Mass spectra of N-[(S)-alpha-methoxy-alpha-(trifluoromethyl)phenylacetyl] derivatives of chiral amines. Stereochemistry of amphetamine metabolism in the rat

Scalemic mixtures preparation for optimized composition of ibuprofen solid dosage forms

The stable and metastable phase diagrams between the sinister and the rectus ibuprofen enantiomers were established by means of thermal analysis and X-ray powder diffraction experiments as a function of temperature. The results obtained allow proving for the first time the existence, for the stable system, of a solid solution by mixing the racemic ibuprofen with one of its enantiomers for low concentration of the enantiomer. Since the rectus ibuprofen is a non-active pharmaceutical agent which can be partially bio-converted into the sinister enantiomer, the present work offers a new approach for scalemic mixtures preparation in order to improve the benefit/risk ratio related to ibuprofen solid dosage form administration.

Simultaneous determination of serine enantiomers in plasma using Mosher's reagent and stable isotope dilution gas chromatography-mass spectrometry

D-Serine is a co-agonist of the N-methyl-D-aspartate receptor in glutamate neurotransmission and has been proposed as a potential therapeutic agent for schizophrenia. However, D-serine also acts as a nephrotoxic substance in rats at high doses. To investigate the pharmacokinetics and toxicokinetics of D-serine, a method for the stereoselective determination of serine enantiomers in rat plasma was developed using GC-MS with selected ion monitoring (GC-MS-SIM). DL-[(2)H(3)]Serine was used as an internal standard to account for losses associated with the extraction, derivatization and chromatography. Serine enantiomers were purified by cation-exchange chromatography using BondElut SCX cartridge and derivatized with HCl in methanol to form methyl ester followed by subsequent N,O-diacylation with optically active (+)-α-methoxy-α-trifluoromethylphenylacetyl chloride to form epimeric amide. Quantitation was performed by SIM of the molecular-related ions of the epimers in the chemical ionization mode. The intra- and inter-day reproducibility of the assay was less than 5% for D-serine and 3% for L-serine. The method was successively applied to study the pharmacokinetics of D-serine in rats.

Stereospecific gas chromatographic/mass spectrometric assay of the chiral labetalol metabolite 3-amino-1-phenylbutane

We previously identified 3-amino-1-phenylbutane (APB) as an oxidative N-dealkylated, metabolite of the antihypertensive agent labetalol. Labetalol has two asymmetric centers and is used clinically as a mixture of the four possible stereoisomers; APB has one asymmetric center. We now report an enantiospecific gas chromatographic/mass spectrometric assay for APB in urine. After adding the internal standard 1-methyl-2-phenoxyethylamine and alkalinizing, the urine samples were extracted with ether. The extracts were derivatized with the optically active acid chloride prepared from (S)-alpha-methoxy-alpha-trifluoromethylphenylacetic acid. The derivatives were separated by capillary gas chromatography and detected by electron capture negative ion chemical ionization mass spectrometry with selected ion monitoring. The derivative of the R enantiomer eluted first, and the [M--32]- ions were monitored for both the drug and the internal standard. The method was linear in the 0.05-2.5 micrograms enantiomer-1 ml-1 range and had inter-assay and intra-assay coefficients of variation of less than 6%. The assay was used in the analysis of urine samples from a patient in labetalol therapy and no interference was found. Further studies are needed to elucidate the oxidative metabolism of labetalol and its stereochemical aspects.

Studies on drug metabolism by use of isotopes. XXIX--Studies of the differences in biological fates of ephedrine isomers by use of a pseudo-racemic mixture

The differences in metabolic fates of the optical isomers of ephedrine were investigated by use of a pseudo-racemic mixture technique. In rats, the (-)-isomer was more easily p-hydroxylated than the (+)-isomer. A stereoselective reaction in the formation of the glucuronides of ephedrine, norephedrine and p-hydroxyephedrine was observed. The (-)-isomers were more easily subjected to glucuronide formation than the (+)-isomers. In human subjects, N-demethylation proved to be a stereoselective reaction. It is also suspected that oxidative dealkylation of norephedrine is a stereoselective reaction.

Simultaneous determination of the enantiomers of tocainide in blood plasma using gas-liquid chromatography with electron-capture detection

Tocainide is a new experimental antiarrhythmic agent used clinically as the racemic mixture of two enantiomers. Since the optical isomers may differ in their efficacy and toxicity, we have initiated studies on the stereoselective disposition of tocainide. For this purpose, an assay was developed for the simultaneous determination of the enantiomers of tocainide in blood plasma. Alkalinized 1-ml plasma samples containing tocainide and an internal standard, 2-amino-2',6'-acetoxylidide, are extracted with ethyl acetate. The organic extract is treated with the chiral reagent (S)-alpha-methoxy-alpha-trifluoromethylphenylacetyl chloride, and the resulting derivatives are resolved and quantified by gas-liquid chromatography with electron-capture detection. Calibration data were fitted by least-squares power curves of the form: drug enantiomer/internal standard peak area ratio = A X CB where A and B were constants and C was the concentration of tocainide enantiomer. The lower limit of sensitivity of the assay was 10 ng/ml of each enantiomer. Intra-assay coefficients of variation were 3.3 and 2.1% for (R)-(-)-tocainide at concentrations of 0.125 and 1.25 micrograms/ml, respectively, and 3.4 and 2.4% for the (S)-(+) enantiomer at the same concentrations. Diazepam may interfere with the determination of (R)-(-)-tocainide if concentrations smaller than 1 microgram/ml of this enantiomer are measured in the presence of higher-than-therapeutic (greater than 1.5 micrograms/ml) concentrations of diazepam.