By David N. Leff

The first rule of warfare is: "Know thine enemy."

One of humanity's major enemies is malaria. Throughout the world's tropics it infects half a billion people a year, and kills 2 million to 3 million of them - mostly children. The endemic disease wages war against the human race via airborne stealth bombers - Anopheles mosquitoes, which drop microscopic smart bomblets - Plasmodium falciparum parasites - into the bites the insect inflicts on its victims.

Malariologists got to know this enemy's heavy weaponry well enough to counterattack with chemical warfare. For centuries, the antimalarial drug of choice was quinine - extracted from the bark of a South American tree, the chinchona. Then in the 1930s, medicinal chemists synthesized a more powerful compound, chloroquine. In fact, it was too powerful for its own good. Chloroquine not only treated malarial attacks but prevented them, so people gobbled the drug to stave off the infection.

That's when P. falciparum deployed its secret weapon: drug resistance.

"Chloroquine resistance developed in about 1960," observed molecular malariologist Alan Cowman," in two parts of the world almost simultaneously - Southeast Asia and South America. And then it spread out from there." Now chloroquine is on the endangered list of antimalarial drugs that the parasite's multidrug resistance genes simply shrug off. (See BioWorld Today, Aug. 20, 1997, p. 1.)

One after another, the latest frontline antimalarial compounds have been added to that list of multidrug resistance (MDR). It includes mefloquine, halofantrine and a worrisome new drug, artemisinin.

Cowman heads the division of Infection and Immunity at the renowned Walter & Eliza Hall Institute of Medical Research in Melbourne, Australia. In 1988, he reported cloning and isolating P. falciparum's multidrug resistance gene, pfmdr1. That gave rise to what he describes as "a fierce debate."

"In 1990," Cowman recalled, "Nature published two papers, back to back. One was from our group on identifying mutations in the P-glycoprotein homologue 1 (Pgh1) - encoded by the pfmdr1 gene - which appeared to be linked to chloroquine resistance. The other was from Tom Wellems' group at the NIH National Institute of Allergy & Infectious Diseases, in which they analyzed a genetic cross between two parasite clones. The title of their paper claims that chloroquine resistance is not linked to multiple drug resistance-like genes.

Gene Manipulation Settled Debate

"So we hadn't been able directly to answer the question," Cowman observed, until the advent just a few years ago of the ability to introduce genes into Plasmodium falciparum meant that we could directly insert mutations into the pfmdr1 gene, and ask the question: What happens to drug resistance when you do that?"

What happened is reported in today's Nature, dated Feb. 24, 2000. The article, of which Cowman is senior author, bears the title: "Pgh1 modulates sensitivity and resistance to multiple antimalarials in Plasmodium falciparum."

"In that paper," Cowman told BioWorld Today, "we showed directly that the mutated protein gives mefloquine resistance, quinine resistance, halofantrine resistance and high levels of chloroquine resistance. Moreover, it affects the new drug, artemisinin - the sensitivity of which is a bit of a real worry, as to the future of that drug in the field.

"We named Pgp1 'Pgp homologue 1,' he recounted, "after the P glycoprotein made in humans, which is involved in multidrug resistance in many cancer cells."

As to the debate, Cowman observed, "Early this month we had a big malaria conference here in Australia, where we presented these results as well. Tom Wellems gave a talk, and he's actually come around, I think, to pfmdr1 being involved, because they have some evidence as well, which they are starting to write up. So the debate is becoming more agreement than disagreement - which is good."

"For an antimalarial to kill P. falciparum," Cowman explained, "the drug has to accumulate in the parasite to a level that's lethal to it. And what pfmdr1/Pgh1 drug resistance does is stop chloroquine or the other compounds from accumulating within the parasite."

However, that's not the only presumed mechanism operating in multiple drug resistance.

"We also believe," Cowman added, "that the parasite may well be transporting - exporting or pumping out - mefloquine and halofantrine directly, though we haven't proven that yet.

"In malaria, the drugs that we've shown to be threatened are all somewhat related. They have a structure that's similar but not identical to that of chloroquine. Whereas," he went on, "artemisinin is totally different. It's a new drug developed by the Chinese from the Artemisia plant. It hasn't been released particularly widely yet, though it's used against malaria in Southeast Asia in particular.

"It's really being recommended only to try and treat severe malaria," he pointed out, "and not let it get out there too much, so that resistance to artemisinin doesn't develop too quickly. The big worry about our results is that we've shown there's a gene out there in the field that can affect artemisinin accumulation, and therefore the level of resistance to that drug already, without artemisinin even being used in the field. So that's a big warning sign for resistance to artemisinin. There's a company in Europe now that's producing large quantities, but not for general availability - to protect it from resistance."

Bearish Market Views Stifle Clinical Potential

Looking toward eventual drug-design antimalarial agents, Cowman made the point, "We now know a lot more about the drug resistance mechanisms, and also about one of the proteins involved in them. Knowledge is power. We now have a target to work with for trying to reverse the effect of the mutations in pfmdr1, in order to restore to usability some of the drugs that P. falciparum is now resistant to.

"But one of the unfortunate things about malaria," he observed, "is that there's not an awful lot of drug companies that are interested, because the market size is not huge.

"When the large drug companies produce their figures," he said, "all they're worried about is the tourist and military markets. I think they forget about the huge middle class that's developing in Southeast Asia, India and in some respects Africa - all potential targets for selling antimalarial drugs and vaccines."