Gentamicin sulphate Ph. Eur. 0331 is supplied as a white to off-white hygroscopic powder with high aqueous solubility and low solubility in organic solvents. The compendial monograph controls identity, specific optical rotation, sulfate content, loss on drying at ≤1.0% w/w, and potency expressed in IU/mg or µg/mg of gentamicin base. Particle size distribution is not harmonised across regions; tablet and capsule operations therefore set internal specifications based on the selected solid-dosage route. Laser diffraction per USP <429> is used for d10, d50 and d90 control, while sieve analysis per USP <786> is used for granule fractions. A micronised API batch with d50 in the range 6–15 µm is typically reserved for low-dose direct compression and capsule blends, whereas a coarser grade with d50 between 30 µm and 50 µm is used where flowability dominates. Handling is performed at room temperature below 40% RH; unbound moisture is checked by USP <921> Karl Fischer titration when the product is exposed to humid conditions.

For oral solid dosage forms containing gentamicin sulphate, the API concentration on a dry basis may range from <1% to 15% w/w depending on the finished label. At the lower end of this range, content uniformity risk increases because the micronised API particles adhere to blender surfaces and form agglomerates with excipients of much larger particle size. The remedy is a staged pre-blend with a fine diluent before final blending; this is confirmed by sampling at 10 or more locations and testing by USP <905> with an acceptance value not greater than 15.

Direct Compression Tablet Blending and the Role of Particle Size Control

Compression of a direct compression blend containing micronised gentamicin sulphate on a D-tooling rotary press at 8–12 kN produces tablets with hardness 6–8 kp and friability below 1.0% per USP <1216>. The preferred diluents are microcrystalline cellulose with d50 of 90–120 µm and spray-dried lactose with d50 of 100–150 µm. The API is first passed through a 500 µm screen with microcrystalline cellulose in a 1:3 ratio to break down cohesive agglomerates. The main blend is then discharged into a 500 L bin blender and mixed at 12 rpm for 15–20 minutes. When the drug load is below 5 mg per tablet, the final blend is sampled in 10 stratified locations; content uniformity acceptance values above 15 per USP <905> are addressed by increasing the pre-blend ratio or reducing the API particle size. Powder flow through the tablet press feed frame is monitored by online weight control; fill depth adjustments are made when weight variation exceeds ±2%.

Lubrication with magnesium stearate at 0.5% w/w is added for the last 3–5 minutes of blending. An addition level above 1.0% w/w lowers tablet tensile strength because the hydrophobic lubricant coats the compressible excipient surfaces; the effect is measurable as a hardness drop at constant compression force. Croscarmellose sodium at 2–4% w/w is used as the superdisintegrant. Disintegration is measured by USP <701> and is generally less than 15 minutes when the compression force is below 12 kN. Dissolution of immediate-release tablets is assessed using USP <711> Apparatus II at 50 rpm in 900 mL purified water; no universal acceptance criterion exists for gentamicin sulfate tablets, so the specification is derived from batch-specific stability and bioavailability data.

What Controls Capsule Fill Weight Variability for Micronized Gentamicin Sulfate?

Filling of a low-dose gentamicin sulfate capsule on a Bosch GKF 2600 at 45,000–60,000 capsules/hour demands a powder bed with a stable density because the dosator picks up a fixed volume of material. A blend with bulk density between 0.45 g/mL and 0.65 g/mL and a d50 between 80 µm and 150 µm produces fill weight relative standard deviation below 3.5% for capsules below 100 mg total fill weight. Fine API with d50 below 10 µm creates electrostatic adhesion and flow interruption; this is mitigated by granulating the API with a water-soluble diluent or by using a low-humidity environment below 30% RH at the filling station.

Capsule size 3 or 4 is typical when the total fill weight does not exceed 180 mg. The diluent should be selected to reduce the particle size mismatch; mannitol and lactose monohydrate with d50 above 80 µm are preferred. The filled capsules are tested for weight variation, content uniformity per USP <905>, and disintegration per USP <701>. Moisture-induced shell brittleness is controlled by keeping the relative humidity in the capsule storage and filling areas below 45% RH; empty gelatin shells may require storage at 35–45% RH to maintain shell integrity before filling.

High-shear wet granulation is introduced when direct compression cannot maintain tablet weight control due to poor flow of the final blend or when the product requires higher API loading and the coarse API fraction is not compressible. The gentamicin sulphate is dry-mixed with a portion of the diluent in a vertical high-shear granulator at impeller speed 150–250 rpm for 3–5 minutes. A binder solution of povidone K30 at 3–5% w/w solids in purified water is added while the impeller is running; the chopper is set at 1,000–1,500 rpm to break large agglomerates. The granulation endpoint is determined by impeller torque increase of 20–40% above the dry-mix baseline. Overwetting produces a smooth paste that adheres to the vessel wall and later yields hard granules with slow dissolution; under-wetting produces a dry mix with visible free powder and poor granule density.

The wet mass is dried in a fluid-bed dryer with inlet air at 50–60°C and product temperature not exceeding 40°C. Final granule loss on drying is set at 1.5–2.5% w/w. Milling is performed with a conical mill fitted with a 1.0 mm screen at 1,500 rpm; the target granule d50 is 180–300 µm by sieve analysis per USP <786>. Extragranular croscarmellose sodium at 2.0% w/w and magnesium stearate at 0.5% w/w are added before compression. The compression force on a 16-station press is 6–10 kN; tablet hardness is 5–7 kp and friability is below 0.8%. When the granule itself is the finished oral dosage form, the dried granules are classified to a target fraction of 150–500 µm and filled into sachets or bottles; loss on drying is then tightened to ≤2.0% w/w.

When Roller Compaction Replaces Wet Granulation for Moisture-Sensitive Aminoglycoside Blends

When aqueous wet granulation of gentamicin sulfate produces sticky granule mass on the impeller blades or when the formulation contains moisture-sensitive coprocessed excipients, dry granulation by roller compaction is evaluated. The powder blend is pre-lubricated with 0.25% w/w magnesium stearate and compacted on an Alexanderwerk WP120 at roll force 8–12 kN/cm, roll speed 3–8 rpm, and gap 2.0–3.0 mm. Ribbon density is measured and controlled to 1.2–1.4 g/cm³. Ribbons below 1.1 g/cm³ are brittle and produce excessive fines after milling; ribbons above 1.5 g/cm³ can cause the final tablets to show slower disintegration and lower dissolution due to reduced porosity.

Granulation after compaction uses a conical mill with a 0.8 mm screen at 1,200 rpm, producing granules with d50 from 180 µm to 250 µm. The granules are blended with extragranular croscarmellose sodium at 2.0% w/w and a final 0.5% w/w lubricant. Because compaction reduces compressibility, the tablet press is operated at 10–14 kN; tablet hardness is 6–9 kp, friability is ≤1.0%, and disintegration is below 15 minutes. The main process boundary is the roll force; above 14 kN/cm, dissolution may slow, and below 6 kN/cm, the ribbon density may be too low to form coherent granules.

Sterile Filtration and pH Control in Aqueous Gentamicin Sulfate Parenterals

Aqueous gentamicin sulfate injection is prepared in a jacketed stainless-steel compounding vessel with water for injection at 20–25°C. The pH is adjusted to 3.5–5.0 with dilute sulfuric acid or sodium hydroxide; pH is measured by USP <791>. The pH window is set to minimise hydrolytic degradation of the aminoglycoside sulfate and to maintain solubility of the API in the final container. Sodium chloride is added to adjust osmolality to 285–295 mOsm/kg. The bulk solution is blanketed with nitrogen where required and transferred through a 0.45 µm prefilter into the filling line. pH adjustment with sulfuric acid can generate local heat; addition is performed slowly under mixing to avoid precipitates.

Sterilising-grade filtration is performed with two 0.22 µm polyvinylidene fluoride or polyethersulfone membranes in series. Filter integrity is tested by bubble point or diffusive flow before and after filtration; a failed post-use integrity test invalidates the lot. The bioburden before the final filter is controlled below 10 CFU/100 mL in accordance with EU GMP Annex 1. The solution is filled in an ISO 5 Grade A unidirectional airflow zone at 0.45 m/s ± 20% into Type I borosilicate glass vials with chlorobutyl rubber stoppers. Vials are depyrogenated at 250°C for a residence time that yields a 3-log endotoxin reduction; the depyrogenation tunnel load is monitored with endotoxin challenge vials containing 1,000 EU.

Terminal sterilisation, where selected, is performed at 121°C for 15 minutes with F0 not less than 15 minutes. The liquid fill volume is limited to 80% of the nominal vial capacity to allow steam penetration and to reduce stopper movement during the pressure equalisation phase. Sterility is verified by USP <71>; bacterial endotoxins are measured by USP <85>; particulate matter is evaluated by USP <788>. Container closure integrity is evaluated by USP <1207> vacuum decay.

For sterile powder for injection presentations of gentamicin sulfate, the aqueous bulk solution is filled into 10 mL or 20 mL Type I borosilicate vials and partially stoppered before loading into a shelf freeze dryer. Freezing is run at shelf temperatures of -40°C to -45°C for 4–6 hours to ensure complete solidification; incomplete freezing leaves liquid in the vial bottom and causes meltback during primary drying. Primary drying is performed at shelf temperatures from -25°C to -20°C and chamber pressure 100–200 mTorr for 12–20 hours. Secondary drying at 30°C for 4–8 hours reduces residual moisture to ≤1.0% w/w by Karl Fischer titration per USP <921>. Published data for the exact collapse temperature of gentamicin sulfate lyophilized formulations is limited; the cycle is developed with differential scanning calorimetry and freeze-drying microscopy for each specific formulation.

The lyophilised cake is stoppered under vacuum or nitrogen and sealed with an aluminium cap. Collapse, shrinkage, or meltback in the cake indicates that the product temperature exceeded the collapse temperature during primary drying and the batch is rejected. Container closure integrity is verified by vacuum decay per USP <1207>. Reconstitution time in water for injection at 20–25°C is measured; a reconstitution time below 2 minutes is expected for a porous cake, but this depends on the freezing protocol and the specific surface area of the dried matrix. The final powder is tested for sterility, bacterial endotoxins, particulate matter, and residual moisture; endotoxin limits are dose-specific and are derived from the maximum adult dose, commonly not exceeding 0.50 EU/mg of gentamicin sulfate.