By David N. Leff

True enough, tumor necrosis factor (TNF) can kill some cancer cells some of the time. That’s why over-optimistic oncologists, in the early 1980s, named it tumor necrosis factor.

Since then, the top-gun cytokine might better be known as joint necrosis factor, for its destruction of arthritic synovial tissue.

“The critical finding about TNF, in terms of the pharmaceutical industry, is that it plays a causative role in rheumatoid arthritis.“ So said protein chemist Roy Black, of Immunex Corp., in Seattle. He heads a project aimed at disarming or defusing TNF’s mayhem against the painfully inflamed joints that torment arthritis sufferers.

The Atlanta-based Arthritis Foundation estimates at 2.1 million the number of Americans with rheumatoid arthritis. Two-thirds of them are older women.

“More generally,“ Black said, “TNF is clearly an early mediator of a variety of immune and inflammatory responses, such as graft-vs.-host disease, inflammatory bowel disease and septic shock. But in terms of the clinical evidence and the excitement of the pharmaceutical industry, it’s the arthritis connection that really is critical.“

He explained: “What TNF does is turn on a variety of tissue-damaging effects. For example, it causes the release of proteolytic enzymes that will destroy the joint tissue. And it causes the release of a variety of other mediators, for example, the chemokines, which then recruit yet more inflammatory cells into the joint. So TNF seems to be at the top of a pyramid of joint destruction.“

Immunex is not the only company aiming its sights at the potential RA market, and its expertise at running TNF to earth.

Macrophages and T lymphocytes of the immune system turn on TNF proteins as an advance party to mobilize infection-fighting cytokines and chemokines. The 26-kiloDalton protein anchors itself in the cell membrane, with two-thirds of its length, 17 kD, projecting out beyond.

“The 157-amino-acid active part of the TNF protein that sticks out,“ Black observed, “interacts with receptors on other cells, but its biology is still somewhat murky. It may actually have some beneficial effects.

“But in any event,“ he continued, “what is clear is that the damage is associated with a high level of the released, soluble form, brought about by a proteolytic cleavage.“

That action begins when a specific enzyme comes along and chops off that transmembrane stretch, releasing TNF to float free in the intercellular matrix soup and mobilize more pro-inflammatory cytokines and chemokines.

Immunex, which discovered that TNF-trimming metalloproteinase in the early 1990s, has named it TACE — tumor necrosis factor-alpha converting enzyme.

TACE Discovery Strategy Paid Off

“Our strategy,“ Black recalled, “was to make a recombinant form of the full-size TNF molecule, and screen for proteases from macrophages that cut it into the correct smaller form. By 1994, we had a rough characterization of the TACE. At that point we published it in Nature, and so did two other companies. Then the whole pharmaceutical industry dived in to try to finally purify the enzyme, then, of course, clone it and do all the things you can do with a cloned enzyme.

“Fortunately,“ Black added, “we seem to have come out on top of the pack, along with Glaxo. Nobody else that we know of was able to clone the enzyme.“

Black is first author of an Immunex paper in the current issue of Nature, dated Feb. 20, 1997. Its title: “A metalloproteinase disintegrin that releases tumor-necrosis factor-a from cells.“

An adjacent paper, with a strangely similar heading, appears in the same issue of Nature. Its title: “Cloning of a disintegrin metalloproteinase that processes precursor tumor-necrosis factor-a.“ It is authored by Glaxo Wellcome Research and Development Inc., in Research Triangle Park, N.C.

“Ours is a much more complete report,“ Black said. “They report the purification, cloning and making of a recombinant form of the enzyme, and showed that it does cut TNF. That’s really all that’s in their paper.“

Immunex, in its article, described generating antibodies to TACE, “and used them to show that the enzyme is actually on the cell surface, where you’d expect it to be.“

Black continued: “More importantly, we also knocked out the gene in mouse cells, and showed that those cells are 90 percent deficient in their ability to release soluble TNF. So our paper contains strong evidence confirming the physiological role of TACE, which Glaxo did not do.“

Big Pharma Vies To Inhibit TACE

Now the race is on to produce a drug that can inhibit TACE, thus preventing the transmembrane TNF sequence from slipping its anchor, while mopping up free-floating copies of the protein’s marauding short form.

Immunex has an injectable entry, trademarked Enbrel, which is now beginning pivotal Phase III trials. The company reported its Phase II study last October to the American College of Rheumatology meeting, in Miami. That multicenter study reported 75 percent of the RA patients on the largest dose of the drug had significantly decreased swollen and tender joints, versus 14 percent in the placebo cohort. (See BioWorld Today, Oct. 23, 1996, p. 1.)

“From the pharmacological point of view,“ Black pointed out, “the heart of the Immunex RA project is to find a small, bioavailable oral compound — a pill.“

Immunex spokeswoman Robin Shapiro added: “We have exclusively licensed our technology to Wyeth-Ayerst Laboratories [in Radnor, Pa.], which is a division of American Home Products [AHP; Madison, N.J.]. So we are working with Wyeth to identify candidates.“ AHP owns a majority interest in Immunex.

“We have a full-fledged, intensive effort,“ Black said, “with the usual tools — high-throughput screening and a team of chemists working on the directed synthesis of compounds. We are working to crystallize the enzyme, to get the structure, so we can employ rational drug design. And we already have a homology-based model of TACE’s active site, which we’re using for designing a small-molecule drug.“

He concluded: “The critical difference from Enbrel would be the oral availability of a TACE inhibitor. We don’t anticipate any greater efficacy.“ *