Routine determination of hydroxyphenytoin in urine by high-performance liquid chromatography using an automatic column-switching technique

Simultaneous determination of p-hydroxylated and dihydrodiol metabolites of phenytoin in urine by high-performance liquid chromatography

Accurate urinary measurements of the two major metabolites of phenytoin, 5-(p-hydroxyphenyl)-5-phenylhydantoin (p-HPPH) and 5-(3,4-dihydroxy-cyclohexa-1,5-dienyl)-5-phenylhydantoin (dihydrodiol, DHD), are necessary for pharmacokinetic and drug-interaction studies of this commonly used antiepileptic drug. We describe a simple, rapid, acid hydrolysis, with liquid-liquid extraction and simultaneous isocratic reversed-phase high-performance liquid chromatography of p-HPPH and 5-(m-hydroxyphenyl)-5-phenylhydantoin (m-HPPH) (hydrolytic end product of DHD). p-HPPH and m-HPPH were quantitated against their separate respective internal standards of alphenal and tolylbarb. The mobile phase consisted of water-dioxane-tetrahydrofuran (80:15:5, v/v/v) at 2 ml/min and at 50 degrees C, with detection at 225 nm. Baseline separation was achieved by use of a 16 cm x 3.9 mm Nova-Pak C18 column and total analysis time of 12 min. p-HPPH and m-HPPH concentrations ranged from 10 to 200 and from 2 to 30 micrograms/ml, respectively, with between-day coefficients of variations of 3.3-4.5% and 2.2-5.1% for controls. All standard curves were linear with r values greater than 0.993. The DHD concentration was determined by multiplying m-HPPH concentrations by 2.3.

Quantification of midodrine and its active metabolite in plasma using a high performance liquid chromatography column switching technique

An automated column-switching HPLC system is described for the simultaneous determination of midodrine, an alpha-adrenergic stimulating drug, and its active metabolite, ST-1059. Serum or plasma (850 microliters) is directly injected onto a RP18 (30 micrograms particle size) pre-column (9 x 4 mm ID) which acts as an on-line liquid-solid extractor and analyte enrichment system. The injection is followed by washing steps. The fraction containing the analytes is transferred onto an analytical RP18 column via step gradient elution where the final analysis is performed. Fluorescence detection is used (lambda ex 290 nm and lambda em 322 nm), and method detection limits of 0.8 ng/mL plasma were reached. These were sufficiently low to determine the plasma concentration-time profiles for both compounds following oral administration of 2.5 mg and 5 mg midodrine hydrochloride. The assay in serum or plasma was linear in the range of 1 to 15 ng analyte/mL, the recovery was greater than 95%, and the reproducibility was sufficient. The assay was rugged and was maintained by routinely changing the home-made, dry packed pre-column every 20th serum injection.

Determination of antiepileptic drugs

The present paper reviews gas and liquid chromatographic methods for the determination of the most commonly monitored antiepileptic drugs: phenobarbital, phenytoin, carbamazepine, primidone, ethosuximide, valproic acid and clonazepam along with a new compound, progabide. The individual classes of drugs are first treated separately to highlight specific aspects of their quantification, and this is followed by an overview of those methods permitting the concomitant analysis of two or more antiepileptic compounds. Sample preparation techniques as well as comparisons between chromatographic and other techniques are treated more fully in separate sections.

Influence of solute polarity in column-switching chromatography for the assay of drugs in plasma and urine

The polarity of a drug is one of the most important parameters for the elaboration of switching systems. If the polarity of the drug is low or medium, "reversed-phase" chromatography is well adapted. The plasma or urine sample is diluted with water, centrifuged and injected first into a column of medium polarity (C2, CN or diol bonded phases). The compounds of interest are stopped on the top of the column and rinsed with water, then eluted and chromatographed on a C8 or C18 analytical column. A third column of still lower polarity can be added to improve the specificity of the system. In each successive step, the polarities of the mobile phases and columns should be decreased to reconcentrate the sample and reduce the band broadening that occurred in the previous step. Compounds of high polarity show almost no retention on reversed-phase columns, and normal-phase chromatography should be used. Aqueous solutions cannot be injected into polar bonded-phase columns as they lead to excessive band broadening. This problem can be solved by diluting plasma or urine with a large volume of a water-miscible organic solvent and injecting the clear supernatant. The compounds to be assayed are first reconcentrated on a polar column (NH2 or N(CH3)2 bonded phase) and then eluted. The selected "heart cut" of the eluate is chromatographed on another, more polar column. The influence of the polarity of drugs on the choice of switching systems is exemplified by assay methods for drugs of low, medium and high polarity.

Routine determination of eight common anti-epileptic drugs and metabolites by high-performance liquid chromatography using a column-switching system for direct injection of serum samples

A simple and rapid high-performance liquid chromatographic method for determining eight common anti-epileptic drugs and metabolites in serum is described. A column-switching system including one analytical column and two precolumns for sample enrichment offers the possibility of directly injecting patients' sera without any pretreatment. The two precolumns are alternately switched over to avoid time loss in analysis due to the sample washing step. The samples are flushed with dilute phosphoric acid, as the purge liquid, onto the precolumns which consist of very short cartridges (length 0.5 cm) filled with spherical ODS silica gel (particle size 30 micron). The retained substances are carried over, after purification, onto the analytical column in the same direction of flow as in the flushing step. A mixture of acetonitrile and phosphoric acid--sodium phosphate buffer solution is thereby used as solvent for the gradient elution. The separation was carried out using an analytical column, which was filled with ODS material of particle size 5 micron.

Fully-automated assay by liquid chromatography for routine drug monitoring in body fluids — method development with biological samples

A fully-automated liquid chromatograph based on alternated pre-column enrichment technique has been developed for routine drug monitoring of body fluids using a column switching technique. Advantages of the method include the facility of direct injection of the body fluid onto the chromatograph and the simultaneous detection of both polar and non-polar metabolites under isocratic or gradient elution conditions.