Validated Reverse Phase HPLC Method for the Determination of DEHP Content in Reconstituting Diluents and in Reconstituted Solutions of Imipenem and Cilastatin for Injection

A simple, rapid, accurate and economic reverse phase HPLC method was developed and validated for determination of di-(2-ethylhexyl)phthalate (DEHP) in reconstituting diluents and reconstituted solutions of imipenem and cilastatin for injection The method was applied to detect any leaching enhancement in presence of imipenem and cilastatin in reconstituted injections. Imipenem and cilastatin for injection is a widely used broad spectrum beta-lactam antibiotic. As per requirements of various regulatory authorities of different countries, DEHP content needs to be monitored in the reconstituting diluents used to reconstitute injections. The proposed method is a unique method wherein DEHP can be determined directly without any sample pre-treatment before analysis. The method can be used as a good quality control tool to control the leaching of DEHP in the reconstituting diluents and reconstituted injections. The method utilizes C18 column (250 x 4.6 mm, 5μ) and a mixture of methanol, propan-2-ol and water as mobile phase. DEHP was detected at 225 nm. The method has low limit of quantification (0.058 μg mL) which is much below the acceptance limit calculated as per USFDA tolerance criteria (3.5 μg mL for adults and 0.3 μg mL for neonates and infants).


Introduction
Di-(2-ethylhexyl) phthalate, more commonly known as DEHP (CAS No. 117-81-7) is an oily, colourless, viscous liquid with chemical formula C 6 H 6 (C 8 H 17 COO) 2 . It is manufactured using phthalic anhydride and 2-ethylhexanol as raw materials. It is most widely used plasticizer in polyvinyl chloride (PVC) devices due to higher boiling point (385 °C) providing good heat sustainability. It has low molecular weight (390.56) giving good lubricany and matrix mobility which in turn provides excellent plasticity and softness to PVC 1 . Moreover, PVC being cheap and having excellent varied properties is one among the most commonly used plastic materials for medical devices. DEHP is prominently used in manufacturing PVC medical devices like infusion bags, blood bags, infusion tubings, feeding tubes, catheters etc. Virtually all PVC medical devices utilize DEHP as plasticizer. It can leach out from PVC matrix resulting in its exposure to body tissues and fluids 1 . Hence, there are high possibilities of DEHP exposure during medical procedures such as haemodialysis, blood and blood products transfusion, extracorporeal membrane oxygenation, heart bypass surgery and administration of intravenous fluids. The highest human exposures to DEHP can occur in newborns and infants undergoing extensive medical procedures which may lead to a body DEHP level between 130-6000 µg kg body weight -1 day 1,2 .
DEHP is rapidly metabolized in human body by the enzymes present mainly in the intestines that break down the DEHP into mono-(2-ethylhexyl) phthalate (MEHP). This enzymatic conversion also occurs in the liver, kidney, lungs, pancreas, and plasma. Metabolism of DEHP involves a complex series of reactions that may produce 30 or even more metabolites, prominently MEHP, 2-ethylhexanol, 2-ethylhexanoic acid and several keto acid derivatives that are responsible for its toxicity [3][4][5][6][7] . DEHP also accumulates in adipose tissues 3 . Its significant amount is secreted through milk in lactating females that adversely affects the infants and new born 2 .
Studies with laboratory rodents show that DEHP and its metabolites adversely effects reproduction and development produce testicular toxicity, underdeveloped reproductive tracts in males, prominently in infants and new borns 1,2,4 . DEHP exposure may also lead to contact utricaria syndrome, bronco-contraction, immunosupression, mutagenecity and carcinogenicity. Liver, kidney and testes are the other main organs targeted by DEHP induced toxicity 1 .
Thus, DEHP leached from PVC medical devices contributing to body fluids and systems is strictly monitored by regulatory agencies. USFDA provides different tolerance limits for adults and neonates of leached DEHP from various PVC medical devices. For intravenous infusion of drugs requiring pharmaceutical vehicles for solubulization, the permissible daily exposure of DEHP is limited to 0.04 mg kg body weight -1 day -1 for adults (70 kg) and 0.03 mg kg body weight -1 day -1 for neonates (4 kg) 8 .
The diluents used for reconstituting imipenem and cilastatin for injection are 10% dextrose, 10% mannitol, 5% mannitol, 0.9% sodium chloride, 5% dextrose with 0.9% sodium chloride, 5% dextrose with 0.225% sodium chloride and 5% dextrose with 0.02% sodium bicarbonate injection 9. These reconstituting diluents are supplied in PVC bags, which may lead to DEHP leaching in the reconstituents. Further, presence of imipenem and cilastatin may also contribute to DEHP leaching to reconstituents when stored in PVC bags. As per the pack insert of imipenem and cilastatin for injection, the reconstituted injection may be stored for three to four hours at room temperature (up to 25 °C) or for 24 h under refrigeration (below 4 °C) 9 . Hence, the concentration of DEHP in reconstituents and reconstituted injections both at the initial stage and at extreme storage time points need to be monitored by an appropriate analytical method.
A variety of analytical methods are available for determination of DEHP in various biological and non biological matrixes. Some analytical methods involve pre-concentration by liquid-liquid extraction of DEHP to organic phase like hexane or methylene chloride or use combination of methanol and acetonitrile as eluent but these methods may not be suitable for determination of leached DEHP in some reconstituting diluents (e.g. mannitol) due to precipitation of osmogens [10][11] . Other methods using hexane and ethyl acetate as mobile phase components to monitor DEHP and its metabolites in plasma are cumbersome as these methods require lengthy and time consuming solvent-solvent extraction of aqueous samples before analysis 12 . A variety of sophisticated and expensive analytical methods are available that utilize techniques like LCMS, GC, GC-MS, electron capture GC [13][14][15] or involve micro-organic ion association phase extraction 16 to determine DEHP and metabolites in plasma or samples from variety of matrices. Most of the mentioned methods are not simple and cannot be used directly without prior sample treatment thus limiting their applicability.
A simple, accurate, sensitive, rapid and economic reversed phase high performance liquid chromatography (RPHPLC) method has been developed that does not involve any preconcentration in organic phases and is capable of analyzing aqueous samples directly. This method involves very simple and fast sample preparation and takes less time for analysis since run time is only 15 minutes. Further, the method has been extended to determine the content of DEHP in reconstituted imipenem and cilastatin injection in various reconstituting diluents 9 . As per dosage regime of imipenem and cilastatin for injection, the maximal acceptable amount of DEHP in reconstituted injection is calculated to be 3.5 µ g mL -1 for adults and 0.3 µ g mL -1 for neonates and infants. The limit of quantification of the proposed analytical method is 0.058 µ g mL -1 which is low enough to quantify DEHP accurately in reconstituted injections much below the tolerance limits 8 .
A mixture of methanol, propan-2-ol and water in a ratio of 250:100:50 respectively was used as mobile phase as well as diluent. The analysis was performed using a chromatograph equipped with zorbax extend C18, 250 mm x 4.6 mm, 5 micrometer. HPLC column was thermostated at 30 °C temperature. Ultraviolet detection was done at 225 nm. The retention time of DEHP peak was obtained about 8 minutes with the mobile phase flow rate of 1.5 mL minute -1 . An injection volume of 100 µL and run time of 15 minutes was found adequate for good response and excellent separation of DEHP. Sample tray temperature was kept ambient. Diluent was used for blank analysis.

Preparation of standard solution
About 1 g of DEHP standard was accurately weighed into a 100 mL dried volumetric flask, previously containing about 80 mL of methanol. Solution was sonicated with constant swirling to mix the contents and volume was made up to the mark with methanol. 1 mL of this solution was transferred into a 100 mL volumetric flask and volume was made up to the mark with diluent. This solution was suitably diluted with diluent to get a final concentration of about 1 µg mL -1 .

Preparation of sample solution
Injection vial was reconstituted with 100 mL of reconstituting diluent to get a clear reconstituted solution and 5 mL of this was further diluted to 10 mL with diluent. During inuse stability studies, samples were stored at different temperatures and analysed at various time points. Reconstituted samples were stored in reconstituent bags as per instructions given on pack insert 9 . Prior to analysis, suitability of the chromatographic system was ensured by injecting blank and standard solutions.
Blank was injected to check any interference at the retention time of DEHP. System suitability parameters, USP tailing (not more than 1.5) and USP plate count (not less than 4000) were monitored for DEHP peak in standard solution injections. The precision of the chromatographic system was ensured by checking the %RSD of area counts of five replicate injections of standard solution (not more than 2.0%). The proposed analytical method was validated for specificity, linearity, precision, accuracy, ruggedness and stability in analytical solution as per ICH 17 guidelines.

Specificity
Specificity of the method was checked by injecting the standard solution, sample solution and sample solution spiked with known related impurities of imipenem and cilastatin in recommended reconstituting diluents.

Linearity of response and limit of quantification (LOQ)
The linearity of the method was checked in the concentration range of 0.07 to 4.4 µg mL -1 of DEHP.

Precision
Six replicates injections of standard solution were given to establish system precision. To establish method precision, six samples of single batch of imipenem and cilastatin for injection were prepared independently using 10% mannitol as reconstituting diluent and analyzed.

Accuracy
Known amount of sample was spiked in triplicates with known quantities of DEHP at different levels using 10% mannitol and 10% dextrose as reconstituting diluents. The samples were analysed by the proposed method and amount of DEHP recovered was calculated after making corrections for the amount already present.

Ruggedness
To demonstrate the ruggedness of the method, interanalyst study was performed using 10% mannitol as reconstituting diluent. Six samples of same batch were reconstituted and analyzed by two different analysts on two different days using different chromatographic systems.

Stability in analytical solution (SIAS)
Sample solutions prepared using the diluents 10% mannitol, 10% dextrose and 5% dextrose with 0.02% sodium bicarbonate were injected repeatedly at various time intervals up to about 15 h keeping the sample solutions at room temperature during the study.

Monitoring DEHP content in reconstituted imipenem and cilastatin for injections
Imipenem and cilastatin for injections were reconstituted separately in 10% mannitol, 10% dextrose and 5% dextrose with 0.02% sodium bicarbonate and stored in PVC bags for 24 h at 4 °C and for 4 h at 25 °C.

Results and Discussion
Since the DEHP has chromophoric groups, attempts were made using HPLC with ultraviolet detection to develop the analytical method for determination of DEHP content in aqueous samples. Initially, a chromatograph equipped with zorbax extend C18, 250 x 4.6 mm, 5 µm, utilizing acetonitrile and methanol combination as mobile phase was used to determine DEHP in aqueous samples 10 . Peak shape of DEHP was symmetrical for samples prepared in dextrose, sodium chloride and 5% dextrose with 0.02% sodium bicarbonate reconstituents. However, sample got precipitated when sample reconstituted using mannitol were diluted using this diluent.
To overcome precipitation issue observed, hexane extraction was tried for extraction of DEHP from reconstituted samples. Again, precipitation was observed using this sample preparation technique. Hence, it was concluded that existing mobile phase composition is not suitable for analysing mannitol based reconstituents.
Several combinations of methanol and acetonitrile were tried as mobile phase to get clear sample solution for mannitol based reconstituents. Precipitation was observed in all the trials. When methanol was used as diluent, initially the sample solution was clear but sample got turbid upon keeping it for some time at room temperature. Hence, a slow precipitation of mannitol was observed with methanol.
Further, water and propan-2-ol were introduced in mobile phase composition along with methanol. A suitable ratio of methanol, propan-2-ol and water was optimized as eluent and diluent to obtain clear solution. Using this ratio, the sample solution remained clear when it was stored for the recommended period. The finalized method was found suitable for direct analysis of aqueous samples without any preconcentration or liquid extraction and was found applicable to all the reconstituents used for reconstitution of imipenem and cilastatin for injection. The proposed method was optimized and validated as per ICH 18 guidelines. There was no interference of DEHP peak from imipenem, cilastatin and related impurity peaks. Peak purity plot of DEHP peak indicated the peak was pure and there were no co-eluting peaks Figure 1-6. This indicated the specificity of the developed method.       Figure 6. Chromatogram of control sample solution in 10% mannitol with peak purity plot.
The system suitability parameters on different days were well met as evident from Table 1. The method was found linear in the concentration range of 0.07 to 4.4 µg mL -1 of DEHP, with a correlation coefficient of 0.9997 (Table 2). The limit of quantification for the analytical method was found to be 0.058 µg mL -1 (Table 3). A % relative standard deviation of 1.19 % for the area counts of DEHP peak for six replicate injections of standard solution indicate the system precision (Table 4). Method was found to be precise as indicated by the % relative standard deviation of 5.26% for DEHP content in six sample preparations ( Table  5). The percentage recoveries were observed to be in the range of 93.40 % to 105.67% (Table 6 & Table 7). An overall % relative standard deviation of 6.02% for DEHP content for twelve samples analyzed in two sets demonstrated the ruggedness of the method (Table  8). DEHP was found to be stable for about 15 h at room temperature in 10% mannitol, 10% dextrose and 5% dextrose with 0.02% sodium bicarbonate (   The maximum DEHP concentration in reconstituted injections of imipenem and cilastatin were observed to be 0.15 µg mL -1 for sample prepared and stored in 10% mannitol solution for 4 h and in 10% dextrose solution for 24 h storage. Both the values were below the tolerance limits (0.3 µg mL -1 ) ( Table 10). There was no increase in content of DEHP upon storing the reconstituted injections for prescribed hours, thereby indicating that DEHP did not leach significantly into the reconstituting solution from PVC.

Conclusion
The proposed method is a convenient, simple and sensitive for direct determination of DEHP in reconstituting diluents and reconstituted imipenem and cilastain for injection. Using this method, analysis time is very short wherein run time is only 15 minutes. Since the same method can be used for various reconstituting dilutents, the proposed method is versatile with respect to its applicability in analysing DEHP content. Since it is required to be controlled within prescribed tolerance limits as per regulatory bodies, this method can be used as a quality control tool by reconstituting diluent manufacturers for monitoring the level of DEHP in reconstituting diluents. Further, this method can be extended to monitor DEHP content in other injectable formulations which require reconstitution prior to administration.