"Every drug is essentially a poison," said J. Lawrence Marsh, professor and director of the developmental biology center at the University of California at Irvine.
"But by combining several drugs whose point of intersection is at one disease, you can use each individual drug at lower doses and perhaps reduce toxicity. It's the chemical equivalent of radiation surgery," he told BioWorld Today.
In the research, published online before print in the Proceedings of the National Academy of Sciences on Feb. 16, 2005, Marsh and colleague Leslie Michels Thompson from UC Irvine's department of psychiatry and human behavior, as well as other scientists from UC Irvine and the University System of Taiwan, test such a combinatorial approach for the treatment of Huntington's disease. Their model system is Drosophila melanogaster, known to its friends as just Drosophila, or the fruit fly.
Asked to explain the advantages of using Drosophila as a model system, Marsh responded, "fast and cheap," then elaborated: "The reason to test in fruit flies is to speed the process and improve the success rate of things you're going to test in mice. So far, [biological processes] in fruit flies have been proven to be very highly conserved in mammals, and you can test things in three to four weeks in fruit flies that will take you the better part of a year in a mouse."
If pre-testing promising compounds in Drosophila can improve the success rate of mouse testing from 1 percent to 2 percent, it would save significant amounts of time and money. Marsh also pointed out that in the fruit fly, drug targets can be validated by genetic manipulation more easily than in mammals.
Huntington's disease is an autosomal dominant neurodegenerative disorder caused by the misfolding and subsequent aggregation of mutant huntingtin protein. Huntington's is best known for the motor impairment it produces in its victims, but it also affects cognitive processes.
Transgenic Drosophila-expressing mutant human huntingtin genes already were known to have degenerated photoreceptors. In the new findings, the researchers first tested whether that photoreceptor damage was indicative of more widespread neurodegeneration. In anatomical investigations, they found flies expressing mutant huntingtin showed neuronal loss and degeneration in several areas of the nervous system, including in their mushroom body, a part of the fly brain that is essential for complex behaviors including both movement and memory.
Triple Combinations Prove Fruitful
The scientists next tested two different triple combinations of drugs for their ability to prevent neurodegeneration, as well as improve motor function in transgenic flies. The first set of drugs, consisting of Congo red, cystamine and the histone deacetylase inhibitor SAHA, "is fairly well described individually as having promise against Huntington's disease, but the mechanisms are not entirely clear in each case," Marsh said. For each compound, the researchers first determined a threshold dose, at which the compound by itself neither reduced neural degeneration, nor improved survival or motor function. When two or three drugs were given in combination, however, both neural degeneration and motor function were improved.
In contrast, the second set of drugs targets have well-known but different cellular pathways. The second set consisted of geldanamycin, which regulates heat-shock proteins; a kinase inhibitor, Y-27632, which suppresses the formation of huntingtin aggregates in mammalian cells; and SAHA again.
In those studies, the triple combination, as well as combinations of either geldanamycin or Y-27632 with SAHA, at threshold doses reduced neurodegeneration, while combining geldanamycin with Y-27632 provided no benefit. The authors concluded that "pre-testing in Drosophila can identify promising pharmacologic combinations and regimens for testing in mammals and can reveal other combinations that either are redundant or for other reasons seem less promising."
Of course, the flip side of combinatorial therapy is that drugs given in combination can also potentiate each other in their side effects. Asked about that possibility, Marsh agreed that, "if you combine some of [those agents] at higher doses, they definitely have added toxicity." He believes the additive toxicity is behind another effect the researchers observed in the studies: When Congo red, cystamine and SAHA were given in combination at individually effective doses, the scientists saw reduced effectiveness compared to individual administration. "And," Marsh pointed out, "you'd like to know that from a fly before you find out from a person in a trial."