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Biomedical subjects

R Kaiser

Publications and source records attributed to R Kaiser.

At least 19 recordsLinked to original sources
PubMed2006

Light transport in cold atoms and thermal decoherence.

We use the coherent backscattering interference effect to investigate experimentally and theoretically how coherent transport of light inside a cold atomic vapor is affected by the residual motion of atomic scatterers. As the temperature of the atomic cloud increases, the interference contrast decreases dramatically. This emphasizes the role of motion-induced decoherence for resonant scatterers even in the sub-Doppler regime of temperature. We derive analytical expressions for the corresponding coherence time.

Journal Article↗
PubMed2006

Double-hadron leptoproduction in the nuclear medium.

The first measurements of double-hadron production in deep-inelastic scattering within the nuclear medium were made with the HERMES spectrometer at DESY HERA using a 27.6 GeV positron beam. By comparing data for deuterium, nitrogen, krypton, and xenon nuclei, the influence of the nuclear medium on the ratio of double-hadron to single-hadron yields was investigated. Nuclear effects on the additional hadron are clearly observed, but with little or no difference among nitrogen, krypton, or xenon, and with smaller magnitude than effects seen on previously measured single-hadron multiplicities. The data are compared with models based on partonic energy loss or prehadronic scattering and with a model based on a purely absorptive treatment of the final-state interactions. Thus, the double-hadron ratio provides an additional tool for studying modifications of hadronization in nuclear matter.

Journal Article↗
PubMed2006

Self-sustained oscillations in a large magneto-optical trap.

We have observed self-sustained radial oscillations in a large magneto-optical trap, containing up to 10(10) Rb85 atoms. This instability is due to the competition between the confining force of the magneto-optical trap and the repulsive interaction associated with multiple scattering of light inside the cold atomic cloud. A simple analytical model allows us to formulate a criterion for the instability threshold, in fair agreement with our observations. This criterion shows that large numbers of trapped atoms N>10(9) are required to observe this unstable behavior.

Journal Article↗
PubMed2006

[Stent therapy with diabetic patients who have coronary artery disease].

When bare-metal stents are used, diabetic patients with coronary artery disease have a poorer prognosis than non-diabetic patients after coronary interventions. In clinical studies, it could be demonstrated that the disadvantage for diabetic patients is not present when drug-eluting stents are used. Today, two types of drug-eluting stents are clinically used: the sirolimus-eluting stent (Cypher) and the paclitaxel-eluting stent (Taxus). Even the high risk group of patients with in-stent stenosis can be efficiently treated with drug-eluting stents.

Coronary Artery Disease↗
PubMed2006

CYP2D6 polymorphism and clinical effect of the antidepressant venlafaxine.

BACKGROUND: Venlafaxine (V) is a mixed serotonin and noradrenaline reuptake inhibitor used as a first-line treatment of depressive disorders. It is metabolized primarily by the highly polymorphic cytochrome P450 (CYP) enzyme CYP2D6 to yield a pharmacologically active metabolite, O-desmethylvenlafaxine (ODV), and to a lesser extent by CYP3A4, to yield N-desmethylvenlafaxine (NDV). OBJECTIVES: The aim of this study was to assess whether the O-demethylation phenotype of V has an impact on the pharmacokinetics and clinical outcome. METHOD: In 100 patients treated with V, serum concentrations of V, ODV and NDV and the ratios of concentrations ODV/V as a measure of O-demethylation were determined. Individuals exhibiting abnormally high or low metabolic ratios of ODV/V were selected for genotyping. Clinical effects were monitored by the Clinical Global Impressions Scale and side effects by the UKU (Udvalg for Kliniske Undersogelser Side Effect Rating Scale) rating scale. RESULTS: There was wide inter-individual variability in ODV/V ratios. The median ratio ODV/V was 1.8 and the 10th and 90th percentiles 0.3 and 5.2, respectively. Individuals with ODV/V ratios below 0.3 were all identified as poor metabolizers (PM), with the genotypes *6/*4 (n = 1), *5/*4 (n = 2) or *6/*6 (n = 1). Individuals with ratios above 5.2 were all ultra rapid metabolizers (UM, n = 6) due to gene duplications. Five individuals with intermediate metabolic activity (ODV/V, 1.1 +/- 0.8) were heterozygotes with the CYP2D6*4 genotype, and one patient with an intermediate metabolic ratio of 4.8 had the genotype *4/2x*1. Clinical outcome measurements revealed that patients with ODV/V ratios below 0.3 had more side effects (P < 0.005) and reduced serum concentrations of sodium (P < 0.05) in comparison with other patients. Gastrointestinal side effects, notably nausea, vomiting and diarrhoea were the most common. Differences in therapeutic efficacy were not significant between the different phenotypes. CONCLUSION: The O-demethylation phenotype of V depends strongly on the CYP2D6 genotype. A PM phenotype of CYP2D6 increases the risk of side effects.

Adolescent↗
PubMed2005

Measurement of the tensor structure function b1 of the deuteron.

The Hermes experiment has investigated the tensor spin structure of the deuteron using the 27.6 GeV/c positron beam of DESY HERA. The use of a tensor-polarized deuteron gas target with only a negligible residual vector polarization enabled the first measurement of the tensor asymmetry A(d)zz and the tensor structure function b(d)1 for average values of the Bjorken variable 0.01< <0.45 and of the negative of the squared four-momentum transfer 0.5 GeV2 < <5 GeV2. The quantities A(d)zz and b(d)1 are found to be nonzero. The rise of b(d)1 for decreasing values of x can be interpreted to originate from the same mechanism that leads to nuclear shadowing in unpolarized scattering.

Journal Article↗
PubMed2005

Single-spin asymmetries in semi-inclusive deep-inelastic scattering on a transversely polarized hydrogen target.

Single-spin asymmetries for semi-inclusive electroproduction of charged pions in deep-inelastic scattering of positrons are measured for the first time with transverse target polarization. The asymmetry depends on the azimuthal angles of both the pion (phi) and the target spin axis (phi(S)) about the virtual-photon direction and relative to the lepton scattering plane. The extracted Fourier component sin((phi+phi(S))(pi)(UT) is a signal of the previously unmeasured quark transversity distribution, in conjunction with the Collins fragmentation function, also unknown. The component sin((phi-phi(S)(pi)(UT) arises from a correlation between the transverse polarization of the target nucleon and the intrinsic transverse momentum of quarks, as represented by the previously unmeasured Sivers distribution function. Evidence for both signals is observed, but the Sivers asymmetry may be affected by exclusive vector meson production.

Journal Article↗
PubMed2005

Association of cyclophosphamide pharmacokinetics to polymorphic cytochrome P450 2C19.

Cyclophosphamide (CP), a widely used cytostatic, is metabolized by polymorphic drug metabolizing enzymes particularly cytochrome P450 (CYP) enzymes. Its side effects and clinical efficacy exhibit a broad interindividual variability, which might be due to differences in pharmacokinetics. CP-kinetics were determined in 60 patients using a global and a population pharmacokinetic model considering functionally relevant polymorphisms of CYP2B6, CYP2C9, CYP2C19, CYP3A5, and GSTA1. Moreover, metabolic ratios were calculated for selected CP metabolites, analyzed by (31)P-NMR-spectroscopy. Analysis of variance revealed that the CYP2C19*2 genotype influenced significantly pharmacokinetics of CP at doses </=1000 mg/m(2), whereas there was no evidence of an association of other genotypes to CP elimination or clearance. Mean (+/-SD) CP elimination constants k(e) (h(-1)) were 0.109+/-0.025 in 44 CYP2C19*1/*1 subjects, 0.088+/-0.018 in 13 CYP2C19*1/*2, and 0.076+/-0.014 in three inactive CYP2C19*2/*2 carriers (P=0.009). At CP doses higher than 1000 mg/m(2), a significantly increase of elimination was observed (P=0.001), possibly due to CYP induction. Further studies should link these findings with the clinical outcome.

Adult↗
PubMed2004

Magnetic field enhanced coherence length in cold atomic gases.

We study the effect of an external magnetic field on coherent backscattering of light from a cold rubidium vapor. We observe that the backscattering enhancement factor can be increased with B. This surprising behavior shows that the coherence length of the system can be increased by adding a magnetic field, in sharp contrast with usual situations. This is mainly due to the lifting of the degeneracy between Zeeman sublevels. We find good agreement between our experimental data and a full Monte Carlo simulation, taking into account the magneto-optical effects and the geometry of the atomic cloud.

Journal Article↗
PubMed2004

Saturation-induced coherence loss in coherent backscattering of light.

We use coherent backscattering of light by cold strontium atoms to study phase-breaking mechanisms in the multiple-scattering regime. As the probe light intensity is increased, the atomic optical transition starts to be saturated. Nonlinearities and inelastic scattering then occur. The latter induces a characteristic phase-breaking time that reduces the wave coherence. In our experiment, this leads to a strong reduction of the enhancement factor of the coherent backscattering cone. The results at different probe detuning are also presented.

Journal Article↗
PubMed2004

Primary drug-resistance in HIV-positive patients on initiation of first-line antiretroviral therapy in Germany.

BACKGROUND: Resistance against antiretroviral drugs in previously untreated HIV-infected persons is of growing relevance. The aim of the study is to determine the prevalence of resistance-associated mutations in this patient group. METHODS: In a prospective multicenter-study in Nordrhein-Westfalen, Germany, genotypic resistance testing was performed in untreated HIV-positive patients before administration of first-line highly active antiretroviral therapy (HAART). RESULTS: Between January 2001 and August 2002 resistance testing was performed in 184 therapy-naive individuals. HAART was initiated in 143 patients, who were included into the study. 70.6 % were males, mean age was 39 years, mean duration of diagnosis of HIV-infection was 1.5 years. The proportion of cases at CDC stage C was 45.4%, mean CD4-cell count was 199 /ml, mean viral load was 206,855 copies/ml. Resistance-associated mutations were detected in 20 patients (14.0%). 10.5% showed mutations indicating nucleoside reverse transcriptase inhibitor- (NRTI) resistance (M41L, E44D, D67N, T69D/N, L74V, V118I, M184V, L210W, K219Q), 2.8% showed non-nucleoside reverse transcriptase inhibitor (NNRTI) resistance (K103N, V108I, Y181C), and 2.1% showed protease-inhibitor- (PI) associated resistance (V82A, L90M), respectively. Multi-class-resistance was found in 2.1%, mutations indicating revertant variants of resistant strains were found in 4.2% (T215C/E/L/S). 86.7% of the isolates showed secondary mutations in the protease gene. No significant difference in the distribution of the parameters age, sex, duration of HIV diagnosis, CDC stage, CD4-cell count, and viral load, between groups with and without resistance was identified. CONCLUSION: The prevalence of primary resistant virus strains can be estimated at 14% in chronically infected HAART-naive HIV-patients in Germany. The majority of these cases show NRTI-associated resistance. Resistance against NNRTI or PI as well as multi-class-resistance is of low prevalence. No risk factor of predictive value can be identified for the diagnosis of resistance mutations in the individual. In conclusion, routine genotypic resistance testing in untreated HIV-positive patients should be performed before administration of first-line HAART in this region.

Adult↗