In oral solid dosage manufacture for human strongyloidiasis therapy, thiabendazole API is handled as a high-dose, weakly basic benzimidazole with a pKa in the range of 4.5–5.0 and a mechanism that disrupts β-tubulin polymerisation in nematodes. The human oral tablet is not a direct compression product, because the API exhibits low bulk density and insufficient compactability at doses of 500 mg per unit. A wet-granulation route is used, with thiabendazole, lactose monohydrate, microcrystalline cellulose, pregelatinized starch, and croscarmellose sodium or sodium starch glycolate at 2.0–5.0 wt% as intragranular disintegrant. Purified water or 5.0 wt% starch paste is added as binder in a high-shear granulator; the wet mass is passed through a 1.25–1.50 mm screen. Fluid-bed drying is performed at inlet air temperature 50–60 °C until loss on drying by USP <731> is ≤2.0%. Milled granules with a target D50 of 150–250 µm are blended with magnesium stearate at 0.5–1.0 wt% for 3–5 min; longer lubrication can reduce dissolution due to hydrophobic film formation. Compression on a rotary press with pre-compression force 4–8 kN is adjusted to tablet hardness of 80–120 N. The terminal tablet must meet disintegration under USP <701> in 0.1 N HCl at 37±0.5 °C with a limit of ≤15 min. Dissolution is tested in 900 mL of 0.1 N HCl using USP <711> Apparatus 2 at 50 rpm, with Q 80% at 30 min. Content uniformity follows USP <905>, and the acceptance value is ≤15.0. Elemental impurities are controlled according to ICH Q3D; residual solvents are controlled by USP <467>. A production failure observed at compression speeds above 60 rpm is sticking when granule moisture exceeds 2.5% w/w; the corrective action is re-drying and reducing magnesium stearate surface area by presieving through a 0.5 mm screen. The final tablet is a film-coated or uncoated 500 mg unit for adult dosing at 25 mg/kg body weight twice daily for 2 days in strongyloidiasis, with a maximum daily dose of 3 g. Published data for film-coating adhesion on this high-dose tablet is limited; if coating is used, a hypromellose-based system is applied at 2.0–3.0 wt% weight gain and pan speed is reduced to avoid edge wear.

Can Veterinary Bolus Tablets Rely on the Same Disintegration Test as Human Oral Tablets?

Veterinary bolus manufacture for thiabendazole in ruminants is driven by dose mass rather than by the same release requirement as human tablets. Historical veterinary pharmacopoeial use lists oral administration at 50–100 mg/kg body weight for gastrointestinal nematodes in cattle, sheep, and goats; a 200 kg calf therefore requires 10–20 g of active substance. A single-tablet product is not feasible within normal die geometry without formulation densification, so the process begins with wet granulation and is followed by slugging or roller compaction. Granules are compressed into oblong or cylindrical bolus units with hardness of 150–250 N to survive rumen handling. The disintegration test in USP <701> with 0.1 N HCl is not predictive of rumen fluid penetration because the rumen has a pH of 5.5–6.5 and high viscosity. Batch rejection events in veterinary plants have been traced to delayed disintegration in water after storage at 40 °C/75% RH for 3 months, a defect that is detected only when a discriminatory disintegration test in pH 5.8 acetate buffer at 39±0.5 °C is added. A target of ≤30 min in this buffer is used for batch release. The bolus formulation includes crospovidone or croscarmellose sodium at 4.0–8.0 wt%, higher than in human tablets, to obtain sufficient wicking in a viscous medium. The terminal product is a flavored bolus for oral administration with a notched shape to allow dose splitting for smaller animals. Content uniformity for large boluses is interpreted using the weight variation provisions of USP <905> only when the active substance is ≥25% of the final dosage unit; if lower, blend uniformity data must be submitted. Residual solvent testing follows VICH GL18 where applicable, and elemental impurities are controlled by ICH Q3D. The main process limitation is granule over-lubrication: magnesium stearate above 1.0 wt% with mixing time above 5 min produces a hydrophobic film that slows rumen fluid ingress and increases disintegration time beyond the 30 min limit.

Capsule Filling of Low-Bulk-Density Thiabendazole Granules Is Not a Direct Scale-Up

Capsule dosage forms are selected when dose flexibility is required in clinical protocols or veterinary practice. Thiabendazole API has a bulk density commonly below 0.45 g/mL, so filling a 500 mg dose into a size 0 or size 1 hard capsule requires dry granulation or wet granulation before the encapsulator. Dry granulation by roller compaction with microcrystalline cellulose and lactose monohydrate, followed by milling to an agglomerate D50 of 150–250 µm, is preferred when moisture-sensitive excipients are not involved. The granule must achieve flow through a 10 mm orifice at a rate of at least 4 g/s measured by USP <1174>; below this value, automatic capsule fillers show weight RSD above 3.0% and unacceptable rejection rates. On a tamping-pin capsule filling machine, pin settings of 1–3 mm and tamping force below 5 N per station are used to avoid crushing the granules into powder. Hard-gelatin capsules are tested for disintegration under USP <701>; however, after storage at 40 °C/75% RH in HDPE bottles without desiccant, gelatin crosslinking can delay release. HPMC capsules reduce the crosslinking failure mode but may have slower dissolution in 0.1 N HCl; capsule shell compatibility is therefore part of the development report. The terminal product is a hard capsule with assay by HPLC and dissolution by USP <711> in 900 mL of 0.1 N HCl; the Q value is not automatically transferable from tablet methods and should be justified with a discriminatory method. The main production failure is granule segregation in the hopper when D90 exceeds three times D10; this is controlled by adjusting the mill screen to 0.8–1.0 mm and mixing the final blend for 10–15 min after adding glidant. Fill weight variation follows USP <905>, and content uniformity acceptance uses AV ≤15.0.

Granules for oral suspension are manufactured as a separate downstream dosage form because thiabendazole is practically insoluble in water and cannot be presented as a simple syrup. The granule product is reconstituted with water at the point of use to deliver 500 mg per 5 mL. Wetting and sedimentation control require a suspending agent such as xanthan gum at 0.5–1.0 wt% and a wetting agent such as polysorbate 80 at 0.1–0.5 wt%; the granulation is performed in a fluid-bed granulator or high-shear granulator with lactose monohydrate, microcrystalline cellulose, and sodium carboxymethylcellulose as the structural excipients. The wet granule is dried to loss on drying ≤1.5% by USP <731>, because overdrying above this limit produces static and particle attrition during packaging. Particle size is controlled by laser diffraction using USP <429>, with a D50 of 120–180 µm and D90 ≤300 µm to avoid grittiness after reconstitution. The reconstituted suspension is buffered to pH 4.0–6.0, where the API is only partially dissolved, and the viscosity at 25 °C is adjusted to 150–400 mPa·s as measured by USP <911>. Preservative efficacy testing follows USP <51>; sodium benzoate at 0.1–0.2 wt% is included if the product is a multi-dose reconstituted suspension. The terminal dosage form is a granule in a sachet or bottle that is reconstituted to a uniform suspension, with a beyond-use date of 7–14 days after reconstitution under refrigeration depending on preservative efficacy data. Stability testing is conducted according to ICH Q1A in the dry granule configuration; after reconstitution, the product must be monitored for changes in particle sedimentation volume, pH, and assay. Published data for the viscosity window of this specific thiabendazole formulation is limited, so the range is confirmed by settlement volume ratio and resuspendability rather than by viscosity alone.

When Aqueous Solubility Falls Below 0.1 mg/mL, Injectable Formulation Routes Require pH Shift or Cyclodextrin

Parenteral thiabendazole development is constrained by aqueous solubility below 0.1 mg/mL at neutral pH. A simple aqueous solution at a target concentration equivalent to oral therapy is unattainable without solubility modification. Two formulation routes are technically feasible: acidification of the aqueous phase with dilute hydrochloric acid to pH 2.0–3.0, where the benzimidazole nitrogen is protonated, or complexation with hydroxypropyl-β-cyclodextrin at 20–40 wt%. The acidified route produces a chemically stable short-term solution but may cause injection-site pain if administered undiluted and may extract leachables from bromobutyl elastomers. The cyclodextrin route avoids extreme pH but raises osmolality and requires filter validation because HPβCD solutions have higher viscosity. Published data for this specific thiabendazole-HPβCD configuration is limited, so formulation screening uses phase-solubility analysis according to the Higuchi-Connors method and a filter compatibility study with 0.22 µm PVDF membranes. Aseptic filtration is preferred over terminal sterilization at 121 °C for 15 min unless thermal challenge studies demonstrate no degradation and no vial headspace pressure increase. The final parenteral product is a sterile solution or suspension packaged in Type I glass vials with bromobutyl rubber stoppers; visible particulates are controlled by USP <790>, subvisible particulates by USP <788> for large-volume parenterals or USP <789> for small-volume parenterals, and bacterial endotoxin by USP <85>. If the target dose is 25 mg/kg in large animals, the endotoxin limit must be calculated from the label dose per kg per hour; a development threshold of ≤0.5 EU/mg is cited when no label exists. Particulate load in a suspension injection is governed by particle size: for intramuscular use, the D90 is maintained at ≤10 µm to avoid capillary blockage; for intravenous use, a solution is required. The most frequent formulation failure is precipitation upon dilution with normal saline at pH above 4.5; package inserts and development reports must state the compatible dilution medium and maximum dilution ratio. Container closure integrity is evaluated by USP <1207>, and terminal sterility assurance follows current GMP guidance for aseptically processed products.