Preparative two-dimensional liquid chromatography/mass spectrometry for the purification of complex pharmaceutical samples

A new preparative two-dimensional liquid chromatography/mass spectrometry system (2D LC-LC/MS) has been designed and implemented to enhance capability and resolving power for the separation and purification of pharmaceutical samples. The system was constructed by modifications of a conventional preparative LC/MS instrument with the addition of a set of switching valves and a sample loop, as well as interfacing a custom software program with MassLynx. The system integrates two chromatographic separations from the first and second dimensions into a single automated run to perform the purification of a target compound from a complex mixture without intermediate steps of sample preparation. The chromatography in the first dimension, operated in the heart-cutting mode, separates the target compound from the impurities by mass-triggered fractionation based on its molecular weight. This purified fraction from the first dimension is stored in the sample loop, and then gets transferred to the second column by using at-column dilution. A control software program, coined Prep 2D LCMS, was designed to integrate with MassLynx to retrieve data acquisition status. All of the chromatographic hardware components used in this preparative 2D LC-LC/MS system are from the original open access preparative LC/MS system, which has high level of robustness and affords easy and user-friendly operation. The new system is very versatile and capable of collecting multiple fractions with different masses under various purification modes as configured in the methods, such as conventional one-dimensional (1D) purification and/or 2D purification. This new preparative 2D LC-LC/MS system is therefore the ideal tool for medicinal chemistry lab in drug discovery environment.

Optimization of normal-phase chromatographic separation of compounds with primary, secondary and tertiary amino groups

The retention behavior of primary, secondary and tertiary amines was studied using normal-phase-HPLC on silica, diol, and cyano stationary phases. Several classes of amines, including benzylamines, anilines, ephedrines, tryptamines, and azatryptamines were chromatographed using mixtures of hexane and ethoxynonafluorobutane with methylene chloride and methanol. Peak tailing, diminished selectivity and low plate count were minimized by the addition of volatile amines to the mobile phase. The optimal additive was n-propylamine at 0.1% concentration. On diol columns, the elution order of free primary, N-N-methyl, and N,N-dimethylamines was predictable, while the elution order of primary and secondary amines on cyano columns varied depending on the alcohol modifier concentration. The feasibility of preparative normal-phase chromatography was demonstrated by the separation of a mixture of primary, secondary and tertiary amines obtained by direct methylation of norephedrine. The procedures described may provide a practical alternative to traditional methods of analysis and purification of potential drug candidates.

Enantiomeric separation and determination of absolute stereochemistry of asymmetric molecules in drug discovery—Building chiral technology toolboxes

The application of Chiral Technology, or the (extensive) use of techniques or tools for the determination of absolute stereochemistry and the enantiomeric or chiral separation of racemic small molecule potential lead compounds, has been critical to successfully discovering and developing chiral drugs in the pharmaceutical industry. This has been due to the rapid increase over the past 10-15 years in potential drug candidates containing one or more asymmetric centers. Based on the experiences of one pharmaceutical company, a summary of the establishment of a Chiral Technology toolbox, including the implementation of known tools as well as the design, development, and implementation of new Chiral Technology tools, is provided.

Normal-phase chiral liquid chromatography–mass spectrometry of non-UV-active compounds: Applications for pharmaceutically relevant racemates

Mixtures of hexane-like ethoxynonafluorobutane with alcohols were used as MS-friendly mobile phases for separation and efficient detection of non-UV-active enantiomers and diastereomers using normal-phase HPLC-APCI-MS. Racemic muscone, camphorsulfonamide, camphorsultam, BOC-protected 1-(3-aminopropyl)-2-pipecoline and diastereomeric 2-methylhexanoyl camphorsultams were resolved on Chiralpak AS and AD and achiral Luna CN columns. The responses of UV and APCI-MS detectors were compared under separation conditions studied, with MS detection achieving lowest detectable quantity in the range of 0.5-2 ng per chromatographic peak. The absolute configuration of crystalline derivatives of racemic 2-methylhexanoic acid with (S)-(-)-2,10-camphorsultam was determined by X-ray analysis after their automatic purification by preparative LC-MS. The technique described can be used to purify and determine the absolute stereochemistry of compounds of unknown structure which contain free carboxy group and lack sufficient UV absorbance.

Evaluation of ethoxynonafluorobutane as a safe and environmentally friendly solvent for chiral normal-phase LC-atmospheric pressure chemical ionization/electrospray ionization-mass spectrometry

Coupling normal-phase LC separation methods to atmospheric pressure ionization (API)-mass spectrometry (MS) for detection can be problematic because of the possible detonation hazard and because nonpolar solvents do not support ionization of the analyte. Unlike achiral separations, enantiomeric separations can be very sensitive to small changes in the separation environment. Thus, completely substituting the main mobile phase component of a normal-phase LC solvent for an environmentally friendly, nonflammable fluorocarbon-ether as a safe and effective solvent must be thoroughly evaluated before it can be recommended for enantioselective separations with API-MS detection. Ethoxynonafluorobutane (ENFB) was used as a normal-phase solvent for the enantioselective separation of 15 compounds on two macrocyclic glycopeptide chiral stationary phases (CSPs) and a new polymeric chiral stationary phase. The chromatographic figures of merit were compared between results obtained with the ENFB mobile phases and traditional heptane-based mobile phases. In addition, the limits of detection (LOD) using the API-MS compatible ENFB were examined, as well as flow rate sensitivities and compatibilities with common polar organic modifier. ENFB is a safe and effective solvent for enantioselective normal-phase/API-MS analyses.