From oncologists to investors, the hot-button word of the late 1990s was "anti-angiogenesis." Both groups were bullish on the hope of starving nascent tumors to death by blocking the angiogenesis that fueled their growth with oxygen-laden lifeblood and other nutrients.

Those wannabe malignancies count on spreading networks of endothelial cells - the new blood vessels that line capillaries, venues and arterioles - to irrigate their swelling cancerous mass with blood, delivered by angiogenesis. But the proliferation of anti-angiogenic drugs in preclinical and human trials have not yet lived up to their promise. One crucial obstacle is finding a way to selectively download a toxic drug in amounts sufficient to destroy those pushy blood vessels.

Today's issue of Science, dated June 28, 2002, carries a research article titled: "Tumor regression by targeted gene delivery to the neovasculature." Its senior author is cellular and molecular biologist David Cheresh, at the Scripps Research Institute in La Jolla, Calif.

"Our overall findings are perhaps somewhat unexpected," Cheresh told BioWorld Today, "yet they make sense. The context is that we're targeting very potent pro-apoptotic genes to the angiogenic blood vessels. So for that reason these studies may provide some insight into what one might expect if we choose the right approach for anti-angiogenic therapy.

"This is the first time, I think," Cheresh continued, "that gene delivery has been targeted to the vasculature. And the first time we've seen a therapeutic benefit by targeting mutant genes to those vessels. I think we've seen a highly selective neovascular guidance system - completely synthetic without bio-based vectors. What's unique about this," he went on, "is that there's no protein here, so there's very little chance of immunogenicity. This is a completely synthetic, organic, lipid-based nanoparticle that can deliver any gene to the antigenic vasculature."

Trio Of Ingredients Cook Up Anti-Angiogenesis

The Scripps concept comprises three key components: a mutant Raf-1 gene; an integrin receptor, avb3; and a nanoparticle delivery vector. Cheresh did an explanatory countdown:

"The Raf gene encodes an enzyme, Raf-1, that transmits information from the outside of the cell to its interior. This kinase sits on the inside of the cell and becomes involved in regulating events such as cell migration, proliferation, survival - all important during angiogenesis. So when we stimulated blood vessels with essentially any growth factor, this particular kinase got called into action. It plays a central molecular role in the signaling events that regulate tissue remodeling - growth, wound healing, angiogenesis, even cancer. So by turning it off, we were looking at a very effective anti-angiogenesis drug. The problem with turning it off everywhere," he added, "is that there are likely to be many side effects.

"Avb3 is an integrin cell-adhesion receptor that is specifically expressed only on angiogenic endothelial cells. And that's a key observation: normal blood vessels don't have it but tumor vessels do. So it's the receptor that helps the cells remodel themselves during wound repair, angiogenesis and cancer. And inhibitors of that receptor can block angiogenesis to some degree. But we extended it well beyond that point by using the alpha v beta 3. It's simply a docking site, an entryway to deliver genes into angiogenic endothelial cells. We used that as our private door to the interior of the blood vessels."

Cellular Rubber Hits The Road

"When a cell migrates, it does so using these integrin receptors to potentiate migration," Cheresh explained. "They are like wheels that make contact with the road. The road is the matrix. The wheels are the receptors that facilitate cell migration. So when a cell moves from point A to point B, as it does during angiogenesis, it uses one or more of these receptors. This particular avb3 receptor is rather unique to angiogenic endothelial cells. That's why we used it.

"The nanoparticle attaches its payload to alpha v beta 3 to turn off angiogenesis. The endothelial cells deliver the Raf gene and from that point on the Raf-1 enzyme causes the tumor cells to die by suicide. The nanoparticle is a delivery vehicle. It's like a bomb that's a guided missile. But it's a smart bomb targeted to the right place. The bomb does the damage, but the guidance system gets the bomb where it needs to go."

The co-authors infused a dose of these nanoparticle packages into the tail veins of mice that had been injected previously with malignant melanoma cells. By six days later, a single treatment had ablated tumors measuring 400 cubic millimeters in size - 1/40th the size of the mouse - or the equivalent of a 2-kilogram (4.4-pound) tumor in an 80-pound person. Animals with metastases to lungs or liver also saw most of their malignancies vanish. In contrast, control mice without the avb3 beacon molecules to guide the nanoparticle vector died after a day or two.

Pediatric surgeon Judah Folkman, at Harvard-affiliated Children's Hospital in Boston, first introduced tumor angiogenesis some three decades ago. He commented on the Scripps report in Science. "It's a very provocative paper, which I think will become a landmark in angiogenesis research."

"Where do we go from here?" Cheresh asked rhetorically.

"The next step," he said, "is to further develop the technique as a general approach toward cancer therapy. We're working not just to turn off angiogenic endothelial cells but to find ways of turning them on, following ischemic stroke and heart attack. These nanoparticles," he added, "may be useful in several other entities besides heart disease and stroke, where angiogenesis plays a major role - like rheumatoid arthritis, and blindness-causing age-related macular degeneration and diabetic retinopathy.

"I would say we're maybe two years away from human trials," Cheresh allowed. "I would estimate we'd first go after cancer, because our preclinical data is very strong. The surprising thing is that we're getting tumor regression - in many cases complete regression after a single treatment. And this is something that's not been seen before.

"Scripps and Stanford have applied for patent coverage," Cheresh observed. "The discoveries were made in my laboratory and that of our Science co-author, Mark Bednarski, at Stanford. They have licensed our inventions," he concluded, to Merck KGaA in Darmstadt, Germany, "which supported the work in my lab."