GLC assay of verapamil in plasma: identification of fluorescent metabolites after oral drug administration.

The fluorometric assay for verapamil in plasma is not useful after oral drug administration because of interference by inactive, but fluorescent, metabolites extracted along with the parent drug. A GLC method using a flame-ionization detector after silylation allows the separation of unchanged active verapamil from the metabolites and quantitation to a sensitivity of 0.025 microgram/ml. After a single oral dose of drug in dogs, up to 80% of "fluorescent verapamil" represented inactive metabolites.     

GLC determination of plasma levels of intact chlorpropamide or tolbutamide.

A GLC procedure was developed for the quantitative estimation of intact chlorpropamide and tolbutamide concentrations in plasma; the drugs are used as mutual internal standards. After extraction of plasma containing the drug and internal standard with toluene, the dried residue is treated with ethereal diazomethane to form the methyl derivatives of tolbutamide and chlorpropamide. Aliquots of the ethereal solution are injected into a gas chromatograph equipped with a glass-lined injection port and glass column packed with a phenyl methyl silicone fluid (OV-25) on Chromosorb W, which facilitates the intact determination of the methyl derivatives of the drugs. The response to the flame-ionization detector was linear over a range of 0.20-25 mug/ml, with a 0.05-mug/ml limit of detectability for both drugs. The method compares favorably with a recently developed high-pressure liquid chromatographic procedure and is adequate for following blood level profiles of single doses of chlorpropamide (125 mg) and tolbutamide (250 mg). Mass spectral evidence showing that intact sulfonylureas are measured is presented.     

GLC determination of indoprofen in plasma.

Implementation of human bioavailability studies with the analgesic indoprofen required a rapid, sensitive, and convenient assay in blood plasma. A procedure based on ether extraction of acidified plasma, derivatization with diazomethane, and GLC analysis using a 3% OV-1 column was sufficiently sensitive for measurement of plasma indoprofen concentrations in the 0.4-16 mug/ml range. An average recovery of 99.0 +/- 7.6% (SD) was achieved when the pentanoic acid homolog was employed as an internal standard.     

Time course of plasma drug levels during once-daily oral administration of clomipramine

Fourteen depressed in-patients were treated with 150 mg clomipramine (CLO) daily, given as one oral dose. Using a gas-chromatographic method, concentrations of CLO and desmethyl clomipramine (DMCLO) were determined in plasma samples taken at frequent intervals during 24h. The plasma level of each compound 12 h after the dose correlated well with the average value in the same patient, calculated over the whole 24-h period. Levels at other times gave poorer correlations, and at 24 h it was particularly poor. Plasma DMCLO concentrations were usually maximum 4–6 h after the dose. The ratios of maximum to minimum levels averaged only 1.31±0.15 SD. Peak CLO levels occurred 3 or 4 h after the dose. Maximum: minimum ratios averaged 2.72±0.73 SD, contrasting with the much smaller fluctuations of plasma nortriptyline (NT) levels observed in patients given this drug once daily. The difference is not due to a shorter half-life of CLO, but to the absorption and/or distribution behaviour of the two drugs. Although not fully understood, this difference between tertiary and secondary amines appears to hold generally among the tricyclic antidepressants.     

Patterns of plasma levels of prednisolone after oral administration in man

A competitive protein-binding method was found suitable for the estimation of plasma prednisolone levels in men in whom endogenous steroids were suppressed by dexamethasone premedication. This method is rapid and precise and allows measurements of prednisolone in small blood samples after usual therapeutic doses of the drug. Using this technique, two preparations of the medication were compared, a conventional form and an enteric-coated formulation; except for the delay due to the coating, they were completely comparable, since maximum levels reached, areas under the plasma concentration-time curves, and apparent disappearance half-lifes were almost identical.This work was supported by the Fonds de la Recherche Scientifique Médicale and was partially realized under contract of the Ministère Belge de la Politique Scientifique, within the framework of the association Euratom-Université de Bruxelles-University of Pisa.     

GLC determination of plasma concentrations of phenprocoumon.

A GLC method for the quantitative estimation of phenprocoumon from plasma is described. Plasma containing phenprocoumon, to which a known amount of phenytoin is added as the internal standard, is acidified and extracted with ethylene dichloride. The drug and the internal standard are then back-extracted into alkali, which is acidified and reextracted with ethylene dichloride. The organic extract is evaporated, and the evaporated residue is mixed with 50 mul of trimethylanilinium hydroxide in methanol. Aliquots (1-2 mul) are injected into a gas chromatography equipped with a flame-ionization detector in which the injection port is held at 325 degrees. The methyl derivatives of phenprocoumon and the internal standard give sharp, well-separated, symmetrical peaks. The method is of sufficient sensitivity to determine 0.125 mug/ml of the drug in plasma with a coefficient of variation of 7%.     

Quantitative GLC determination of codeine in plasma.

A sensitive and accurate GLC method for quantitating codeine in plasma at levels of 50 ng/ml, with limits of detection as low as 5 ng/ml, is described.     

GLC determination of griseofulvin in human plasma.

A specific and quantitative GLC method for the determination of griseofulvin in human plasma is described. The method involves extraction with ether, evaporation, addition of the internal standard dissolved in benzene, and GLC analysis using an electron-capture detector. The sensitivity of the method is 0.05 mug/ml of plasma. The results obtained with this specific GLC method were compared with the results obtained with the more frequently used spectroflurometric method by analyzing duplicate plasma samples obtained from 12 subjects following a single dose of griseofulvin. It was deduced that the 30% higher plasma levels obtained spectrofluormetrically were due to the coextraction and presence of the metabolite 6-demethylgriseofulvin in the assay solutions.     

GLC determination of quinidines in human plasma.

Human plasma was made alkaline and extracted with methylene chloride. To the extract was added the internal standard, cinchonine, followed by evaporation to dryness. The resultant residue was dissolved in a methanolic solution containing trimethylanilinium hydroxide. This solution was assayed by GLC for quinidines (quinidine and hydroquinidine). Evaluation of the method over a 0.5-10-mug/ml range in human plasma gave an overall precision and accuracy of +/- 4.5% (RSD and RE). Plasma of several patients was analyzed by the present method as well as by a fluorometric method for the level of quinidines. Results from the two methods were comparable.     

Improved GLC determination of plasma nitroglycerin concentrations.

A specific and quantitative assay for determining nanogram levels of nitroglycerin in plasma was developed. The method involves stabilization of the drug in plasma with silver nitrate, followed by multiple extraction using purified hexane. Isosorbide dinitrate is added as the internal standard. The hexane extract is subsequently concentrated and injected into a glass column packed with 3% SP-2401 on 100--120-mesh Supelcoport at 140 degrees. A 63Ni-electron-capture detector gives a linear response over the range of 0.1--50 ng of nitroglycerin/ml of rat plasma. From spiked samples, the procedure gave a recovery of about 90%. There was little or no interference from the isomeric glyceryl dinitrates and endogenous compounds in rat or human plasma.     

Determination of desmethyldiazepam in plasma by electron-capture GLC: application to pharmacokinetic studies of clorazepate.

Plasma desmethyldiazepam concentrations were quantitated by a rapid and sensitive technique using electron-capture GLC. Following addition of diazepam as the internal standard, plasma is extracted at physiological pH into benzene-isoamyl alcohol. The extract is evaporated to dryness and reconstituted with toluene-isoamyl alcohol prior to chromatography. Both diazepam and desmethyldiazepam are quantitatively extracted. The variation of identical samples is 5%, and the sensitivity is 5 ng of desmethyldiazepam/ml of original sample. The method is applicable to pharmacokinetic studies of clorazepate, a benzodiazepine derivative transformed to desmethyldiazepam prior to absorption.     

Naproxen plasma levels in volunteers after single-dose administration by oral and rectal routes.

The bioavailability (plasma concentrations, AUC) of a rectal formulation (suppository) of naproxen was investigated in six healthy volunteers by comparison with an oral preparation (tablets). Plasma half-lives after both formulations were identical 10 hr 15 min+/-25 min (S.D.). Determined by the AUC the bioavailability of naproxen in the suppositories was 94.8%+/-6.3% of the bioavailability of naproxen in the tablets. This paper describes also a new gas-liquid chromatographic method for determining unchanged naproxen in human plasma which is quick, sensitive, and specific.