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D G Covell

Publications and source records attributed to D G Covell.

At least 55 records · Page 3Linked to original sources
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↗
PubMed1987

Does myoglobin contribute significantly to diffusion of oxygen in red skeletal muscle?

We have examined the role of myoglobin to facilitate O2 diffusion to active mitochondria in skeletal muscle by constructing computer-simulation experiments. Steady-state mitochondrial O2 consumption under different conditions of supply partial pressure of O2 (PO2) in a system with and without myoglobin were examined for a one-dimensional slab of tissue. O2 consumption by mitochondria was saturable with the mitochondria located in bands at uniform intervals throughout the tissue. Under these conditions, myoglobin provides a measurable increase in O2 transport for supply PO2 below 10 Torr and diffusion lengths expected for skeletal muscle fibers. We conclude that under circumstances where hypoxia lowers PO2 below 10 Torr that myoglobin begins to provide a measurable increase in O2 delivery to mitochondria.

Homeostasis↗
PubMed1986

Pharmacokinetics of monoclonal immunoglobulin G1, F(ab')2, and Fab' in mice.

The pharmacokinetics of an immunoglobulin G1 (IgG1) and its F(ab')2 and Fab' fragments following i.v. administration in mice has been studied by constructing a physiologically based, organ-specific model to describe antibody biodistribution, catabolism, and excretion. The antibody selected for study (MOPC-21) has no known binding sites in the body and therefore is useful for defining antibody metabolism by nontumor tissues. Whole IgG remains in the body for 8.3 days, the majority of time in the carcass (53.0% of the total residence time); has a distribution volume exceeding that of plasma plus interstitial fluid; distributes into these volumes rapidly for most enteral organs (equilibration time less than 2.6 min for liver, spleen, kidney, and lung), slower for the gut (less than 20 min), and slowest for the carcass (less than 260 min); produces interstitial:plasma concentration ratios of greater than 0.5 for enteral organs and 0.18 for carcass; has the greatest percentage of its catabolism due to the gut (72.8%), followed by the liver (20.5%), then the spleen (3.6%); has the highest extraction on a single pass by the gut (0.14%) and cycles through the interstitial spaces of the body at least 2.8 times/g of organ weight before being metabolized or excreted. When compared with whole IgG, the Fab' fragment is cleared from the body 35 times faster; has a larger total distribution volume; distributes more rapidly into this volume; produces higher interstitial:plasma concentration ratios; is catabolized principally by the kidney (73.4% of total catabolism), followed by the gut (22.9%), then the spleen (3.1%); is extracted from the circulation to the extent of 3.4% on each pass through the kidney, and less by gut (1.0%) and spleen (0.14%) and cycles through non-kidney interstitial spaces at least 0.4 cycles/g of tissue weight before metabolism or excretion. The F(ab')2 fragment has pharmacokinetic characteristics that fall between those of whole IgG and Fab'. These results provide pharmacokinetic criteria for selecting whole IgG, F(ab')2, or Fab' for various in vivo applications; provide a framework for predicting cumulative tissue exposure to antibody labeled with different isotopes; and provide a reference metabolic state for the analysis of more complex systems that do include antibody binding.

Animals↗
PubMed1986

Optimization of monoclonal antibody delivery via the lymphatics: the dose dependence.

After interstitial injection in mice, antibody molecules enter local lymphatic vessels, flow with the lymph to regional lymph nodes, and bind to target antigens there. Compared with i.v. administration, delivery via the lymphatics provides a more efficient means for localizing antibody in lymph nodes. An IgG2a (36-7-5) directed against the murine class I major histocompatibility antigen H-2Kk has proved useful for studying the pharmacology of lymphatic delivery. The antibody specifically binds to most cells in Kk-positive strains of mice and to none in Kk-negative mice. At very low doses, most of the antibody remains at the injection site in Kk-positive animals. As the dose is progressively increased, most effective labeling occurs first in nodes proximal to the injection site and then in the next group of nodes along the lymphatic chain. At higher doses, antibody overflows the lymphatic system and enters the blood-stream via the thoracic duct and other lymphatic-venous connections. Once in the blood, antibody is rapidly cleared, apparently by binding to Kk-bearing cells. These findings indicate that the single-pass distribution of monoclonal antibodies in the lymphatics can be strongly dose dependent, a principle which may be of clinical significance in the improvement of immunolymphoscintigraphic imaging, especially with antibodies directed against normal and malignant lymphoid cells. Monoclonal antibodies directed against normal cell types in the lymph node may be useful for assessing the integrity of lymphatic chains by immunolymphoscintigraphy or, more speculatively, for altering the status of regional immune function. The results presented here indicate that a low or intermediate antibody dose may optimize the signal:noise ratio for imaging. In Kk-negative animals, the percentage of dose taken up in the major organs was essentially independent of the dose administered; there was no evidence for saturable sites of nonspecific binding. These findings provide background for attempts to use antitumor antibodies via the lymphatic route. Specific binding to target cells (and any cross-reaction with normal tissues) would presumably be superimposed on the nonspecific pharmacology of the antibody in vivo.

Animals↗
PubMed1985

Kinetic model for disposition of 6-mercaptopurine in monkey plasma and cerebrospinal fluid.

The antipurine 6-mercaptopurine (6-MP) is effective in the induction and maintenance of remission in patients with acute lymphocytic leukemia. This report presents a compartmental model that describes the kinetics of 6-MP in the plasma and cerebrospinal fluid (CSF) of the monkey. Analysis is based on simultaneously measured plasma and CSF 6-MP concentrations after intravenous and intraventricular bolus administration. Results indicate that 6-MP administered intraventricularly remains largely in the CSF. Disappearance of 6-MP from CSF is principally due to convective losses at a rate equivalent to CSF turnover. Diffusion of 6-MP across the ependymal surface accounts for only 7% of the 6-MP appearing in the plasma. Conversely the dominant route for entry of 6-MP into the CSF from the plasma is entrainment in choroidally formed CSF. Only 12% of 6-MP in the CSF after intravenous administration can be accounted for by permeation of cerebral capillaries and diffusion through brain parenchyma and across the ependymal surface into CSF. These results indicate that the choroid plexus is not a significant barrier for the transfer of molecules like 6-MP from plasma to CSF.

Animals↗