Fortified Procaine Benzylpenicillin Pharma Grade API is a co-processed or blended penicillin G derivative manufactured for downstream conversion into sterile injectable suspensions and, under restricted formulation conditions, oral granules, tablets, or capsules. The non-proprietary procurement code FPPB-API identifies the non-sterile crystalline powder described in this technical note; it is not a pharmacopoeial model number. The fortified designation refers to the presence of a soluble benzylpenicillin salt, typically benzylpenicillin sodium or potassium, combined with the poorly water-soluble procaine benzylpenicillin depot fraction. The soluble component provides rapid parenteral bactericidal concentrations, while the procaine salt dissolves slowly from an intramuscular depot and extends measurable plasma penicillin activity. The API is released against pharmacopoeial monographs and a certificate of analysis that includes total potency in international units per milligram, procaine mass fraction, water content, residual solvents, particle-size distribution, and, for injectable-grade material, bacterial endotoxin data. Although the product is specified for both oral and injectable routes, the oral route is constrained by the known gastric acid lability of benzylpenicillin; direct oral administration without enteric protection or buffering is not supported by published stability data. API manufacture is performed under EU GMP Part II and 21 CFR 210/211, with release specifications aligned to Ph. Eur. monograph 0333 for Benzylpenicillin procaine and the USP Penicillin G Procaine monograph.

What Release Specifications and Physical Limits Govern the Fortified API?

Release specifications for FPPB-API are divided into compendial identity and purity limits and application-specific physical attributes. Identity is confirmed by infrared absorption against a qualified reference standard and by thin-layer chromatography or HPLC retention time. Assay for benzylpenicillin is expressed as total penicillin G activity because the fortified composition contains two benzylpenicillin salts. A representative non-sterile certificate of analysis records total potency 1,010–1,050 IU/mg on a dried basis; the procaine fraction is separately quantified by liquid chromatography at 38.5–41.5% w/w. Water determined by USP <921> is controlled to ≤3.0% because higher moisture accelerates β-lactam ring hydrolysis and reduces powder flow. Residual solvent levels comply with ICH Q3C, with ethanol ≤0.5% and acetone ≤0.1% in the released powder. Heavy metals are controlled under ICH Q3D Option 1 elemental impurity limits rather than legacy colorimetric tests. The pH of a 10% aqueous suspension is monitored at 5.5–7.5 because both acidic and alkaline excursions degrade the β-lactam ring. Particle-size distribution is not fixed by the monograph but is controlled by the intended downstream process; injectable-grade material is milled to D90 ≤75 µm, while solid-dose material is typically supplied with D50 35–55 µm. The limits in Table 1 are representative of a non-sterile solid-dose grade. Injectable grades may be supplied with lower bioburden and endotoxin limits calculated from the maximum intended daily dose.

Shifts in procaine mass fraction of ≥1.0% w/w alter the depot-to-immediate release ratio and can fail uniformity of dosage units under USP <905>. Because the soluble benzylpenicillin salt ratio is not fixed by monograph, it is declared on the manufacturer’s certificate of analysis and must be reconciled with the total potency result before formulation.

For tablet and capsule manufacturing, the API is milled to a particle-size distribution suitable for dry granulation or direct compression. The needle-like habit of procaine benzylpenicillin reduces free-flow and increases die-fill variation; bulk density 0.45–0.60 g/mL and Carr index values 25–35 indicate passable-to-poor flow. Hopper vibration or forced feeders are therefore required on rotary tablet presses. Direct compression is typically reserved for low-dose tablets or capsule blends because the high therapeutic dose and the poor compressibility of the raw API can exceed main compression force limits. For tablets and capsules, dry granulation by roller compaction is preferred to aqueous granulation. Roller compaction operated at roll pressure 8–12 kN/cm and mill screen 1.0–1.5 mm densifies the powder without adding water and limits heat exposure. Granule drying after any aqueous step must not exceed 45 °C because procaine benzylpenicillin degrades in humid heat; at relative humidity above 60%, pre-drying of the API is required before blending. The formulation must avoid acidic granulating fluids below pH 4.0 and alkaline fluids above pH 8.0, because both conditions hydrolyse the β-lactam ring and the procaine ester. Contact surfaces should be 316L stainless steel; copper and iron ions accelerate penicillin degradation. Magnesium stearate is limited to ≤0.5% w/w because higher levels retard dissolution of the procaine fraction. Capsule filling with dosator or tamping-pin equipment requires flow aids such as colloidal silicon dioxide at 0.5–1.0% w/w; however, high-surface-area silicas can adsorb the soluble benzylpenicillin fraction and should be tested by HPLC recovery studies. Tablet hardness for enteric-coated cores is typically targeted at 8–14 kp, while uncoated cores may require 5–8 kp but must still pass friability testing under USP <1216>.

Injectable Suspension Manufacturing Requires Particle-Size and Endotoxin Control

For injectable suspensions, the API is either supplied as a sterile milled powder or prepared by validated aseptic processing because terminal steam sterilization is incompatible with β-lactam thermal stability. The procaine fraction cannot be sterile-filtered through a 0.2 µm membrane because it is a suspension; only the soluble benzylpenicillin fraction can be sterilised by filtration. The final suspension must meet USP <1> injections, USP <788> particulate matter, and USP <85> bacterial endotoxin requirements. Particle-size reduction to D90 ≤75 µm is necessary to allow passage through 21G or 23G needles without excessive plugging. Colloid milling or high-shear rotor-stator dispersion is used at controlled jacket temperatures below 30 °C to avoid thermal degradation. The aqueous vehicle typically contains a buffer to pH 5.5–7.5, a suspending agent, and a preservative where permitted; benzyl alcohol or parabens are used only after compatibility screening because the procaine ester can interact with certain preservative systems. Syringeability testing follows ISO 7886-1:2017 and may include force-to-plug and glide-force measurements on production syringes. The soluble benzylpenicillin fraction in the fortified API dissolves into the vehicle, increasing osmolality; the osmolality of the final injection should be monitored and kept within physiologically acceptable limits. If the soluble fraction is too high, the suspension can become viscous and difficult to resuspend. Aseptically processed injectable-grade API requires a sterility assurance level of 10⁻⁶, and final sterile suspension batches are released only after sterility testing under USP <71> and bacterial endotoxin assay. Published batch data for this specific fortified configuration are limited, so each manufacturer must qualify suspension resuspendability and particle aggregation under ICH Q1A accelerated stability conditions.

Handling of the fortified API requires dry, temperature-controlled storage below 25 °C in sealed aluminium-lined bags under nitrogen or dry air. Procaine benzylpenicillin is sensitive to moisture and heat; repeated opening of containers at ambient relative humidity above 60% can increase water activity and reduce potency. The powder should not be exposed to direct sunlight because benzylpenicillin undergoes photodegradation. Incompatible substances include strong oxidisers, acids, alkalis, copper and iron salts, and certain surfactants that disrupt the procaine depot. For solid-dose manufacturing, avoid pre-blending with acidic effervescent components or amine-releasing additives until compatibility is demonstrated by HPLC impurity profiling. Personnel handling injectable-grade material require validated gowning and environmental monitoring according to EU GMP Annex 1.

When Oral Granules, Tablets, or Capsules Are Required Despite Gastric Acid Instability

Benzylpenicillin has poor stability in gastric acid, with the β-lactam ring undergoing rapid acid-catalysed hydrolysis below pH 4.0. In in vitro gastric simulation at pH 1.2, published degradation studies report substantial loss within 15–30 min. Consequently, an oral tablet, capsule, or granule containing fortified procaine benzylpenicillin cannot be a conventional immediate-release product. Enteric coating is the minimum intervention; an enteric film based on methacrylic acid copolymers with a dissolution threshold of pH 5.5–6.8 can protect the API until the small intestine. A buffering layer may be added between the API core and the enteric coat to prevent local acid migration. Dissolution testing should use USP apparatus II with a two-stage medium: 0.1 M HCl for 2 h, followed by phosphate buffer pH 6.8. Granules for oral liquid or sachet use are often wet-granulated with a buffering excipient system that maintains the reconstituted vehicle at pH 6.0–7.0. If the granule is dispersed in acidic juice or cola, pH falls below 4.0 and rapid hydrolysis is likely; this incompatibility is a common field failure in veterinary oral products. Dry granule blends should therefore include an alkalizing buffer such as sodium citrate at a level sufficient to resist acid challenge for 30 min at pH 1.2. Published pharmacokinetic data for oral fortified procaine benzylpenicillin in humans are limited; equivalence to oral phenoxymethylpenicillin should not be assumed. Operators must confirm that the enteric polymer does not interact with the procaine component and that residual water in the core remains below 3.0% during stability.

Distinguishing Fortified Procaine Benzylpenicillin from Other Penicillin Salts

The product differs from unfortified procaine benzylpenicillin by the deliberate addition of a soluble benzylpenicillin salt. That addition changes the release profile from a purely slow-release depot to a biphasic profile with an immediate peak and a sustained tail. Compared with benzylpenicillin sodium or potassium, the fortified API is less water-soluble and cannot be presented as a rapid intravenous solution; it is intended for intramuscular suspension or protected oral administration. Intravenous administration is contraindicated because the procaine component can cause neurotoxicity. Benzathine benzylpenicillin is even less soluble than procaine benzylpenicillin and is used for repository treatment at intervals of weeks, whereas procaine-based products are reappraised at 12–24 h intervals in many veterinary protocols. Phenoxymethylpenicillin is acid-stable and is the preferred oral penicillin for treating susceptible infections; fortified procaine benzylpenicillin is not a direct substitute for oral phenoxymethylpenicillin because of its acid lability, procaine content, and depot behaviour. Unlike amoxicillin/clavulanate or ampicillin/sulbactam, fortified procaine benzylpenicillin contains no β-lactamase inhibitor and is not active against β-lactamase-producing staphylococci. Cross-allergenicity with other β-lactams must be considered in labelling; a documented penicillin allergy is a contraindication unless skin testing and medical evaluation support use.

Table 2 summarises formulation-relevant differences among penicillin salts.