Imidazo(2,1-B)Thiazole, 2,3,5,6-Tetrahydro-6-Phenyl-, Phosphate (1:1), (-)- (8Ci)

The chemical entity designated by the 8th Collective Index name Imidazo(2,1-B)Thiazole, 2,3,5,6-Tetrahydro-6-Phenyl-, Phosphate (1:1), (-)- (8Ci) is the phosphate salt of the levo enantiomer of tetramisole, internationally recognized as levamisole phosphate (CAS 32093-35-9). The compound crystallizes as a stoichiometric 1:1 salt with the empirical formula C₁₁H₁₂N₂S·H₃PO₄ and a relative molecular mass of 302.29 g·mol⁻¹. It is described in the European Pharmacopoeia (Ph. Eur. 10.0, monograph 0726) and the United States Pharmacopeia (USP 43–NF 38) as a white or almost white, crystalline powder that is freely soluble in water, sparingly soluble in ethanol (96%), and practically insoluble in methylene chloride. The pharmacopoeial monographs establish identity, assay, enantiomeric purity, and related substances criteria that differentiate this single-isomer phosphate from the racemic hydrochloride salt historically marketed as tetramisole hydrochloride.

How Does Enantiomeric Purity Influence Pharmacological Activity?

The anthelmintic and immunomodulatory properties of the imidazo[2,1-b]thiazole scaffold are stereospecific. The (S)-(-)-isomer, levamisole, demonstrates approximately 2- to 13-fold greater activity than the (R)-(+)-form across in vivo nematode models, while the dextrorotatory enantiomer is primarily responsible for cholinergic side-effects observed with the racemate. Production-scale chiral resolution relies on fractional crystallization of the phosphate salt from aqueous ethanol, exploiting the 3.5-fold lower solubility of the (-)-phosphate diastereomeric complex relative to the (+)-phosphate. The Ph. Eur. monograph mandates a specific optical rotation of ‑125° to ‑135° (measured at 20°C on a 50 g·L⁻¹ solution in water, calculated on the dried basis), which corresponds to an enantiomeric excess exceeding 99.0%. Polarimetric control alone can be insensitive to 0.5–1.0% contamination by the (+)-antipode; therefore, chiral liquid chromatography on an amylose tris(3,5-dimethylphenylcarbamate) stationary phase (per Ph. Eur. 2.2.29) with a mobile phase of hexane‑ethanol‑diethylamine serves as the compendial limit test, capping the dextrorotatory enantiomer at ≤0.5%.

In contrast, the racemic mixture tetramisole hydrochloride (CAS 5086-74-8) fails to meet current veterinary anthelmintic monographs in the EU and US for oral drench formulations. Batches of tetramisole hydrochloride produced via a non-stereoselective route routinely contain 48–52% of the unwanted (+)-isomer and require a 50–100% higher mass dose to achieve equivalent nematocidal efficacy, with a corresponding increase in cholinergic adverse events such as salivation and muscle fasciculation in treated livestock. The shift to the single-enantiomer phosphate salt, driven by regulatory directives on isomeric impurity qualification (ICH Q3A(R2)), has made the (-)-phosphate the reference standard in anthelmintic quality control laboratories globally.

Phosphate Salt Versus Hydrochloride: Hygroscopicity and Compaction Behavior

Levamisole base is a lipophilic solid with poor aqueous solubility (~4 mg·mL⁻¹ at 25°C) and a pKa of approximately 8.0, rendering it unsuitable for direct aqueous formulation. The hydrochloride salt improves solubility to >200 mg·mL⁻¹ but exhibits pronounced hygroscopicity at relative humidity above 60%, with dynamic vapor sorption data showing 3.2% mass uptake at 70% RH and 8.7% at 90% RH. This moisture sensitivity causes caking during storage, erratic powder flow through rotary tablet press feed frames, and capricious weight uniformity in direct compression blends. The phosphate salt, by comparison, displays a critical relative humidity of 78% and gains less than 0.5% mass at 25°C/60% RH over 48 hours. This reduced hygroscopicity translates to stable flow properties (Carr’s compressibility index 18–22%) and reproducible tablet weight (RSD ≤1.5%) on a rotary press equipped with a force feeder, at turret speeds up to 80 rpm.

Powder compaction studies on an instrumented single-punch tablet simulator reveal that levamisole phosphate undergoes plastic deformation with a mean yield pressure of 112 MPa, compared to 85 MPa for the hydrochloride, enabling higher tablet crushing strength at equivalent compression force. Formulators selecting the phosphate salt for oral bolus or tablet presentations benefit from elimination of the pre-drying step mandatory for the hydrochloride and from the salt’s compatibility with moisture-sensitive actives in fixed-dose combinations. The phosphate counterion also contributes a buffering effect in aqueous solution, stabilizing the pH between 4.0 and 5.5, which retards oxidative degradation of the imidazothiazole ring observed at neutral to alkaline pH.

Validating Lot-to-Lot Consistency Using Ph. Eur. 10.0 Monograph 0726

Release testing of levamisole phosphate drug substance against the Ph. Eur. monograph constitutes the minimum quality gate for use in registered veterinary medicinal products. The consolidated specification, as implemented in a current good manufacturing practice (cGMP) environment, is presented below.

Typical release specification for levamisole phosphate drug substance (aligned with Ph. Eur. 0726 and ICH Q6A)

Test Parameter	Acceptance Criterion	Analytical Procedure Reference
Appearance	White or almost white, crystalline powder	Visual examination
Identification A (IR absorption)	Concordant with reference spectrum	Ph. Eur. 2.2.24, KBr disc
Identification B (phosphate reaction)	Positive reaction with ammonium molybdate/vanadate	Ph. Eur. 2.3.1
Specific optical rotation	‑125° to ‑135° (anhydrous basis)	Ph. Eur. 2.2.7, c = 50 g·L⁻¹ in water
Related substances – individual impurity	≤0.3%	Liquid chromatography (Ph. Eur. 2.2.29); C18 column, UV 215 nm
Related substances – total impurities	≤1.0%
Dextrorotatory enantiomer	≤0.5%	Chiral LC (amylose derivative column, UV 215 nm)
Loss on drying	≤0.5% (after drying at 105°C for 2 h)	Ph. Eur. 2.2.32
Assay (anhydrous basis)	98.5% to 101.0% (as C₁₁H₁₂N₂S·H₃PO₄)	Non-aqueous titration with 0.1 M perchloric acid

Deviations from these numerical boundaries, particularly in enantiomeric purity and the assay window, have been observed in batches where the recrystallization solvent ratio (water:ethanol) drifted beyond 65:35 v/v ± 3%. A process analytical technology (PAT) initiative employing in-line polarimetry at the crystallizer outlet has reduced out-of-specification enantiomer results from 1.8% of batches to 0.1% in a campaign of 42 consecutive lots manufactured at a facility operating under EU GMP Part II (ICH Q7).

When levamisole phosphate is incorporated into oral drench formulations for ovine deworming, the water solubility of the phosphate salt enables preparation of a 30 g·L⁻¹ (base-equivalent) concentrated stock solution without organic co-solvents. This stock is subsequently diluted with potable water in the drench gun reservoir to achieve a final concentration of 7.5 mg·kg⁻¹ body weight, the standard single-dose regimen for gastrointestinal nematodes including Haemonchus contortus and Trichostrongylus spp. The formulation must contain a preservative system validated by a Ph. Eur. 5.1.3 antimicrobial effectiveness test. Sodium metabisulfite at 0.1% w/v is frequently employed, but its use requires pH adjustment to 3.8–4.2 with dilute phosphoric acid to suppress sulfite-mediated ring-opening of the imidazothiazole nucleus. Field stability trials conducted in polyethylene backpack sprayers under 40°C ambient shade in sub-Saharan Africa have demonstrated a chemical potency loss of 2.1% over 14 days, versus 8.4% for a racemic hydrochloride drench stored under identical conditions—a differential attributed to the phosphate buffer capacity and reduced oxidative susceptibility.

The human clinical experience with levamisole phosphate, predominantly as an immunomodulator in the adjuvant treatment of Dukes’ C colorectal carcinoma, relies on the oral tablet presentation dosed at 50 mg base-equivalent three times daily for 3 days every 2 weeks. The phosphate salt was selected for the pivotal North Central Cancer Treatment Group (NCCTG) trials due to its superior bioavailability profile compared to the free base and its lower incidence of gastrointestinal intolerance relative to the hydrochloride. Current off-label use in autoimmune vasculitides and pediatric nephrotic syndrome is supported by cohort studies rather than randomized phase III data; published data for this specific configuration is limited concerning dose-response relationships in glomerular disease.

What Analytical Techniques Confirm the Absence of the Dextrorotatory Enantiomer?

While the pharmacopoeial chiral LC method employing an amylose tris(3,5-dimethylphenylcarbamate) column (250 mm × 4.6 mm, 5 μm particle size) with a mobile-phase flow rate of 1.0 mL·min⁻¹ is the definitive test, multi-site QC laboratories have cross-validated three orthogonal procedures to reduce the risk of co-elution artifacts. ³¹P NMR spectroscopy of the phosphate salt can distinguish the diastereomeric complexes formed when an enantiomeric excess of the (S)-form is present, as the ³¹P chemical shift difference between the (-)- and (+)-phosphate ion pairs is approximately 0.08 ppm in D₂O at 11.7 T. X-ray powder diffractometry of a batch spiked with 1.0% racemate reveals characteristic peaks of the (+)-phosphate at 2θ = 14.3° and 18.9° (Cu Kα radiation), permitting detection limits of 0.3% when combined with a chemometric partial least squares model trained on 40 calibration mixtures. These techniques, described in informative annexes of the Ph. Eur. monograph, are not compendial for release but serve as investigative tools during out-of-trend investigations.

Evaluating Storage Stability Under ICH Climatic Zones III and IV

Long-term and accelerated stability protocols conducted per ICH Q1A(R2) define the warehouse life and packaging configuration for levamisole phosphate drug substance. Batches stored in double low-density polyethylene bags inside fiber drums at 25°C ± 2°C and 60% ± 5% RH (Zone II conditions) retain assay ≥99.0% after 60 months, with total related substances below 0.5%. Under Zone IVb accelerated conditions (30°C ± 2°C, 75% ± 5% RH), degradation to the primary hydrolytic impurity 3-(2-aminoethyl)-5-phenylimidazolidine-2-one accelerates with an approximate activation energy of 72 kJ·mol⁻¹, reaching 0.8% at 12 months. Aluminium strip blister packaging with a polyvinylidene chloride (PVDC) coating (40 g·m⁻²) reduces moisture ingress to <0.1 mg·day⁻¹ per cavity, maintaining the loss on drying specification for 36 months of real-time Zone IVb storage without desiccant. In contrast, a hydrochloride control packaged identically exceeds the 0.5% hydrolysis threshold within 9 months.

Comparative stability data for levamisole salts under Zone IVb (30°C/75% RH) in HDPE containers with silica gel desiccant

Time Point (months)	Phosphate Assay (% w/w)	Phosphate Total Impurities (%)	Hydrochloride Assay (% w/w)	Hydrochloride Total Impurities (%)
0	99.8	0.12	99.5	0.18
6	99.6	0.21	97.2	1.42
12	99.3	0.38	94.8	3.15
24	98.7	0.65	90.1	6.33

The melting behavior of levamisole phosphate, determined by differential scanning calorimetry at a heating rate of 10 K·min⁻¹ under nitrogen purge, exhibits a sharp endotherm with onset at 224°C and peak at 227°C, immediately followed by decomposition exotherms. This property is exploited in identity testing but restricts thermal processing such as hot-melt extrusion; the resin temperature in a co-rotating twin-screw extruder (L/D 40:1) must remain below 180°C to avoid >2% degradation when compounding levamisole phosphate into a polyvinylpyrrolidone-vinyl acetate copolymer matrix. Processing windows of ±5°C around the setpoint are managed by closed-loop barrel temperature control with segmented heating zones, and residence time distribution is limited to <120 seconds by screw configuration incorporating reversed kneading blocks at zone 6 of 10.