In a paper published in the January issue of the Journal of Pharmacology and Experimental Therapeutics, a University at Bufflo team reports that rats that were administered methylprednisolone on a chronic basis significantly decreased their synthesis of receptors that bind the drug and alter the expression of genes.

Following a significant effect just after the steroid administration, the animals' response to the drug dropped to just one-quarter of what it would have been in the absence of the tolerance.

"What was surprising here was that this tolerance process was as severe as it is," said William J. Jusko, Ph.D., UB professor of pharmaceutical sciences and director of the study involving scientists in UB's School of Pharmacy and Pharmaceutical Sciences and the College of Arts and Sciences.

"We saw a big effect during the first several hours of exposure to the drug and then about 75 percent of that effect was lost."

The researchers discovered that this tolerance response occurs as a result of a feedback mechanism triggered by the initial binding to the drug of the corticosteroid receptor in cells, causing an initial increase in the production of the enzyme tyrosine aminotransferase (TAT), and the TAT gene.

"When TAT production goes up, there's an offsetting reduction in receptor synthesis, which, in turn, ends up reducing TAT production," Jusko explained.

The researchers found that about 12 hours after the drug is administered, there is 75 percent less TAT gene being produced.

Quantifying drug effects to that level of detail is not usually the goal of most pharmacologic studies. But for decades, the UB researchers, led by Jusko, have gone a critical step further, taking the results of their basic and clinical pharmacologic studies and then putting them into sophisticated mathematical models to quantify what happens at the whole-body, tissue, molecular and gene levels at different time intervals after a drug is administered.

For the past 25 years, Jusko, who also is interim chair of the UB Department of Pharmaceutical Sciences, has been continuously funded on the same National Institutes of Health grant to use various pharmacodynamic methods to study the complex effects that occur after exposure to a drug. (Pharmacodynamics is the study of the time course of drug effects on the body.)

The NIH funding has steadily increased each year to the present annual level of $346,500.

"The question we've been working on for many years is how can you best control the dosing regimen to maximize the efficacy of steroids and minimize their toxicity," said Richard R. Almon, Ph.D., UB professor of biological sciences and co-author.

Managing side effects is an urgent concern for physicians whose patients take steroids to treat chronic conditions. Some, such as transplant patients, have to take them for as long as they live, since without steroids to suppress the immune system, their bodies will reject a transplanted organ.

Serious side effects of steroid use include muscle wasting, hypertension, adrenal suppression, fat redistribution, hyperglycemia and diabetes.

Over the past decade, the UB team has compiled a database of what is probably the world's largest in vivo time series of gene-mediated effects that result from exposure to steroids under different dosing regimens.

This wealth of data provides the researchers with the most complete picture of what the drug is doing to the body at the level of genomic changes.

"For each separate point in time we have studied following drug exposure, we have asked, how does the expression of messenger RNA change for each of 12,000 genes?" said Almon.

"What does knowing these time patterns of gene change tell us?" Almon asked. "To be biblical, it tells us what changes begat other changes. Drugs don't just do one thing. The drug influences one factor, which becomes an influence on another. It cascades. To understand what can be the effect or cause of the next thing, you need the time sequence."

According to Jusko, it has been known that the body has several types of tolerance mechanisms that alter drug effects.

"This latest finding is the best example so far of a type of tolerance that is genomically controlled -- that is, controlled through the expression of genes," Jusko said. "It shows for the first time how dramatically the body down-regulates both messenger RNA and receptor synthesis and how that down-regulation so markedly blunts the effects of the steroid."

Jusko described this down-regulation of receptor synthesis as a type of homeostatic process that is likely to be present to some extent for many drugs that have a genomic mechanism of action.

In the experiments, rats were administered a steady-state infusion of methylprednisolone over the course of seven days. The marked decrease in the production of the corticosteroid receptor was observed within twelve hours after chronic dosing began.

In addition to Jusko and Almon, co-authors on the study include Rohini Ramakrishnan, Ph.D. former UB doctoral student, Debra C. DuBois, Ph.D., UB research associate professor of biological sciences, and Nancy A. Pyszczynski, research specialist.

For this research, Ramakrishnan was selected to be one of five recipients of a graduate Student Award from the American Association of Pharmaceutical Scientists last fall.


[Contact: Ellen Goldbaum]

15-Feb-2002