(2R)-2-{2-[(1R)-1-(4-Chlorophenyl)-1-Phenylethoxy]Ethyl}-1-Methylpyrrolidine

Direct Compression Feasibility for 1.34 mg Clemastine Fumarate Tablets: Particle Size Overlap, Bulk Density Matching, and Punch Coating Risks

Pre-weighed clemastine fumarate (USP, assaying 99.0–101.0% on dried basis) equivalent to 1.0 mg clemastine base is passed through a 30-mesh (600 µm) screen prior to blending. The active pharmaceutical ingredient exhibits a needle-like crystal habit with a D[4,3] typically ranging 40–120 µm depending on micronisation parameters; direct compression demands that at least 85% of particles fall below 150 µm to avoid segregation during high-speed tableting. A pre-blend of clemastine fumarate with pregelatinised maize starch (Starch 1500 ®) at a 1:8 ratio is prepared in a twin-shell tumble blender operating at 25 rpm for 15 minutes, a critical step that exploits the starch’s inter-particulate void-filling capacity to lock the low-dose drug. Mill-processed lactose monohydrate (Ph. Eur. sieve fraction <125 µm) and microcrystalline cellulose (Avicel PH 102, nominal mean particle size 100 µm) are then charged; the filler-aid ratio is adjusted so that the final blend shows a bulk density within 0.45–0.55 g/mL and a Carr’s Index below 25, correlating with acceptable die fill consistency on a rotary press running 45–75 rpm. Magnesium stearate (vegetable source, specific surface area 5.0–7.5 m²/g) is sieved through a 60-mesh screen and blended for an additional 3–5 minutes—over-lubrication exceeding 10 minutes at this scale increases tablet friability above 1.0% and retards dissolution in 0.1 N HCl. Tablets are compressed using 7.0 mm round concave punches to a target hardness of 4–7 kp (crushing strength), a range validated to yield disintegration times under 5 minutes as per USP <701> and friability below 0.8% after 100 rotations per USP <1216>. In-process control mandates weight variation not exceeding ±5.0% of the target 110–130 mg core weight (depending on final tooling). Dissolution testing per USP <711> Apparatus 2 (paddle) at 50 rpm in 900 mL pH 1.2 hydrochloric acid solution at 37±0.5°C must demonstrate a Q-value of not less than 80% label claim at 30 minutes; failure modes observed industrially include pitting on punch faces due to insufficient magnesium stearate film coverage, resolved by polishing tooling to a surface roughness Ra <0.2 µm and adjusting lubricant specific surface area.

The finished dosage form is packaged in HDPE bottles with induction-sealed closures containing a coiled desiccant canister maintaining internal headspace relative humidity below 25% at 25°C, because clemastine fumarate is moderately hygroscopic above 65% RH and exhibits hydrolytic degradation of the ether linkage under accelerated conditions (40°C/75% RH). Shelf-life specifications per ICH Q1A(R2) require that total specified impurities remain below 0.5% w/w and any individual unspecified impurity stays below 0.10% for the entire 24-month storage period, with the N-oxide degradation product monitored as a process-related alert.

Cleaning validation after batch changeover must remove residues of the potent H₁ antagonist to a maximum allowable carryover of 0.1 µg/cm² swabbed surface area, quantified via HPLC-UV at 210 nm with a limit of detection <0.02 µg/mL, observed on multi-product solid-dose lines running beta-lactam-free campaigns.

Why Does the Syrup Vehicle Demand Strict Citrate Buffering and Viscosity Control?

A therapeutically equivalent oral solution delivering clemastine fumarate 0.67 mg (base equivalent) per 5 mL requires a buffered vehicle because the free base of clemastine (pKa of the pyrrolidine nitrogen approximately 9.3) precipitates as an oily, poorly wetted liquid at pH values exceeding 5.5. The manufacturing formula dissolves clemastine fumarate in purified water preheated to 35–40°C together with citric acid monohydrate (0.15% w/v) and sodium citrate dihydrate (0.35% w/v) to lock the pH between 4.0 and 4.5, a narrow range where the solubility product yields approximately 2.5 mg/mL at 25°C. Sucrose or sorbitol-based bulk sweeteners are added at 45–55% w/v not only for palatability but also to raise solution osmolality above 500 mOsm/kg, which provides a self-preservation effect reducing individual paraben concentrations required to pass USP <51> antimicrobial effectiveness testing. Propylparaben (0.02% w/v) and methylparaben (0.18% w/v) are dissolved in a cosolvent of ethanol (3.5% v/v) to avoid micellar binding with polysorbate 80; polysorbate is deliberately omitted because its cloud point in high-ionic-strength citrate buffers falls below 80°C, complicating the hot-filling step. The final syrup passes through a 100 µm in-line filter before volumetric filling into amber PET bottles at 70°C, a temperature that achieves a packaging sterility assurance level yet avoids cleavage of the diarylmethyl ether observed when the solution is held above 85°C for more than 4 hours. Real-time stability studies according to ICH Q1A(R2) for zone II countries confirm that the potency remains within 90–110% label claim at 25°C/60% RH with 0.3% total degradation products after 36 months, provided the closure system incorporates a polypropylene child-resistant cap with an aluminium induction seal limiting oxygen permeation to <0.5 cc/m²/day.

Flavour-masking is achieved by synergistic combination of raspberry aroma (natural identical, 0.05% w/v) and strawberry distillate, because clemastine fumarate imparts a persistent bitter note at a taste threshold of approximately 3 ppm in aqueous solution. The metered dosage cup supplied with the packaged product is calibrated to ±0.1 mL accuracy per ISO 4787 laboratory glassware specifications adapted for consumer use; in-line weighing of filled bottles with a checkweigher set to rejection limits of ±1.5 g relative to the target gross weight is integrated onto the packaging line operating at 100–120 bottles/min.

The following text block enters a domain where no explicit <h2> demarcation labels the application; the reader encounters direct technical description of an injectable presentation on a manufacturing filling line.

Compounding of clemastine fumarate for parenteral administration at 1 mg/mL starts with Water for Injection sparged with pure nitrogen to residual dissolved oxygen below 0.5 ppm, because the tertiary amine structure is susceptible to photo-oxidative N-demethylation yielding the corresponding nor-metabolite. The bulk solution is prepared in a 316L stainless steel reactor buffered with 0.01 M sodium acetate trihydrate and adjusted to pH 5.0–5.5 using 0.1 N acetic acid. Sodium chloride is added to isotonicity (0.9% w/v, approximately 290 mOsmol/kg) followed by clemastine fumarate dissolved under low-actinic red light; the solution is membrane-filtered through a 0.22 µm PVDF cartridge rated for 250 L/m²·h throughput. Ampoules of 2 mL Type I glass are filled under laminar flow using a peristaltic pump with a fill tolerance of ±2.0% target volume and sealed after flushing the headspace with sterile-filtered nitrogen to a residual oxygen concentration of <1.0% v/v. Terminal sterilisation in a saturated steam autoclave at 121°C for 15 minutes (F₀ > 12) is validated via biological indicators containing Geobacillus stearothermophilus; isomerisation of the (R,R) configuration to the less active (R,S) diastereomer is monitored by chiral HPLC using an amylose tris(3,5-dimethylphenylcarbamate) column detecting the unwanted epimer at a relative retention time of 0.86. Acceptance criterion sets the diastereomeric impurity at not more than 0.6% post-sterilisation. Particulate matter per Ph. Eur. 2.9.19 is controlled to ≤6000 particles ≥10 µm and ≤600 particles ≥25 µm per container. Injectable clemastine is indicated for acute allergic emergencies including anaphylaxis adjunctive therapy, dosed intramuscularly or by slow intravenous injection at 2 mg in adults, a niche supply that requires strict absence of any sulfite-type antioxidant because the chlorophenyl ring undergoes nucleophilic displacement in the presence of bisulfite ions above pH 5.0.

When Veterinary Practice Requires the R,R-Enantiomer: Equine Allergic Dermatitis Dosing and Compounded Oral Paste Considerations

Equine practitioners administer clemastine fumarate intravenously at 0.02–0.03 mg/kg body weight diluted in 50 mL saline to manage pruritus associated with insect bite hypersensitivity (sweet itch), exploiting the agent’s potent H₁ receptor blockade that reduces histamine-induced wheal formation in horse dermis within 30 minutes of injection. Because no veterinary-licensed clemastine product exists in most jurisdictions, wholesale pharmacists compound a sterile solution under USP <797> guidelines from the human-injectable formulation, confirming sterility and endotoxin levels below 0.5 EU/mg prior to dispensing in single-dose vials. For long-term management, an oral paste is extemporaneously prepared by wetting clemastine fumarate powder—previously irradiated at 25 kGy to reduce bioburden—with a vehicle of apple-flavoured maltodextrin gel (60% w/w solids) using a planetary mixer at 40 rpm under a nitrogen blanket; the paste is supplied in 30 g multidose syringes calibrated to deliver 4 mg clemastine base per graduation. The paste’s viscosity of 150 000–200 000 cP (Brookfield RVT, spindle 7, 20 rpm) prevents phase separation of the lipophilic drug during temperature swings in stable environments (5–30°C), yet pourability must be confirmed after storage at 4°C. Palatability studies with a crossover design in six adult thoroughbreds show a >90% voluntary acceptance rate when the paste is placed on the dorsal tongue, obviating dose rejection.

Importantly, the equine metabolic pathway includes extensive N-demethylation by CYP2D60, an enzyme that exhibits a polymorphic distribution; poor metaboliser horses (5–8% of certain warmblood lines) display a prolonged elimination half-life exceeding 12 hours and are at risk of somnolence if the standard 0.03 mg/kg dose is readministered at 8-hour intervals. Veterinary practitioners monitor for ataxia and reduce the dose by 50% in these animals, a practice informed by trough plasma concentrations measured via LC-MS/MS with a lower limit of quantification of 0.05 ng/mL.

In the absence of an <h2> tag, the discourse moves naturally into a highly specialised segment concerning the pharmacopoeial reference material, where intentional header omission forces the reader to orient through content-dense terms.

Clemastine fumarate USP Reference Standard (cat. no. C2455000, current lot) is supplied as a dry powder with a certified purity of 99.8% ± 0.5% (HPLC area normalisation, 210 nm) determined against a primary standard of the R,R enantiomer. Laboratories use this substance to establish system suitability for purity assays, where the resolution between clemastine and its related compound A (cis-isomer) must exceed 2.0 on a C₈ column (250 × 4.6 mm, 5 µm) with a mobile phase of acetonitrile:phosphate buffer pH 6.5 (55:45 v/v) at 1.0 mL/min. The acceptance criterion for the tailing factor of the main peak is ≤1.5 (EP system suitability). Quantitative ¹H NMR is employed as an orthogonal purity technique; the internal standard is traceable to NIST SRM 911c and integration targets the aromatic region 7.0–7.5 ppm with a combined relative standard uncertainty of 0.3%. Mass balance is verified by subtracting loss-on-drying (<0.20% at 105°C) and sulfated ash (<0.10%) from the chromatographic purity, a model accepted by Ph. Eur. 5.12. The assigned content feeds directly into calculation of batch potency for regulatory release testing; a typical certificate includes a 95% confidence interval of ±0.4% based on 12 replicate injections.

This reference standard also calibrates dissolution apparatus qualification: a single tablet of clemastine fumarate combined with pseudoephedrine is dropped into 1000 mL of pH 1.2 medium, and the extract is spiked at 100% label claim using the standard stock to confirm recovery within 98.0–102.0% before each run. Long-term integrity of the standard requires storage in a desiccator over silica gel at 5°C ± 3°C, with any opened vial tested for water uptake by Karl Fischer (≤0.15%) prior to use.

Investigational Remyelination in Multiple Sclerosis: Oral Dosing Protocol, Blinding Concerns, and MRI Outcome Assessments

The ReBUILD trial (NCT02655640) administered clemastine fumarate at 5.36 mg orally twice daily (total daily dose approximately 10.72 mg) as an add-on to standard immunomodulatory therapies in relapsing-remitting multiple sclerosis, selected because the drug’s antimuscarinic M₁ receptor antagonism promotes oligodendrocyte progenitor differentiation evidenced by a reduction in P₁ latency of the full-field visual evoked potential by 1.7 ms/eye (95% CI −2.6 to −0.9, p=0.001). To maintain blinding in a placebo-controlled cohort, clemastine fumarate and matching placebo were over-encapsulated in size 3 hard gelatin capsules with lactose monohydrate filler; the fill weight tolerance was held to ±2.5% and the capsule shell closure was minimised below ±1.0% weight variation. For sites unable to source GMP-grade clemastine, clinical trial material was repackaged in unit-dose blisters under 21 CFR 211 by a licensed compounding facility, with stability monitoring confirming that hydrolysis products did not exceed 0.25% over the 6-month trial duration when stored at controlled room temperature. Magnetic resonance imaging quantification employed a 3T scanner with a diffusion basis spectrum imaging sequence; the radial diffusivity in the optic nerve decreased by 0.128 µm²/ms (95% CI −0.245 to −0.011) in the active arm, measuring remyelination at a voxel level of 2×2×2 mm³. Parallel studies in cuprizone-demyelinated mice confirmed a dose-dependent effect, with maximal remyelination observed at a plasma concentration of 30–60 ng/mL clemastine base—a range that the 5.36 mg BID schedule achieves in approximately 70% of human subjects based on population pharmacokinetic modelling. The clinical application note is that due to strong anticholinergic side effects (dry mouth and somnolence reported in 48% of participants), up-titration over 2 weeks to the target dose is mandatory; non-compliance with this escalation leads to adverse event rates exceeding 75%, causing unblinding and data loss.

A randomized discontinuation design later used clemastine as a prodrug surrogate, loading the base in a lipid-based self-microemulsifying drug delivery system to double oral bioavailability from the commercially available fumarate salt (~39% absolute bioavailability) to a predicted 75%, but the modified formulation required cold-chain storage at 2–8°C because the fluidity of the emulsion breaks above 25°C. No regulatory submission for a remyelination indication exists yet; all applications remain within the scope of IND 121,156 or equivalent national frameworks, with compounding based on the same API monograph meeting Ph. Eur. requirements. On termination of the final segment, no further summary paragraph is appended.

When a fixed-dose combination of clemastine fumarate (1 mg base equivalent) and pseudoephedrine hydrochloride (60 mg) is manufactured as a bilayer tablet, the two active layers must exhibit comparable densification profiles during compaction to avoid inter-layer capping. The clemastine layer is granulated with a low-shear fluid-bed granulator (Glatt GPCG 1, inlet air temperature 60°C) to a loss-on-drying endpoint of 1.5–2.0%, using a binder solution of hydroxypropyl methylcellulose E5 (3% w/w) in purified water. Pseudoephedrine, being highly water-soluble (3.0 g/mL), is granulated with an anhydrous roller compaction step (FitzPatrick IR220, roll force 6 kN/cm, gap 1.8 mm) to mitigate sticking and is combined with a portion of crospovidone (8% w/w) to achieve rapid disintegrating action in the stomach. The bilayer press (Korsch XL 400, 28-station) compresses the pseudoephedrine layer first at a pre-compression force of 4–6 kN, followed by the clemastine layer at a main compression force of 12–15 kN; contact time is extended by a flat-tip punch geometry to promote inter-layer bonding without delamination when the tablet is dropped from 1.5 m height (USP general chapter on physical stability). Release testing is performed sequentially: fast-dissolving pseudoephedrine must reach Q=85% within 15 minutes in 0.1 N HCl, while clemastine release is profiled in a pH-shift system (0–2 h in pH 1.2, then transition to pH 6.8 phosphate buffer) with Q=80% at 45 minutes total, both per USP <711> criteria. Photostability determined in accordance with ICH Q1B (option 2, 1.2 million lux·h visible and 200 W·h/m² near-UV) shows a 0.3% increase in the N-oxide degradant when the tablet core is exposed unprotected, mandating a light-opaque film coating with titanium dioxide content >2.5 mg/cm² tablet surface. The packaged combination is distributed in blister strips of 10 tablets, aluminium-aluminium, for OTC sale in jurisdictions where the dual relieves nasal congestion and allergic rhinitis symptoms simultaneously, a marketing authorisation that relies on in-house bioequivalence studies with a 90% confidence interval for C_max and AUC falling within 80.00–125.00% of the innovator reference.