Bethanechol chloride, (2-hydroxypropyl)trimethylammonium chloride carbamate, CAS 590-63-6, is supplied as a white to off-white crystalline powder under the model designations BTH-VET-API-01 (non-micronized) and BTH-VET-API-02M (micronized). The molecular formula is C7H17ClN2O2, with a molar mass of 196.67 g/mol. The API is intended for incorporation into veterinary tablets, injectable solutions, capsules, powders for reconstitution, granules, feed premixes, and oral solutions. The molecule is a direct-acting muscarinic receptor agonist whose quaternary ammonium structure limits blood-brain barrier penetration and whose carbamate ester confers resistance to acetylcholinesterase-mediated hydrolysis relative to acetylcholine. It is freely soluble in water and practically insoluble in nonpolar solvents; this solubility profile makes aqueous granulation, direct compression with dry aqueous binders, and sterile solution compounding feasible, while lipid-based premix vehicles require adsorption of the API onto a hydrophilic carrier prior to dispersion.

The principal veterinary use is the management of non-obstructive urinary retention and gastrointestinal hypomotility after exclusion of mechanical obstruction. Because the drug acts directly on muscarinic receptors of the detrusor muscle and gastrointestinal smooth muscle, it does not require endogenous acetylcholine accumulation for activity. This differs from neostigmine, an indirect parasympathomimetic that requires cholinergic nerve terminals. The product is contraindicated in urinary or gastrointestinal tract obstruction, suspected bladder rupture, or known hypersensitivity to choline esters.

Particle-Size Control Across Solid and Liquid Dosage Platforms

For solid dosage forms, BTH-VET-API-02M is micronized to a volume-median particle size D50 of 10–20 µm and D90 ≤ 50 µm when measured by laser diffraction according to USP <429>. Non-micronized BTH-VET-API-01 is controlled to D90 ≤ 150 µm. The micronized grade supports blend uniformity in direct compression at API loadings as low as 2.5% w/w, but the increased specific surface area raises hygroscopicity. At relative humidity above 60%, water uptake can exceed 0.5% within 2 h, leading to punch filming, weight variation, and capping on rotary tablet presses. Pre-drying at 40 °C for 2 h in a forced-air oven is applied when moisture ingress is suspected.

For liquid and injectable platforms, particle-size specification is less relevant after dissolution, but insoluble particulate matter must be controlled. Aqueous solutions are prepared with Water for Injection or purified water at a bethanechol chloride concentration of 5 mg/mL under acidic conditions, followed by filtration through 0.22 µm polyvinylidene fluoride or polyethersulfone membranes. The API should not be combined with alkaline buffers, carbonate salts, or primary amines because the carbamate ester undergoes pH-dependent nucleophilic attack; published kinetic data for this specific veterinary-grade material in multi-component solutions is limited, but hydrolysis increases above pH 7.5.

When Bethanechol Chloride Is Used in Sterile Injection Compounding

In sterile compounding, BTH-VET-API-02M is dissolved and the solution pH is adjusted to 4.0–5.5 before terminal sterilization or aseptic filtration. The acid pH suppresses carbamate hydrolysis and minimizes carbon dioxide formation. Terminal sterilization by autoclaving at 121 °C for 15 min may be used only after confirming that the container-closure system withstands thermal stress and that the chosen buffer does not shift pH during heating. Aseptic filtration through 0.22 µm membranes is an alternative where terminal sterilization is not validated. For parenteral-grade material, bacterial endotoxin control follows USP <85>; a limit of not more than 0.5 EU/mg is a typical starting point for small-volume injectables with single doses below 10 mg, but the final limit is derived from the maximum animal dose on a body-weight basis.

Tonicity adjustment is made with sodium chloride to an osmolality of 280–320 mOsm/kg. The solution is clear and colorless; visible particles are controlled by 100% visual inspection after filling. Extractables and leachables studies should be conducted for any rubber stoppers or plastic ampoules used with the solution, because quaternary ammonium compounds may alter the surface properties of elastomer materials. Solutions below pH 3.0 should be avoided for parenteral administration because of injection-site discomfort; this is independent of chemical stability.

For medicated feed premixes, bethanechol chloride is not dispersed directly into soybean oil or liquid molasses carriers because the quaternary ammonium salt partitions poorly into lipid phases. A dry adsorption step onto lactose monohydrate or microcrystalline cellulose is used before the blend is incorporated into mineral oil or molasses binders. Dust control during sifting and blending requires local exhaust ventilation and containment because quaternary ammonium dusts are irritating to mucous membranes. Final premix homogeneity is assessed by sampling at least 10 points per mixer load and assaying the active by HPLC; acceptance is typically a relative standard deviation below 5.0% for the target concentration.

Which Pharmacopeial Tests Are Applied to the Veterinary-Grade API?

Release specifications follow the general chapters of the United States Pharmacopeia and ICH Q3D where applicable. Identification is confirmed by infrared absorption against a reference standard and by chromatographic retention time; the chloride salt is confirmed by a silver nitrate precipitation test. Assay on the anhydrous basis is 98.0–102.0% by validated HPLC, corresponding to compendial quality for bethanechol chloride API. Loss on drying is not more than 0.5% by USP <731>, and residue on ignition is not more than 0.1% by USP <281>.

Direct Compression Versus Wet Granulation in Tablets and Premix Feed Additives

Direct compression with BTH-VET-API-02M is preferred for low-dose tablets where content uniformity at 2.5% w/w active is critical. A 27-station rotary press with 8 mm round flat-faced beveled-edge tooling can be operated at pre-compression force below 4 kN to avoid sticking. The formulation should include microcrystalline cellulose as diluent, croscarmellose sodium as disintegrant at 2.0–3.0% w/w, and magnesium stearate at 0.5% w/w; magnesium stearate above 1.0% w/w may retard dissolution by hydrophobic film formation, particularly at low tablet hardness. Ejection force should be monitored; values above 1.0 kN for an 8 mm tool indicate excessive friction from insufficient lubricant or moisture-related adhesion.

Wet granulation with BTH-VET-API-01 is more suitable for higher-dose tablets and granules because particle-size reduction is not required and bulk handling generates less dust. A top-spray fluid-bed granulator with inlet air temperature 55–65 °C and product temperature 28–32 °C is typical. Hydroxypropyl methylcellulose at 3.0% w/w or povidone K30 at 2.0–4.0% w/w can serve as binder. After drying, loss on drying should be below 2.0% before compression or capsule filling. Overgranulation produces hard granules that resist compression and may lower tablet tensile strength.

Compared with carbachol, a direct-acting cholinergic agonist with both muscarinic and nicotinic activity, bethanechol chloride has a narrower muscarinic profile and produces less nicotinic ganglionic stimulation at therapeutic doses. Compared with pilocarpine, a tertiary amine muscarinic agonist, bethanechol does not cross the blood-brain barrier to a meaningful extent because of its permanent positive charge. Compared with neostigmine, an indirect cholinesterase inhibitor, bethanechol remains effective when cholinergic nerve terminals are depleted or when acetylcholinesterase inhibition would complicate therapy. These differences are material in veterinary patients with urinary retention after pelvic surgery or spinal cord injury, where direct detrusor stimulation is desired without initial reliance on neural acetylcholine release.

Release Specifications and Stability-Limiting Impurities

The main degradation route is hydrolysis of the carbamate ester to choline chloride and carbon dioxide, with the rate increasing above pH 7.5 and at temperatures above 40 °C. Storage is therefore specified at controlled room temperature 20–25 °C in tight, light-resistant containers, with excursions limited to 15–30 °C. The API should be protected from moisture; at relative humidity above 60%, the crystalline powder may cake and harden. No antioxidant is required for solid-state storage, but aqueous solutions should be preserved or used within a validated hold time because microbial growth can occur in non-sterile oral solutions.

Impurity control includes total impurities not more than 1.0% by HPLC, with any single unspecified impurity not more than 0.10%. The principal related compound is choline chloride, and resolution between bethanechol and choline in the chromatographic system should be not less than 2.0. Residual solvents are controlled to USP <467> Class 3 limits. Elemental impurities are controlled according to ICH Q3D; for an oral or injectable veterinary product, the risk assessment should consider residues from catalysts or processing equipment, and published data for this specific API source is limited where custom manufacturing routes deviate from standard carbamoylation chemistry.

The product differs from human-grade bethanechol chloride primarily in documentation, not necessarily in chemical identity; the veterinary-grade API may not be manufactured under a drug master file that supports human marketing authorization, but it should be produced under current good manufacturing practice for veterinary active pharmaceutical ingredients. For international distribution, compliance with VICH GL18 residual solvents and VICH GL10 impurities is relevant; if EU feed use is intended, the API must be assessed under Regulation (EC) No 1831/2003 for feed additives, and if used in food-producing animals, maximum residue limit considerations under Regulation (EU) No 37/2010 may apply. Published data for bethanechol chloride as a feed additive in food-producing species is limited, so extrapolation from companion-animal clinical use should not be done without target-species withdrawal-period data.