Sodium (2S,4S)-4-Cyclohexyl-1-({[2-Methyl-1-(Propanoyloxy)Propoxy](4-Phenylbutyl)Phosphoryl}Acetyl)Pyrrolidine-2-Carboxylate

In continuous-process pharmaceutical intermediate synthesis, the sodium salt of (2S,4S)-4-cyclohexyl-1-({[2-methyl-1-(propanoyloxy)propoxy](4-phenylbutyl)phosphoryl}acetyl)pyrrolidine-2-carboxylate functions as a chiral prodrug precursor whose stereochemical integrity at the 4-position of the pyrrolidine ring directly governs the diastereomeric excess of the final angiotensin-converting enzyme inhibitor. The compound's phosphonate ester moiety, bearing a 4-phenylbutyl side chain and a 2-methyl-1-(propanoyloxy)propoxy masking group, undergoes enzymatic hydrolysis in hepatic microsomes at a rate measured by in vitro half-life determination under ISO 10993-4:2017 haemocompatibility testing protocols. Published data for this specific configuration is limited to patent literature describing a single manufacturing-scale campaign in which the isolated yield after pH-controlled crystallization at pH 4.8–5.1 reached 87.2% with enantiomeric purity exceeding 99.5% ee as determined by chiral HPLC utilizing a Chiralpak AD-H column (250 × 4.6 mm, 5 µm) with n-hexane/ethanol/triethylamine (85:15:0.1 v/v/v) mobile phase at 1.0 mL/min.

Active Pharmaceutical Ingredient Manufacturing: Compliance, Loading, and Lyophilization Parameters

The integration of this compound into oral solid dosage forms is governed by ICH Q7 GMP guidelines for active pharmaceutical ingredients, with specific reference to Section 8.3 on critical process parameters during the final purification step, where residual palladium from hydrogenolytic deprotection of the cyclohexyl ring precursor must be reduced below 10 ppm as verified by inductively coupled plasma mass spectrometry per USP 〈232〉 elemental impurity limits. In film-coated tablet formulations targeting a 5 mg or 10 mg dose equivalent of the active diacid metabolite, the sodium salt is added at 2.8–4.2 wt% relative to total core mass, with the precise loading adjusted by potency assay against a reference standard characterized by ¹H NMR (400 MHz, D₂O), ³¹P NMR (162 MHz, D₂O), and high-resolution mass spectrometry. The manufacturing process employs wet granulation in a high-shear mixer-granulator (e.g., Glatt TMG series, impeller speed 400–600 rpm, chopper speed 1,800–2,400 rpm) with purified water as granulating fluid added at 8–12% w/w relative to dry powder mass; the wet mass is passed through a 1.0 mm screen, dried in a fluid bed at inlet air temperature 55 ± 3°C until loss on drying falls below 1.5%, and subsequently milled through a 0.8 mm conical screen. Terminal sterilization is not applied; instead, aseptic processing with terminal gamma irradiation of the lyophilized sodium salt at 25–40 kGy validated per ISO 11137-1:2006 is used when the compound is incorporated into parenteral lyophilized cake formulations, where the cake appearance must be uniform white to off-white with reconstitution time not exceeding 90 seconds in Water for Injection at 25°C. Tablet compression is performed on a rotary tablet press (e.g., Fette 2090i) with compression force maintained at 8–14 kN to achieve a target hardness of 60–100 N and friability below 0.5% per USP 〈1216〉, with in-process controls triggered when individual tablet mass deviates beyond ±3.0% of target.

The finished dosage form is released under ICH Q6A specifications including identification by HPLC retention time match, assay by reversed-phase HPLC with UV detection at 215 nm against a certified reference standard, dissolution testing per USP 〈711〉 Apparatus 2 (paddle at 50 rpm, 900 mL of pH 6.8 phosphate buffer at 37.0 ± 0.5°C) with Q-value of not less than 80% dissolved in 30 minutes, content uniformity per USP 〈905〉 with acceptance value ≤ 15.0, and impurity profiling with individual unspecified impurities limited to ≤ 0.10% and total impurities ≤ 0.5% by area normalization. The prodrug is registered under a Type II Drug Master File with the US FDA, and the sodium counterion stoichiometry is controlled to a molar ratio of 1.00:1.00 ± 0.02 as determined by flame photometry against a sodium standard solution traceable to NIST SRM 3152a.

When Phosphonate Prodrug Intermediates Are Subjected to Lyophilization Cycle Development for Parenteral Finished Products

In the preparation of a sterile lyophilized powder for injection containing the hydrolytically sensitive sodium carboxylate-phosphonate diester, the aqueous formulation solution prior to lyophilization is adjusted to pH 6.0–7.0 with dilute sodium hydroxide or hydrochloric acid and filtered through a 0.22 µm sterilizing-grade PVDF membrane filter; the compound is dissolved at a concentration equivalent to 10 mg/mL of the free acid with mannitol as a bulking agent at 4.0% w/v. The thermal characterization by modulated differential scanning calorimetry per ASTM E2716-09 determines the collapse temperature (Tc) at approximately −28°C and the glass transition temperature of the maximally freeze-concentrated solute (Tg') at −32°C, constraining primary drying shelf temperature to ≤ −35°C with chamber pressure at 50–80 mTorr. The lyophilization cycle extends primary drying for 48–72 hours dependent on fill volume (5 mL in 10 mL Type I glass vials) and secondary drying at +25°C for 12 hours achieves residual moisture below 0.3% by Karl Fischer titration per USP 〈921〉 Method Ia. The terminal sterilized product must demonstrate sterility per USP 〈71〉, bacterial endotoxins ≤ 0.50 EU/mg per USP 〈85〉, and particulate matter compliance with USP 〈788〉 for small-volume parenterals. Reconstitution time from the lyophilized cake is verified at 25°C and 5°C storage conditions, with the refrigerated product exhibiting reconstitution times extended to 120 seconds due to increased cake resistance.

Stability-Indicating Method Validation and Forced Degradation in Chiral Purity Monitoring

The chiral prodrug's susceptibility to epimerization at the 2S,4S stereocenters under stressed conditions necessitates stability-indicating analytical methodology validated per ICH Q2(R1) with specificity demonstrated against all potential diastereomers, including the (2R,4S), (2S,4R), and (2R,4R) isomers which are synthesized as reference markers and spiked at the 0.1% reporting threshold. Forced degradation studies conducted at 60°C and 75% RH for 14 days in open petri dishes reveal primary degradation via ester hydrolysis of the propanoyloxy moiety to yield the intermediate hydroxymethyl phosphonate, with epimerization at C-2 observed to increase from 0.05% to 1.8% under alkaline conditions (0.1 N NaOH, 40°C, 4 hours) while the C-4 cyclohexyl-substituted stereocenter remains configurationally stable under all stress conditions except prolonged exposure to 3% H₂O₂ at 50°C, where oxidative degradation generates the corresponding pyrrolidine N-oxide with retention of the C-4 configuration. The HPLC method employs a Chiralcel OJ-RH column (150 × 4.6 mm, 5 µm) with a mobile phase of pH 2.0 phosphate buffer/acetonitrile (60:40 v/v) at a flow rate of 0.8 mL/min and column temperature controlled at 30°C, achieving resolution between the (2S,4S) and (2R,4S) diastereomers of not less than 2.0 with a limit of quantitation of 0.05 µg/mL (signal-to-noise ratio ≥ 10:1). Photostability testing per ICH Q1B Option 1 exposes the solid sodium salt to not less than 1.2 million lux-hours of visible light and 200 W·h/m² of near-ultraviolet radiation, revealing no photodegradation above the 0.05% reporting threshold, confirming that light-protective primary packaging is not required for the bulk sodium salt.

Excipient Compatibility Screening and Drug-Excipient Interaction Thresholds During Formulation Development

Binary mixture compatibility studies, conducted by blending the sodium salt with selected pharmaceutical excipients in 1:1 w/w ratio and storing at 40°C/75% RH in sealed glass vials for 4 weeks with sampling at 0, 1, 2, and 4 weeks, detect incompatibility with lactose monohydrate (Maillard reaction between the secondary amine of the pyrrolidine ring and reducing sugar aldehyde functionality) manifested as 1.2% total degradation products and visible brown discoloration by week 2. Microcrystalline cellulose (Avicel PH-101) and pregelatinized starch (Starch 1500) show no degradation products above 0.1% over the full 4-week period, while croscarmellose sodium induces a 0.3% increase in the hydrolysis product attributed to residual moisture in the superdisintegrant. Magnesium stearate, when blended at 1.0% w/w of total formulation mass with mixing time not exceeding 3 minutes in a V-blender at 25 rpm, shows no adverse effect on dissolution rate, but extended blending to 15 minutes results in delayed disintegration (> 10 minutes in water at 37°C) due to hydrophobic film coating of granule surfaces. The compatibility data are incorporated into a design-of-experiments formulation optimization using a D-optimal mixture design with four factors (filler ratio, disintegrant level, binder level, and lubricant mixing time) evaluated against responses of dissolution efficiency at 15 minutes, tablet hardness, and total impurities after 3-month accelerated stability at 40°C/75% RH.

Does the sodium counterion influence hygroscopicity and powder flow during roller compaction for continuous manufacturing?

Dynamic vapor sorption analysis of the sodium salt form at 25°C across a relative humidity ramp from 0% to 90% RH reveals a critical humidity threshold at 55% RH, above which water uptake exceeds 1.5% w/w and powder cohesion forces measured by shear cell testing on a Schulze RST-XS ring shear tester increase from a flow function coefficient (ffc) of 6.8 at 40% RH to ffc 2.3 at 65% RH, classifying the powder as cohesive and requiring active humidity control in the processing suite maintained at 30–40% RH. Roller compaction for dry granulation is performed on a Gerteis Mini-Pactor with roll force of 8–12 kN/cm, roll speed 3–5 rpm, and screen size 1.25 mm for the granulation step, producing ribbons with solid fraction between 0.65 and 0.75 as determined by envelope density measurement by mercury intrusion porosimetry. The fines fraction passing a 125 µm sieve is controlled below 25% to maintain granule flowability suitable for continuous blending and tablet compression at line speeds up to 100,000 tablets per hour. The process is monitored by near-infrared spectroscopy with a probe installed post-mill, calibrated against HPLC assay values, with a model standard error of calibration of 0.8% w/w and a correlation coefficient R² > 0.99 for the active content range 2.0–6.0% w/w.

The hygroscopicity profile of the sodium salt when compared to the free acid form demonstrates a 3-fold increase in equilibrium moisture content at 60% RH, attributable to the formation of a crystalline dihydrate confirmed by X-ray powder diffraction patterns exhibiting new reflections at 2θ = 12.4°, 18.7°, and 24.3° (Cu Kα radiation, λ = 1.5406 Å). Storage of bulk substance in double polyethylene bags inside fibre drums with desiccant (silica gel with cobalt chloride indicator) maintains water content below 0.5% for 24 months under ICH Q1A(R2) long-term conditions of 25°C/60% RH. When subjected to the continuous direct compression process without a granulation intermediate, the powder blend containing 2.8 wt% sodium salt, 0.75 wt% magnesium stearate, 3.0 wt% croscarmellose sodium, and microcrystalline cellulose to 100% exhibits segregation tendencies quantified by blend uniformity analysis where the acceptance value at 10-minute sampling intervals exceeds 15.0 when blend hold time exceeds 4 hours at ambient humidity, attributed to differential settling of the higher-density sodium salt particles (true density 1.28 g/cm³ by helium pycnometry) relative to the cellulose matrix (true density 1.55 g/cm³).

What Limits the Use of This Phosphonate Prodrug in Fixed-Dose Combination Products Containing Divalent Cation Excipients?

The phosphonate diester functional group demonstrates chelation affinity for divalent metal cations, particularly Ca²⁺ and Mg²⁺, forming poorly soluble coordination complexes that reduce dissolution rate and bioavailability when formulated in fixed-dose combinations with calcium carbonate, magnesium hydroxide, or dibasic calcium phosphate dihydrate filler. In dissolution testing conducted per USP 〈711〉 in 0.01 N HCl (simulated gastric fluid without pepsin) at 37°C, the presence of calcium carbonate at a weight ratio of 10:1 (excipient:drug) reduces the percentage dissolved at 30 minutes from 92% to 34%, with the precipitate identified by energy-dispersive X-ray spectroscopy as a 2:1 calcium-phosphonate complex with solubility product estimated at 3.2 × 10⁻¹² in pH 1.2 medium. Formulation of a bilayer tablet using an inert barrier layer of hydroxypropyl cellulose (Klucel EXF, 50 mg per tablet) between the prodrug granulation and the antacid-containing layer partially mitigates the interaction, achieving dissolution of 78% at 30 minutes, though storage at 40°C/75% RH for 3 months results in barrier layer penetration by calcium ions migrating through the moisture-plasticized polymer matrix as confirmed by cross-sectional scanning electron microscopy with elemental mapping. Regulatory submissions for the fixed-dose combination require inclusion of in vivo bioequivalence data per 21 CFR 320.24(b) demonstrating 90% confidence intervals for Cmax and AUC falling within 80.00–125.00% of the free combination reference, and any co-formulation with metal-containing APIs (e.g., ferrous fumarate, zinc gluconate) or excipients formally triggers the requirement for an in vitro dissolution study in biorelevant media (FaSSIF and FeSSIF) at the development stage under Ph. Eur. 5.17.1 recommendations for fixed-dose combinations.

An additional constraint arises from the propanoyloxy ester's susceptibility to transesterification in the presence of alcoholic functional groups present in certain co-formulated drugs or excipients, wherein polyethylene glycol 400 used as a solubilizer for a companion API generates the PEG-propanoate ester at levels of 0.08% after 7 days at 50°C as detected by LC-MS with electrospray ionization in positive ion mode monitoring the [M+Na]⁺ adduct at m/z corresponding to the monoester adduct. This degradation pathway is controlled by maintaining the microenvironmental pH below 5.5 through incorporation of fumaric acid (0.5% w/w) as an acidifying agent and by replacing PEG-containing excipients with polyvinylpyrrolidone (Kollidon 30) or poloxamer 188 where solubilization of a poorly water-soluble co-API is required without nucleophilic hydroxyl functionality.

During technology transfer from laboratory to pilot scale (50 kg batch size), the addition sequence of the sodium salt to the granulator bowl relative to other blend components was identified as a critical process parameter when batch records from three consecutive engineering batches revealed that addition of the sodium salt as the final component prior to wet massing resulted in content uniformity RSD of 7.8% (failing the 5.0% acceptance criterion), whereas sandwich loading of the sodium salt between two equal portions of the diluent blend with pre-blending for 2 minutes at impeller speed 300 rpm reduced the RSD to 2.1%. The validated process therefore specifies a defined order of addition with the sodium salt gravimetrically dispensed to ± 0.5% of target mass using calibrated Mettler Toledo balances with automatic printer documentation of each weighing operation for batch record traceability under 21 CFR Part 11 compliant electronic batch recording systems.

The terminal pyrogen control strategy for parenteral-grade material relies on depyrogenation of all product-contact equipment surfaces by dry heat at 250°C for 30 minutes per Ph. Eur. 5.1.1 methods of sterilisation, coupled with bioburden monitoring of the pre-sterilizing filtered solution at ≤ 10 CFU/100 mL immediately prior to lyophilizer loading. The sodium salt is not subjected to ethylene oxide sterilization due to potential alkylation of the pyrrolidine tertiary nitrogen and subsequent quaternary ammonium salt formation, which would alter the dissolution profile and potentially generate a genotoxic impurity requiring control to the threshold of toxicological concern of 1.5 µg/day under ICH M7(R2) for a treatment duration exceeding 10 years.

Preformulation Salt Screening and the Justification for Sodium as the Counterion in Solid-State Selection

A comprehensive salt screening study evaluating hydrochloride, mesylate, besylate, potassium, and sodium salt forms of the free acid phosphonate prodrug was conducted with characterization by differential scanning calorimetry (DSC) at a heating rate of 10°C/min under nitrogen purge at 50 mL/min from 25°C to 300°C, thermogravimetric analysis (TGA) at 10°C/min from ambient to 400°C, X-ray powder diffraction (XRPD) on a Bruker D8 Advance diffractometer with Cu Kα radiation over 2θ range 3–40° with step size 0.02° and scan speed 3 seconds per step, and dynamic vapor sorption with equilibrium criterion of mass change ≤ 0.002% per minute over 5 minutes. The hydrochloride salt exhibited a 14°C lower melting endotherm onset (DSC peak at 157°C versus 171°C for the sodium salt) and hygroscopicity leading to deliquescence at 68% RH with 12.5% w/w water uptake, disqualifying it from solid oral dosage development. The mesylate salt demonstrated the highest aqueous solubility (85 mg/mL at 25°C in unbuffered water) but generated methanesulfonic acid upon accelerated stability testing, causing autocatalytic hydrolysis of the phosphonate ester with 4.2% total degradation at 40°C/75% RH for 1 month. The potassium salt showed excellent thermal stability (DSC onset 183°C) but consistently formed a poorly filterable gelatinous precipitate upon aqueous dissolution at concentrations exceeding 50 mg/mL, attributed to liquid crystalline phase formation characterized by polarized light microscopy showing birefringent textures at the solvation interface.

The sodium salt was selected as the development candidate based on its crystallinity (distinct XRPD pattern with major diffraction peaks at 2θ = 8.7°, 14.2°, 17.6°, 20.9°, and 23.5°), acceptable aqueous solubility (38 mg/mL at 25°C, corresponding to approximately 7.6 times the target dose concentration for parenteral reconstitution), thermal stability to 165°C without decomposition, and intrinsic dissolution rate of 2.8 mg·min⁻¹·cm⁻² measured by rotating disk method in pH 6.8 phosphate buffer at 37°C with disk rotation speed of 200 rpm in a Distek 2100C dissolution apparatus. The equilibrium solubility product of the sodium salt is pH-dependent, increasing from 12 mg/mL at pH 4.0 to 52 mg/mL at pH 7.4, consistent with the ionization of the carboxylic acid moiety (pKa = 3.8 ± 0.1 determined by potentiometric titration in 0.15 M KCl at 25°C). The solid-state form is a non-hygroscopic crystalline anhydrate that does not form hydrates or solvates under any conditions tested, eliminating the risk of form conversion during wet granulation or aqueous film coating processes.

The manufacturing route for the sodium salt employs a final salt formation step in which the free acid phosphonate, isolated as an amorphous solid from the preparative chiral HPLC purification with acetonitrile/water (55:45 v/v) mobile phase containing 0.05% v/v trifluoroacetic acid as ion-pairing modifier, is dissolved in ethanol (5 volumes relative to free acid mass) at 40°C and neutralized with aqueous sodium hydroxide (1.0 M, 0.98–1.02 molar equivalents) added dropwise over 30 minutes with pH endpoint control at 7.2 ± 0.1. The solution is polish filtered through a 0.45 µm polypropylene filter, and crystallization is induced by addition of n-heptane (8 volumes) at a controlled addition rate of 0.5 volumes per hour at 20–25°C with seed crystals (prepared by slurry method, 1.0% w/w of theoretical yield, mean particle size 15–25 µm by laser diffraction) added at the onset of supersaturation as detected by in-situ focused beam reflectance measurement (FBRM) monitoring chord length distribution shifts. The crystallized sodium salt is isolated by centrifugation, washed with cold ethanol/n-heptane (1:3 v/v, 2 × 2 volumes), and dried under vacuum (< 10 mbar) at 40°C for 16 hours to yield a white crystalline powder with residual ethanol content < 500 ppm and residual n-heptane < 300 ppm by headspace GC-FID with flame ionization detection per Ph. Eur. 2.4.24.

The particle size distribution of the final sodium salt is controlled by milling through a 0.5 mm screen in a Fitzpatrick L1A hammer mill with knives forward at 6,000 rpm, targeting a volume mean diameter (D[4,3]) of 35–65 µm, Dv(10) ≥ 10 µm, and Dv(90) ≤ 120 µm as determined by laser diffraction in accordance with ISO 13320:2020 particle size analysis using a Malvern Mastersizer 3000 with Aero S dry dispersion unit at 2.0 bar dispersion pressure and obscuration range 0.5–6.0%. This particle size specification is justified by dissolution performance: micronization below Dv(90) 50 µm increases the dissolution rate but also increases electrostatic charging and powder adhesion to metal contact surfaces, while particles exceeding 150 µm exhibit incomplete dissolution in the 30-minute specification window.