Pharmacodynamics and Pharmacogenomics of Methylprednisolone during 7-Day Infusions in Rats ROHINI RAMAKRISHNAN, DEBRA C. DUBOIS, RICHARD R. ALMON, NANCY A. PYSZCZYNSKI, and WILLIAM J. JUSKO Department of Pharmaceutical Sciences, School of Pharmacy and Pharmaceutical Sciences, State University of New York at Buffalo, Buffalo, New York Received June 14, 2001; accepted August 24, 2001 This paper is available online at http://jpet.aspetjournals.org ABSTRACT An array of adverse steroid effects was examined on a whole body, tissue, and molecular level. Groups of male adrenalec- tomized Wistar rats were subcutaneously implanted with Alzet mini-pumps giving zero-order release rates of 0, 0.1, and 0.3 mg/kg/h methylprednisolone for 7 days. The rats were sacri- ficed at various times during the 7-day infusion period. A two- compartment model with a zero order input could adequately describe the kinetics of methylprednisolone upon infusion. Blood lymphocyte counts dropped to a minimum by 6 h and were well characterized by the cell trafficking model. The time course of changes in body and organ (liver, spleen, thymus, gastrocnemius muscle, and lungs) weights was described us- ing indirect response models. Markers of gene-mediated ste- roid effects included hepatic cytosolic free receptor density, receptor mRNA, tyrosine aminotransferase (TAT) mRNA, and TAT levels. Our fifth-generation model of acute corticosteroid pharmacodynamics was used to predict the time course of receptor/gene-mediated effects. An excellent agreement be- tween the expected and observed receptor dynamics sug- gested that receptor events and mRNA autoregulation are not altered upon 7-day methylprednisolone dosing. However, the model indicated a decoupling between the receptor and TAT dynamics with this infusion. The strong tolerance seen in TAT mRNA induction could be partly accounted for by receptor down-regulation. An amplification of translation of TAT mRNA to TAT and/or a reduction in the enzyme degradation rate could account for the observed exaggerated TAT activity. Our results exemplify the importance of biological signal transduction vari- ables in controlling receptor/gene-mediated steroid responses during chronic dosing. The clinical use of steroids is extensive and frequently chronic. Corticosteroids currently are among the most impor- tant drugs used for the treatment of a variety of immunolog- ical conditions such as lupus erythematosus, rheumatoid arthritis (Canvin and el-Gabalawy, 1999), organ transplan- tation, bronchial asthma (Boushey, 1998), and inflammatory bowel disease (Selby, 1993), to name a few. Short-term treat- ment in acute or transient illness is generally not associated with major side effects. However, the multiple and potent metabolic effects of steroids become prominent upon chronic dosing, leading to an increased risk of toxicity thus limiting their usefulness. The undesirable metabolic effects of corti- costeroids cannot be completely separated from their favor- able anti-inflammatory effects because most actions are manifested using the same glucocorticoid receptor. The inte- grated effects result in hyperglycemia, negative nitrogen bal- ance, and fat redistribution leading to complications, includ- ing diabetes, muscle wasting, hypertension, cataracts, and peptic ulcers (David et al., 1970; Baxter and Forsham, 1972; Swartz and Dluhy, 1978). Another important clinical conse- quence of long-term use of steroid is adrenal insufficiency after withdrawal of therapy (Swartz and Dluhy, 1978). Because the principal undesirable effects of steroids are manifested only upon long-term dosing, it can be expected that there might be additional factors contributing to the dynamics of steroid responses under these dosing conditions. Results (Ramakrishnan, 2001) from steady-state studies con- ducted previously in our laboratory suggested that a decou- pling between receptor and enzyme dynamics occurs upon long-term continuous dosing of steroid in male adrenalecto- mized rats. It is possible that nuclear molecular events (re- ceptor translocation, chromatin binding) are altered upon chronic dosing. On the other hand, signal transduction pro- cesses involved in generation of the response can be ampli- fied/diminished due to global systemic effects of the steroid. Corticosteroid responses may be countered by other hor- mones whose circulating levels are affected by prolonged steroid exposure (Baxter and Forsham, 1972). For instance, the release of insulin is stimulated in response to hypergly- This study was supported by Grant GM24211 from the National Institutes of Health. ABBREVIATIONS: TAT, tyrosine aminotransferase; GRE, glucocorticoid response element; MPL, methylprednisolone; CL, clearance; ABEC, area between the baseline and effect curve; GCR, glucocorticoid receptor. 0022-3565/02/3001-245–256$3.00 THE JOURNAL OF PHARMACOLOGY AND EXPERIMENTAL THERAPEUTICS Vol. 300, No. 1 Copyright © 2002 by The American Society for Pharmacology and Experimental Therapeutics 4236/952706 JPET 300:245–256, 2002 Printed in U.S.A. 245 at ASPET Journals on June 13, 2016 jpet.aspetjournals.org Downloaded from