Markus Fluck

Unified enantioselective capillary chromatography on a Chirasil-DEX stationary phase. Advantages of column miniaturization

Immobilized Chirasil-DEX (mono-6-O-octamethylenepermethyl-beta-cyclodextrin chemically linked to dimethylpolysiloxane) can be employed as a versatile chiral stationary phase in chromatography. The chiral polymer has a long lifetime and is configurationally and thermally stable. The concept of unified enantioselective chromatography has been demonstrated for the enantiomer separation of hexobarbital by gas chromatography, supercritical fluid chromatography, liquid chromatography and capillary electrochromatography on a single open-tubular column (1 m x 50 microns I.D.) coated with Chirasil-DEX. The advantages of miniaturization in contemporary chromatographic enantiomer separation are demonstrated. Chirasil-DEX coated on porous silica is also useful for enantiomer separation in high-performance liquid chromatography.

Determination of enantiomerization barriers by computer simulation of experimental elution profiles obtained by high-performance liquid chromatography on a chiral stationary phase

The enantiomers of oxazepam, chlorthalidone and phenylthiohydantoin-phenylalanine (PTH-phenylalanine) were separated by HPLC using chemically bonded β-cyclodextrin (ChiraDex) as chiral stationary phase. The obtained chromatograms showed typical plateaus between the peaks of the separated enantiomers, indicating enantiomerization, i.e. reversible enantiomer interconversion. Computer simulation of the experimental chromatographic elution profiles was employed for the determination of rate constants and corresponding enantiomerization barriers for the above compounds. The data were compatible with independently determined values, but a very strong dependence on the solvent was observed.

On the enantiomerization barrier of atropisomeric 2,2′,3,3′,4,6′-hexachlorobiphenyl (PCB 132)

Abstract The rotational barrier of chiral, tri-ortho-substituted 2,2′,3,3′,4,6′-hexachlorobiphenyl (PCB 132) was determined. After semipreparative HPLC separation of PCB 132 on polysiloxane-anchored permethyl-β-cyclodextrin (Chirasil-Dex), one single enantiomer was subjected to an enantiomerization kinetic and the enantiomeric ratio was determined by chiral gas chromatography. Furthermore, on-column inversion of PCB 132 was investigated in the presence of Chirasil-Dex via stopped-flow gas chromatography.

Dynamic phenomena involving chiral dimethyl-2,3-pentadienedioate in enantioselective gas chromatography and NMR spectroscopy

Abstract The chiral allene dimethyl-2,3-pentadienedioate 1 undergoes enantiomerization during gas-chromatographic enantiomer separation on the two polymeric chiral stationary phases Chirasil-Nickel 2a and Chirasil-Dex 3 at ambient temperatures. The enantiomerization barrier of 1 is determined by dynamic gas chromatography via computer simulation of interconversion elution-profiles and by multidimensional stopped-flow gas chromatography. The previously reported deracemization of 1 in the presence of a chiral paramagnetic lanthanide shift reagent by NMR spectroscopy is confirmed.

Enantiomer separation by complexation SFC on immobilized Chirasil-nickel and Chirasil-zinc☆

The use of complexation SFC for enantiomer separation of Lewis base selectands on chiral nickel(II)- and zinc(II)-bis[(3-heptafluorobutanoyl)-10-methylene-(1R)-camphora te] chemically bonded to poly(dimethylsiloxane) (Chirasil-nickel and Chirasil-zinc) and employed as Lewis acid selectors is described. The method is especially suited for less volatile and configurationally labile racemates. The variation of the experimental parameters temperature T, pressure p and density rho of the mobile phase carbon dioxide on the retention factor k, relative retention r and chiral separation factor alpha is studied, providing insights into the mechanisms of chiral recognition under supercritical conditions. For mecoprop methyl ester (methyl 2-(4-chloro-2-methylphenoxy)propanoate) an unusual increase of alpha at increased temperature is observed on Chirasil-nickel. Supercritical carbon dioxide does not inadvertently affect the complexation equilibria between Lewis donor selectands and the Lewis acid metal selectors during complexation SFC.

In vitro characterization of sarizotan metabolism: hepatic clearance, identification and characterization of metabolites, drug-metabolizing enzyme identification, and evaluation of cytochrome p450 inhibition.

In vitro biotransformation studies of sarizotan using human liver microsomes (HLM) showed aromatic and aliphatic monohydroxylation and dealkylation. Recombinant cytochromes P450 (P450) together with P450-selective inhibitors in HLM/hepatocyte cultures were used to evaluate the relative contribution of different P450s and revealed major involvement of CYP3A4, CYP2C9, CYP2C8, and CYP1A2 in sarizotan metabolism. The apparent Km, u and Vmax of sarizotan clearance, as investigated in HLM, were 9 μM and 3280 pmol/mg/min, predicting in vivo hepatic clearance of 0.94 l/h, which indicates that sarizotan is a low-clearance compound in humans and suggests nonsaturable metabolism at the targeted plasma concentration (≤1 μM). This finding is confirmed by the reported human clearance (CL/F of 3.6–4.4 l/h) and by the dose-linear area under the curve increase observed with doses up to 25 mg. The inhibitory effect of sarizotan toward six major P450s was evaluated using P450-specific marker reactions in pooled HLM. Ki, u values of sarizotan against CYP2C8, CYP2C19, and CYP3A4 were >10 μM, whereas those against CYP2D6 and CYP1A2 were 0.43 and 8.7 μM, respectively. Based on the estimates of sarizotan concentrations at the enzyme active sites, no clinically significant drug-drug interactions (DDIs) due to P450 inhibition are expected. This result has been confirmed in human DDI studies in which no inhibition of five major P450s was observed in terms of marker metabolite formation.

Correction to: The Clinical Pharmacology of Cladribine Tablets for the Treatment of Relapsing Multiple Sclerosis

The cladribine prodrug is phosphorylated intracellularly to its active product, 2-chlorodeoxyadenosine triphosphate (Cd-ATP), by deoxycytidine kinase.