Biomedical subjects
Publications and source records attributed to A Kapur.
All-carbon 1,3-dipoles derived from allenic esters/ketones, by catalytic interaction with triphenylphosphine, undergo an unusual [8 + 2] annelation with tropone, leading to 8-oxa-9-(ethoxycarbonyl/acylalkylidene)bicyclo[5.3.0]deca-1,3,5-trienes. Dipoles derived from allenic ketones as well as an alpha-methyl-substituted allenic ester display high reactivity and selectivity.
Photon beams of 4, 6 and 15 MV from Varian Clinac 2100C and 2300C/D accelerators were simulated using the EGS4/BEAM code system. The accelerators were modelled as a combination of component modules (CMs) consisting of a target, primary collimator, exit window, flattening filter, monitor chamber, secondary collimator, ring collimator, photon jaws and protection window. A full phase space file was scored directly above the upper photon jaws and analysed using beam data processing software, BEAMDP, to derive the beam characteristics, such as planar fluence, angular distribution, energy spectrum and the fractional contributions of each individual CM. A multiple-source model has been further developed to reconstruct the original phase space. Separate sources were created with accurate source intensity, energy, fluence and angular distributions for the target, primary collimator and flattening filter. Good agreement (within 2%) between the Monte Carlo calculations with the source model and those with the original phase space was achieved in the dose distributions for field sizes of 4 cm x 4 cm to 40 cm x 40 cm at source surface distances (SSDs) of 80-120 cm. The dose distributions in lung and bone heterogeneous phantoms have also been found to be in good agreement (within 2%) for 4, 6 and 15 MV photon beams for various field sizes between the Monte Carlo calculations with the source model and those with the original phase space.
The purpose of this work was to use Monte Carlo simulations to verify the accuracy of the dose distributions from a commercial treatment planning optimization system (Corvus, Nomos Corp., Sewickley, PA) for intensity-modulated radiotherapy (IMRT). A Monte Carlo treatment planning system has been implemented clinically to improve and verify the accuracy of radiotherapy dose calculations. Further modifications to the system were made to compute the dose in a patient for multiple fixed-gantry IMRT fields. The dose distributions in the experimental phantoms and in the patients were calculated and used to verify the optimized treatment plans generated by the Corvus system. The Monte Carlo calculated IMRT dose distributions agreed with the measurements to within 2% of the maximum dose for all the beam energies and field sizes for both the homogeneous and heterogeneous phantoms. The dose distributions predicted by the Corvus system, which employs a finite-size pencil beam (FSPB) algorithm, agreed with the Monte Carlo simulations and measurements to within 4% in a cylindrical water phantom with various hypothetical target shapes. Discrepancies of more than 5% (relative to the prescribed target dose) in the target region and over 20% in the critical structures were found in some IMRT patient calculations. The FSPB algorithm as implemented in the Corvus system is adequate for homogeneous phantoms (such as prostate) but may result in significant under or over-estimation of the dose in some cases involving heterogeneities such as the air-tissue, lung-tissue and tissue-bone interfaces.
A hybrid approach for commissioning electron beam Monte Carlo treatment planning systems has been studied. The approach is based on the assumption that accelerators of the same type have very similar electron beam characteristics and the major difference comes from the on-site tuning of the electron incident energy at the exit window. For one type of accelerator, a reference machine can be selected and simulated with the Monte Carlo method. A multiple source model can be built on the full Monte Carlo simulation of the reference beam. When commissioning electron beams from other accelerators of the same type, the energy spectra in the source model are tuned to match the measured dose distributions. A Varian Clinac 2100C accelerator was chosen as the reference machine and a four-source beam model was established based on the Monte Carlo simulations. This simplified beam model can be used to generate Monte Carlo dose distributions accurately (within 2%/2 mm compared to those calculated with full phase space data) for electron beams from the reference machine with various nominal energies, applicator sizes, and SSDs. Three electron beams were commissioned by adjusting the energy spectra in the source model. The dose distributions calculated with the adjusted source model were compared with the dose distributions calculated using the phase space data for these beams. The agreement is within 1% in most of cases and 2% in all situations. This preliminary study has shown the capability of the commissioning approach for handling large variation in the electron incident energy. The possibility of making the approach more versatile is also discussed.
Phosphatidylcholine (PtdCho) is the most abundant phospholipid in mammalian cell membranes and is essential for cell viability. The levels of this lipid must be tightly controlled to maintain homeostasis. Therefore, changes in the rate of PtdCho synthesis are generally balanced by changes in PtdCho catabolism and vice versa. It is commonly accepted that the rate of PtdCho synthesis is regulated by CTP:phosphocholine cytidylyltransferase (CT). However, it is not certain if PtdCho mass is regulated by specific catabolic enzyme(s). Our goal is to determine if PtdCho homeostasis is regulated by a phospholipase A2 (PLA2). To this end, we have prepared Chinese hamster ovary (CHO) cell lines that overexpress CT. CT activity is 7-10-fold higher in the transfected cells than in parental CHO cells. This increase in CT activity is associated with increases in both PtdCho synthesis and PtdCho catabolism. Glycerophosphocholine is the PtdCho catabolite that accumulates in the transfected cells, which suggests that PtdCho turnover is mediated by a phospholipase A2 (PLA2). Indeed, higher levels of calcium-independent PLA2 activity are measured in the cytosols of the CHO cells that overexpress CT, compared to parental CHO cells. The elevated calcium-independent PLA2 activity is associated with increases in the expression of the 80-kDa calcium-independent PLA2 (iPLA2). Together, these data suggest that the 80-kDa iPLA2 may be modulated in response to changes in PtdCho levels and therefore is involved in the regulation of PtdCho homeostasis in CHO cells.
The noncovalent binding of the antitumour drugs daunomycin and nogalamycin to duplex DNA has been studied using electrospray ionisation mass spectrometry (ESI-MS). The conditions for the preparation of drug/duplex DNA complexes and for their detection by ESI-MS have been optimised. Ions corresponding to these complexes were most abundant relative to free DNA when prepared in the pH range 8-9, and using gentle ESI interface conditions. Self-complementary oligonucleotides, 5'-d(GGCTAGCC)-3' or 5'-d(CGGCGCCG)-3', annealed in the presence of a 5-fold molar excess of either nogalamycin or daunomycin gave ESI mass spectra in which the most intense ions corresponded to three molecules of drug bound to duplex DNA, with some evidence for four drug molecules bound. For binding to 5'-d(TGAGCTAGCTCA)(2)-3', complexes containing up to four nogalamycin and six daunomycin molecules were observed. These data are consistent with the neighbour exclusion principle whereby intercalation occurs between every other base pair such that up to four bound drugs would be expected for the 8 mers and up to six for the 12 mer. Competition experiments involving a single drug in an equimolar mixture of two oligonucleotides (5'-d(TGAGCTAGCTCA)(2)-3' with either 5'-d(CGGCGCCG)(2)-3' or 5'-d(GGCTAGCC)(2)-3') showed ions arising from complexes of drug/5'-d(CGGCGCCG)(2)-3' were more intense than complexes of drug/5'-d(GGCTAGCC)(2)-3', relative to those from the 12 mer in each mixture. While this suggests ESI-MS has the potential to detect differences in sequence selectivity, more detailed experiments involving a comparison of the relative ionisation efficiency of different oligonucleotides and a wider range of intercalators are required to establish this definitively. ESI mass spectra from experiments in which both drugs were reacted with the same oligonucleotide were more complex, such that a clear preference for one drug could not be established.
We investigated long-term potentiation (LTP) at mossy fiber synapses on CA3 pyramidal neurons in the hippocampus. Using Ca2+ imaging techniques, we show here that when postsynaptic Ca2+ was sufficiently buffered so that [Ca2+]i did not rise during synaptic stimulation, the induction of mossy fiber LTP was prevented. In addition, induction of mossy fiber LTP was suppressed by postsynaptic injection of a peptide inhibitor of cAMP-dependent protein kinase. Finally, when ionotropic glutamate receptors were blocked, LTP depended on the postsynaptic release of Ca2+ from internal stores triggered by activation of metabotropic glutamate receptors. These results support the conclusion that mossy fiber LTP and LTP at other hippocampal synapses share a common induction mechanism involving an initial rise in postsynaptic [Ca2+].
Restricted mass collision stopping-power ratios for electron beams from a scatter-foil medical linear accelerator (Varian Clinac 2100C) were calculated for various combinations of beams, phantoms and detector materials using the Monte Carlo method. The beams were of nominal energy 6, 12 or 20 MeV, with square dimensions 1 x 1 cm2 to 10 x 10 cm2. They were incident at nominal SSDs of 100 or 120 cm and inclined at 90 degrees or 30 degrees to the surface of homogeneous water phantoms or water phantoms interspersed with layered lung or bone-like materials. The broad beam water-to-air stopping-power ratios were within 1.3% of the AAPM TG21 protocol values and consistent with the results of Ding et al to within 0.2%. On the central axis the stopping-power ratio variations for narrow beams compared with normally incident broad beams were 0.1% or less for water-to-LiF-100, graphite, ferrous sulfate dosimeter solution, polystyrene and PMMA, 0.5% for water-to-silicon and 1% for water-to-air and water-to-photographic-film materials. The transverse variations of the stopping-power ratios were up to 4% for water-to-silicon, 7% for water-to-photographic-film materials and 10% for water-to-air in the penumbral regions (where the dose was 10% of the global dose maximum) at shallow depths compared with the values at the same depths on the central axis. In the inhomogeneous phantoms studied, the stopping-power ratio correction factors varied more significantly for air, followed by photographic materials and silicon, at various depths on the central axis in the heterogeneous regions. For the simple layered phantoms studied, the estimation of the stopping-power ratio correction factors based on the relative electron-density derived effective depth approach yielded results that were within 0.5% of the Monte Carlo derived values for all the detector materials studied.
The purpose of this study was to implement the Monte Carlo method for clinical radiotherapy dose calculations. We used the EGS4/BEAM code to obtain the phase-space data for 6-20 MeV electron beams and 4, 6, and 15 MV photon beams for Varian Clinac 1800, 2100C, and 2300CD accelerators. A multiple-source model was used to reconstruct the phase-space data for both electron and photon beams, which retained the accuracy of the Monte Carlo beam data. The multiple-source model reduced the phase-space data storage requirement by a factor of 1000 and the accelerator simulation time by a factor of 10 or more. Agreement within 2% was achieved between the Monte Carlo calculations and measurements of the dose distributions in homogeneous and heterogeneous phantoms for various field sizes, source-surface distances, and beam modulations. The Monte Carlo calculated electron output factors were within 2% of the measured values for various treatment fields while the heterogeneity correction factors for various lung and bone phantoms were within 1% for photon beams and within 2% for electron beams. The EGS4/DOSXYZ Monte Carlo code was used for phantom and patient dose calculations. The results were compared to the dose distributions produced by a conventional treatment planning system and an intensity-modulated radiotherapy inverse-planning system. Significant differences (>5% in dose and >5 mm shift in isodose lines) were found between Monte Carlo calculations and the analytical calculations implemented in the commercial systems. Treatment sites showing the largest dose differences were for head and neck, lung, and breast cases.
Streptogramin antibiotics represent a unique class of antibacterials in the each member of the class consists of at least 2 structurally unrelated molecules: group a streptogramins (macrolactones) and group B streptogramins (cyclic hexadepsipeptides). Both group A and group B streptogramins inhibit protein synthesis at the ribosomal level, and they act synergistically against many isolates their combination generating bactericidal activities and reducing the possibility of emergencies of resistant strains. The mechanisms of acquired resistance to group B streptogramins remain unaffected by target modifications and active efflux. The pharmacokinetic parameters of group A and group B streptogramins in blood are quite similar. In addition, both the A and B group penetrate and accumulate in macrophages and in the bacterial gegetations of experimental endocarditis. Until recently, the complex and irregular composition of naturally occurring pristinamycin and virginiamycin, as well as the unavailability of soluble forms, have limited the clinical development of streptogramins. The synthesis of water soluble derivatives of pristinamycin IA and IIB has now allowed the development of injectable streptogramins with fixed compositions. This unique class of antibacterials will have a significant clinical impact in a world of increasing multidrug resistance affecting the Gram-positive cocci, especially staphylococci and pneumococci. The absence of cross-resistance to macrolides in many of these isolates and the rapid antibacterial killing against these species bright future for this class of antibiotics.
The small heat-shock protein, alpha-crystallin, has chaperone ability whereby it stabilises proteins under stress conditions. In this study, alterations in the structure of alpha-crystallin during its interaction with a variety of substrate proteins (insulin, alpha-lactalbumin, ovotransferrin and serum albumin) under stress conditions have been examined using visible absorption, 31P-NMR and 1H-NMR and fluorescence spectroscopy. The fluorescence and 31P-NMR data imply that during the chaperone action of alpha-crystallin under reducing conditions, there is a slight increase in hydrophilicity of its N-terminal region and an alteration in flexibility of its C-terminal region, but overall, alpha-crystallin does not undergo a gross structural change. The fluorescence data suggest that substrate proteins interact with alpha-crystallin in a molten globule or intermediately folded state. The same conclusion is made from 1H-NMR spectroscopic monitoring of the interaction of alpha-crystallin with substrate proteins, e.g. the insulin B chain. The stoichiometry of interaction between alpha-crystallin and the various substrate proteins reveals that steric factors are important in determining the efficiency of interaction between the two proteins, i.e. on a molar subunit basis, alpha-crystallin is a more efficient chaperone protein with smaller substrate proteins. Comparison is also made between the high-molecular-mass (HMM) complexes formed between alpha-crystallin and ovotransferrin when reduced and heat stressed. Under heating conditions, fluorescence spectroscopy indicates that the HMM complex has a greater exposure of hydrophobicity to solution than that formed by reduction. Furthermore, in interacting with heated ovotransferrin, the C-terminal extension of the alphaB-crystallin subunit preferentially loses its flexibility suggesting that it is involved in stabilising bound ovotransferrin. By contrast, this extension is only partially reduced in flexibility in the HMM complex formed after reduction of ovotransferrin. The functional role of the C-terminal extensions in the chaperone action and the overall quaternary structure of alpha-crystallin is discussed.
BACKGROUND: Laxative pretreatment decreases the volume of polyethylene glycol electrolyte lavage solution (PEG-ELS) required for colonoscopy without compromising preparation quality. We compared the use of 4 L of PEG-ELS with the use of 2 L plus a laxative. METHODS: One hundred fifty consecutive patients (148 men) undergoing outpatient colonoscopy were randomly selected for one of three preparations (Prep 1: 4 L PEG-ELS; Prep 2: 2 L PEG-ELS plus 296 mL magnesium citrate 1 hour prior; Prep 3: 2 L PEG-ELS plus bisacodyl 20 mg). Endoscopists were blinded as to the type of preparation. RESULTS: Colonoscopy times were 37, 33, and 29.5 minutes (p = 0.02). Satisfaction scores (0 to 11) during preparation were 2.75, 1.84, and 2.54 (p = 0.05). Preparation times were 519, 397, and 379 minutes (p < 0.001). Preparation satisfaction scores (0 to 10) were 6.2, 7.7, and 7.4 (p < 0.001). Endoscopists' scores of preparation quality (1 to 10) were 7.3, 7.8, and 8.1 (p = 0.03). Volumes of liquid stool aspirated were 181, 103, and 90 mL (p < 0.001). Twenty-four patients receiving Prep 2 and 16 receiving Prep 3 had previous colonoscopy using full volume PEG-ELS; 88% who received Prep 2 and 56% who received Prep 3 preferred the newer preparation (p = 0.006). CONCLUSIONS: Two liters of PEG-ELS plus laxative improved preparation quality and patient satisfaction and reduced preparation time. Magnesium citrate pretreatment had fewer symptoms and was preferred to bisacodyl. PEG-ELS in 2 L quantities could reduce costs, and consideration should be given to making it available commercially.
Bacterial lipopolysaccharide (LPS) is a potent activator of cells of the macrophage/monocyte lineage. Two mature macrophage cell lines, P388D1 and RAW264.7, exhibit very different biological responses to LPS. Although RAW264.7 cells release arachidonic acid from phospholipid in response to LPS stimulation, P388D1 cells do not respond in this manner. However, LPS primes P388D1 cells to release arachidonic acid in response to other stimuli. The goal of this work is to contrast the biochemical events that occur in LPS-treated P388D1 and RAW264.7 macrophages. Enzyme assays indicate that LPS treatment induces the activation of cytosolic PLA2 in RAW264.7, but not in P388D1 cells. Phorbol ester (PMA), a receptor-independent stimulus, also fails to induce arachidonic acid release from P388D1 cells, suggesting that these cells may have a defect in the signal transduction machinery that is common to LPS and PMA. This hypothesis is supported by the observation that the expression of the LPS receptors CD14 and CD11b/CD18 is similar on P388D1 and RAW264.7 cells. Western blot analyses indicate that the erk kinases are activated upon LPS treatment of RAW264.7 but not P388D1 cells. LPS-induced arachidonic acid release is reduced in cells treated with the MEK inhibitor PD98059, suggesting that activated erk kinases mediate the phosphorylation and activation of cPLA2 in this system. Interestingly, the p42 isoform of erk (erk2) appears to be activated in resting P388D1 cells. This observation indicates that the MAP kinase cascade may be constitutively activated in P388D1 cells which may in turn limit their ability to respond to LPS. Together, these data provide evidence that mature macrophages from different sources can exhibit variable responses to LPS and highlight the danger of making generalizations regarding the effects of LPS on macrophages.
Explore the source record for details and available documents.
The purpose of this study was to investigate the application of the Monte Carlo technique to the calculation and analysis of output factors for electron beams used in radiotherapy. The code EGS4/BEAM was used to obtain phase-space files for 6, 12 and 20 MeV clinical electron beams from a scattering-foil linac (Varian Clinac 2100C) for a clinically representative range of applicator and square or rectangular insert combinations. The source-to-surface distance used was 100 cm. The field sizes ranged from 1 x 1 cm2 to 20 x 20 cm2. These phase-space files were analysed to study the intrinsic beam characteristics and used as source input for relative dose and output factor computations in homogeneous water phantoms using the code EGS4/DOSXYZ. The calculated relative central-axis depth-dose and transverse dose profiles at various depths of clinical interest agreed with the corresponding measured dose profiles to within 2% of the maximum dose. Calculated output factors for the fields studied agreed with measured output factors to about 2%. This demonstrated that for the Varian Clinac 2100C linear accelerator, electron beam dose calculations in homogeneous water phantoms can be performed accurately at the 2% level using Monte Carlo simulations.
The requirement of postsynaptic calcium influx via L-type channels for the induction of long-term potentiation (LTP) of mossy fiber input to CA3 pyramidal neurons was tested for two different patterns of stimulation. Two types of LTP-inducing stimuli were used based on the suggestion that one of them, brief high-frequency stimulation (B-HFS), induces LTP postsynaptically, whereas the other pattern, long high-frequency stimulation (L-HFS), induces mossy fiber LTP presynaptically. To test whether or not calcium influx into CA3 pyramidal neurons is necessary for LTP induced by either pattern of stimulation, nimodipine, a L-type calcium channel antagonist, was added during stimulation. In these experiments nimodipine blocked the induction of mossy fiber LTP when B-HFS was given [34 +/- 5% (mean +/- SE) increase in control versus 7 +/- 4% in nimodipine, P < 0.003]; in contrast, nimodipine did not block the induction of LTP with L-HFS (107 +/- 10% in control vs. 80 +/- 9% in nimodipine, P > 0.05). Administration of nimodipine after the induction of LTP had no effect on the expression of LTP. In addition, B- and L-HFS delivered directly to commissural/associational fibers in stratum radiatum failed to induce a N-methyl--aspartate-independent form of LTP, obviating the possibility that the presumed mossy fiber LTP resulted from potentiation of other synapses. Nimodipine had no effect on calcium transients recorded from mossy fiber presynaptic terminals evoked with the B-HFS paradigm but reduced postsynaptic calcium transients. Our results support the hypothesis that induction of mossy fiber LTP by B-HFS is mediated postsynaptically and requires entry of calcium through L-type channels into CA3 neurons.
Stimulation of afferent fibers with current pulse trains has been reported to induce long-term potentiation (LTP) in piriform cortex in vitro but not in vivo. LTP has been observed in vivo only when trains are paired with behavioral reinforcement and as a consequence of kindled epileptogenesis. This study was undertaken in the urethan-anesthetized rat to determine if the reported failures to observe pulse-train evoked LTP in vivo may be related to a lesser persistence rather than lack of occurrence, if disinhibition might facilitate induction, and to examine the nature of the relationship between seizure activity and LTP. Stimulation of afferent fibers in the lateral olfactory tract with theta-burst trains under control conditions potentiated the monosynaptic field excitatory postsynaptic potential (EPSP) by approximately the same extent (20.3 +/- 2%; n = 12) as reported for the slice. However, in contrast to the slice, potentiation in vivo decayed to a low level within 1-2 h after induction (70% loss in 1.5 h, on average). The N-methyl--aspartate (NMDA)-receptor antagonists -APV and MK-801 blocked the induction of this decremental potentiation. Pharmacological reduction of gamma-aminobutyric acid-mediated inhibition at the recording site did not increase the duration of potentiation. In contrast, theta-burst stimulation applied after recovery from a period of epileptiform bursting induced stable NMDA-dependent potentiation. Mean increase in the population EPSP was approximately the same as under control conditions (21 +/- 2%; n = 6), but in five of six experiments there was little or no decay in potentiation for the duration of the monitoring period (</=6 h). It is concluded that seizure activity has an enabling action on the induction of persistent synaptic potentiation by stimulus trains that bypasses the need for behavioral reinforcement.