Corticosteroid pharmacodynamics: models for a broad array of receptor-mediated pharmacologic effects.
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Adrenal Cortex Hormones↗
Biomedical subjects
W J Jusko
Publications and source records attributed to W J Jusko.
Michaelis-Menten metabolite formation kinetics: equations relating area under the curve and metabolite recovery to the administered dose.
A computational approach which concomitantly determines the capacity-limited rate constants of parent drug elimination and metabolite formation is presented. The approach applies both the presently derived total excretory recovery versus dose relationships of the metabolite and the AUC versus dose relationships of the parent drug to identify the parameters. Three parent drug elimination conditions were assessed: pooled first-order, pooled Michaelis-Menten, and parallel first-order and pooled Michaelis-Menten kinetics. Model and parameter identification criteria are discussed. Literature data for theophylline and two of its metabolites in rats were examined to reveal pooled Michaelis-Menten elimination kinetics of theophylline and capacity-limited formation of the metabolites. The proposed technique is useful for quantitating commonly obtained nonlinear drug disposition data such as AUC and amount of metabolites excreted.
Animals↗
Nonlinear pharmacokinetics and interconversion of prednisolone and prednisone in rats.
The pharmacokinetics of prednisolone and prednisone were examined in 32 rats at four intravenous doses (5, 10, 25, and 50 mg/kg). Each rat was given one dose of either intravenous prednisolone or prednisone, and plasma concentrations of both compounds were measured by HPLC. Mammillary moment analysis showed the apparent clearance and volume of distribution of prednisolone and the apparent clearance of prednisone to be dose-dependent. Further diagnostic analysis using a linear interconversion model revealed modest interconversion between the two steroids and at least two saturable clearance processes: the conversion of prednisolone to prednisone and the irreversible elimination of prednisone. A comprehensive model which incorporates the nonlinear clearances of prednisolone and prednisone plus the additional feature of nonlinear tissue distribution of prednisolone was then constructed. Four differential equations describing the rate of change of each steroid in each compartment were used to numerically fit by nonlinear least squares analysis all plasma concentration-time profiles simultaneously. The final estimates from the full model only partly agreed with the results from the two moment analyses but confirmed the general structure of the model. The nonlinear tissue distribution of prednisolone was reinforced by assay of muscle tissue. This study demonstrates the utility of the model-building process where simpler models yield insights into more elaborate schemes with complex nonlinear features.
Analysis of Variance↗
Receptor-mediated prednisolone pharmacodynamics in rats: model verification using a dose-sparing regimen.
Our receptor/gene-mediated model of corticosteroid action was tested and extended by examining the pharmacokinetics/dynamics of multiple low doses vs. a single higher dose of intravenously administered prednisolone in adrenalectomized male Wistar rats. Low-dose rats received 3 bolus doses (5 mg/kg) of prednisolone at 0, 0.5 and 1.0 hr. High-dose animals were given a single 25 mg/kg dose of prednisolone. Both regimens were expected to produce equivalent net responses based on model predictions. Control rats were not dosed. The profiles of free hepatic cytosolic glucocorticoid receptors and the hepatic tyrosine aminotransferase (TAT) enzyme were examined. Plasma prednisolone concentrations showed bi-exponential decline for both doses using pooled animal data. Clearance of total plasma prednisolone was 4.16 and 3.21 L/hr per kg in low- and high-dose groups. Volume of distribution at steady state (approximately 1.50 L/kg) and central volume (approximately 0.6 L/kg) were similar for both groups. Receptor levels from 5-16 hr stabilized at 64% of the 0-hr control value. Receptor and TAT profiles were essentially superimposable for both dosing groups. Our previous model was used to simultaneously describe prednisolone plasma concentrations, hepatic receptors, and TAT activity. The ability of total plasma prednisolone (Cp), corticosteroid binding globulin (CBG)-free plasma prednisolone (CCBG), and free plasma prednisolone (CF) to describe the kinetics/dynamics were examined. The CF values produced optimum fitting of all receptor data. The similarity of the two dosing groups supports the view that appropriately timed doses of a steroid can be used in an optimally efficacious manner by first filling all receptor sites and then replacing steroid as receptors are expected to recycle from nuclear/DNA binding sites as the steroid is eliminated.
Adrenalectomy↗
Steroid dose sparing: pharmacodynamic responses to single versus divided doses of methylprednisolone in man.
Inhibitory drug interactions affecting the metabolism of methylprednisolone (MP) may produce either steroid sparing or adverse effects partly by increasing the exposure time to the steroid. This phenomenon can be mimicked by administering MP in divided doses. Two types of responses were compared after a single MP dose (40 mg bolus) and a divided regimen (20 mg bolus and a 5 mg bolus 8 hours later) in six healthy male volunteers. The suppression of basophils measured as whole blood histamine and plasma cortisol concentrations was assessed during 32 hours. The 37.5% reduction in dose produced a 23% overall decreased blood histamine response. A pharmacodynamic model for basophil cell distribution to and from an extravascular compartment describes the effects of MP after both regimens. A slower initial decline in blood histamine after the divided regimen may be related to incomplete suppression of basophil cell return to blood. The 50% inhibitory concentrations of MP of about 5 ng/ml were similar for both regimens. The decline and return of cortisol concentrations were similar between MP treatments with suppression continuing for 24 hours. The 50% inhibitory concentrations of MP values for adrenal suppression were about 1 ng/ml. Pharmacodynamic modeling is useful in quantitating corticosteroid responses and generally predicted the "dose-sparing" effects that were achieved by prolonging MP plasma concentrations. This study supports previous clinical observations that patients may require morning through evening exposure to MP to optimize efficacy while adrenal suppression is being minimized.
Adult↗
Mean residence times of multicompartmental drugs undergoing reversible metabolism.
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Biotransformation↗
Effects of variation in drug elimination on five methods for assessing zero-order drug absorption rates.
The area function method for assessing zero-order (ko) drug absorption rates was compared to four other methods under conditions where variation occurs in the plasma concentration data and in the elimination rate constant (kel or k10) for one- or two-compartment models. For deviant kel values of a one-compartment model, the most accurate recovery of ko occurred with the area function method and nonlinear least-squares regression, followed by the Wagner-Nelson and moment analysis methods. With deviant k10 values for a two-compartment model, the order of superiority of the methods was: area function approximately nonlinear regression greater than Loo-Riegelman greater than moment analysis. Moment analysis should generally be reserved for use as an estimation rather than calculation technique. The area function procedure offers particular advantages in ease of data analysis and accuracy of recovered parameters.
Administration, Oral↗
Constant-rate intravenous infusion methods for estimating steady-state volumes of distribution and mean residence times in the body for drugs undergoing reversible metabolism.
Equations for the steady-state volumes of distribution (Vss) and the mean residence times in the body (MRT) are derived for a drug and its metabolite subject to reversible metabolism and separately infused intravenously at a constant rate to steady state of both compounds. The Vss and MRT parameters are functions of the integrals of plasma concentrations, plasma concentrations at steady state, and times to reach steady state of both drug and metabolite. In addition, the MRT values are functions of the infusion rates. These equations were validated by computer simulations and comparison with IV bolus dose parameters. These relationships extend the ability to assess the pharmacokinetics of linear reversible metabolic systems.
Infusions, Intravenous↗
Mean interconversion times and distribution rate parameters for drugs undergoing reversible metabolism.
The mean interconversion time and recycling numbers are introduced as intrinsic metabolic interconversion and distribution parameters for drugs undergoing linear reversible metabolism. Equations for these parameters, the distribution clearance, and the mean transit time in the central and peripheral compartments are derived for a metabolic pair where interconversion and elimination occur in central compartments. These parameters can be calculated from plasma concentration versus time slopes and intercepts, AUC, and AUMC data of parent drug and its metabolite partner following iv administration of each compound. The mean time analysis is illustrated with disposition data obtained previously for methylprednisolone and methylprednisone in the rabbit. Examination of mean times and additional properties of the system reveals that total exposure time of methylprednisolone is weakly influenced by the metabolic interconversion process, whereas the total exposure time of methylprednisone is strongly influenced by the process. In addition, the tissue distribution process moderately influence the total exposure times of both compounds. The derived mean time parameters, along with previously evolved equations for clearances, volumes of distribution, moments, and mean residence times allow comprehensive analysis of linear, multicompartmental reversible metabolic systems.
Animals↗
Pharmacokinetics of methylprednisolone sodium succinate and methylprednisolone in patients undergoing cardiopulmonary bypass.
The pharmacokinetics of methylprednisolone sodium succinate (MPHS) and methylprednisolone (MP) were determined in six patients undergoing open heart surgery with cardiopulmonary bypass. Plasma concentrations of both compounds were measured by high-performance liquid chromatography after doses of MPHS of 1.7-2.4 g. The prodrug ester MPHS yields MP with an average formation rate constant of 0.70 +/- 0.29 hr-1. Peak concentrations of MP occur around 1-2 hours after loading and additional administration of MPHS. The pharmacokinetic values of the two drugs in patients having cardiopulmonary bypass were compared to those in younger, healthy subjects. The volume of distribution of MPHS was lower in the patients, and that of MP was similar to the value in controls. Total clearances of both agents were reduced by about 5 and 2 times. The elimination half-life of MPHS was increased slightly, whereas that of MP increased more than twice in the patients. Significant alterations in clearances occurred in patients, but concentrations of MP were appreciable and prolonged MP due to the extensive formation of MP from MPHS and reduced clearance of MP.
Adult↗
Reversed-phase high-performance liquid chromatographic assay for the determination of mezlocillin in human and rat biological samples.
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Adult↗
Pharmacokinetics of capacity-limited systems.
Nonlinearities are commonplace in pharmacokinetics and most frequently occur because of a limited concentration of biological material available for interaction with relatively high concentrations of drugs. The Michaelis-Menten-type function can be applied to metabolism, transport, and binding phenomena that display capacity-limitation. Nonlinear systems can be handled with differential equations (directly or converted to linear form), integrated equations (in non-closed form) or with relatively new area/moment or MRT relationships. Drug disposition studies should include several dose levels and recommendations are provided for detecting the presence of one or more nonlinear absorption or disposition processes in typical experimental data.
Humans↗
Factors affecting ceftriaxone plasma protein binding during open heart surgery.
Factors most likely contributing to reduced ceftriaxone plasma protein binding in patients undergoing open heart surgery (OHS) were examined. Binding was determined by equilibrium dialysis. It was found that ceftriaxone does not bind significantly to red blood cells, alpha 1-acid glycoprotein, or to protamine, and that the pH of serum did not significantly affect binding. Albumin is the major protein to bind ceftriaxone, and binding decreases with lower albumin concentrations due to fewer binding sites. The binding of ceftriaxone was not affected by the in vitro addition of heparin or methylprednisolone, but high concentrations of methylprednisolone hemisuccinate increased the free fraction of ceftriaxone. Increased concentrations of free fatty acids (FFA) were demonstrated in several patients undergoing OHS. The in vitro addition of palmitic, stearic, linoleic, and oleic acids in high concentrations decreased the binding of ceftriaxone. Ceftriaxone binding in patient samples correlated with the molar ratio of FFA to albumin, but not to either individually. The dual effect of increased FFA and decreased albumin concentrations in OHS patients appears responsible for most of the observed binding alterations.
Binding, Competitive↗
Ion-paired reversed-phase high-performance liquid chromatography assay for determination of ceftriaxone in human plasma and urine.
A rapid, sensitive, and specific ion-paired reversed-phase HPLC assay for ceftriaxone in human plasma and urine is described. Small volumes (50 microL) of sample are deproteinized with acetonitrile and are directly injected on a C18 analytical column. The UV absorbance is monitored at 280 nm. The assay is linear between 1 and 125 micrograms/mL of ceftriaxone, with less than 10% coefficient of both intra- and interday variation. Chromatography was specific for ceftriaxone as endogenous compounds and 30 common drugs did not interfere. The assay was used in open heart surgery patients where potential interference from corticosteroids was overcome.
Buffers↗
Second generation model for prednisolone pharmacodynamics in the rat.
An improved model describing receptor/gene-mediated pharmacodynamics of prednisolone is presented which consists of seven differential equations. Data for plasma prednisolone concentrations, free hepatic glucocorticoid receptors, and hepatic tyrosine aminotransferase activity (TAT) following low (5 mg/kg) and high (50 mg/kg) doses of prednisolone are used to quantitate the kinetics and dynamics of this synthetic steroid in the rat. In contrast to the earlier model, the newer model provides for a coupling and simultaneous fitting of receptor and TAT data and is able to describe the recycling of receptors between cytosol and nucleus and the return of cytosolic receptors to baseline following glucocorticoid elimination. A numerical technique to determine the efficiency of TAT induction based on area under the curve calculations is presented, which supports the hypothesis that nonlinear dose-response effects are due to dose and time-dependent receptor depletion in the cytosol. Simulations are presented to examine the major determinants of corticosteroid effects and to compare the effects of single- and multiple-dose regimens in maximizing drug effects.
Animals↗
An area function method for calculating the apparent elimination rate constant of a metabolite.
A simple method for determination of the apparent elimination rate constant of a metabolite (km) has been developed. This procedure requires calculation of area intervals under the plasma concentration-time curves of the parent drug and its derived metabolite. The method has been evaluated and compared with the Chan moment method using both errorless and errant data. The approach is accurate for various ratios of elimination rate constants of drug and metabolite, allows several values of km to be averaged, but works best using data prior to the metabolite tmax.
Biotransformation↗
Application of mean residence-time concepts to pharmacokinetic systems with noninstantaneous input and nonlinear elimination.
Equations describing the mean residence time (MRT) of drugs in the body are derived for drugs that are administered by first- and zero-order rates into systems with Michaelis-Menten elimination. With computer simulations, the validity of these equations, the differences between them, and the conventional approach using the AUMC/AUC or the summation of mean times are demonstrated by examining calculations of the percentage of the administered dose eliminated at the MRT and AUMC/AUC. The effects of the absorption rate on the AUC and on the approximate and true MRT values in a nonlinear pharmacokinetic system are also illustrated with computer simulations. It was previously found that the true MRTiv = Vss.AUCiv/dose for an iv bolus. The total MRT (sum of input and disposition) of a drug after noninstantaneous administration was found to be a function of the MRTiv, two values of AUC (iv and non-iv), and exactly how the drug is administered expressed as the mean absorption time (MAT). In addition, a theoretical basis is proposed for calculation of the bioavailability of drugs in both linear and nonlinear pharmacokinetic systems.
Mathematics↗
The area function method for assessing the drug absorption rate in linear systems with zero-order input.
A noncompartmental approach for determination of the apparent zero-order absorption rate constant (k0) has been developed. The procedure evolves from the convolution integral and requires individual oral-dose plasma concentrations values and calculation of area intervals under the plasma concentration-time curves after intravenous administration. The proposed method was evaluated and compared with the Wagner-Nelson, Loo-Riegelman, deconvolution, nonlinear regression, and moment methods using errorless and errant simulation data from one- or two-compartment models. The area function method is generally equal to the best of these techniques (nonlinear regression) and superior to the weaker methods (moment, deconvolution, Loo-Riegelman), especially for errant two-compartment data. Coupled with a companion procedure for constructing fraction absorbed versus time plots and assessing first-order absorption rate constants, the area function methods offer direct and accurate means of discerning drug absorption kinetics without the need for assignment of a disposition model for drugs with linear elimination kinetics.
Intestinal Absorption↗