Solution A— Prepare a filtered and degassed mixture of 0.17 M acetic acid and methanol (125:8). Make adjustments if necessary (see System Suitability under Chromatography 621).

Solution B: methanol, filtered and degassed.

Mobile phase— Use variable mixtures of Solution A and Solution B as directed for Chromatographic system. Make adjustments to either solution as necessary (see System Suitability under Chromatography 621).

System suitability solution— Dissolve suitable quantities of purine and USP Acyclovir RS in Solution A to obtain a solution containing about 0.5 µg of each per mL.

Acyclovir standard solution— Dissolve an accurately weighed quantity of USP Acyclovir RS in Solution A, and dilute quantitatively, and stepwise if necessary, with Solution A to obtain a solution having a known concentration of about 5 µg per mL.

Guanine solution— Dissolve about 25 mg of guanine, accurately weighed, in 50 mL of 0.1 N sodium hydroxide in a 500-mL volumetric flask, dilute with water to volume, and mix.

Standard solution 1— Transfer 5.0 mL of Acyclovir standard solution to a 50-mL volumetric flask, dilute with Solution A to volume, and mix.

Standard solution 2— Transfer 5.0 mL of Guanine solution to a 50-mL volumetric flask, dilute with Solution A to volume, and mix.

Test solution— Constitute and combine not fewer than 10 vials of Acyclovir for Injection. Transfer an accurately measured quantity, equivalent to about 100 mg of acyclovir, to a 200-mL volumetric flask, dilute with Solution A to volume, and mix.

Chromatographic system (see Chromatography 621)— The liquid chromatograph is equipped with a 254-nm detector and a 4.6-mm × 25-cm column that contains packing L1. The flow rate is about 1 mL per minute. The chromatograph is programmed as follows. Chromatograph the System suitability solution, and record the peak responses as directed for Procedure: the resolution, R, between purine and acyclovir is not less than 2.0. Chromatograph Standard solution 1 and Standard solution 2, and record the peak responses as directed for Procedure: the typical retention times for guanine and acyclovir are about 5.8 minutes and 14 minutes, respectively; and the relative standard deviation of the acyclovir peak area and the guanine peak area for replicate injections is not more than 1%.

Procedure— Separately inject equal volumes (about 50 µL) of the Standard solution and the Test solution into the chromatograph, record the chromatograms, and measure the peak area responses. Calculate the percentage of guanine in the Acyclovir for Injection by the formula: in which C is the concentration, in mg per mL, of guanine in the Standard solution; W is the total weight, in mg, of acyclovir in the Test solution based on the label claim; rg is the peak response for guanine, if present, in the Test solution; and rsg is the peak response of guanine in the Standard solution: not more than 1.0% is found. Calculate the percentage of each other impurity in the Acyclovir for Injection by the formula: in which C is the concentration, in mg per mL, of USP Acyclovir RS in the Standard solution; W is the total weight, in mg, of acyclovir in the Test solution based on the label claim; ri is the peak response for each impurity; and rS is the peak response of acyclovir in the Standard solution: not more than 0.15% for any peak having a relative retention time of about 0.7 compared to the acyclovir peak is found; not more than 0.5% of any other individual impurity is found; and the total of all other impurities is not more than 1.0%.

Mobile phase— Prepare a filtered and degassed solution of 0.02 M acetic acid. Make adjustments if necessary (see System Suitability under Chromatography 621).

System suitability preparation 1— Dissolve accurately weighed quantities of USP Acyclovir RS and guanine in 0.1 N sodium hydroxide, and dilute quantitatively, and stepwise if necessary, with 0.1 N sodium hydroxide to obtain a solution having known concentrations of about 0.1 mg of each per mL.

System suitability preparation 2— Dissolve an accurately weighed quantity of guanine in 0.1 N sodium hydroxide, and dilute quantitatively, and stepwise if necessary, with 0.1 N sodium hydroxide to obtain a solution having a known concentration of about 2.0 µg per mL.

Standard preparation— Dissolve accurately weighed quantities of USP Acyclovir RS in 0.1 N sodium hydroxide, and dilute quantitatively, and stepwise if necessary, with 0.1 N sodium hydroxide to obtain a solution having a known concentration of about 0.1 mg per mL.

Assay preparation— Constitute, with water, 1 vial of Acyclovir for Injection. Transfer an accurately weighed amount of this solution, equivalent to about 10 mg of acyclovir, to a 100-mL volumetric flask, and dilute with water to volume.

Chromatographic system (see Chromatography 621)— The liquid chromatograph is equipped with a 254-nm detector and a 4.2-mm × 25-cm column that contains packing L1. The flow rate is about 1.5 mL per minute. Chromatograph System suitability preparation 1, and record the peak responses for acyclovir as directed for Procedure: the relative retention times are about 0.6 for guanine and 1.0 for acyclovir; the resolution, R, between guanine and acyclovir is not less than 2.0; and the relative standard deviation for replicate injections for the acyclovir peak is not more than 2.0%. Chromatograph System suitability preparation 2, and record the peak responses as directed for Procedure: the relative standard deviation for replicate injections is not more than 2.0%.

Procedure— Separately inject equal volumes (about 20 µL) of the Standard preparation and the Assay preparation into the chromatograph, record the chromatograms, and measure the peak responses. Calculate the quantity, in mg, of acyclovir (C8H11N5O3) in the portion of Acyclovir for Injection taken by the formula: in which C is the concentration, in mg per mL, of USP Acyclovir RS in the Standard preparation; and rU and rS are the peak responses obtained from the Assay preparation and the Standard preparation, respectively.