One of the greatest challenges neurologists face is successful delivery of drugs to the brain. This is because a special filtering layer of tissue, called the blood brain barrier, protects the brain and spinal cord. The barrier acts like a molecular sieve, allowing only properly sized molecules through. This means that any medication needing to reach the brain (for example, to kill a brain tumor) needs to be small enough, and even then, it is difficult to target the drug to specifically reach the brain.
Kumar and his colleagues from Harvard Medical School have developed a potentially revolutionary drug delivery method, taking advantage of a known master infiltrator of the brain: the virus responsible for rabies, also known as the rhabdovirus. Rabies viruses travel from the site of infection (a local wound bite) to the nerves, through which it gains access to the brain. It is one of the few viruses known to be nearly 100% deadly to mankind, when vaccination has not been administrated. Kumar and colleagues took advantage of the virus’ neurotropic ability by isolating a protein from the viral outer layer used to bind to the brain cells. They then attached an experimental drug to the purified fragment of protein, a small-interfering RNA. This RNA-peptide complex showed highly specific ability to access neurons in the brain that expressed receptors to the neurotransmitter acetylcholine. This high specificity of drug action was demonstrated to only occur in the brain, and not in other tissues of the body.