Determination of linezolid in growth media by high-performance liquid chromatography with on-line extraction

Development and Validation of an HPLC-UV Method for Quantitation of Linezolid: Application to Resistance Study Using in vitro PK/PD Model

Background

Linezolid (LNZ), an oxazolidinone antibiotic, has 100% oral bioavailability and favorable activities against gram-positive pathogens. The in vitro PK/PD model was developed based on concentrations obtained with routine doses in humans can be used to guide dose optimization in the clinic.

Methods

In this study, we employed an in vitro PK/PD model to simulate the changes in the plasma concentration of linezolid in the human body against a clinical isolate of MRSA in vitro. A high-performance liquid chromatography (HPLC)-UV method was applied to measure the concentration of linezolid. Bacterial samples were collected at different times from the central compartment for count.

Results

The chromatographic separation was carried out with an AichromBond-AQC18 column(250mm×4.6mm, 5μm), using a mobile phase of water with 0.1% formic acid:acetonitrile 70:30 (v/v), followed by detection at 254 nm, and a single detection run was completed within 10 min. The method was validated by estimating the precision and accuracy for the inter- and intra-day analyses in the concentration range of 0.25-32 mg/L. The method was linear over the investigated range of 0.125-32 mg/L, with all correlation coefficients R² = 0.9999. The intra-day and inter-day precisions were within 7.598%, and the method recovery ranged from 90.912% to 106.459%. In vitro PK/PD model, both the absorption and elimination of linezolid being simulated can be precisely controlled by computer. In the control group, the bacterial reached 7.9 Log10CFU/mL in the first 48h and maintained until the end, indicating that the colonies grew well in vitro PK/PD model. In the linezolid 600 mg q12h administration group, the colony decreased to 2.39 Log10CFU/mL at 24h, showing a good bactericidal effect; however, the colonies resumed growth to the initial level in 48h, indicating an emergence of resistance.

Conclusion

We successfully established an in vitro infection PK/PD model and developed an HPLC-UV method to determine linezolid concentration for resistance investigation. The results suggest that the 600 mg q12h dosing regimen may no longer be applicable and requires optimization.

Pharmacokinetics and Pharmacodynamics of Linezolid in Patients With Sepsis Receiving Continuous Venovenous Hemofiltration and Extended Daily Hemofiltration

BACKGROUND

This prospective study compared pharmacokinetics (PK) and pharmacodynamics (PD) of linezolid in patients with sepsis receiving continuous venovenous hemofiltration (CVVH) with patients receiving extended daily hemofiltration (EDH).

METHODS

Patients with sepsis treated with linezolid and CVVH or EDH were included. Serial blood samples were collected and linezolid concentrations measured. PKs were analyzed using Pmetrics. Monte Carlo simulations were used to evaluate PD target achievement.

RESULTS

From 20 patients, 320 blood samples were collected for PK and PD analysis. PK profiles of linezolid were best described by a 2-compartment model. PK parameters were not significantly different between EDH and CVVH groups and were associated with body weight, renal replacement therapy (RRT) duration, and sequential organ failure assessment score. Monte Carlo simulations showed poor fractional target attainment for a minimum inhibitory concentration (MIC) of 2 mg/L with standard 600 mg intravenous administration every 12 hours.

CONCLUSIONS

Patients with sepsis receiving RRT exhibited variability in PK/PD parameters for linezolid. PK parameters were not significantly different between CVVH- and EDH-treated patients. Higher probability of target attainment would be achievable at a MIC of 2 mg/L in EDH patients. Higher linezolid doses should be considered for patients on RRT to achieve adequate blood levels.

A critical review of HPLC-based analytical methods for quantification of Linezolid

Linezolid is a synthetic antimicrobial agent belonging to the oxazolidinone class. Since its approval in the year 2000 until now, linezolid remains the main representative drug for the oxazolidinone class of drugs, which is used in therapy due to its unique mode of action, which involves inhibition of protein synthesis. As linezolid holds great importance in antimicrobial therapy, it is necessary to compile the various analytical methods that have been reported in the literature for its analysis. Analytical techniques used for pharmaceutical analyses and therapeutic drug monitoring play an important role in comprehending the aspects regarding bioavailability, bioequivalence, and therapeutic monitoring during patient follow-ups. Even though linezolid has had the approval for clinical use for more than 18 years now, most of the analytical methods for its determination reported in the scientific literature are the ones which utilize HPLC. Therefore, the present review provides a summary of the HPLC-based methods used in the determination and quantification of linezolid in different matrices since the time of its discovery.

Linezolid-A Review of Analytical Methods in Pharmaceuticals and Biological Matrices

Bacterial resistance to antibiotics is a growing phenomenon in the world. Considering the relevance of antimicrobials for population and the reduction in the registration of new antimicrobials by regulatory agencies, proper quality control is required to minimize the spread of bacterial resistance and ensure the effectiveness of a treatment, as well as safety for the patient. The recent addition to the antimicrobial world is the oxazolidinone classes of antibiotics, especially useful to treat infections caused by Gram-positive bacteria. Eperezolid and linezolid (LIN) are the two members of the oxazolidinone class of antibiotics. LIN was the first oxazolidinone approved by the Food and Drug Administration. The present review focuses on the analytical methods for the assessment of LIN in pharmaceuticals and biological matrices. The critical validation parameters like the linearity, limit of detection, limit of quantification are discussed for the individual method. Also the critical quality attributes like the sensitivity and the sample preparation techniques for bioanalytical methods are also discussed. Furthermore, some future trends that can be incorporated in the determination of similar drugs are also suggested.

Two and three way spectrophotometric-assisted multivariate determination of linezolid in the presence of its alkaline and oxidative degradation products and application to pharmaceutical formulation

Linezolid (LIN) is determined in the presence of its alkaline (ALK) and oxidative (OXD) degradation products without preliminary separation based on ultraviolet spectrophotometry using two-way chemometric methods; principal component regression (PCR) and partial least-squares (PLS), and three-way chemometric methods; parallel factor analysis (PARAFAC) and multi-way partial least squares (N-PLS). A training set of mixtures containing LIN, ALK and OXD; was prepared in the concentration ranges of 12-18, 2.4-3.6 and 1.2-1.8 μg mL(-1), respectively according to a multilevel multifactor experimental design. The multivariate calibrations were obtained by measuring the zero-order absorbance from 220 to 320 nm using the training set. The validation of the multivariate methods was realized by analyzing their synthetic mixtures. The capabilities of the chemometric analysis methods for the analysis of real samples were evaluated by determination of LIN in its pharmaceutical preparation with satisfactory results. The accuracy of the methods, evaluated through the root mean square error of prediction (RMSEP), was 0.058, 0.026, 0.101 and 0.026 for LIN using PCR, PLS, PARAFAC and N-PLS, respectively. Protolytic equilibria of LIN and its degradation products were evaluated using the corresponding absorption spectra-pH data obtained with PARAFAC. The obtained pKa values of LIN, ALK and OXD are 5.70, 8.90 and 6.15, respectively. The results obtained were statistically compared to that of a reported HPLC method, and there was no significant difference between the proposed methods and the reported method regarding both accuracy and precision.

Comparison of the pharmacokinetics of linezolid in burn and non-burn rabbits

Linezolid is effective on many resistant organisms for the treatment of severe infections in burns. However, its pharmacokinetics was difficult to predict after major burns. The study aimed to describe the pharmacokinetic properties of linezolid administered intravenously at a dose of 10 mg/kg in severely burned rabbits in comparison to that in non-burns. Linezolid concentrations were quantitatively analyzed by high-performance liquid chromatography. The direct consequence of the physiological changes after burn injury was lower plasma linezolid concentrations. In addition, burn injury induced significantly altered pharmacokinetic parameters with higher inter-individual variability. The distribution volume and clearance rate were increased (2.88 vs. 1.92 L/kg, P > 0.05; 0.28 vs. 0.20 L/h/kg, P < 0.05), and the AUC0-∞ was significantly lower (37.99 vs. 51.47 mg/L h, P < 0.05). However, there were almost no changes in half-life and mean residence time. These results suggested that therapeutic drug monitoring and dosage individualization of linezolid in patients with severe burns were necessary.

Validation of a new HPLC-UV method for determination of the antibiotic linezolid in human plasma and in bronchoalveolar lavage

A rapid and selective HPLC-UV method was developed for the quantification of linezolid (LNZ) in human plasma and bronchoalveolar lavage (BAL) at the concentrations associated with therapy. Plasma samples were extracted by solid-phase extraction followed by evaporation to dryness and reconstitution in mobile phase solution. The chromatographic separation was carried out on a C18 column with an isocratic mobile phase consisting of dihydrogen phosphate buffer 50 mm (pH 3.5) and acetonitrile (60:40 v/v). The detection was performed using a photodiode array. Under these conditions, a single chromatographic run could be completed within 12 min. The method was validated by estimating the precision and the accuracy for inter- and intra-day analysis in the concentration range of 25-25600 ng/mL. The method was linear over the investigated range with all the correlation coefficients R > 0.999. The intra- and inter-day precision was within 8.90% and the accuracy ranged from -4.76 to +5.20%. This rapid and sensitive method was fully validated and could be applied to pharmacokinetic study for the determination of LNZ levels in human plasma and BAL samples.

A validated stability-indicating LC method for the separation of enantiomer and potential impurities of Linezolid using polar organic mode

Although a number of methods are available for evaluating Linezolid and its possible impurities, a common method for separation if its potential impurities, degradants and enantiomer in a single method with good efficiency remain unavailable. With the objective of developing an advanced method with shorter runtimes, a simple, precise, accurate stability-indicating LC method was developed for the determination of purity of Linezolid drug substance and drug products in bulk samples and pharmaceutical dosage forms in the presence of its impurities and degradation products. This method is capable of separating all the related substances of Linezolid along with the chiral impurity. This method can also be used for the estimation of assay of Linezolid in drug substance as well as in drug product. The method was developed using Chiralpak IA (250 mm×4.6 mm, 5 μm) column. A mixture of acetonitrile, ethanol, n-butyl amine and trifluoro acetic acid in 96:4:0.10:0.16 (v/v/v/v) ratio was used as a mobile phase. The eluted compounds were monitored at 254 nm. Linezolid was subjected to the stress conditions of oxidative, acid, base, hydrolytic, thermal and photolytic degradation. The degradation products were well resolved from main peak and its impurities, proving the stability-indicating power of the method. The developed method was validated as per International Conference on Harmonization (ICH) guidelines with respect to specificity, limit of detection, limit of quantification, precision, linearity, accuracy, robustness and system suitability.

Activity of linezolid in an in vitro pharmacokinetic-pharmacodynamic model using different dosages and Staphylococcus aureus and Enterococcus faecalis strains with and without a hypermutator phenotype

The influence of antibiotic dosages and bacterial mutator phenotypes on the emergence of linezolid-resistant mutants was evaluated in an in vitro pharmacokinetic-pharmacodynamic model. A twice-daily 0.5-h infusion of a 200-, 600-, or 800-mg dose for 48 h was simulated against four strains (MIC, 2 microg/ml): Staphylococcus aureus RN4220 and its mutator derivative MutS2, Enterococcus faecalis ATCC 29212, and a mutator clinical strain of E. faecalis, Ef1497. The peak concentrations (4.38 to 4.79, 13.4 to 14.6, and 19.2 to 19.5 microg/ml) and half-lives at beta-phase (5.01 to 6.72 h) fit human plasma linezolid pharmacokinetics. Due to its bacteriostatic property, the cumulative percentages of the dosing interval during which the drug concentration exceeded the MIC (T > MIC), 66.6 and 69.1% of the dosing interval, were not significant, except for Ef1497, with an 800-mg dose and a T > MIC of 80.9%. At the standard 600-mg dosage, resistant mutants (2- to 8-fold MIC increases) were selected only with Ef1497. A lower, 200-mg dosage did not select resistant mutants of E. faecalis ATCC 29212, but a higher, 800-mg dosage against Ef1497 did not prevent their emergence. For the most resistant mutant (MIC, 16 microg/ml), characterization of 23S rRNA genes revealed the substitution A2453G in two of the four operons, which was previously described only in in vitro mutants of archaebacteria. Nevertheless, this mutant did not yield further mutants under 600- or 200-mg treatment. In conclusion, linezolid was consistently efficient against S. aureus strains. The emergence of resistant E. faecalis mutants was probably favored by the rapid decline of linezolid concentrations against a strong mutator, a phenotype less exceptional in E. faecalis than in S. aureus.