It’s already being targeted in the clinic; now, scientists have figured out what the receptor for urokinase plasminogen activator looks like, a possible harbinger of rationally designed therapeutics.

Plasminogen activators are a family of proteins that cleave plasminogen to plasmin. Plasmin, in turn, helps dissolve blood clots, which is the basis of its success in the clinic to date: Tissue plasminogen activator, or tPA, marketed by South San Francisco-based Genentech Inc. under the name Activase, is an FDA-approved ischemic stroke treatment (See BioWorld Today, July 8, 2005.)

But plasmin also affects other proteins that can weaken the extracellular matrix. And that weakening is part of what allows tumor cells to leave their original site in cancer metastasis; urokinase-type plasminogen activators are overexpressed on some aggressive tumors. UPA receptor activation also contributes to primary tumor growth.

In the Feb. 3, 2006, issue of Science, scientists from Harvard Medical School in Boston; San Diego’s Attenuon LLC; University of Pennsylvania Medical Center in Philadelphia; Fujian Institute of Research on the Structure of Matter in Fuzhou, People’s Republic of China; the University of Alabama at Huntsville; and D. E. Shaw Research and Development in New York have published a high-resolution structure of uPA receptor with a uPA fragment bound to it.

The authors used the uPA fragment instead of the whole molecule because it "contains all of the determinants required for binding to its receptor," they wrote in their paper. The receptor, too, was incomplete: It was a soluble form (or suPAR) but again, contained all parts of the receptor that interact with uPA, making it an appropriate stand-in for the membrane-anchored form to study the interaction of uPA with the receptor.

Mingdong Huang, lecturer at Harvard Medical School and the paper’s senior author, described the structure as having "a teacup shape, with a middle cavity that is the shape of a funnel." That funnel "is the ideal target for rationally designed inhibitors," he told BioWorld Today.

More formally, the funnel is known as domain one (of three.) The scientists found that how the three domains are oriented in relationship to each other depends a lot on what is bound to them. While a low-resolution structure of the receptor bound to an inhibitor had been published previously, the structure of the receptor bound to uPA itself "turned out to be very different," co-author Bruce Furie, director of Harvard Medical School’s center for hemostasis and thrombosis research, told BioWorld Today. Huang added that this shape change "shows that the structure can be regulated by inhibition."

Munich, Germany-based Wilex AG has uPA receptor inhibitors WX-UK1 and WX-671 in several Phase I/II trials for the treatment of breast cancer and other solid tumors that bind to domain one. (See BioWorld Today, July 28, 2005.)

A Wilex spokesman described WX-UK1 as "an active site reversible competitive inhibitor. It binds exactly where natural substrates bind," and added that "WX-671 is a non-active prodrug of WX-UK1" that is metabolized to UK1.

The scientists crystallized uPA with its receptor and an antibody; the antibody stabilizes the structure, enabling determination of the structure at high resolution. Furie called the use of an antibody "a standard trick of the trade."

The proteins for the study were supplied by Attenuon LLC. Andrew Mazar, chief scientific officer at Attenuon, told BioWorld Today that the company is working on an antibody that will inhibit the urokinase receptor. Mazar said a conservative estimate is that the antibody will enter clinical trials in 18 to 24 months. Attenuon also is working on what Mazar termed "a second-generation approach" to inhibiting the receptor, which will use small molecules instead of antibodies.

Mazar said that from a drug discovery standpoint the structure determination identifies other druggable sites that could be targets for a small-molecule approach. Furie, in turn, noted that the scientists hope studying the structure will help them solve a more basic biological mystery: "It’s a very curious receptor in that it binds to a host of other proteins," he said. "From a biological standpoint, it’s interesting how it can be so promiscuous."

In a follow-up email, Huang added that while the scientists need to analyze more different configurations of receptor and ligands, "our current published structure shows us that the inter-domain flexibility of the uPA receptor is one reason that accounts for uPAR’s capability to recognize multiple ligands."