Carbepentanten (Pentoxyverine) Veterinary Grade API is supplied as the citrate salt pentoxyverine citrate, 1-phenylcyclopentanecarboxylic acid 2-[2-(diethylamino)ethoxy]ethyl ester citrate, with CAS registry 23142-01-0 for the citrate salt and 77-23-6 for the base. The salt has molecular formula C₂₀H₃₁NO₃·C₆H₈O₇ and relative molecular mass 525.6 g/mol; the base has relative molecular mass 333.5 g/mol. The veterinary-grade material is a white to almost white crystalline powder intended as an active pharmaceutical ingredient in solid oral, liquid oral, parenteral, and medicated feed dosage forms. Supplier grade designations typically encode the target particle-size upper limit: PTV-CIT-D90/25, PTV-CIT-D90/50, and PTV-CIT-D90/100 correspond to laser-diffraction D90 targets of 25 µm, 50 µm, and 100 µm, respectively. The product is manufactured under current good manufacturing practice and is not presented as sterile; terminal sterilization of finished injections remains a downstream responsibility. Compendial adoption must be checked against the current Ph. Eur. monograph for pentoxyverine citrate where applicable, and against any relevant national veterinary pharmacopoeia monographs.

The citrate salt imposes distinct handling limits during wet granulation

Because the citrate salt has a reported melting range of 93–96 °C by differential scanning calorimetry, the material is unsuitable for dry-heat sterilization and for hot-melt extrusion processes exceeding the melting onset. In high-shear wet granulation, the API should be dry-blended with a portion of the filler before water addition; aqueous binder at temperatures above 40 °C can soften the salt and generate fines-to-wall adhesion. Loss on drying after tray drying at 55–60 °C should be controlled at 0.5–1.0% for compressible granules, but published data for this specific veterinary formulation configuration are limited and each formulation must be confirmed by near-infrared moisture mapping. The product is hygroscopic at relative humidity above 70%; open handling in non-humidity-controlled suites can increase water content by 0.3–0.5% within 30 min, based on common citrate salt behavior. Batch-to-batch particle-size variation is observed when the final milled product is discharged from hammer mills operating at different rotor-tip speeds; a milling step with classifier control is preferred over single-pass pin milling.

Dry-mix uniformity for low-dose veterinary tablets is influenced less by the nominal assay and more by the particle-size tail above 150 µm. Milled material with a D90 below 50 µm can be mixed directly with lactose monohydrate or dicalcium phosphate dihydrate, but agitator speed in bin blenders should remain below the point that induces air entrainment. For tablet compression, use of a low-compressibility filler such as spray-dried lactose improves ejection force, while magnesium stearate at 0.5–1.0 wt% reduces sticking. Content uniformity testing according to Ph. Eur. 2.9.40 or USP <905> is required for tablets with active content below 2 mg; acceptance value should meet the compendial limit, not a looser veterinary in-house value.

How does the API behave under terminal sterilization in aqueous injection compounding?

Pentoxyverine citrate contains an ester linkage in the 2-(diethylamino)ethoxyethyl side chain; therefore, aqueous injection solutions are sensitive to pH-dependent hydrolysis, particularly above pH 6.0 and under prolonged steam sterilization at 121 °C for 15 min. Terminal sterilization can reduce assay by measurable amounts, with the hydrolytic degradation product being the corresponding acid and amino alcohol. Published data for this specific veterinary configuration is limited; development batches require a stability-indicating high-performance liquid chromatographic method with gradient separation of the acid degradant. Aseptic filtration through a 0.2 µm sterilizing-grade filter is preferred when the solution cannot tolerate terminal moist-heat exposure. If terminal sterilization is required by regulatory submission, use of an overage is not a substitute for validation of the terminal process. Nitrogen purging and protection from light during compounding reduce oxidative discoloration.

Powders and granules for oral solutions are frequently dry-blended with citric acid, sodium citrate, mannitol, and a dispersed binder. Because the citrate salt is water-soluble, dissolution rate in sachet presentation is controlled mainly by granule disintegration and not by API intrinsic solubility. For medicated feed premix, the API is generally first adsorbed onto precipitated silica or spray-dried onto a maltodextrin carrier before blending with ground corn or soybean meal. In bin-transfer operations, segregation occurs when the carrier-particle size exceeds the API D90 by a factor greater than 10; stepwise premix ratios of 1:5, 1:20, and final mix are preferable to single-stage addition.

Sieve retention and segregation thresholds in low-dose tablet blends

Sieve analysis according to Ph. Eur. 2.9.38 or laser diffraction according to Ph. Eur. 2.9.37 should be used to monitor the D10, D50, and D90 of the API before dry granulation. Retention on a 250 µm sieve should be below 0.1% for direct compression; material with tail particles above 250 µm is difficult to distribute uniformly in tablet cores below 80 mg. Roller compaction of the API with a brittle binder such as microcrystalline cellulose improves flow, but dry granulation increases the fraction of fines; a subsequent milling step with a 0.8 mm screen is typical for veterinary granulations. The granulated intermediate should be characterized by Hausner ratio below 1.25 and Carr index below 20% to avoid weight variation in high-speed rotary tablet presses. Press speeds above 60,000 tablets/h may require forced feeding, but the low melting point of the salt can cause localized heating in the turret if dwell time is high.

Film coating with an aqueous hydroxypropyl methylcellulose system at inlet air temperature below 60 °C avoids melting-related core sticking in the coating pan. Deep-bed drying of granules above 55 °C for more than 2 h can produce case-hardening and uneven moisture distribution, which later reduces tablet hardness by 10–15%; split drying cycles are preferred. For tablet cores containing pentoxyverine citrate and acidic lubricants, sodium stearyl fumarate is a non-acidic alternative to magnesium stearate if disintegration time exceeds 15 min in purified water at 37 °C.

Specification and grade matrix for the veterinary API is shown in Table 1.

When solutions require pH adjustment before mixing with other actives

In aqueous oral solution compounding, the citrate salt introduces a buffering effect due to the citrate counterion; addition of sodium hydroxide to raise pH above 6.5 may precipitate the free base and cause turbidity. Acidic buffers below pH 3.0 increase protonation and aqueous solubility but also accelerate ester hydrolysis over the shelf life. Compatibility with benzalkonium chloride, disodium edetate, and sulfites should be assessed using binary-mixture stability screens because sulfite reducing agents can attack the ester linkage under elevated temperature. The product should not be mixed with strongly alkaline opioids or phenothiazine solutions unless the final pH is held between 4.0 and 5.5; development data should confirm this range by forced-degradation study rather than by theoretical pKa alone.

Hard gelatin or hydroxypropyl methylcellulose capsules containing pentoxyverine citrate can be filled directly from the dry blend when the final blend density is above 0.55 g/mL; lower-density blends segregate during tamping. Lactose monohydrate and pregelled starch are effective diluents, whereas starch 1500 can increase disintegration time due to the water-swellable matrix. Capsule content uniformity should be tested at start, middle, and end of a filling run with sample size per USP <905>; fill weights below 100 mg require tighter control of powder rheology. The use of a low-shear tumble mixer after the final addition of magnesium stearate should be limited to 5 min to avoid over-lubrication and delayed disintegration.

Controlling residual solvent profiles across crystallization routes

Residual solvent profiles for veterinary-grade pentoxyverine citrate depend on the final recrystallization solvent system. Ethanol, isopropanol, and ethyl acetate are common Class 3 solvents; their residual levels should comply with ICH Q3C options and be confirmed by headspace gas chromatography per Ph. Eur. 2.2.28. Class 2 solvents such as dichloromethane or toluene, if used in the final synthetic step, require limit demonstration in each batch; a specification below 600 ppm for toluene is aligned with the ICH concentration limit, but the exact limit must be calculated from the maximum daily dose of the veterinary formulation. Elemental impurities are controlled under ICH Q3D; medicinal products for companion animals and food-producing species may have different parenteral or oral PDE allowances. The API is not inherently sterile, so endotoxin contamination in injectable-grade material should be below 2.5 EU/mg when the final injection requires a dose-dependent limit derived from the finished product monograph.

Lyophilization of the API as a citrate salt is possible but requires attention to the glass transition temperature of the freeze-concentrate; mannitol or glycine as bulking agents protect the cake structure. Primary drying shelf temperature above the collapse temperature can cause cake collapse; published data for this specific veterinary formulation is limited. Consequently, freeze-drying cycle design should start with a conservative ramp and use comparative pressure measurement or freeze-drying microscopy. Assay by non-aqueous titration may overestimate potency if hydrolytic acid degradation products are present; a stability-indicating HPLC method with UV detection at 210 nm or 220 nm is therefore preferred for release testing of veterinary formulations. Forced-degradation studies under acid, base, peroxide, heat, and light should be conducted per ICH Q1A/Q1B principles to demonstrate peak purity of the active peak. Injection solutions should be packaged in amber glass or opaque closures because ultraviolet exposure accelerates discoloration and free-radical oxidation.

The formulation-relevant distinction from other veterinary antitussive APIs is primarily the citrate salt and the hydrolyzable ester side chain. Table 2 summarizes comparative handling parameters; pharmacodynamic selection is outside the scope of the API technical profile.

Veterinary premix manufacturing in the European Union must follow Directive 2004/28/EC and the Cascade; in the United States, extralabel use in food-producing animals is permitted only under AMDUCA and with a veterinarian’s withdrawal period assignment. Mass homogeneity in feed mills should be verified by sampling at the mixer discharge and final bagged feed; theoretical homogeneity coefficient of variation should be below 5% for medicated feed. Because published data for pentoxyverine citrate in target species is not uniformly available across all intended dosage forms, laboratory-scale compatibility and pilot-batch stability are required before process scale-up.