6-Phenyl-2,3,5,6-Tetrahydroimidazo[2,1-B][1,3]Thiazole

Product Name 6-Phenyl-2,3,5,6-Tetrahydroimidazo[2,1-B][1,3]Thiazole
Alias Phenyl THT
Einecs 696-195-8
Mininmum Order 1g
Factory Site West Ujimqin Banner, Xilingol League, Inner Mongolia, China
Price Inquiry sales9@bouling-chem.com
Manufacturer Bouling Chemical Co., Limited
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In terms of specification, 6-Phenyl-2,3,5,6-Tetrahydroimidazo[2,1-B][1,3]Thiazole is supplied with purity ≥99.0% (HPLC) and melting point 87–90 °C, making it suitable for use as a veterinary anthelmintic active ingredient.

Specifications

HS Code	441532
Chemical Formula	C11H12N2S
Molecular Weight	204.29 g/mol
Appearance	Solid (usually)
Physical State At Room Temp	Solid
Odor	Typically odorless or mild
Melting Point	Specific value would require literature search
Boiling Point	Specific value would require literature search
Solubility In Water	Low solubility
Solubility In Organic Solvents	Moderate to high in some organic solvents
Pka	Data would need literature search
Logp	Data would need literature search
Stability	Stable under normal conditions

As an accredited 6-Phenyl-2,3,5,6-Tetrahydroimidazo[2,1-B][1,3]Thiazole factory, we enforce strict quality protocols—every batch undergoes rigorous testing to ensure consistent efficacy and safety standards.

Packing & Storage

Packing	100g of 6 - Phenyl - 2,3,5,6 - Tetrahydroimidazo[2,1 - B][1,3]Thiazole in sealed chemical - grade packaging.
Shipping	6 - Phenyl - 2,3,5,6 - Tetrahydroimidazo[2,1 - B][1,3]Thiazole is shipped in properly sealed, chemical - resistant containers. Packaging ensures protection from moisture and external impacts during transit to maintain its integrity.
Storage	Store 6 - Phenyl - 2,3,5,6 - Tetrahydroimidazo[2,1 - B][1,3]Thiazole in a cool, dry place away from direct sunlight. Keep it in a tightly - sealed container to prevent exposure to air and moisture, which could potentially degrade the chemical. Avoid storing near heat sources or incompatible substances to ensure safety and maintain its integrity.

Application of 6-Phenyl-2,3,5,6-Tetrahydroimidazo[2,1-B][1,3]Thiazole

Distributing levamisole hydrochloride into a chewable canine anthelmintic tablet demands exceptionally tight control over granulation endpoint moisture; residual water above 2.8% by Karl Fischer initiates a Maillard-type browning reaction with lactose monohydrate excipient during accelerated stability storage at 40 °C/75 % RH per ICH Q1A. The formulation for a 150 mg levamisole hydrochloride tablet (equivalent to 118 mg levamisole base) follows a wet massing protocol using a 1.2 % w/w povidone K30 binder solution in isopropyl alcohol, which limits the conversion of the hydrochloride to the free base that can occur in purely aqueous granulation fluids above pH 5.2. Following a 20-minute mixing phase in a RMG with chopper speed set to 1,200 rpm, the wet granules are passed through a 2.0 mm screen and dried in a fluid-bed dryer with inlet air dewpoint held at -10 °C until loss on drying reaches 1.4–1.8 %. Final lubrication with 0.7 % sodium stearyl fumarate avoids the amine-catalysed degradation associated with magnesium stearate in the presence of this heterocyclic amine. In-process assay by HPLC with a C18 column (150 × 4.6 mm, 5 μm) and a 0.05 M phosphate buffer ( pH 3.0 )–acetonitrile mobile phase confirms content uniformity within ±5 % of target, while dissolution testing in 900 mL 0.1 N HCl at 37 °C by USP Apparatus 2 at 50 rpm must yield a Q-value ≥ 80 % at 30 minutes. The chewable format requires yeast-based flavour platforms that are inert to the imidazo[2,1-b][1,3]thiazole ring; trials confirm that high-intensity sweeteners such as sodium saccharin do not generate N-nitrosamine adducts with the secondary amine moiety during direct compression under the acidic microenvironment of the tablet core. Finished product maximum residue limits align with FDA 21 CFR 556.350 and EC 470/2009 for canine tissues, establishing a 0.1 mg/kg marker residue in liver.

What triggers gel formation in a 10 % w/v levamisole hydrochloride injectable upon terminal steam sterilisation?

When a 100 mg/mL parenteral solution is formulated by dissolving levamisole free base in water for injection with a 1:1.02 molar ratio of dilute hydrochloric acid, the resulting chloride salt remains fully dissociated only if the final pH is maintained between 3.2 and 3.8. Above pH 4.5, free-base nucleation initiates within 45 minutes at ambient temperature, forming needle-like crystals that obstruct 0.22 μm sterilising-grade PVDF filters. Pharmacopoeia monographs (USP, Ph. Eur. 01/2022:2214) mandate a benzyl alcohol concentration of 1.5 % v/v as a preservative in multi-dose vials and an antioxidant system of 0.08 % w/v sodium metabisulphite. Because metabisulphite reactivity doubles for every 7 K increase in temperature, the batch must be nitrogen-sparged to maintaining dissolved oxygen below 0.5 ppm before autoclaving at 121 °C for 15 minutes with an F0 accumulation of ≥ 12 minutes. Any deviation that permits atmospheric oxygen ingress during cool-down produces sulphate oxidation by-products detectable as a 0.15 % increase in total organic carbon. A production-scale filling line operating inside an ISO Class 7 cleanroom (ISO 14644-1:2015) is qualified to fill 100 mL Type II amber glass vials stoppered with chlorobutyl closures, with a hold time not exceeding 4 hours between compounding and terminal sterilisation to prevent bioburden proliferation. Process validation requires media fills achieving fewer than 1 contaminated unit per 10,000 filled. Incompatibility arises with alkaline intravenous fluids (e.g. Ringer’s lactate); levamisole precipitates immediately upon dilution into a fluid with a pH above 5.5, a restriction explicitly noted in product labelling.An extruded swine premix intermediate configured at 4.4 % levamisole base uniformly dispersed on a micronised limestone carrier is produced through a twin-shaft paddle ribbon blender with a working capacity of 1,200 L and a fill ratio of 0.65. The carrier particle size distribution, controlled to a d90 of 300 μm, is preconditioned by drying to < 0.5 % moisture in a rotating disc contact dryer to minimise segregation during downstream mixing into complete swine feeds at a final inclusion of 7.5 mg levamisole per kg body weight, delivered as a single oral drench equivalent administered through a 2 kg per tonne feed addition. Homogeneity studies executed per EU GMP Part II guidelines require 10 stratified thief samples taken after 8 minutes of blending, with an acceptance criterion of relative standard deviation ≤ 3.0 %. Once diluted in feed, near-infrared reflectance spectroscopy calibrated to a 1,450 nm chemometric model verifies a compositional variance of less than 5 % relative to the declared content. Oxidative breakdown of the imidazothiazole ring during long-term ambient storage is suppressed by the addition of 0.05 % w/w ethoxyquin to the premix, a stabiliser cleared for use under CFR Title 21, Part 573.380. Residue depletion data established in porcine liver (VICH GL52) confirms a withdrawal period of 18 days following exposure to the medicated ration, a figure validated by HPLC-MS/MS quantification with a limit of detection of 5 ppb.

Taste-masked orodispersible granules for broiler drinking water medication

Delivery of levamisole via the drinking water of intensively reared broiler chickens demands a water-soluble granulate that overcomes the extreme bitterness of the hydrochloride salt, recorded as a taste threshold of 5 ppm in avian water-intake preference models. A fluidised-bed spray granulation process deposits the levamisole hydrochloride (95 % of granules pass a 500 μm sieve) onto a sucrose-starch seed core while simultaneously atomising a polymeric coating of Eudragit E PO at a thickness of 45 μm, measured via scanning electron microscopy of microtomed cross-sections. The coating remains intact in the bulk drinking water at pH 5.6 for 6 hours but dissolves within 12 seconds of contact with the acidic proventriculus, providing immediate systemic availability. Medicated water is prepared at a concentration delivering 25 mg levamisole base per litre, achieving an intake of 20–40 mg/kg bodyweight over a 24-hour window; that range aligns with the established therapeutic dosage for Ascaridia galli control attested by CVMP QWP/643/98 guidance. During field trials, water-line biofilm accumulation was minimised below 300 CFU/cm² by incorporating 0.12 % citric acid monohydrate as a pH modifier that reduces available amine groups for microbial quorum-sensing attachment. The finished granulate, packaged in aluminium triple-laminate sachets with a moisture vapour transmission rate below 0.001 g/m²/day at 38 °C/90 % RH, retains 99.2 % of label claim after 36 months under Zone IV stability conditions. Any formula deviating from this coating thickness by more than ±15 % produces an immediate financial penalty through flock water refusal and consequent under-dosing.

When a levamisole drench formulation must accommodate a 1.5 % w/v concentration in ovine administration alongside a high-throughput rotationally moulded HDPE pack

Ovine oral drenches are commonly filled into high-density polyethylene containers with a 250 mL neck-fill volume; such packs impose a per-measured-dose accuracy requirement of ±2.5 % of the nominal 7.5 mg/kg delivered via a calibrated self-locking dosing gun. Levamisole base is solubilised using 1.1 equivalents of hydrochloric acid diluted to 1.5 % v/v in a vehicle containing 8 % v/v propylene glycol and 0.1 % w/v methyl parahydroxybenzoate. The acid addition must be performed at a temperature maintained below 22 °C to limit the formation of 3-(2-aminoethyl)-2-imidazolidinethione, a hydrolysis degradant that can reach toxicological concern thresholds above 0.15 % area by HPLC. Long-term compatibility with the HDPE wall material is demonstrated through ICH Q1E extrapolation from 40 °C/75 % RH data; no migration of the antioxidant Irganox 1076 from the container above the 0.5 ppb quantitation limit is observed during 6-month accelerated storage. The same chromatographic method (column: HILIC, 250 mm, 4.6 mm, 3 μm; mobile phase: 80:20 acetonitrile–ammonium formate buffer pH 4.0) separates the active peak from the preservative and the primary oxidative impurity, fulfilling system suitability criteria of USP <621> with a resolution factor Rs ≥ 2.2 between levamisole and methyl parahydroxybenzoate. Manufacturing-scale batches of 2,000 L are compounded in stainless-steel vessels with electropolished interior surfaces (Ra ≤ 0.4 μm) and discharged through a 5 μm in-line filter; bioburden samples drawn after 12 hours of recirculation must return < 50 CFU/mL.

Species	Dosage form	Levamisole base equivalent	Key excipient restriction	Stability-indicating method
Ovine / Caprine	Oral drench 1.5 % w/v	7.5 mg/kg	Free of polyethylene glycol > 400 MW	USP <621> HILIC, Rs ≥ 2.2
Porcine	Injectable 100 mg/mL	7.5 mg/kg	Antioxidant: sodium metabisulphite 0.08 %	Ph. Eur. 2.2.29 chiral purity
Canine	Chewable tablet 150 mg HCl salt	118 mg base per tablet	Avoid magnesium stearate; use sodium stearyl fumarate	USP <711> dissolution, 0.1 N HCl
Gallus domesticus	Water-soluble granulate	25 mg/L in drinking water	Coating thickness 45 ± 7 μm	NIR calibration, 1,450 nm

The immunomodulatory application of levamisole in human adjuvant oncology, specifically for Stage III colon carcinoma in combination with 5-fluorouracil, historically required a 50 mg oral tablet administered three times daily for three consecutive days every two weeks – a schedule derived from the seminal INT‑0035 protocol. While current clinical guidelines have largely replaced this with oxaliplatin-based regimens, levamisole hydrochloride remains in circulation as a compounded option governed by USP <795> and Ph. Eur. 2619 regulation of non-sterile preparation when manufactured under a hospital exemption. The direct compression blend integrates 50.0 mg levamisole hydrochloride (39.4 mg base equivalent), 98.0 mg microcrystalline cellulose PH‑102, 6.5 mg croscarmellose sodium, and 1.5 mg colloidal anhydrous silica, all precision-weighed to ±2 % on a validated dispensing bench inside a Grade D environment. A 10-station rotary tablet press equipped with 8 mm flat-faced bevel-edge tooling yields a tablet hardness of 6–9 kp and a friability of < 0.8 % after 100 rotations in a USP <1216> tester. Bioavailability studies reported under FDA NDA 019873 indicate a peak plasma concentration of 0.38 μg/mL at 2.1 hours post‑dose following a 150 mg oral load, with a volume of distribution of 120 L. Critically, genotoxic potential from N‑nitroso‑levamisole necessitates routine spiking of the HPLC mobile phase with 0.01 % v/v formic acid and mass spectrometric single-ion monitoring at m/z 206 during release testing to confirm an impurity level below the 1.5 μg/day acceptable intake defined in the ICH M7(R2) framework.An industrial user requiring a chiral resolution standard deploys levamisole free base recrystallised from 95 % ethanol to yield an enantiomeric excess exceeding 99.5 % ee, confirmed by polarimetric measurement at the sodium D‑line ([α]D²⁰ = –85° to –88°, c = 1 in water as the hydrochloride). This material is subsequently milled in a zirconia ball mill under liquid nitrogen cooling to a particle size d50 of 12 μm and portioned into 100 mg sealed ampoules under an argon overlay inside a ISO Class 5 laminar‑flow workstation. Purity assignment follows a mass balance approach subtracting organic impurities by HPLC‑UV at 215 nm, inorganic residue by sulphated ash Ph. Eur. 2.4.14, and residual solvents by static headspace GC‑FID calibrated against a Class 2 solvent mixture, reporting a combined purity of 99.94 % ± 0.05 %. The certified value is traceable to NIST SRM 1883b through a metrological hierarchy documented in the batch‑specific certificate conforming to ISO 17034:2016. When this reference material is incorporated into a chiral HPLC method for the quantification of the destrorotatory impurity in tetramisole racemate, the system suitability test on an immobilised amylose tris(3,5‑dimethylphenylcarbamate) column demands resolution of Rs ≥ 3.0 between the (–)‑ and (+)‑enantiomers at a retention time window of 13–15 minutes. Off‑label reliance on a non‑certified in‑house standard has been documented to inflate d‑isomer estimates by 0.8 % absolute bias, underscoring the necessity of a monograph‑grade external reference.

Pharmacopoeial Monograph	Identification test	Assay limit (dried basis)	Optical rotation (hydrochloride)	Loss on drying
USP 2023 Levamisole Hydrochloride	IR <197K>; HPLC retention time	98.5–101.0 %	–82.5° to –86.5°	≤ 0.5 %
Ph. Eur. 11.5 Levamisole hydrochloride	IR (2.2.24); TLC (2.2.27)	98.5–101.0 %	–85° to –88°	≤ 0.5 %
JP Levamisole Hydrochloride	UV; IR	98.5–101.0 %	–82° to –87°	≤ 0.5 %

A deep‑freeze vaccine adjuvant preparation where levamisole acts as a non‑specific immune potentiator in an experimental Mycoplasma hyopneumoniae bacterin for porcine respiratory disease formulated at 2.0 mg/mL presents unique cold‑chain performance constraints. The active is dissolved together with the aluminium hydroxide gel adjuvant (1.3 mg Al³⁺ per dose) by high‑shear mixing at 8,000 rpm for 4 minutes using an Ultra‑Turrax T50 basic equipped with a S50N‑G45G dispersing tool; this step must occur at 5 ± 2 °C because levamisole adsorbs to the gel surface at higher temperatures, causing a drop in free drug recovery to 62 % at 20 °C. The sterile‑filtered bulk is filled into 2R borosilicate vials, freeze‑dried in a lyophiliser with shelf temperature ramped from –45 °C to +25 °C over 36 hours under a chamber pressure of 0.08 mbar, then sealed under dry nitrogen. When reconstituted with 0.9 % NaCl, the product must meet a viscosity of < 15 cP at 25 °C (Brookfield LV, spindle #2 at 60 rpm) to pass through a 23‑gauge needle without clogging. Field efficacy trial data published under EMA/CVMP/852/16 indicate that inclusion of levamisole elevates the seroconversion titre against M. hyopneumoniae by a factor of 3.7 relative to the adjuvant‑alone control at 21 days post‑vaccination, though this benefit is nullified if the lyophilised cake residual moisture exceeds 2.5 %, above which levamisole undergoes hydrolysis to the pharmacologically inactive 2‑oxo derivative.

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Certification & Compliance

6-Phenyl-2,3,5,6-Tetrahydroimidazo[2,1-B][1,3]Thiazole is manufactured under an ISO 9001 quality system and complies with relevant regulatory requirements.
COA, SDS/MSDS, and related certificates are available upon request. For certificate requests or inquiries, contact: sales9@bouling-chem.com.

More Introduction

The heterocyclic scaffold 6-Phenyl-2,3,5,6-tetrahydroimidazo[2,1-b][1,3]thiazole, with the racemic base identified under CAS 14769-73-4 and the hydrochloride salt under CAS 16595-80-5, forms the pharmacophoric core of tetramisole. The molecule contains a single asymmetric carbon at the 6-position; industrial-grade material is thereby produced as an equimolar mixture of levamisole (S(−)-isomer) and dexamisole (R(+)-isomer). The hydrochloride salt, a white to faintly cream crystalline powder, decomposes within a melting interval of 264–266 °C and exhibits an aqueous solubility exceeding 20% w/v at 20 °C, whereas the free base remains practically insoluble in water. Large-scale synthesis proceeds through condensation of 2-imino-3-(2-chloroethyl)thiazolidine with styrene oxide, followed by sulfuric acid-catalyzed cyclization. The resulting racemic base is subsequently resolved via diastereomeric salt formation with (−)-dibenzoyl-L-tartaric acid or with L-(+)-tartaric acid under controlled solvent conditions, enabling isolation of the levamisole enantiomer at an enantiomeric excess routinely above 99.0%. Residual dexamisole in the levamisole fraction is minimized through multiple recrystallization steps performed in methanol-water mixtures; the mother liquors from the first crystallization—enriched in the R-isomer—are typically directed toward tetramisole production or discarded after neutralization. Process deviations in cooling rate or seed crystal addition have been observed in production-scale crystallizers exceeding 2,000 L capacity to increase fine-particle generation, which occludes mother liquor and raises the diastereomeric impurity burden in the isolated cake.

What Distinguishes Levamisole from Dexamisole at the Nicotinic Receptor?

The anthelmintic activity of 6-phenyl-2,3,5,6-tetrahydroimidazo[2,1-b][1,3]thiazole is virtually stereospecific: levamisole acts as a potent, selective agonist at nematode nicotinic acetylcholine receptors (nAChRs) located on somatic muscle cells, producing sustained depolarization and spastic paralysis, whereas dexamisole possesses negligible affinity for these receptor subtypes and contributes predominantly to off-target cholinergic side effects in the host. Electrophysiological recordings on Ascaris suum muscle strips have demonstrated that levamisole at concentrations of 3–10 µM induces rapid, irreversible membrane depolarization, an effect entirely absent with dexamisole at equimolar concentrations. This pharmacological divergence is attributable to the stereochemical fit within the agonist-binding pocket of the L-type nAChR; molecular docking studies indicate that the phenyl ring orientation of the S-enantiomer permits a hydrogen-bond network with loop C residues that the R-enantiomer cannot replicate. Consequently, when the racemic product tetramisole is administered at a dose of 15 mg/kg body weight in sheep, only half the dose—the levamisole fraction—drives the therapeutic effect, while the dexamisole half imposes an unnecessary metabolic load. This half-dose inefficiency, combined with the narrower therapeutic index of dexamisole, has driven the global veterinary market to replace tetramisole with pure levamisole hydrochloride in all oral, injectable, and pour-on formulations. Moreover, the immunomodulatory property of levamisole—restoration of depressed T-lymphocyte and phagocyte function—is similarly enantiospecific and completely absent in dexamisole. This dual-use capacity (anthelmintic and immunostimulant in adjunctive cancer therapy or chronic infections) further differentiates levamisole from other imidazothiazole derivatives such as butamisole, which never attained marketing authorization due to a narrower safety margin and lack of immunotropic activity. Conformance to the relevant pharmacopoeial monographs is a prerequisite for active pharmaceutical ingredient release into both human and veterinary supply chains. The United States Pharmacopeia (USP) monograph for Levamisole Hydrochloride and the European Pharmacopoeia (Ph. Eur.) monograph 01/2008:1640 prescribe a battery of identity, purity, and assay tests. Quantitative determination of the active content is carried out by non-aqueous titration with perchloric acid, as described under USP <541>, with an acceptance range of 98.0–102.0% calculated on the dried basis. The enantiomeric purity is verified by measuring the specific optical rotation of a 2% w/v aqueous solution at the sodium D-line (589 nm), which must fall between −115° and −123° for the S-isomer hydrochloride. Alternative chiral HPLC methods employing a cellulose tris(3,5-dimethylphenylcarbamate) stationary phase and a mobile phase of hexane-ethanol-diethylamine achieve baseline separation of the enantiomers, enabling a quantitative limit for the unwanted R-isomer of not more than 1.0%. The table below aggregates the core specification parameters applicable to pharmaceutical-grade levamisole hydrochloride.

Test Parameter	Method Reference	Acceptance Criterion
Appearance	Visual / USP <631>	White or almost white crystalline powder
Solubility	Ph. Eur. 2.2.32	Freely soluble in water (≥20% w/v at 20 °C), soluble in ethanol
Melting point (decomposition)	USP <741>	264–266 °C
Specific optical rotation [α]_D²⁰	USP <781>	−115° to −123° (2% in water)
pH (aqueous solution)	USP <791>	4.0–5.5 (5% w/v)
Loss on drying	USP <731>	≤0.5% (105 °C, 4 h)
Assay (anhydrous basis)	Non-aqueous titration	98.0–102.0%
2,3-Dihydro-6-phenylimidazo[2,1-b]thiazole impurity	HPLC / TLC	≤0.5%
Any other single impurity	HPLC	≤0.2%
Total impurities	HPLC	≤1.0%
Residual solvents (methanol, ethanol)	USP <467> / ICH Q3C	Methanol ≤3,000 ppm; ethanol ≤5,000 ppm
Sulfated ash	USP <281>	≤0.1%

When Enantiomeric Excess Falls Below 99.0%—Clinical and Regulatory Consequences

Maintenance of an enantiomeric excess (ee) of ≥99.0% in levamisole batches destined for immunomodulatory indications is enforced not merely by compendial limits but by clinical safety boundaries. At ee values between 95.0% and 99.0%, the presence of 1–2.5% dexamisole remains pharmacologically silent in most anthelmintic applications; however, when levamisole is prescribed as an adjuvant in colorectal carcinoma therapy at a human dose of 50 mg three times daily for three days every two weeks, even low-level R-isomer contamination has been associated with a higher incidence of dose-limiting agranulocytosis and leukopenic events. This toxicity is attributed to dexamisole’s inhibition of dihydrofolate reductase, an activity absent in the S-enantiomer, and to its longer plasma half-life—approximately 1.5–2 hours greater than that of levamisole—resulting from slower hepatic oxidation by cytochrome P450 1A2. In the resolution plant, the ee of the final levamisole hydrochloride is governed by the crystallization solvent ratio (methanol:water typically 85:15 v/v), cooling ramp rate (−0.3 °C/min from 65 °C to 5 °C), and the molar ratio of resolving agent (0.50–0.52 equivalents of L-(+)-tartaric acid). Process analytical technology employing on-line polarimetry or near-infrared spectroscopy is increasingly integrated into commercial manufacturing trains to detect ee drift during kilogram-scale crystallizations. When a batch falls below the 99.0% ee threshold, it is either reprocessed through a second tartrate salt formation or downgraded to non-pharmaceutical uses, such as a research chemical or an anthelmintic for non-food-producing animals in territories that still accept racemic tetramisole. Stability of levamisole hydrochloride in finished veterinary dosage forms, particularly oral drenches and drinking-water formulations, depends critically on the pH and the presence of transition metal ions that catalyze oxidative ring-opening. The imidazothiazole nucleus undergoes hydrolytic cleavage in alkaline media; therefore, buffering the vehicle to a pH between 3.5 and 4.5 with citric acid or sodium dihydrogen phosphate is mandatory. Accelerated stability testing conducted at 40 °C / 75% relative humidity over 6 months pursuant to ICH Q1A(R2) has shown that the primary degradation product, 2,3-dihydro-6-phenylimidazo[2,1-b]thiazole, increases from an initial 0.1% to approximately 0.8% in unbuffered solution, but remains below 0.3% when the pH is maintained below 4.5. Inclusion of sodium metabisulfite as an oxygen scavenger at 0.05% w/v further retards discoloration and precipitate formation. In high-concentration pourable suspensions, the wet-milling process must be conducted with predried excipients (moisture content below 0.2% by Karl Fischer titration) to avoid agglomerate formation during storage. Incompatibilities have been documented with strong alkalis, with oxidizing agents such as hydrogen peroxide-based disinfectants, and with amine-functional polymers used in sustained-release matrices, which induce premature racemization. Consequently, blend validation protocols in solid dosage forms specify exclusion of crospovidone grades with residual peroxide numbers above 200 ppm.

Comparative Anthelmintic Efficacy Against Major Nematode Species

Within the broad-spectrum veterinary anthelmintics, the imidazothiazoles occupy a distinct mechanistic niche—selective agonism of nematode nicotinic receptors—that sets them apart from benzimidazoles (β-tubulin polymerization inhibitors) and macrocyclic lactones (glutamate-gated chloride channel potentiators). This mechanistic differentiation translates into a unique cross-resistance profile: nematode populations resistant to benzimidazoles and ivermectin frequently retain susceptibility to levamisole, making it an indispensable rotation agent in integrated parasite management programs for sheep, cattle, and swine. The table below contrasts key attributes of levamisole/tetramisole with representative alternatives.

Agent	Chemical Class	Primary Mode of Action	Label Dose (cattle, oral)	Ovicidal Activity	Withdrawal Period (meat/milk)	Documented Resistance Profile
Levamisole HCl	Imidazothiazole	nAChR agonist (L-type)	7.5 mg/kg	Negligible	7 d / 60 h (cattle)	Prevalent in Haemonchus contortus; largely independent of BZ/ML resistance
Tetramisole HCl (racemic)	Imidazothiazole	nAChR agonist (50% active)	15 mg/kg	Negligible	14 d / prohibited in many jurisdictions	Same loci as levamisole; broader side-effect burden
Albendazole	Benzimidazole	Inhibition of β-tubulin polymerization	7.5 mg/kg	Partly ovicidal	14 d / 60 h	Widespread in strongyles; F200Y/T167Y mutations common
Ivermectin	Macrocyclic lactone	Glutamate-gated Cl⁻ channel agonist	0.2 mg/kg	Negligible	35 d / not for use in lactating dairy	Multi-drug resistance reported in Cooperia and Ostertagia spp.
Praziquantel	Pyrazinoisoquinoline	Ca²⁺ permeability disruption in cestodes	Not applicable (cestocide)	N/A	7 d	No cross-resistance with nematocides

The absence of ovicidal activity in levamisole necessitates repeat dosing or co-administration with a larvicidal benzimidazole when targeting arrested larvae of Ostertagia ostertagi. Furthermore, the therapeutic index of levamisole is narrower than that of macrocyclic lactones; the single-dose oral LD₅₀ in cattle is approximately 40 mg/kg, resulting in a margin of safety of only roughly 5.3—substantially lower than the >30-fold margin typically offered by ivermectin. This constraint has direct operational implications in high-throughput feedlot dosing systems, where accidental double-dosing can precipitate cholinergic crisis manifesting as salivation, muscle fasciculations, and respiratory distress. Veterinarians are therefore trained to keep atropine sulfate (0.25 mg/kg intramuscularly) available as an antidote during mass-treatment campaigns. From a regulatory toxicology standpoint, levamisole is classified under the Globally Harmonized System (GHS) as Acute Toxicity Category 4 (Harmful if swallowed, H302) and as a suspected human reproductive toxicant (H361d) based on rodent teratogenicity studies. The compound is not considered genotoxic in the Ames test (OECD 471) or in the in vivo micronucleus assay (OECD 474). Its environmental fate is governed by rapid photodegradation in surface waters, with a half-life of less than 24 hours under natural sunlight; nevertheless, its ecotoxicity to aquatic invertebrates (Daphnia magna EC₅₀ at 48 h of 2.3 mg/L) obligates containment of manufacturing effluents and adherence to a Predicted No Effect Concentration (PNEC) of 0.23 µg/L in freshwater environments as per REACH Annex I. The drug substance and its formulations are controlled under 21 CFR 520.1242 for veterinary use in the United States, with a requirement that treated animals not be slaughtered for food within the prescribed withdrawal period. Trace detection methods employing LC-MS/MS have been validated to a limit of quantification of 1 µg/kg in bovine muscle tissue, ensuring compliance with Codex Alimentarius maximum residue limits. Occupational exposure during pharmaceutical compounding warrants particular attention: airborne particulates of levamisole hydrochloride can induce contact urticaria and, upon repeated inhalation, have been associated with reversible agranulocytosis in cases documented through pharmacovigilance in human drug manufacturing facilities. Engineering controls including local exhaust ventilation achieving a capture velocity of 0.5 m/s at powder charging stations, coupled with the use of powered air-purifying respirators assigned an assigned protection factor of 50 in accordance with OSHA 29 CFR 1910.134, are integral to routine handling procedures. Powder flowability, measured by a Hausner ratio exceeding 1.40 for micronized grades, demands the incorporation of glidants such as colloidal silicon dioxide below 0.5% w/w during direct compression tableting, as severe segregation has been repeatedly observed in rotary presses operating at turret speeds above 40 rpm when flow aids are omitted.