Biomedical Frontiers: Winter/Spring 1996, Vol.3, No.2
Research Advance
Targeting Enveloped Viruses

Scientists have long sought a way to harness targeted toxins to use them therapeutically against pathological cells. This approach, however, has been fraught with problems. Most toxins aren't well targeted, or, if they reach their targets, cannot make it past cell membranes to do damage. Now, a new strategy that uses a toxin from the venom of the snake Bungarus multicinctus is helping Dr. Peter D. Kwong, postdoctoral research scientist, and Dr. Wayne A. Hendrickson, Howard Hughes Medical Institute investigator and professor, biochemistry and molecular biophysics, design a targeted toxin with the potential to act on enveloped viruses.

The toxin, b-bungarotoxin, is a presynaptic neurotoxin with targeting and enzymatic action contained in separate units. The Kunitz subunit of the toxin guides it to its site of action at the neuromuscular junction because of its high affinity interaction with a voltage sensitive potassium channel. Once the toxin is properly located, a phospholipase subunit catalytically degrades the presynaptic membrane, blocking nerve transmission. The snake toxin is highly specific, which makes it suitable for therapeutic adaptation.

In step 1, the potassium channel binding subunit is removed, making the phospholipase subunit non-toxic and safe for therapeutic use. The receptor for HIV, CD4, is then linked to the purified phospholipase in step 2, targeting it against HIV.

In a design for which a patent application has been filed, Dr. Kwong and colleagues propose switching the targeting unit of the toxin so that it is aimed not against presynaptic membranes but against enveloped viruses. Such a targeting is ideal, says Dr. Kwong, because phospholipases degrade extracellular membranes, and the virions of enveloped viruses do not have cellular biosynthetic mechanisms to repair the degradation. This makes enveloped viruses, which include HIV, herpes, influenza, and leukoviruses, particularly susceptible, says Dr. Kwong. Dr. Kwong prepared a conjugate of the separated bungarotoxin phospholipase linked to a recombinant soluble form of CD4, the primary receptor for HIV. When CD4 binds to its receptor on HIV, the phospholipase would attack the membrane of the virus, killing it. Preliminary tests by AIDS researcher Dr. Quentin J. Sattentau show that this conjugate works at least 10 times better than dextran sulfate, a common benchmark of HIV inhibition. The researchers are preparing more potent conjugates.

CD4 binds to gp120, the HIV envelope protein, positioning the phospholipase which degrades the HIV lipid membrane, inactivating the virion.

"If we could use this therapy to reduce the number of active virions, we might change AIDS from a fatal disease to one that is merely chronic."

"If we could use this therapy to reduce the number of active virions, we might be able to change AIDS from a fatal disease to one that is merely chronic and manageable," says Dr. Kwong. The idea of using targeted toxins is not new, he adds, but this method allows it to target virions, which are resistant to conventional drugs