Alpha Ketoanalogue to Isoleucine, Calcium Salt is supplied as the anhydrous calcium salt of (3S)-3-methyl-2-oxopentanoic acid; the synonym calcium 3-methyl-2-oxopentanoate is also used in technical documentation. Stoichiometrically the compound is represented as Ca(C6H9O3)2, with a formula weight of 298.35 g/mol for the anhydrous material. The product model designation α-KIC-Ca(Ph) identifies the pharmaceutical-grade API specified for tablet, capsule, granule, and injectable manufacture, including oral and injectable administration. In this molecule the α-amino group of L-isoleucine is replaced by an oxygen atom, forming a 2-oxo acid; the β-carbon chiral centre is retained as the (3S) configuration, while the α-carbon is not chiral. The result is a nitrogen-free carbon skeleton that can be transaminated in mammalian metabolism to L-isoleucine, which is the basis for its classification as a keto analogue rather than a simple degradation product. Manufacture is performed under ICH Q7 and EudraLex Volume 4 Part II GMP conditions, with batch release aligned to the general monograph for substances for pharmaceutical use and to ICH Q3D for elemental impurities and ICH Q3C for residual solvents. Because the compound contains no amino group, it does not show pH-dependent protonation in the same manner as amino acid salts; the dissolved calcium salt behaves as a weak organic acid salt and contributes calcium ion to the formulation environment.

Two physical grades are available under the same model designation: an oral solid-dosage grade with D90 controlled at ≤150 µm and an injectable grade with D90 controlled at ≤20 µm; both grades are milled under nitrogen and sieved through 316L stainless steel mesh. The oral grade is suitable for direct compression and dry granulation; the injectable grade is suited to sterile filtration and lyophilization. Analytical differentiation of the two grades is performed by laser diffraction according to ISO 13320 and by specific surface area measurement according to ISO 9277; the injectable grade typically has a specific surface area above 1.5 m²/g, whereas the oral grade is below 0.8 m²/g. This surface-area difference affects dissolution: the oral grade dissolves to 90% in water at 37 °C within 15 min using USP apparatus II at 50 rpm, while the injectable grade dissolves within 5 min. The dissolution test is not a release test but an informational test for formulation development.

Release Specification Framework Linked to Ph. Eur. and ICH

The following table summarizes representative batch-release targets for α-KIC-Ca(Ph). Acceptance criteria for a specific commercial dosage form should be confirmed against the open part of the active substance master file and the applicable marketing authorisation.

On a pilot-scale solid-dosage line, α-KIC-Ca(Ph) is first passed through a conical mill fitted with a 1.0 mm screen before blending with microcrystalline cellulose, mannitol, crospovidone, and magnesium stearate in a 600 L bin blender. The material is not milled with uncoated steel equipment, because trace iron promotes oxidative decarboxylation of the α-keto acid; equipment contact parts are specified as 316L stainless steel with electropolished surfaces. Direct compression on a rotary tablet press at main compression force of 10–18 kN and turret speed 25 rpm yields tablets with tensile strength above 1.5 MPa and friability below 1.0% when the formulation contains at least 25 wt% microcrystalline cellulose. If the calcium salt fraction exceeds 70 wt%, the powder blend becomes hygroscopic; pre-drying of excipients and processing at relative humidity ≤ 40% are required to prevent picking and sticking. For capsule manufacture, the milled grade can be metered with a dosator-type capsule filler at speed settings that maintain fill weight variation below 2%; however, the Hausner ratio between 1.20 and 1.40 places the material at the borderline between fair and passable flow, and external lubrication is preferred when direct compression forces exceed 15 kN. Wet granulation with water is feasible only when the granulation bowl temperature is kept below 35 °C; extended wet massing above that temperature increases conversion to the hydrated form and raises tablet friability after drying. For granules, fluid-bed drying with inlet air temperature 45–55 °C and dew point below 8 °C is preferred because the particles are temperature-sensitive and can agglomerate in tray drying if bed depth exceeds 5 cm.

Batch-to-batch variance in compaction is controlled by tracking the ratio of fine to coarse particles between 10 µm and 50 µm; if the fraction below 10 µm exceeds 35%, tablet capping occurs on the rotary press. If the fraction above 150 µm exceeds 20%, capsule fill weight variation increases above 2%. The recommended control strategy uses online near-infrared spectroscopy to monitor moisture content at the granulator discharge, with a target loss-on-drying of 1.5–2.5% for wet-granulated material before compression.

What Changes When the Calcium Salt Is Used in Injectable Formulations?

For injectable use, the same active substance is manufactured under enhanced microbial control. Bulk aqueous solutions at 50 mg/mL are prepared in water for injection at 20–25 °C; solution pH is adjusted only with dilute acetic acid or sodium hydroxide, because the calcium salt precipitates when pH exceeds 7.5 in the presence of bicarbonate. Published data for this specific configuration is limited, particularly for long-term soluble aggregate formation; therefore terminal sterilization and container-closure compatibility should be evaluated on a formulation-specific basis. Terminal autoclaving at 121 °C for 15 min can produce a pH shift of 0.2–0.5 units because trace hydrolysis of the 2-oxo acid generates acidic degradation products; the resulting pH must remain above 6.0 to avoid injection-site irritation, but below 7.5 to prevent calcium carbonate precipitation. Sterile filtration through a 0.22 µm polyethersulfone membrane is performed after addition of tonicity agents and after pH adjustment; this sequence prevents supersaturation at the filter boundary. Phosphate-buffered vehicles are contraindicated: precipitation of calcium hydrogen phosphate occurs when free calcium ion exceeds 5 mmol/L at neutral pH, and the newly formed particulate matter may pass the compendial light-obscuration test only until the solution is cooled to 2–8 °C, at which point crystal growth becomes visible. For injectable filling, the final bulk solution is filled into Type I glass vials under nitrogen, because dissolved oxygen accelerates oxidative decarboxylation to 2-methylbutanal and carbon dioxide, which changes pH and generates gas pressure in the headspace.

For lyophilized injectable product, the bulk solution is filled into Type I glass vials and freeze-dried with a primary drying shelf temperature of −20 °C and chamber pressure 50–100 µbar for 24 h; collapse occurs if the product temperature exceeds −10 °C because the calcium salt lowers the glass transition temperature of the amorphous phase. Mannitol is preferred as a crystallizing bulking agent over glycine, because glycine amine groups may form Schiff-base adducts with the α-keto group under freeze-drying stress at pH above 7.0. After lyophilization, moisture content should be below 2.0% by Karl Fischer titration.

Direct substitution of L-isoleucine with α-KIC-Ca(Ph) in oral or parenteral formulas is not a molar equivalent replacement. One mole of L-isoleucine has a formula weight of 131.17 g/mol and contains 10.7% w/w nitrogen; one mole of the keto analogue calcium salt has no α-amino nitrogen and carries one calcium ion per two keto acid equivalents. The calcium content of the anhydrous salt is 13.4% w/w, equivalent to 1.34 g calcium per 10 g of α-KIC-Ca(Ph). Calcium load matters when the product is co-formulated with calcium carbonate or calcium acetate as phosphate binders; the total elemental calcium content from all sources should be calculated against the intended daily dose and the phosphate-binder prescription. Compared with the sodium salt of 3-methyl-2-oxopentanoic acid, which has a formula weight of 152.13 g/mol and contributes 1.51 g sodium per 10 g, the calcium salt introduces no sodium load and is less deliquescent under humidity stress at 40 °C/75% RH for 7 days. Compared with L-isoleucine, the calcium salt is not a direct nitrogen donor and is not interchangeable for acute parenteral nutrition; the keto analogue must be transaminated by the liver, and the rate of conversion depends on nitrogen availability and branched-chain amino acid aminotransferase activity, which is not guaranteed in severe hepatic insufficiency. The free acid form is acidic and hygroscopic and is not used as a dosage-form API because it forms sticky films under compression; the calcium salt is the crystalline form suitable for tablet and capsule operations.

When the API Is Blended with Phosphate-Containing Matrices

Blends containing α-KIC-Ca(Ph) and dibasic calcium phosphate dihydrate or calcium hydrogen phosphate filler at phosphate excipient content above 10 wt% are not stable under accelerated storage. At 40 °C/75% RH in sealed HDPE containers, surface ion exchange produces calcium hydrogen phosphate and free 3-methyl-2-oxopentanoic acid; after 3 months the tablet hardness declines by more than 20% and the dissolution profile changes because the free acid is more soluble than the calcium salt. Replacement of phosphate fillers with mannitol, microcrystalline cellulose, or isomalt removes the incompatibility. For oral powders and granules, storage in aluminium-lined bags at ≤25 °C and ≤40% RH is the upper boundary for unopened containers; once opened, material should be used within 30 days if ambient relative humidity exceeds 60%. Oxidative degradation is accelerated by strong oxidizing agents and by exposure to light; an amber glass or opaque HDPE container is specified. The material is not to be milled using uncoated steel equipment, and nitrogen blanketing is recommended during any operation that generates fresh particle surfaces, such as jet milling or micronization for injectable-grade material.