Simultaneous blood and brain sampling of cephalexin in the rat by microdialysis and microbore liquid chromatography: application to pharmacokinetics studies

To circumvent the need for laborious sample clean-up and multiple blood sampling, a system was developed consisting of on-line microdialysis coupled to microbore liquid chromatography and ultraviolet detection. The system was designed for the simultaneous and continuous monitoring of unbound blood and brain cephalexin in the rat following single bolus intravenous administrations (10 mg/kg, n = 6). Microdialysis probes were inserted into the jugular vein and brain striatum, respectively, for blood and brain sampling. Chromatographic conditions consisted of a mobile phase of methanol-100 mM monosodium phosphoric acid (20:80, v/v, pH 5.0) pumped through a microbore reversed-phase column at a flow-rate of 0.05 ml/min. Detection wavelength was set at 260 nm. The method was validated for response linearity as well as intra- and inter-day variabilities. Rapid appearance of cephalexin in the striatal dialysate suggested good blood-brain barrier penetration. This study provided pharmacokinetics information for cephalexin as well as demonstrated the applicability of this continuous sampling method for pharmacokinetics studies.

Spectrofluorometric determination of alpha-aminocephalosporins in biological fluids and pharmaceutical preparations

A selective and highly sensitive fluorometric method was developed for the determination of four alpha-aminocephalosporins, namely cefaclor, cefadroxil, cephalexin and cephradine. The method involves the reaction of the target compounds with fluorescamine at a specific pH, ranging from 7.8 to 8.4. The produced derivatives exhibit maximum fluorescence intensities at 472-478 nm after excitation at 370-372 nm. The method is highly specific because other alpha-aminocephalosporins whose alpha amino group was blocked do not react similarly and hence do not interfere. At the specific pH range of the reaction where no degradation may occur with that medium the proposed method can be utilised as a stability-indicating assay. The different experimental parameters affecting the derivatisation reaction were carefully studied and incorporated into the procedure. Under the described conditions, the proposed method is linear over the concentration range of 0.05(-1) microg/ml(-1) for both cefaclor and cephalexin, and 0.05-0.65 and 0.025-0.5 microg/ml(-1) for cefadroxil and cepharadine, respectively and the coefficients of determination were greater than 0.999 (n = 3). The recoveries of the title compounds from spiked serum ranged from 88.6 to 89.7% and from spiked urine from 92.2 to 93.3% with a limit of quantitation (LOQ) of 25-50 ng/ml(-1) and limit of detection (LOD) of 5 ng/ml(-1) (S/N = 2) for all drugs. The mechanism of the fluorometric reaction is proposed and the advantages of the proposed method are discussed.

Automated stand-alone flow injection immunoanalysis system for the determination of cephalexin in milk

A fully automated stand-alone flow injection immunoanalysis (FIIA) device for the determination of cephalexin in milk is developed with a main focus on the investigation of the influence of the sample matrix. The system is based on principles of flow-through immunoassays and on sequential addition of the assay components to an immunoreactor. Protein G is immobilised on the surface of the immunoreactor serving as affinity matrix for the polyclonal anti-cephalexin antibodies. A cephalexin-alkaline phosphatase conjugate is mixed with the analyte-containing sample and binds in a competitve manner to the corresponding antibodies in the immunoreactor. After substrate addition enzymatically generated p-aminophenol is detected at a carbon electrode at +150 mV vs. Ag/AgCl. One assay cycle takes 16 min including regeneration of the immunoreactor. The large excess of protein G allows for more than 150 regenerations without significant loss of signal height. Due to the high specificity of the anti-cephalexin antibodies, other beta-lactam antibiotics like penicillin, amoxicillin and cloxacillin do not interfere in the measurements, even when added at 10 mg l-1. To deactivate alkaline phosphatase present in milk, samples are heat-treated for 3 min prior to measurements. Cephalexin recoveries from two milk samples are 90 and 110%. The detection limit in milk is 1 microgram l-1 (mean relative standard deviation of 3%), less than the maximum residue level of 4 micrograms per kg milk fixed for some beta-lactam antibiotics in the European Union. The device is suitable for fast quantitative data generation from consecutively measured samples and thus adds to analytical screening methods.

[Studies on the simultaneous measurement of several cephalosporins by RP-HPLC (I)]

This paper reported the research on the simultaneous separation and determination of six cephalosporins by RP-HPLC. Six cephalosporins are cefalcor, cefalexin, cefradine, cefadroxil, cefominox and cefoxitin. The analytical conditions for this method were as follows: a Hypersil ODS C18(200 mm x 4.6 mm i.d., 5 microns), detection wavelength: 254 nm; a mobile phase solution of 50 mmol/L monopotassium phosphate (pH 3.4)-acetonitrile (87.5:12.5) and DAD detector. The flow rate was 1.0 mL/min. The calibration curves of the six compounds were linear, the correlation coefficients were 0.9951 for cefominox, 0.9999 for the others, the range were 164 ng-16.4 micrograms for cefominox, 99 ng-9.934 micrograms for cefadroxil, 104 ng-10.358 micrograms for cefalcor, 122 ng-12.224 micrograms for cefalexin, 107 ng-10.702 micrograms for cefradine and 115 ng-11.506 micrograms for cefoxitin. The recovery rates were 103.5% for cefominox, 99.3% for cefadroxil, 101.4% for cefalcor, 101.5% for cefalexin, 98.7% for cefradine and 97.6% for cefoxitin. Six cephalosporins were all stable in 50 mmol/L monopotassium phosphate (pH 3.4-4.6). When preparations of these cephalosporins were determined, it is indicated there were no difference between the results by using this method and the pharmacopoeia methods. The total separation time of these cephalosporins was within fifteen minutes. This method is simple, sensitive, rapid and accurate.

[Purity analysis of cephalosporins with capillary zone electrophoresis]

A capillary zone electrophoresis method is proposed for the purity determination of nine cephalosporin drugs. A background electrolyte comprising of either 20 mmol/L pH 9.20 borate buffer or 20 mmol/L pH 6.86 phosphate buffer was used for most drugs studied except for cefaloridine which formed neutral molecules at these pH values. For it 50 mmol/L pH 2.05 phosphate buffer was used instead. Internal normalization method was employed for quantitation. The method is simple, rapid and versatile. Analysis was completed within 8 min. The merits and limitations of the method were also discussed.

Transport studies of beta-lactam antibiotics and their degradation products across electrified water/oil interface

An electrochemical method for quantifying beta-lactam antibiotics (cephalexin and ampicillin) and their hydrolysis products is described. Cyclic voltammetry at the water/nitrobenzene interface in a four-electrode system was used. The zwitterionic compounds were ionized to the necessary electrochemical form by pH adjustment. The pH change, however, resulted also in hydrolysis of the antibiotics. Hydrolysis products were characterized across UV-vis spectrum. The various hydrolysis products as well as the ionized antibiotics were studied in voltammetric transfer from water to nitrobenzene using the method of the interface between two immiscible electrolyte solutions (ITIES). It was concluded that this electrochemical method is suitable for the quantification of beta-lactam antibiotics and their hydrolysis products.

Development of a densitometric method for the determination of cephalexin as an alternative to the standard HPLC procedure

A HPTLC-densitometric method was developed in order to obtain a reliable procedure for routine analysis of cephalexin in pharmaceutical formulations. Optimization of TLC conditions for the densitometric scanning was reached by eluting HPTLC silica gel plates in an horizontal developing chamber. Quantitation of cephalexin was performed in single beam reflectance mode by using a computer-controlled densitometric scanner and applying a five-point calibration. A linear regression has been found in the 200-1000 ng range. The setup method is precise, reproducible and accurate. Recovery was also assessed by comparison with the HPLC USP XXIII alternate method. In this case HPTLC-densitometry appears worth of consideration as being relatively inexpensive and time-saving (up to 12 samples can be determined simultaneously in less than 15 min with a solvent consumption of about 15 ml). The results suggest that the proposed method may be used in place of HPLC for the routine quantitation of cephalexin in both pure and dosage forms.

HPTLC assay of cephalexin and cefaclor in pharmaceuticals

A simple and reliable HPTLC method for the simultaneous determination of cephalexin and cefaclor is developed and validated. The methanol-ethyl acetate-acetone-water (5:2.5:2.5:1.5 v/v/v/v) solvent system is used for the quantitative evaluation of chromatograms. The chromatographic zones, corresponding to the spots of cephalexin and cefaclor on the silica gel plates, are scanned in the reflectance/absorbance mode at 265 nm. The method is found to be reproducible and convenient for the quantitative analysis of cephalexin and cefaclor in its dosage forms.

Antibiotic delivery system using bioactive bone cement consisting of Bis-GMA/TEGDMA resin and bioactive glass ceramics

A novel drug delivery system containing cephalexin (CEX) as a model drug using a new bioactive bone cement consisting of 15% bisphenol-alpha-glycidyl methacrylate (Bis-GMA), 15% triethylene-glycol dimethacrylate (TEGDMA) resin and 70% apatite- and wollastonite-containing glass-ceramic (A-W GC) powder was investigated. A-W GC powder containing CEX powder hardened within 5 min after mixing with Bis-GMA/TEGDMA resin, and furthermore its compressive strength was expected to be higher than that of polymethylmethacrylate cement. In vitro CEX release from bioactive bone cement pellets in a simulated body fluid at pH 7.25 and 37 degrees C continued for more than 2 weeks. The drug release rate increased with increasing amount of CEX in the mixture. All of the drug release profiles followed the Higuchi equation at the initial stage, but not at later stages. As hydroxyapatite was precipitated out on the cement surface, the drug release rate decreased. These results suggest that the CEX release rate from bioactive bone cement could be controlled by varying the amount of drug in the cement system.

A comparative study of LC methods for analysis of cefradine

A comparative study of two isocratic liquid chromatographic methods for the analysis of cefradine is described. The first method is prescribed by the European Pharmacopoeia for the assay of cefradine, using classical alkyl bonded phase (C18) as the stationary phase. Poor reproducibility of the selectivity towards cefradine and its related substances was observed when this method was used and none of the C18 columns examined was able to separate cefradine completely from its potential related substances under the prescribed LC conditions. On the other hand, the second method, which uses poly(styrene-divinylbenzene) as the stationary phase, shows good selectivity even when using columns from different manufacturers and of different age. Four bulk samples of cefradine were analysed following both methods and the results were compared.

The development of an enzyme-linked immunosorbent assay (ELISA) for cephalexin

Cephalexin was structurally modified by the attachment of a spacer at the carboxylic acid through which it was subsequently covalently attached to BSA. This method permitted the molecule to be attached without cleavage of the beta-lactam ring giving a conjugate distinct from previously described immunogenic preparations of penicillins and cephalosporins. This approach required the development of a novel spacer molecule, and its synthesis and characterisation are reported. Rabbits were used to raise antisera and the antibodies produced were characterised with respect to their reactivity with cephalexin and various analogues, other cephalosporins, and a number of penicillins.

Pharmaceutical properties of loracarbef: the remarkable solution stability of an oral 1-carba-1-dethiacephalosporin antibiotic

Loracarbef is an oral 1-carba-1-dethiacephalosporin antibiotic structurally related to cefaclor. Like many beta-lactam antibiotics, loracarbef exists in several hydrated crystalline forms. The pH-solubility profile curve for loracarbef monohydrate is U-shaped, resembling those of other zwitterionic cephalosporins. Loracarbef was found to be much more stable in solution than cefaclor. For example, in pH 7.4 phosphate buffer, loracarbef was unexpectedly found to be 130-150 times more stable than cefaclor and 10-12 times more stable than cephalexin, depending on the phosphate concentration. The pH-stability profile is U-shaped, similar to that of other zwitterionic cephalosporins, and shows maximum stability at the isoelectric point. At any given pH, loracarbef is more stable in solution than any other therapeutically useful cephalosporin. Acetate, borate, citrate, and especially phosphate buffers have catalytic effects on the rate of loracarbef hydrolysis.

Residue determination of two co-administered antibacterial agents--cephalexin and colistin--in calf tissues using high-performance liquid chromatography and microbiological methods

Residues of two antibacterial agents, cephalexin and colistin, co-administered by intramuscular injection to calves, were quantified in four different tissues (muscle, fat, liver and kidney) by column switching HPLC and by a microbiological method. For cephalexin assay, tissue samples with cephradin as internal standard were homogenized in a 5% trichloroacetic acid solution and filtrates were injected onto a concentration precolumn filled with LiChroprep RP-18 (25-40 microns). A clean-up step was incorporated by flowing a mobile phase (methanol-0.01 M phosphate buffer (pH 3.0); 15:85, v/v) through the enrichment column before elution on a LiChrospher RP-18e (5 microns) column with a methanol-phosphate buffer (30:70, v/v) at a flow rate of 1 ml min-1. Spectrometric detection was at 260 nm. An additional "off-line" washing step of extracts with methylene chloride was operated to achieve higher selectivity in the case of liver and kidney samples. The limit for quantitative assay was 0.045 micrograms g-1 with relative standard deviations in the range 5-8% and recoveries within 70%. For microbiological assay of colistin, samples were homogenized in 0.1 M hydrochloric acid-acetonitrile mixtures (3:1, v/v, for kidney and liver; 3:2, v/v, for fat and muscle). The supernatants were assayed by the cylinder plate method after evaporation to dryness under vacuum. Bordetella bronchiseptica ATCC 4617 was chosen as test organism. After a 3-h diffusion step at room temperature, the medium was incubated at 37 degrees C for 18 h and then the diameter of the growth inhibition zones was measured. Sensitivity reached 0.10-0.15 micrograms g-1. Results from the analysed samples over a 7-28 day period after drug administration show that no cephalexin was found at concentrations higher than the quantitation limit in the four test tissues and that colistin was found in muscle (injection site only) for 15 days and in kidney for 21 days.

[Spectral and thermal characterization of cephalosporins. I. Cefadroxil and cefalexin]

Cefadroxil and cefalexine were characterized by thermal and spectral analysis. A vibrational study by infrared and Raman spectroscopies was made to connect the structural data with the antibacterial activity.

Sensitive enzyme immunoassay of cephalexin residues in milk, hen tissues, and eggs

A sensitive enzyme immunoassay for cephalexin (CEX) was developed using the rabbit antiserum to CEX, beta-D-galactosidase-labeled CEX, and a double-antibody separation method. The immunogen of CEX was prepared by coupling the amino group of CEX to thiol groups introduced into bovine serum albumin by the use of N-(m-maleimidobenzoyloxy)succinimide as a cross-linker. Highly titered antiserum to CEX was produced in rabbits immunized with the immunogen. Enzyme labeling of CEX with beta-D-galactosidase was done by using N-(gamma-maleimidobutyryloxy)succinimide as the cross-linker. The limit of detection was 30 ng CEX/mL sample solution. Application of the method to CEX drug residues detected 30 ng/mL in milk, 60 ng/g in egg yolk, and 400 ng/g in hen tissue.

Stability of cephalexin monohydrate suspension in polypropylene oral syringes

The stability of cephalexin monohydrate suspension in plastic oral syringes was studied. Commercially available cephalexin monohydrate powder for oral administration was reconstituted according to the manufacturer's instructions and stored in the original containers or drawn into 5-mL clear polypropylene oral syringes. The original containers and syringes were divided into groups and stored at -20, 4, 25, 40, 60, or 80 degrees C. Powder from two additional lots was similarly reconstituted and packaged; these original containers and syringes were stored at 80 degrees C only to assess interlot variability. Immediately after reconstitution and at specified times during storage, three syringes and the corresponding three original containers stored at each temperature were removed, and their contents were analyzed for cephalexin concentration using the standard USP iodometric assay for antibiotics. The stability-indicating nature of the assay was documented. Cephalexin monohydrate followed a first-order rate of degradation at temperatures of 40, 60, and 80 degrees C. At temperatures of -20, 4, and 25 degrees C, cephalexin monohydrate exhibited no appreciable degradation during the 90-day study period. Cephalexin monohydrate suspension reconstituted from powder as a suspension and repackaged in clear polypropylene oral syringes was stable for 90 days when stored under ambient, refrigerated, and frozen conditions.

Analysis of cephalexin from canine skin biopsy by liquid chromatography with ultraviolet-visible photodiode-array detection

A sensitive and selective ion-paired liquid chromatographic method with UV-VIS photodiode-array detection was developed to measure cephalexin in skin biopsy samples. The method involved a sonication of minced canine skin with ethanol-acetonitrile-water (30:20:50, v/v/v) and ultrafiltration of received extract through 10,000 daltons. Separation of cephalexin from other components was by liquid chromatography using a reversed-phase column which was eluted with an ion-paired acetonitrile-water solution. Detection was achieved with a UV-VIS photodiode-array detector scanning from 230 to 320 nm. Cephalexin in the eluate was quantitated at its wavelength maximum of 260 nm. The evaluation of chromatographic peak homogeneity was performed by absorbance ratios, contour maps, first-derivative spectra and a three-dimensional spectrochromatogram. Additionally, the cephalexin peak identity was confirmed by liquid chromatography-mass spectrometry.

High performance liquid chromatographic analysis of cephalexin in serum and urine

A rapid, highly sensitive high performance liquid chromatographic method has been developed for the determination of cephalexin in serum and urine. Serum protein was precipitated with 1% zinc sulphate solution containing cephradine as the internal standard. The drugs were eluted from a 5 micron, C-18 reversed-phase column at ambient temperature with a mobile phase consisting of acetonitrile-methanol-acetate buffer of pH 4.2 (10:10:80%), at a flow rate of 1.4 ml/min with ultraviolet detection at 254 nm. Each analysis lasted 9 min. Quantification was achieved by the measurement of the peak-height ratio and the relative and absolute recoveries varied from 98 to 103%. Detection limits for cephalexin were 1 microgram/ml in serum and 5 micrograms/ml in urine. Within-run coefficient of variation ranged from 0.73 to 5.63% at three different concentrations for the serum and urine assay.

Rapid chromatographic determination of cefotaxime and its metabolite in biological fluids

A reversed-phase high-performance liquid chromatographic assay for the simultaneous determination of cefotaxime and its metabolite desacetylcefotaxime in plasma and urine was developed. Plasma was deproteinized with small amounts of acetonitrile. After separation of the proteins the supernatant was extracted with a mixture of chloroform and 1-butanol. A phase separation was obtained leaving the cephalosporin and its metabolite in the aqueous part and extracting most of the interfering endogenous material. The aqueous phase was injected directly into the chromatograph. As part of the plasma water was dissolved in the acetonitrile--1-butanol--chloroform layer, the concentration of the cephalosporin in the aqueous phase was significantly higher than in the original plasma sample. Therefore, the usual diluting effect of the deproteinization could be avoided. In a similar way the assay was applicable to measure cefotaxime and its metabolite in urine. Calibration curves were set up and were linear up to 25 micrograms/ml for desacetylcefotaxime and 250 micrograms/ml for cefotaxime. The assay was applied to study the pharmacokinetics of cefotaxime and its metabolite in a healthy volunteer. In a similar way this deproteinization and extraction method was also applied to assay for ceftazidime, cephalexin, cephazolin and cefoxitin.