Closantel pharma grade API is a synthetic halogenated salicylanilide anthelmintic supplied in two model forms: the free base, CAS 57808-65-8, and the sodium salt dihydrate. Both models are intended for veterinary dosage forms, including tablets, capsules, granules for oral administration, oral drench suspensions, and sterile injectable suspensions for sheep, cattle, and goats. The molecule contains two iodine atoms, two chlorine atoms, a phenolic hydroxy group, and a nitrile-substituted aromatic system; these substitutions confer high lipophilicity, high plasma protein binding, and low aqueous solubility in the neutral free-acid state. The free base has the molecular formula C₂₂H₁₄Cl₂I₂N₂O₂ and a relative molecular mass of 663.08 g/mol.

The free base is selected when anhydrous or non-aqueous granulation is required, while the sodium salt is selected for aqueous oral suspension, granule reconstitution, and injectable suspension because it disperses rapidly in water and reduces surface wetting time. The API is not intended for human use and should be handled as a potent veterinary active pharmaceutical ingredient. Manufacture is conducted under ICH Q7 conditions, with batch release supported by a certificate of analysis that includes route-specific controls. For tablet and capsule use, the API is commonly supplied as milled or micronized powder; for injectable suspension, the API is supplied as a low-bioburden or sterile grade with particle size and endotoxin data. The base and sodium salt are not interchangeable in all formulations because the sodium salt imparts wetting and pH behaviour, while the free base can precipitate if the vehicle pH is reduced.

Closantel sodium and free base: specification profile under compendial alignment

Release specifications are aligned with the current European Pharmacopoeia monograph for closantel sodium for veterinary use where applicable, supplemented by route-specific controls for injectable grade. Assay is determined by reversed-phase HPLC using Ph. Eur. 2.2.29; the acceptance window is 98.0%–102.0% on the dried basis. Identification is confirmed by infrared spectrophotometry and by liquid chromatographic retention time against the reference standard. Related substances are limited by area normalisation or external standard; unspecified impurities are controlled to ≤0.10% where adopted in the monograph, and total impurities are reported against compendial thresholds. Residual solvents are tested per Ph. Eur. 2.4.24 and comply with ICH Q3C(R8) limits for the solvents used in manufacture. Elemental impurities are risk-assessed under ICH Q3D; the oral and injectable permitted daily exposure values are selected from the route-specific tables. Water content is measured by Karl Fischer titration per Ph. Eur. 2.5.12, with typical release limits of ≤0.5% for the free base and a stoichiometric range for the sodium salt dihydrate.

Quality risk management under ICH Q9 is used to rank process parameters for each dosage form. For solid oral products, the highest risks are particle-size distribution, moisture, and segregation; for injectable products, the highest risks are endotoxin load, sterility assurance, particle growth, and resuspendability. The active substance master file or DMF should include route-specific particle-size data, residual solvent profiles, impurity batch data, and ICH Q1A(R2) stability data to support the finished-product dossier. Incoming particle size is confirmed by laser diffraction; batch-to-batch shifts in D90 outside the approved range can alter wet granulation endpoint torque and should trigger a formulation review rather than a fixed-process continuation.

Closantel free base is hydrophobic and frequently shows poor flow and segregation when used as a micronized direct-compression blend. Production-scale tablet and capsule manufacture therefore relies on wet granulation or dry granulation. In a high-shear mixer, the aqueous binder is added at a controlled rate; localized overwetting converts the API into dense, poorly compressible agglomerates. End-point control by impeller torque or main power consumption is more reproducible than fixed time because batch-to-batch particle-size variation shifts granulation kinetics. Granules are dried to the target loss on drying, typically below 2.0%, before compression or capsule filling. Tablet hardness and disintegration are then assessed per Ph. Eur. 2.9.8 and 2.9.1. For capsule filling, blend flow is improved when the API is pre-granulated with a hydrophilic filler; electrostatic charging of micronized API on rotary press feed frames and punches is managed by maintaining relative humidity below 50% and using static elimination equipment. A twin-shell blender with a preblend of colloidal silicon dioxide is more effective than direct addition of the API to the final blend.

The selection between the base and sodium salt is not cosmetic. The base is less hygroscopic and may be preferred when dry granulation by roller compaction is used. The sodium salt, because of its ionisable phenolate function, hydrates rapidly but can raise moisture sensitivity in the dry blend if stored in unlined containers. For aqueous suspension manufacture, the sodium salt permits hydration without a separate wetting step; for tablet manufacture, the base can be co-milled with lactose or microcrystalline cellulose to improve blend uniformity. In roller compaction, the base is often dry-granulated with a binder; the fines recycle fraction should be controlled to avoid loss of compressibility. For aqueous granulation, the sodium salt may be dissolved partially in the binder solution, but this can lead to pH-dependent recrystallization and should be confirmed by X-ray powder diffraction per Ph. Eur. 2.9.33.

Why does aqueous solubility govern granulation and capsule dissolution?

The free acid exhibits limited solubility in water over the pH range relevant to oral absorption. This creates dissolution-limited exposure after oral administration. Capsule and tablet dissolution methods are therefore product-specific; they commonly include a surfactant concentration above the critical micelle concentration or use a two-stage pH gradient to avoid precipitation of the free acid from the sodium salt. Dissolution testing is performed according to Ph. Eur. 2.9.3, and the method is validated under ICH Q2(R1). When the sodium salt is used in granules for oral suspension, rapid wetting is obtained, but physical stability must be controlled because dispersed particles may settle and cake. Sedimentation volume, zeta potential, and resuspendability after storage at 25°C and 4°C are more informative than visual appearance alone.

The manufacture of granules for reconstitution uses fluid-bed granulation or high-shear granulation; it should avoid strongly acidic binder solutions because a drop in pH below the apparent precipitation point of the ionised molecule may cause localized recrystallization and non-uniform drug distribution. A nonionic or anionic wetting agent at 0.05%–0.5% w/w is usually sufficient to reduce foaming and improve dispersion, but the exact concentration is formulation-specific. For immediate-release capsules, the API and hydrophilic filler are granulated to produce a dissolution surface that is independent of the pristine hydrophobic crystal morphology; the process is monitored by loss on drying, sieve analysis, and dissolution profile comparison against the reference product.

When sterile injectable suspension is the target dosage form, particle size and sterility become simultaneous release constraints

Injectable closantel suspensions are prepared as sterile aqueous or non-aqueous suspensions. The sodium salt is preferred for aqueous vehicle compatibility, but precipitation can occur if the vehicle pH is adjusted below the apparent precipitation threshold. Terminal moist-heat sterilisation of the final suspension is possible only where stability data demonstrate no particle growth, pH shift, or degradation; otherwise the API is sterilised by gamma irradiation or aseptically crystallised and then aseptically compounded with a terminally sterilised vehicle. Bacterial endotoxin limits are derived from the maximum single dose and are tested per Ph. Eur. 2.6.14; sterility is tested per Ph. Eur. 2.6.1.

Particle size control is critical for syringeability. The D90 is usually kept below 30 µm and the D50 between 5 µm and 15 µm for subcutaneous administration through a 21G needle; however, published data for this specific API configuration is limited, and the final range is established during formulation development rather than by monograph. Extremely fine particles increase dissolution but also raise Ostwald ripening and caking risk. The suspension viscosity is adjusted with suspending agents; post-storage resuspendability is tested because caking is temperature-dependent. The injectable API grade also has a lower bioburden specification and may be packaged in gamma-compatible sealed containers for aseptic transfer. The route of injection is product-specific; subcutaneous administration is common in sheep and goats, whereas intramuscular use is less frequent due to potential local irritation.

Distinctions relative to flukicides and nematocides in the same therapeutic class

Published pharmacokinetic data in sheep commonly report terminal half-life values in the range 14–23 days and plasma protein binding above 99%; product-specific values vary with formulation and route. Closantel is differentiated from triclabendazole by its combination of adult fluke activity, persistent activity against certain nematodes, and high plasma protein binding. Triclabendazole is primarily a flukicide with activity against multiple liver fluke stages but lacks the same breadth against blood-feeding nematodes. Compared with oxyclozanide, closantel has a longer terminal half-life and higher protein binding, which can reduce dose frequency in registered products; oxyclozanide is narrower in spectrum and is often combined with levamisole. Compared with rafoxanide, both are halogenated salicylanilides, but differences in substitution pattern lead to different pharmacokinetic and residue kinetics. Compared with niclosamide, closantel is systemically absorbed and therefore not limited to lumenal cestode activity.

Because closantel is highly protein-bound and slowly cleared, tissue and milk residue depletion is prolonged; authorised withdrawal periods are product-specific and must be observed. Overdosage in small ruminants has been associated with neurotoxic and visual effects, so species-specific dose margins from approved labels should be used. The sodium salt may precipitate in acidic vehicles or in the presence of divalent cations; compatibility with other veterinary medicinal products should be confirmed before mixing. Handling and milling require local exhaust ventilation, respiratory protection where dust is generated, and adherence to the safety data sheet. The API should be stored in tightly closed containers protected from light and at controlled room temperature, with retesting intervals assigned from ICH Q1A(R2) stability data.