In oral solid-dose development for a highly water-soluble, hygroscopic antibiotic salt, the dissolution-rate limitation is replaced by a flowability and moisture-control limitation before the tablet press is started. Fosfomycin sodium at an addition level of 65% w/w in a direct-compression blend is not the rate-limiting component with respect to dissolution but is the primary cause of sticking, capping, and weight variation if ambient moisture exceeds 30% RH. A representative immediate-release tablet formula contains 65% w/w fosfomycin sodium, 28% w/w microcrystalline cellulose, 4% w/w crospovidone, and 1% w/w sodium stearyl fumarate; magnesium stearate is usually restricted to 0.5% w/w because the sodium salt exhibits high ejection force and extended lubricant blending suppresses tablet hardness. The blend is sieved through a 0.5 mm oscillating screen, blended in a V-shaped blender for 10 min at 15 rpm, and compressed on a 16-station rotary tablet press with punch face roughness below 0.1 µm Ra. Compression force is maintained between 8 kN and 18 kN; above 18 kN, elastic recovery after ejection produces capping at the edge of 250 mg and 500 mg tablets. Environmental controls in the compression suite are set at 20–25°C and ≤30% RH, and pre-drying of microcrystalline cellulose at 60°C to a loss-on-drying below 1.0% is required when ambient relative humidity exceeds 60%. Finished tablet testing follows USP <905> for uniformity of dosage units, USP <701> for disintegration, and USP <711> for dissolution using purified water at 37°C; because the sodium salt is freely soluble, disintegration is typically complete in 5–15 min, and dissolution does not require a surfactant-based medium. Terminal products are immediate-release fosfomycin sodium tablets at 250 mg and 500 mg strengths for oral administration in regulated markets where this salt form is authorized.

How Does Roller Compaction Force Affect the Compression Window of Fosfomycin Sodium Capsule Granules?

When the same sodium salt is filled into hard capsules, direct powder filling is not feasible because the bulk density of the crystalline powder can fall below 0.35 g/cm³, producing cohesive bridging in the dosator bowl and fill-weight drift exceeding ±5% at speeds above 30 cycles/min. Dry granulation by roller compaction is therefore selected to increase bulk density and reduce the fine fraction. The intragranular charge comprises 55–70% w/w fosfomycin sodium, 25–35% w/w microcrystalline cellulose, and 2% w/w crospovidone; after compaction the milled granules are blended with 3% w/w extragranular crospovidone and 0.5% w/w magnesium stearate. Roller compactor settings are critical: hydraulic roll force is maintained at 5–12 kN/cm, roll gap at 2 mm, roll speed at 3–8 rpm, and the mill screen is 0.8 mm with rotor speed 50–80 rpm. Ribbon density between 1.1 g/cm³ and 1.3 g/cm³ is targeted; ribbons below 1.1 g/cm³ yield excessive fines, while ribbons above 1.3 g/cm³ produce hard granules that resist disintegration and lower capsule dissolution. On a production-scale roller compactor with side sealing and vacuum deaeration, batch-to-batch variance in the fraction below 45 µm is held below 20% to prevent segregation during transfer into the encapsulation hopper. Fill weights for 500 mg capsules are typically 750–850 mg depending on granule bulk density, and fill weight is monitored every 15 min against USP <905> uniformity requirements. Compliance testing includes USP <711> dissolution in 900 mL purified water at 37°C, USP <921> water content, and ICH Q3D elemental impurity screening; the capsule shell is selected as HPMC rather than bovine gelatin to avoid crosslinking from residual aldehydes in dry-granulation binders. Terminal products are powder-filled hard capsules in 250 mg and 500 mg strengths for oral dose flexibility.

Single-dose oral granules impose a different physical requirement from tablets because the complete sachet content is dispersed in water and swallowed, so the API must wet rapidly and the excipients must not foam excessively when the suspension is stirred. When the sodium salt is used instead of the trometamol salt, the corrected fill mass is determined by the salt factor because the sodium salt delivers more fosfomycin free acid per gram than the trometamol salt; each sachet is filled to deliver the intended fosfomycin base dose, commonly 1.5 g or 3.0 g base equivalent. A representative sachet formulation contains 35–50% w/w fosfomycin sodium, 40–55% w/w sucrose or mannitol, 3–8% w/w citric acid-sodium citrate buffer to maintain reconstituted pH near 7.2–7.8, 0.5–1.0% w/w colloidal silica as a flow aid, and 0.2–0.5% w/w saccharin sodium plus dry orange flavour. The blending sequence screens the API through a 0.5 mm sieve, mixes it with silica for 5 min in a 500 L bin blender, then adds the remaining excipients in two stages to avoid flavour segregation. The powder is packed on a vertical form-fill-seal stick-pack machine with a dewpoint below −20°C in the sealing area; the laminate is a PET/AL/PE triple-layer film with moisture vapor transmission below 0.1 g/m²/day because fosfomycin sodium sorbs moisture above 40% RH and becomes non-flowable. In-process control includes sachet fill weight every 10 min, seal integrity by dye-leak test, and final content uniformity according to USP <905> adapted for unit-dose powders. Compliance for elemental impurities follows ICH Q3D, and the package must meet USP <671> moisture vapor transmission requirements for solid oral unit-dose containers. Terminal products are single-dose soluble oral granules in stick packs, yielding a clear-to-slightly cloudy oral solution after 30 s of stirring at 20–25°C.

Aseptic Filling of 4 g Fosfomycin Sodium Vials Is Governed by Particulate and Endotoxin Limits Rather than Dissolution

Because injectable fosfomycin sodium is filled without excipients, the addition ratio at the filling stage is 100% w/w API, with fill weights adjusted to 2.0 g or 4.0 g per vial. The sterile powder is prepared by dissolution and aseptic crystallization from sterile-filtered aqueous solvent, followed by vacuum drying at 40°C until the water content determined by USP <921> Method I is below 0.5%. The dry powder is passed through a nitrogen-purged conical mill with a 0.5 mm screen to control particle size for reconstitution; a 2.0 g fill is required to dissolve in 20 mL water for injection at 20–25°C within 2 min without visible agglomerates. Powder filling is performed on an auger-based aseptic filling line with ±2% fill accuracy, and vials are closed with bromobutyl rubber stoppers under first-air unidirectional flow. The critical compendial controls are USP <71> sterility, USP <85> bacterial endotoxins, USP <788> particulate matter in injections, USP <790> visible particulates, and USP <1> injection volume requirements; for small-volume parenterals, USP <788> light obscuration limits are 6,000 particles per container at ≥10 µm and 600 particles per container at ≥25 µm. Environmental monitoring follows EU GMP Annex 1:2022 for grade A zones, with continuous airborne particulate monitoring at ≥0.5 µm and settle plates exposed for the entire filling campaign. Batch release also requires 21 CFR 211.84 excipient testing and 21 CFR 211.110 in-process sampling. Terminal sterilization by dry heat is not feasible due to thermal degradation; terminal sterilization by gamma irradiation is also not applied to the sodium salt because of free-radical generation, so the process remains aseptic throughout. Terminal products are powder for injection vials of 2.0 g and 4.0 g fosfomycin sodium, reconstituted before intravenous infusion.

When Fosfomycin Sodium Injection Is Transferred into 100 mL PVC Infusion Bags, Which In-Use Hold Times Apply?

Hospital pharmacy compounding of a ready-to-administer intravenous fosfomycin sodium solution requires a controlled environment and a defined dilution that avoids both particulate formation and excessive hypertonicity at the catheter site. The reconstituted vial is transferred through a 0.22 µm filter into a polyvinyl chloride bag containing 100 mL of 0.9% sodium chloride injection or 5% dextrose injection; a 2.0 g vial diluted to 100 mL produces 20 mg/mL, and a 4.0 g vial diluted to 250 mL produces 16 mg/mL. Concentrations above 20 mg/mL are not recommended outside a validated infusion center because published data for higher concentrations in PVC containers are limited. Compounding is performed in an ISO 14644-1:2015 Class 5 laminar airflow workstation within a controlled non-hazardous drug area, following USP <797> beyond-use-date rules. The admixture is administered through a 0.22 µm inline filter to trap any particulate introduced during spiking, and the infusion pump rate is set according to the prescribed dose. In-use stability at 20–25°C is generally acceptable for up to 24 h when the bag is protected from light, but prolonged storage at 2–8°C beyond 24 h is not supported by sufficient published stability data for all container systems. The terminal product is a ready-to-administer IV bag containing fosfomycin sodium in 0.9% sodium chloride or 5% dextrose at 16–20 mg/mL. No mixing with other anti-infectives is performed in the same bag unless a documented compatibility study specific to the container and temperature is available; otherwise the infusion line is flushed with 0.9% sodium chloride before and after administration.