Closantel base, pharma grade API, product code CLT-B-663, is the non-salt salicylanilide anthelmintic supplied for oral solid dosage forms and as the starting material for soluble salts used in injectable formulations. The chemical formula is C₂₂H₁₄Cl₂I₂N₂O₂, with molar mass 663.07 g/mol and CAS registry number 57808-65-8. The material is a white to off-white crystalline powder; the unionized base is practically insoluble in water and soluble in polar organic solvents such as acetone and methanol. It is manufactured under ICH Q7 active pharmaceutical ingredient GMP and is suitable for tablet, capsule, granule, and oral powder processing after appropriate particle-size control. For aqueous injection, the base is converted to closantel sodium or used to prepare a non-aqueous injectable vehicle; direct use of the neutral base in aqueous injection is not feasible because of insufficient solubility. The product code distinguishes the anhydrous base from closantel sodium dihydrate, which is assigned CLT-B-663S and is preferred for aqueous injectable finished products. Veterinary indications include fascioliasis and haemonchosis in ruminants; the API itself is not a finished product, and dosing is defined by the marketing authorization of the formulated veterinary medicinal product.

Table 1. Representative release specification for CLT-B-663 base.

Solid-state control for the base is assigned through X-ray powder diffraction and differential scanning calorimetry. The material should be crystalline and match the reference diffractogram; amorphous content is not routinely specified unless the API is milled under high energy, where an X-ray amorphous halo may appear. If micronization is performed, specific surface area increases and the powder may become more cohesive; conditioning at 25 °C and 40–50% RH is used to dissipate static charge.

Which Physicochemical Parameters Govern Oral Solid Dose Processability?

For tablet and capsule manufacture, the critical input from the API is not chemical purity alone but particle size distribution, bulk and tapped density, flow function, and surface energetics. A milled closantel base with D90 ≤100 µm is normally specified to provide content uniformity in direct compression or dry granulation. If the base is milled below D90 30 µm, electrostatic adhesion and cohesive arching may increase, and the powder may require conditioning at 40–50 °C in a fluidized-bed dryer before blending. Flowability is assessed by Ph. Eur. 2.9.36 or USP 1174; a Hausner ratio between 1.25 and 1.40 indicates fair flow, while values above 1.40 typically require glidant addition. Colloidal silicon dioxide at 0.5–1.0% w/w is used to improve flow without substantial dilution. Bulk density determined by Ph. Eur. 2.9.34 is used to set die fill depth and capsule disc volume; lots with bulk density below 0.30 g/mL may require overfilling or a compression aid.

Because the unionized base is hydrophobic, dissolution testing in 0.1 M HCl or pH 6.8 phosphate buffer may require a surfactant such as 0.1% w/v polysorbate 80 to discriminate batch differences. The low aqueous solubility has a direct consequence for wet granulation: water alone wets the powder poorly, and the granulating fluid should contain a wetting agent or an alcohol-water mixture. If wet granulation is used, drying should not exceed 60 °C to avoid sintering or agglomeration of waxy fines; loss on drying after drying is routinely below 0.5%. High-shear dispersion of the hydrophobic base in water is improved by adding polysorbate 80 to the granulating fluid; without a wetting agent, the granulation endpoint is difficult to reach because the powder floats and produces bimodal agglomerates. On a top-driven high-shear granulator with a chopper speed of 1,500–3,000 rpm, the wet mass is processed until a median granule diameter of 120–250 µm is achieved; overdrying below 1.0% moisture can increase friability and reduce tabletability.

Granulation behaviour for closantel base is dominated by its hydrophobic surface and low aqueous solubility. Dry granulation by roller compaction is preferred when the API level is high or when aqueous binder creates processing or stability concerns. Ribbon solid fraction is maintained between 0.65 and 0.75; screen size 0.8–1.25 mm is used after compaction to limit oversized granules. Capsule filling on tamping-pin equipment benefits from a granule D50 below 500 µm and a flow function coefficient above 4, measured by ring shear or Jenike cell. Tablets compressed from these granules should be monitored for tensile strength and ejection force; if ejection force exceeds 8 kN, magnesium stearate level should be revised to below 1.0% w/w or lubrication time shortened to avoid overlayering. Published data for this specific compound in continuous roller-compaction lines is limited; pilot-scale studies should define the exact parameter window.

When the Base Is Converted to a Soluble Salt for Aqueous Injection

Aqueous injectable formulations of closantel do not use the neutral base directly because the unionized form remains practically insoluble at physiological pH. The base is converted to the sodium salt by reaction with sodium hydroxide, typically in aqueous solution or a water-miscible cosolvent, and the resulting solution is adjusted to pH 9.0–10.5. The converted solution is clarified through 0.22 µm PVDF or polyethersulfone membrane filters and filled into amber glass vials or ampoules. Terminal sterilization at 121 °C for 15 min may be applied if formulation stability data support moist-heat sterilization; otherwise aseptic filtration is used. For injectable salt formation, the base feedstock should meet low bioburden and endotoxin limits as defined by the finished parenteral specification; typical in-process targets are ≤10 CFU/g bioburden and a bacterial endotoxin limit aligned with the maximum permitted dose.

Non-aqueous injectable vehicles such as propylene glycol or glycerol formal have been used for salicylanilide products; injection site tolerance and precipitation at the injection depot must be assessed. The base particle size becomes less critical for salt formation because the compound is dissolved, but milling to D90 50 µm can accelerate reaction and reduce residual base in the salt solution. In oral drench granules, the base is milled and blended with dispersing agents; the granular product is dispersed in water before administration.

Thermal and Hydrolytic Stability Limits during Processing

Manufacturing operations for closantel base should respect two boundaries: dry heat input during drying or granulation and alkaline hydrolysis during salt formation. Differential scanning calorimetry of the base shows the melting endotherm at 213–217 °C; processing temperatures are kept below 60 °C in wet granulation to avoid phase changes or agglomeration. Hydrolytic degradation of the amide linkage can occur under strongly acidic or strongly alkaline conditions at elevated temperature; therefore the pH during salt formation is controlled below 10.5 and the reaction temperature is maintained at 20–40 °C. In-process HPLC should monitor the appearance of degradation products; if total impurities exceed 1.0%, the batch is reprocessed or rejected according to the site quality procedure.

Comparative Pharmacology and Formulation Differentiation from Other Anthelmintics

Closantel is a salicylanilide anthelmintic whose primary mechanism is uncoupling of oxidative phosphorylation in helminth mitochondria, leading to depletion of adenosine triphosphate and immobilization. This mechanism differs from the beta-tubulin binding of benzimidazoles such as albendazole and from the glutamate-gated chloride channel modulation of macrocyclic lactones such as ivermectin. The pharmacological difference is relevant to control programs because benzimidazole-resistant Haemonchus contortus populations may remain susceptible to closantel; however, cross-resistance within the salicylanilide class, including rafoxanide, should be assumed unless demonstrated otherwise. Published pharmacokinetic studies in sheep report terminal elimination half-lives of approximately 14–31 days, which supports prolonged plasma exposure and persistent activity against blood-feeding nematodes.

Compared with rafoxanide, closantel base contains a 4-chlorobenzoyl substituent on the anilide ring, while rafoxanide contains a p-chlorophenoxy substituent; both are heavily halogenated and highly protein-bound. This structural difference shifts the octanol-water partition coefficient and may alter tissue distribution, but both require salt formation or cosolvents for injectable products. Albendazole is typically formulated as an oral suspension because of its low solubility and first-pass sulfur oxidation to the active sulfoxide metabolite; closantel does not require hepatic bioactivation for primary anthelmintic activity. Ivermectin is usually formulated as a solvent-based or oil-based injection or as an oral solution, with dose volumes and excipients selected for its lipophilic but solvent-soluble profile.

Table 2. Comparative positioning of closantel base and selected anthelmintic substances.

Storage of CLT-B-663 is recommended in tightly closed, light-resistant containers at controlled room temperature not exceeding 25 °C. The powder should be protected from relative humidity above 60% RH; if the container is opened in humid conditions, pre-drying at 40–50 °C is required before blending or compression. The base is incompatible with strong oxidizing agents and with strong alkalis in dry blends because premature salt formation can produce sticky agglomerates. Stability data should be generated according to ICH Q1A(R2) for the intended zone; published data for this specific product code is limited, so retest periods are established from batch-to-batch stability studies. Residual solvent levels are controlled under ICH Q3C Option 1, and the manufacturer's certificate of analysis should be verified against the marketing authorization specification before release.