Fosfomycin phenylethylamine is a crystalline stoichiometric salt of the epoxide phosphonate antibiotic fosfomycin with 2-phenylethylamine. The API retains the same (2R,3S)-3-methyloxiran-2-ylphosphonic acid pharmacophore as the sodium, calcium, and trometamol presentations, but the counter-ion is an aromatic primary amine rather than an alkali metal, alkaline-earth metal, or trometamol. The molecular formula is C11H18NO4P and the theoretical relative molecular mass is 259.24 g/mol. The product is supplied as a pharmaceutical-grade active pharmaceutical ingredient for tablet, capsule, granule, and injectable finished dosage forms, including oral and intravenous presentations where local marketing authorisation permits. The term “model” is not used in pharmacopoeial nomenclature; the material is identified by the salt name, the manufacturer’s ASMF/DMF product code, and the batch number on the certificate of analysis. Because no dedicated pharmacopoeial monograph for fosfomycin phenylethylamine has been published in Ph. Eur. or USP-NF, the API is controlled against the fosfomycin active moiety monograph, general monograph Ph. Eur. 2034 Substances for Pharmaceutical Use, and ICH Q6A decision-tree requirements.

Is the Phenylethylamine Salt Controlled by a Dedicated Fosfomycin Monograph?

No dedicated monograph exists for the phenylethylamine salt in Ph. Eur. or USP-NF; the sodium, calcium, and trometamol salts are monographed separately. The release specification therefore applies active moiety methods and general monograph Ph. Eur. 2034. The specification framework in Table 1 aligns with ICH Q3A, Q3C, and Q3D and is intended for pharmaceutical development; batch-specific limits on the certificate of analysis take precedence.

Powder flow and hygroscopicity are critical because the phenylethylamine counter-ion is larger than the sodium ion and may alter crystal habit, surface energy, and compaction behaviour. The material should be conditioned at 25±2 °C and 40±5% RH for 24 h before flow testing under USP <1174>. If the Carr index exceeds 25% or the Hausner ratio exceeds 1.35, direct encapsulation is not recommended without a glidant. Colloidal silicon dioxide at 0.2–0.5% w/w is blended in a low-shear bin blender at 10–25 rpm for 10–15 min; magnesium stearate is added last at 0.25–0.75% w/w and blended for 2–5 min because over-lubrication can reduce tablet tensile strength and slow dissolution under USP <711>/Ph. Eur. 2.9.3.

Epoxide Ring Stability Constrains Aqueous Granulation and Requires pH-Buffered Binders

The fosfomycin scaffold contains an epoxide ring that undergoes hydrolytic opening under alkaline pH stress and elevated temperature. Aqueous granulation of the phenylethylamine salt is therefore restricted to binder solutions buffered between pH 3.5 and 5.5 and maintained below 40 °C. For acid-sensitive formulations, roller compaction is preferred because it avoids aqueous binder contact and permits dry reprocessing of fines. High-shear granulation with an impeller tip speed of 2–8 m/s, chopper speed 1000–2500 rpm, and wet massing time 30–120 s is used to limit granule overwetting. Fluidised-bed drying is operated at inlet air 50–70 °C with product temperature not exceeding 35 °C. These ranges are representative pharmaceutical granulation equipment practice and do not replace design-of-experiment optimisation for a specific formulation; published degradation kinetics for this exact phenylethylamine salt remain limited.

On production-scale high-shear granulators, overwetting of the epoxide-containing granulate can produce a torque rise above 10 N·m and adhesive fouling of the bowl. Stagewise binder addition is preferred over single-shot addition to control this failure mode. Roller compaction at pilot scale typically operates at roll speed 5–15 rpm, hydraulic pressure 30–80 bar, and screen mill sieve size 1.0–2.0 mm. Granule moisture after drying should be controlled below 1.5% before tablet compression unless stability data support a higher limit.

Tablet and capsule formulations containing the API are generally dry-compressed or roller-compacted. Tablet hardness is targeted at 60–120 N only after friability under USP <1216> or Ph. Eur. 2.9.7 is below 0.5%. Disintegration is tested under USP <701>/Ph. Eur. 2.9.1; dissolution should not be assumed from the fosfomycin trometamol monograph because salt-specific solubility and wettability can shift the dissolution profile. Capsule filling requires powder flow suitable for the selected machine type; dosator systems may require greater bulk density control than tamping-pin systems. Direct compression is acceptable only when the particle size distribution, Carr index, and conditioned bulk density demonstrate reproducible flow and segregation resistance.

When Injectable Presentations Are Compounded, Sterile Filtration and Endotoxin Control Follow Parenteral Chapters

For injectable dosage forms, the phenylethylamine salt is dissolved in Water for Injection and the solution pH is selected to balance chemical stability and physiological compatibility. The solution is filtered through a validated sterilising-grade membrane with a nominal pore size of 0.22 µm. Filter compatibility is demonstrated by testing sub-visible particulate matter under USP <788>/Ph. Eur. 2.9.19 and visible particles under USP <790>/Ph. Eur. 2.9.20. Terminal sterilisation at 121 °C for 15 min is acceptable only if the related substances increase remains within the registered specification; otherwise aseptic processing is mandatory. Endotoxin limits are calculated from the maximum bolus or infusion dose using the K/M calculation in USP <85> and Ph. Eur. 2.6.14. Sterility is verified by membrane filtration under USP <71>/Ph. Eur. 2.6.1. Because published solubility and stability data for the phenylethylamine salt in common parenteral vehicles are limited, a pre-formulation solubility study at 5±3 °C, 25±2 °C, and 40±2 °C is required before batching.

Counter-Ion Effects Across Sodium, Calcium, Trometamol, and Phenylethylamine Salts

The principal commercial salts differ in counter-ion identity, not in the fosfomycin pharmacophore. Trometamol and phenylethylamine have nearly identical relative molecular masses of 121.14 g/mol and 121.18 g/mol, so the theoretical fosfomycin acid content is approximately 53.3% for both. Fosfomycin calcium and disodium have lower formula masses and therefore higher active moiety mass fractions, approximately 78.4% and 75.9% on an anhydrous basis. The phenylethylamine salt introduces an aromatic lipophilic amine, which is expected to reduce aqueous solubility and may reduce hygroscopicity relative to the highly water-soluble sodium or trometamol salts; peer-reviewed comparative solubility data for this salt remain scarce. It is not therefore a direct processing substitute for fosfomycin trometamol or calcium in oral formulations, nor a direct substitute for fosfomycin disodium in parenteral formulations, without side-by-side dissolution and stability testing.

The decision point in formulation development is not active content, which is nearly identical between trometamol and phenylethylamine salts, but rather processing compatibility and solid-state behaviour. The phenylethylamine salt is evaluated only after pre-formulation data demonstrate acceptable hygroscopicity, powder flow, compaction, and dissolution; direct substitution for fosfomycin trometamol or calcium without these data is outside the registered use.