Untitled

Abstract

Dose-dependent and repeated-dose effects of methylprednisolone (MPL) on down-regulation of glucocorticoid receptor messenger RNA (GR mRNA) and GR density, as well as tyrosine aminotransferase (TAT) mRNA and TAT induction by receptor/gene-mediated mechanisms in rat liver were examined. A previously developed pharmacokinetic/pharmacodynamic (PK/PD) model was used to design these studies which sought to challenge the model. Three groups of male adrenalectomized Wistar rats received MPL by i.v. injection: low-dose (10 mg/kg at Time 0), high-dose (50 mg/kg at Time 0), and dual-dose (50 mg/kg at Time 0 and 24 hr). Plasma concentrations of MPL, and hepatic content of free GR, GR mRNA, TAT mRNA, and TAT activity were determined. The P-Pharm program was applied for population analysis of MPL PK revealing low interindividual variation in CL and Vc values (3-14%). Two indirect response models were applied to test two competing hypotheses for GR mRNA dynamics. Indirect Pharmacodynamic Response Model I (Model A) where the complex in the nucleus decreases the transcription rate of GR mRNA better described GR mRNA/GR down-regulation. Levels of TAT mRNA began to increase at 1-2 hr, reached a maximum at 5-6 hr, and declined to the baseline at 12-14 hr after MPL dosing. The induction of TAT activity followed a similar pattern with a delay of about 1-2 hr. The low-dose group had 50-60% of the TAT mRNA and TAT induction compared to the high-dose group. Since the GR density returned to about 70% of the baseline level before the second 50 mg/kg dose at 24 hr, tolerance was found for TAT mRNA/TAT induction where only 50-60% of the initial responses were produced. Our fourth-generation model describes the dose dependence and tolerance effects of TAT mRNA/TAT induction by MPL involving multiple-step signal transduction controlled by the steroid regimen, free GR density, and GR occupancy. This model may provide the foundation for studying other induced proteins or enzymes mediated by the similar receptor/nuclear events.

Pharmacodynamic Modeling of Acute and Chronic Effects of Methylprednisolone on Hepatic Urea Cycle Genes in Rats

“…No suitable model could explain these differences simultaneously and therefore, only the acute dataset were fitted with Model F. The estimated parameters for these three genes are given in Table 5. The estimated degradation rate constant for GR mRNA was very similar to previously reported values based on data from Quantitative Northern Hybridization measurements (Sun et al 1998a; Sun et al 1998b). Most of the parameters could be estimated with reasonable precision with CV% less than 30%.…”

“…Although the experiment continued for 72 hours, the figure is truncated at 20 hours because MPL reached its lower limit of quantification by 6 hr and the drug-receptor complexes returned to their baseline prior to this time. Figure 4b shows the simulation of GR and GR mRNA which is also based on previously published data (Sun et al 1998b). Figure 4c shows the simulations of MPL kinetics, DR, and DR(N) during chronic infusion of MPL.…”

“…TAT, which facilitates the transfer of an amino group from amino acid breakdown to the urea cycle and is also one of the most highly studied biomarkers, could be classified as one of the Class 1 genes. Both TAT and glucocorticoid receptor ( Class 4A) dynamics had extensively been studied in our lab (both at mRNA and protein levels) and five generations of highly mechanistic, quantitative PK/PD models were developed for MPL at various dose levels (Ramakrishnan et al 2002; Sun et al 1998a; Sun et al 1998b). Thus the presence of these two genes in our database not only had importance with respect to their involvement in the urea cycle, but also allowed us to compare the results from the microarrays to previous measurements by quantitative Northern Hybridization.…”

“…Rats were sacrificed by aortic exsanguination at 0.25, 0.5, 0.75, 1, 2, 4, 5, 5.5, 6, 7, 8, 12, 18, 30, 48 and 72 hr. The sampling time points were selected based on previous studies of receptor dynamics and enzyme induction in muscle and liver (Ramakrishnan et al 2002; Sun et al 1998a; Sun et al 1998b). Four vehicle-treated rats were designated as controls and were considered as sacrificed at time point zero.…”

“…The values for the baselines of both the GR mRNA and the receptors were fixed and were obtained from Sun et al (Sun et al 1998a; Sun et al 1998b). The baseline values for DR and DR ( N ) are zero.…”