Benzathine benzylpenicillin is incorporated into aqueous intramuscular depot suspensions for the treatment of Treponema pallidum infections because the dibenzylethylenediamine salt dissociates slowly from the injection site and sustains measurable serum penicillin activity over an extended interdose interval. The API is dry-milled before wetting; a spiral jet mill operating with compressed nitrogen at an inlet pressure of 6–8 bar is a common production configuration because the beta-lactam ring is shear- and temperature-sensitive, and cryogenic or nitrogen-assisted milling reduces thermal degradation. The milled solids are then dispersed into a sterile aqueous vehicle containing buffering salts, lecithin, polysorbate 80, and sorbitol under vacuum using a high-shear rotor-stator disperser. Tip speeds in the range of 5–10 m/s are generally sufficient to break agglomerates, while higher energy input raises the suspension temperature and accelerates hydrolytic degradation of the penicillin nucleus. The finished suspension viscosity is measured at 20 °C with a rotational viscometer according to USP <911> or Ph.Eur. 2.2.10; the target viscosity range is formulation-specific, and published data for this exact suspension is limited. Syringeability through a 21-gauge needle is assessed by measuring the force required to expel the suspension at a constant rate, with acceptance criteria based on needle gauge, fill volume, and patient comfort. Clinical treatment recommendations for primary, secondary, and early latent syphilis specify a single intramuscular dose of 2.4 million units; late latent syphilis uses the same dose at 7-day intervals for 3 weeks. These dosing schedules require a suspension that remains resuspendable after prolonged storage because incomplete redispersion can leave a subtherapeutic dose in the vial and compromise patient outcomes.

Why Does Resuspension Time Increase After Accelerated Storage of Benzathine Penicillin Suspensions?

Accelerated stability protocols conducted under ICH Q1A(R2) at 40 °C ± 2 °C and 75% ± 5% RH frequently show an increase in resuspension time for benzathine penicillin suspensions, and the underlying cause is usually physical rather than purely chemical degradation. The low aqueous solubility of the API means that small particles with high surface energy can dissolve and reprecipitate on larger crystals during temperature cycling; repeated partial dissolution and recrystallization promotes Ostwald ripening and the formation of solid bridges between settled particles. This produces a hard sediment that resists manual redispersion. Particle size distribution before and after storage is measured by laser diffraction using USP <429>; a shift in D90 from ≤20 µm to ≥45 µm is used in some production sites as a trigger for corrective action, although published data for this specific configuration is limited. Formulators control sedimentation by adding a polymeric suspending agent at a concentration between 0.2% and 0.5% w/v; the resulting low-shear viscosity is kept low enough to remain syringeable through a 21-gauge needle. On a filling line, resuspendability is evaluated with a roller mixer at 30 rpm, and the time to eliminate wall adhesion is recorded; excessive resuspension time is a known cause of batch rejection because the patient may fail to receive the labeled dose. The stability chamber data are correlated with particle morphology by scanning electron microscopy, and the presence of elongated or fused crystals is considered a physical instability marker that justifies reformulation.

Secondary prevention of acute rheumatic fever relies on the same depot presentation because continuous serum penicillin exposure over a 3–4 week interdose interval reduces the risk of recurrent group A streptococcal infection. The standard adult dose is 1.2 million units administered deep into the gluteal muscle every 4 weeks; a dose of 2.4 million units every 3 weeks may be used in selected clinical settings. The injection technique requires needle aspiration before injection to reduce the risk of inadvertent intra-arterial or intravascular placement, and this clinical constraint has a manufacturing analogue: the suspension must be free of oversized particles that could obstruct the needle or irritate muscle tissue. Fill volume accuracy for single-dose vials is controlled to ±1% by periodic gravimetric checks on the filling line, and the container closure system is subjected to integrity testing using vacuum decay or dye ingress per USP <1207>; any leak can compromise sterility and reduce product shelf life. Endotoxin load is verified by USP <85> or Ph.Eur. 2.6.14, and the acceptance limit is derived from the maximum intramuscular dose per kilogram. The manufacturing dossier for this indication must demonstrate consistency of particle size distribution, potency, and resuspendability across multiple batches because the clinical outcome depends on reproducible depot formation after injection.

When Benzathine and Procaine Penicillin G Are Combined for Biphasic Release

Combination products containing benzathine and procaine penicillin G are manufactured as aqueous suspensions that provide biphasic release: the procaine component dissolves more rapidly, whereas the benzathine component remains as a prolonged-release depot. The two penicillin salts must be dispensed by potency rather than gravimetric weight because the theoretical potencies and molecular weights differ, and the water content of each API batch is determined by loss on drying or Karl Fischer titration before calculation. The vehicle is prepared by hydrating the suspending agents in water for injection; the order of addition of the two salts changes the dispersion state because simultaneous wetting of two low-solubility powders can produce heteroagglomerates that do not fully disperse. During mixing, the temperature is maintained below 25 °C using a jacketed vessel because benzylpenicillin degrades by hydrolysis and rearrangement in aqueous media at elevated temperatures. The finished suspension is tested for potency and related substances by high-performance liquid chromatography using the compendial monograph for penicillin G benzathine and procaine penicillin G; the related substances limit includes benzylpenicilloic acid, benzylpenilloic acid, and other degradation products. Resuspendability is measured as time to complete redispersion under controlled inversion, and subvisible particulate matter is measured per USP <790> or Ph.Eur. 2.9.19. Syringes filled from the bulk suspension are inspected for visible particles, and the fill line is stopped if particle counts exceed the compendial limits for large-particle contamination. The final product is filled aseptically because terminal moist-heat sterilization would alter the suspension structure and accelerate penicillin degradation.

Veterinary injectable formulations of benzathine benzylpenicillin are produced for intramuscular or subcutaneous administration in cattle, sheep, and swine, often with broader particle size specifications than human parenteral products because syringeability through 16-gauge or 18-gauge needles is the critical handling requirement. Suspending agents such as aluminum monostearate are used to extend physical stability; the concentration must be controlled within a narrow range because insufficient suspending agent permits rapid sedimentation, while excess suspending agent increases injection-site reactions and may prolong the withdrawal period. Stability batches are placed at 30 °C ± 2 °C and 75% ± 5% RH, corresponding to WHO climatic zone IVb long-term conditions, and resuspendability is tested after storage in both upright and inverted orientations. Residue depletion studies are required to establish meat and milk withdrawal periods; maximum residue limits for benzylpenicillin are set by Codex Alimentarius and by regional authorities such as the European Medicines Agency. Cross-contamination control between veterinary and human beta-lactam production areas is validated by cleaning verification, with beta-lactam-specific analytical methods used to detect residual penicillin residues on equipment surfaces. The veterinary production line is usually segregated from human API processing because the same molecule must not enter the human supply chain without full GMP documentation and release testing.

Oral Tablet, Capsule, and Granule Feasibility Boundaries

Benzathine benzylpenicillin is not considered suitable for conventional immediate-release oral tablet or capsule dosage forms because the dibenzylethylenediamine salt is very slightly soluble in water and the beta-lactam ring undergoes acid-catalyzed hydrolysis in the gastric environment. The anhydrous molecular weight of the salt is approximately 909.1 g/mol, and its low aqueous solubility limits dissolution rate and oral absorption. Solid oral dosage form development would require micronization, wet granulation, and enteric coating to bypass gastric acid, but these interventions do not overcome the low intestinal permeability of the salt; published data for this specific configuration is limited. If a tablet or capsule formulation is nevertheless required, the API must be characterized for flowability, compressibility, and crystal habit; needle-like crystals can require wet granulation with povidone or another binder to achieve acceptable tablet hardness. Granule intermediates would be dried to a controlled loss on drying, then milled and blended with disintegrant before compression or encapsulation. Dissolution would be evaluated using USP <711> apparatus 2 in pH 6.8 phosphate buffer after an acid-stage pretreatment, but no pharmacopoeial dissolution monograph exists for benzathine benzylpenicillin oral dosage forms, and bioequivalence reference products are not established in major jurisdictions. The oral route therefore remains a formulation feasibility study rather than a recognized commercial application for this penicillin salt.

Aseptic processing of benzathine benzylpenicillin suspensions requires that all components are sterile before compounding because the final suspension cannot be sterilized by filtration after the particles are dispersed. The aqueous vehicle is sterilized by filtration through a 0.22 µm polyvinylidene fluoride or polyethersulfone membrane, while the API is introduced as a sterile powder with a validated sterility assurance level arising from crystallization, milling, and drying under controlled conditions. Compounding and filling occur in an isolator or a Grade A unidirectional airflow environment; aseptic process simulations using media fill are performed to verify that the filling line can maintain a contamination rate of zero. The final product is tested for sterility according to USP <71> or Ph.Eur. 2.6.1, bacterial endotoxins according to USP <85> or Ph.Eur. 2.6.14, particulate matter according to USP <790> or Ph.Eur. 2.9.19, and pH according to USP <791> or Ph.Eur. 2.2.3. The API supplier must provide an audit trail for particle size distribution, residual solvents, elemental impurities according to ICH Q3D, and penicillin degradation products, because the finished product manufacturer cannot reprocess a sterile suspension once it has been formed. In this application, the central technical risk is not chemical potency loss alone but the combination of particle instability, microbial contamination risk, and the inability to terminal-sterilize a non-filterable suspension.