Immunoaffinity Ultrafiltration with Ion Spray HPLC/MS for Screening Small-Molecule Libraries

Methods of screening combinatorial libraries using immobilized or restrained receptors

The screening of combinatorial libraries for compounds with high affinity toward drug receptors is currently a major center of attention. We describe methods recently developed for library screening that involve "constrained" receptors (either immobilized onto a surface or restrained to a compartment by some physical means). These include affinity selection chromatography, ultrafiltration assays, the scintillation proximity assay, a variety of interfacial optical techniques (surface plasmon resonance and its relatives, among others), the quartz crystal microbalance, the jet ring cell, and new interferometric assays using porous silicon to immobilize the receptor. We note some trends in assay development involving assays of membrane-bound complexes, and the coupling of two analytical methods to expand the assay resolution.

Rapid drug metabolite profiling using fast liquid chromatography, automated multiple-stage mass spectrometry and receptor-binding

Rapid drug metabolite profiling can be achieved using fast chromatographic separation and fast mass spectrometric scanning without compromising the separation efficiency. Fast chromatographic separations of drug and its metabolites can be achieved by eluting from a short narrow-bore guard cartridge column (20 x 2 mm I.D., 3 microns BDS Hypersil C8) at flow-rate of 1.0 ml/min and with a gradient volume greater than 90 column volumes. The need for chromatographic separation is important for automated data dependent multiple-stage mass spectrometry (MSn) experimentation. The total analysis time of 8 min permits profiling of metabolites in a 96-well plate in 13 h. The narrow chromatographic peaks resulting from the high flow-rate require the use of a mass spectrometer capable of fast scan speed due to the need to perform multiple MS experiments within the same chromatographic analysis. A method has been developed for screening potentially biologically active in vitro microsomal metabolites by affinity binding with a receptor. After separation by centrifugal ultrafiltration, the bound ligands are released and characterized by LC-MS. In vitro microsomal metabolites of tamoxifen, raloxifene and adatanserin were screened for potential biological activity using this method. The in vitro metabolites of tamoxifen captured by the receptor include N-demethyltamoxifen and three species of hydroxytamoxifen; these data are consistent with those from a conventional binding study and bioassay. In addition, both hydroxyraloxifene and dihydroxyraloxifene are also recognized by the receptor. The specificity of the molecular recognition process is illustrated by the absence of binding with control microsomal incubate and with adatanserin and its metabolites. Therefore, active metabolites can be rapidly profiled by fast LC, automated MSn, and receptor binding. This information can be obtained quickly and can add value to the drug discovery process.

Determining affinity-selected ligands and estimating binding affinities by online size exclusion chromatography/liquid chromatography-mass spectrometry

Size exclusion chromatography (SEC) isolation of affinity-selected ligands combined with reverse phase liquid chromatography-mass spectrometry (LC-MS) is an effective means for identifying members of mixtures which form tightly bound noncovalent complexes with target proteins. A potential liability of the approach is that the SEC isolation is carried out under nonequilibrium conditions favoring protein/ligand complex dissociation. At long SEC isolation times and/or for complexes with fast off-rates the extent of dissociation can jeopardize the ability to detect the affinity-selected components. Additionally, equilibrium binding affinities cannot be exactly determined from the measured distribution of isolated ligands. We present here an online SEC/LC-MS system for determining affinity-selected members of active mixtures which reduces this liability. A kinetic model of the SEC isolation process is developed to determine the practical limits for the application of the method and to extrapolate equilibrium binding affinities from the nonequilibrium data. The utility of online SEC/LC-MS for identifying affinity-selected ligands and for estimating binding affinities is demonstrated for a small molecule mixture of compounds with known binding affinities and for a simple combinatorial mixture.

Screening for high-affinity ligands to the Src SH2 domain using capillary isoelectric focusing-electrospray ionization ion trap mass spectrometry

A solution-based microscale approach for determination of high-affinity noncovalent complexes from mixtures of compounds is presented, based on capillary isoelectric focusing coupled on-line with electrospray ionization ion trap mass spectrometry. The studies are performed using the src homology 2 domain and tyrosine-phosphorylated peptide ligands as a model system. Tight complexes are formed in solution, preconcentrated up to 2 orders of magnitude and separated on the basis of their isoelectric points. The complexes are then dissociated in the mass spectrometer and the freed ligands identified. Picomole or less amounts of protein reagent are consumed per experiment. Structural information for the ligands involved in tight complex formation may be obtained using the MSn capabilities of the ion trap. The methodology can potentially be used to screen rapidly combinatorial mixtures of compounds for high-affinity ligands.

Survey of recent advances in analytical applications of immunoaffinity chromatography

Methods that use immunoaffinity chromatography (IAC) for sample preparation or detection are becoming increasingly popular as tools in the analysis of biological and nonbiological compounds. This paper presents an overview of immunoaffinity chromatography and examines some recent developments of this technique in analytical applications. The emphasis is placed on HPLC-based IAC methods or those that combine IAC with other instrumental techniques; however, novel approaches that employ low-performance IAC columns for chemical quantitation are also considered. Particular applications that are examined include (1) the use of IAC in the direct detection of analytes, (2) the extraction of samples by IAC prior to on- or off-line detection by other methods, (3) the use of IAC in chromatographic-based immunoassays, and (4) the development of postcolumn reactors based on IAC for the detection of analytes as they elute from other types of chromatographic columns. The advantages and limitations for each approach are considered. In addition, a summary is provided of reports in the literature that have used IAC for these various formats.