In-situ microscale spectrophotometric determination of phenytoin by using branched gold nanoparticles

A rapid method for the sensitive detection of phenytoin (PHT) by branched gold nanoparticles (B-AuNPs) is described. These nanoparticles were synthesized by adding methanol as the reducing agent and poly(ethylene glycol) as the stabilizer at 70 °C. The B-AuNPs are red in color with an absorption maximum at 540 nm when prepared in situ. However, the color becomes increasingly weaker when PHT is present in increasing concentrations. This method can determine PHT over the 67-670 ng·mL^-1 concentration range, with detection limit of 21 ng·mL^-1. The relative standard deviation for five replicate measurements at 68 and 530 ng·mL^-1 of PHT was 3.2% and 1.2%, respectively. The method was applied to the determination of PHT in plasma samples of epileptic patients, and the results were in agreement with those obtained by a standard official method. Graphical abstract Branched gold nanoparticles (AuNPs) prepared in situ have a red color with an absorption maximum at 540 nm. The color becomes increasingly weaker with decreasing the intensity of the characteristic SPR band when PHT is present in increasing concentration. The current assay is capable of determining PHT over the 67-670 ng·mL^-1 concentration range with a limit of detection of 21 ng·mL^-1.

Characterization of interaction kinetics between chiral solutes and human serum albumin by using high-performance affinity chromatography and peak profiling

Peak profiling and high-performance columns containing immobilized human serum albumin (HSA) were used to study the interaction kinetics of chiral solutes with this protein. This approach was tested using the phenytoin metabolites 5-(3-hydroxyphenyl)-5-phenylhydantoin (m-HPPH) and 5-(4-hydroxyphenyl)-5-phenylhydantoin (p-HPPH) as model analytes. HSA columns provided some resolution of the enantiomers for each phenytoin metabolite, which made it possible to simultaneously conduct kinetic studies on each chiral form. The dissociation rate constants for these interactions were determined by using both the single flow rate and multiple flow rate peak profiling methods. Corrections for non-specific interactions with the support were also considered. The final estimates obtained at pH 7.4 and 37°C for the dissociation rate constants of these interactions were 8.2-9.6 s(-1) for the two enantiomers of m-HPPH and 3.2-4.1 s(-1) for the enantiomers of p-HPPH. These rate constants agreed with previous values that have been reported for other drugs and solutes that have similar affinities and binding regions on HSA. The approach used in this report was not limited to phenytoin metabolites or HSA but could be applied to a variety of other chiral solutes and proteins. This method could also be adopted for use in the rapid screening of drug-protein interactions.

Analysis of free drug fractions using near-infrared fluorescent labels and an ultrafast immunoextraction/displacement assay

A chromatographic method was developed for measuring free drug fractions based on the use of an ultrafast immunoextraction/displacement assay (UFIDA) with near-infrared (NIR) fluorescent labels. This approach was evaluated by using it to determine the free fraction of phenytoin in serum or samples containing the binding protein human serum albumin (HSA). Items considered in the design of this method included the dissociation rate of HSA-bound phenytoin, the rate of capture of free phenytoin by immunoextraction microcolumns, the behavior of NIR fluorescent labels in a displacement format, and the overall response and stability of the resulting assay. In the final UFIDA method, the free fraction of phenytoin was extracted in approximately 100 ms by a microcolumn containing a small layer of anti-phenytoin antibodies. This gave a displacement peak for a NIR-fluorescent-labeled analogue of phenytoin that appeared within 2-3 min of sample injection, creating a signal proportional to the amount of free phenytoin in the sample. The UFIDA method provided results within 1-5% of those determined by ultrafiltration for reference samples. The lower limit of detection was 570 pM, and the linear range extended up to 10 microM. This approach is not limited to phenytoin but can be adapted for other analytes through the use of appropriate antibodies and labeled analogues.

Studies by biointeraction chromatography of binding by phenytoin metabolites to human serum albumin

Biointeraction studies based on high performance affinity chromatography were used to investigate the binding of human serum albumin (HSA) to two major phenytoin metabolites: 5-(3-hydroxyphenyl)-5-phenylhydantoin (m-HPPH) and 5-(4-hydroxyphenyl)-5-phenylhydantoin (p-HPPH). This was initially examined by conducting self-competition zonal elution experiments in which m-HPPH or p-HPPH were placed in both the mobile phase and injected sample. It was found that each metabolite had a single major binding site on HSA. Competitive zonal elution experiments using l-tryptophan, warfarin, digitoxin, and cis-clomiphene as site-selective probes indicated that m-HPPH and p-HPPH were interacting with the indole-benzodiazepine site of HSA. The estimated association equilibrium constants for m-HPPH and p-HPPH at this site were 3.2 (+/-1.2)x10(3) and 5.7 (+/-0.7)x10(3)M(-1), respectively, at pH 7.4 and 37 degrees C. Use of these metabolites as competing agents for injections of phenytoin demonstrated that m-HPPH and p-HPPH had direct competition with this drug at the indole-benzodiazepine site. However, the use of phenytoin as a competing agent indicated that this drug had additional negative allosteric interactions on the binding of these metabolites to HSA. These results agreed with previous studies on the binding of phenytoin to HSA and its effects on the interactions of HSA with site-selective probes for the indole-benzodiazepine site.

Simultaneous determination of phenytoin, barbital and caffeine in pharmaceuticals by absorption (zero-order) UV spectra and first-order derivative spectra--multivariate calibration methods

The quantitative predictive abilities of partial least squares (PLS-1) and principle component regression (PCR) on absorption (zero-order) UV spectra are compared with the results obtained by the use of these multivariate calibration methods on first-order derivative spectra. Both approaches were satisfactorily applied to the simultaneous determination of these drugs in synthetic and pharmaceutical mixtures. Significant advantages were found in the simultaneous determination of phenytoin, barbital and caffeine in binary and ternary mixtures, by application of different multivariate calibration methods when the calibration matrix was performed using the first-order derivative spectra. The proposed method was validated by applying it to the analysis of binary and ternary mixtures of phenytoin, barbital and caffeine. Determinations were made over the concentration ranges of 0.24-22.0, 0.01-27.0 and 0.049-27.0 microg ml(-1) for phenytoin, barbital and caffeine, respectively, in the binary and 0.45-22.0, 0.05-26.0 and 0.05-20.0 microg ml(-1) for phenytoin, barbital and caffeine, respectively, in the ternary mixtures. The relative standard errors in the determinations were less than 3% in most cases.

High-throughput screening assays for the assessment of CYP2C9*1, CYP2C9*2, and CYP2C9*3 metabolism using fluorogenic Vivid substrates

CYP2C9 is a genetically polymorphic human cytochrome P450 isozyme involved in the oxidative metabolism of many drugs, including nonsteroidal anti-inflammatory compounds. Individuals genotyped heterozygous or homozygous for CYP2C9 allelic variants have demonstrated altered metabolism of some drugs primarily metabolized by CYP2C9. The ability to expand screening of CYP2C9 allelic variants to a larger set of drugs and pharmaceutical agents would contribute to a better understanding of the significance of CYP2C9 polymorphisms in the population and to predictions of possible outcomes. The authors report the development of an in vitro fluorescence-based assay employing recombinant CYP2C9 variants (CYP2C9*1, CYP2C9*2, and CYP2C9*3) and fluorogenic Vivid(R) CYP2C9 substrates to explore the effects of CYP2C9 polymorphisms on drug metabolism, using drugs primarily metabolized by CYP2C9. Several chemically diverse fluorogenic substrates (Vivid(R) CYP2C9 blue, green, and red substrates) were used as prototypic probes to obtain in vitro CYP2C9 metabolic rates and kinetic parameters, such as apparent K(m), V(max), and V(max)/K(m) ratios for each allelic variant. In addition, a diverse panel of drugs was screened as assay modifiers with CYP2C9*1, CYP2C9*2, CYP2C9*3, and the fluorogenic Vivid(R) CYP2C9 substrates. The inhibitory potential of this large group of chemically diverse drugs and compounds has been assessed on the basis of their ability to compete with Vivid(R) CYP2C9 substrates in fluorescent reporter assays, thus providing a sensitive and quick assessment of polymorphism-dependent changes in CYP2C9 metabolism.

Determination of antiepileptic drugs in biological material

Current analytical methodologies applied to the determination of antiepileptic drugs in biological material are reviewed. The role of chromatographic techniques is emphasized. Special attention is focused on new chemical entities as well as current trends such as high-speed liquid chromatographic techniques, hyphenated techniques and electrochromatography techniques. A review with 542 references.