By David N. Leff
By a recent count, there are 27 non-steroidal, over-the-counter pain relievers on the market, and more joining them, even as we watch TV.
Pain, of course, is only one of the Four Horsemen of Inflammation — dolor, calor, rugor, turgor. The other three are fever, redness and swelling.
Prostaglandins are the main perpetrators of the ouch in inflammation. Normally, these ubiquitous substances carry out housekeeping duties all over the body’s tissues, from protecting the mucosal membrane lining stomach and gut to minding the water-salt balance in kidneys, to keeping the blood’s clotting and clot-busting functions in steady state.
Prostaglandins (PG) owe their existence to a fatty acid, arachidonic acid, which an enzyme called cyclooxygenase-1 (COX-1) turns into PG as needed. Sometimes, trauma or disease triggers an inflammatory state that calls for a whole lot more PG than usual.
To fill that emergency order, the body turns to COX-1’s twin enzyme, COX 2, which floods the scene of the injury with enormous overloads of pain-inflicting PG.
From aspirin and ibuprofen to naproxen and piroxicam, virtually all the non-steroidal analgesics on the market work by inhibiting the PG-promoting, pain-provoking action of COX-2. But in the process, they also block COX-1, which causes side-effects, such as stomach irritation and kidney damage.
“All along,“ observed structural biologist Ravi Kurumbail of Monsanto Co.’s G. D. Searle division, “when pharmaceutical companies in the 1960s and 70s were looking for drugs to reduce inflammation, they screened compounds against the only PG synthase then known, COX-1. Every one they came up with was invariably a good COX-1 inhibitor.
“Then in 1991,“ Kurumbail told BioWorld Today, “people got really excited when COX-2 was cloned. If we could selectively inhibit COX-2, we could do even better.“
Curbing PG’s Side Effects; Keeping Its Good Ones
To make it really better, the goal was “to inhibit COX-2 selectively, so as to reduce inflammation without upsetting the normal hemostasis in the body maintained by COX-1.“
Searle now has such a drug in Phase III trials worldwide, the company’s COX-2 project leader, pharmacologist Karen Seibert, told BioWorld Today.
The compound’s name is Celecoxib, Kurumbail said, adding, “With it we are going after rheumatoid arthritis and osteoarthritis.“
He is first author of an article in the current issue of Nature, dated Dec. 19, 1996, titled: “Structural basis for selective inhibition of cyclooxygenase-2 by anti-inflammatory agents.“
The team’s X-ray crystallographic analysis of Celecoxib determined that the secret of its selective down-grading of COX-2 was that “in the active site of the COX-2 enzyme, the protein pocket where non-steroidal anti-inflammatory drugs bind is significantly larger than in COX-1. So when you have a drug as small as aspirin or flurbiprofen, it can bind pretty effectively in both enzyme forms But a somewhat larger inhibitor, which is what we have, binds specifically to COX-2 only.“
Kurumbail went on to point out that “most of the COX-2-inhibiting compounds that we have tested preclinically in animals clearly showed that they do not cause stomach irritation.“
First off the mark with a COX-2-targeting non-steroidal drug, Kurumbail said, “is Boehringer-Mannheim GmbH of Waldhof, Germany. They have come out with a COX-2 inhibitor that they call meloxicam. In our hands, we haven’t seen significant selectivity of COX-2 versus COX-1. They are already on the market in Europe,“ he added, “and I have heard that they have approached the FDA for approval in the U.S.“
He observed that “Merck, our biggest competitor, has made similar observations.“
A First’ In Membrane-Structure Imaging
Searle’s feat in solving the structure of COX-2, a membrane protein, by X-ray crystallography, Kurumbail said, “was unusually difficult. You could probably find not more than ten structures of membrane proteins so far in the literature.“ He added, “I’m pretty sure ours is the first membrane protein that has been successfully used as a target in structure-based design.“
Kurumbail concluded: “We are using the structural information we have acquired to design future generations of COX-2 inhibitors, with some different properties than what we have in existing molecules.“
Seibert — Searle’s COX-2 drug discovery leader — pointed out: “To be sure, steroidal anti-inflammatories, such as dexamethasone and hydrocortisone, also selectively inhibit COX-2. However,“ she continued, “they do so, not by direct inhibition at the enzyme level, but at the genetic level, by blocking transcription of the RNA message for the enzyme. In fact, steroids have effects on hundreds and hundreds of genes. And then,“ Seibert observed, “you get lots of other effects, not all of them good.“