Laser-plasma accelerators of only a centimetre's length have produced nearly monoenergetic electron bunches with energy as high as 1 GeV. Scaling these compact accelerators to multi-gigaelectronvolt ...energy would open the prospect of building X-ray free-electron lasers and linear colliders hundreds of times smaller than conventional facilities, but the 1 GeV barrier has so far proven insurmountable. Here, by applying new petawatt laser technology, we produce electron bunches with a spectrum prominently peaked at 2 GeV with only a few per cent energy spread and unprecedented sub-milliradian divergence. Petawatt pulses inject ambient plasma electrons into the laser-driven accelerator at much lower density than was previously possible, thereby overcoming the principal physical barriers to multi-gigaelectronvolt acceleration: dephasing between laser-driven wake and accelerating electrons and laser pulse erosion. Simulations indicate that with improvements in the laser-pulse focus quality, acceleration to nearly 10 GeV should be possible with the available pulse energy.
Depleted uranium is of current programmatic interest at Los Alamos National Lab due to its high density and nuclear applications. At room temperature, depleted uranium displays an orthorhombic ...crystal structure with highly anisotropic mechanical and thermal properties. For instance, the coefficient of thermal expansion is roughly 20
×
10
−6
°C
−1 in the
a and
c directions, but near zero or slightly negative in the
b direction. The innate anisotropy combined with thermo-mechanical processing during manufacture results in spatially varying residual stresses and crystallographic texture, which can cause distortion, and failure in completed parts, effectively wasting resources. This paper focuses on the development of residual stresses and textures during deformation at room and elevated temperatures with an eye on the future development of computational polycrystalline plasticity models based on the known micro-mechanical deformation mechanisms of the material.
Metabolism by aldehyde oxidase (AO) has been responsible for a number of drug failures in clinical trials. The main reason is the clearance values for drugs metabolized by AO are underestimated by ...allometric scaling from preclinical species. Furthermore, in vitro human data also underestimates clearance. We have developed the first in silico models to predict both in vitro and in vivo human intrinsic clearance for 8 drugs with just two chemical descriptors. These models explain a large amount of the variance in the data using two computational estimates of the electronic and steric features of the reaction. The in vivo computational models for human metabolism are better than in vitro preclinical animal testing at predicting human intrinsic clearance. Thus, it appears that AO is amenable to computational prediction of rates, which may be used to guide drug discovery, and predict pharmacokinetics for clinical trials.
Knowledge of free drug intracellular concentration is necessary to predict the impacts of drugs on intracellular targets. The goal of this study was to develop models to predict free intracellular ...drug concentrations in the presence of apical efflux transporters. The apical efflux transporter P-glycoprotein (P-gp), encoded by human gene multidrug resistance 1 (MDR1), was studied. Apparent permeabilities for 10 compounds in Madin-Darby canine kidney (MDCK) and MDR1-MDCK cell monolayers were obtained experimentally. Six of these compounds were evaluated additionally in the presence of the P-gp inhibitor cyclosporine A. A three-compartment model was developed, and passive and apical efflux clearances (CL(d) and CL(ae), respectively) were estimated. Endogenous canine transporters also were delineated. The three-compartment model was unable to simulate experimentally observed lag times and exhibited systematic bias across the simulations. Next, a five-compartment model with explicit membrane compartments was developed. This model resulted in lower systematic errors and simulated the lag time observed experimentally. Apical efflux was modeled out of the cell or out of the membrane. The five-compartment model with apical efflux out of the membrane predicted marked differences in unbound intracellular concentrations between the apical-to-basolateral and the basolateral-to-apical directions. Upon apical drug addition, large decreases in intracellular concentrations were observed with the efflux transporter. No such difference was predicted upon basolateral drug addition. This is consistent with experimental differences in the impact of P-gp on hepatic and brain distribution and supports the hypothesis that apical efflux occurs out of the apical membrane.
Many pharmacokinetic analyses require unbound plasma concentrations, including prediction of clearance, volume of distribution, drug−drug interactions, brain uptake analysis, etc. It is most often ...more convenient to measure the total drug concentration in plasma rather than the unbound drug concentration. To arrive at unbound plasma concentrations, separate in vitro determinations of the plasma protein binding of a drug are usually carried out in serum or in plasma, and the plasma pharmacokinetic results are then mathematically adjusted by this fraction unbound (f u,p). Plasma protein binding or the drug fraction unbound in plasma (f u,p) is known to be affected by protein, drug, free fatty acid concentrations, lipoprotein partitioning, temperature, pH, and the presence or absence of other drugs/displacing agents within plasma samples. Errors in f u,p determination caused by lack of adequate pH control in newer assay formats for plasma protein binding (e.g., 96-well equilibrium thin walled polypropylene dialysis plates) will have significant drug-specific impact on these pharmacokinetic calculations. Using a diverse set of 55 drugs and a 96-well equilibrium dialysis plate format, the effect of variable pH during equilibrium dialysis experiments on measured values of f u,p was examined. Equilibrium dialysis of human plasma against Dulbecco’s phosphate buffered saline at 37 °C under an air or 10% CO2 atmosphere for 22 h resulted in a final pH of approximately 8.7 and 7.4, respectively. The ratio of f u,p at pH 7.4 (10% CO2) vs pH 8.7 (air) was ≥2.0 for 40% of the 55 compounds tested. Only one of the 55 compounds tested had a ratio <0.9. Select compounds were further examined in rat and dog plasma. In addition, physicochemical properties were calculated for all compounds using ACD/Labs software or Merck in-house software and compared to plasma protein binding results. Changes in plasma protein binding due to pH increases which occurred during the equilibrium dialysis experiment were not species specific but were drug-specific, though nonpolar, cationic compounds had a higher likely hood of displaying pH-dependent binding. These studies underscore the importance of effectively controlling pH in plasma protein binding studies.
Some cytochrome P450 catalyzed reactions show atypical kinetics, and these kinetic processes can be grouped into five categories: activation, autoactivation, partial inhibition, substrate ...inhibition, and biphasic saturation curves. A two-site model in which the enzyme can bind two substrate molecules simultaneously is presented which can be used to describe all of these observed kinetic properties. Sigmoidal kinetic characteristics were observed for carbamazepine metabolism by CYP3A4 and naphthalene metabolism by CYPs 2B6, 2C8, 2C9, and 3A5 as well as dapsone metabolism by CYP2C9. Naphthalene metabolism by CYP3A4 and naproxen metabolism by CYP2C9 demonstrated nonhyperbolic enzyme kinetics suggestive of a low K m, low V max component for the first substrate molecule and a high K m, high V max component for the second substrate molecule. 7,8-Benzoflavone activation of phenanthrene metabolism by CYP3A4 and dapsone activation of flurbiprofen and naproxen metabolism by CYP2C9 were also observed. Furthermore, partial inhibition of 7,8-benzoflavone metabolism by phenanthrene was observed. These results demonstrate that various P450 isoforms may exhibit atypical enzyme kinetics depending on the substrate(s) employed and that these results may be explained by a model which includes simultaneous binding of two substrate molecules in the active site.
Aldehyde oxidase is a molybdenum hydroxylase that catalyzes the oxidation of aldehydes and nitrogen-containing heterocycles. The enzyme plays a dual role in the metabolism of physiologically ...important endogenous compounds and the biotransformation of xenobiotics. Using density functional theory methods, geometry optimization of tetrahedral intermediates of drugs and druglike compounds was examined to predict the likely metabolites of aldehyde oxidase. The calculations suggest that the lowest energy tetrahedral intermediate resulting from the initial substrate corresponds to the observed metabolite ≥90% of the time. Additional calculations were performed on a series of heterocyclic compounds where the products resulting from metabolism by xanthine oxidase and aldehyde oxidase differ in many instances. Again, the lowest energy tetrahedral intermediate corresponded to the observed product of aldehyde oxidase metabolism ≥90% for the compounds examined, while the observed products of xanthine oxidase were not well predicted.
Aims The present study was conducted to evaluate metabolism of the enantiomers of verapamil and norverapamil using a broad range of cytochrome P450 isoforms and measure the kinetic parameters of ...these processes.
Methods Cytochrome P450 cDNA‐expressed cells and microsomes from a P450‐expressed lymphoblastoid cell line were incubated with 40 μm concentrations of R‐ or S‐verapamil and R‐ or S‐norverapamil and metabolite formation measured by h.p.l.c. as an initial screening. Those isoforms exhibiting substantial activity were then studied over a range of substrate concentrations (2.5–450 μm ) to estimate the kinetic parameters for metabolite formation.
Results P450s 3A4, 3A5, 2C8 and to a minor extent 2E1 were involved in the metabolism of the enantiomers of verapamil. Estimated Km values for the production of D‐617 and norverapamil by P450 s 3A4 and 3A5 were similar (range=60–127 μm ) regardless of the enantiomer of verapamil studied while the Vmax estimates were also similar (range=4–8 pmol min−1 pmol−1 P450). Only nominal production of D‐620 by these isoforms was noted. Interestingly, P450 2C8 readily metabolized both S‐ and R‐verapamil to D‐617, norverapamil and PR‐22 with only slightly higher Km values than noted for P450s 3A4 and 3A5. However, the Vmax estimates for P450 2C8 metabolism of S‐ and R‐verapamil were in general greater (range=8–15 pmol min−1 pmol−1 P450) than those noted for P450 s 3A4 and 3A5 with preference noted for metabolism of the S‐enantiomer. Similarly, P450 s 3A4, 3A5 and 2C8 also mediated the metabolism of the enantiomers of norverapamil with minor contributions by P450 s 2D6 and 2E1. P450s 3A4 and 3A5 readily formed the D‐620 metabolite with generally a lower Km and higher Vmax for S‐norverapamil than for the R‐enantiomer. In contrast, P450 2C8 produced both the D‐620 and PR‐22 metabolites from the enantiomers of norverapamil, again with stereoselective preference seen for the S‐enantiomer.
Conclusions These results confirm that P450s 3A4, 3A5 and 2C8 play a major role in verapamil metabolism and demonstrate that norverapamil can also be further metabolized by the P450s.
PDF
