Alpha ketoanalogue to phenylalanine, calcium salt is supplied as a pharma-grade active pharmaceutical ingredient under the model designation AKPC-PH-01, with oral subgrade AKPC-PH-01-O and injectable subgrade AKPC-PH-01-I. The substance is the calcium salt of 2-oxo-3-phenylpropanoic acid, Ca(C₉H₇O₃)₂, with relative molecular mass 366.38 g/mol. The CAS registry number commonly cited for the hydrated calcium salt is 51828-95-6. The molecule is the nitrogen-free ketoacid precursor of L-phenylalanine and is used in chronic kidney disease regimens to provide phenylalanine without adding non-protein nitrogen. The calcium salt form is selected over sodium salt or free-acid forms for oral and parenteral products where sodium intake is restricted and where acid-neutralizing capacity is required. The API is released as a white to off-white crystalline powder; typical bulk density for the oral grade is controlled between 0.40 g/mL and 0.60 g/mL, and the material is qualified for tablet, capsule, granule, and injectable solution manufacture under current good manufacturing practice.

Which release parameters are applied to oral and injectable grades?

Release specifications are established in accordance with Ph. Eur. general methods and ICH Q2(R1) validation principles. The oral and injectable subgrades share chemical purity requirements but differ in microbiological and particulate controls. Table 1 summarizes the release profile.

All numerical limits are release criteria; in-process controls are established for particle size distribution and bulk density because these properties are not adjusted after final crystallization.

Chromatographic impurity control and degradation chemistry

Impurity profiling for the phenylalanine ketoacid is performed by reversed-phase high-performance liquid chromatography on a C18 column with dimensions 250 mm × 4.6 mm, particle size 5 µm, maintained at 30 °C. The mobile phase consists of phosphate buffer at pH 3.0 and acetonitrile in gradient mode, with UV detection at 210 nm and 257 nm. The aromatic ring provides a chromophore at 257 nm that permits selective detection of phenylpyruvate-related impurities without derivatization. The method is validated for specificity, linearity over 0.05–2.0 µg/mL, accuracy, precision, and quantitation limit per ICH Q2(R1). The primary oxidative degradation product is benzaldehyde, and acid hydrolysis releases phenylpyruvic acid. The single unknown impurity limit is 0.10%; total degradation products are controlled below 1.0% for the oral subgrade and 0.8% for the injectable subgrade. Mass balance in forced degradation studies is required to be 95.0–105.0%. Peroxide contamination from excipients such as povidone or crospovidone can accelerate oxidative degradation; therefore peroxide-controlled excipient grades with peroxide number below 400 ppm are specified for tablet and capsule formulations.

Dry blending of AKPC-PH-01-O with microcrystalline cellulose and dicalcium phosphate requires control of bulk density between 0.45 g/mL and 0.65 g/mL and tapped density below 0.85 g/mL. Laser diffraction per ISO 13320 controls D90 at ≤150 µm for capsule filling and ≤75 µm for direct compression; a bimodal distribution outside these limits can cause segregation in low-dose tablet blends. The powder is moderately cohesive, and flow through a 2 mm funnel orifice is not reliable without a glidant. Addition of 0.5–1.5% colloidal silicon dioxide and 1.0–2.0% magnesium stearate achieves a Carr index below 20 in direct compression formulations. Wet granulation with purified water is possible when the binder is hypromellose, but the granulation fluid is maintained at pH 6.5–7.5; acidic granulating fluid below pH 4.0 converts the salt to phenylpyruvic acid, which increases stickiness and causes picking during tablet compression. Fluid-bed drying inlet air temperature is limited to 60 °C; thermal degradation above this temperature can elevate benzaldehyde content. Loss on drying at 105 °C for 3 h is required to be ≤0.5% before dry granulation or roller compaction.

When injectable-grade material is specified, which controls change?

The injectable subgrade AKPC-PH-01-I is controlled for bacterial endotoxins, sub-visible particles, and dissolution homogeneity. Bacterial endotoxin limit is 0.25 EU/mg by limulus amebocyte lysate test per Ph. Eur. 2.6.14 or USP <85>. The API is dissolved and sterile-filtered through a 0.22 µm membrane; therefore the bulk API is not required to be sterile but must have low bioburden and meet endotoxin limits. Reconstituted solution particulate matter is controlled per Ph. Eur. 2.9.19 with not more than 6000 particles ≥10 µm and 600 particles ≥25 µm per container. The calcium salt has pH-dependent aqueous solubility. At 25 °C in unbuffered water, solubility is below 20 mg/mL; adjustment to pH 7.0–7.5 with sodium hydroxide or tromethamine raises solubility above 50 mg/mL. Phosphate buffers are avoided because calcium phosphate precipitates within the neutral pH range. Injectable formulations are commonly prepared in citrate or acetate buffer systems to maintain calcium solubility. Terminal sterilization at 121 °C for 15 min is applicable only if solution pH remains above 6.0; heat sterilization below pH 4.0 accelerates decarboxylation and should not be used.

Compared with calcium salts of the branched-chain ketoanalogues such as ketoleucine and ketoisoleucine, the phenylalanine analogue retains an aromatic side chain that contributes hydrophobic character and alters dissolution. The phenyl ring gives a UV maximum near 257 nm, enabling HPLC quantitation without pre-column derivatization; the branched-chain ketoacid calcium salts are typically monitored at low wavelengths near 210 nm or require derivatization for adequate sensitivity. The aromatic ketoacid is more prone to oxidative ring-opening under peroxide stress, so antioxidant-free and peroxide-controlled excipients are specified. In oral combination products, the phenylalanine ketoanalogue is blended with other ketoanalogue calcium salts and selected amino acids in defined stoichiometric ratios; assay variation outside 98.0–102.0% shifts the nitrogen-sparing ratio and may alter plasma amino acid profiles. The product is not interchangeable with L-phenylalanine on a milligram basis because the ketoacid form requires transamination and does not contribute to nitrogen load; this property is the basis for its use in protein-restricted renal diets.

The calcium salt form differs from a sodium salt or free acid by its lower hygroscopicity and reduced sodium load. Sodium salt forms, where used, can contribute to sodium burden; the calcium salt is preferred in renal patients. However, the calcium salt is less water-soluble than the sodium salt, so injectable grades require pH adjustment or complexation for high-concentration solutions.

Solvent and elemental impurity limits follow ICH Q3C and ICH Q3D Option 1

Residual solvents are controlled by headspace gas chromatography per Ph. Eur. 2.4.24. Class 1 solvents are not detected, Class 2 solvents are maintained below the corresponding ICH Q3C Option 1 limits, and Class 3 solvents are limited to 0.5% w/w each. Elemental impurities are assessed by inductively coupled plasma mass spectrometry per Ph. Eur. 2.4.20 or USP <233>. Limits for cadmium, lead, arsenic, mercury, cobalt, vanadium, and nickel are derived from ICH Q3D permitted daily exposure values and the maximum intended daily dose of the API; the finished product manufacturer calculates the final limit from the dosing table. No universal concentration limit is assigned without the maximum daily dose. For the injectable subgrade, parenteral limits apply and require lower nickel and cobalt controls because of parenteral bioavailability assumptions.

Defining direct compression feasibility through moisture uptake and flow constraints

Dynamic vapour sorption shows mass increase below 0.5% at 25 °C and 60% RH for AKPC-PH-01. Storage above 75% RH increases moisture and reduces flow; material should be kept in double polyethylene bags within a sealed container with desiccant. Re-drying at 50 °C under vacuum at -0.08 MPa for 8–12 h is effective if moisture exceeds 0.5%. Tablets produced by direct compression on a rotary press at main compression force 10–18 kN require target hardness 50–80 N and friability ≤1.0% per Ph. Eur. 2.9.7. Capsule filling with a dosator or tamping pin machine is limited by flow index; the addition of 0.5% colloidal silicon dioxide is recommended to achieve mass variation below 3.0% for size 3 hard gelatin capsules. Roller compaction is preferred over slugging because of reduced dust generation and improved content uniformity; ribbon density is controlled between 1.1 g/mL and 1.4 g/mL, and milled granules are sized through a 800 µm screen.

For oral solid dosage, particle size distribution and bulk density are critical quality attributes because they control flow, blend uniformity, and dissolution. The oral grade is milled and classified to D90 ≤150 µm. For injectable solution manufacture, particle size is less critical because the API is dissolved before filtration; however, the injectable grade is subjected to additional endotoxin and bioburden reduction steps. The injectable grade is crystallized from water-for-injection-compatible solvents and dried in a controlled environment with local protection for final packing.

Stability data support storage at 15–25 °C in well-closed containers protected from light. Photostability testing per ICH Q1B indicates the drug substance is sensitive to light; amber glass or opaque polyethylene containers are specified. Bulk retest period is 24 months when stored below 25 °C and 60% RH. In-use stability for injectable solution is formulation-dependent; buffered solutions at pH 6.5–7.5 are typically used within 24 h at 2–8 °C unless terminal sterilization and closed-container stability data support longer hold times.