Addressing the metabolic activation potential of new leads in drug discovery: a case study using ion trap mass spectrometry and tritium labeling techniques

Metabolic activation of drug candidates to electrophilic reactive metabolites that can covalently modify cellular macromolecules may result in acute and/or idiosyncratic immune system-mediated toxicities in humans. This presents a significant potential liability for the future development of these compounds as safe therapeutic agents. We present here an example of an approach where sites of metabolic activation within a new drug candidate series were rapidly identified using online liquid chromatography/multi-stage mass spectrometry on an ion trap mass spectrometer. This was accomplished by trapping the reactive intermediates formed upon incubation of compounds with rat and human liver microsomes as their corresponding glutathione conjugates and mass spectral characterization of these thiol adducts. Based on the structures of the GSH adducts identified, potential sites and mechanisms of bioactivation within the chemical structure were proposed. These metabolism studies were interfaced with iterative structural modifications of the chemical series in order to block these bioactivation sites within the molecule. This strategy led to a significant reduction in the propensity of the compounds to undergo metabolic activation as evidenced by reductions in the irreversible binding of radioactivity to liver microsomal material upon incubation of tritium-labeled compounds with this in vitro system. With the efficiency and throughput achievable with such an approach, it appears feasible to identify and address the metabolic activation potential of new drug leads during routine metabolite identification studies in an early drug discovery setting.     

Synthesis of sugar oxime ether surfactants

A straightforward synthesis of a novel class of sugar surfactants is described. The key step is the chemoselective condensation of a hydrophobic alkoxyamine with the resident aldehyde/ketone moiety on a hydrophilic sugar. Neither protection/deprotection of the sugars nor extensive product purification is required. The method allows for the facile adjustment of hydrophobic and hydrophilic domains of the sugar oxime ether surfactant and uses inexpensive, readily accessible, and renewable materials.     

The reaction of atomic oxygen O(3P) with isobutane

The reaction of O(³P) atoms with isobutane has been studied by using the discharge-flow system described previously [1]. The rate constant was measured from determinations of the isobutane concentration in the presence of an excess of O atoms and is given by k₁ = (7.9 ± 1.4) × 10⁷ dm³/mol·s at 307 K. In order to explain the observed reaction products, the mechanism requires that the principal process be the successive abstraction of H atoms from isobutane and from the t-butyl radical to give isobutene. A minor part of the reaction between O(³P) and the t-butyl radical gives the t-butoxy radical, which decomposes to acetone. The branching ratios are .     

Microbiological proficiency testing: a personal perspective

The author considers the fundamental differences between analyses in microbiology and those in chemistry and physics, deducing special issues for microbiological proficiency testing. He concludes that the variability and uncertainty implicit in microbiological analysis requires a broader range of proficiency scheme providers providing a broader range of services than in chemistry and physics. Received: 10 November 2000 Accepted: 3 December 2000     

Non-catalytic hydroamination of alkenes: a computational study

The detailed reaction profiles of the neutral-neutral as well as the cation-neutral direct hydroamination reactions between ethylene and ammonia are analyzed using MP2 (Full)/6-31++G(2df,2p) and B3LYP/6-31++G(2df,2p) methodologies. Analysis shows that both neutral-neutral, as well as the cation-neutral reactions are exothermic and the latter is >100 kJ/mol more exothermic than the former. Calculations show that a very large barrier height (>200 kJ/mol), and very large negative reaction entropy prevent the neutral-neutral reaction from proceeding in the forward direction. Analysis of the cation-neutral reaction, which is barrierless (the transition state is more stable than the reactants) and highly exothermic, indicates that the direct hydroamination reaction is thermodynamically attainable via a cation-neutral reaction pathway without a catalyst. Our calculations also suggest that although the cation-neutral direct hydroamination reaction is very fast, the cation of either ethylene or ammonia goes through a structural relaxation process before reacting with the other neutral reactant.     

Fission process and its application in fissile material identification

Summary In this work, prompt gamma-rays emitted by thermal neutron-induced fission decay using fast coincidence techniques are used to identify both light and heavy fragments and obtain gamma-ray signatures and radiation multiplicity. The measurements were carried out with ²³⁵U and ²³⁹Pu targets using the Intense Pulsed Neutron Source (IPNS) at Argonne National Laboratory. This study reports the recent results of a preliminary analysis of experimental data collected from the induced fission of ²³⁵U and ²³⁹Pu. In this paper, we discuss how these results could be used to characterize an unknown sample of fissile material.     

Spontaneous transmission of chirality through multiple length scales

The hierarchical transfer of chirality in nature, from the nano‐, to meso‐, to macroscopic length scales, is very complex, and as of yet, not well understood. The advent of scanning probes has allowed chirality to be monitored at the single molecule or monolayer level and has opened up the possibility to track enantiospecific interactions and chiral self‐assembly with molecular‐scale detail. This paper describes the self‐assembly of a simple, model molecule (naphtho[2,3‐a]pyrene) that is achiral in the gas phase, but becomes chiral when adsorbed on a surface. This polyaromatic hydrocarbon forms a stable and reversibly ordered system on Cu(111) in which the transmission of chirality from single surface‐bound molecules to complex 2D chiral architectures can be monitored as a function of molecular packing density and surface temperature. In addition to the point chirality of the surface‐bound molecule, the unit cell of the molecular domains was also found to be chiral due to the incommensurate alignment of the molecular rows with respect to the underlying metal lattice. These molecular domains always aggregated in groups of three, all of the same chirality, but with different rotational orientations, forming homochiral “tri‐lobe” ensembles. At a larger length scale, these tri‐lobe ensembles associated with nearest‐neighbor tri‐lobe units of opposite chirality at lower packing densities before forming an extended array of homochiral tri‐lobe ensembles at higher converges. This system displayed chirality at a variety of size scales from the molecular (≈1 nm) and domain (≈5 nm) to the tri‐lobe ensemble (≈10 nm) and extended array (>25 nm) levels. The chirality of the tri‐lobe ensembles dictated how the overall surface packing occurred and both homo‐ and heterochiral arrays could be reproducibly and reversibly formed and interchanged as a function of surface coverage. Finally, these chirally templated surfaces displayed remarkable enantiospecificity for naphtho[2,3‐a]pyrene molecules adsorbed in the second layer. Given its simplicity, reversibility, and rich degree of order, this system represents an ideal test bed for the investigation of symmetry breaking and the hierarchical transmission of chirality through multiple length scales.