A Validated RP-HPLC Method for the Determination of Recombinant Human Insulin in Bulk and Pharmaceutical Dosage Form

A modified RP-HPLC method was developed for the quantitative determination of recombinant human insulin in bulk and pharmaceutical dosage form with reduced retention time. Study of the effects of the column temperature, pH of the mobile phase and presence of vial additives (phenol and m-cresol), or impurities (A-21 Disamido) on the accuracy of the assay were assessed. Separation was achieved using a Hypersil BDS C-18 column and the mobile phase was composed of solution A (aqueous solution of 28.3 anhydrous Na2SO4g/L, pH 2.3) and solution B (28.5 g anhydrous Na2SO4 g/L in 50:50 mixture of water and acetonitrile, pH 2.3) in a ratio 48:52 (v/v) at 45– 50 oC. The column temperature was 40 oC, the flow rate was 1 mL/min and detection was performed at 216 nm. The procedures were validated according to international conference on harmonization (ICH) guidelines. Recovery study was done applying standard addition technique for further validation of the procedure. The retention time of recombinant human insulin was 19.7 min as compared to 29 min obtained by the reference method. Analytical conditions fluctuations or presence of vial additives or impurities did not show any significant effect on the accuracy of the method. The prepared standard insulin solution in 0.01 N HCl was found to be stable for 5 days. Statistical comparison showed no significant difference between the described method and reference method regarding the accuracy and precision. The modified method can be applied for routine quality control applications for determination of recombinant human insulin.


Introduction
Insulin is composed of two peptide chains, referred to as A chain (21 amino acids) and B chain (30 amino acids) with molecular weight of 5.8 kDa.Two disulfide bridges link these chains together and another intra-peptide disulfide bond exists in the A chain 1,2 .Human insulin produced by recombinant DNA technology is the first commercial health care product derived from this technology.A weakened form of Escherrichia coli is used as a vector for the synthesis of recombinant human insulin.The protein is produced in the bacterial cell as two chains A and B. The two chains are mixed and reconnected in a reaction that forms the disulfide cross bridges, resulting in pure recombinant human insulin.Insulin is administered subcutaneously for the treatment of type I diabetes mellitus when pancreatic β-cells daily production of insulin is insufficient.Several methods have been reported for the determination of insulin in the pharmaceuticals and plasma including voltametric 3 , spectroscopic 4 , electrochemical 5 and chromatographic methods 6,7 .
RP-HPLC is probably the most used analytical method for separation and determination of peptides and proteins in an extensive range of applications 8 .The United States Pharmacopeia (USP) describes a RP-HPLC method for the determination of recombinant human insulin with insulin peak detected at 214 nm and appearing 9 after 29 min.The aim of the present study is to provide a RP-HPLC method suitable for industrial application with reduced analysis time that can withstand fluctuations in analytical conditions, presence of vial additives (phenol and mcresol) or impurities (A-21 disamido) without significant effect on its accuracy.

Experimental
Chromatographic separation was performed on Shimatzu 2010 HPLC system (Germany) equipped with an auto-sampler, a Hypersil BDS C-18 (30 x 4.6 mm, 3µm) column and a UV detector.A Kontes Ultra-Ware ® filtration system with 5 liter reservoir part no.31862(USA) was used.

Pure standards
Recombinant human insulin USP reference standard and bulk crystals were obtained from the egyptian company for biological products and vaccines (VACSERA, Egypt).

Pharmaceutical dosage forms of insulin
Commercially available insulin-VACSERA R ® 40 IU was obtained from public pharmacy.

Chemicals and reagents
All chemicals and reagents used throughout this work were of analytical grade and the solvents were of HPLC grade.Water for injection (WFI) was obtained from VACSERA (Egypt).Hydrochloric acid, o-phosphoric acid, sodium sulphate, acetonitrile, phenol and m-cresol were obtained from Fisher (USA).

Standard solutions Recombinant human insulin standard solutions
Into a series of 50 mL volumetric flasks, different concentrations of recombinant human insulin USP reference standards (20, 40, 80, 90, 100, 10 and 120 IU/mL) were prepared by dissolving 35.3 mg, 70.6 mg, 141.2 mg, 158.8 mg, 176.5 mg, 194.2 mg and 211.8 mg of the recombinant human insulin USP reference standard crystals, respectively, in 0.01N HCl.

Phenol and m-cresol standard solution
In a 50 mL volumetric flask, 65 mg of phenol and 150 mg of m-cresol were dissolved in acetonitrile.This was the manufacturer reported concentration in the final dosage form.

Preparation of A-21 disamido (degradation product of insulin)
In a suitable clean capped vial, 1 mg of insulin crystals was dissolved in 1 mL 0.01 N HCl and incubated at 35 ºC for 4 days 9 .

Insulin, phenol and m-cresol mixture
In suitable clean capped vials, 5 mL from the 80, 100 and 120 IU/mL insulin standard solutions were separately mixed with 5 mL of the prepared phenol and m-cresol standard solution.

Chromatographic conditions
Chromatographic separation was performed using a Hypersil BDS C-18 (30x4.6 mm, 3 µm) analytical column.The flow rate was maintained at 1 mL/min and the detection was performed at 216 nm.The column temperature was kept at 40 °C.The mobile phase consists of solution A and B of pH 2.3 in a ratio of 48:52 (v/v) at 45-50 ºC.

Preparation of mobile phase
Solution A was prepared by dissolving 56.8 g of anhydrous Na 2 SO 4 in WFI then the pH was adjusted to 2.3 with o-phosphoric acid and the volume was completed to 2 L with WFI and filtered.Solution B was prepared by dissolving 28.5 g of Na 2 SO 4 in 400 mL WFI, the pH was adjusted to 2.3 with o-phosphoric acid, volume was completed with WFI to 1 L then 1 L of acetonitrile was added.The two solutions (A & B) were kept on hot-plate magnetic stirrer at a temperature 45-50 ºC with continuous stirring and filtered before being introduced into the HPLC system.

Determination of method specificity
Identification 20 µL of 0.01 N HCl, 100 IU/mL insulin standard, A-21 disamido preparation, phenol & m-cresol were separately injected into the HPLC system and chromatographed under the previously mentioned chromatographic conditions.The retention time for each was recorded.

Assay
The effect of vial additives was tested by carrying out the experiment on the prepared insulin, phenol and m-cresol mixtures.Area under the peak (AUP) corresponding to insulin concentration was recorded for each mixture and insulin recovery percent was calculated using the regression equation as described below (linearity).

Linearity
A series of different concentrations of recombinant human insulin ranging from 80-120 IU/mL were injected into the HPLC system and analyzed using the specified chromatographic conditions.The retention times and the AUPs were recorded from the resulting chromatograms.A calibration curve plotted for the AUP versus insulin concentration and the respective regression equation was computed.

Accuracy
Different aliquots of insulin standard solutions (80, 100 and 120 IU/mL) were injected into the HPLC system and the same procedures used for establishing linearity were followed.AUP for each insulin solution was recorded and the recovery percent of insulin was calculated using the regression equation.

Inter-assay precision (Repeatability)
Insulin standard solutions (80, 100 and 120 IU/mL) were analyzed three times each intradaily.The AUPs were recorded and the relative standard deviation (RSD) was calculated.

Intermediate precision
Intermediate precision expresses within-laboratory variations including different days, analysts, equipments, etc.According to the International Conference on Harmonization (ICH) guidelines it is not necessary to study these effects individually; the use of an experimental design (matrix) is encouraged 10 .The previous procedures were repeated interdaily (on different days) using different insulin standards concentrations (80, 100 and 120 IU/mL) three times each for each concentration using freshly prepared mobile phase prepared by another analyst.The AUPs were recorded and RSD was calculated.

Limit of detection (LOD) and limit of quantitation (LOQ)
LOD and LOQ were calculated based on the standard deviation of the analytical response represented by AUP and the slope of the calibration curve.The following equations LOD = 3.3 σ/S, LOQ = 10 σ/S were used, where σ is the standard deviation of the response and S is the slope of the calibration curve.

Robustness
To determine the robustness of the method, small variations in the experimental conditions were introduced and the AUP was recorded followed by calculation of the RSD.In the above mentioned experiments, the pH of the mobile phase was 2.3 and the temperature of the column was 40 ºC.The effect of small variations in pH of the mobile phase and the temperature of the column on the method accuracy were separately tested on the 100 IU/mL insulin standard solution.The pH was varied between 2.2 and 2.4 in 0.1 unit increments using o-phosphoric acid, while the column temperature was varied between 39 ºC and 41 ºC in 1 ºC increments.The procedure was repeated accordingly, the AUP was recorded and RSD was calculated.

Insulin solution stability
The stability of the insulin standard preparation was tested by analyzing freshly prepared 100 IU/mL insulin solution using the modified RP-HPLC method.The prepared solution was stored at 2 -8 °C and analyzed every 24 h for 5 successive days.The AUPs were recorded and RSD was calculated.

Testing purity of bulk recombinant human insulin from commercial suppliers
Recombinant human insulin solutions of concentrations (80-120) IU/mL were prepared according to their labeled potency as per the supplier's certificate of analysis.The prepared concentrations were analyzed using the modified RP-HPLC, AUPs were recorded and recovery percent of pure recombinant human insulin was calculated using the regression equation.

Application on vial dosage form
40 µL of 6N HCl were added to a commercial 40 IU/mL insulin vial to obtain a clear solution which was injected into the HPLC system and analyzed.Recovery study was performed by applying standard addition for further validation of the procedure.

Results and Discussion
Figure 1 showed that recombinant human insulin had a peak with retention time of 19.7 min.
The insulin peak was resolved from the peaks of the additives (phenol & m-cresol; 6.5 and 9.8 min, respectively) and degradation product (A-21 disamido; 25.8 min) (Table 1).The additives had no effect on average recovery percent of recombinant human insulin which was 99.76±0.15(Table 2), hence the proposed method was found to be of sufficient specificity for assay of recombinant insulin.The relationship between AUP and concentration of recombinant human insulin was linear over the concentration range 20-120 IU/mL and the correlation coefficient was 0.9993.Parameters of the regression equation are shown in Table 3.There was good correlation between AUP and recombinant human insulin concentration.
The AUPs corresponding to insulin concentrations of 80, 90, 100 and 120 IU were used to calculate the recovery percent of insulin using the regression equation.The recovery percent was 99.85±1.39(Table 4).The results demonstrate that the modified method was suitable for quantitative determination of recombinant insulin.RSDs of analysis for the inter-assay precision 0.3, 0.78 and 0.1; and those for the intermediate precision were 0.2, 0.2 and 0.1 for the three chosen concentrations (80, 100 & 120 IU/mL) (Table 5).
LOD and LOQ were calculated based on standard deviation of the analytical response represented by AUP and slope of the calibration curve and were 4.17 and 13.9 IU/mL, respectively.
In the robustness study, the effect on AUP was studied and RSDs of the response were 1.8 and 0.6 for pH of the mobile phase and column temperature variations, respectively.Small fluctuations in analytical conditions had no significant effect on method performance (Table 6).

Conclusion
The objectives of this study were to reduce the analysis time of recombinant insulin using RP-HPLC method and investigate the effect of analytical condition fluctuations and presence of vial additives or impurities on the accuracy of modified method.This is in order to make the RP-HPLC method more amenable for routine quality control procedures of bulk crystals, in-process quality control and finished product analysis.The retention time of recombinant insulin was 19.7 min as compared to 29 min obtained by the reference method.Analytical conditions fluctuations or presence of vial additives or impurities did not show any significant effect on the accuracy of the modified method.The insulin standard was found to be stable for 5 days and validation experiments were satisfactory for all parameters tested.
The modified RP-HPLC method for determination of recombinant human insulin can be applied for routine quality control procedures with no interference from vial additives or impurities and can withstand normal fluctuation in the analysis conditions that may arise during extended operation of the HPLC system.

Table 1 .
Specificity of the insulin RP-HPLC assay

Table 2 .
Specificity results in presence of vial additives

Table 3 .
Parameters of regression equation relating insulin concentration to analytical response (AUP)

Table 4 .
Accuracy of the modified RP-HPLC method for determination of recombinant human insulin

Table 10 .
Statistical comparison between the modified and reference RP-HPLC methods for determination of recombinant insulin