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.     

Delta-electron emission in fast heavy ion — atom collisions: observations of new phenomena and breakdown of common scaling laws

Double differential cross sections for the emission of Delta-electrons have been measured in fast uranium-rare gas collisions. The well-known Binary Encounter peak reveals unexpected structures for certain observation angles and its intensity increases towards smaller angles, which is in contradiction to results and scaling laws obtained by experiments with light ion impact. The observed dependencies are fairly well described by recent calculations in the framework of IA and CTMC. From systematic experimental as well as theoretical studies we can derive that the potential of the partially stripped projectile ion gives rise to rainbow and glory scattering of the target electron in the field of the projectile. The rainbow scattering is observed in the laboratory frame as pronounced interference structures, whereas the glory scattering is responsible for the steep increase of the cross sections for binary-encounter electrons towards small laboratory ejection angles. The observed effects have a dramatic influence on the commonq ² scaling laws derived from experiments with light ions. Furthermore, since the binary-encounter electrons ejected at forward angles have approximately twice the projectile velocity, these new phenomena have an important influence on the electronic stopping power of heavy ions and therefore have to be taken into account for the investigation of radiation damage by these ions e.g. in biological matter.     

Electron-electron interaction in strong electromagnetic fields: the two-electron contribution to the ground-state energy in He-like uranium

Radiative recombination transitions into the ground state of cooled bare and hydrogenlike uranium ions were measured at the storage ring ESR. By comparing the corresponding x-ray centroid energies, this technique allows for a direct measurement of the electron-electron contribution to the ionization potential in the heaviest He-like ions. For the two-electron contribution to the ionization potential of He-like uranium we obtain a value of 2248+/-9 eV. This represents the most accurate determination of two-electron effects in the domain of high-Z He-like ions, and the accuracy reaches already the size of the specific two-electron radiative QED corrections.     

Electron correlations observed through intensity interferometry

Intensity interferometry was applied to study electron correlations in doubly ionizing ion-atom collisions. In this method, the probability to find two electrons emitted in the same double ionization event with a certain momentum difference is compared to the corresponding probability for two uncorrelated electrons from two independent events. The ratio of both probabilities, the so-called correlation function, is found to sensitively reveal electron correlation effects, but it is rather insensitive to the collision dynamics.     

Synthesis and acylation of 2-nitro-11H-dibenzo[b,e][1,4]dioxepin

A one-pot synthesis of the 11H-dibenzo[b,e]1,4]dioxepin ring system from catechol and an o-chlorobenzyl chloride is described. Friedel-Crafts acylation occurs at the 7-position as shown by X-ray analysis.     

Inner-Shell X-ray production in adiabatic iodine-Z 2 collisions,withZ 1+Z 2≧100

K-, L- andM-x ray production cross sections for the molecular excitation regime in heavy ion-atom collisions (Z ₁+Z ₂≧100) are reported for the projectile velocity range 1.3–3.7 a.u. Outer-shell vacancy configurations of the collision partners having experienced inner shell vacancy creating collisions are deduced from the x-ray spectra. The influence of outer shell vacancy occupation on fluorescence yields is discussed and vacancy production cross sections are derived from x-ray production cross sections.     

An algorithm for the electromagnetic scattering due to an axially symmetric body with an impedance boundary condition

Let B be a body in R³ and let S denote the boundary of B. The surface S is described by $\text{S = {(x, y, z): (x}{}^2\text{+ y}{}^2\text{)}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{= f(z), ?1 ? z ? 1}}$, where f is an analytic function that is real and positive on (?1, 1) and f(±1) = 0. An algorithm is described for computing the scattered field due to a plane wave incident field, under Leontovich boundary conditions. The Galerkin method of solution used here leads to a block diagonal matrix involving 2M + 1 blocks, each block being of order 2(2N + 1). If, e.g., N = O(M²), the computed scattered field is accurate to within an error bounded by $\text{Ce}{}^{\mathrm{<mtext>?cN</mtext><mtext>1</mtext><mtext>2</mtext>}}$, where C and c are positive constants depending only on f.     

Inner-shell x-ray production in adiabatic iodine -Z 2 collisions withZ 1+Z 2≧100

For heavy collision systems withZ _UA ≧100 the excitation mechanism for electrons in molecular orbitals from 2p _{1/2 1/2} up to 3d_{3/2 1/2}, which end up inK andL levels of the collision partners, is investigated for Iodine projectile velocityv between 1.3 and 3.7 a.u. (i.e. 6.3 MeV≦E _i ≦45 MeV). For 2p _{1/2 1/2} excitation a prevailing radial coupling excitation mechanism is found. In a two state approximation coupling strengths are derived for decisive isolated crossings; for the strong variation of the projectileL x-ray spectra with targetZ and impact velocity a Landau Zener type model is suggested.     

Study of the 2p 1/2 σ excitation probability in collision systems withZ 1+Z 2>150 using line width analysis

The 90°-line width ofK x-ray lines induced in heavy ion collisions is mainly determined by the kinematical broadening. Its conversion into mean impact parameters together with total excitation cross section allows to obtain mean excitation probabilities. Within the energy range (1.4–5.9) MeV/u mean excitation probabilities for 2p _1/2 σ-excitation are deduced. Collision systems in theZ-range (150–180) are investigated. A very poor agreement between experiment and theoretical predictions has been found. The excitation probabilities are larger than predicted and are nearly independent of the atomic numberZ of the united atom. The collision energy dependence is much weaker than predicted. Even a decrease of the mean excitation probabilities at high collision energies has been found.