Sulphadoxine Pharma Grade API is a long-acting sulfonamide supplied for the manufacture of tablet, capsule, granule, and injection dosage forms by both oral and injectable routes. The substance is identified by CAS 2447-57-6, molecular formula C12H14N4O4S, and relative molecular mass 310.33 g/mol. Pharmacopoeial material is controlled against the current Ph. Eur. Sulfadoxine monograph and the applicable USP monograph, with release testing performed under ICH Q7. The API is a white or almost white crystalline powder; aqueous solubility is pH-dependent and increases in dilute alkaline solution, while neutral aqueous solubility is low. Sulphadoxine inhibits dihydropteroate synthase in the folate biosynthetic pathway. In fixed-dose combination with pyrimethamine, which inhibits dihydrofolate reductase, the two active substances act sequentially on folate biosynthesis. Oral presentations are commonly 500 mg sulphadoxine with 25 mg pyrimethamine; injectable presentations are sterile solutions for deep intramuscular use. Plasma protein binding is approximately 90% to 95%, and the reported terminal elimination half-life is 100 h to 230 h in healthy adults, supporting once-weekly dosing in antimalarial regimens.

For solid oral dosage forms, the crystallised API is not typically ready for direct compression. Bulk density and tapped density are determined by USP <616>; the derived Carr compressibility index is used as a flow indicator. Compressibility indices above 25% are generally interpreted as poor flow and require wet granulation or roll compaction. A high-dose 500 mg tablet is therefore manufactured by high-shear wet granulation or fluid-bed granulation rather than direct compression. The granulation is dried to a loss on drying endpoint of 1.0% to 2.0% w/w, then milled through a screen with apertures from 0.5 mm to 1.25 mm. The milled granulate is blended with crospovidone, microcrystalline cellulose, and magnesium stearate before compression on a rotary press. For capsule filling, dosator and tamping-pin machines require granules in the 100 µm to 800 µm range; fines below 50 µm increase dusting, flow variability, and fill weight variation. Roller compaction may be used where solvent-free granulation is required; ribbon solid fraction is maintained between 0.60 and 0.80 to balance granule strength and tablet compressibility, although published data for this specific configuration is limited.

Polymorphic form and crystallinity are controlled where relevant because changes in crystal form can alter dissolution and processability. X-ray powder diffraction is performed according to USP <941> or Ph. Eur. 2.9.33. Unprocessed sulfonamide crystals may form agglomerates that do not flow; particle size reduction by jet milling can be used to obtain D90 values below 75 µm for dissolution-limited formulations. However, micronised API may increase static charge and require more intensive mixing. The particle size specification is consequently agreed between the API manufacturer and the finished-dose manufacturer; no universal D90 acceptance criterion is stated in the pharmacopoeial monograph.

What limits direct compression of sulphadoxine API in tablet manufacture?

Direct compression is limited primarily by powder flow, bulk density, and segregation risk. Cohesive, high-fines API from crystallisation can produce compressibility indices above 25% and high tablet weight variability on high-speed rotary presses. Sulphadoxine has low neutral aqueous solubility, but dry blending problems occur before dissolution limitations. At API mass fractions above 50% w/w, extragranular flow aids such as spray-dried lactose or dibasic calcium phosphate dihydrate may be required in a direct compression formulation. Content uniformity for a 500 mg tablet is sensitive to segregation in the feed frame; fine API particles can move through the granulation and produce assay variation. Production-scale correction normally uses wet granulation or roll compaction. The compression force is set to achieve tablet hardness of 80 N to 120 N and friability below 1.0% according to USP <1216>. Tablet disintegration time is controlled to 15 min or less in water at 37°C according to USP <701>, although the finished product specification should be consulted.

At production scale, high-shear granulation endpoint is frequently monitored by impeller torque or power consumption. The endpoint is reached when the torque curve plateaus after binder solution addition. Overgranulation produces dense granules that resist disintegration; undergranulation produces weak granules and segregation. Dried granules are milled through a conical mill with a round-hole screen of 0.8 mm to 1.0 mm at impeller speed 1000 rpm to 2000 rpm; oversized material is recycled. The final blend is lubricated with 0.5% w/w magnesium stearate for 3 min to 5 min. Prolonged lubrication can reduce tablet hardness and increase dissolution time. Fluid-bed drying uses inlet air at 55°C to 65°C and product temperature below 50°C to avoid colour change.

For injectable presentations, the free acid form of sulphadoxine is not suitable for direct aqueous formulation because of low neutral pH solubility. Solubilisation is achieved by pH adjustment with sodium hydroxide or by forming the sodium salt, followed by aseptic filtration through 0.22 µm polyethersulfone or polyvinylidene fluoride membrane cartridges. The fill solution is protected from light and filled under nitrogen to limit oxidative discoloration. Terminal sterilisation by autoclaving at 121°C for 15 min may be employed if solution stability permits; combination solutions containing pyrimethamine may require aseptic filtration because of heat sensitivity. Bacterial endotoxin is controlled under USP <85> or Ph. Eur. 5.1.10, with the limit derived from the maximum dose and route of administration; no universal monograph value applies. Particulate matter for injectable grade is tested by light obscuration according to USP <788> or Ph. Eur. 2.9.19. Sterility is tested according to USP <71> or Ph. Eur. 2.6.1. The solution is also controlled for pH, color, and clarity; the release pH is fixed by stability and compatibility data, and is not a single compendial value across all manufacturers.

A pharmacopoeial release specification for sulphadoxine API is anchored to USP and ICH methods.

The release specification combines compound-specific limits from the Sulphadoxine monograph with general tests for residual solvents, elemental impurities, and microbiology. HPLC assay and related substances are performed using USP <621> or Ph. Eur. 2.2.29; assay on the dried basis is typically 99.0–101.0%. Loss on drying is controlled by USP <731> with a limit of ≤0.5%; residue on ignition is controlled by USP <281> with a limit of ≤0.1%. Residual solvent control follows ICH Q3C and USP <467>; methanol is limited to 3000 ppm and acetone to 5000 ppm where these solvents are used in the final crystallisation. For nonsterile oral-grade API, microbial enumeration limits follow USP <61> or Ph. Eur. 2.6.12, with total aerobic microbial count ≤100 CFU/g and total yeasts/moulds ≤10 CFU/g. Elemental impurities are controlled according to ICH Q3D; no single monograph value applies. The API is requalified after any change in final crystallisation solvent, drying profile, or micronisation step.

Dissolution testing for sulphadoxine-containing oral dosage forms is conducted using USP <711> apparatus 2 at 75 rpm in 900 mL of degassed aqueous buffer. Because sulphadoxine is weakly acidic, dissolution media at higher pH generally provide higher sink conditions than acidic media. The exact medium and acceptance criterion are set by the finished product monograph or regulatory filing; published data for this specific configuration is limited. A formulation with granule particle size D50 of 150 µm to 400 µm is usually sufficient for rapid tablet disintegration, while finer particles improve dissolution at the cost of flow. Dissolution failure can occur if granule drying above 60°C converts residual binder to a hydrophobic film or if magnesium stearate mixing time exceeds 5 min because of excessive hydrophobisation.

Storage for the API is in well-closed containers, protected from light. Long-term stability is generated under ICH Q1A at 25°C/60% RH and accelerated at 40°C/75% RH. Sulphadoxine should not be exposed to strong oxidising agents or to prolonged strongly acidic or strongly alkaline heating, because the sulfonamide linkage can undergo hydrolytic degradation. The API is re-tested for appearance, assay, related substances, and loss on drying at intervals defined by the stability protocol. Bulk handling should avoid temperatures above 60°C where drying is not required, to limit discoloration and crystallinity changes.

Compared with other sulfonamide APIs, sulphadoxine is distinguished by its prolonged elimination and once-weekly administration in combination with pyrimethamine. Sulfamethoxazole has an elimination half-life of approximately 10 h and is co-formulated with trimethoprim at a 5:1 ratio; sulphadoxine is co-formulated with pyrimethamine at a 20:1 ratio. The long residence time reduces dosing frequency but means that adverse reactions can persist after discontinuation because elimination continues over several weeks. Sulphadoxine is not interchangeable with sulfadiazine, sulfamethoxazole, or sulfalene in clinical protocols; substitution requires bioequivalence and therapeutic equivalence data. The manufacturing specification also differs because sulphadoxine is formulated as the free acid in oral solids and as an alkaline solution for injectable use, whereas more water-soluble sulfonamides may be converted to sodium salts for parenteral administration. The fixed combination with pyrimethamine is specific to antiprotozoal therapy and is not a first-line antibacterial combination.