CURE IN A HAYSTACK
Analysis is a complex process which attempts to separate the plant sample into smaller and smaller parts until the chemical of interest is isolated. The problem is that plants make so many compounds—up to five hundred or six hundred different compounds in the same leaf, each with fifty or sixty different biological activities. The real trick is to identify which one is performing the miracles.
First the sample is milled, distilled into a tarry sludge, and then treated with chemicals to separate out the essence of the plant. This essence is then pitted against many known human diseases to see whether it will take any action. The National Cancer Institute, for instance, is testing forty-five hundred samples a year, seeing if they have an effect on HIV-infected cells and sixty different kinds of tumor cell lines representing the various types of cancers, such as brain tumors, leukemias, and melanomas. (Ultimately the institute hopes to test twenty thousand substances a year against one hundred cell lines.) If a certain extract looks promising, it is further separated into its component chemicals, each of which is tested again. The most active ones are mapped on a molecular scale to see how their chemical structure may be contributing to what they do.
Once a promising molecule is identified, scientists can try to synthesize it in the lab, adding different twists in hopes of making it more effective. If a facsimile can’t be made artificially, plant-tissue culture techniques may come to the rescue. A plant-tissue culture is a vat of plant cells all grown from a few starter cells. The cells produce copious amounts of the product, which is then separated out of solution. If the product passes all the tests of effectiveness, a company or government might try to invest the money needed to bring it to market—about $230 million for the average drug.
In the past, this assaying for bioactivity was a slow procedure—you injected the extract into a rabbit and waited to see what happened. Bioassaying in the test tube has speeded up the process, but it’s still a needle-in-a-haystack procedure. For every twelve thousand samples, only one becomes a drug, and its development (tweaking, enhancing, and testing the substance) can take ten years or more. In short, we’re spending precious time in the lab screening unpromising compounds—and we don’t have that kind of time. Experts agree we need to develop some sort of prescreening procedure to narrow our search and help us quickly key in on the promising compounds before the species that hold the recipes disappear.
How we have gone about narrowing that search tells a lot about us as a culture. At first, we simply dragged our collecting nets across the whole jungle in an indiscriminate approach. Collecting everything was easy, but the holdup was back at the lab—samples piled up waiting for analysis, and in the jungle, species were going extinct before we could even sample them. The fear was that by the time we found the cure for cancer or AIDS, then went back out to find more of the sample to study, it would be gone, plowed out by a bulldozer to make room for cattle or housing. There had to be a way to speed the search.
Next, we thought we’d be logical and try tracing the family tree of a sample that we found promising, hoping that related species would also contain powerful compounds. (For example, lilies are rich in alkaloids, so let’s investigate the closely related orchid family. Bingo, they’re rich in alkaloids, too.) This narrowing approach is called the phylogenetic strategy, but it’s limited as well. Not all plant relatives arrive at the same chemical solutions to sticky predator problems.
Finally (and reluctantly), we in the Western world decided to officially solicit the help of shamans, indigenous folk healers from tribes that have been using the jungle pharmacy for centuries. We had relied on folk medicine heavily in the past, although this fact was never advertised. As Deputy Editor Philip H. Ableson writes in an April 1994 Science editorial: “Of the 121 clinically useful prescription drugs worldwide that are derived from higher plants, 74% of them came to the attention of pharmaceutical houses because of their use in traditional medicine.” But we rarely advertised our sources, nor did we formally seek their help. Today, schoolchildren know the names Fleming and Pasteur and Salk, but the names of shamans in the Amazon and in Africa are on the tip of no one’s tongue.
Finally, ethnobotany has begun to lose the stigma of a fringe discipline, and is now attracting both funding and professional personnel. A handful of organizations are trying to contact the last remaining indigenous cultures that have lived close to the Earth. Dialogues with their shamans have yielded several important compounds, including an oral hypoglycemic for diabetics, a respiratory virus fighter, and a possible antidote for herpes simplex. All three are reaching clinical trials thanks to a fleet-footed firm called Shaman Pharmaceuticals in south San Francisco that employs ten ethnobotanists on three continents. Another exciting prospect coming from folk remedies is prostratin, shown to be active in the test tube against HIV.
Fieldgoing ethnobotanists often speak of being outclassed by the native people, who have uncanny plant knowledge. The legendary Richard Evans Schultes, who has combed the Amazon for healing strategies for over forty years, writes that natives in the Amazon are able to differentiate between chemivars—plants that appear similar in form and yet have quite different chemical properties. Although Western botanists can’t find any morphological differences among chemivars, the Indians can identify them by sight, even from many paces away. They say they base their identification not only on the physical look of the plant but also on its age, its size, and the kind of soil in which it grows. This sort of knowledge is dying, says Schultes, especially if healers do not have apprentices, or if their people have adopted pills over plants.
Cultural Survival, a culture advocacy group, estimates that the world has lost 90 of its 270 Indian cultures since 1900, about a tribe a year, and with them, all their knowledge. As Schultes writes in a March/April 1994 article in The Sciences, “…the Earth is losing not only the biodiversity of the forest; it is also losing what I call its crypto-diversity, the hidden chemical wealth of the plants.” He calls on us to use native cultures as rapid-assessment teams already on the ground, but warns that as “civilization” encroaches, we can lose, in only one generation of acculturation, botanical knowledge acquired over millennia.
Ethnobotanists, then, like the biomimics, are also in a race. To narrow their search, they concentrate on cultures that are in floristically diverse areas, that transmit their healing knowledge through the generations, and that have resided in one place for long enough to explore and experiment with local vegetation. Based on those criteria, is there any culture that we’re forgetting? Any source of local expert knowledge that we might be overlooking?
After spending time with Wrangham, Strier, and Glander, I immediately think of chimps and muriquis and howlers. All are local experts, passing knowledge from mother to offspring, and living in floristically diverse areas. Instead of thousands of years, these animals have been conducting millions of years of field trials. Their self-medication is more ancestral than that of indigenous peoples, and comes without the overlay of religious taboos or tribal customs. Why not let their “nose” for what is curative help us zero in on bioactive compounds, making the screening process more efficient?
Daniel Janzen, a tropical ecologist at the University of Pennsylvania, explains it this way: “I think there are better ways to spend money [than random sampling]—it’s too broad shot. How do you know what to collect? How do you know which tree of the same species to collect? They differ in chemical composition—one tree may be stressed and one not stressed. Primates and birds and lizards know.”
By getting to know them, say the zoopharmacognosists, we may begin to know, too.
ECOLOGICAL SLEUTHING:
BIORATIONAL DRUG DISCOVERY
One of the most promising ways to explore the natural world, and to further narrow our search, is called biorational drug prospecting, a strategy advocated by Dan Janzen and Tom Eisner. The biorational route goes beyond simply following chimps and howlers around the jungles. It challenges us to use i
nformation from the entire ecosystem to find our target molecules. It requires that we know something about the relationships around us—the coevolutionary tangos of herbivore and herb, the community webs, the interlacing of population with bioregion. “I would use the whole set of animals out there,” says Janzen. “Humans are just one animal…and they only pick the stuff that doesn’t give them a stomachache or make them go blind.” It is a detective game that dares us to use all our senses as well as our sense of ecology to ferret out the bio-clues.
Tom Eisner’s father was a chemist who used to make cosmetics in his basement, leaving the lower floor “smelling in the most interesting ways.” The younger Eisner developed an uncanny nose, allowing him to actually smell insects as he walked, identifying those full of potent chemicals. “Flying molecules,” he calls them.
As an outgrowth of his work with insects, Eisner perfected the art of seeing what isn’t obvious, finding, as he calls it, the “unforeseen from the unexpected.” When prospecting for possible drugs, for instance, Eisner will look for plants that are notably free of damage. Plants that insects avoid eating must have potent defenses, he reasons, and should be screened for bioactive secondary compounds. Similarly, a tree that has no plant growth around its stem or is conspicuously free of disease should be checked for growth inhibitors or antibiotics that can serve as models for new herbicides and antimicrobials. If ants reject a fallen leaf, or predators avoid an insect’s egg when it is covered with its mother’s saliva, chemistry is at work, and ecology has handed us a clue.
Ecological sleuthing has already helped us zero in on compounds that naturally repel or kill insects. Commercially available formulations of natural plant-derived insecticides include nicotin, pyrethins, and rotenoids. These natural products are a welcome addition to a field of disturbingly less effective pesticides synthesized from petroleum. May Berenbaum of the University of Illinois at Urbana-Champaign describes the rat race we have entered with synthetic pesticides and the pests that learn to resist them. “The use of increasingly higher concentrations of existing insecticides has led to a fourfold increase in agricultural pests that manifest resistance to at least one type of insecticide. Where are the new pesticides? Not many have been developed since 1960, and the standing prescription is simply to spray more and more chemicals.” A new crop of insecticides—those without a deadly residue that collects in animal tissues—would provide needed relief.
Other ways to find drugs from bugs is to watch how venomous animals handle both their enemies and their prey. Any substance that can have such a profound effect on the victim—paralyzing, poisoning, or even breaking down its cellular matter with a single dose—is bound to have powerful biochemical or pharmaceutical properties. Natural Product Sciences in Salt Lake City, with funding from the large pharmaceutical firm Pfizer, is looking into the toxins of spiders, snakes, and scorpions. These compounds, which attack specific neurochemical targets, are already helping researchers identify tiny openings in the membranes of human neurons that admit charged molecules called ions. Since ion channel activity is important in the signaling of nerve cells, the company hopes to develop drugs for relieving anxiety and depression, strokes, and degenerative neurological diseases.
Besides looking at individual organisms, biorational prospectors are also identifying settings that they believe will be particularly rich in toxins. Environments where animals must always be on their guard against high levels of disease or parasitism are like giant breeding grounds of chemical inventiveness. The defenses that animals evolve in these settings may yield magic shields for us as well.
The ocean tops the list of promising settings for biodiscovery, says D. John Faulkner, professor of marine chemistry at the Scripps Institution of Oceanography. Here, the sheer diversity of plants and animals far exceeds what you can find on land. Marine creatures are literally awash in the chemical byproducts of other creatures, and their watery world is teeming with microbes. To stave off poisons or diseases, they have had to defend themselves in novel ways.
A doctor named Michael Zasloff began to appreciate this when he noticed an extraordinary defensive immunity in dogfish sharks (Squalus acanthias); though they were often scarred in fights, they didn’t develop infections. Looking closer, Zasloff isolated a powerful new antibiotic called squalamine from the shark. Zasloff also discovered—and was later able to synthesize—two slightly different strains of a powerful new antibiotic produced in frog’s skin. The discovery grew out of his observation that surgical wounds in frogs healed without inflammation and were rarely infected after the frogs were thrown into a murky aquarium. The antibiotics, which Zasloff calls magainins (from the Hebrew for “shield”) are the first chemical defense other than the immune system to be found in vertebrates. This doctor-turned-biomimic has since left his post as chief of human genetics at the Children’s Hospital of Philadelphia to form Magainin Inc., a company founded on the idea of biorational drug discovery.
Zasloff isn’t the only hunter at sea. C. M. Ireland of the University of Utah reports that during the 1980s alone, seventeen hundred compounds with bioactive properties were isolated from marine invertebrates. Despite this obvious wealth, it’s only been in the last two decades that scientists have started systematically scouring the world’s oceans for helpful chemicals.
As a general rule in biorational discovery, says Charles Arneson, of the Coral Reef Research Foundation, biologist-divers look for creatures that should be vulnerable, but aren’t. For example, Spanish dancer (Hexabranchus sanguineus), a tasty-looking six-inch sea slug, is rarely bothered, despite the fact that it is not protected by a shell and moves at a slug’s pace. Its secret shield turned out to be a noxious chemical that now forms the basis for an anti-inflammatory drug. Spanish dancer also puts out flowerlike egg masses that biochemist Faulkner says “look good enough to eat” but have no takers. Upon investigation, Faulkner and his students found that the sea slug sequesters powerful compounds from a sponge that it eats and concentrates them in its eggs. These compounds do more than repel predators; they also have antifungal properties, and have shown some activity against human tumors!
Other examples of drugs from the deep that are being explored by U.S. scientists:
Discodermolide, from the Bahamian sponge Discodermia dissoluta, is a powerful immunosuppressive agent that may have a future role in suppressing organ rejection after transplant surgery.
Bryostatin, from the West Coast bryozoan (moss animal) Bugula neritina, and didemnin B, from a Caribbean tunicate (sea squirt) of the genus Trididemnum, are both in clinical trials as cancer treatments.
Pseudopterosin E, from the Caribbean gorgonian coral (Pseudopterogorgia elisabethae), and scalaradial, from dictyoceratid sponges found in the western Pacific, are both being studied as anti-inflammatory agents.
On land, biorational drug prospectors can find crowded, oceanlike conditions wherever colonies of organisms gather to breed in close quarters. Seals breeding by the thousands on the same beach, for instance, would provide a fertile environment for disease microbes to flourish, which would in turn encourage the evolution of microbial foes. Presumably, the individuals that managed to fight off infections in such crowded settings would be chock-full of ingenious antibiotics, some of which may benefit us.
Finally, we’d be smart to pay attention to “extremophiles”—creatures that survive scorching temperatures, months of being frozen, or extreme salinity. These tough cookies are the special forces of living creatures, thriving in environments that would wilt lesser species. By deliberately looking for creatures that awe us, we may just stumble upon a whole new chemistry—the spoils of survival.
SWALLOWING OUR HUBRIS
Not surprisingly, there is a great hue and cry against this biorational approach to drug prospecting. It seems that the idea of animals being wise about their world is something that is hard for some Sons and Daughters of Baconian Science to stomach. Robert M. Sapolsky, associate professor of biological science and neuroscience at Stan
ford University and author of Why Zebras Don’t Get Ulcers, came down hard on the zoopharmacognosists in an opinion piece in the journal The Sciences in early 1994. He cautioned readers that the medicinal effects of pith drinking and leaf swallowing, which he called “eating episodes,” were at the moment no more than anecdotal evidence. He claimed that zoopharmacognosists were treading into the New Age realm by attributing wisdom to animals without having the scientific evidence to back it up. How do we know the animal is actually getting medical doses from what it eats? asked Sapolsky. How can we tie cause to effect?
Wrangham, Huffman, Strier, and Rodriguez defended their new field in a subsequent article in The Sciences. They admitted that animal self-medication has not yet been proven, nor has it been shown that animals have innate knowledge of medicinal plants. They know there is a lot more work to do. But while Sapolsky says whoa, his targets say, look, just because it’s complex doesn’t mean we should abandon the possibility that there’s some wisdom here that we could learn from. As the four zoopharmacognosists wrote in their rebuttal, “Even if this endeavor doesn’t show us new drugs, we think it worthwhile merely because there’s a range of animal skills waiting to be discovered [emphasis mine]. Of course it’s tough when you depend not only on intimate knowledge of a population’s behavior, but also on rare events that are difficult to manipulate experimentally. But we can take the anecdotes and build them into evidence.”
Their closing remark could well become the rallying cry of biomimics everywhere: “In an era of shrinking biological resources, we don’t think it’s such a good idea to rely only on studies of laboratory rats. Let’s keep an open mind about ways to explore the natural world.” Amen.
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