By David N. Leff
To discover a new drug and bring it to market takes a decade or more and can cost north of $100 million.
PCR in the mid-1980s and combinatorial chemistry in the mid-1990s have enormously accelerated and refined this still-costly process.
When combinatorial chemistry burst on the drug-designing scene, biotekkies used a single word to acclaim its effect: explosive.
Briefly stated, this post-genomic stage of biotechnology involves creating astronomically vast libraries of random organic molecules and querying them with a compound of interest. Analogs that pop up can then be screened for bioactivity and other desired properties, as well as harmful side effects, specific to the disease of interest.
It took the proto-human progenitors of Homo sapiens a couple of million years to advance from the first stone tools to the technological sophistication of combinatorial chemistry.
Beetles had a head start.
Coleoptera — beetles — are the largest order of insects; they emerged around 100 million years ago. Now, one Coleoptera species, the squash beetle (Epilachna borealis) has been caught in the act of practicing combinatorial chemistry without a license.
Bugs Scramble Chemicals Just Like Folks Do
A first report of this surrealist discovery appears in the current issue of Science, dated July 17, 1998. Its title is "Combinatorial chemistry in insects: A library of defensive macrocyclic polyamines." The paper's co-senior authors are biologist Thomas Eisner and organic chemist Jerrold Meinwald, both at Cornell University, in Ithaca, N.Y.
"This paper reports a beetle doing combinatorial chemistry," Meinwald told BioWorld Today. "God knows when, but obviously a very long time before humans came on the scene."
Squash beetles, once a wide-ranging agricultural pest, are now found, for reasons unknown, only on Long Island, N.Y., where they munch on squash, melons and suchlike plants.
Like its immediate genomic relative, the ladybird or ladybug, E. borealis is about a centimeter long with a dome-like shape. "It has a yellowish-brown background with dark spots," said chemical ecologist Scott Smedley, a co-author of the Science paper. "Its pupa," he told BioWorld Today, "is also strikingly colored, a brighter yellow than the ladybug, with spiny black hairs coming off the body."
In an insect's life cycle, pupae are the stage between larvae and the winged mature form.
"A pupa," Meinwald pointed out, "is defenseless. It can't run away or fly away. Unlike the larva which comes before it and the adult which comes after it, a pupa is sort of a sitting duck. Therefore it would benefit greatly from protection against predators.
"Squash beetle pupae have a dense forest of fine, hollow hairs coating almost all of their body surface," Meinwald went on. "And there's a tissue at the base that's secreting chemicals through those hollow hairs. They pile up as a beautiful, clear little spherical droplet at the end of each hair."
He continued: "When an ant or other predator comes up to a pupa, if he touches a droplet with his antennae or mandibles, then he is repelled. These chemicals are repellant to potential predators."
Smedley described a laboratory experiment he conducted to characterize this squash beetle's pupal defense mechanism:
"The predators we used in our bioassays," he recounted, "are a common North American ant, dark with a typical heart-shaped abdomen. On contact with the pupa, the ants didn't curl up and die. Contact didn't have any lethal effects, but it certainly didn't lead to a happy ending.
"The ants backed off and began cleaning," Smedley went on. "They have a rather stereotypical cleaning behavior, where they comb their antennae with their front legs, then proceed to clean those legs off by running them through their mouth. At this point, ants became very ill, from that contact with the pupa.
"The oily secretion that they had scraped off it and put into their mouths caused them to vomit. And then they spent a fair amount of time dragging their heads across the bottom of the dish for our bioassay."
That potent insect-repellant, as described in the Science paper, consists of macrocyclic (large closed-ring) molecules rich in nitrogen, belonging to the alkaloid class of compounds. Caffeine, nicotine, morphine and many other plant-produced compounds, — usually ending in "-ine" — are all alkaloids.
"The ladybird beetle," Smedley observed, "happens to belong to a terribly skilled group of alkaloid chemists."
Genetics Of Defense Mechanism Still Unknown
The beetles manifested that aptitude by resorting to combinatorial chemistry.
"They secrete this very complex mixture of chemicals," Meinwald observed, "20 percent of which is vitamin E. We had found that part several years ago and published it. But we knew we were missing something because vitamin E doesn't repel ants.
"So we looked again," the Cornell chemist recalled, "and found the 80 percent that we'd missed before. And that is this new combinatorial library of varied compounds.
"It's composed of chemicals that themselves are a bit unstable. So they're slowly changing into other things, on a time scale such that 10 percent of it is changed in a couple of days. It's slowly converting to something else, in fact into quite a mixture of something elses. So each one is giving rise to many many chemical children.
"The mixture is so complicated," Meinwald allowed, "that we have very little idea of the action of the individual chemicals. We did synthesize one member of this secretion and we know what it does."
As for the size of that combinatorial library, Meinwald guesses "there are certainly several hundred discrete chemicals, and it might be several thousand. It's very hard to tell. The analysis goes up to the detection limits of what's the smallest amount of things we can detect.
"I think it would be very interesting to know the mechanism whereby the beetle is producing this array of compounds," he said. "Ideally, what we'd like to know is the genetics behind what makes those enzymes, so one could see the genetic control of this process.
"It's unlikely that we ourselves would go into the genetics," he observed, "because that's not a thing in which any of our group has expertise. We might do that with colleagues who do have those interests. *