During a heart attack, cardiomyocytes become ischemic and will die through necrotic processes if the blockage isn't promptly removed. Once they are reperfused, though, many of the formerly ischemic cells will go on to die through apoptosis, causing irreversible damage to the heart muscle. Despite lingering questions about the exact mechanisms of this secondary damage, drugs that block ischemia-reperfusion-related apoptosis might be entering clinical trials as early as next year.At the American College of Cardiology meeting on 30 March, for instance, LXR Biotechnology, Inc. (Richmond, CA, USA) presented data from a study conducted in collaboration with researchers at the Cleveland Clinic Foundation (Cleveland, OH, USA) of LXR015-2 (Elirex) in a canine model of ischemia-reperfusion. In this study, the left anterior descending coronary artery in 28 dogs was clamped for 90 minutes to create an occlusion similar to that seen in myocardial infarction. Fifteen minutes before the occlusion was removed, either Elirex or placebo was infused into the artery of 20 of the animals, with leakage and collateral circulation carrying the compounds into the ischemic area. Superoxide dismutase (SOD), which scavenges free radicals and is highly effective in blocking post-myocardial infarction apoptosis in dogs but not in humans, was used as a positive control in eight other dogs. In the hearts treated with Elirex, they found a 53% reduction in the size of the infarction compared with placebo, while the SOD-treated hearts showed only a 40% reduction.`This is very strong evidence in support of the hypothesis that during a heart attack a portion of the myocardium becomes ischemic, and the damage is not necessarily lethal damage,' says David Tomei, chief scientific officer at LXR. `We're guessing that about 15% of the damage following a heart attack is due to processes we may not be able to suppress with our drugs. But about 85% is due to programmed cell death. We're shooting to suppress all of that.'Just how Elirex works is not completely clear. The active ingredients are lysophosphatidic acid (LPA) and a type of polyethylene glycol with linear side chains and a molecular weight of 20 000 (PEG-L20). The LPA binds to specific receptors that are found on the surfaces of many cell types. By a signal transduction pathway that hasn't been completely elucidated, LPA appears to suppress the caspase-dependent proteolytic pathway, in which a cascade of proteases related to interleukin 1β-converting enzyme (ICE) trigger programmed cell death (Fig. 1Fig. 1). `We've got the receptor for LPA cloned, we've got the ligand, and we've got suppression of apoptosis. Now we have to prove that those receptors are mediating it,' says Tomei.Fig. 1Targets for anti-apoptotic therapies. Bax and related proteins activate the caspase cascade, whereas Bcl-2 and related proteins are anti-apoptotic. Blocking Bax, augmenting the activity of Bcl-2, or directly inhibiting caspases should block apoptosis. Agonists at the lysophosphatidic acid (LPA) receptor also appear to block caspase-dependent apoptosis; PEG-L20 both augments the efects of LPA and prevents permeabilization of the plasma membrane.View Large Image | Download PowerPoint SlideSeparate studies of PEG-L20 alone show that it has at least two apoptosis-blocking effects: (1) it significantly inhibits increases in membrane permeability; and (2) it enhances the anti-apoptotic effect of LPA.Tomei says he would like to begin clinical trials of Elirex starting in late 1998, but because of financial and resource restrictions inherent in a small biotech company, he sees 1999 as a more realistic target.Another company hot on the cardiac apoptosis trail is Idun Pharmaceuticals (San Diego, CA, USA). While LXR has a drug candidate in development, Idun, which has made apoptosis its main line of business, has taken the tactic of acquiring the rights to a number of critical patents covering molecules that suppress apoptosis, such as Bcl-2, and molecules that promote it, such as Bax. Now the company is actively developing small synthetic molecules that bind to these molecules to inhibit apoptosis.It is still a mystery why ischemia induces apoptosis, but blocking caspase activity certainly seems to work. `Cells are designed to know when something's not right, and when something's not right, they kill themselves,' says Kevin J. Tomaselli, Idun's vice president, science and technology. `If you can prevent the cells from killing themselves until the stress is removed, then the cells can carry on their normal business. It's almost as if the cells give up prematurely.'Besides acute myocardial infarction and stroke, Idun is working on inhibiting apoptosis in neurodegenerative diseases, in collaboration with Novartis Ltd (Basel, Switzerland). They are also working on acute liver disease, in particular alcoholic hepatitis. If all goes as planned, Tomaselli foresees his company's anti-apoptotic drugs entering the clinic in one to two years for liver applications, and in two to three years in the CNS and cardiovascular areas.