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

Characterized as (S)-6-phenyl-2,3,5,6-tetrahydroimidazo[2,1-b]thiazole hydrochloride, the compound is registered under CAS RN 16595-80-5 and formally recognized as Levamisole Hydrochloride in the Ph. Eur. monograph 01/2017:0027 and USP Levamisole Hydrochloride monograph. The chemical structure consists of a fused imidazo[2,1-b]thiazole bicycle with a tetrahydro core, a phenyl substituent at the 6-position, and a single chiral center in the S configuration; the hydrochloride salt yields a molecular formula C₁₁H₁₂N₂S·HCl (molecular weight 240.75 g·mol⁻¹). The monohydrochloride form is preferred over the free base for pharmaceutical handling because it provides a crystalline, non-hygroscopic solid with a melting point of 227–230°C with decomposition, whereas the free base is a low-melting solid with limited water solubility. Aqueous solubility of the salt at 25°C exceeds 200 mg·mL⁻¹ in purified water, facilitating formulation as injectable solutions and oral drenches.

What Distinguishes the (S)-Enantiomer from Racemic Tetramisole in Biological Activity?

The racemate of 2,3,5,6-tetrahydro-6-phenylimidazo[2,1-b]thiazole hydrochloride is known as Tetramisole Hydrochloride, and its anthelmintic activity resides almost exclusively in the S-enantiomer. Levamisole hydrochloride demonstrates a nicotinic acetylcholine receptor (nAChR) agonist action on nematode muscle, causing sustained depolarization and spastic paralysis. Electrophysiology recordings on Ascaris suum muscle strips indicate that the S-enantiomer exhibits an EC₅₀ of approximately 0.5 µM, while the R-enantiomer (dexamisole) requires concentrations exceeding 100 µM to elicit comparable depolarization. This stereoselectivity is reflected in in vivo efficacy: single oral doses of levamisole hydrochloride at 7.5 mg·kg⁻¹ body weight reduce adult Haemonchus contortus burden in sheep by 95–99%, whereas the R-enantiomer at equivalent dose shows negligible worm count reduction. Tetramisole hydrochloride, containing 50% of the inactive enantiomer, requires doses roughly twice that of the pure S-enantiomer to achieve equivalent anthelmintic effect, but the presence of the R-isomer introduces off-target cholinergic side effects (salivation, muscle fasciculations) at a higher incidence due to its interaction with mammalian nicotinic receptors. The toxicological differentiation is critical for veterinary product safety. Acute oral LD₅₀ values in rats: levamisole hydrochloride 180 mg·kg⁻¹; dexamisole hydrochloride 84 mg·kg⁻¹; tetramisole hydrochloride 130 mg·kg⁻¹. The S-enantiomer is therefore not only more potent but also exhibits a wider therapeutic index. Consequently, pharmacopoeial monographs mandate a minimum enantiomeric purity: the Ph. Eur. specifies a specific optical rotation of +115° to +120° (c = 2 in water, 20°C, sodium D line), corresponding to an enantiomeric excess of not less than 99.0% of the S-isomer by chiral HPLC (e.g., Chiralpak AD-H, 250 × 4.6 mm, hexane:ethanol:diethylamine 70:30:0.1, detection at 254 nm).

Pharmacopoeial Compliance and Purity Specifications

Commercial levamisole hydrochloride is supplied as a white or almost white crystalline powder and must satisfy the compendial requirements summarized below. The assay (non-aqueous titration with perchloric acid) is specified as 98.0–102.0% on the anhydrous basis (USP, Levamisole Hydrochloride monograph). Loss on drying at 105°C for 2 hours must not exceed 0.5%. Heavy metals are controlled to ≤ 20 ppm (Ph. Eur. method 2.4.8, reference solution prepared from lead standard). Sulfated ash ≤ 0.1%. Residual solvents—methanol, acetone, isopropanol—are monitored to levels compliant with ICH Q3C Class 2 and Class 3 guidelines; a typical specification for methanol ≤ 3000 ppm, acetone ≤ 5000 ppm. Water content determined by Karl Fischer titration (Ph. Eur. 2.5.12) is typically below 0.5% for material stored under dry conditions, though in packaging not hermetically sealed under inert atmosphere, moisture uptake can elevate water activity, especially at relative humidity above 60%. In such environments, pre-drying in a vacuum oven at 40–50°C for 4 hours is recommended before use in non-aqueous processes. Identity testing is confirmed by infrared absorption spectrophotometry (KBr disc, comparison against a certified reference standard) and by the characteristic chloride reaction (Ph. Eur. identification A). Related substances determination by liquid chromatography (C18 column, 250 × 4.6 mm, 5 µm; mobile phase acetonitrile:phosphate buffer pH 3.0 20:80) resolves the principal degradation product, 3-(2-aminoethyl)-5-phenylimidazolidine-2-thione, which is limited to ≤ 0.3%. Any unspecified impurity is capped at ≤ 0.1%, and total impurities ≤ 1.0%. Enantiomeric purity, as described, is verified using a chiral stationary phase, with the R-enantiomer quantified as a chiral impurity ≤ 0.5% to meet the optical rotation acceptance window. When formulated as an Anthelmintic Bolus: Excipient Interactions and Processing Constraints Processing of levamisole hydrochloride into solid oral dosage forms for ruminants (typically boluses containing 600 mg or 1.5 g of the active) is dominated by the compound’s moderate hygroscopicity above 60% RH and its susceptibility to Maillard-type browning in the presence of reducing sugars. Wet granulation using aqueous binders can initiate a color change from white to pale beige if lactose or dextrose is present in the diluent system and granule drying temperatures exceed 55°C. Consequently, direct compression or dry granulation (roller compaction) with microcrystalline cellulose/dicalcium phosphate dihydrate (USP) as filler is the preferred manufacturing route on rotary tablet presses operating at 30–60 rpm with a target tablet hardness of 5–8 kp. For extruded boluses, a twin-screw extruder (L/D ratio 20:1) is used with a lipid-based binder (e.g., hydrogenated vegetable oil) that melts at 50–60°C, allowing processing below the compound’s decomposition onset (200°C by DSC at 10°C·min⁻¹). Release kinetics from such matrices, tested by USP Apparatus 1 (baskets) at 100 rpm in 900 mL 0.1 N HCl, typically yield 60–70% dissolution within 2 hours and complete release by 6 hours, a pattern that maintains therapeutic plasma levels above the paralysing threshold of 0.1 µg·mL⁻¹ for 12–18 hours. Incompatibilities are observed with strong alkalis (free base precipitates) and oxidizing agents (sulfur in the thiazole ring is prone to oxidation, raising sulfoxide impurity levels). Formulations containing peroxide-generating excipients (e.g., povidone of high peroxide value) must be avoided unless a free-radical scavenger such as butylated hydroxytoluene (0.01% w/w) is incorporated. Lot-to-lot particle size consistency of the active ingredient impacts content uniformity of direct compression blends; a D₉₀ value ≤ 150 µm (laser diffraction, Malvern Mastersizer) is specified, and incoming lots with D₉₀ above 250 µm require pre-milling through a 0.5 mm screen (Comil 197S) to achieve blend CV ≤ 3% in 500 kg production batches. Beyond its classical use as an anthelmintic, levamisole hydrochloride functions as an immunomodulator in a fundamentally different dose-response relationship. In the adjuvant treatment of stage III colon carcinoma, the clinical trial MOF-2 (Moertel et al., N Engl J Med 322: 352–358, 1990) demonstrated that levamisole hydrochloride administered at 50 mg orally three times daily for 3 days every 2 weeks, combined with 5-fluorouracil, reduced the recurrence rate by 41% compared with surgery alone. This application capitalizes on the compound’s ability to potentiate T-cell proliferation and macrophage activation through a mechanism independent of its cholinergic receptor activity. The precise mechanism remains incompletely characterized, but published ex vivo data indicate that levamisole mimics thymic hormones, enhancing cyclic nucleotide phosphodiesterase activity and shifting the cAMP/cGMP ratio in lymphocytes. Crucially, this immunoenhancing window is narrow: doses exceeding 2.5 mg·kg⁻¹ per day paradoxically suppress splenic plaque-forming cell responses, a biphasic effect not observed with the racemate, likely because the R-enantiomer exerts an inhibitory tone on the same pathway. The difference in stereochemical dependence of anthelmintic versus immunomodulatory activity necessitates that material intended for immunological research carries a certificate of enantiomeric purity ≥ 99.5% and is free of endotoxins (< 0.05 EU·mg⁻¹) by LAL test Ph. Eur. 2.6.14.

Chiral Chromatographic Performance as a Reference Material

Because levamisole hydrochloride is one of the most extensively characterized single-enantiomer imidazo[2,1-b]thiazole derivatives, it is routinely deployed as a system-suitability reference substance in the development of chiral separation methods. On an amylose tris(3,5-dimethylphenylcarbamate)-coated silica column (Chiralpak AD-H, 4.6 × 250 mm, 5 µm) with a normal-phase eluent of n-hexane:2-propanol:diethylamine (80:20:0.1 v/v/v), the S-enantiomer elutes at a retention time of approximately 8.2 minutes and the R-enantiomer at 14.1 minutes, affording a resolution factor R_s > 2.5 under 1.0 mL·min⁻¹ flow. This consistent separation provides a benchmark for column qualification and for validating transfer of compendial methods across laboratories. The dynamic range for quantification of the R-enantiomer is linear from 0.05% to 2.0% of the main peak (r² ≥ 0.999), with a limit of detection of 0.02%. Laboratories using alternative chiral selectors (e.g., Chiralcel OD, Lux Cellulose-1) adjust the mobile phase composition—introducing ethanol replacing 2-propanol—to achieve equivalent resolution, but the S-enantiomer retention time shifts to 10–12 minutes. Such method adaptability illustrates the compound’s role as a transferrable chiral standard.

Comparative Properties: Levamisole HCl, Racemate, and Free Base

To aid selection among product forms, the following table contrasts key physicochemical and bioactivity parameters of the hydrochloride salts and the free base. Data are drawn from reference collections and compendial sources.

Property	Levamisole HCl (S)	Tetramisole HCl (RS)	Free Base (S)
CAS RN	16595-80-5	5086-74-8	14769-73-4
Specific rotation [α]D20 (c=2, H₂O)	+115° to +120°	≈ 0°	+92° to +96° (c=1, methanol)
Melting range (°C)	227–230 (dec.)	264–265 (dec.)	87–90
Water solubility (25°C)	> 200 mg·mL−1	> 200 mg·mL−1	< 1 mg·mL−1
LD50 oral rat (mg·kg−1)	180	130	Published data limited
Anthelmintic ED90 sheep (mg·kg−1)	5–7.5	10–15	Inequivalent; studied as salt

The monohydrochloride salt imparts sufficient crystallinity and stability to withstand standard pharmaceutical processing, whereas the free base is seldom utilized due to low aqueous solubility and a propensity to oxidize at ambient storage. When the free base is required for synthetic transformations (e.g., N-alkylation at the thiazole nitrogen), it is typically generated in situ by neutralization of the hydrochloride salt with ammonium hydroxide, extracted with dichloromethane, and used immediately to avoid discoloration.