(6S)-6-Phenyl-2,3,5,6-Tetrahydroimidazo[2,1-B][1,3]Thiazole Phosphate (1:1)

Levamisole phosphate, systematically designated (6S)-6-phenyl-2,3,5,6-tetrahydroimidazo[2,1-b][1,3]thiazole phosphate (1:1), CAS 32093-35-9, is the single-enantiomer phosphate salt of the imidazothiazole anthelmintic chemotype. The substance presents as a white to off-white crystalline powder with a molecular formula of C₁₁H₁₂N₂S · H₃PO₄ and a molecular weight of 306.3 g·mol⁻¹. Pharmaceutical-grade lots released under current Good Manufacturing Practice routinely exhibit a purity by HPLC (area-%) of not less than 98.5%, specific optical rotation [α]²⁰_D in water (c = 2) tightly controlled within the range −85° to −90°, and a water content by Karl Fischer titration below 1.0%. The compound is the active moiety in a range of veterinary oral drenches, injectable solutions, and medicated feed premixes targeting gastrointestinal and pulmonary nematodes in cattle, sheep, swine, and poultry. Its immunological activity as a type-1 cytokine response modifier, exploited in adjunctive chemotherapy protocols, is preserved only when the (6S) configuration is essentially free of the (6R)-enantiomer.

How Does the (6S)-Enantiomer Phosphate Salt Differ from Racemic Tetramisole and Hydrochloride Analogs?

Racemic tetramisole contains equimolar amounts of the (6S) and (6R) isomers; only the (6S)-form contributes the desired nematocidal and immunostimulatory pharmacodynamics, while the (6R)-enantiomer is associated with a higher incidence of cholinergic side effects and lacks meaningful anthelmintic efficacy. The phosphate salt offers a deliberately moderated aqueous solubility profile relative to the hydrochloride. At 20°C, dissolution of levamisole hydrochloride readily yields solutions above 500 mg·mL⁻¹, whereas levamisole phosphate saturates near 200 mg·mL⁻¹, a property that reduces the risk of concentration-dependent gastric irritation when delivered as a bolus drench and retards deliquescence in high-humidity storage conditions. The absence of chloride ion eliminates the corrosion potential on 316L stainless steel contact surfaces observed with the hydrochloride during prolonged wet mixing, a factor directly relevant to multi-tonne batch processing in ribbon blenders and fluid-bed granulators. Additionally, the phosphate counterion modifies the crystal lattice such that the neat solid carries a lower cohesive energy density, a characteristic exploited in direct compression tablet formulations where punch adhesion and capping tendency must be suppressed. Without a preceding header, the manufacturing integration of levamisole phosphate into 0.08–0.16% (w/w) medicated feed premixes begins with a geometric preblend on a rotary drum mixer operated at 60% fill volume. A typical premix carrier is ground maize or soya hulls, conditioned to a moisture content of not more than 12% before addition, because excess free moisture accelerates agglomeration of the phosphate crystals and drives segregation during subsequent final feed incorporation. The addition sequence—carrier first, then a 1% light mineral oil binder, followed by the levamisole phosphate—achieves a coefficient of variation (CV) of drug content across 10 stratified samples below 5.0%, as measured per the procedures outlined in USP〈905〉Uniformity of Dosage Units. Mix times exceeding 15 min at 25 rpm have been observed on production-scale horizontal ploughshare mixers to elevate the fines fraction of the phosphate crystals through attrition, increasing dust generation and causing a drift in the batch-averaged potency by 0.8–1.2% relative to the label claim.

Granulation Parameters and Compression Behavior in 300-mg Tablet Formulations

Wet granulation of a mass containing 60% levamisole phosphate, 25% microcrystalline cellulose (Avicel PH-102), and 10% pregelatinized starch is performed in a high-shear mixer-granulator with an impeller speed of 200 rpm and a chopper at 1500 rpm. Purified water is sprayed at a rate of 60 mL·min⁻¹ to a final granulation moisture content of 7–9%, as determined by loss-on-drying at 105°C using a halogen moisture analyzer. Drying in a fluid-bed drier at an inlet air temperature of 60°C until the product temperature plateaus at 38°C yields granules with a bulk density of 0.48–0.52 g·cm⁻³ and a Carr’s compressibility index below 15%. Compression on a 16-station rotary tablet press with 9 mm round, flat-faced bevel-edged tooling at a main compression force of 18–22 kN and a turret speed of 35 rpm produces tablets of target hardness 7–9 kp and friability consistently under 0.4% after 100 drops in a USP〈1216〉fraibilitator. The dissolution profile in 900 mL of 0.1 M HCl at 37°C using USP apparatus 2 at 50 rpm demonstrates not less than 85% release within 30 min, a behavior attributable to the moderate wetting rate conferred by the phosphate anion. A comparative dissolution data set obtained on pilot-scale batches demonstrates the kinetic divergence between the phosphate and hydrochloride salt forms under identical test conditions.

Comparative Dissolution of Levamisole Salts from 300-mg Immediate-Release Tablets (USP Apparatus 2, 0.1 M HCl, 37°C, 50 rpm)

Time (min)	Phosphate Salt (% Released ± SD, n=12)	Hydrochloride Salt (% Released ± SD, n=12)
5	27.4 ± 3.1	52.8 ± 4.6
10	58.9 ± 2.8	81.3 ± 3.9
15	79.2 ± 2.1	93.5 ± 2.4
30	93.6 ± 1.7	99.1 ± 1.2
45	98.2 ± 1.1	99.7 ± 0.8

Stability under ICH Q1A(R2) Accelerated Conditions: Degradation Product Profile

Sealed double polyethylene bags placed inside fibre drums containing 25 kg of levamisole phosphate were subjected to 40°C ± 2°C and 75% ± 5% relative humidity over 6 months. Assay by validated HPLC (C₁₈ column, 250 × 4.6 mm, 5 μm; mobile phase acetonitrile–phosphate buffer pH 3.0 (30:70 v/v); UV detection at 215 nm) fell from 99.3% to 98.7%, while total impurities rose from 0.12% to 0.29%. The primary degradant, 6-phenyl-2,3-dihydroimidazo[2,1-b]thiazole, generated by hydrolytic ring-opening, remained below the 0.15% identification threshold specified in the relevant veterinary pharmacopoeial monograph. No significant isomerisation to the (6R)-enantiomer was observed; chiral purity, determined by HPLC on a Chiralpak AGP column with aqueous ammonium acetate–isopropanol mobile phase, stayed above 99.0% enantiomeric excess. Water uptake under these conditions was 0.3%, confirming the non-hygroscopic character of the phosphate crystal lattice. Exposure to 365 nm UV light at 25°C for 120 h in a photostability chamber (ICH Q1B option 2) produced no photodegradant exceeding 0.05%, a stark contrast to the hydrochloride salt that exhibited a 0.4% rise in coloured by-products under the same exposure. When the phosphate salt replaces the hydrochloride in medicated drinking water premixes delivered through nipple-drinker lines, the lower chloride load mitigates stress-corrosion cracking in 304 stainless steel fittings that has been documented in field installations after 6–12 months of continuous use. Solubilisation in hard water (250 mg·L⁻¹ CaCO₃ equivalent) at a target levamisole concentration of 800 mg·L⁻¹ proceeds to a clear, particle-free solution within 15 min of propeller agitation at 200 rpm and a water temperature of 25°C; the hydrochloride typically achieves dissolution in 5 min, but the slightly extended wetting time of the phosphate eliminates transient supersaturation peaks that occasionally cause precipitation in the delivery lines overnight. Field-test data collected from a 2500-bird poultry house over a 5-day treatment cycle showed variation in delivered dose at the farthest drinker of ±4% relative to the header tank concentration, compared to ±9% for a historical hydrochloride-based formulation, a difference attributed to the absence of localized hygroscopic crusting around the metering device diaphragm.

What Analytical Markers Confirm Chiral Purity and Freedom from the (R)-Isomer?

Enantiomeric purity is verified by a chiral HPLC method employing a Chiralpak AGP (150 × 4.0 mm, 5 μm) column thermostatted at 25°C, with a mobile phase of 10 mM ammonium acetate buffer (pH 5.5) containing 2% (v/v) isopropanol at a flow rate of 0.8 mL·min⁻¹. Under these conditions, the retention time for the (6R)-enantiomer is approximately 8.2 min, while the (6S)-form elutes at 10.5 min. System suitability requires resolution of not less than 2.0 between the peaks and a limit of quantitation for the unwanted enantiomer of 0.05%. Pharmacopoeial acceptance criteria adapted from the Ph. Eur. monograph for Levamisole Hydrochloride (01/2023:0660) establish that the (6R)-impurity does not exceed 0.5%. Routine release testing of commercial phosphate batches consistently reports values of 0.05–0.10%. An orthogonal polarimetric measurement at 589 nm and 20°C with a 2% aqueous solution serves as the identity confirmation; acceptance limits are −85° to −90°, a specification that precludes dilution with racemate and invalidates any lot where inadvertent racemisation occurred during synthesis.

Typical Release Specifications for (6S)-6-Phenyl-2,3,5,6-Tetrahydroimidazo[2,1-B][1,3]Thiazole Phosphate (1:1)

Attribute	Acceptance Criterion	Test Method Reference
Assay (anhydrous, solvent-free basis)	98.5–101.0%	HPLC UV 215 nm, external standard
Specific optical rotation ([α]20D, c=2, H2O)	−85.0° to −90.0°	Ph. Eur. 2.2.7
Water content (Karl Fischer)	≤ 1.0%	Ph. Eur. 2.5.12
Heavy metals	≤ 20 ppm	Ph. Eur. 2.4.8 Method C
Sulphated ash	≤ 0.1%	Ph. Eur. 2.4.14
Related substances – any unspecified impurity	≤ 0.10%	HPLC area-%, same as assay
Enantiomeric purity (content of (6R)-isomer)	≤ 0.5%	Chiral HPLC (AGP column)
Residual solvents (ethanol)	≤ 5000 ppm	GC-FID, Ph. Eur. 5.4

The limited aqueous solubility of the phosphate form necessitates careful consideration when formulating injectable solutions intended for subcutaneous administration. A terminal sterilisation cycle of 121°C for 20 min in a saturated steam autoclave reduced the pH of a 10% w/v solution from 4.2 to 3.8, accompanied by an increase in the imidazoline ring-opened hydrolysis product by 0.08 area-%. To maintain hydrolytic stability, the formulation pH is buffered with 10 mM citrate buffer at pH 4.5 ± 0.2, and the filled vials are sterilised by filtration through a 0.22 μm PVDF membrane prior to aseptic filling, avoiding thermal stress entirely. This process, validated in accordance with EU GMP Annex 1, yields a product with a 24-month shelf life at 25°C/60% RH when stored in Type I glass vials with coated rubber stoppers.