Sulphamethoxazole Pharma Grade API for Tablet / Capsule / Granule / Injection, Oral & Injectable is the compendial active substance form supplied to pharmaceutical manufacturing sites for conversion into finished dosage forms. The material is identified by CAS 723-46-6, molecular formula C₁₀H₁₁N₃O₃S, and molecular weight 253.28 g/mol. The product designation itself serves as the grade identifier; no proprietary model number is applied to compendial sulfamethoxazole. It is a white or almost white crystalline powder with pH-dependent aqueous solubility and is synthesized as a sulfonamide antibacterial for oral and injectable use. The powder is non-sterile in standard commercial form, and the injectable route requires downstream salt formation, pH adjustment, and terminal sterilization or aseptic filtration. This distinction is critical because “injectable grade” refers to bioburden, endotoxin, particle, and impurity control, not to a sterile API claim.

In fixed-dose formulations, sulfamethoxazole is normally combined with trimethoprim at a 5:1 sulfamethoxazole-to-trimethoprim weight ratio. Common adult oral strengths contain 400 mg sulfamethoxazole with 80 mg trimethoprim in immediate-release tablets, or 800 mg sulfamethoxazole with 160 mg trimethoprim in double-strength tablets. Oral suspension is available at 200 mg/5 mL plus 40 mg/5 mL; injectable concentrate contains 80 mg/mL sulfamethoxazole and 16 mg/mL trimethoprim. These strengths establish the formulation envelope: the API has high dose, moderate aqueous solubility, and is typically present at 80–90% of tablet mass when combined with trimethoprim and common excipients, so raw material particle size and flow are operationally decisive.

Pharmacopoeial Release Attributes, Residual-Solvent Limits, and Elemental Impurity Control

Release is controlled against current Ph.Eur. and USP monographs for sulfamethoxazole. Identification is by infrared absorption and chromatographic retention, with the melting range falling at 169–172 °C. Assay is calculated on the dried basis and must lie between 99.0% and 101.0% of the molecular formula. Loss on drying is limited to ≤ 0.5%, and sulfated ash is controlled at ≤ 0.1%. Related substances are resolved by liquid chromatography; known impurities include the sulfanilamide-related starting-material impurity and N⁴-acetyl sulfamethoxazole, which is also the major human metabolite and a potential degradation marker. Individual and total impurity limits are set by the monograph and are stability-indicating. Residual solvent control follows ICH Q3C; methanol and dichloromethane, when used, carry limits of ≤ 3000 ppm and ≤ 600 ppm under the common options. Elemental impurity control is established under ICH Q3D and differs by route of administration; parenteral products impose lower permitted daily exposures for elements such as arsenic, cadmium, lead, and mercury. The API manufacturer must provide a risk assessment for elemental impurities unless the route-specific finished-product validation demonstrates no significant contribution.

Pharmacopoeial monographs do not define a universal particle-size window, because the required size distribution is dependent on the dosage form and manufacturing train. For direct compression of 400 mg-strength tablets, a controlled fraction with D90 below 100 µm and D10 above 5 µm is commonly handled to limit segregation and excessive dusting; for oral suspension granules, the same D90 may reduce grittiness but does not by itself guarantee content uniformity in the dry blend. The D-values are measured by laser diffraction according to ISO 13320 or by sieving, and the release limits are specified in the drug master file because no single distribution is compendial. A change in particle size can alter blend uniformity, powder flow, and dissolution without changing the chemical assay; therefore, it is a critical material attribute under ICH Q8.

What Separates Injectable-Grade Sulphamethoxazole from Solid Oral Powder?

The primary difference is not chemical identity but the control of microbial and particulate load. Injectable-grade sulfamethoxazole is released with a low bioburden and a validated bacterial endotoxin limit, tested by the limulus amebocyte lysate method according to Ph.Eur. 2.6.14 or USP <85>. The acceptance limit is derived from the maximum intended parenteral dose under Ph.Eur. 5.1.10 or harmonized guidance and is not automatically identical across suppliers. Because the free acid is practically insoluble in water, parenteral manufacture uses sodium salt formation or alkaline pH adjustment. The molecule is amphoteric with reported pK_a values near 1.6 and 5.7; solubility increases at acidic and alkaline extremes but is lowest near neutral pH. The injectable solution is therefore maintained at a pH that ionizes the sulfonamide group, and the finished product can be diluted into 0.9% sodium chloride or 5% dextrose infusion fluids. Precipitation may occur if the solution is mixed with acidic drug solutions or if the pH is allowed to fall below the designed range.

Terminal sterilization of the dry sulfamethoxazole powder is not a routine option; heat or irradiation can produce degradation products and color change. Instead, the finished injectable solution is filtered through 0.22 µm sterilizing-grade membrane filters or terminally sterilized after filling, provided that thermal stability data support the selected cycle. Injectable-grade API therefore requires a stability-indicating method capable of separating sulfamethoxazole from its sodium salt-related impurities and from oxidation products generated during alkaline processing. Packaging also must protect the solution from light and oxygen; amber glass or oxygen-barrier containers are used. The API powder should be stored in well-closed, light-resistant containers at controlled room temperature unless the validated retest period supports other conditions.

For solid oral products, bacterial endotoxin limits are not usually applied, but total aerobic microbial count and total combined yeasts and molds are controlled according to the finished-product monograph and local pharmacopoeial requirements. For injectable powder, the total aerobic microbial count is commonly specified at not more than 10² CFU/g and endotoxins at a limit derived from the maximum dose; however, exact values are product-specific and may be lower. The API manufacturer must validate the bioburden test method for the powder because sulfamethoxazole can exhibit antimicrobial activity that inhibits microbial recovery in direct plate or membrane filtration tests.

When Wet Granulation, Direct Compression, and Capsule Filling Operate Near the Particle-Size Threshold

The high drug load of co-trimoxazole tablets makes wet granulation the most common processing route. In a high-shear granulator the dry blend containing sulfamethoxazole, trimethoprim, binders, and disintegrant is granulated with purified water or a binder solution. The endpoint is determined by impeller power draw, torque, or amperage on the granulator motor; overdrying creates friable granules, while residual moisture above 2–3% can cause picking and sticking during compression. Drying is executed in a fluid-bed dryer with inlet air temperature typically between 50 °C and 60 °C, depending on the dryer load and air flow, to keep the product below the softening or degradation threshold. The dried granulate is milled through a 1.0–1.5 mm screen and blended with lubricant; magnesium stearate is used at low concentration and for limited mixing time because hydrophobic lubricant films can delay tablet disintegration and dissolution in a poorly soluble API system.

Direct compression is possible only when the API powder has adequate compactability and flow. Sulphamethoxazole as a crystalline material can exhibit poor flow and high electrostatic charging; slugging or dry granulation is often inserted when direct compression cannot meet dose uniformity. Capsule filling on tamping-pin or dosing-disk machines is sensitive to bulk density. If the sulfamethoxazole fraction has a low bulk density or high fines content, the fill weight varies beyond 5% relative standard deviation, and flow aids such as colloidal silicon dioxide or pregelatinized starch must be optimized. Granules for oral suspension add a further constraint: they must wet, disperse, and remain suspendable. The suspending medium requires viscosity control, and the API particle size must be small enough to avoid gritty mouthfeel; however, micronization to below 10 µm can increase surface area, static charge, and possible degradation. The chosen particle-size range is therefore a compromise among content uniformity, dissolution, suspension appearance, and chemical stability.

For injectable processing, the solid-state particle-size issues are mostly replaced by dissolution, pH, and filterability constraints. The API is dissolved as sulfamethoxazole sodium or by adding the free acid to an alkaline solution of sodium hydroxide or trometamol. The batch is mixed until fully dissolved, and the solution is sampled for clarity, pH, assay, related substances, color, and bioburden before filtration. A 0.22 µm membrane filter removes bioburden but not endotoxin; the API must already meet the endotoxin specification. Holding times for the alkaline solution are critical because pH-induced degradation can increase; the maximum holding time and temperature must be established during process validation. The solution is filled into ampoules or vials under nitrogen or another inert atmosphere when oxidative discoloration is observed, and the finished product is terminally sterilized or aseptically processed according to the formulation’s thermal sensitivity.

Stability studies for solid oral forms indicate that sulfamethoxazole is relatively stable under dry, light-protected conditions, but the combination with trimethoprim can exhibit moisture-sensitive discoloration if excipients or packaging allow water uptake. Aqueous granulation introduces temporary moisture stress; the drying endpoint is therefore monitored by loss-on-drying rather than by time alone. Oxidative degradation can occur if the wet granulate is held for extended periods at elevated temperature before drying. The API itself may develop a yellow or pink discoloration upon exposure to light and certain metals; stainless-steel contact surfaces are preferred, and copper or iron contamination should be avoided because trace metals can accelerate oxidative color formation.

Comparative Attributes Against Other Sulfonamide Active Ingredients

Sulphamethoxazole is distinguished from other sulfonamide actives by the methyl-isoxazole ring at the sulfonamide nitrogen. This substituent slows metabolism and renal excretion relative to short-acting sulfisoxazole; the elimination half-life of sulfamethoxazole is approximately 10 h, enabling twice-daily administration. Sulfadiazine differs in solubility and acetylation; silver sulfadiazine and sulfadiazine-trimethoprim are separate products with different dose ratios and clinical uses. Sulfamethoxazole is the sulfonamide partner in co-trimoxazole because its 10 h half-life is sufficiently matched to trimethoprim’s 8–11 h half-life to maintain the synergistic ratio in plasma; the two actives inhibit sequential steps in microbial folate biosynthesis—sulfamethoxazole at dihydropteroate synthase and trimethoprim at dihydrofolate reductase. The consequence at the API level is that sulfamethoxazole cannot be replaced with sulfadiazine on an equal-mass basis without redeveloping the formulation because solubility, dose, absorption, and protein binding differ.

Compared with sulfasalazine, sulfamethoxazole is not a prodrug requiring azoreduction in the colon; sulfasalazine delivers mesalamine and sulfapyridine, whereas sulfamethoxazole is itself the antibacterial. Compared with sulfamethizole, sulfamethoxazole has a longer dosage interval and different renal handling. In raw material terms, these differences appear as divergent impurity profiles, residual solvent origins, crystallinity, and particle morphology. A manufacturer purchasing “sulfonamide API” without compound-specific controls risks selecting a material whose compactability, pH-solubility curve, and compendial limits are not interchangeable. The sulfamethoxazole monograph and the finished-product monographs for co-trimoxazole should be treated together with the approved drug master file when qualifying a new source.

Interchangeability between two Sulphamethoxazole Pharma Grade API sources is established by comparative testing, not inferred from compendial compliance alone. A supplier change can preserve the assay and impurity limits while altering the crystal habit, surface area, bulk density, residual solvent profile, or bioburden. For tablets and capsules, the comparative evaluation should include particle-size distribution, flow, compactability, and dissolution of a pilot batch. For granules and oral suspension, sedimentation volume and redispersibility are added. For injectable solutions, the evaluation includes clarity, color, pH stability, and endotoxin recovery; the API supplier must provide validation data for the limulus test showing the absence of assay interference. Processing conditions such as granulation water amount, dryer inlet temperature, or alkaline holding time may require adjustment even when the new material meets the same pharmacopoeial specification. These parameters are managed under ICH Q11 and ICH Q9 and are documented in the pharmaceutical quality system of the manufacturing site.