Thrombosis, or the formation of blood clots, can happen for many reasons - everything from atherosclerosis to sitting on an airplane for too long. On a molecular level, one of the things that can lead to clotting is shear stress, the friction that is caused by the flow of blood through vessels - a phenomenon which, among other things, has complicated the use of artificial hearts and heart valves.

Shear stress is self-reinforcing; that is, once clot formation has started, that in itself will affect shear stress and thus, further clot formation. "The fluid mechanics get very complicated. But by its very nature, the clot always leads to high shear force" in the blood vessel, Shaun Jackson, professor at Monash University, told BioWorld Today.

One problem with treating thrombosis, whether caused by shear stress or other factors, is that it can be done too successfully. In other words, preventing the formation of blood clots can lead to excessive bleeding. That is, of course, not an altogether surprising consequence of interfering with normal clotting mechanisms, but its predictability doesn't make it any less medically problematic.

But in the June 2005 issue of Nature Medicine, researchers from Monash University, the University of Melbourne in Australia and University College London in the UK, scientists reported on an antithrombotic compound that does not appear to increase bleeding times in response to injury. Even more encouraging, the compound does not show synergistic effects on bleeding times with the classic anti-clotting drug heparin.

"I frankly never expected that," said Jackson, who is senior author of the paper.

The compound, known as TGX-221, appeared to work by blocking a specific isoform of PI3 kinase, a lipid kinase that normally activates platelets in response to shear stress.

For blood clots to form, platelets must stick to each other, and to the blood vessel. The scientists first tested their compound by perfusing fibrinogen-coated microcapillaries with platelets at a speed that allowed the bonding of platelets to fibrinogen, via integrin alphaIIb-beta3. When the blood flow speed suddenly was increased, increasing both shear and the mechanical stress on the integrin-fibrinogen bond, pretreatment with a PI3 kinase inhibitor reduced the ability of platelets to maintain their bonds with the artificial blood vessel.

The scientists next designed a series of inhibitors that target different forms of the PI3 kinase, to pinpoint the isoform responsible for maintaining the platelet-vessel bond. TGX-221, which targets the beta-isoform, had the strongest effect on clotting in response to shear stress. Further experiments in cell culture showed that the compound was able to loosen platelet's death grip on cell walls. In animals, TGX-221 prevented the formation of clots in response to shear stress.

To Jackson, one of the most exciting aspects of TGX-221 is that it shows promise for use as a combination agent. While it did reduce the formation of blood clots in response to shear stress, it did not affect clotting time in response to a tail injury, either by itself or in conjunction with the anti-clotting agent heparin.

"Current anti-clotting drugs such as aspirin or heparin are not ideal - many patients still have clotting problems. The prospect that you could add another anti-clotting agent without affecting bleeding is one of the real breakthroughs of this research, one for which there is a lot of clinical need," Jackson said.

Analogues of TGX-221 are being developed clinically by Melbourne-based Cerylid Biosciences Ltd. Gregg Smith, vice president of drug development at Cerylid, told BioWorld Today via email that "the company's second-generation antithrombotic agent, CBL1309, is nearing the end of its formal preclinical toxicology program. It is due to enter Phase I trials in Australia in Q4 this year. Data from the tox program collected to date suggest that it is very well tolerated." CBL1309 is a TGX-221 analogue that shares its mechanism of action and broad pharmacological properties; it is not partnered, but Smith said that Cerylid is in "advanced discussions with a number of large pharma and biotechs" about partnering possibilities.