By David N. Leff
Some gourmets may pick mahi mahi off the seafood menu. The dogfish shark (Squalus acanthias) is a less chic member of that ferocious but edible fish family.
"The dogfish," said molecular biologist Michael Zasloff, "is a food shark, fished on both coasts of the U.S., to the tune of several million pounds a year."
That number suited Zasloff's needs back in 1993. "It's the only shark you can get ahold of," he told BioWorld Today, "in the quantities that we needed to isolate sufficient amounts of the material to do the studies we had to do."
Zasloff is vice chairman and executive vice president of Magainin Pharmaceuticals Inc., in the Philadelphia suburb of Plymouth Meeting, Pa.
Struck by the fact that antibiotics are present all over the animal kingdom, he decided to take a look at the dogfish shark's chemical defenses. "That shark, about two to four feet long, has been studied by physiologists for many years," he explained, "and they noted how hardy it was."
Zasloff subsequently learned "about the extraordinarily primitive immune system of the dogfish, representing sharks in general. It doesn't have a functional thymus. It cannot produce highly specific antibodies, and would be for all the world one of the extreme immunological knockouts one can imagine. Yet it's very hardy."
He and his colleagues began to search in the fish's tissues for antibiotics. In its liver they discovered a steroid, which they christened squalamine, "after the shark genus Squalus, and the fact that it had a completely novel chemical structure. We went on to demonstrate that this was quite an impressive antibiotic."
How squalamine made the leap from antibiotic to anticancer agent, Zasloff recounted, "came initially from scientists around the country who told me: 'This molecule has features of a number of substances that Judah Folkman identified in the past as anti-angiogenic. It has a steroid, a polyamine, and a sulfate — all in one molecule.'"
Folkman, a surgical oncologist at the Harvard-affiliated Children's Hospital, in Boston, is widely acknowedged at the founding father of anti-angiogenesis — cutting off a tumor's potential for growth by abrogating its growing blood-vessel network. (See BioWorld Today, Dec. 2, 1997, p. 1.)
'Most Potent Anti-Angiogenic Yet'
Zasloff picked up on those hints. " We began to do some studies, and very soon discovered that we had in our hands what I think we can say is the most potent anti-angiogenic agent yet described."
He and neurosurgeon Henry Brem, at The Johns Hopkins University, in Baltimore, are co-senior authors of an early report on that defense mechanism in the current issue of Cancer Research, dated July 1, 1998. Its title is "Squalamine inhibits angiogenesis and solid tumor growth in vivo and perturbs embryonic vasculature." (Brem was one of Folkman's students.)
When a malignant tumor starts to grow, it puts out an SOS to the endothelial cells that line blood-vessel walls. This piratical distress signal, carried by cytokines, growth factors and suchlike chemical messengers, triggers the endothelium to begin putting out branching capillaries to feed the cancer with oxygen and nutrients.
How does squalamine scotch this deadly process?
"We don't have a full picture of how it works," said molecular biologist Jon Williams. "That's true," he added, "for the entire class of compounds that have been discovered and are now being developed as anti-angiogenic agents."
Williams is executive director of biological research at Magainin, project leader for squalamine, and a co-author of the Cancer Research paper.
"Our best understanding," he went on, "is that it works through a receptor-mediated event specific for activated endothelial cells. And it acts pretty early in what we call the angiogenic cascade. It's a multistep process, and we believe that what it does is blunt the effect of a variety of those tumor-triggered growth factors."
As described in their paper, the co-authors tested squalamine's ability to cut off burgeoning tumor-headed blood vessels in a menagerie of animal models — rats, mice, rabbits and embryonic chicks.
Squalamine Worked In Animals; Next Trials In Humans
Fortified by uniformly positive results, showing squalamine to be a highly effective player in the game of anti-angiogenesis, the company has begun two Phase I trials of its safety in humans.
"One site," Williams told BioWorld Today, "is at the Cancer Therapy and Research Center in San Antonio, Texas, which is informally affiliated with the University of Texas. The first patient was enrolled in December of last year; there are now nine so far." The second study, at Georgetown University, in Washington, accrued its first patient in January, and now has 13.
"The reason we're running two Phase I studies," Williams pointed out," is that they have different treatment protocols. We're examining the effect of varying modes of dosage escalation.
"Participating cancer patients are of all kinds. They all have advanced malignancies beyond conventional therapy. We see a scattering of lung, breast, melanoma, sarcoma, colon cancers, and others. Ages range from 21 all the way to an 81-year-old."
The Phase I treatments consist of squalamine infused intravenously nonstop for five days, followed by time out for two or three weeks, then resumed.
"These being safety studies," Williams pointed out, "the primary endpoint is to determine two things: Maximum tolerated dose, and pharmacokinetics of the squalamine in the patients.
"Along the way, we're looking for potential clinical benefit, to see what sorts of hints might be there that would enable us to conduct formal Phase II studies."
On February 24, 1998, the U.S. Patent and Trademark Office issued patent No. 5,721,226 to Magainin, headed, "Method for inhibiting angiogenesis using squalamine and squalamiune steroid derivatives."
Besides squelching a tumor's blood supply, the Magainin scientists are working toward other clinical uses for anti-angiogenic squalamine. "Three immediate areas come to mind," Williams suggested.
"For those of us who work in angiogenesis, the next most important one has to do with eye disease, characterized by abnormal blood vessel growth in the eye. For example, macular degeneration affects literally millions of aged people. At the other end of the spectrum, a small fraction of premature babies experience retinopathy of prematurity, with a very high frequency of blindness.
"Another area is heterotopic ossification, in which abnormal blood vessel growth allows abnormal bone formation where bone shouldn't be. This condition afflicts about 5 percent of people who receive artificial hip replacement, or other physical implants," Williams concluded. *