PubMed Health⌕ Search

Biomedical subjects

D G Covell

Publications and source records attributed to D G Covell.

At least 19 recordsLinked to original sources
PubMed1994

Analysis of hydrophobicity in the alpha and beta chemokine families and its relevance to dimerization.

The chemokine family of chemotactic cytokines plays a key role in orchestrating the immune response. The family has been divided into 2 subfamilies, alpha and beta, based on the spacing of the first 2 cysteine residues, function, and chromosomal location. Members within each subfamily have 25-70% sequence identity, whereas the amino acid identity between members of the 2 subfamilies ranges from 20 to 40%. A quantitative analysis of the hydrophobic properties of 11 alpha and 9 beta chemokine sequences, based on the coordinates of the prototypic alpha and beta chemokines, interleukin-8 (IL-8), and human macrophage inflammatory protein-1 beta (hMIP-1 beta), respectively, is presented. The monomers of the alpha and beta chemokines have their strongest core hydrophobic cluster at equivalent positions, consistent with their similar tertiary structures. In contrast, the pattern of monomer surface hydrophobicity between the alpha and beta chemokines differs in a manner that is fully consistent with the observed differences in quaternary structure. The most hydrophobic surface clusters on the monomer subunits are located in very different regions of the alpha and beta chemokines and comprise in each case the amino acids that are buried at the interface of their respective dimers. The theoretical analysis of hydrophobicity strongly supports the hypothesis that the distinct dimers observed for IL-8 and hMIP-1 beta are preserved for all the alpha and beta chemokines, respectively. This provides a rational explanation for the lack of receptor crossbinding and reactivity between the alpha and beta chemokine subfamilies.

Amino Acid Sequence↗
PubMed1992

Folding protein alpha-carbon chains into compact forms by Monte Carlo methods.

A method is presented for generating folded chains of specific amino acid sequences on a simple cubic lattice. Monte Carlo simulations are used to transform extended geometries of simplified alpha-carbon chains for eight small monomeric globular proteins into folded states. Permitted chain transitions are limited to a few types of moves, all restricted to occur on the lattice. Crude residue-residue potentials derived from statistical structure data are used to describe the energies for each conformer. The low resolution structures obtained by this procedure contain many of the correct gross features of the native folded architectures with respect to average residue energy per nonbonded contact, segment density, and location of surface loops and disulfide pairs. Rms deviations between these and the native X-ray structures and percentage of native long-range contacts found in these final folded structures are 7.6 +/- 0.7 A and 48 +/- 3%, respectively. This procedure can be useful for predicting approximate tertiary interactions from amino acid sequence.

Models, Molecular↗
PubMed1990

Conformations of folded proteins in restricted spaces.

A new method is presented to examine the complete range of folded topologies accessible in the compact state of globular proteins. The procedure is to generate all conformations, with volume exclusion, upon a lattice in a space restricted to the individual protein's known compact conformational space. Using one lattice point per residue, we find 10(2)-10(4) possible compact conformations for the five small globular proteins studied. Subsequently, these conformations are evaluated in terms of residue-specific, pairwise contact energies that favor nonbonded, hydrophobic interactions. Native structures for the five proteins are always found within the best 2% of all conformers generated. This novel method is simple and general and can be used to determine a small group of most favorable overall arrangements for the folding of specific amino acid sequences within a restricted space.

Animals↗
PubMed1990

Pharmacokinetic analysis of blood distribution of intravenously administered 153Gd-labeled Gd(DTPA)2- and 99mTc(DTPA) in rats.

Rat plasma distribution data obtained following IV administration of 99mTc(DTPA) alone or after co-administration of 99mTc(DTPA) and 153Gd-labeled Gd(DTPA)2- at 0.001, 0.1, and 1.0 mmol Gd/kg were evaluated using compartmental modeling techniques. A three-compartment open model was found to fit the data significantly better (P less than 0.01) than a two- or four-compartment open model. This model incorporates and links the plasma and urine data and includes a delay to account for the transit time through the kidneys/ureters. The two nonplasma compartments of the model were assumed to be related to rapidly and slowly equilibrating tissues. Tc(DTPA) and Gd(DTPA)2- had nearly identical pharmacokinetic profiles in plasma and the rate constants were essentially the same. No significant dose dependent pharmacokinetic differences were found for the range of Gd(DTPA)2- doses tested. Simulations of the proposed three-compartment model were used to generate concentration-time curves for each of the three compartments.

Animals↗
PubMed1990

Conformational analysis of the tachykinins in solution: substance P and physalaemin.

A determination of the solution conformational behavior of two tachykinins, substance P and physalaemin, is described. Two-dimensional homonuclear Hartmann-Hahn (HOHAHA) and rotating-frame cross relaxation spectroscopy (ROESY) are used to obtain complete proton resonance assignments. Interproton distance restraints obtained from ROESY spectroscopy are used to characterize the conformational behavior. These data show that in solution both substance P and physalaemin exist in a mixture of conformational states, rather than as a single three-dimensional structure. In water both peptides prefer to be in an extended chain structure. In methanol, their behavior is described as a mixture of beta-turn conformations in dynamic equilibrium. Solvent titration data and chemical shift temperature coefficients complement the NMR estimate of interproton distances by locating hydrogen bonds and serving to identify predominant conformational states. The C-terminal tetrapeptide segment has the same conformational behavior for both substance P and physalaemin. In physalaemin, the midsegment of the peptide may also be constrained by formation of a salt bridge. The conformational behavior of substance P and physalaemin is discussed in relation to potency and receptor binding properties.

Magnetic Resonance Spectroscopy↗
PubMed1990

Recent studies of human calcium metabolism using stable isotopic tracers.

Stable isotopes of calcium are used safely as tracers for calcium in human populations ranging in age from infants to postmenopausal women. Thermal ionization mass spectrometry is used to measure calcium isotope ratios with relative accuracies of about 1% for natural abundance ratios at precisions of about 1% relative to the mean. Perturbations of natural abundance ratios are determined for the calcium in blood, urine, and feces with a limit of detection of about 2 delta % excess. The mathematical rationale for clinical studies of fractional absorption of dietary calcium and the kinetics of calcium's internal distribution are presented.

Animals↗
PubMed1990

A modeling analysis of monoclonal antibody percolation through tumors: a binding-site barrier.

For successful use of radiolabeled monoclonal antibodies (MAbs) for diagnosis and therapy, it is helpful to understand both global and microscopic aspects of antibody biodistribution. In this study, antibody distribution in a tumor is simulated by splicing together information on global pharmacokinetics: transport across the capillary wall, diffusive penetration through the tumor interstitial space, and antigen-antibody interaction. The geometry simulated corresponds to spherical nodules of densely packed tumor cells. This modeling analysis demonstrates that: 1) antigen-antibody binding in tumors can retard antibody percolation; 2) high antibody affinity at a given dose tends to decrease antibody percolation because there are fewer free antibody molecules. The result is a more heterogeneous distribution; 3) the average antibody concentration in the tumor does not increase linearly with affinity; and 4) increasing antibody dose leads to better percolation and more uniform distribution. This mathematical model and the general principles developed here can be applied as well to other biologic ligands.

Antibodies, Monoclonal↗
PubMed1989

Modeling analysis of the global and microscopic distribution of immunoglobulin G, F(ab')2, and Fab in tumors.

In order to understand the pharmacology of monoclonal antibodies and their conjugates, one must consider both global and microscopic aspects of antibody distribution. Here we present an analysis of antibody distribution in tumors based on the following factors: (a) molecular weight and valence of the antibody; (b) global pharmacokinetic profile following i.v. bolus injection; (c) penetration through the vascular wall; (d) diffusive and convective transport through interstitial space in the tumor; (e) antigen-antibody interaction; (f) antibody metabolism. Partial differential equations were developed to incorporate these factors and then solved numerically using parameter values from animal experiments, from clinical protocols at our institution, from studies of antibody binding characteristics in vitro, and from the literature. Salient findings from this model are that (a) antigen-antibody interaction in the tumor can retard antibody percolation away from blood capillaries, thus constituting a "binding site barrier"; (b) high antibody affinity tends to decrease antibody penetration and result in a more heterogeneous distribution; (c) high molecular weight [IgG greater than F(ab')2 greater than Fab] slows percolation and results in less uniform spatial distribution; (d) the average antibody concentration in the tumor does not increase linearly with antibody dose; (e) raising the rate of antibody metabolism results in low concentration and poor percolation; (f) perhaps most interesting, there is predicted to be a range of antibody dose and affinity within which the specificity ratio and average concentration could be kept high while limiting the heterogeneity of distribution. PERC, the computer program package developed for these analyses, provides a convenient and flexible way to assess the impact of global and microscopic parameters on the distribution of immunoglobulin in tumors. For calculations presented here, the input data were obtained from experimental sources, and qualitative features of the output proved consistent with the few interpretable observations available. However, detailed validation would require much more data than are currently at hand. The mathematical findings should therefore be considered as aids to concept development and as a set of null hypotheses with which to guide experimentation. Experiments and simulations will continue in tandem. It should be noted that the PERC package (and also the general principles delineated here) can be applied as well to biological ligands other than antibodies.

Antibody Affinity↗
PubMed1989

Influence of the routes of continuous intrahepatic infusion of 5-fluorouracil on its pharmacokinetics.

Continuous infusion chemotherapy via hepatic artery using newly available mechanical devices is frequently used to treat hepatic metastases to achieve a high concentration of 5-fluorouracil (5-FUra) in the hepatic circulation while minimizing systemic exposure. We compared four routes of intrahepatic administration to find out the best one in the canine model. To ascertain this data, 5-FUra (30 mg/kg) was given as a continuous infusion over a 3 hr period into either a systemic vein (femoral), portal vein, hepatic artery, or hepatic artery distal to its ligation after hepatic dearterialization. A total of eight dogs were studied. During 5-FUra infusion, concomitant blood samples were taken from the inferior vena cava and hepatic vein at 1, 2, 3, 5, 10, 15, 30, 60, 120, and 180 min. 5-FUra levels were determined in plasma by high-performance liquid chromatography. Blood flow in the portal vein and hepatic artery was measured by an electromagnetic flowmeter. The data described by a multicompartmental model, including the measured flows, had separate hepatic arterial and portal compartments with elimination from each described by linear kinetics. Mean area under the curve values in microgram/ml X min and the ratios of the systemic/hepatic vein areas following 5-FUra infusion via systemic, portal vein, hepatic artery, or hepatic artery after dearterialization routes were: 975/539 (R = 1.80), 939/748 (R = 1.35), 211/454 (R = 0.46), and 562/1,424 (R = 0.39). The results indicated that the administration of 5-FUra via the hepatic arterial route distal to its ligation results in the highest hepatic vein drug levels with the smallest systemic/hepatic vein exposure ratio, followed by intra-arterial route, while systemic and portal vein routes were not nearly as advantageous as the intra-arterial routes.

Animals↗
PubMed1988

Applications of a general method for deconvolution using compartmental analysis.

A method of deconvolution is illustrated using compartmental models. The approach can be used to determine an arbitrary unknown input function from a measured response and the impulse response of the system. Compartmental models are constructed to specify (a) the function fitting the response data and (b) the impulse response of the system. Simulation of these models is then used to construct the unknown input function.

Alanine↗
PubMed1988

Mass action kinetics of virus-cell aggregation and fusion.

A simple approximate solution for the mass action kinetics of small particles (viruses or vesicles) binding to large particles (cells) and their subsequent fusion has been derived. The solution is evaluated in terms of the measurable fluorescence changes expected when the virus or vesicles are labeled with fluorescent probes, which are diluted into the cellular membrane by fusion. Comparison with numerical integrations shows that the approximate solution is extremely accurate. Analytic simplifications for a variety of special cases of this general problem are also shown.

Cell Membrane↗
PubMed1988

Measurement of true calcium absorption in premature infants using intravenous 46Ca and oral 44Ca.

We have developed a method for measuring true fractional calcium absorption (alpha) in premature infants using two stable isotopes of calcium and tested it in seven studies in seven infants (birth weight 1543 +/- 65 g, gestation 32.8 +/- 7 wk). A total of 7.5 micrograms/kg 46Ca was given as a single intravenous bolus. Immediately thereafter 1.25 mg/kg of 44Ca was given in a single gavage feeding of standard infant formula (Enfamil). A metabolic isolette was used to obtain 4-h collections of urine for 24 h total. 46Ca and 44Ca were measured in urine by thermal ionization mass spectroscopy and expressed as the ratio to naturally occurring 48Ca. The differences in the 46Ca/48Ca and 44Ca/48Ca ratios from natural levels (delta % excess 46Ca and delta % excess 44Ca) were calculated. Percent absorption (alpha) equals a constant times cumulative delta % excess 44Ca/delta % excess 46Ca. The calculation of alpha is independent of urine volume or concentration. The delta % excess 46Ca, showed the expected multiexponential decline as a function of time, and delta % excess 44Ca usually peaked during a 4- to 8-h urine collection. Calculations of alpha using increasingly long sampling times showed that a plateau had been reached by 12 h. alpha values calculated after 16-24 h in the seven infants at 2 wk of age were 41, 48, 45, 46, 25, 55, and 51%. Repeat studies at 3 wk of age were 46, 60, and 54%. These values are somewhat higher than net percent calcium absorption values reported for standard formula and thus appear very appropriate.(ABSTRACT TRUNCATED AT 250 WORDS)

Absorption↗
PubMed1987

Biodistribution of monoclonal IgG1, F(ab')2, and Fab' in mice after intravenous injection. Comparison between anti-B cell (anti-Lyb8.2) and irrelevant (MOPC-21) antibodies.

Quantitative pharmacokinetic measurements of uptake and metabolism for two murine immunoglobulin G1 (IgG1) monoclonal antibodies (anti-Lyb8.2, MOPC-21) and their F(ab')2 and Fab' fragments were obtained following i.v. administration into C57BL/6 mice. Anti-Lyb8.2 antibody, reactive with the allelic Lyb8.2 murine B cell antigen, was labeled with 125I, and MOPC-21, an antibody with no known target antigen, was labeled with 131I. The two IgG or their fragments were co-injected, and all major organs were analyzed. Specific uptake of anti-Lyb8.2 IgG, F(ab')2, and Fab' was observed in the spleen with maximum peak values occurring at 1 to 2 hr. For MOPC-21, blood and organ kinetics was indicative of a nontargeted IgG molecule. The kidneys showed significant and rapid uptake of anti-Lyb8.2 and MOPC-21 Fab' fragments. This uptake by kidneys attenuated the maximum peak values of anti-Lyb8.2 Fab' in spleen. Multiexponential data fitting provided mean residence times (MRT) for each organ. The MRT for anti-Lyb8.2 in all organs were greater than those for its F(ab')2 and Fab' fragments. Only the kidneys showed greater MRT for Fab' than for F(ab')2. Blood, spleen, kidneys, and carcass exhibited substantial differences across fragments. When total body MRT for each fragment was compared with that of the respective parent IgG molecule, a progressive decline was observed. For MOPC-21, the decrease in total body MRT for F(ab')2 demonstrates the influence of the Fc portion of the molecule on IgG1 metabolism. This organ-by-organ data set may be pertinent to other monoclonal antibodies and their fragments and should help in optimizing delivery of these molecules to specific sites in vivo for immunologic and clinical purposes.

Animals↗
PubMed1987

Kinetic model for the biodistribution of an 111In-labeled monoclonal antibody in humans.

Using data from 12 patients, we have analyzed the pharmacokinetics of 111In-9.2.27, an antimelanoma monoclonal antibody, following i.v. infusion. Plasma data and scintillation camera images obtained from patients receiving either 1, 50, or 100 mg of monoclonal antibody indicated dose-dependent (i.e., saturable) kinetics. Based on these observations and known immunoglobulin kinetics, we developed a nonlinear compartmental model to describe the biodistribution of 111In-9.2.27 and the other coinjected 111In-associated compounds. The model included (a) three compartments representing intact 111In-9.2.27 ("plasma," "nonsaturable," and "saturable binding" compartments), (b) four compartments representing 111In-diethylenetriaminepentaacetic acid, and (c) one compartment representing 111In in an undetermined chemical form ("extravascular delay" compartment). Analysis of the rate of urinary excretion relative to plasma concentration indicated that the saturable binding compartment was a site for catabolism of monoclonal antibody. Further examination of the urinary data, together with previous studies of the site(s) of immunoglobulin catabolism, suggested that additional elimination took place from either the plasma or the nonsaturable compartment. The model indicated that to fill the saturable sites would require a dose of approximately 0.5 mg and suggested that greater than 3.5 mg would maintain saturation for 200 h. Computer integration of gamma camera counts over the spleen revealed a clear saturable component of uptake, whereas integration over the liver showed no such pattern. The proposed model was fitted to the liver and spleen imaging data by summing fractions of model simulations of each compartment. That analysis confirmed the suspected saturable uptake by the spleen (21% of the saturable binding compartment) and revealed a quantitatively important component of saturation in the liver (35% of the saturable binding compartment) that was not obvious from initial examination of the images. When the results were expressed on a concentration basis, the spleen accounted for 247% of the saturable compartment per kg, whereas the liver accounted for 25%/kg. The bone marrow also showed saturable uptake; hence, the saturable uptake may relate to the sinusoidal blood supply characteristic of liver, spleen, and marrow. The model predicts the dose levels required to overcome saturable background, suggests appropriate doses and schedules for cold loading strategies, and provides a format for explicit inclusion of tumor antigen.

Antibodies, Monoclonal↗
PubMed1987

Direct measurement of dietary fractional absorption using calcium isotopic tracers.

Fractional dietary Ca absorption, 'a', is measured by determining the ratio of two stable isotopic tracers, one of them orally (44Ca + 0.2-0.5 mg/kg) and the other intravenously (42Ca / 0.02-0.1 mg/kg). Thermal ionization mass spectometry (TIMS) is used to measure the perturbation of natural abundance isotope ratios (delta % excess). Typical sensitivity of the TIMS permits detection of a 2.5 delta % excess change from the natural Ca isotope ratio with relative standard deviations of about 0.5%. At sufficiently long times absorption becomes constant so that 'a' is determined by a product of constants and a measured ratio.

Adolescent↗
PubMed1987

Inhibition of the catalytic properties of Staphylococcus aureus nuclease by monoclonal antibodies.

Monoclonal antibodies (Mab) specific for Staphylococcus aureus nuclease (nuclease) were examined for their capacity to inhibit the enzyme-mediated cleavage of DNA. Within a panel of 22 anti-nuclease Mab produced by hybridoma cell lines derived from SJL/J, A/J or BALB/c mice, only five were capable of modifying nuclease activity. Of the five, only one protected DNA from enzymatic degradation whereas the others reduced the rate of the enzymatic reaction. When mixed together, partially inactivating Mabs were frequently more efficient inhibitors than when used individually. It was shown by competitive binding assay that nuclease could be bound simultaneously to more than one Mab. Mixtures of five inactivating Mabs were able to completely block the nuclease activity. Although the actual mechanism for Mab nuclease inactivation is not known, the present data are consistent with simple steric hindrance for the formation of the DNA-nuclease complex by bulky Mab molecules bound to epitopes close to, but distinct from, nuclease catalytic sites. A mathematical model for Mab binding and inactivation of nuclease, taking into account multiple binding events for one or two Mabs interacting with nuclease, was used to derive affinities and maximum reductions of the enzymatic rate (details on the derivation of the equations and on the hypotheses of the model are given in an appendix). This analysis showed that the observed cooperative effects were dependent on the formation of multi-molecular complexes in which nuclease is bound simultaneously to two (or more) different Mabs. It also shows that the formation of cyclic complexes, if allowed, might result in very high apparent affinities. Since in screening of hybridoma fusions, the probability of finding such pairs of monoclonal antibodies would be low, this phenomenon may explain the fact that no Mab, or mixture of Mabs, matched the polyclonal antisera in capacity to block nuclease enzymatic activity.

Antibodies, Monoclonal↗
PubMed1987

Pharmacokinetic analysis of biosynthetically radiolabeled mouse monoclonal antibodies in normal rhesus monkeys.

Two mouse monoclonal IgG2b antibodies that differ in their CH2 domains have been compared for their blood mean residence times (MRTs) in normal female Rhesus monkeys. A biosynthetically incorporated radiolabel (S-35 methionine) was used to follow blood elimination of the antibodies and urinary excretion. Results indicate that the blood (plasma) MRTs for the Ar13.4 and the ArM16.1 antibodies were remarkably similar in normal Rhesus monkeys. This observation varies greatly from those obtained with these same antibodies in separate studies in normal Balb/c mice. In Balb/c mice, the ArM16.1 antibody was found to be removed from the blood six times faster than the Ar13.4. The CONSAM program was used to fit a multiexponential function to the plasma data obtained from mice and monkeys. The sum of exponentials was then used to calculate the MRT values. These results demonstrate that mouse monoclonal antibody pharmacokinetics significantly differ between normal primates and mice. Presumably the Rhesus monkey better reflects the clinical behavior and pharmacokinetics of mouse monoclonal antibodies than do rodents. Therefore, the use of normal primates for the preclinical evaluation of monoclonal antibodies for in vivo use is suggested.

Animals↗