Putting A New Spin On Old Drugs
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At the recent symposium of the American College of Veterinary Surgeons, there was an interesting abstract presented about liposome technology. Liposomes are basically teeny-tiny "bubbles" made up of the same basic components as cell membranes. They are sometimes call nanoparticles because they are so small, but they are still much bigger than even large molecules, which gives them some interesting biological abilities. When liposomes are injected into the bloodstream, they go all over the body, but they tend to accumulate where there is tissue inflammation, because blood vessels become "leaky" in inflamed tissue, allowing these nanoparticles to escape the bloodstream. What's great about liposomes is you can put different things inside them (such as drugs), and they help deliver their contents to inflamed sites in greater concentration and over a longer period of time, while reducing the exposure of the other body tissues to whatever they're carrying. This can ultimately help improve the efficacy and safety of the drugs they carry. Liposomes also tend to accumulate in tumors, so they are sometimes used to deliver anti-neoplastic chemotherapy drugs to these sites.
Liposomes have actually been around for a long time. They were first described by Dr. Alec Bangham in 1961, and since then have become valuable tools in biology, biochemistry and medicine. The technology still has its share of problems that need to be worked out. Some people may have acute adverse reactions when liposomes are injected intravenously. Sometimes the body's immune system will "attack" the liposomes, taking them out of circulation prematurely. Much work is still being done to find better ways to help liposomes target particular tissues.
In terms of treating infectious diseases, liposomes can be used to deliver antibiotics to infected tissues. Because of the targeted delivery system, the toxic/side effects of drugs on the rest of the body can be reduced while still achieving the same (or higher) concentrations of drug at the site of infection. An example of this is the drug Abelcet (Enzon Pharmaceuticals), which is a liposomal preparation of the very toxic antifungal drug amphotericin B. Abelcet has been licensed for use in the US and Europe since 1995.
The abstract presented at the ACVS (C. Underwood et al.) described the use of liposomes containing a radioactive marker (99-m Technetium) in horses. They injected 10 healthy horses with a dose of liposomes, monitored them for adverse effects (of which there were none detected), and then used the radioactive marker to determine where the liposomes accumulate in a horse under normal conditions. Their ultimate objective is to determine if liposomes can be used to better deliver antimicrobials to areas of infected bone (osteomyelitis), which are typically very difficult to treat. Liposome technology still has a long way to go before it's being used widely in horses, but this was an important first look at how liposomes can possibly be applied to equine medicine in the future.
Image credit: Kosi Gramatikoff (1999)
