An improved micromethod for vancomycin determination by high-performance liquid chromatography

An Overview of Analytical Methodologies for Determination of Vancomycin in Human Plasma

Vancomycin is regarded as the last resort of defense for a wide range of infections due to drug resistance and toxicity. The detection of vancomycin in plasma has always aroused particular concern because the performance of the assay affects the clinical treatment outcome. This article reviews various methods for vancomycin detection in human plasma and analyzes the advantages and disadvantages of each technique. Immunoassay has been the first choice for vancomycin concentration monitoring due to its simplicity and practicality, occasionally interfered with by other substances. Chromatographic methods have mainly been used for scientific research due to operational complexity and the particular requirement of the instrument. However, the advantages of a small amount of sample needed, high sensitivity, and specificity makes chromatography irreplaceable. Other methods are less commonly used in clinical applications because of the operational feasibility, clinical application, contamination, etc. Simplicity, good performance, economy, and environmental friendliness have been points of laboratory methodological concern. Unfortunately, no one method has met all of the elements so far.

An LC-MS/MS method to determine vancomycin in plasma (total and unbound), urine and renal replacement therapy effluent

AIM

Critical illness and medical interventions, such as renal replacement therapy, can cause changes to vancomycin pharmacokinetics and lead to suboptimal dosing. To comprehensively characterize vancomycin pharmacokinetic a method must measure vancomycin in a range of clinical matrices.

RESULTS

A LC-MS/MS method was developed using hydrophilic interaction liquid chromatography and microsample volumes, where possible. For all matrices, the linear concentration range was 1-100 μg/ml, interassay accuracy and precision was within 15%, and recovery above 80%. No matrix effects were observed. Calibration equivalence may be applied for some matrix combinations.

CONCLUSION

A method for the analysis of vancomycin in plasma (total, unbound), urine and renal replacement therapy effluent, suitable for use in any patient pharmacokinetic study, has been developed and validated.

New liquid chromatography-tandem mass spectrometry method for routine TDM of vancomycin in patients with both normal and impaired renal functions and comparison with results of polarization fluoroimmunoassay in light of varying creatinine concentrations

A new LC-MS/MS method with simple sample extraction and a relatively short period of vancomycin analysis for routine therapeutic drug monitoring was developed and validated. 50μL serum was precipitated using 20μL 33% trichloroacetic acid and 0.5mol/L NH₄OH was added to increase pH before analysis. A RP BEH C18, 1.7μm, 2.1×50mm column maintained at 30°C and tobramycin as internal standard were used. Mass detection was performed in positive electrospray mode. The results obtained with LC-MS/MS method were correlated with an FPIA assay (Abbott AxSYM) using mouse monoclonal antibody. Subjects were divided into three groups according to creatinine levels (53.5±19.1, 150.2±48.4, 471.7±124.7μmol/L) and Passing-Bablok regression analysis and Bland-Altman analysis were used to compare vancomycin concentrations. The results of subjects with both normal and higher creatinine levels correlated very well and the linear regression model equations were near ideal (LC-MS_VAN=0.947×Abbott_VAN+0.192 and LC-MS_VAN=0.973×Abbott_VAN-0.411 respectively). Dialyzed patients with the highest creatinine levels showed about 14% greater vancomycin concentration with the FPIA assay (LC-MS_VAN=0.866×Abbott_VAN+2.127). This overestimation probably due to the presence of the metabolite CDP ought not to be of clinical relevance owing to the wide range of recommended vancomycin concentration.

Selection of appropriate analytical tools to determine the potency and bioactivity of antibiotics and antibiotic resistance

Antibiotics are the chemotherapeutic agents that kill or inhibit the pathogenic microorganisms. Resistance of microorganism to antibiotics is a growing problem around the world due to indiscriminate and irrational use of antibiotics. In order to overcome the resistance problem and to safely use antibiotics, the correct measurement of potency and bioactivity of antibiotics is essential. Microbiological assay and high performance liquid chromatography (HPLC) method are used to quantify the potency of antibiotics. HPLC method is commonly used for the quantification of potency of antibiotics, but unable to determine the bioactivity; whereas microbiological assay estimates both potency and bioactivity of antibiotics. Additionally, bioassay is used to estimate the effective dose against antibiotic resistant microbes.

Simultaneously, microbiological assay addresses the several parameters such as minimal inhibitory concentration (MIC), minimum bactericidal concentration (MBC), mutation prevention concentration (MPC) and critical concentration (Ccr) which are used to describe the potency in a more informative way. Microbiological assay is a simple, sensitive, precise and cost effective method which gives reproducible results similar to HPLC. However, the HPLC cannot be a complete substitute for microbiological assay and both methods have their own significance to obtain more realistic and precise results.

Application of microbiological assay to determine pharmaceutical equivalence of generic intravenous antibiotics

Background

Demonstration of equivalent amounts of the same active pharmaceutical ingredient (API) between generic and innovator products (pharmaceutical equivalence) is a basic requirement of regulatory agencies for intravenous generic drugs prior to clinical use, and constitutes the pivotal point to assume therapeutic equivalence. Physicochemical methods are preferred instead of biological assays to determine concentration of drugs in biological fluids, but it does not permit direct quantification of potency. Here, we report a microbiological assay using large plates designed to determine potency and concentration of pharmaceutical-grade antibiotics for injection and a statistical method to assess the in vitro equivalence of generic products with respect to the innovator.

Methods

The assay is based on the concentration-dependent variation of the inhibitory effect of antibiotics on reference bacteria (B. subtilis ATCC 6633, S. aureus ATCC 6538p and S. epidermidis ATCC 12228) in a seeded agar (Difco™ Antibiotic Media), producing a concentration-response linear relationship with two parameters: y-intercept (concentration) and slope (potency). We compared the parameters of 22 generic products (amikacin 4, gentamicin 15, and vancomycin 3 products) against the innovator and the reference powder by Overall Test for Coincidence of the Regression Lines (Graphpad Prism 5.0).

Results

The validation method yielded excellent results for linearity (r² ≥ 0.98), precision (intra-assay variation ≤ 11%; inter-assay variation ≤ 10%), accuracy, and specificity tests according to international pharmacopoeial requirements. Except for one generic of vancomycin that had 25% more API (P_y-intercept = 0.001), the pharmaceutical equivalence was demonstrated in 21 generics with undistinguishable slopes and intercepts (P > 0.66). Potency estimates were 99.8 to 100.5, 99.7 to 100.2 and 98.5 to 99.9% for generic products of amikacin, gentamicin and vancomycin, respectively.

Conclusion

The proposed method allows rapid, cost-saving, precise, and accurate determination of pharmaceutical equivalence of drugs in pharmaceutical dosage-form, and may be used as a technique for testing generic antibiotics prior to their approval for human use.

Investigation of vancomycin and related substances by liquid chromatography/ion trap mass spectrometry

Liquid chromatography (LC) methods compatible with mass spectrometry (MS) that are suitable for impurity profiling of vancomycin mixtures have not been described in the literature. The mobile phases of the existing methods contain non-volatile additives and/or solvents that give problems in combination with MS. In this paper, a reversed-phase LC/tandem mass spectrometry method is described for the investigation of vancomycin and related substances. The LC method uses a Zorbax Extend C18 column (250 x 4.6 mm i.d.), 5 microm, and a mobile phase consisting of methanol, water and ammonium acetate solution (pH 9.0). This method allows us to separate vancomycin and its impurities. Mass spectral data are acquired on an LCQ ion trap mass spectrometer equipped with an electrospray interface operated in the positive and negative ion modes. The LCQ is ideally suited for identification of impurities and related substances because it provides on-line LC/MSn capability, which allows efficient identification without time-consuming isolation and purification procedures. Using this method, the fragmentation of vancomycin and known derivatives was studied and the structures of six substances occurring in commercial samples were elucidated.

Successful management of discovered pH dependence in vancomycin recovery studies: novel HPLC method for microdialysis and plasma samples

Vancomycin is a glycopeptide antibiotic approved for the treatment of serious infections or patients allergic to beta-lactams. A rapid HPLC assay using UV detection for the determination in microdialysate and human plasma was developed. After sample preparation, using methanol and trichloroacetic acid for plasma and water for microdialysate, 20 microL were injected and separated on a RP(18) column. Overall, the assay exhibited good precision and accuracy. The diffusion properties of vancomycin investigated in in vitro microdialysis experiments revealed an unfavourable concentration dependence avertable by keeping a constant pH using phosphate buffer as perfusate. The mean relative recoveries were 27.8% [coefficient of variation (CV) 11.1%] and 33.2% (CV 8.3%) for retrodialysis and recovery experiments, respectively. Following characterization of vancomycin in in vitro microdialysis, the developed setting is suitable for application in (pre-)clinical studies.

Regional delivery of vancomycin using pluronic F-127 to inhibit methicillin resistant Staphylococcus aureus (MRSA) growth in chronic otitis media in vitro and in vivo

For the effective treatment of chronic otitis media caused by methicillin resistant Staphylococcus aureus (MRSA), the local delivery of vancomycin was attempted based on a formulation with pluronic F-127. The phase transition property of pluronic F-127, which is liquid at room temperature and becomes solid at body temperature, enabled convenient injection and reduced the side effects of the potent antibiotic. The temperature responsive polymer-vancomycin matrix system was also shown to effectively deliver vancomycin locally to the infected inner ear, plus the MRSA growth was inhibited by the formulation. The otitis media was completely cured by the sustained local delivery of vancomycin from the matrix in vivo. Accordingly, this system has potential clinical application for treating chronic otitis media with improved patient compliance and better therapeutic efficacy.

An automated analyzer for vancomycin in plasma samples by column-switching high-performance liquid chromatography with UV detection

An automated analyzer for vancomycin in rat plasma by column-switching high-performance liquid chromatography (HPLC) with UV detection was developed. The method includes in-line extraction of vancomycin by ion-exchange cartridge column and a separation on a reversed-phase column with UV detection at 215 nm. Plasma samples were diluted by mobile phase solution and directly injected to HPLC. Vancomycin was quantitatively recovered from rat plasma samples. The separation was completed within 15 min. The calibration curve was linear over the range from 0.5 to 100 microg/mL with the detection and quantification limits of 0.5 microg/mL (2.5 ng on column; signal-to-noise ratio = 3). The values of precision in intra- and inter-day assays (n = 3) were less than 1.92 and 3.69%, respectively. This method does not require time-consuming pre-treatment and is suitable for the routine assay of plasma samples.

Development and validation of an improved method for the analysis of vancomycin by liquid chromatography selectivity of reversed-phase columns towards vancomycin components

The current method prescribed in official monographs for the purity control of vancomycin is inappropriate in that several components are not separated from each other and other components are coeluted with the main component vancomycin B. The method uses an ODS column at pH 3.2. In this study, several changes were introduced in order to improve the separation. The optimization of the separation method at low pH indicated that pH 1.7 was optimum and that the use of dioxane as organic modifier drastically improved the separation. These conditions were used to test a set of more than 40 reversed-phase columns for their selectivity towards vancomycin components. The selection of the most suitable columns was performed by means of principal component analysis. Most of these columns did not allow the separation of didechlorovancomycin from monodechlorovancomycin 1. It was found that neutral to slightly alkaline mobile phases allowed better separation. Further optimization of the separation method and a robustness study were performed by means of experimental design. This optimization indicated that pH 7.7 was optimum and gradient elution was also used to effect complete analysis. The final method uses a Kromasil column and the mobile phase comprises dioxane, water and ammonium formate solution pH 7.7. The separation of monodechlorovancomycin 2 and of some unknown impurities from the main component vancomycin B is described for the first time. The method shows good repeatability, linearity and sensitivity.

New sensitive assay of vancomycin in human plasma using high-performance liquid chromatography and electrochemical detection

A method using reversed-phase high-performance liquid chromatography with electrochemical detection for the analysis of vancomycin in human plasma was developed. Chromatographic conditions included an octadecyl column, a mobile phase of acetonitrile-sodium phosphate buffer (pH 7) (12:88), a total run time of 12 min, and coulometric electrochemical detection at +700 mV. Linear detector response was found in the range 5-100 microg ml(-1) after a 1:80 dilution or from 0.5 to 50 microg ml(-1) after a 1:20 dilution of the samples. In both cases the correlation coefficient (r) of the calibration curve standard was better than 0.995. Vancomycin determination was based on a denaturation of plasma proteins with methanol, then a dilution with mobile phase was performed. Recovery of vancomycin from plasma was 103.1+/-3.9%, and no interference from commonly used drugs or endogenous compounds was observed. A significant correlation was shown with the EMIT assay (r=0.92, P<0.001) using clinical samples from children. This HPLC technique is simple, sensitive, rapid, precise, selective and requires only 100 microl of plasma for completion.

Quantitation of vancomycin and its crystalline degradation product (CDP-1) in human serum by high performance liquid chromatography

The delayed clearance of vancomycin results in accumulation of vancomycin crystalline degradation product, CDP-1, in the bodies of renally impaired patients. The 2 isomers, CDP-1-M (major) and CDP-1-m (minor), of CDP-1 are antibiotically inactive but cross-react with some immunoassays that use polyclonal antibodies resulting in falsely elevated results. A high performance liquid chromatographic (HPLC) method was developed to quantitate vancomycin and CDP-1 in the serum of renal patients. After solid phase extraction of 200 microliters serum, the separation of vancomycin, the 2 isomers of CDP-1 and the internal standard (cefazolin) was accomplished by gradient HPLC on a reversed phase C18 column with detection at 210 nm. Linearity was established from 1 to 25 and 25 to 100 micrograms ml-1 vancomycin and 1 to 25 micrograms ml-1 CDP-1. Coefficients of variation for vancomycin and CDP-1 were 3.3-8.6% (n = 10) and 2.8-5.2% (n = 8).

Abbott AxSYM Vancomycin II assay: multicenter evaluation and interference studies

The authors evaluated the performance characteristics of the Abbott AxSYM Vancomycin II immunoassay in sera of patients with (n = 93 samples) and without (n = 327 patients) renal dysfunction. Correlation of vancomycin measurements with the Abbott AxSYM Vancomycin, Abbott TDx/TDxFLx, Syva enzyme-multiplied immunoassay technique (EMIT), DuPont automated chemistry analyzer (ACA), and high-performance liquid chromatography methods showed acceptable correlation as indicated by: slope values >0.95, r-values >0.97, y-intercepts <1.7 microg/ml, and S(y/x) ranging from 9% to 15% of the average vancomycin value. The AxSYM Vancomycin II assay showed acceptable correlation with AxSYM vancomycin, TDx/TDxFLx, and high-performance liquid chromatography methods in 93 samples from patients with renal dysfunction. This monoclonal antibody-based assay showed no apparent interference from the presence of human antimouse antibody (HAMA) or the microbiologically inactive vancomycin crystalline degradation product (CDP). The authors conclude that the AxSYM Vancomycin II assay showed satisfactory agreement with other methods tested in this study.

Development of a quantitative vancomycin immunoassay for the Abbott AxSYM analyzer

A novel fluorescence polarization immunoassay for vancomycin on Abbott AxSYM analyzer is described. The immunoassay allows for the accurate quantification of vancomycin in the presence of the crystalline degradation product (CDP). It displays dilution linearity from 1.0 microg/ml to 100.0 microg/ml, coefficients of variation ranging from 2.94% to 4.26%, recovery from 98% to 105%, and a sensitivity of <2.0 microg/ml. The assay demonstrates no cross-reactivity to crystalline degradation product, and to commonly-prescribed and over-the-counter drugs, as well as a minimum interference from endogenous substances.