Cephalexin
C16H17N3O4S·H2O 365.40
C16H17N3O4S 347.40
5-Thia-1-azabicyclo[4.2.0]oct-2-ene-2-carboxylic acid, 7-[(aminophenylacetyl)amino]-3-methyl-8-oxo-, monohydrate, [6R-[6
,7
(R*)]]-;
(6R,7R)-7-[(R)-2-Amino-2-phenylacetamido]-3-methyl-8-oxo-5-thia-1-azabicyclo[4.2.0]oct-2-ene-2-carboxylic acid monohydrate
[23325-78-2].
Anhydrous[15686-71-2].
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C16H17N3O4S·H2O 365.40
C16H17N3O4S 347.40
5-Thia-1-azabicyclo[4.2.0]oct-2-ene-2-carboxylic acid, 7-[(aminophenylacetyl)amino]-3-methyl-8-oxo-, monohydrate, [6R-[6
,7 (R*)]]-;
(6R,7R)-7-[(R)-2-Amino-2-phenylacetamido]-3-methyl-8-oxo-5-thia-1-azabicyclo[4.2.0]oct-2-ene-2-carboxylic acid monohydrate
[23325-78-2].Anhydrous
[15686-71-2].DEFINITION
Cephalexin has a potency of NLT 950 µg/mg and NMT 1030 µg/mg of C16H17N3O4S, calculated on the anhydrous basis.
IDENTIFICATION
• B.
The retention time of the major peak of the Sample solution corresponds to that of the Standard solution, as obtained in the Assay.
ASSAY
• Procedure
Mobile phase:
0.985 g/L of sodium-1-pentanesulfonate in a mixture of acetonitrile, methanol, triethylamine, and water (20:10:3:170), adjusted with phosphoric acid to a pH of 3.0 ± 0.1
Standard stock solution:
1 mg/mL of USP Cephalexin RS in water
Standard solution:
0.4 mg/mL of cephalexin in Mobile phase from Standard stock solution
Sample stock solution:
1 mg/mL of Cephalexin in water
Sample solution:
0.4 mg/mL of Cephalexin in Mobile phase from Sample stock solution
Chromatographic system
Mode:
LC
Detector:
UV 254 nm
Column:
4.6-mm × 25-cm; packing L1 of low acidity
Flow rate:
1.5 mL/min
Injection size:
20 µL
System suitability
Sample:
Standard solution
Suitability requirements
Relative standard deviation:
NMT 2.0%
Analysis
Samples:
Standard solution and Sample solution
Calculate the quantity, in µg, of cephalexin (C16H17N3O4S) per mg of the Cephalexin taken:
Result = (rU/rS) × (CS/CU) × P
| rU | = | = peak response from the Sample solution |
| rS | = | = peak response from the Standard solution |
| CS | = | = concentration of USP Cephalexin RS in the Standard solution (mg/mL) |
| CU | = | = concentration of Cephalexin in the Sample solution (mg/mL) |
| P | = | = potency of cephalexin in USP Cephalexin RS (µg/mg) |
Acceptance criteria:
950–1030 µg/mg on the anhydrous basis
IMPURITIES
Organic Impurities
• Procedure 1
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.
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, and add 350 mL of acetonitrile and 350 mL of methanol.
Mobile phase:
See the gradient table below.
| Time (min) |
Solution A (%) |
Solution B (%) |
|---|---|---|
| 0 | 100 | 0 |
| 1 | 100 | 0 |
| 33.3 | 0 | 100 |
| 34.3 | 0 | 100 |
Diluent:
18 mg/mL of monobasic potassium phosphate in water
Standard solutions:
0.08 mg/mL and 0.16 mg/mL of cephalexin (C16H17N3O4S) from USP Cephalexin RS in Diluent, taking into account the stated potency of the USP Cephalexin RS
Sample solution:
5 mg/mL of Cephalexin in Diluent
Chromatographic system
Mode:
LC
Detector:
UV 254 nm
Column:
4.6-mm × 25-cm; packing L1 of low acidity
Flow rate:
1 mL/min
Injection size:
20 µL
Analysis
Samples:
Standard solutions and Sample solution
Plot the responses of the cephalexin peaks from the Standard solutions versus their concentrations, calculated on the anhydrous basis, in mg/mL, and draw a straight line through the two points and zero. From the line and the peak responses of the Sample solution, determine the concentration, I, in mg/mL, of each cephalexin-related substance of the Sample solution other than the cephalexin peak.
Calculate the percentage of each cephalexin-related substance:
Result = I/C × 100
| I | = | = concentration of each cephalexin-related substance in the Sample solution as determined from the calibration curve (mg/mL) |
| C | = | = concentration of cephalexin from the Sample solution (mg/mL) |
Acceptance criteria
Individual impurities:
NMT 1.0% of any individual cephalexin-related substance
Total impurities:
NMT 5.0%
• Procedure 2: Dimethylaniline 
223
:
Meets the requirement
223:
Meets the requirement
SPECIFIC TESTS
• Optical Rotation, Specific Rotation 781S:
+149
to +158
781S:
+149 to +158
Sample solution:
5 mg/mL, in pH 4.4 neutralized phthalate buffer (see Reagents, Indicators, and Solutions—Buffer Solutions)
• Crystallinity 695:
Meets the requirements
695:
Meets the requirements
• pH 791:
3.0–5.5, in an aqueous suspension containing 50 mg/mL
791:
3.0–5.5, in an aqueous suspension containing 50 mg/mL
• Water Determination, Method I 921:
4.0%–8.0%
921:
4.0%–8.0%
ADDITIONAL REQUIREMENTS
• Packaging and Storage:
Preserve in tight containers.
• USP Reference Standards 11
11
USP Cephalexin RS 
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Auxiliary Information—
Please check for your question in the FAQs before contacting USP.
| Topic/Question | Contact | Expert Committee |
|---|---|---|
| Monograph | Ahalya Wise, M.S.
Senior Scientific Liaison 1-301-816-8161 |
(SM12010) Monographs - Small Molecules 1 |
| Reference Standards | RS Technical Services 1-301-816-8129 rstech@usp.org |
USP35–NF30 Page 2584
Pharmacopeial Forum: Volume No. 36(1) Page 41