When a dedicated television viewer of the underwater world, or even a sport diver, thinks of marine life, he is likely to think first of the brightly colored fish that abound in tropical reefs and next of massive corals, orange and red sponges, or purple fronds of sea fans waving with the current. What he almost certainly will not think of are sea squirts. Yet they appear to provide exciting new tools in battles against cancer, viral disease, and other stubborn maladies.
Sea squirts are also called ascidians or tunicates; the latter term is indicative of their tough outer coat, or tunic. Like corals, sponges, and sea fans (gorgonians), they are invertebrate marine animals. However, in their larval stage they have a primitive spinal cord. This places them in a special category of sea animals, the subphylum Urochordata, which is well above sponges on the evolutionary ladder. Some sea squirts grow as individuals and can become as large as a fist. Others grow in colonies, where the individual animals are much smaller.
Colonial tunicates are providing promising new medicines. The most advanced of the ascidian-derived drugs is didemnin B, which takes its name from the Didemnidae, the family of tunicates where it was found. The tunicate, whose full name is Trididemnun solidum, was first shown to produce an active agent against a herpes virus on an expedition to the western Caribbean in 1978. On board the National Science Foundation's ship, R/V Alpha Helix, specimens of every available invertebrate were tested--at sea--against the herpes virus, bacteria, and fungi. Each time Trididemnum solidum was tested, its extract proved to inhibit the herpes virus. It also killed some of the monkey-kidney cells that the virus was grown in. This suggested its potential as an anticancer agent, since most anticancer agents kill cells.
In the laboratory back on land, didemin B was shown to provide protection against leukemia, melanoma, and one kind of sarcoma. Now, some 10 years after its discovery, didemin B is in the middle of clinical trials, sponsored by the National Cancer Institute, as an anticancer agent. The compound also shows some promise as an immunosuppressive agent in transplant surgery and as an antiviral drug.
This tunicate is an unprepossessing candidate for "wonder drug" production. It belongs to a special class, the compound tunicates, in which myriad tiny animals--600 per square inch--share one common tunic. Trididemnum resembles a pale gray, rubbery pancake that overlaps or encrusts rocks and other invertebrates, including the more prominent corals, sponges, and sea fans. It probably survives by chemical defense, using its didemnins to kill the invertebrates that it grows on.
Other colonial tunicates are showing medicinal promise. Eudistoma olivaceum, more obviously a colony, grows especially well on mangrove roots and looks like greenish kernels of corn growing together. Eudistome produces a series of at least 16 compounds, the eudistomins, which include some especially potent antiviral agents.
Another colonial tunicate that grows on mangrove roots is Ecteinascidia turbinata. This sea squirt looks like a bunch of miniature translucent orange-tipped grapes, which can be quite lovely in clear water but are often coated with silt. Ecteinsascidia was shown almost 20 years ago to produce a very powerful antitumor extract.
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