Supplied as a white to off-white crystalline powder, the chiral active pharmaceutical ingredient designated (4S)-3-[5-(4-fluorophenyl)-1,5-dioxopentyl]-4-phenyl-2-oxazolidinone (C20H18FNO4; monoisotopic mass 355.12199 Da; nominal molecular weight 355.36 g/mol) is released under a pharma-grade specification covering oral tablet, capsule, granule, and injectable formulation. The molecule contains a 2-oxazolidinone ring substituted at the 4-position with phenyl and N-acylated by a 5-(4-fluorophenyl)-1,5-dioxopentyl moiety. Unlike unsubstituted 4-phenyl-2-oxazolidinone, the additional 4-fluorophenyl ketone side chain increases the number of hydrogen-bond acceptors and alters chromatographic retention of polar degradation products on reversed-phase HPLC columns. Because the API is intended for both oral and injectable use, the release program includes solid-state identity, chemical purity, chiral purity, residual solvent, elemental impurity, water, and microbiological controls. The material is considered a non-sterile active ingredient; terminal sterilization or aseptic filtration is applied at the drug-product stage.

The model designation for this material is the single (4S)-configured oxazolidin-2-one enantiomer. It should not be interchanged with the corresponding (4R)-enantiomer, the racemic mixture, the non-fluorinated phenyl analog, or the 5-hydroxy reduction product without full comparative validation. The 4-fluorophenyl ketone side chain distinguishes the product from common oxazolidinone chiral auxiliaries and from 4-phenyl-2-oxazolidinone itself in both molecular weight and polarity. Solid-state variability is monitored by X-ray powder diffraction, differential scanning calorimetry, and hot-stage microscopy. If multiple polymorphs are present in development lots, the API is classified according to ICH Q6A and the most thermodynamically stable form is recommended for tablet, granule, capsule, and injectable development unless solubility performance requires a metastable form. Storage in double low-density polyethylene liners inside aluminium foil bags under nitrogen is used to limit moisture uptake and oxidative degradation of the fluorophenyl ketone.

What release controls are required when one API grade is supplied to both oral and parenteral lines?

Because no dedicated pharmacopoeial monograph exists for this single oxazolidinone, the specification is derived from ICH Q6A decision-tree principles for new drug substances and from general chapters applicable to neutral aromatic APIs. Identification is confirmed by mid-infrared absorption against a qualified reference standard and by HPLC retention time in the assay procedure. Enantiomeric purity is controlled because the 4S configuration is chiral; a chiral HPLC method using amylose- or cellulose-based stationary phase separates the undesired (4R)-enantiomer from the active (4S)-form. The routine release limit for enantiomeric excess is ≥99.0%, with any single unidentified impurity limited to ≤0.10% and total related substances limited to ≤1.0%. Residual solvent compliance follows ICH Q3C Option 2; water content is determined by USP <921>; residue on ignition is controlled by USP <281>; elemental impurities are evaluated against ICH Q3D relative to the intended maximum daily dose. For injectable use, a bacterial endotoxin limit is established in accordance with USP <85> and the maximum endotoxin burden is calculated from the maximum bolus dose.

The specification is a baseline control set; injectable grades may require lower bioburden, sub-visible particle counting in the final drug product per USP <788>, and reduced water content to limit hydrolysis of the oxazolidinone ring during terminal sterilization. Because the N-acyl carbonyl can undergo hydrolytic attack in aqueous solution, the API is not held as a long-term aqueous solution. Instead, it is dissolved in a suitable co-solvent system and sterile-filtered through 0.22 µm polyether sulfone or polyvinylidene fluoride membranes. Heat-labile behavior should be confirmed by forced degradation at 40 °C, 60 °C, and 80 °C in pH 1.2, 4.5, 6.8, and 7.4 buffers before selecting a terminal sterilization cycle. Method validation for the HPLC and chiral procedures follows ICH Q2(R2) with specificity, linearity, accuracy, precision, and range determined on spiked stressed samples.

At the point of solid oral formulation, particle-size distribution is the primary route-dependent control. Direct compression of high-dose tablets on a rotary press requires a d90 below 150 µm and a flow function coefficient sufficient to prevent feed-frame segregation; if the API lot contains acicular crystals, the formulator should first pass the material through a comil equipped with a 0.5 mm screen. Wet granulation with purified water and a binder such as povidone or hypromellose may be preferred when the API content exceeds 30% w/w, because oxazolidinone-like neutral molecules frequently display poor powder flow at high loadings. Published data for this specific configuration is limited; therefore, preformulation studies should include bulk density, tapped density, Carr index, and shear cell measurements on each batch before selecting a process path. Dry granulation by roller compaction is suitable when the drug load is high and the compacted ribbon density is between 1.10 g/cm³ and 1.30 g/cm³; ribbon milling should be performed with a low-energy rotor to limit generation of fines below 50 µm.

Process-transfer limits that govern direct compression and wet granulation

For tablet manufacture, the API is blended with a diluent selected from microcrystalline cellulose, lactose monohydrate, or dibasic calcium phosphate dihydrate. The choice is determined by drug-loading, disintegration, and compatibility with the fluorophenyl ketone. A planetary mixer or high-shear granulator is used when wet granulation is required; a high-shear granulator with impeller tip speed below 5 m/s is preferred to avoid over-wetting and localized hydrolysis during binder addition. Granule drying is carried out in a fluid-bed dryer with inlet air temperature not exceeding 60 °C for non-aqueous granulation; for aqueous granulation, the moisture endpoint is controlled by loss-on-drying rather than time because the oxazolidinone ring may be sensitive to prolonged humid conditions above 60% RH. The dried granule is milled through a 1.0 mm screen and lubricated with 0.5–1.0% w/w magnesium stearate. Over-lubrication should be avoided because magnesium stearate can reduce tablet tensile strength and prolong disintegration. Tablet compression is performed on a rotary tablet press with pre-compression force between 4 kN and 8 kN and main compression force between 12 kN and 20 kN, depending on tooling diameter and tablet hardness target. Hardness is monitored with USP <1217> as guidance, and friability is tested according to USP <1216> with a target below 1.0%.

For capsule filling, the API may be dry-milled or granulated before filling on a dosator or tamping-pin capsule machine. Low-density flocculent API should be densified by slugging or roller compaction to improve weight uniformity on high-speed capsule equipment. The final powder blend is filled into hard gelatin or hypromellose capsules at relative humidity below 50% RH to prevent cross-linking of gelatin and moisture uptake by the API. Granule formulations intended for sachet or suspension can be prepared by wet granulation followed by fluid-bed drying; the granule fraction between 180 µm and 710 µm is often retained to achieve acceptable flow and dispersibility. Dissolution performance is evaluated in 900 mL aqueous media containing 0.1 M hydrochloric acid, pH 4.5 acetate buffer, pH 6.8 phosphate buffer, and pH 7.4 phosphate buffer, with agitation at 50 rpm or 75 rpm using USP <711> Apparatus II. The dissolution criterion is selected from the discriminatory ability of the method; if the compound exhibits low aqueous solubility, a surfactant such as sodium lauryl sulfate may be added at a validated concentration.

Injectable formulation requires a separate solubility and stability assessment. The API is dissolved in a co-solvent system such as propylene glycol, polyethylene glycol, or a buffered saline mixture containing a safe amount of an organic co-solvent. The solution is protected from light if the fluorophenyl ketone undergoes photo-oxidation during accelerated studies. The final solution is filtered through 0.22 µm sterile membrane filters and filled into vials under aseptic conditions. If terminal sterilization is desired, steam sterilization at 121 °C for 15 min may be evaluated, but the oxazolidinone ring must be shown to remain within specification after the cycle. For lyophilized formulations, the API is dissolved in a bulk solution and freeze-dried with a controlled primary drying shelf temperature below the collapse temperature of the formulation. Reconstitution volume, pH, and particulate matter are controlled using USP <788> and USP <790>; visible particles are unacceptable, and sub-visible counts must comply with the compendial limits for large-volume or small-volume injections. The final injectable dose is tested for sterility by USP <71>, bacterial endotoxins by USP <85>, and container closure integrity by dye ingress or vacuum decay.

When the 4-fluorophenyl ketone side chain is compared with related oxazolidinone materials

The principal difference between this product and 4-phenyl-2-oxazolidinone is the presence of the N-acyl chain terminating in a 4-fluorophenyl ketone. That side chain increases molecular mass from 163.17 g/mol for 4-phenyl-2-oxazolidinone to 355.36 g/mol and introduces a conjugated aryl ketone chromophore. HPLC detection at 254 nm is therefore stronger for this derivative than for the unacylated precursor. Compared with the corresponding racemic mixture, this product is controlled as the (4S)-enantiomer; the racemate and the (4R)-enantiomer are not considered interchangeable in pharmaceutical use without chiral stability data showing no inversion. Compared with the 5-hydroxy reduction product (4S)-3-[(5S)-5-(4-fluorophenyl)-5-hydroxypentanoyl]-4-phenyl-2-oxazolidinone, the ketone-containing product is more electrophilic at the side-chain carbonyl and may have different hydrolytic stability in pH 7.4 buffer. The two molecules also differ in retention time, mass fragmentation, and infrared carbonyl stretching frequency.

The second table gives the route-readiness comparison for solid oral and injectable use. It is a compliance matrix rather than a release specification; the acceptance criteria are qualitative design limits unless otherwise indicated.

In forced degradation studies, the main degradation pathways to monitor are hydrolysis of the oxazolidinone ring, cleavage of the N-acyl bond, and oxidation of the fluorophenyl ketone. Hydrolysis may be accelerated under alkaline pH conditions above pH 8.0; oxidation may be accelerated in the presence of transition metal ions or peroxide. The API should therefore be protected from strong bases, strong oxidizing agents, and prolonged exposure to aqueous solvents above neutral pH. If the API is milled with a nitrogen purge, the oxygen level in the mill is maintained below 5% v/v to limit surface oxidation. Bags are purged with nitrogen and sealed immediately after sampling. For transport, temperature is controlled at 15–25 °C and humidity is maintained below 40% RH; short excursions must be justified by stability data. The product is not considered a controlled substance under typical pharmaceutical chemical regulations, but the manufacture is subject to FDA 21 CFR Part 211 for drug substance production, ICH Q7 for good manufacturing practice, and registration requirements under REACH where applicable.

Process validation for tablet, capsule, granule, and injectable applications includes lot-to-lot variability of crystallinity, particle-size distribution, and residual solvent profile. Three consecutive production batches are generally analyzed using the approved release specification to demonstrate reproducibility. The critical process parameters identified for this API include the final recrystallization solvent ratio, cooling rate, vacuum drying temperature, milling screen size, and the duration of nitrogen grinding. Changes in the crystallization solvent from ethyl acetate to acetone or methyl tert-butyl ether can alter particle shape and residual solvent species; therefore, solvent changes require comparative stability and powder-rheology studies. The acceptable residual solvent levels are set according to ICH Q3C for the actual solvent system used. If dichloromethane is used as a reaction solvent, its limit is controlled at ≤600 ppm for general oral use; if ethanol is used, the limit is set at ≤5000 ppm. Final API must be free from visible extraneous matter and must pass the identity, assay, and impurity tests before release to oral or injectable formulation.