Allopurinol, 1H-pyrazolo[3,4-d]pyrimidin-4-ol, CAS 315-30-0, is supplied as a pharma-grade active pharmaceutical ingredient for the manufacture of tablets, capsules, granules, and injectable preparations under oral and parenteral administration. The product line comprises three model designations: ALP-101 for standard-milled allopurinol, ALP-102-M for micronized allopurinol, and ALP-103-I for sterile injectable-grade allopurinol sodium. Other active pharmaceutical ingredients in the same portfolio are released under the same quality-system controls for oral solid-dose and injectable applications and are available for fixed-dose combination development. The free acid is practically insoluble in water; the sodium salt is freely soluble, creating a formulation-specific distinction between oral and injectable routes. All grades are tested against USP-NF, Ph.Eur., BP, and JP monographs where applicable, with additional customer-specific particle-size, residual-solvent, and elemental-impurity profiles.

The term pharma grade refers to compliance with current good manufacturing practice for active substances under ICH Q7, with batch record review, change control, and stability commitments. This distinguishes the material from laboratory reagents and from technical-grade allopurinol sold for non-human applications. Each shipment is accompanied by a certificate of analysis containing assay, related substances, residual solvents, elemental impurities, particle-size data where applicable, and retest date.

What Pharmacopoeial Limits Apply to Assay, Related Substances, and Residual Solvents?

The release specification for ALP-101 and ALP-102-M follows the current monograph for allopurinol in USP-NF, Ph.Eur., BP, and JP. Assay calculated on the dried basis is controlled within 98.0–101.0%. Loss on drying is limited to not more than 0.5%, residue on ignition to not more than 0.1%, and sulfated ash to not more than 0.1% when the Ph.Eur. procedure is used. Where the monograph retains the heavy metals limit, the result is not more than 0.002%; otherwise elemental impurities are controlled according to ICH Q3D and measured by USP <232>/<233> or Ph.Eur. 2.4.20. Residual solvents are determined by headspace gas chromatography according to USP <467> or Ph.Eur. 2.4.24, with Class 2 solvents managed under ICH Q3C Option 2. Related substances are determined by HPLC with UV detection: pharmacopoeial impurity A is not more than 0.1%, any unspecified impurity is not more than 0.10%, total impurities are not more than 0.5%, and the disregard limit is 0.05%. Identification is confirmed by infrared absorption spectrophotometry against the USP reference standard; the melting point of allopurinol is typically above 350°C with decomposition, so differential scanning calorimetry is used as an identity tool in some laboratories. For ALP-103-I, the sodium salt is tested as allopurinol content on the anhydrous basis, with bacterial endotoxins and sterility assigned according to the finished injectable product because allopurinol sodium is not a sterile finished dosage form until converted and filled.

Micronized, Sterile Injectable, and Direct Compression Grade Differentiation

ALP-101 is a standard-milled powder intended for wet granulation, capsule filling, and direct compression when the formulated product does not require particle-size reduction to meet dissolution acceptance. ALP-102-M is a jet-milled grade with D90 not more than 20 µm and D50 between 5 µm and 10 µm for oral solid formulations in which disintegration and dissolution are rate-limiting. ALP-103-I is allopurinol sodium, sterile injectable-grade API, supplied with reduced bioburden and data for bacterial endotoxins and particulate matter according to Ph.Eur. 2.6.14, USP <85>, Ph.Eur. 2.9.19, and USP <788> as applicable.

In direct compression of ALP-101 on a production-scale rotary tablet press, segregation of fines can occur when the poured bulk density varies by more than 0.05 g/mL between batches. On a bin blender with 600 L capacity, filling to 60–70% of working volume provides adequate particle mobility for ALP-101; micronized ALP-102-M may require lower fill and longer blend time to achieve uniformity because of increased cohesive forces. For ALP-102-M, high-shear wet granulation at 70–80% bowl fill volume may generate a cohesive wall layer unless colloidal silicon dioxide at 0.5–1.0% w/w is pre-blended before binder addition. Magnesium stearate levels above 2.0% w/w are not recommended for dissolution-sensitive formulations because hydrophobic film formation on granule surfaces retards drug release. Published data for allopurinol micronized wet granulation on production-scale twin-screw equipment is limited; process qualification should therefore include at least three batches and full finished-product release testing.

For granule dosage forms, the API is dry-mixed with lactose monohydrate or mannitol and granulated with purified water or a polymeric binder solution. Wet mass end point is determined by impeller torque or power consumption; overgranulation increases bulk density and can extend disintegration time beyond the pharmacopoeial limit. Fluid-bed drying inlet air is typically set at 50–60°C, with product temperature not exceeding 45°C, and the dried granules are milled through a 1.0–1.5 mm screen before final blending. After drying, moisture content is monitored by Karl Fischer titration or loss on drying; a moisture level above 0.5% can cause sticking during tablet compression.

When Xanthine Oxidase Inhibition Requires a Change From Oral Tablets to Injectable Compounding

Oral allopurinol is formulated as tablets and capsules for chronic hyperuricaemia and gout. Common tablet strengths are 100 mg and 300 mg, with maintenance dosing in adults usually 100–300 mg/day. The dose is adjusted when creatinine clearance falls below 30 mL/min. Allopurinol has oral bioavailability of approximately 80–90%; the active metabolite oxipurinol has a terminal plasma half-life of approximately 18–30 h in adults with normal renal function. Tablet and capsule production uses wet granulation or direct compression, with in-process controls for blend uniformity, hardness, friability, and dissolution according to the relevant pharmacopoeial apparatus.

Parenteral allopurinol is used when oral administration is not feasible, particularly to prevent hyperuricaemia induced by cytotoxic chemotherapy. The injectable form is based on allopurinol sodium because the free acid has negligible aqueous solubility. Commercially available allopurinol sodium for injection is presented as 500 mg allopurinol equivalent per vial; reconstitution with 25 mL Sterile Water for Injection yields 20 mg/mL. Further dilution in 0.9% sodium chloride injection or 5% dextrose injection should be used promptly because published stability data for extended storage of diluted admixtures is limited. Allopurinol sodium injection should not be mixed with certain acidic diluents if the pH falls below the solubility boundary, because precipitation of allopurinol may occur. The solution should be inspected visually after reconstitution; any visible particles indicate incompatibility or incomplete reconstitution. ALP-103-I is supplied as sterile API for compounding or terminal sterilisation and is not a ready-to-use injection.

Compared with other active pharmaceutical ingredients of the same therapeutic class, allopurinol is a purine analogue and mechanism-based inhibitor of xanthine oxidase. Febuxostat is a non-purine thiazole carboxylic acid selective inhibitor, with hepatic metabolism rather than predominantly renal clearance; topiroxostat is a hybrid-type inhibitor approved in Japan with a different urate-lowering potency and safety profile. This structural difference affects drug-interaction boundaries: allopurinol inhibits xanthine oxidase and increases azathioprine and mercaptopurine exposure, an interaction that is not shared to the same degree by all urate-lowering APIs. Technical-grade allopurinol is not interchangeable with pharma-grade API because it may fail residual solvent, elemental impurity, related-substance, and particle-size controls.

Comparing Allopurinol API With Non-Pharmacopoeial and Alternative Xanthine Oxidase Inhibitors

The pharma-grade models ALP-101, ALP-102-M, and ALP-103-I differ from non-pharmacopoeial allopurinol in three main respects: documented monograph compliance, controlled particle-size distribution, and injectable-grade quality attributes. Non-pharmacopoeial material is typically sold without a full certificate of analysis matching USP-NF or Ph.Eur., and its particle-size distribution may vary in a way that changes blend uniformity and dissolution in finished solid oral dosage forms. For injectable compounding, technical-grade material is unsuitable because endotoxin, bioburden, particulate matter, and elemental impurity data are absent. Other active pharmaceutical ingredients in the portfolio are supplied with the same model logic: a standard oral grade, a micronized or controlled-PSD grade, and a sterile injectable grade where the molecule and its salt form permit. Their monographs, residual-solvent classes, and particle-size targets differ; therefore the allopurinol specifications provided here do not apply to other portfolio APIs.

Febuxostat is not a purine analogue and does not cause the same degree of azathioprine interaction, but its hepatic metabolism introduces different cytochrome P450-related considerations. Allopurinol is contraindicated in patients with severe renal impairment unless dose is adjusted; febuxostat has alternative renal precautions. These therapeutic differences are formulation-independent but influence the choice of API in product development.

Storage of ALP-101 and ALP-102-M should be in tightly closed containers below 25°C and protected from moisture, consistent with ICH Q1A(R2) long-term conditions. If moisture content approaches 0.5% before dry granulation, pre-drying is required; open storage at relative humidity above 60% should be avoided. ALP-103-I should be stored per label and is intended only for further pharmaceutical processing under validated aseptic or terminal sterilisation conditions.