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Biomedical subjects

M G Choc

Publications and source records attributed to M G Choc.

18 recordsLinked to original sources
PubMed1996

The pharmacokinetics of a microemulsion formulation of cyclosporine in primary renal allograft recipients. The Neoral Study Group.

This study was a randomized, double-blind, 12-week comparison of the pharmacokinetics, safety, and tolerability of two cyclosporine (CsA) formulations, cyclosporine emulsion capsules and oral solution for microemulsion and cyclosporine, in the postoperative management of renal transplant patients. Of the 101 patients, aged 18 to 65, who entered the study, 89 were evaluable for pharmacokinetics. Initial dosage was 10 mg/kg per day, administered twice daily in two equal doses. Dosages were adjusted to achieve target CsA concentrations. The pharmacokinetic (PK) parameters (dose-normalized) of greatest interest were maximum blood concentration (C(max)/dose), time to reach maximum concentration (t(max), area under the blood concentration-vs.-time curve (AUC/dose), and trough blood concentrations (Co h/dose). The relative CsA bioavailabilty was found to be significantly enhanced with cyclosporine emulsion compared with cyclosporine with a 16% to 31% increase in AUC and a 32% to 42% increase in C(max). Intrapatient variability of PK parameters was significantly lower with cyclosporine emulsion than with cyclosporine for AUC, C(oh), t(max), and C(max) in many instances. This indicates a more consistent, rapid, and more complete total absorption of CsA. Despite higher CsA C(max) levels and AUCs with cyclosporine emulsion, safety and tolerability (detailed in a parallel report) were comparable to those of cyclosporine. The PK advantages of cyclosporine emulsion over cyclosporine are either independent of food conditions or possibly reflective of more consistent absorption of CsA with cyclosporine emulsion. The findings suggest that de novo use of cyclosporine emulsion may simplify and improve management of organ transplant recipients and that the PK advantages of cyclosporine emulsion may translate into clinical benefits.

Adolescent↗
PubMed1996

The safety and tolerability of cyclosporine emulsion versus cyclosporine in a randomized, double-blind comparison in primary renal allograft recipients. The Neoral Study Group.

A 12-week, randomized, double-blind, multicenter pharmacokinetics study was conducted to compare the clinical safety and tolerability of cyclosporine capsules and oral solution for microemulsion and cyclosporine in 101 primary renal transplant recipients Cyclosporine emulsion has more complete absorption and improved bioavailability compared with cyclosporine, and dosing of both cyclosporine formulations was adjusted to achieve comparable whole-blood trough levels. Mean serum creatinine values were higher in the cyclosporine emulsion group at baseline, 8, and 12 weeks (P<0.05). The incidence of acute rejection was similar in both treatment groups although fewer patients required monoclonal antibody therapy in the cyclosporine group (31% vs. 82%, respectively). Despite the increased bioavailability of cyclosporine emulsion, no significant differences in the incidence of adverse events were observed; the safety, tolerability, and efficacy of cyclosporine emulsion and cyclosporine were comparable.

Administration, Oral↗
PubMed1995

Application issues in bioanalytical method validation, sample analysis and data reporting.

Although some degree of consensus has been reached concerning the requirements for acceptable method validation, the procedures used to establish them vary significantly between laboratories. Also, issues arising from application of these requirements during validation and subsequent sample analysis need to be addressed. The purpose of this paper is to discuss application issues concerning prerequisites to method validation, and all validation criteria for evaluation of method reliability and overall performance. Other poorly addressed issues such as re-validation, cross-validation, partial sample volume, multicomponent analysis and reporting will also be discussed. Although many issues discussed are of a general nature, the scope of this presentation is primarily to address issues arising from the validation and routine application of chromatographic methods.

Animals↗
PubMed1993

High-performance liquid chromatographic method for the determination of fluvastatin in human plasma.

The method outlined in this paper utilizes internal standardization and is simple, reliable, and sensitive for the determination of fluvastatin in plasma. Fluvastatin sodium and the internal standard are extracted from buffered plasma into methyl tert.-butyl ether, followed by evaporation of an aliquot of the organic phase. After reconstitution of the dried sample into a small volume of mobile phase (methanol-13 mM tetrabutylammonium fluoride, 3:2, v/v), the sample is chromatographed on an LC-18 column thermostated at 50 degrees C. Fluorescence detection (excitation at 305 nm and emission at 380 nm) is used to monitor both fluvastatin (free acid) and the internal standard. The method can accurately detect 1 ng/ml fluvastatin using a 1.0-ml plasma sample. The precision and reproducibility over the linear range of the method are 5.57 and 7.32%, respectively. This method has been used to measure fluvastatin plasma concentrations in support of bioavailability/pharmacokinetic studies with no indication of interference.

Chromatography, High Pressure Liquid↗
PubMed1990

Infrared spectra of carbonyl hemoglobins: characterization of dynamic heme pocket conformers.

The infrared spectra for carbon monoxide complexed to hemoglobins were examined in the C-O stretch region. Deconvolution of the spectra requires four bands and supports the presence of four distinct conformers at the ligand binding site. Most typical hemoglobins exhibit only one predominant conformer for each subunit represented by a band at 1951 cm-1 in contrast to myoglobins, which typically exist in two major conformations. Several hemoglobins with an enlarged heme pocket are shown to shift the C-O frequency into the higher frequency conformer regions. Many factors, including pH, temperature, solvents, and divalent metals, are also shown to be capable of expanding the heme pocket. Only very specific structural changes that can reduce the size of the heme pocket will result in the lower frequency conformers. The weighted averages of the multiple CO vibrational frequencies are linearly related to the single 13CO NMR chemical shift values and to the exponential of fast CO on-rates. Conformer interconversion occurs at a rate greater than 10(4) s-1. The infrared C-O stretch spectra provide qualitative and quantitative information on the structural dynamics, stability, and ligand binding properties of hemoglobins.

Animals↗
PubMed1990

Single- vs multiple-dose pharmacokinetics of clozapine in psychiatric patients.

Clozapine plasma levels were monitored in 16 patients during a series of three consecutive treatments (single dose-multiple dose-single dose). Each patient received a single 75-mg dose (3 x 25 mg) with clozapine tablets, and serial plasma samples were collected over 48 hr after the dose. At 48 hr, a multiple-dose regimen was started, consisting of an initial dose escalation period followed by dosing at a constant regimen for at least 6 days. After the last dose, serial plasma samples were again obtained over 72 hr. Drug was then withheld for at least 7 days, a final single 75-mg dose was given, and plasma sampling was repeated. A subset of the patient population (N = 7) was used to test for a food effect during the single-dose treatments. The pharmacokinetic parameters between the initial and the final single dose periods were not significantly different. Similarly, there were no differences within patients when given the dose after fasting (fed 1 hr after dose) or with a meal. In contrast, the terminal elimination rate differed between the single-dose and the multiple-dose treatments (t1/2 m3 = 7.9 hr single dose and 14.2 hr multiple dose) (P less than 0.05) and the dose-normalized area under the plasma concentration/time curves increased 27% with multiple dosing. Since a previous study in patients (Choc et al., Pharm. Res. 4:402-405, 1987) showed dose proportionality of clozapine plasma concentrations during multiple-dose regimens, the present results cannot be described by Michaelis-Menten kinetics.

Adult↗
PubMed1989

Single dose kinetics of thioridazine and its two psychoactive metabolites in healthy humans: a dose proportionality study.

Dose proportionality in some pharmacokinetic parameters for thioridazine and its two active metabolites (mesoridazine and sulforidazine) was investigated in 11 healthy human subjects following oral administration of three single doses (25, 50, and 100 mg) of thioridazine hydrochloride separated in each case by an interval of two weeks. Also, after a further two weeks, another 100-mg dose of thioridazine (divided as 5 mg every 0.5 h) was administered to all the volunteers to investigate the effect of a slow rate of dosage input on the pharmacokinetic parameters of this drug. An HPLC method was used to measure concentrations of thioridazine, mesoridazine, and sulforidazine in plasma samples collected up to 72 h following each dose. Dose proportionality for the three single doses of thioridazine was observed for all three analytes in the area under the plasma concentration versus time curves (AUC infinity 0 or AUCt0) and the maximum plasma concentration (Cmax) in that the relationships between the dose and these parameters were each describable by an equation for a straight line (r2 greater than or equal to 0.8). However, the mean apparent distribution and elimination rate constants for thioridazine and mesoridazine and the mean apparent oral clearance for thioridazine decreased significantly with increasing dose. This suggests nonlinear trends in the elimination kinetics at high doses of thioridazine. When a 100-mg divided oral dose of thioridazine was administered, no statistically significant differences between single and divided doses were observed in the mean AUC infinity 0 or AUCt0 for thioridazine or sulforidazine. A significant decrease in the mean AUC infinity 0 or AUCt0 was observed for mesoridazine after the administration of the divided dose.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1988

The site and mechanism of dioxygen reduction in bovine heart cytochrome c oxidase.

The site and mechanism of dioxygen reduction in cytochrome c oxidase from bovine heart muscle have been investigated. The rate of cytochrome c2+ oxidation by O2 is shown to be affected by several factors: 1) pH, with optima at 5.65 and 6.0, 2) temperature between 0 and 29 degrees C, with E alpha = 13 kcal mol-1, 3) D2O exchange, with a reduction in rate of 50% or more at the pH optima, and 4) the addition of ethylene glycol or glycerol, which significantly lowers the rate. The extremely narrow (delta vCO approximately 4 cm-1) infrared stretch bands at approximately 1964 and approximately 1959 cm-1 for liganded CO are only slightly affected by factors 1-4 or by changes in the oxidation state of metals other than the heme alpha 3 iron. These results indicate a stable, unusually immobile O2 reduction site well-isolated from the external medium, a characteristic expected to be important for oxidase function. Precise stereochemical positioning of hydrogen donors adjacent to O2 liganded to heme alpha 3 iron can be expected in order to achieve the optimization of the time/distance relationships required for enzyme catalysis. These findings support a novel mechanism of O2 reduction via a hydroperoxide intermediate within a reaction pocket that experiences little change in conformation during the hydrogen and electron transfer steps.

Animals↗
PubMed1988

S-oxidation of thioridazine to psychoactive metabolites: an oral dose-proportionality study in healthy volunteers.

Thioridazine has two major active metabolites, which are formed from S-oxidation of its 2-methylthio group; the sulphoxide, mesoridazine, and the sulphone, sulforidazine. Dose proportionality of the three compounds was investigated for the first time in 11 males after administration of three single oral doses (25, 50, and 100 mg) of thioridazine hydrochloride separated in each case by two weeks. Based on the plasma concentrations of the three analytes over 72 h following each dose, large intersubject variabilities in such parameters as AUCot and Cmax were observed for each of the three compounds. The relationships between dose and parameters such as AUCot and Cmax for each analyte were described by an equation for a straight line (r2 greater than or equal to 0.8). However, the mean apparent distribution and elimination rate constants for thioridazine and mesoridazine and the mean apparent oral clearance for thioridazine decreased significantly with increasing dose, suggesting non-linearity in the elimination of thioridazine at high dose.

Adult↗
PubMed1987

Multiple-dose pharmacokinetics of clozapine in patients.

After a 2-day buildup, patients were dosed continuously with clozapine solution at three ascending dose levels (37.5, 75, and 150 mg bid for 7 days at each dose level). Following the morning administration on the twenty-third day of dosing a drug holiday was instituted which lasted for a minimum of 48 hr. Serial plasma samples were obtained during each of the periods and during the drug holiday for the calculation of the steady-state parameters AUCSS, CSSmax, and CSSmin at each dose level as well as for the assessment of the terminal elimination rate. Mean parameter values for AUCSS, CSSmax, and CSSmin showed a linearly increasing response with the dose, well described by a straight line passing through the origin. The terminal elimination appeared to follow linear kinetics and had a mean half-life of 15.8 hr (range, 5.8-33 hr).

Adolescent↗
PubMed1984

Respiratory proteins from the extremely thermophilic aerobic bacterium, Thermus thermophilus. Purification procedures for cytochromes c552, c555,549, and c1aa3 and chemical evidence for a single subunit cytochrome aa3.

We have developed a chemically defined, minimal growth medium for Thermus thermophilus which is suitable for nutritional studies, isotopic enrichment, and genetic manipulation of the organism. Reliable procedures are described for the large scale purification of cytochrome c552 from the periplasm and for cytochrome c555,549 and cytochrome c1 aa3 from the plasma membrane. In contrast to a previous report (Fee, J. A., Choc, M. G., Findling, K. L., Lorence, R., and Yoshida, T. (1980) Proc. Natl. Acad. Sci. U. S. A. 77, 147-151) which suggested a molecular weight near 200,000, the cytochrome c1aa3 complex was shown by protein and amino acid analyses to have Mr approximately 93,000. Sodium dodecyl sulfate-urea-polyacrylamide gel electrophoresis and reversed phase high performance liquid chromatography, combined with amino acid analyses, revealed the presence of only two proteins in a 1:1 ratio: C-protein has Mr approximately 33,000, binds heme C, and is thought to correspond to cytochrome c1. A-protein has Mr approximately 55,000 and is thought to bind the four redox components (2 heme A and 2 Cu) of cytochrome aa3.

Amino Acids↗
PubMed1981

Evidence from infrared and 13C NMR spectra for discrete rapidly interconverting conformers at the carbon monoxide binding sites of hemoglobins A and Zurich.

Fully liganded hemoglobin carbonyls in solution exhibit infrared and 13C NMR spectra that indicate the normal presence of discrete, rapidly interconverting conformers at the carbon monoxide binding sites. The CO ligands bound to Hb A give infrared bands with vCO = 1968 and 1951 cm-1. With Hb Zurich [beta 63 His leads to Arg], vCO values of 1968, 1958, and 1951 cm-1 are observed. Changing either the temperature (3-32 degrees C) or the pH (4 to 11.9) produces reversible redistributions of individual infrared band intensities without a significant change in the total integrated intensity for all bands; only small shifts in frequency occur. In 13C NMR spectra for 13C16O ligands only one resonance is observed for each type of subunit: alpha A and alpha Zh at approximately 206.4 ppm, beta A at approximately 206.0 ppm, beta Zh at approximately 205.5 ppm, each from tetramethylsilane. Since the multiple conformers observed in infrared spectra are not evident in the 13C NMR spectra, the interconversions among the conformers are considered to be too rapid for the individual conformers to be detected by NMR spectroscopy. Estimated differences in enthalpy and in entropy between the separate conformers range from 1.5 to 5.2 kcal/mol and 6.6 to 11.5 entropy units, respectively. The structural differences between carbonyl conformers are sufficiently great that iron (II)--CO bonding and the reactivity of the individual conformers are expected to vary significantly. These findings demonstrate a dynamic aspect of structure due to the flexibility of the protein at the ligand binding site that is important in any consideration of structure-function or structure-property relationships.

Binding Sites↗
PubMed1981

Dynamic protein structures: infrared evidence for four discrete rapidly interconverting conformers at the carbon monoxide binding site of bovine heart myoglobin.

Infrared spectra for the carbon monoxide complex with myoglobin isolated as the oxygenyl species from bovine heart muscle were carefully examined in the C--O stretch region as either the pH or the temperature was varied. Deconvolutions of these spectra into bands of Gaussian shape suggest the presence of four bands near 1938(I), 1944(II), 1954(III), and 1965(IV) cm-1 with halfband widths of about 18, 9, 9, and 10 cm-1, respectively. The relative intensities of the four bands varied with changes in pH or temperature. 13C NMR spectra and other evidence indicate that the four C--O stretch bands arise from four discrete rapidly interconverting conformers: CI, CII, CIII, and CIV. Under conditions of physiological pH and temperature, the relative stabilities are CI approximately CII much greater than CIII approximately CIV. The delta H and delta S values for conformer interconversions are estimated to range from -8 to 34 kJ/mol and -27 to 87 J.mol-1 K-1, respectively; therefore the structures of the conformers may be expected to vary significantly. These findings provide evidence for a highly flexible, dynamic structure at the ligand-binding site of bovine myoglobin, even when ligands are bound.

Animals↗
PubMed1980

Properties of a copper-containing cytochrome c1aa3 complex: a terminal oxidase of the extreme thermophile Thermus thermophilus HB8.

From the plasma membrane of Thermus thermophilus HB8 we have partially purified a detergent-solubilized complex of cytochromes a and c1 that actively catalyzes the transfer of electrons from ascorbate via a redox dye to oxygen. The complex is composed of two types of polypeptides, with molecular weights of approximately 55,000 and 33,000. Quantitative analysis revealed the presence of heme a, heme c, and copper in a ratio of 2:1:2, with the heme a being present at 10 +/- 1.3 nmol/mg of protein. The heme c was shown to be associated with the molecular weight 33,000 peptide and is suggested to be of the c1 type. The optical and electron paramagnetic resonance properties of this complex were found to be similar to those of eukaryotic cytochrome oxidase, suggesting the following arrangement of chromophores: a magnetically isolated cytochrome c1 and an oxygen-reducing functional unit consisting of two heme a groups and two copper ions associated with one or more larger peptides.

Copper↗