Film-coated methimazole tablets for feline hyperthyroidism are produced by direct compression after geometric dilution of the API into a microcrystalline cellulose-based diluent. The API addition is expressed as a unit dose of 2.5 mg or 5.0 mg per core; in a low-core-weight formulation this corresponds to an active fraction of approximately 2.5% w/w to 5.0% w/w. At these low fractions, blend segregation is the main process failure mode; if the neat API is discharged directly into a bin blender without a staged pre-blend, static adhesion to the scoop and early stratification along the blender wall can produce superpotent and subpotent cores in the same compression run. The API is therefore screened through a 500 µm stainless steel sieve and triturated with a portion of microcrystalline cellulose in a 1:3 to 1:10 ratio before the premix is added to the main diluent charge. Dispensing and blending are carried out at controlled relative humidity, typically below 60% RH at 20–25°C, to limit static charge. The lubricant magnesium stearate is added at 0.5–1.0% w/w and blended for a short, validated interval of 3–5 min because overlubrication can reduce tablet hardness and slow dissolution. Compression is performed on a rotary tablet press with 7 mm round concave tooling; the compression force is adjusted to keep friability at or below 1.0% while allowing disintegration to meet the pharmacopoeial method. In-production content uniformity is evaluated according to USP <905> with an acceptance value not exceeding 15.0. Film coating is applied in a perforated pan coater using an aqueous non-functional coating dispersion to a weight gain of 2–3% w/w; core hardness must be sufficient to prevent edge erosion during pan transit because the low tablet weight makes edge defects visible and can affect terminal packaging. Regulatory controls include 21 CFR 211.84 for component testing, 21 CFR 211.110 for in-process blend sampling, and 21 CFR 211.165 for finished product release. The API itself is released against the Ph. Eur. monograph for thiamazole and the USP Methimazole monograph where a US filing is involved. Terminal finished product types are 2.5 mg and 5.0 mg film-coated tablets in aluminum/PVC/PVDC blister packs, with bulk HDPE container formats using a desiccant for multi-dose dispensing to veterinary clinics. The compound is not directed to medicated feed premix or food-producing animal administration because no maximum residue limit for methimazole in edible tissues has been established.

What Physicochemical Constraints Govern a 5 mg/mL Veterinary Oral Solution?

Unlike tablet manufacture where segregation is the principal risk, an oral solution containing methimazole at 5 mg/mL (0.5% w/v) presents a different set of control points: dissolution thermodynamics, pH-dependent chemical stability, preservative efficacy, and light exposure. Methimazole is freely soluble in water, with a solubility of approximately 1 g in 5 mL at room temperature, so dissolution is not the bottleneck. The API is dissolved in purified water or a buffered aqueous vehicle in a stainless steel mixing vessel equipped with a bottom-mounted overhead impeller; the batch is not held for extended periods before pH adjustment if forced degradation data show pH drift. The solution pH is adjusted with hydrochloric acid or sodium hydroxide to the registered formulation target, and a preservative load is selected according to Ph. Eur. 5.1.3 efficacy of antimicrobial preservation. During scale-up, foaming caused by air entrainment at the impeller can lead to visible bubbles and fill-weight variation; therefore the bulk solution is deaerated under vacuum after the mixing phase. The solution is clarified through a 20 µm filter before filling to remove undissolved excipient particles, then filled into amber Type III glass or polyethylene terephthalate bottles fitted with low-density polyethylene dropper tips. Fill volume is monitored by weight rather than level to maintain dose accuracy at volumes as low as 0.5 mL. Microbiological quality is confirmed by Ph. Eur. 2.6.12 and 2.6.13; the acceptance criterion for oral liquids is total aerobic microbial count not more than 10² CFU/g, total combined yeasts and moulds not more than 10¹ CFU/g, and absence of Escherichia coli in 1 g. Terminal finished product types are 5 mg/mL oral solutions in 15 mL or 30 mL packs, supplied with graduated oral syringes calibrated in 0.1 mL increments. The main incompatibility to avoid is incorporation of strong oxidising agents or uncontrolled metal-catalysed oxidation, which can reduce the thioamide content during shelf life.

Extemporaneously compounded methimazole oral suspensions are produced in veterinary compounding pharmacies when the approved oral solution or tablet dose cannot be adjusted precisely, particularly at doses below 2.5 mg per administration. The relevant compliance framework is USP <795> for nonsterile compounding, FDA CPG 608.400 for animal use, and state-level pharmacy board oversight. The formulation addition ratio is commonly expressed as 1 mg/mL, 2.5 mg/mL, 5 mg/mL, or 10 mg/mL; a 5 mg/mL suspension is produced by triturating 500 mg of API with a small portion of glycerin to form a particle-wetted paste, then incorporating a suspending vehicle containing microcrystalline cellulose/carboxymethylcellulose sodium, sorbitol, and xanthan gum in geometric portions to a final volume of 100 mL. The suspension is mixed on an electronic mortar and pestle or an overhead stirrer until visually homogeneous, then deaerated under vacuum to reduce air-binding in the dosing syringe. A common process failure is caking at the bottle neck when the suspension is not shaken before dispense; the formulation is therefore labelled with a shake-well instruction and packaged with an oral syringe rather than a simple pour adapter. Packaging is into amber polyethylene terephthalate bottles having child-resistant closures; beyond-use dating is assigned according to USP <795>. Because water-containing oral liquids carry a default beyond-use date of 14 days refrigerated unless product-specific stability data support a longer interval, many pharmacies rely on this conservative default rather than generating site-specific stability studies. Terminal finished product types are compounded oral suspensions in 10 mL to 100 mL amber bottles, supplied with single-use or reusable oral dosing syringes calibrated in 0.1 mL increments. The main incompatibility to avoid is combination with strong oxidising agents in the formulation, because the thioamide group of methimazole is susceptible to oxidation; such combinations are generally excluded during formula development.

When Transdermal Gel Replaces Oral Administration in Feline Patients

Administration of methimazole as a compounded pluronic lecithin organogel is limited to feline patients in which oral dosing is not feasible due to vomiting, dysphagia, or owner inability to administer tablets; it is not an approved commercial veterinary product in most regulatory jurisdictions. The API is commonly incorporated at 5 mg/0.1 mL (5% w/v) or 10 mg/0.1 mL (10% w/v) into a base prepared from a 20% w/w poloxamer 407 aqueous phase and a lecithin/isopropyl palmitate phase, mixed in a ratio that yields thermoreversible gelation near skin temperature. In compounding practice, methimazole is first dissolved in a small volume of ethoxy diglycol or propylene glycol; the resulting solution is levigated into the organogel base using a three-roll ointment mill or a vacuum planetary mixer to remove air pockets and prevent dose-weight variation in the dispensing syringe. The gel is filled into 1 mL or 3 mL dosing syringes or airless pumps, and the dose is applied to the inner pinna. Compliance for this extemporaneous semisolid follows USP <795> and FDA CPG 608.400, with a category default beyond-use date for water-containing semisolids generally limited to 30 days unless site-specific stability data support extension; published beyond-use data specific to methimazole PLO gels remain limited. Terminal finished product types are compounded transdermal gels in 1 mL to 3 mL syringes, usually labelled with strength per 0.1 mL to avoid confusion with oral milligram doses. The operational boundary is strict: transdermal absorption is not bioequivalent to oral administration on a mg-for-mg basis, so dose adjustment must be confirmed by serum total thyroxine monitoring rather than by direct substitution of the oral dose.

In feline patients that require solid-dose titration without discontinuation of oral dosing, compounded methimazole capsules are prepared when a clinician requires strengths such as 1.25 mg, 2.5 mg, 5 mg, or unusual intermediate doses that are not commercially available as a film-coated tablet or oral solution. The API is handled as a powder and diluted by geometric dilution with a dry carrier such as lactose monohydrate or microcrystalline cellulose; a typical first-pass trituration combines 1 part API with 9 parts carrier to form a 10% w/w premix, and further dilutions are prepared in 1:10 or 1:100 steps until the target capsule strength is reached. The blend is passed through a 300–500 µm stainless steel screen between dilution stages to break aggregates, then loaded into a semiautomatic capsule-filling machine using size 3 or 4 hard gelatin or hydroxypropylmethylcellulose capsules. Weight uniformity is checked during filling by repeated capsule weight sampling; a documented in-process weight variation limit, often not more than 5% relative standard deviation, is used to justify content uniformity confidence. The finished capsules are packaged in amber vials with desiccant and assigned a beyond-use date according to USP <795>, generally a conservative limit for dry non-aqueous powder-filled capsules of 180 days unless stability data support otherwise. The relevant compliance standards are USP <795>, FDA CPG 608.400, and state board of pharmacy rules; because this is a compounded preparation rather than an approved drug product, cGMP requirements under 21 CFR Part 211 do not apply directly, but the pharmacy must meet the compounding standard’s operational and documentation requirements. Terminal finished product types are hard capsules in strengths from 1.25 mg to 10 mg, packaged with desiccant in amber vials or unit-dose blisters for feline outpatient use.