPE bottles and are administered directly, mixed with feed, or reconstituted in water. This form is used for anthelmintics in horses, anticoccidials in poultry, and antimicrobials in pigs. In-process sampling follows 21 CFR 211.110, and finished product testing follows 21 CFR 211.165.

When a Medicated Premix Is Mixed in a Ribbon Blender, Carryover Is Controlled by Sequence, Not Time

Premix manufacture from a paste-grade veterinary API requires geometric dilution before any large-scale mixing. Direct addition of the paste to a ribbon mixer creates agglomerates, and active assay relative standard deviation can exceed 15% even after extended blending. The paste is first adsorbed onto ground corn, wheat middlings, or calcium carbonate at a ratio of 1:2 to 1:5 by weight. This intermediate is then combined with the full carrier in a horizontal ribbon or double-shaft paddle mixer. The mixer is operated at 70%–80% of gross volume, with mixing times of 10–20 minutes. Homogeneity is verified by sampling 10–12 points across the mixer after 5, 10, and 15 minutes; the relative standard deviation of the active substance is required to be below 5.0%. If the RSD exceeds 5.0%, the corrective action is to increase the number of carrier premix steps rather than to lengthen mixing time, because over-mixing can raise electrostatic segregation. The final medicated premix contains 0.5%–10% active equivalent. Farm feed is prepared at an inclusion rate of 0.5–10 kg premix per tonne of complete feed, depending on the licensed dose. Carryover control requires the subsequent non-medicated batch to contain less than 2.5% of the previous batch's active level after flushing with 50–100 kg of ground corn or calcium carbonate. Analytical quantification is performed by LC-MS/MS with a limit of quantification below 0.5 mg/kg in feed. The regulatory frame includes EU Regulation 2019/4, FAMI-QS Code of Practice, GMP+ BA2, 21 CFR 225, and 21 CFR 558.3. Terminal products are 1%, 2.5%, or 5% medicated premix bags for feed mills and farm mixing units.

Buffering Sequence, Not Mixing Speed, Governs Final Solubility

Oral liquids, drenches, and drinking water concentrates are prepared from paste-grade veterinary APIs by solubilization or suspension. The most common manufacturing failure is the addition of the paste directly to water before the co-solvent or buffer system is fully dispersed. This sequence produces a gelatinous precipitate or a reversible floc that does not clear with mixing. The correct order is to charge propylene glycol, glycerin, or PEG 400 into a jacketed vessel at 10%–30% v/v, add the paste API under high-shear mixing at 1,000–2,500 rpm, and then add the aqueous phase slowly. The pH is adjusted with citric acid, sodium citrate, or phosphate buffer after the API has dissolved; the target range is read from the solubility curve and is often pH 3.0–5.0 for weak bases or 7.0–9.0 for weak acids. The final solution or suspension is passed through a 100 µm nylon bag filter; a solution may be passed through a 0.45 µm membrane if filtration loss is acceptable. The active content is commonly 1%–10% w/v; co-solvent is 10%–30% v/v; sweetener such as sodium saccharin is 0.1%–0.5% w/v; preservative such as sodium benzoate or potassium sorbate is 0.1%–0.2% w/v; and antioxidant is 0.05%–0.1% w/v. pH is tested by USP <791> or Ph. Eur. 2.2.3; viscosity by USP <911>; deliverable volume by USP <698>; and content uniformity by weight or volume. The container closure system is high-density polyethylene with induction-sealed caps; light-sensitive APIs require amber polyethylene terephthalate. Terminal products include 1 L or 5 L drinking water concentrates, 100 mL to 1 L drench bottles, and graduated syringes for sheep, cattle, pigs, and poultry. Because published solubility data for a given paste-grade active in mixed solvent systems are often limited, the co-solvent ratio is confirmed by pilot-scale phase-solubility screening before scale-up. The paste vehicle is included in the final active concentration calculation because it contributes volume and may alter density.