By David N. Leff
By the end of 1999 - the last year of this century and millennium - some 1,221,800 new cases of cancer will be diagnosed in the U.S. But that annual estimate by the National Cancer Institute (NCI) is just the tip of an enormously larger statistic. Multiply that million-plus by 10 or a hundred to count the number of people who die with - not of, but with - malignant tumors in their bodies.
These tiny cancers, the size of a pencil point, were found during autopsies on people who lost their lives in accidents, and never had cancer during their lifetimes. Pediatric surgeon Judah Folkman and his associate, oncologist Michael O'Reilly, at Boston's Children's Hospital speculate these incipient but never-developed tumors were strangled in their cradles, as it were, by natural substances in their hosts' bodies. These had denied those mini-malignancies the network of blood vessels - angiogenesis - that a tumor must have to grow, spread and metastasize.
Folkman and O'Reilly pioneered the discovery three years ago - in the walls of human blood vessels - of two such natural antiangiogenic factors - endostatin and angiostatin. While grooming them for now-imminent human trials, they and their co-workers continued to search for other such inborn compounds by which the body puts a hold on unbridled blood-vessel proliferation - not only in cancers but in normal sources of bloodletting, such as menstruation and wound repair.
In today's Science, dated Sept. 17, 1999, they report an unexpected new tumor inhibitor of angiogenesis, this one generated paradoxically by the tumors themselves. Their paper's title tells the story: "Antiangiogenic activity of the cleaved conformation of the serpin antithrombin." Folkman is the report's senior author, O'Reilly its lead author.
"It's not a tumor's goal, so to speak, to make an angiogenesis inhibitor," O'Reilly told BioWorld Today. "Instead it's just a byproduct of something else that tumors do - and that they need to do. So that's why it's mobilizing all these inhibitors." On this point, he added, "So our thoughts are that these inhibitors, which are all around in the tissues, are normally having a protective response. And that the tumor has to overcome the threshold that is in the tissues around it, in order to overcome the angiogenesis inhibitors before it can grow."
"The phenomenon of a primary tumor inhibiting a secondary tumor has been known to surgeons and immunologists for decades," O'Reilly pointed out. "We hypothesized that the primary tumor was producing - along with epithelial growth factors - angiogenesis inhibitors, which suppress the growth of the tiny secondary sites."
Clotting Cascade Yields Angiogenesis Blocker
Antithrombin III, the latest entry in the tumor-starving sweepstakes, is a well-known player in the body's anti-coagulation-factor cascade. It's a slow-acting plasma protease inhibitor that neutralizes blood-clotting enzymes. But adding heparin, the powerful blood-thinner, revs up antithrombin to rapidly disarm the cascade's clotting factors IX and X.
In their Science paper, the co-authors described how they altered the molecular conformation of antithrombin III, and galvanized it into a compound, aaAT, that prevented wannabe tumors from breaking out of their speck-sized latency and launching angiogenesis. Then they pitted this candidate drug against two human tumors - a neuroblastoma and a lung carcinoma - implanted in nude mice.
These animals received daily injections of aaAT in escalating doses, which inhibited tumor growth in direct proportion to the drug's dose level. At the highest dose, the cancers regressed to small nodules under the skin, without bleeding or toxicity. Meanwhile, in untreated control mice, the tumors grew steadily on an extensive network of arteries, veins and capillaries.
The Science article reported, "This same pattern of regression of established tumors has been demonstrated previously for human angiostatin and endostatin but required substantially higher and more frequent dosing." In 1996, Children's Hospital licensed endostatin and angiostatin exclusivity to EntreMed Inc., of Rockville, Md. That firm will launch a Phase I clinical trial of endostatin at Boston's Dana-Farber Cancer Center by the end of this month, the hospital has said, "with additional trials beginning at M.D. Anderson in Houston and the University of Wisconsin in Madison in the following weeks."
"We do not yet know," O'Reilly pointed out, "how aaAT compares in potency to the entirely unrelated angiostatin or endostatin. The efficacy of these three inhibitors will only be known after Phase II human clinical trials are conducted."
Genzyme Companies Get aaAT License
In January 1999, Children's Hospital licensed aaAT to Genzyme Molecular Oncology (GMO), in Framingham, Mass. "Our goal for aaAT," O'Reilly observed, "would be to try to translate it into something that could be used clinically. And for that reason we've set up a partnership with Genzyme and GMO. The reason for that is that Genzyme Transgenics Corp. [of Framingham] has been making antithrombin III for the past 10 years in transgenic goats." (See BioWorld Today, April 28, 1999, p. 1.)
"So what really made Genzyme the ideal partner," O'Reilly went on, "was the fact they have so much experience with the molecule. It's our hope - which we don't know if we can do yet but are trying - to add a few steps to the protocol they already have. So instead of ending up with an antithrombin that works with an antithrombin, come up with what we want, the antiangiogenic form, aaAT. Our goal is to get this produced in sufficient quantity so we can characterize it and get it to the clinic."
On this score, Genzyme's vice president of medical affairs for oncology, Mark Goldberg, said, "Pending successful completion of replication and preclinical studies, we could file our IND [investigational new drug application] for aaATIII clinical trials as early as the year 2000."
NCI notes that as of August, 20 angiogenesis inhibitors were in clinical trials for cancer patients in the U.S., and seven of them had reached Phase III.
Meanwhile, O'Reilly and his co-authors are "still looking for more angiogenesis inhibitors - to see if tumors are doing the same thing. We now have another human tumor," he added, "a pancreatic cancer, that makes antithrombin III. And other tumors we've been finding," he concluded, "make angiostatin and other inhibitors as well."