By David N. Leff
A body's best friend can become its worst enemy.
Take Lyme disease, for example: When a tick's bite squirts the Borrelia burgdorferi spirochete into its human victim's bloodstream, that invasion rings alarm bells in the immune system.
Apparently first to answer the call is a cytokine called interleukin 1 (IL-1). It summons bug-gobbling macrophage cells to the scene, to ingest and dispose of those pathogenic bacteria. That's the good news.
The bad news is that over-expression of that same IL-1 cytokine dumps destructive immune complexes into joints, nerves and other organ systems inflamed by Lyme disease.
Wearing its white hat, IL-1 triggers the inflammatory responses that counter infection and promote wound-healing. Black-hatted, the same front-line cytokine provokes the chronic, destructive inflammation of autoimmune diseases.
To be sure, IL-1 is not a molecular loner. It acts initially in concert with other cytokines, notably tumor necrosis factor. But as IL-1 is an early instigator of the inflammatory response — for better or worse — a prime goal of therapeutic drug designers is finding ways to curb its over-
"We try to understand the distinction between inflammation and inflammatory disease," observed molecular biologist Michael Bevilacqua, vice president for research at the Boulder, Colo. outpost of Amgen Inc. "Inflammation is a good thing — pretty well controlled. It starts, it acts and even tends to clean up after itself."
He went on, "Inflammatory disease is distinguished by the damage that occurs to tissue. So in arthritis, a little bit of inflammation is no problem. But when it becomes ingrained in the joints, invading cells ultimately erode cartilage and bone.
"And IL-1 seems to be a major player in keeping those macrophages, endothelial cells and synoviocytes activated, and promoting their destruction of arthritic joints," Bevilacqua pointed out, adding: "We think we can come in with IL-1 receptor antagonists and control it."
Two strikingly similar articles in this week's Nature, dated March 13, 1997, advance that cause by exposing to X-ray crystallography the molecular structure of IL-1 complexed to its receptor.
One paper, by Amgen scientists at Boulder, bears the title: "Crystal structure of the type-1 interleukin-1 receptor complexed with interleukin-1b."
The adjoining article, "A new cytokine-receptor binding mode revealed by the crystal structure of the IL-1 receptor with an antagonist," is by authors at Affymax in Palo Alto Calif., and the Marion Merrell Dow Research Institute, Strasbourg, France.
Companies Reveal Complementary Structures
Receptor pharmacologist Ronald Barrett, of Affymax, is a senior co-author of the second paper. "Both articles," he told BioWorld Today, "are additions to the growing number of studies on how cytokines bind to their receptors. Amgen has the co-crystal structure of the IL-1 agonist; we, of the antagonist. Comparing these two may give some insight on how the signal to this cytokine receptor is initiated."
Barrett added: "The other hope is that by understanding how these rather large proteins bind to their receptors, we may be able to design small molecules capable of doing the same."
A key to this prospect is the difference between IL-1 agonists and antagonists. "There are three naturally occurring proteins that bind to the IL-1 receptor," Barrett explained. "Two of them, IL-1* and IL-1ß are agonists. The third is a natural receptor antagonist, IL-1RA. Agonists are capable of initiating signal transduction, and causing an effect on cells, whereas an antagonist binds to the receptor but instead of initiating a signal, it can block agonists from binding."
This blocking action has the feedback effect of modulating the IL-1 system — nature's way of doing naturally what the drug discoverers seek to achieve biochemically.
Crystallographer Barbara Brandhuber, senior author of the Amgen paper, told BioWorld Today of "an unexpected finding in our crystallization of the IL-1 agonist: The molecule binds to its receptor in a way that's different from all other co-crystal structures that have been published — such as tumor necrosis factor, human growth hormone, interferon. This is in a completely different binding mode. You have two sites, and the molecule sitting snugly between them. The implications for drug discovery would be identifying which of the two sites could be potentially more important for binding, and which for interrupting that binding."
Brandhuber made the point, anent the two Nature papers: "Our structure of the IL-1 agonist and theirs' of the antagonist represent definitely complementary work."
To which Bevilacqua added: "Comparing the binding differences between these two structures should definitely advance drug discovery efforts for small-molecule antagonist mimetics. But in our view it looks like a pretty challenging task."
Amgen's Antagonist In the Clinic
Meanwhile, he pointed out, Amgen has in Phase II clinical trials for rheumatoid arthritis the naturally occurring large IL-1 receptor antagonist, "which has showed positive clinical outcomes." But he went on: "The molecule needs to be present in the extracellular fluid to work. So it doesn't have a long-lasting effect."
Back at the preclinical drawing board, Bevilacqua said, "on animal models, we're looking to improve the product by sustained release."
Affymax, Barrett observed, "has had a long-standing collaboration with Marion Merrell Dow, with the goal of producing small-molecule antagonists to the IL-1 receptor. In the course of that work, we published last year that we have identified — by screening combinatorial peptide libraries — of a high-affinity molecule that binds to the receptor.
"That's one approach to finding high-affinity antagonists," he continued. "The other is structure-based design.
"We don't have the co-crystal structure of that small peptide yet," he continued, "but we're trying to understand how it can mimic, or in some ways achieve, the same antagonistic effect as this large IL-1RA. Now with its structure in hand, we hope that by understanding how the two big molecules bind to each other, we'll be able to design small ones that block the interaction.
"Without the structure," Barrett concluded, "you're really shootin' in the dark." *