By David N. Leff

After two atomic bombs exploded over Hiroshima and Nagasaki half a century ago, the world woke up to the fact that humanity had perfected the means of its own decimation.

Now, molecular parasitologists must face the fact that malarial parasites are perfecting new strategies to disarm antimalarial drugs, not just one at a time but wholesale—and faster than new drugs can be developed. This threat to further decimate the human victims of malaria, warned biologist Pradip Rathod, has "public health implications that are ominous."

As it is, malaria infects some 300 million people a year worldwide, and one to two million of them, mostly children, die each year of the tropical disease.

Rathod, a faculty member at The Catholic University of America, in Washington, pointed out that "it took more than 30 years for malarial parasites to develop widespread resistance to the drugs chloroquine and Fansidar, developed after World War II. Yet, resistance to new antimalarial drugs Mefloquine and atovaquone occurred within one year after their introduction." (See BioWorld Today, Jan. 12, 1996, p. 1.)

A research report in the current Proceedings of the National Academy of Sciences (PNAS), dated Aug. 19, 1997, of which Rathod is principal author, carries the bland title: "Variations in frequencies of drug resistance in Plasmodium falciparum." Those variations spell out the "accelerated resistance to multiple drugs" (ARMD) now found increasingly, and alarmingly, in the parasites' anti-human arsenal.

"Continual exposure of malarial parasite populations to different drugs," the PNAS paper suggests up front, "may have selected not only for resistance to individual drugs but also for genetic traits that favor initiation of resistance to novel unrelated antimalarials."

Two such novel and potent drugs are 5-fluoroorotate and atovaquone. When Rathod and his co-authors challenged large populations of P. falciparum clones from Africa and Southeast Asia in vitro, they discerned an intriguing geographic gradient of resistance.

At one extreme, P. falciparum clones from the West African nation of Sierra Leone proved entirely sensitive (susceptible) to 5-fluoroorotate, as well as to chloroquine and all other traditional antimalarial agents, and the two new entries. At the opposite end of the scale, parasites from Indochina displayed total resistance to all drugs thrown at them, including the newest, 5-fluoroorotate, which had not yet reached field testing.

The Indochina strain acquired resistance to both novel agents 1,000 times more frequently than did the West African.

The grim conclusion of Rathod's ARMD research: The ability of individual malarial parasite clones is related to their prior drug-resistance history.

Plumbing The Depths Of Parasite's Cellular Plumbing

"The malarial parasite," said molecular and cell biologist Mark Wiser, "is a fascinating organism."

P. falciparum's round-trip peregrination in its human victim begins with the bite of an anopheles mosquito. This squirts a dollop of the parasite into the bloodstream, whence it makes its way to the liver, then on to the erythrocytes (red blood cells), and finally—via a sex change—back to the bloodstream, to be taken up again by the mosquito.

The stops and starts along this intricate route offer targets of opportunity to malariologists seeking sites and substances for possible vaccines or antimalarial drugs. A report of the latest such discovery appears in the same current PNAS. Its title: "A novel alternate secretory pathway for the export of Plasmodium proteins into the host erythrocyte."

Wiser, the paper's lead author, told BioWorld Today: "As far as I know, it's a unique finding. No one has ever described an organism that has two distinct endoplasmic reticula [ER]. There appear to be two distinct ER in the malarial parasite."

He explained: "The ER is an organelle found in all eukaryotic cells. Its function over all is involved either in ferrying proteins to other organelles within the cell, such as the Golgi apparatus, or secreting proteins from the cell. But what we think we're seeing," Wiser added, "is two ER in P. falciparum. One does all the normal ER things; the other, the novel ER we're describing, specializes in exporting proteins from the parasite, which it pumps into specific sites within the erythrocyte."

Potential Drug-Discovery Target

Wiser, a researcher at Tulane University's department of tropical medicine, in New Orleans, expressed his conviction that "the process by which the parasite modifies its red blood cell host is crucial for its development. If you could somehow stop them, if you could design drugs targeted at this second ER organelle, but non-toxic to humans, and prevent the export of the proteins and modification of the host cell by the organism, that would be a target by which you could kill the parasites.

"The question we were pursuing is: How does the parasite actually get the proteins out? They don't just magically appear out there. There has to be a movement from the parasite to the erythrocyte membrane," Wiser said.

"What we stumbled across was that one of the first steps is that there appears to be a very novel organelle that the numerous exported proteins go to first. From there they exit the parasite and take up housekeeping in the erythrocyte it's just entered."

At this stage, "the parasite is like a fixer-upper. That red blood cell will be its home for the next 48 hours, and it will raise a family there.

In this erythrocyte-inhabiting, trophozoite stage, Wiser said, the parasite measures about three to four microns across, for starters; the red blood cell, seven.

Wiser's "two major goals for the future are to ask questions about the signals on the exported proteins that target them to this novel compartment instead of to the ordinary ER, and to biochemically characterize the process. This," he concluded, "would involve recombinant DNA and transfections, and fairly complex experimental things." *