Amoxicillin Trihydrate (sterile) Pharma Grade API, model designation Amoxicillin Trihydrate (sterile), CAS 61336-70-7, is the stoichiometric trihydrate of the beta-lactam antibacterial amoxicillin. The molecular formula is C₁₆H₁₉N₃O₅S·3H₂O, and the nominal molecular weight is 419.45 g/mol. The active moiety is (2S,5R,6R)-6-[[(2R)-2-amino-2-(4-hydroxyphenyl)acetyl]amino]-3,3-dimethyl-7-oxo-4-thia-1-azabicyclo[3.2.0]heptane-2-carboxylic acid. The material is released as a pharma-grade starting material for tablets, capsules, granules, and sterile oral or injectable preparations where the trihydrate form is acceptable. Unlike non-sterile amoxicillin trihydrate, the sterile grade carries a documented sterility assurance profile and, for parenteral applications, defined bacterial endotoxin control.

The crystal contains three moles of water per mole of amoxicillin, corresponding to approximately 12.9% w/w water; the release specification therefore includes a Karl Fischer water range of 11.5–14.5% w/w as a compendial marker of hydrate integrity. Sterility is evaluated according to USP <71> or Ph. Eur. 2.6.1. Bacterial endotoxin is evaluated according to USP <85> or Ph. Eur. 2.6.14 when the intended use is injectable. The product is not a chemical derivative or salt substitution of amoxicillin; it is the same active moiety as non-sterile amoxicillin trihydrate but is manufactured, packaged, and released under a sterile-product quality system.

Compliance for the sterile API is linked to the current USP Amoxicillin monograph and the Ph. Eur. monograph for Amoxicillin trihydrate, supplemented by ICH Q3C for residual solvents and ICH Q3D for elemental impurities where applicable. The beta-lactam ring undergoes hydrolytic ring opening under alkaline aqueous conditions and at elevated temperature, which imposes specific constraints on terminal sterilization and on aqueous granulation or reconstitution steps.

What Limits the Use of Amoxicillin Trihydrate in Terminal Sterilization and High-Shear Wet Granulation?

Terminal steam sterilization is generally unsuitable for dry amoxicillin trihydrate because moist heat accelerates beta-lactam degradation and can alter the hydrate water content. Aseptic processing is therefore used for the sterile grade, with crystallization, filtration, drying, milling, and packaging conducted in controlled cleanroom zones aligned with EU GMP Annex 1 and FDA 21 CFR 211.113. Sterile-filtered solutions are not a downstream option for the trihydrate because of its limited aqueous solubility; therefore, sterility is built into the powder through process design rather than terminal treatment.

For oral solid dosage forms, the dominant processing constraint is powder flow rather than sterility. Direct compression of micronized amoxicillin trihydrate is often limited by high fines content and low bulk density. In such cases, the API is densified by roller compaction and milled to a granule fraction suitable for a rotary tablet press with a forced feeder. Wet granulation with water can be used only with controlled liquid addition and low-temperature drying; the final water content must return to the trihydrate hydrate range. High-shear wet granulation may create hard agglomerates that resist disintegration if over-wetted or if the impeller speed and chopper time are not matched to the binder viscosity.

Tablet and capsule manufacture typically uses a low-shear tumble blender, a roller compactor, and a rotary tablet press. Blend uniformity is monitored by HPLC according to USP <621>. Finished dosage-unit uniformity is assessed by USP <905>. Disintegration testing is performed according to USP <701>, and dissolution testing is performed according to the amoxicillin dosage-form monograph using USP <711> apparatus. For granules and powder for oral suspension, particle size distribution is controlled by laser diffraction according to USP <429> and Ph. Eur. 2.9.31. A fine particle fraction is needed to ensure rapid dispersion and acceptable sedimentation after reconstitution.

For injectable preparations, the trihydrate has limited aqueous solubility; amoxicillin sodium is the freely soluble salt used for intravenous or intramuscular solution. The trihydrate is appropriate only for sterile suspension presentations or non-intravenous sterile preparations where a slow dissolution rate is intended. In such cases, the reconstituted suspension must meet injectable particulate matter limits according to USP <788> and the endotoxin limit derived from the maximum dose. Particle size, zeta potential, and suspension viscosity control syringeability and resuspendability; the exact specifications are fixed in the regulatory submission.

Specification, Sterility Assurance, and Endotoxin Release Requirements

The sterile grade is released against a specification matrix that combines chemical, physical, and microbial attributes. The following table lists the compendial test areas typically applied to the API, with the understanding that product-specific release limits are finalized under the marketing authorization.

Sterility testing of beta-lactam APIs requires a validated method-suitability step because the active moiety can inhibit growth of compendial challenge organisms. Membrane filtration with neutralization or enzymatic inactivation is preferred over direct inoculation. The bacterial endotoxin limit is not a single universal value; it is derived from the maximum dose, route, and patient body weight using the compendial threshold of 5 EU/kg for intravenous administration. For a 1 g adult dose in a 70 kg patient, the calculated limit is 0.35 EU/mg. If the intended product is a sterile oral powder, the endotoxin test may not be required, but the sterility test remains a release attribute.

Aseptic manufacturing includes on-site bioburden monitoring before and after drying, particulate control, and sterilizing-grade filtration of all liquid streams that contact the API after the final crystallization. The beta-lactam ring is sensitive to gamma irradiation and ethylene oxide; published data for this specific configuration is limited, and aseptic processing is therefore preferred over terminal sterilization. The packaging system is selected to maintain the sterile barrier and limit moisture ingress, with the seal integrity of each primary container closure verified as part of the drug application.

Related substances are controlled by gradient HPLC according to the compendial monograph; open-ring degradation products include amoxicilloic acid and amoxicillin diketopiperazine. The capacity factor and resolution between amoxicillin and its related substances must meet the system suitability requirements of the parent monograph. Compendial tests for residual solvents according to USP <467> and Ph. Eur. 2.4.24 apply if solvents are used in the final crystallization, and elemental impurity limits are assigned according to ICH Q3D and measured by USP <233> or equivalent plasma techniques.

In a typical dry granulation suite, the sterile API is handled in an isolator-based weigh room and transferred to a bin blender. Production-scale experience with beta-lactam powders has shown that micronized trihydrate lots with high fines fractions can segregate during hopper discharge and produce tablet weight variation if the tablet press feed frame is run with low powder levels. To mitigate this, the API is compacted into ribbons and milled under low-humidity conditions. The granule fraction is then blended with a disintegrant such as crospovidone at 2–5% w/w and a lubricant such as magnesium stearate at 0.25–1.0% w/w. Magnesium stearate above 2% w/w may reduce tablet hardness and increase disintegration time because it reduces interparticulate bonding and can hydrophobize the blend. Disintegration time is measured according to USP <701>, and tablet friability is evaluated according to USP <1216>.

At the tablet press, tooling is often run with precompression force to reduce lamination, especially when the granule contains a high proportion of API. The compression profile is monitored by tablet weight, hardness, thickness, and disintegration. Process capability studies on beta-lactam lines frequently demonstrate that weight variability is an early indicator of segregation, and near-infrared spectroscopy may be used to monitor blend potency in real time. The final water content of granules is verified by Karl Fischer procedure in USP <921> to confirm that the trihydrate crystal water remains within specification.

When Amoxicillin Sodium or Non-Sterile Trihydrate Is Replaced by the Sterile Trihydrate Grade

Formulators must differentiate among at least three materials that share the amoxicillin pharmacophore but differ in salt form, solvation state, and microbiological quality. The sterile trihydrate is not a direct substitution for amoxicillin sodium in injectable solutions. Amoxicillin sodium is freely soluble in water and is used for intravenous or intramuscular injection after reconstitution; the trihydrate is very slightly soluble and will form a suspension at clinically relevant concentrations. Replacing the sodium salt with the trihydrate in a parenteral formulation therefore changes the pharmaceutical form, syringeability, and release profile, and is not a simple excipient-equivalent change.

Compared with non-sterile trihydrate, the sterile grade may have a shifted particle-size distribution because aseptic milling and containment affect powder surface energy and flow. A robust qualification program should compare particle-size D10, D50, and D90 by laser diffraction, bulk and tapped density by USP <616>, and specific surface area where dissolution rate is critical. Published data for this specific configuration is limited, so the comparison must be performed on the actual lots intended for commercial use. Anhydrous amoxicillin, where used, can absorb water and convert partially to the trihydrate in aqueous suspension, altering physical stability and dissolution profile; the trihydrate is therefore preferred where hydrate integrity is required.

Switching from non-sterile amoxicillin trihydrate to sterile trihydrate in a solid oral product may not alter dissolution if the particle-size distribution is matched, but the additional microbial quality controls are unnecessary unless the intended product is sterile. Conversely, using the sterile trihydrate as the starting material for an injectable solution is not feasible without solubility enhancement because the trihydrate will not yield a true solution at clinically relevant concentration; amoxicillin sodium remains the appropriate starting material for parenteral solutions.

Compared with ampicillin trihydrate, amoxicillin trihydrate carries an additional hydroxyl substituent on the phenyl ring. This changes oral absorption and antimicrobial spectrum, but it does not alter the beta-lactam processing constraints governing sterile manufacture. The sterile API shares the same compendial sterility and endotoxin test platforms applied to other penicillin-class APIs.