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Cephalexin contains not less than 95.0 per cent and not more than 101.0 per cent of C₁₆H₁₇N₃O₄S, calculated on the anhydrous basis.

Category Antibacterial (first-generation cephalosporin)

Description White or off-white, crystalline powder.

Solubility Slightly soluble in water; practically insoluble in chloroform, in ethanol and in ether.

Stability It is slightly hygroscopic. In neutral or alkaline solutions, cephalexin degrades rapidly. In acid solution, it is relatively more stable especially when kept at pH 4.5 under refrigeration.

Contra-indication It is contra-indicated in patients who have shown hypersensitivity to any member of the cephalosporins.

Warning
          1. It should be avoided in patients who have had an immediate-type (anaphylactic) hypersensitivity reaction to penicillins and should be administered with caution in patients who have had a delayed-type (e.g., rash, fever, eosinophilia) reaction to penicillins or other drugs.
          2. It should be used with caution in patients with a history of gastro-intestinal diseases, particularly colitis.
          3. It should be administered with caution and in reduced dosage in the presence of markedly impaired renal function.
          4. It may cause gastro-intestinal disturbances, headache, hypersensitivity reactions, pseudomembranous colitis, and hematological abnormalities.
          5. Its overdosage can cause CNS irritation leading to seizures.
          6. Prolonged use may result in the overgrowth of nonsusceptible organisms, especially Enterobacter, Pseudomonas, Enterococci, or Candida.
          7. Risk-benefit should be considered if it is to be used in pregnancy or nursing women.

Additional information
          1. Group A beta-hemolytic streptococcal infections should be treated for at least 10 days to prevent the development of acute rheumatic fever or acute glomerulonephritis.
          2. Continue administration for a minimum of 48 to 72 hours after fever abates or after evidence of bacterial eradication has been obtained.

Packaging and storage Cephalexin shall be kept in tightly closed containers, protected from light, and stored at a temperature not exceeding 25º.

Labelling The label on the container states storage condition.

Identification
          A. The infrared absorption spectrum is concordant with the spectrum obtained from Cephalexin RS (Appendix 2.1) or with the reference spectrum of Cephalexin.
          B. Place 2 mg in a test-tube about 150 mm long and 15 mm in diameter. Moisten with one drop of water and add 2 ml of sulfuric-formaldehyde TS. Mix the contents of the tube by swirling; the solution is pale yellow. Place the test-tube in a water-bath for 1 minute: a dark yellow colour develops.
          C. Mix 20 mg with 5 drops of a 1 per cent v/v solution of glacial acetic acid and add 2 drops of a 1 per cent w/v solution of copper(II) sulfate and 1 drop of 2 M sodium hydroxide: an olive-green colour is produced.

Crystallinity It is crystalline (Method I, Appendix 4.14).

pH 3.0 to 5.5, in a 0.5 per cent w/v solution (Appendix 4.11).

Water Not less than 4.0 per cent w/w and not more than 8.0 per cent w/w (Karl Fischer Method, Appendix 4.12). Use 300 mg.

Sulfated ash Not more than 0.2 per cent w/w (Appendix 5.3).

Specific rotation +149º to +158º, calculated on the an hydrous basis, determined in a 0.5 per cent w/v in phthalate buffer solution pH 4.4 prepared by dissolving 2.042 g of potassium hydrogenphthalate in 50 ml of water, adding 7.5 ml of 0.20 M sodium hydroxide and diluting to 200 ml with water (Appendix 4.8).

Absorbance Dissolve 50 mg in water and dilute to 100.0 ml with the same solvent. The absorbance of the solution determined at 330 nm is not greater than 0.05. Dilute 2.0 ml of the solution to 50.0 ml with water. Examined between 220 nm and 300 nm, the diluted solution shows an absorption maximum at about 262 nm. The specific absorbance at this maximum is 220 to 245, calculated on the anhydrous basis (Appendix 2.2).

Related substances Not more than 1.0 per cent w/w of any individual cephalexin-related substance and not more than 5.0 per cent w/w of the sum of all cephalexin- related substances. Carry out the test as described in the “High-pressure Liquid Chromatography” (Appendix 3.5).
          Solution A Dissolve 1 g of sodium 1-pentanesulfonate in a mixture of 1000 ml of water and 15 ml of triethylamine. Adjust with phosphoric acid to a pH of 2.5±0.1. Make adjustments if necessary.
          Solution B Dissolve 1 g of sodium 1-pentanesulfonate in a mixture of 300 ml of water and 15 ml of triethylamine. Adjust with phosphoric acid to a pH of 2.5±0.1, add 350 ml of acetonitrile and 350 ml of methanol, and mix. Make adjustments if necessary.
          Mobile phase Use variable mixtures of Solution A and Solution B as directed for Chromatographic system.
          Solvent Dissolve 18 g of potassium dihydrogenphosphate in 1000 ml of water.
          Standard solution Dissolve accurately weighed quantities of Cephalexin RS quantitatively in Solvent to obtain solutions having known concentrations of about 80 and 160 μg of cephalexin per ml, respectively, taking into account the stated potency of the Cephalexin RS.
          Test solution Transfer about 25 mg of the substance being examined in, accurately weighed, to a 5-ml volumetric flask, dissolve in and dilute with Solvent to volume, and mix.
          Chromatographic system The chromatographic procedure may be carried out using (a) a stainless steel column (4.6 mm × 25 cm) packed with low acidity octadecylsilane chemically bonded to porous silica or ceramic microparticles (3 to 10 μm), (b) Mobile phase at a flow rate of about 1 ml per minute, and (c) an ultraviolet photometer set at 254 nm. The chromatograph is programmed as follows:

       Procedure Separately inject equal volumes (about 20 μl) of Standard solutions and Test solution into the chromatograph, record the chromatograms, and measure the responses for the cephalexin peaks in the chromatograms obtained from Standard solutions and for all of the peaks, other than that from cephalexin, in the chromatogram obtained from Test solution. Plot the responses of the cephalexin peaks in the chromatograms obtained from Standard solutions versus their  concentrations, calculated on the anhydrous basis, in mg per ml, and draw a straight line through the two points and zero. From the line so obtained and the peak responses obtained from Test solution, determine the concentration, I, in mg per ml, of each cephalexin-related substance obtained from Test solution other than the cephalexin peak. Calculate the percentage of each cephalexinrelated substance by the expression:

500I/W,

in which W is the quantity, calculated on the anhydrous basis, in mg, of Cephalexin taken to prepare the Test solution.

N,N-Dimethylaniline Not more than 20 ppm (Appendix 5.16).

Assay Carry out the determination as described in the “High-pressure Liquid Chromatography” (Appendix 3.5).
          Mobile phase Prepare a mixture of 5 volumes of acetonitrile, 2 volumes of methanol, 10 volumes of a 1.36 per cent w/v solution of potassium dihydrogenphosphate and 83 volumes of water. Make adjustments if necessary.
          Resolution solution Dissolve about 10 mg of Cephalexin RS and 10 mg of Cephradine RS, accurately weighed, in water and dilute to 100.0 ml with the same solvent.
          Standard preparation Dissolve about 50 mg of Cephalexin RS, accurately weighed, in water and dilute to 100.0 ml with the same solvent.
          Assay preparation Dissolve about 50 mg of Cephalexin, accurately weighed, in water and dilute to 100.0 ml with the same solvent.
          Chromatographic system The chromatographic procedure may be carried out using (a) a stainless steel column (25 cm × 4.6 mm) packed with octadecylsilane chemically bonded to porous silica or ceramic microparticles (5 μm or 10 μm), (b) Mobile phase at a flow rate of about 1.5 ml per minute, and (c) an ultraviolet photometer set at 254 nm.
          To determine the suitability of the chromatographic system, chromatograph Resolution solution, and record the peak responses as directed under Procedure: the resolution factor between cephalexin and cephradrine peaks is not less than 4.0. Chromatograph Standard preparation, and record the peak responses as directed under Procedure: the relative standard deviation for six replicate injections is not more than 1.0 per cent.
          Procedure Separately inject equal volumes (about 20 μl) of Standard preparation and Assay preparation into the chromatograph, record the chromatograms, and measure the responses for the major peaks.
          Calculation Calculate the content of C₁₆H₁₇N₃O₄S of the Cephalexin taken, using the declared content of C₁₆H₁₇N₃O₄S in Cephalexin RS.