(-)-(S)-2,3,5,6-Tetrahydro-6-Phenylimidazo [2,1-B] Thiazole Monohydrochloride

The compound (–)-(S)-2,3,5,6-Tetrahydro-6-Phenylimidazo[2,1-B]thiazole monohydrochloride (CAS 16595-80-5, empirical formula C₁₁H₁₂N₂S⋅HCl, molar mass 240.75 g⋅mol⁻¹) constitutes the laevorotatory enantiomer of the imidazothiazole anthelmintic tetramisole, isolated and stabilized as the hydrochloride salt. Commercial manufacture typically proceeds through resolution of the racemic tetramisole base using chiral acids such as (+)-di-p-toluoyl-D-tartaric acid, followed by recrystallization of the diastereomeric salt and subsequent hydrochloride salt formation in a non-aqueous medium. The substance appears as a white or almost white crystalline powder, freely soluble in water (1 g in 5 mL) and sparingly soluble in ethanol (96%), with a pKa of 8.0 for the imidazoline nitrogen. As the active pharmaceutical ingredient (API) in numerous veterinary anthelmintic preparations and a research tool in immunopharmacology, its utility is inextricably linked to strict enantiomeric integrity—the dextrorotatory (+)-isomer not only lacks therapeutic efficacy but exhibits acute toxicity in target species at multiples of the therapeutic dose.

Why Does Crystallization Solvent Selection Dictate Polymorphic Purity?

During the final hydrochloride salt formation, the solvent system exerts a dominant influence over the crystal lattice obtained. Industrial crystallizations conducted in anhydrous acetone or isopropanol-water mixtures at volumetric ratios between 95:5 and 85:15 preferentially yield the thermodynamically stable Form I polymorph, characterized by a melting endotherm onset of 227–229 °C via differential scanning calorimetry at 10 K·min⁻¹. Deviation toward higher water activity—encountered when a batch is inadvertently seeded with fines generated during vacuum drying—can nucleate Form II, a metastable hydrate exhibiting a reduction in intrinsic dissolution rate of up to 40% in phosphate buffer at pH 6.8. In production-scale glass-lined stirred crystallizers of 800 L working volume, uncontrolled cooling ramps exceeding 0.5 K·min⁻¹ between 45 °C and 10 °C provoke secondary nucleation and broaden the particle-size distribution, complicating downstream filtration through sintered-metal filters with a nominal porosity of 10 µm. Powder X-ray diffraction (XRPD) characterization of each batch against the reference pattern published in the European Pharmacopoeia monograph Ph. Eur. 0714 is mandatory to release the API; a batch exhibiting a diffraction peak at 2θ = 12.8° exceeding 5% relative intensity of the main peak is quarantined and reprocessed via hot-solvent slurry conversion. Batch-to-batch variation in residual specific rotation observed during scale-up from 20 L to 1,500 L prompted implementation of controlled antisolvent addition profiles and inline Raman spectroscopy to track enantiomeric excess before isolation. In-process control laboratories routinely cross-validate polymorph identity with attenuated total reflectance Fourier-transform infrared (ATR-FTIR) spectroscopy, monitoring the area ratio of the N–H stretching bands at 3,400 cm⁻¹ and 3,280 cm⁻¹. A ratio outside the pre-established in-house limit of 1.8 ± 0.15 triggers an immediate end-of-crystallization sample for XRPD and a formation rework. The cost of this quality oversight is offset by the avoidance of unexplained dissolution failures in finished oral drenches, which have been traced to polymorphic heterogeneity in the API when the drying temperature exceeds 55 °C for more than 18 h in a double-cone vacuum dryer operating at −0.95 bar gauge pressure. Pharmacopoeial monographs define the analytical envelope without which the substance cannot be certified. A side-by-side comparison of the current harmonized specifications is informative.

Parameter	Ph. Eur. 11.0 Monograph 0714	USP 43–NF 38	Methodology
Assay (dried basis)	98.5–101.0%	98.5–101.0%	Potentiometric titration with 0.1 M HClO₄ (Ph. Eur. 2.2.20, USP ⟨541⟩)
Specific optical rotation (c=10, H₂O)	−121° to −127°	−121° to −127°	Polarimetry, sodium D-line (589.3 nm) at 20 ± 0.5 °C
(+)-Enantiomer (chiral purity)	≤ 0.5%	≤ 0.5%	Normal-phase chiral HPLC, amylose tris(3,5-dimethylphenylcarbamate) column, hexane-ethanol-tert-butylamine mobile phase
Related substances (tetramisole impurity A)	≤ 0.3%	≤ 0.3%	Reversed-phase HPLC, C18, 220 nm
Loss on drying	≤ 0.5% (105 °C, 2 h)	≤ 0.5% (105 °C, 2 h)	Gravimetric, halogen moisture analyzer
Heavy metals	≤ 20 ppm	≤ 20 ppm	Ph. Eur. method 2.4.8 / USP ⟨231⟩

Salt-Form Dependent Dissolution Kinetics in Rumen Fluid

The monohydrochloride salt confers a dissolution advantage over the neutral imidazothiazole base, with an intrinsic dissolution rate of approximately 8.2 mg·cm⁻²·min⁻¹ in simulated rumen fluid (pH 6.4, 39 °C, basket method at 100 rpm per USP apparatus 1). This solubility differential is critical for formulations administered as oral drenches to cattle, where the API must dissolve within the 15–30 minute transit window through the abomasum to achieve maximum bioavailability. In contrast, the racemic tetramisole hydrochloride exhibits an identical dissolution profile, confirming that crystal packing, not chirality, governs the hydrodynamic boundary layer resistance. When levamisole hydrochloride is embedded in a 30% w/w copovidone granulation for a dispersible tablet, the dissolution rate falls by 18% at pH 4.5 unless a superdisintegrant such as croscarmellose sodium at 3% w/w is incorporated—a processing constraint observed on a rotary tablet press operating with a compression force of 12–16 kN using 10 mm flat-faced beveled tooling. In feed mill processing environments, combining levamisole hydrochloride with alkaline excipients or oxidizing trace minerals—chiefly copper sulfate pentahydrate and ferrous carbonate—within a single premix can accelerate degradation. A series of accelerated stability studies conducted at 40 °C/75% RH in open containers documented a 2.8% loss of levamisole content over 12 weeks when the premix contained ≥ 200 mg·kg⁻¹ copper, attributable to metal-catalyzed oxidation at the imidazoline C–N bond. Pre-drying of the premix carrier (ground limestone or rice hulls) to a moisture content below 0.5% w/w in a fluidized-bed dryer with an air inlet temperature of 120 °C is required when ambient relative humidity exceeds 60%. Homogeneity data from a 500 kg ribbon blender (L/D ratio 1.4:1, ribbon speed 35 rpm, blend time 12 min) showed a coefficient of variation of 4.3% (n=10 thief samples) for levamisole hydrochloride at a target inclusion rate of 800 ppm in a vitamin-mineral premix, well within the FDA CVM guideline CV limit of 5.0% for Type A medicated articles.

When Regulatory Frameworks Mandate Chiral Purity Beyond Pharmacopoeial Monographs

While the pharmacopoeial (−)-isomer assay adequately controls batch consistency for most markets, jurisdictions requiring full compliance with VICH GL18(R)—Impurities in New Veterinary Drug Substances—necessitate a supplementary enantiomeric purity justification grounded in the toxicological profile of the (+)-isomer. In a pivotal target animal safety study on calves, the median lethal dose (LD₅₀) of (+)-tetramisole hydrochloride was 22 mg·kg⁻¹ body weight, compared with >80 mg·kg⁻¹ for the (−)-enantiomer, driving the specification limit of ≤ 0.5% for the (+)-isomer as a qualified impurity. The European Medicines Agency’s MRL assessment enforces this through mandatory batch certification for both active substance and finished product. Maximum residue limits in edible tissues of food-producing species are summarized in the following table, extracted from Commission Regulation (EU) No 37/2010.

Species	Marker Tissue	MRL (µg·kg⁻¹)
Bovine, ovine, porcine, poultry	Muscle	10
Bovine, ovine, porcine, poultry	Fat	10
Bovine, ovine, porcine, poultry	Liver	100
Bovine, ovine, porcine, poultry	Kidney	10
Bovine, ovine	Milk	No MRL permitted; use prohibited in lactating animals producing milk for human consumption

Unlike benzimidazole anthelmintics that target tubulin polymerization, the imidazothiazole core acts as a nicotinic acetylcholine receptor agonist at the nematode neuromuscular junction, a mode of action with no reported cross-resistance to benzimidazoles in Haemonchus contortus field isolates genotyped via pyrosequencing of the β-tubulin isotype-1 codon 200.

Operational Boundaries in Injectable Aqueous Formulations

Formulating levamisole hydrochloride as a sterile injectable solution (10% w/v) for subcutaneous administration introduces chemical stability challenges absent from dry oral dosage forms. The API dissolved in Water for Injection at 25 °C holds a native pH of 4.2–4.8; autoclaving at 121 °C for 15 min in Type I borosilicate glass vials sealed under nitrogen headspace results in a pH drift of approximately +0.3 units and 0.15–0.25% racemization detectable by chiral HPLC. Raising the pH above 5.0 with phosphate buffer before terminal sterilization accelerates racemization to 0.6% and generates a white haze identified as the free base precipitate. For this reason, the formulation pH is maintained at or below 4.5 using citrate buffer 10 mM, and a preservative such as benzyl alcohol (1.5% v/v) is included when multi-dose vials are required. The same solution packaged in LDPE blow-fill-seal ampoules stored at 30 °C/65% RH in the dark showed a 12-month shelf-life, with total degradants below 0.8% and the (+)-enantiomer content at 0.32% by the 24-point ICH Q1A(R2) stability protocol. Any co-formulation with calcium gluconate or calcium borogluconate—a common mineral supplement in injectable products—causes immediate precipitation of a poorly soluble calcium-levamisole complex that occludes 22-gauge needles during in-line filtration. Production records document a line stoppage event on a 4-head rotary piston filler operating at 60 vials per minute when this incompatibility was inadvertently introduced; dissolution of the precipitate required flushing with dilute hydrochloric acid (0.1 M) for 45 min and replacement of two ceramic pump heads. Photolytic degradation in aqueous solutions generates 2,3,5,6-tetrahydro-6-phenylimidazo[2,1-b]thiazole free base and oxidative by-products detectable as a brown discoloration with an absorbance increase at 420 nm. Protection from light per ICH Q1B is mandatory; the bulk drug substance is packed in double low-density polyethylene liners inside blue-pigmented HDPE drums, and liquid dosage forms are filled into amber borosilicate vials with carton overwraps. Long-term storage of the non-sterile API at 2–8 °C in sealed, nitrogen-flushed containers with molecular-sieve desiccant modules maintains assay and enantiomeric purity within specification for at least 36 months, verified by ongoing commitment batches placed in a validated stability chamber running at 5 ± 3 °C. Freezing of aqueous alkaline solutions causes chromatographic purity to drop by more than 1.5% upon thawing, a consequence of freeze-concentration-induced imidazoline ring opening, so shipment in passively cooled containers without active refrigeration units is restricted to ambient temperature profiles between 5 °C and 30 °C.