Injectable Solution Processing and the Role of Tonicity in Field Dosing

Diminazene aceturate (triazoamidine; Berenil) is handled as a water-soluble veterinary-grade API for sterile injectable solutions, and the most widely encountered field formulation is a 70 mg/mL ready-to-use product, equivalent to 7% w/v. The API is first passed through a 250 μm stainless-steel sieve to remove agglomerates, then charged into a jacketed dissolution vessel containing Water for Injection at 20–25°C. Low-shear agitation at 150–300 rpm is maintained because high-shear mixing above 600 rpm can entrain air and generate a persistent foam layer that interferes with downstream sterile filtration. Dissolution is completed when visual inspection shows a clear, pale-yellow solution and in-process sampling confirms the absence of undissolved crystalline material by light obscuration. Because the molecule contains two amidine groups, aqueous stability depends on pH, and formulators are required to establish the narrowest safe pH window through forced degradation at pH 2, 4, 7, and 9 before fixing the final specification. The pH is adjusted with 0.1 M sodium hydroxide or acetic acid after complete dissolution, and the bulk is brought to final volume with WFI. Tonicity is controlled to 280–320 mOsm/kg with sodium chloride for the standard presentation, but hypertonic field products with osmolalities above 500 mOsm/kg are sometimes used to improve injection-site tolerance at the expense of increased muscle irritation; no universal osmolality specification exists, so the chosen range must be justified by injection-site irritation studies. Phosphate buffers are not presumed compatible because the diamidine molecule can interact with polyvalent anions and form sparingly soluble complexes; compatibility must be confirmed by visual inspection and assay after 24 h of quiescent storage. The bulk solution is prefiltered through a 0.45 μm polypropylene depth filter and then sterilized through two 0.22 μm polyethersulfone membrane filters in series. Terminal steam sterilization at 121°C for 15 min is generally avoided because published data for this specific configuration are limited and the amidine groups may undergo hydrolytic degradation; aseptic filtration is the standard alternative. Vials and stoppers are prepared separately: Type II amber glass vials are washed with WFI and depyrogenated at 250°C for 30 min in a dry-heat tunnel, while chlorobutyl stoppers are steam-sterilized at 121°C for 15 min. Filling is conducted in an ISO 14644-1 Class 7 suite with a Class 5 unidirectional airflow critical zone, and the line performs 100% weight verification on every filled container. A preservative is omitted for single-dose vials; where a multi-dose claim is intended, antimicrobial preservation must satisfy Ph. Eur. 5.1.3 and the closure must be validated for repeated puncture. The terminal product is released against assay, sterility, bacterial endotoxins, and subvisible particulate matter.

Equine piroplasmosis caused by Theileria equi and Babesia caballi introduces a narrower therapeutic margin than bovine trypanosomiasis, and the same 70 mg/mL injectable solution is often used by deep intramuscular injection at 3.5–5 mg/kg. Published adverse-event reports describe transient ataxia, sweating, and respiratory distress when the dose is pushed rapidly or when the horse is dehydrated, so presentation size and fill volume are treated as safety-critical rather than commercial parameters. Small-volume vials of 10 mL or 20 mL reduce the number of stopper punctures during field use and lower the particulate burden. Pre-filtration bioburden is controlled below 10 CFU/100 mL as an internal limit aligned with EU GMP Annex 1, because high bioburden can elevate the endotoxin burden even when the sterilizing filter achieves sterility. Filling speed is reduced to 30–60 vials/min for the small equine presentation; at higher speeds, silicone oil from the stopper track can transfer to the closure bore and form subvisible particles that are later detected by Ph. Eur. 2.9.19. The closure is therefore specified as a low-silicone chlorobutyl component, and the stopper wash program is qualified by particle screening of finished units. Photostability is a critical stability parameter: the equine product is exposed in a photostability chamber under ICH Q1B conditions of not less than 1.2 million lux hours and 200 Wh/m² ultraviolet irradiation, with acceptance criteria for yellow discoloration and related substances. Real-time stability is run at 25°C/60% RH for 24 months on the commercial pack, and accelerated stability is run at 40°C/75% RH for 6 months. The terminal product is filled with an extractable volume excess of 0.2–0.5 mL per vial to compensate for multiple withdrawal sequences, and extractable volume is verified by Ph. Eur. 2.9.17. Formulators are warned not to change the salt composition or buffer system of the registered equine product without a full bioequivalence and target-species safety study; published data for specific alternative buffers in this configuration are limited.

What Limits Oral Bioavailability of Diminazene Aceturate in Compounded Tablets and Capsules?

Oral-tablet and capsule dosage forms containing diminazene aceturate are not approved for food-producing animals in the European Union or the United States; they appear only as compounded preparations under veterinary prescription for captive wildlife or small ruminants where repeated injection is not practical. The limiting factor is the drug substance itself: diminazene aceturate is a diamidine carrying two cationic amidine groups at physiological pH, so its octanol-water partition coefficient is low and its permeability across enterocyte membranes is poor. Dissolution testing alone therefore cannot predict systemic exposure, and a permeability assessment such as the Caco-2 cell assay or a parallel artificial membrane permeability assay is required before any oral efficacy claim can be justified. If a tablet formulation is selected for development, direct compression is preferred over wet granulation because the hygroscopic API is exposed to aqueous binder during wet granulation and may begin to hydrolyze. A starting formula for a 100 mg tablet may contain 30–40% w/w diminazene aceturate, 45–55% microcrystalline cellulose, 3–5% croscarmellose sodium, 1–2% colloidal silicon dioxide, and 0.5–1% magnesium stearate; this ratio is a development starting point only and must be revalidated against the reference dissolution profile under Ph. Eur. 2.9.3. Blending is performed in a tumble blender at 25 rpm for 10–15 min, with total blending time fixed by blend uniformity data from Ph. Eur. 2.9.40. Direct compression is run on a rotary tablet press at 8–15 kN compression force and a target tablet hardness of 40–80 N; the friability limit is set at not more than 1.0% using Ph. Eur. 2.9.7. Capsule filling requires a powder with a Carr’s index below 25 and a Hausner ratio below 1.35 to ensure uniform die filling on an automatic capsule machine; for moisture-sensitive products, hard gelatin capsules are replaced by HPMC shells. The provisional dissolution acceptance criterion commonly used in development is not less than 75% release in 45 min in 900 mL of 0.1 M hydrochloric acid at 37°C with paddle speed at 50 rpm, but this is not an official compendial limit. Packaging uses 60 mL HDPE bottles with a desiccant canister and an induction-sealed liner, and stability is assigned on real-time data at 25°C/60% RH. The operational boundary is explicit: oral administration is not bioequivalent to the 3.5 mg/kg intramuscular injection used in cattle, and oral doses cannot be copied from injectable formularies without target-species pharmacokinetic data.

Non-sterile water-soluble powders containing diminazene aceturate are supplied to veterinarians for reconstitution into oral drench fluids in small ruminants, but the main production risk is not dissolution rate; it is moisture uptake and electrostatic dusting during sachet filling. The aceturate salt dissolves rapidly, and a properly milled powder with particle size below 500 μm normally yields a clear solution in potable water at 20–25°C within 5 min. A representative reconstitutable powder formula contains 70% w/w diminazene aceturate, 27% lactose monohydrate, and 3% anhydrous colloidal silica, but this is a technical starting point and not an approved label claim. The process begins with the API passed through a 500 μm stainless-steel sifter and charged into a 150 L ribbon blender; mixing is conducted at a tip speed of 1.5 m/s for 12–18 min, and blend uniformity is confirmed by sampling 10 positions under Ph. Eur. 2.9.40. The filling room is set to 35–40% relative humidity and 18–22°C, with dew-point monitoring at ≤8°C, because the API can cake when relative humidity exceeds 60% and form lumps that fail a 250 μm reconstitution screen. An auger filler with automatic net weight check is used, and sachet fill weight variation is held within ±2% of the nominal 20 g or 50 g fill. Sealing is performed by heat-sealing a trilaminate foil structure, and seal integrity is tested by vacuum dye penetration according to ASTM F3039-15. Each sachet carries a desiccant sachet, and the moisture vapor transmission rate of the foil is controlled by the packaging specification. The terminal product is labeled as a veterinary-use powder for oral drench preparation, not as an injectable, and the label states that the reconstituted solution is to be used immediately under veterinary supervision. Real-time stability at 25°C/60% RH supports a provisional shelf life of 24 months provided that moisture content remains below 0.5% by the Karl Fischer method in Ph. Eur. 2.5.12 and the reconstituted solution remains clear through a 250 μm screen.

When Medicated Feed Premix Carries Over, Homogeneity and Cleaning Validation Become Critical

Medicated feed premixes containing diminazene aceturate are not a recognized therapeutic route for trypanosomiasis in the European Pharmacopoeia; technical batches are nevertheless produced for research feeding trials and for captive wildlife where parenteral administration is impractical. A development premix containing 10% w/w diminazene aceturate on a corncob or wheat middling carrier is used at inclusion rates of 1–5 kg per tonne of finished feed, but the inclusion rate must be justified by target-species pharmacokinetic data because oral absorption of the diamidine is low. The manufacturing sequence starts with stepwise geometric dilution: 1 part API is mixed with 3 parts carrier in a high-shear mixer at 100–200 rpm for 5 min, then this fraction is transferred to a double-ribbon mixer and the remaining carrier is added to complete the batch. Before the API is added, 1–2% light mineral oil is sprayed onto the carrier to reduce dusting; overlubrication must be avoided because it can impair blend flow and create non-uniform feed inclusion. Homogeneity is evaluated by sampling 10 positions and assaying by HPLC using the in-house method; the relative standard deviation must remain below 5% to meet process capability expectations. The exit screen of the mixer is set at 850 μm, and a magnetic grid is placed downstream to remove tramp metal. Carryover is the main operational boundary: after a batch, the mixer, sifter, and conveying lines are dry-vacuumed and then swabbed for residual API, with the swab limit derived from a maximum permitted daily exposure assessment for the following non-target feed. The cleaning validation protocol follows VICH GL18 principles, and when the next feed batch contains no diminazene aceturate the carryover limit is normally set at not more than 1% of the lowest therapeutic concentration used in the previous batch. Terminal premix is packed in 20 kg woven polypropylene bags with an inner polyethylene liner, labeled as veterinary-use premix, and stored at 20–25°C in a dry, ventilated warehouse. Any commercial medicated feed application must comply with Regulation (EU) 2019/4 where applicable. Published data for oral feed premix efficacy in food-producing species are limited, so this presentation is not represented as a substitute for injectable therapy without regulatory approval.

Bulk API Repackaging and Cleanroom Humidity Control

Bulk repackaging of veterinary-grade diminazene aceturate is performed for licensed compounding pharmacies, veterinary wholesalers, and research institutions, and the central constraints are cross-contamination control and humidity management. The incoming API is sampled under an ISO 14644-1 Class 7 downflow booth, visually inspected, and tested for identity, assay, loss on drying, and related substances before repackaging is approved. The API is dispensed into 100 g, 500 g, and 1 kg black polyethylene inner bags inside metalized aluminum outer pouches; the packaging area is maintained at 35–40% relative humidity and 18–22°C with continuous dew-point monitoring at ≤8°C to prevent hygroscopic caking. Powder transfer uses a stainless-steel gravity-fed splitter with a 500 μm mesh screen, and each changeover is followed by a swab test for residual API and a visual inspection under 1,000 lux lighting. Loss on drying is controlled at not more than 0.5% by the Karl Fischer method in Ph. Eur. 2.5.12, and related substances are measured by HPLC using Ph. Eur. 2.2.29 liquid chromatography; where a compendial monograph for diminazene aceturate is absent in a given jurisdiction, the manufacturer’s internal specification functions as the release document. Each aluminum pouch is heat-sealed and checked by vacuum leak testing according to ASTM F3039-15, and the label carries the manufacturer name, batch number, storage conditions, and the statement “for veterinary use only.” The operational boundary is explicit: repackaged API is not a sterile material and must not be used directly for compounding sterile injectables unless the receiving facility has a qualified cleanroom and performs its own aseptic filtration or terminal sterilization on the finished product.