Procaine benzylpenicillin (sterile), identified by CAS Registry Number 6130-64-9, is a white to off-white crystalline monohydrate used as a pharma-grade active pharmaceutical ingredient for oral solid-dose and injectable suspension products. Commercial material is supplied under a manufacturer-specific grade code that designates sterile micronized, non-pyrogenic powder; the pharmacopoeial descriptor remains benzylpenicillin procaine. The compound is an equimolar salt of benzylpenicillin and procaine. With the molecular formula C13H20N2O2·C16H18N2O4S·H2O and a relative molecular mass of approximately 588.7 g/mol, the theoretical procaine content is approximately 40.1% by mass and the theoretical water of hydration is approximately 3.1% by mass. The aqueous solubility at 20 °C is approximately 4 mg/mL, which is substantially lower than the freely soluble benzylpenicillin potassium and sodium salts and gives the procaine salt its prolonged-release behaviour after intramuscular injection. The sterile grade is distinguished from non-sterile procaine benzylpenicillin by compliance with Ph. Eur. 2.6.1 and USP <71> sterility tests, by a validated bacterial endotoxin limit, and by particle-size distribution controls appropriate to aseptic dispersion. The product is intended for formulation into tablets, capsules, granules, and injectable suspensions; it is not suitable for intravenous, intrathecal, or ophthalmic administration.

How Does the Procaine Salt Modify Physicochemical Handling Versus Alkali-Metal Penicillins?

The replacement of the alkali-metal counterion with procaine alters dissolution kinetics, aqueous stability, and injection-site tolerability. Benzylpenicillin potassium and sodium salts dissolve rapidly in water, generate high early plasma concentrations after parenteral administration, and are suitable for intravenous infusion after reconstitution. In contrast, procaine benzylpenicillin hydrates slowly from a microcrystalline depot after intramuscular injection, yielding measurable plasma concentrations for approximately 12 to 24 hours while limiting peak plasma levels. The procaine cation provides a local anaesthetic effect that reduces injection-site pain in aqueous suspension formulations. The low aqueous solubility introduces processing constraints: the powder cannot be reconstituted into a true solution at clinically useful concentrations and cannot be sterilised by terminal steam processes without beta-lactam hydrolysis. The solubility difference also affects oral dosage design: the procaine salt is less hygroscopic than the potassium salt but exhibits slower dissolution in gastric fluid, so oral tablets and capsules require particle-size and formulation controls to achieve pharmacopoeial dissolution acceptance ranges. Benzathine benzylpenicillin, a benzylpenicillin salt with even lower aqueous solubility, typically extends intramuscular release to 2–4 weeks, while procaine benzylpenicillin occupies an intermediate duration position. This intermediate release profile makes the sterile API suitable for formulations where prolonged, but not depot-length, systemic exposure is required.

In oral tablet and capsule manufacture, the sterile-grade powder is typically introduced after pre-blending with dried excipients and then processed by dry granulation or roller compaction to avoid free-water exposure. Direct compression of procaine benzylpenicillin monohydrate may be limited by electrostatic charging and insufficient flow in high-speed rotary presses; roller compaction with sieve milling is therefore used to improve bulk density uniformity and reduce hopper bridging. Aqueous wet granulation is avoided unless the binder application rate is minimal and the drying endpoint is controlled at a loss on drying below 2.0% w/w, because residual moisture accelerates beta-lactam ring hydrolysis. Content uniformity is assessed according to Ph. Eur. 2.9.40 or USP <905> using high-performance liquid chromatography with ultraviolet detection. Dissolution testing for oral solid products is performed using apparatus II or IV at pH values corresponding to the intended release environment; published data for the dissolution behaviour of procaine benzylpenicillin granules under fed-state gastric conditions are limited, so formulation qualification should include a pH-switching dissolution method. The product is not inherently enteric-protected; when gastric acid protection is required, the granules or tablets must receive a compounding-specific polymer coating. Batch-to-batch variance in particle size can alter segregation in twin-shell V-blenders; colloidal silicon dioxide at 0.5%–1.5% w/w is commonly evaluated as a glidant.

Release Specifications and Pharmacopoeial Alignment

Pharmacopoeial monographs for benzylpenicillin procaine, including Ph. Eur. and USP, define identity, assay, related substances, water content, and bacterial endotoxins for sterile parenteral grades. The release specification for the sterile API includes infrared absorption spectrophotometry according to Ph. Eur. 2.2.24 and high-performance liquid chromatography according to Ph. Eur. 2.2.29 for identification and assay. The assay is calculated and reported as the sum of benzylpenicillin and procaine contents on the anhydrous basis; the theoretical percentages are approximately 56.8% for benzylpenicillin and 40.1% for procaine, and monograph limits are applied to both moieties. Related substances are controlled by reversed-phase HPLC with detection of benzylpenicilloic acid, p-aminobenzoic acid, and other specified degradation products. Residual solvents are controlled according to Ph. Eur. 2.4.24 or USP <467>; the product is normally supplied as a non-pyrogenic powder with a bacterial endotoxin limit determined according to Ph. Eur. 2.6.14 or USP <85>. Water content is determined by Karl Fischer titration according to Ph. Eur. 2.5.12 and should remain within the hydration range appropriate for the monohydrate. Particle-size distribution is measured by laser diffraction according to Ph. Eur. 2.9.37; the oral solid-dose and injectable suspension variants normally carry different particle-size targets because flow, blend uniformity, sedimentation, and syringeability impose different requirements.

The bacterial endotoxin limit for the injectable grade is not a single universal value; it is calculated as K/M, where K is 5 EU/kg for intramuscular products and M is the maximum dose per kilogram. For a 1.2 g dose in a 70 kg adult, M is approximately 17.1 mg/kg and the corresponding endotoxin threshold is approximately 0.29 EU/mg. The exact limit is product-specific and must be justified in the marketing authorisation dossier. Microbiological purity of non-sterile oral grades is assessed according to Ph. Eur. 2.6.12 or USP <61> and <62>.

When Aseptic Dispersion Replaces Terminal Sterilisation for Intramuscular Suspensions

For injectable suspension manufacture, terminal sterilisation of procaine benzylpenicillin is not feasible because moist-heat exposure accelerates beta-lactam hydrolysis and procaine salt decomposition, and dry-heat exposure at temperatures above 50 °C increases related substances. The sterile API is therefore processed by aseptic dispersion: the micronized powder is weighed and transferred within an isolator or restricted-access barrier system operating under Grade A particle conditions with Grade B background according to EU GMP Annex 1 and USP <71> compliance. Inline particle-size reduction through a sterile cone mill or jet mill may be employed to control D90 below 50 µm, but the equipment must be qualified for yield, for the absence of metal particulate contamination, and for the absence of heat-induced agglomeration. The aqueous vehicle is sterilised by autoclaving and cooled before dispersion; suspending agents, buffers, and preservatives are selected to maintain pH between 6.0 and 6.8, where penicillin G stability is maximal. After dispersion, the suspension is filled into siliconised glass vials or prefilled syringes, and the filled product is tested for sterility, bacterial endotoxins, particulate matter according to Ph. Eur. 2.9.19, and syringeability through a 21 G needle. Because the salt is not soluble at therapeutic concentrations, reconstitution cannot be used as a clarity check; instead, suspension uniformity and sedimentation volume are part of batch release. Production-line failure modes observed during sterile powder handling include powder bridging in hoppers due to electrostatic charge, caking after exposure to relative humidity above 60%, and particle-size increase from pressure-induced agglomeration in screw-fed mills. These failures are controlled by humidity-controlled weighing suites and by periodic sieve or laser-diffraction checks after milling.

Compliance with current good manufacturing practice for sterile active substances is required under EU GMP Part II and ICH Q7 because the product is destined for aseptic processing. The API manufacturer's process validation should include media fills for aseptic powder handling, lyophilisation-free drying, and environmental monitoring data for Grade A and Grade B zones. Aseptic APIs are not released solely on the basis of sterility testing; parametric release may be used only where a defined sterilising stage exists, which is not applicable to the final procaine benzylpenicillin powder after crystallisation. Consequently, batch release is based on terminal sterility test results together with bioburden monitoring at pre-filtration and pre-filling stages, and on environmental monitoring trend reports.

In low-dose oral granule and powder formulations, the sterile API may be dry-blended with lactose, microcrystalline cellulose, croscarmellose sodium, and magnesium stearate; however, the blend must not include amine-containing excipients such as certain shellac or gelatin hydrolysates in the presence of free moisture, because primary amines can open the beta-lactam ring by nucleophilic substitution. Contact with copper, zinc, or iron surfaces in wet granulation equipment should be avoided because trace metal ions catalyse degradation and can reduce potency below the assay limit. Storage of the sterile API should be maintained at 2–8 °C in double-layer polyethylene bags inside sealed aluminium-laminated outer packaging; repeated opening of the package in an uncontrolled humidity environment above 60% RH requires prior moisture sorption verification because the monohydrate surface can adsorb free water and increase hydrolysis risk. Batch-to-batch variance in particle-size distribution and residual procaine-related impurities can alter suspension sedimentation behaviour and content uniformity; therefore incoming raw-material qualification should include laser diffraction, scanning electron microscopy, and HPLC impurity profiling against a qualified reference lot. The sterile API is not interchangeable with benzylpenicillin potassium or sodium on a weight-for-weight basis; dosing must be expressed in penicillin G units or in the procaine salt mass specified by the relevant pharmacopoeial monograph and converted using the assay value of the specific batch.